"Science, Faculty of"@en . "Earth, Ocean and Atmospheric Sciences, Department of"@en . "DSpace"@en . "UBCV"@en . "Greene, Andrew R."@en . "2009-02-27T21:58:38Z"@en . "2008"@en . "Doctor of Philosophy - PhD"@en . "University of British Columbia"@en . "The Wrangellia flood basalts are parts of an oceanic plateau that formed in the\r\neastern Panthalassic Ocean (ca. 230-225 Ma). The volcanic stratigraphy presently extends\r\n>2300 km in British Columbia, Yukon, and Alaska. The field relationships, age, and\r\ngeochemistry have been examined to provide constraints on the construction of oceanic\r\nplateaus, duration of volcanism, source of magmas, and the conditions of melting and\r\nmagmatic evolution for the volcanic stratigraphy.\r\nWrangellia basalts on Vancouver Island (Karmutsen Formation) form an\r\nemergent sequence consisting of basal sills, submarine flows (>3 km), pillow breccia and\r\nhyaloclastite (<1 1cm), and subaerial flows (>1.5 km). Karmutsen stratigraphy overlies\r\nDevonian to Permian volcanic arc (~380-355 Ma) and sedimentary sequences and is\r\noverlain by Late Triassic limestone. The Karmutsen basalts are predominantly\r\nhomogeneous tholeiitic basalt (6-8 wt% MgO); however, the submarine part of the\r\nstratigraphy, on northern Vancouver Island, contains picritic pillow basalts (9-20 wt%\r\nMgO). Both lava groups have overlapping initial EHf and ENd, indicating a common, ocean\r\nisland basalt (OIB)-type Pacific mantle source similar to the source of basalts from the\r\nOntong Java and Caribbean Plateaus. The major-element chemistry of picrites indicates\r\nextensive melting (23-27%) of anomalously hot mantle (~1500\u00C2\u00B0C), which is consistent\r\nwith an origin from a mantle plume head.\r\nWrangellia basalts extend ~450 km across southern Alaska (Wrangell Mountains\r\nand Alaska Range) and through southwest Yukon where <3.5 km of mostly subaerial\r\nflows (Nikolai Formation) are bounded by Pennsylvanian to Permian volcanic (312-280\r\nMa) and sedimentary strata, and Late Triassic limestone. The vast majority of the Nikolai\r\nbasalts are LREE-enriched high-Ti basalt (1.6-2.4 wt% Ti0\u00E2\u0082\u0082) with uniform plume-type\r\nPacific mantle isotopic compositions. However, the lowest ~400 m of stratigraphy in the\r\nAlaska Range, and lower stratigraphy in Yukon, is light rare earth element (LREE)\r\ndepleted low-Ti basalt (0.4-1.2 wt% Ti0\u00E2\u0082\u0082) with pronounced negative-HFSE anomalies\r\nand high Elf values that are decoupled from Nd and displaced well above the OIB mantle\r\narray. The low-Ti basalts indicate subduction-modified mantle was involved in the\r\nformation of basalts exposed in Alaska and Yukon, possibly from mechanical and\r\nthermal erosion of the base of the lithosphere from an impinging mantle plume head."@en . "https://circle.library.ubc.ca/rest/handle/2429/5282?expand=metadata"@en . "14967289 bytes"@en . "application/pdf"@en . "WRANGELLIA FLOOD BASALTS IN ALASKA, YUKON, AND BRITISH COLUMBIA: EXPLORING THE GROWTH AND MAGMATIC HISTORY OF A LATE TRIASSIC OCEANIC PLATEAU By ANDREW R. GREENE A THESIS SUBMITTED iN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY in THE FACULTY OF GRADUATE STUDIES (Geological Sciences) UNIVERSITY OF BRITISH COLUMBIA (Vancouver) August 2008 \u00C2\u00A9Andrew R. Greene, 2008 ABSTRACT The Wrangellia flood basalts are parts of an oceanic plateau that formed in the eastern Panthalassic Ocean (ca. 230-225 Ma). The volcanic stratigraphy presently extends >2300 km in British Columbia, Yukon, and Alaska. The field relationships, age, and geochemistry have been examined to provide constraints on the construction of oceanic plateaus, duration of volcanism, source of magmas, and the conditions of melting and magmatic evolution for the volcanic stratigraphy. Wrangellia basalts on Vancouver Island (Karmutsen Formation) form an emergent sequence consisting of basal sills, submarine flows (>3 km), pillow breccia and hyaloclastite (<1 1cm), and subaerial flows (>1.5 km). Karmutsen stratigraphy overlies Devonian to Permian volcanic arc (\u00E2\u0080\u0094\u00E2\u0080\u0098380-355 Ma) and sedimentary sequences and is overlain by Late Triassic limestone. The Karmutsen basalts are predominantly homogeneous tholeiitic basalt (6-8 wt% MgO); however, the submarine part of the stratigraphy, on northern Vancouver Island, contains picritic pillow basalts (9-20 wt% MgO). Both lava groups have overlapping initial and ENd, indicating a common, ocean island basalt (OIB)-type Pacific mantle source similar to the source of basalts from the Ontong Java and Caribbean Plateaus. The major-element chemistry of picrites indicates extensive melting (23 -27%) of anomalously hot mantle (\u00E2\u0080\u0098\u00E2\u0080\u00941500\u00C2\u00B0C), which is consistent with an origin from a mantle plume head. Wrangellia basalts extend \u00E2\u0080\u0094-\u00E2\u0080\u0098450 km across southern Alaska (Wrangell Mountains and Alaska Range) and through southwest Yukon where <3.5 km of mostly subaerial flows (Nikolai Formation) are bounded by Pennsylvanian to Permian volcanic (312-280 Ma) and sedimentary strata, and Late Triassic limestone. The vast majority of the Nikolai basalts are LREE-enriched high-Ti basalt (1.6-2.4 wt% Ti02)with uniform plume-type Pacific mantle isotopic compositions. However, the lowest \u00E2\u0080\u0098-.400 m of stratigraphy in the Alaska Range, and lower stratigraphy in Yukon, is light rare earth element (LREE) depleted low-Ti basalt (0.4-1.2 wt% Ti02)with pronounced negative-HFSE anomalies and high Elf values that are decoupled from Nd and displaced well above the OIB mantle array. The low-Ti basalts indicate subduction-modified mantle was involved in the formation of basalts exposed in Alaska and Yukon, possibly from mechanical and thermal erosion of the base of the lithosphere from an impinging mantle plume head. 11 TABLE OF CONTENTS fi Table of Contents ffl List of Tables vii List of Figures ix Acknowledgements Dedication xvi Co-authorship statement xvii CHAPTER 1 Introduction 1 Introduction and motivation for this study 2 The Wrangellia oceanic plateau 7 Methodology and rationale for this study 10 Previous research 11 The importance of LIPs and mantle plumes 12 An overview of the four manuscripts in this dissertation and additional references...15 Contributions to this project 18 References 19 CHAPTER 2 Wrangeffia Flood Basalts on Vancouver Island: Significance of Picritic and Tholeiitic Lavas for the Melting History and Magmatic Evolution of a Major Oceanic Plateau 26 Introduction 27 Geologic setting 28 Wrangellia on Vancouver Island 28 Age of the Karmutsen Formation 31 Volcanic stratigraphy and petrography 31 Sample preparation and analytical methods 37 Whole-rock chemistry 44 Major- and trace-element compositions 44 Sr-Nd-Hf-Pb isotopic compositions 56 Alteration 62 Olivine accumulation in picritic lavas 63 Discussion 63 Melting conditions and major-element composition of primary magmas 65 Source of Karmutsen lavas 68 REE modeling: Dynamic melting and source mineralogy 71 Magmatic evolution of Karmutsen tholeiitic basalts 75 Conclusions 78 Acknowledgements 79 References 79 111 CHAPTER 3 Wrangeffia Flood Basalts in Alaska: A Record of Plume-Lithosphere Interaction in a Late Triassic Accreted Oceanic Plateau 86 Introduction 87 Geologic setting 89 Wrangellia in Alaska 89 Wrangell Mountains 89 Eastern Alaska Range 91 Age of the Nikolai Formation 93 Volcanic stratigraphy and petrography 93 Whole-rock chemistry 104 Major- and trace-element compositions 104 Sr-Nd-Hf-Pb isotopic compositions 115 Alteration 115 Flood basalt chemostratigraphy 123 Discussion 125 Source ofNikolai basalts 125 Lithospheric involvement in derivation of the low-titanium basalts 128 Nature of underlying Paleozoic arc lithosphere 128 Trace-element and isotopic source constraints of the low titanium basalts... 129 Origin of decoupled Hf and Nd isotopic compositions of low-titanium basalts 134 Melting conditions and estimated major-element composition of primary low-Ti magma 138 Conclusions 142 Acknowledgements 143 References 143 CHAPTER 4 Geochemistry of Flood Basalts from the Yukon (Canada) Segment of the Accreted Wrangeffia Oceanic Plateau 151 Introduction 152 Geologic setting and age constraints 153 Field relations and petrography 156 Whole-rock chemistry 161 Major- and trace-element compositions 161 Sr-Nd-Hf-Pb isotopic compositions 171 Discussion 171 Effects of alteration and comparison to Nikolai basalts in Alaska 171 Relationship between chemistry and stratigraphic position 179 Source characteristics ofNikolai basalts in Yukon 180 Melting of arc mantle in formation of the low-titanium basalts 182 Conclusion 188 Acknowledgements 189 References 189 iv CHAPTER 5 The Age and Volcanic Stratigraphy of the Accreted Wrangeffia Oceanic Plateau in Alaska, Yukon and British Columbia 195 Introduction 196 Wrangellia flood basalts: The volcanic stratigraphy of an accreted oceanic plateau 198 Geographic distribution and aerial extent of the Wrangellia flood basalts 200 Geologic history of Wrangellia 200 Stratigraphy of Wrangellia 203 Wrangellia of southern Alaska 203 Talkeetna Mountains and eastern Alaska Range 205 Wrangell Mountains 209 Wrangellia in southwest Yukon 215 Wrangellia in southeast Alaska 218 Wrangellia in the Queen Charlotte Islands (Haida Gwaii) 221 Wrangellia on Vancouver Island 222 Central and southern Vancouver Island 222 Northern Vancouver Island 228 Geochronology of Wrangellia 233 Previous geochronology for Wrangellia flood basalts and related plutonic rocks 233 Information about samples analyzed by40Ar/39r in this study 235 4O,39 geochronological results 236 Summary of isotopic age determinations for Wrangellia flood basalts 243 Paleontological studies 243 Discussion 245 Overview of geology and age of Northern and Southern Wrangellia 245 Eruption environment for Wrangellia flood basalts 248 Northern Wrangellia 248 Southern Wrangellia 249 The accumulation and subsidence of the Wrangellia flood basalts 251 Conclusion 254 Acknowledgements 255 References 255 CHAPTER 6 Conclusions Conclusions and Directions for Future Research 267 References 271 v Appendices. Appendix A. Geologic map of the Mount Arrowsmith area 273 Appendix B. XRF whole-rock analyses of a subset of Karmutsen basalts, Vancouver Island, B.C 274 Appendix C. PCIGR trace-element analyses of Karmutsen basalts, Vancouver Island, B.C 276 Appendix D. Sample preparation and analytical methods for Alaska samples 279 Appendix E. Sample preparation and analytical methods for Yukon samples 284 Appendix F. Major element (wt% oxide) and trace element (ppm) abundances in whole rock samples of Late Paleozoic Station Creek Formation, Yukon 288 Appendix G. Previous research on Wrangellia 290 Appendix H. 40Ar/39ranalytical methods 306 Appendix I. Analytical results of reference material from Actlabs whole-rock analyses for Vancouver Island and Yukon 308 Appendix J. Description of supplementary electronic files on CD-ROM 310 Supplementary electronic files on CD-ROM Supplementary data files (SD) SD 1- Endnote database for Wrangellia (.enl file) SD 2- Reference list for Wrangellia (.doc file) SD 3- Geochemistry for40Ar/39r samples (.xls file) SD 4-40Ar/39r analytical data (.xls file) SD 5- Wrangellia ages and biostratigraphy (.xls file) Supplementary Google Earth ifies (SGE) SGE 1- Mapped Wrangellia flood basalts (.kniz file) SGE 2- Major faults in Alaska and Yukon (.kml file) SGE 3- Major faults in southwest B.C. (.kml file) SGE 4- Alaska sample locations (.kml file) SGE 5- Yukon sample locations (.kml file) SGE 6- Vancouver Island sample locations (.kml file) SGE 7- Alaska Range photograph locations (.kmz file) SGE 8- Wrangell Mountains photograph locations (.kmz file) SGE 9- Yukon photograph locations (.kmz file) SGE 10- Vancouver Island photograph locations (.kmz file) Supplementary photo ifies (SP) SP 1- Alaska Range photographs (.pdf file) SP 2- Wrangell Mountains photographs (.pdf file) SP 3- Ed MacKevett Jr. Wrangell Mountains photographs (.pdf file) SP 4- Yukon photographs (.pdf file) SP 5- Vancouver Island photos (.pdf file) Greene_2008_PhD_dissertation_UBC (.pdf file of complete dissertation) vi LIST OF TABLES CHAPTER 2 Table 2.1 Summary of petrographic characteristics and phenocryst proportions of Karmutsen basalts on Vancouver Island, B.C 39 Table 2.2 Major element (wt% oxide) and trace element (ppm) abundances in whole rock samples of Karmutsen basalts, Vancouver Island, B.C 46 Table 2.3 Sr and Nd isotopic compositions of Karmutsen basalts, Vancouver Island, B.C 59 Table 2.4 Hf isotopic compositions of Karmutsen basalts, Vancouver Island, B.C 60 Table 2.5 Pb isotopic compositions of Karmutsen basalts, Vancouver Island, B.C 61 Table 2.6 Estimated primary magma compositions for Karmutsen basalts and other oceanic plateaus/islands 67 CHAPTER 3 Table 3.1 Summary of petrographic characteristics and phenocryst proportions ofNikolai basalts in Alaska 102 Table 3.2 Major element (wt% oxide) and trace element (ppm) abundances in whole rock samples ofNikolai basalts, Alaska 106 Table 3.3 Sr and Nd isotopic compositions ofNikolai basalts, Alaska 118 Table 3.4 Hf isotopic compositions ofNikolai basalts, Alaska 119 Table 3.5 Pb isotopic compositions ofNikolai basalts, Alaska 120 Table 3.6 Estimated primary magma compositions for Nikolai basalts and other oceanic plateaus/islands 141 CHAPTER 4 Table 4.1 Summary of petrographic characteristics and phenocryst proportions ofNikolai basalts in Yukon 160 Table 4.2 Major element (wt% oxide) and trace element (ppm) abundances in whole rock samples ofNikolai basalts, Yukon 163 vii Table 4.3 Sr and Nd isotopic geochemistry ofNikolai basalts, Yukon 174 Table 4.4 Hf isotopic compositions ofNikolai basalts, Yukon 175 Table 4.5 Pb isotopic compositions ofNikolai basalts, Yukon 176 CHAPTER 5 Table 5.1 Areal extent and volumetric estimates for the Wrangellia flood basalts 201 Table 5.2 Compilation of previous geochronology of Wrangellia flood basalts and associated plutonic rocks 234 Table 5.3 40Ar/39rdating results for 13 samples of Wrangellia flood basalts and 6 samples from the Wrangellia Terrane 241 Table 5.4 Comparison of geology and ages ofNorthern and Southern Wrangellia 246 vi\u00E2\u0080\u009D LIST OF FIGURES CHAPTER 1 Figure 1.1 Distribution of Phanerozoic LIPs on Earth 3 Figure 1.2 Schematic diagrams of mantle plumes in the Earth\u00E2\u0080\u0099s interior 4 Figure 1.3 \u00E2\u0080\u009CPin-pricking\u00E2\u0080\u009D the giant Ontong Java Plateau 6 Figure 1.4 Map showing distribution of Wrangellia flood basalts in Alaska, Yukon, and British Columbia 8 Figure 1.5 Estimated distribution of the continental landmasses in the Middle to Late Triassic (226 Ma) 9 Figure 1.6 Age difference between peak eruption ages of LIPs from the last 260 Myr versus the age of a stratigraphic boundary 14 Figure 1.7 Photographs showing the different modes of transportation used for field work in remote areas of Alaska, Yukon, and BC as part of this project 17 CHAPTER 2 Figure 2.1 Simplified map of Vancouver Island showing the distribution of the Karmutsen Formation 30 Figure 2.2 Geologic map and stratigraphy of the Schoen Lake Provincial Park and Karmutsen Range areas 33 Figure 2.3 Photographs of picritic and tholeiitic pillow basalts from the Karmutsen Range area (Alice and Nimpkish Lake area), northern Vancouver Island 34 Figure 2.4 Photographs showing field relations from the Schoen Lake Provincial Park area, northern Vancouver Island 35 Figure 2.5 Photomicrographs of picritic pillow basalts, Karmutsen Formation, northern Vancouver Island 38 Figure 2.6 Whole-rock major-element, Ni, and LOl variation diagrams for the Karmutsen Formation 45 Figure 2.7 Whole-rock REE and trace-element concentrations for the Karmutsen Formation 53 ix Figure 2.8 Whole-rock trace-element concentrations and ratios for the Karmutsen Formation 55 Figure 2.9 Whole-rock Sr, Nd, and Hf isotopic compositions for the Karmutsen Formation 57 Figure 2.10 Pb isotopic compositions of leached whole-rock samples by MC-ICP-MS for the Karmutsen Formation 58 Figure 2.11 Relationship between abundance of olivine phenocrysts and whole-rock MgO contents for Keogh Lake picrites 64 Figure 2.12 Estimated primary magma compositions for three Keogh Lake picrites using the forward and inverse modeling technique of Herzberg et a!. (2007) 66 Figure 2.13 Comparison of age-corrected (230 Ma) Sr-Nd-Hf-Pb isotopic compositions for Karmutsen flood basalts on Vancouver Island to age-corrected OIB and MORB 69 Figure 2.14 Evolution of8Hf with time for picritic and tholeiitic lavas for the Karmutsen Formation 72 Figure 2.15 Trace-element modeling results for incongruent dynamic mantle melting for picritic and tholeiitic lavas from the Karmutsen Formation 74 Figure 2.16 Forward fractional crystallization modeling results for major elements from MELTS (Ghiorso & Sack, 1995) compared to picritic and tholeiitic lavas from the Karmutsen Formation 77 CHAPTER 3 Figure 3.1 Simplified map of south-central Alaska showing the distribution of the Nikolai Formation 90 Figure 3.2 Geologic map and stratigraphy of the Wrangell Mountains, Alaska 92 Figure 3.3 Photographs of the base of the Nikolai Formation in the Wrangell Mountains, Alaska 94 Figure 3.4 Photograph of \u00E2\u0080\u00941000 m of continuous flood basalt stratigraphy at the top of the Nikolai Formation along Glacier Creek in the Wrangell Mountains, Alaska 95 Figure 3.5 Photographs showing the top of the Nikolai Formation in the Wrangell Mountains, Alaska 97 x Figure 3.6 Simplified geologic map and stratigraphy of the Amphitheater Mountains, Alaska 98 Figure 3.7 Photographs of the base of the Nikolai Formation in the Amphitheater Mountains, east-central Alaska Range (Tangle Lakes, West), Alaska 99 Figure 3.8 Geologic map and stratigraphy of the Clearwater Mountains, Alaska 101 Figure 3.9 Representative photomicrographs ofNikolai basalts, Alaska 103 Figure 3.10 Whole-rock major-element and Ni variation diagrams vs. MgO for the Nikolai Formation in Alaska with data for the Nikolai Formation in Yukon 105 Figure 3.11 Whole-rock REE and trace-element concentrations for the Nikolai Formation in Alaska 113 Figure 3.12 Whole-rock trace-element concentration variations and ratios for the Nikolai Formation in Alaska, with data for the Nikolai Formation in Yukon 114 Figure 3.13 Whole-rock Sr, Nd, and Hf isotopic compositions for the Nikolai Formation in Alaska 116 Figure 3.14 Pb isotopic compositions of leached whole-rock samples by MC-ICP-MS for the Nikolai Formation in Alaska 117 Figure 3.15 Loss-on-ignition versus MgO and isotopic ratios for the Nikolai Formation in Alaska 122 Figure 3.16 Chemostratigraphy of the Nikolai Formation in three areas of Alaska (Clearwater, Amphitheater, and Wrangell Mountains) 124 Figure 3.17 Comparison of age-corrected (230 Ma) Sr-Nd-Hf isotopic compositions for Nikolai basalts in Alaska to age-corrected OIB and MORE 126 Figure 3.18 Comparison of Pb isotopic compositions of the Nikolai Formation in Alaska to OIB and MORE 127 Figure 3.19 Trace-element ratios and isotopic compositions of the Nikolai Formation in Alaska 130 Figure 3.20 Th-Nb and Ti-Yb proxies of the Nikolai Formation in Alaska with data compilation and modeling results 132 xi Figure 3.21 Global Hf-Nd isotope systematics with age-corrected data of the Nikolai Formation in Alaska 136 Figure 3.22 Comparison of initial Hf, Nd, and Sr isotopic compositions of the Nikolai Formation in Alaska to Pacific arcs (Tonga Kermadec, Mariana, Vanuatu, New Britain) and Pacific MORB 137 Figure 3.23 Estimated primary magma compositions for two picrites from the Nikolai Formation using the forward and inverse modeling technique of Herzberg et al. (2007) 140 ChAPTER 4 Figure 4.1 Simplified map of southwest Yukon showing the distribution of the Nikolai Formation 154 Figure 4.2 Geologic map and stratigraphy of the northern part of the Kluane Ranges, Yukon 155 Figure 4.3 Photographs of the Nikolai Formation in the Kluane Ranges, Yukon 158 Figure 4.4 Photomicrographs of representative Nikolai basalts in southwest Yukon 159 Figure 4.5 Whole-rock major-element variation diagrams for the Nikolai Formation in Yukon with data for the Nikolai Formation in Alaska 162 Figure 4.6 Whole-rock REE and trace-element concentrations for the Nikolai Formation in Kluane Ranges, Yukon 170 Figure 4.7 Whole-rock trace-element concentrations for the Nikolai Formation in Yukon, with data for the Nikolai Formation in Alaska 172 Figure 4.8 Whole-rock Sr, Nd, and Hf isotopic compositions for the Nikolai Formation in Yukon, with fields for the Nikolai Formation in Alaska 173 Figure 4.9 Comparison of trace-element compositions of the Nikolai basalts in Yukon to averages for basalts from Alaska 178 Figure 4.10 Comparison of age-corrected (230 Ma) Sr-Nd-Hf isotopic compositions of the Nikolai Formation in Yukon and Alaska to age-corrected OIB and MORB 181 Figure 4.11 Th-Nb and Ti-Yb systematics for the Nikolai Formation in Yukon with data compilation and modeling results 183 xii Figure 4.12 Trace-element ratios of high- and low-titanium basalts of the Nikolai Formation in Yukon with Paleozoic arc samples and fields for Nikolai basalts in Alaska 184 Figure 4.13 Schematic diagrams of two stages of melting ofNikolai basalts in Yukon and Alaska that produced the low- and high-titanium basalts in Yukon and Alaska 185 Figure 4.14 Trace-element abundances of low-titanium basalts from Yukon compared to arc mantle compositions of the Early to Middle Jurassic Talkeetna arc and incongruent dynamic melting modeling results 187 CHAPTER 5 Figure 5.1 Map showing the distribution of Phanerozoic large igneous provinces 197 Figure 5.2 Simplified map showing the distribution of Wrangellia flood basalts in Alaska, Yukon, and British Columbia 199 Figure 5.3 Simplified map of eastern south-central Alaska showing the distribution of Wrangellia flood basalts and stratigraphic columns 204 Figure 5.4 Geology and magnetic map of the Amphitheater Mountains 207 Figure 5.5 Photographs ofbase ofNikolai Formation in Tangle Lakes area of the Amphitheater Mountains 208 Figure 5.6 Photographs of the base ofNikolai Formation north of Skolai Creek in Wrangell-St. Elias National Park 210 Figure 5.7 Photographs of flood basalts in the Glacier Creek area in Wrangell-St. Elias National Park and map of the southern part of the Wrangell Mountains 211 Figure 5.8 Photographs of the top of the Nikolai Formation around Hidden Lake Creek in Wrangell-St. Elias National Park 212 Figure 5.9 Photographs and map of the Nikolai Formation in southwest Yukon 216 Figure 5.10 Simplified map of southeast Alaska showing the distribution of Triassic basalts that may be correlative with Wrangellia flood basalts 219 Figure 5.11 Simplified map of Vancouver Island showing the distribution of the Karmutsen Formation 223 xlii Figure 5.12 Field notes, photographs, and geologic map for the Schoen Lake area, Vancouver Island 226 Figure 5.13 Generalized geology and photographs of Buttle Lake area, Vancouver Island 227 Figure 5.14 Generalized geology of the Karmutsen Formation on northern Vancouver Island in the Port Alice-Robson Bight area 229 Figure 5.15 Photographs of intra-Karmutsen sedimentary lens near the top of the Karmutsen southwest of Nimpkish Lake, northern Vancouver Island 231 Figure 5.16 Photographs from near Holberg Inlet, northern Vancouver Island 232 Figure 5.17 40ArI39rage spectra for 6 analyses of plagioclase separates from Vancouver Island 237 Figure 5.18 40Ar/39r age spectra of 1 biotite and 2 plagioclase separates from Yukon 238 Figure 5.19 Ar/Ar age spectra for 6 analyses of plagioclase separates from Alaska 239 Figure 5.20 40Ar/39r age spectra for 6 analyses of homblende separates from Alaska 240 Figure 5.21 40Ar/Ar and UiPb ages of Wrangellia flood basalts and plutomc rocks 244 Figure 5.22 Summary of geochronology and biostratigraphy for Wrangellia 247 xiv ACKNOWLEDGEMENTS This project has involved help from numerous people who I am extremely grateful to. James Scoates and Dominique Weis proposed and guided this study. I am grateful for the opportunity they gave me and for their enthusiastic guidance and support. I also have greatly appreciated help from Graham Nixon of the BC Geological Survey and Steve Israel of the Yukon Geological Survey. Advice was also offered from Jeanine Schmidt (USGS Anchorage), David Brew (USGS Menlo Park), Nick Massey (BCGS), Mikkel Schau, and Don Carlisle. Kelly and Natalie Bay of Wrangell Mountain Air in McCarthy, Alaska provided extensive knowledge of the Wrangell Mountains. Jeff Trop (Bucknell University) and Danny Rosenkrans (Wrangell-St. Elias National Park) offered assistance with backcountry advice about Wrangell-St. Elias National Park. Great thanks go to Bruno Kieffer, Frederico Henriques, and James Scoates for help with field work and making time in the field lots of fun. Katrin Breitsprecher was helpful with reviewing and discussions. Bruno Kieffer, Jane Barling, and Bert Mueller provided training for analytical work. I would also like to thank Mark Jellinek and Jim Mortensen for being a part of my thesis committee and posing thought-provoking questions that inspired me to explore different areas. xv DEDICATION This dissertation is dedicated to my Mom and Dad, Sue and Art Greene. It is special to share this achievement with them. xvi CO-AUTHORSifiP STATEMENT The four manuscripts in this dissertation are all co-authored by colleagues. My supervisors James Scoates and Dominique Weis are co-authors on each of the manuscripts and they significantly contributed to each manuscript. Their contributions included ideas, advice, comments, and reviewing, as well as financial support. The contributions from other co-authors on each manuscript are described below. CHAPTER 2 Wrangellia flood basalts on Vancouver Island: Significance of picritic and tholeiltic lavas for the melting history and magmatic evolution of a major oceanic plateau Authors: Andrew R. Greene, James S. Scoates, Dominique Weis, Graham T. Nixon, Bruno Kieffer Bruno Kieffer analyzed a suite of 24 samples from Vancouver Island for isotopic and trace-element compositions. Graham Nixon helped with field work and advice on field relationships and geochemistry. CHAPTER 3 Wrangeffia flood basalts in Alaska: A record of plume-lithosphere interaction in a Late Triassic accreted oceanic plateau Authors: Andrew R. Greene, James S. Scoates, and Dominique Weis James Scoates and Dominique Weis contributed to many aspects of this manuscript. CHAPTER 4 Geochemistry of flood basalts from the Yukon (Canada) segment of the accreted Wrangeffia oceanic plateau Authors: Andrew R. Greene, James S. Scoates, Dominique Weis, Steve Israel Steve Israel assisted with field work, and contributed funding for field work and analytical data. Steve provided major- and trace-element whole-rock analyses of twenty- six samples ofNikolai basalt and 8 Station Creek samples. CHAPTER 5 The age and volcanic stratigraphy of the accreted Wrangeffia oceanic plateau in Alaska, Yukon and British Columbia Authors: Andrew R. Greene, James S. Scoates, Dominique Weis, Erik C. Katvala, Steve Israel, and Graham T. Nixon Erik Katvala assisted with field work on Vancouver Island, provided fossil determinations, and helped with revisions on the manuscript. Steve Israel contributed a single40Ar/Ar age date and provided revisions on the manuscript. Graham Nixon offered advice and revisions on the manuscript. xvii Iuornporn IHLJAVID INTRODUCTION AND MOTIVATION FOR THIS STUDY The largest melting events on Earth have led to the formation of transient large igneous provinces (LIPs). LIPs are features that can extend over millions of square kilometers of the Earth\u00E2\u0080\u0099s surface and consist of flood basalt sequences up to 6 km thick. These voluminous outpourings of lava occur over short intervals of geologic time (i.e. several million years) and have been linked to major changes in the global climate and biosphere in Earth history. This project is an extensive field and geochemical study of one of the major transient LIPs on Earth, the Triassic Wrangellia oceanic plateau. Transient and persistent LIPs cover large areas of the oceans and continents (Fig. 1.1). Approximately 13 major transient LIPs have formed in the last 260 Myr (Courtillot & Renne, 2003), including the Siberian (ca. 251 Ma) and Deccan traps (ca. 65 Ma); volcanism usually takes place in less than a few million years. Examples of persistent LIPs, or hotspots, are Ninetyeast Ridge and the Hawaiian-Emperor chain, where volcanism occurs over tens of millions of years. Large igneous province is an umbrella term that includes continental flood basalts (CFBs), volcanic rifled margins, oceanic plateaus, and aseismic ridges (Saunders, 2005). Common attributes of these provinces are that they form from an unusually high magmatic flux, they are mostly basaltic in composition, and they are not directly associated to seafloor spreading processes. The origin of most LIPs is best explained by mantle plumes. Mantle plumes are buoyant upwellings of hot mantle that may rise through the surrounding mantle as large spherical heads fed by narrower conduits (Griffiths & Campbell, 1990), or a variety of plume shapes, possibly without heads (Farnetani & Samuel, 2005). Transient LIPs are the initial eruptive products that form from melting of a new mantle plume head at the base of the lithosphere, and persistent LIPs form from melting within the narrower plume tail (Campbell, 2005) (Fig. 1.2). Continental flood basalts are closely associated with continental breakup. The formation of the Paran\u00C3\u00A1-Etendeka (ca. 133 Ma) preceded the opening of the South Atlantic Ocean and formation of the North Atlantic Volcanic Province (ca. 62-60 Ma) preceded the opening of the North Atlantic Ocean. The Ethiopian traps (ca. 30 Ma) are an example of the eruption of flood basalts prior to rifling of Arabia and Africa. CFBs are at least 1 km thick and greater than 100,000 km2 and are emplaced as hundreds (or over a 2 (Ca. 230 Ma) (16\u00C2\u00B11 Ma) (201\u00C2\u00B11 Ma) (56\u00C2\u00B11 Ma; 61\u00C2\u00B12 Ma) wrangellia Columbia River \u00E2\u0080\u00A2 , North Atlantic Figure 1.1 Distribution of Phanerozic LIPs on Earth. Transient LIPs are indicated in yellow (continental) and orange (oceanic) with peak eruption ages. Persistent LIPs are mostly in red. Map modified from base map by A. Goodlife and F. Martinez in Mahoney and Coffin (1997). The peak eruption ages of each LIP are from the compiled references within Courtillot and Renne (2003) and Taylor (2006). (30\u00C2\u00B11 Ma) (65.5\u00C2\u00B10.5 Ma) (259\u00C2\u00B13 Ma) (250\u00C2\u00B11 Ma) Ethiopia Deccan Emeishan Siberia a Ontong Java-H (122\u00C2\u00B11 Ma) Caribbean Paran\u00C3\u00A1-Etendeka (123\u00C2\u00B11.5 Ma) (89\u00C2\u00B11 Ma) (133\u00C2\u00B11 Ma) 3 Figure 1.2 Schematic diagrams of mantle plumes in the Earth\u00E2\u0080\u0099s interior. (a) A view of the deep-mantle complexities beneath the central Pacific Ocean showing the Hawaiian hotspot, a D\u00E2\u0080\u009D high-velocity reflector, and heterogeneity at the base of the lower mantle where mantle plumes may originate. Mantle plume head is hypothetical. Drawing modified from Gamero (2004) with parts of a plume model by Farnetani and Samuel (2005). (b) A drawing of a model of plume- lithosphere interaction beneath oceanic lithosphere that produces basalts erupted in an oceanic plateau. Dashed line marks the base of the mechanical boundary layer. Arrows indicate flattening of the plume head. Model developed primarily after work of Saunders et al. (1992). Parts of the diagrams were adapted from a drawing by J. Holden in Fodor (1987) and a plume model by Fametani and Samuel (2005). _._r . lox vertical exaggeration 4 thousand) of inflated compound pahoehoe flow fields that form a tabular flow stratigraphy (Self et a!., 1997). CFBs have been well-studied due to their accessibility as subaerial land-based features. Oceanic plateaus are not nearly as well-studied as CFBs because they are mostly submerged in the oceans. Several extant oceanic plateaus in the ocean basins have been broken up by seafloor spreading since their formation (Fig. 1.1). The Ontong Java, Manihiki and Hikurangi plateaus may have formed together at ca. 120 Ma and together cover 1% Earth\u00E2\u0080\u0099s surface (Taylor, 2006). Ontong Java (2 x 106 k2 in area; 30-35 km thick) formed from the largest magmatic event recorded on Earth and covers an area of the western Pacific Ocean comparable to the size of western Europe, or approximately one-third the size of the conterminous United States (Fitton eta!., 2004). The Kerguelen (2 x 106 km2 in area; 20 km thick) and Broken Ridge Plateaus formed together, beginning ca. 118 Ma, and have since been separated by seafloor spreading along the Southeast Indian Ridge (Weis & Frey, 2002; Fig. 1.1). An important feature of the basalts erupted in some oceanic plateaus is that they are unaffected by continental lithosphere and are thus better suited for understanding the composition of the mantle source of LIPs than CFBs. The basalt stratigraphy of oceanic plateaus remains largely unsampled because of the difficulty in sampling more than the uppermost few hundred meters of flows from drilling ships (Fig. 1.3). For example, the Ontong Java Plateau, which rises to depths of 1700 m below sea level, has been the focus of several Deep Sea Drilling Project (DSDP; Site 289) and Ocean Drilling Program (ODP; Leg 192, Sites 803 and 807) legs to sample the basaltic basement of the plateau (Fig. 1.3; Mahoney et a!., 2001). These efforts have been described as \u00E2\u0080\u009Cpin-pricking the elephant\u00E2\u0080\u009D (Tejada eta!., 2004); drilling of Ontong Java has only penetrated a maximum 338 m of approximately 6 km or more of the basalt stratigraphy (Mahoney et a!., 2001). Drilling of the Kerguelen Plateau has only penetrated a maximum 233 m of the basalt stratigraphy (Frey et a!., 2000). Ocean drilling of extant plateaus submerged in the ocean is an extremely difficult and expensive tool to study oceanic plateaus. The most efficient and comprehensive way to study oceanic plateaus is by observing and sampling exposures of oceanic plateaus where they have been accreted at 5 Figure 1.3 \u00E2\u0080\u009CPin-pricking\u00E2\u0080\u009D the giant Ontong Java Plateau. (a) Representation of drilled stratigraphic sections from 8 DSDP and ODP drilling sites (locations shown in panel b). Diagram adapted from Mahoney et al. (2001). The most volcanic rock sampled from drilling is 338 m of volcaniclastic rock at Site 1184 and 217 m of pillowed and massive basalt at Site 1185. (b) Predicted bathymetry (after Smith & Sandwell, 1997) of the Ontong Java Plateau and surrounding region showing the location of drill sites from ODP Leg 192 (stars) and previous drill sites (white and black circles). Map adapted from Mahoney et a!. (2001). The broken black line (labeled W-E) indicates a transect where multichannel seismic reflection (MCS) investigations acquired data for the plateau by RJV Hakuho Maru KH98-1 Leg 2 in 1998 (shown in panel c). (c) Composite east-west MCS transects of the Ontong Java Plateau with reflecting horizons (location shown in panel b). Figure modified from Inoue et a!. (2008). Sediments and the top of the igneous basement are indicated. Vertical red lines indicate depth of drilling at 3 labelled sites on the transect. Red circle indicates location of \u00E2\u0080\u0098eye structure\u00E2\u0080\u0099 (see Inoue et a!., 2008). Vertical exaggeration = 1 00:1. This figure serves to illustrate the difficulty of sampling and studying extant oceanic plateaus in the ocean basins. The black rectangle in panel c indicates the extent of stratigraphy that is exposed and accessible in obducted oceanic plateaus on land, such as the Wrangellia oceanic plateau. Distance (km) 6 continental margins or accreted onto island arcs, as in the case of the Ontong Java Plateau and the Solomon island arc. However, Phanerozoic examples of accreted oceanic plateaus are rare. Accreted sections of flood basalts around the Caribbean Sea and in Central and South America are several kilometers thick in areas (e.g. Kerr et a!., 1998; R\u00C3\u00A9villon et al., 1999; Kerr, 2003). Several small sections of the Ontong Java Plateau are exposed on land in the Solomon Islands (Tejada et a!., 1996; Babbs, 1997; Petterson et a!., 1999; Tejada et a!., 2002). These are the two major oceanic plateaus where obducteci parts of the plateau have been studied. THE WRANGELLL4 OCEANIC PLATEAU Wrangellia flood basalts in the Pacific Northwest ofNorth America are part of one of the best exposed accreted oceanic plateaus on Earth and can provide information about oceanic plateaus that is rarely accessible elsewhere on Earth. Wrangellia flood basalts formed as part of a transient LIP in the Middle to Late Triassic, with accretion to westem North America occurring either in the Late Jurassic or Early Cretaceous. Although their original areal distribution was likely considerably larger, current exposures of the flood basalts extend over 2300 km in British Columbia (BC), Yukon, and Alaska (Fig. 1.4). Parts of the entire volcanic stratigraphy, from the base to the top, are accessible for close examination on land. From a geochemical perspective, the Wrangellia flood basalts have remained relatively poorly studied and have been the focus of only one study in the last twenty years using modem analytical geochemistry (Lassiter eta!., 1995). In this study, the Wrangellia oceanic plateau has been intensively studied to establish the stratigraphical and geochemical architecture of an oceanic plateau and the nature of the episodic melting events that lead to the formation of oceanic plateaus. The Wrangellia flood basalts erupted mostly within the Late Ladinian and Camian stages of the Triassic (ca. 230 Ma; e.g. Carlisle & Suzuki, 1974; Parrish & McNicoll, 1992), as the continents were gathered into a great landmass referred to as Pangaea (Fig. 1.5). They erupted atop different-aged Paleozoic arc volcanic and marine sedimentary sequences and are overlain by Late Triassic limestone. Paleontological and paleomagnetic studies indicate that the Wrangellia flood basalts probably erupted in the eastem Panthalassic 7 Figure 1.4 Map showing distribution of Wrangellia flood basalts in Alaska, Yukon, and British Columbia. Map derived from Wilson et a!. (1998), Israel (2004), Massey et a?. (2005a, b), Wilson et a!. (2005), and Brew (2007, written comm.). Outlines for the Peninsular and Alexander Terranes are shown in orange and blue, respectively. As labeled, the Wrangellia Terrane is referred to as Northern and Southern Wrangellia in this study. Major faults in Alaska and parts of Yukon and BC are shown with purple lines. The Wrangellia, Alexander, Peninsular terranes, which are part of the Wrangellia Composite Terrane, share similar elements or have a linked geologic history [as defined by Plafker et aL (1989), Nokleberg et a?. (1994), Plafker and Berg (1994), and Plaficer et a?. (1994)]. Geochronology from a single pluton in Alaska is proposed to link the Proterozoic to Triassic Alexander Terrane to Wrangellia by late Pennsylvanian time (Gardner et a?., 1988). The Wrangellia and Peninsular Terranes may have been in close proximity by the Late Triassic or Early Jurassic (Rioux eta!., 2007). 8 Figure 1.5 Estimated distribution of the continental landmasses in the Middle to Late Triassic (226 Ma). Map modified from Ogg (2004), based on the reconstruction of Scotese (2004). Global Stratotype Sections and Points (GSSP) are indicated for stages of the Triassic. Blue arrow indicates estimated location of the Wrangellia oceanic plateau. 9 Ocean in equatorial latitudes (Jones et al., 1977; Katvala & Henderson, 2002). The eruption of Wrangellia flood basalts broadly coincides with major biotic and environmental changes worldwide that occurred at the end of Carnian time (Furin et al., 2006). A global-scale environmental crisis, referred to as the Reingrabener turnover, occurred in the Carnian (e.g. Furin et a!., 2006) This event is preserved by the collapse of rimmed carbonate platforms, a global shift in sedimentological and geochemical proxies, and a strong radiation of several groups, including scleractinian reef builders, calcareous nannoplankton, and dinosaurs (Furin et al., 2006 and references therein). The accretion of the Wrangellia oceanic plateau to western North America was a major tectonic event and represents a significant addition of oceanic mantle-derived material to the North American crust (Condie, 2001). The volcanic stratigraphy of the Wrangellia plateau is defined as the Karmutsen Formation on Vancouver and Queen Charlotte Islands (Haida Gwaii), and as the Nikolai Formation in southwest Yukon and south-central Alaska (Fig. 1.4). On Vancouver Island, the volcanic stratigraphy is a tripartite succession of submarine, volcaniclastic, and subaerial flows approximately 6 km thick. In Alaska and Yukon, the volcanic stratigraphy is predominantly massive subaerial flows with a small proportion of submarine flows along the base. Smaller elements in southeast Alaska may be correlative with the Wrangellia flood basalts. Rapid eruption of the Wrangellia flood basalts is supported by the absence of intervening sediments between the flows, except in the uppermost part of the stratigraphy as volcanism waned. Paleomagnetic measurements of the Wrangellia flood basalts have not revealed any magnetic reversals, which also testifies to the short duration of eruptions (Hillhouse, 1977; Yole & Irving, 1980; Hilihouse & Coe, 1994). METHODOLOGY AND RATIONALE FOR THIS STUDY This study represents the first comprehensive study of the Wrangellia oceanic plateau. The integration of geochemical, volcanological, stratigraphic, and geochronological results provide constraints on the formation of the Wrangellia oceanic plateau. The volcanological and stratigraphic studies yield insights about the emplacement of flows, eruptive environment, original tectonic setting, and construction 10 of the plateau. The geochronological and paleontological studies allow for estimation of the age and duration of volcanism. The petrologic and geochemical studies provide information about the composition of the source, conditions of melting, and the magmatic evolution of lavas that formed the Wrangellia plateau. The interpretation of these different results will help to develop our understanding of the physical and chemical processes that occur in mantle plumes, as they decompress and impinge on the base of oceanic lithosphere and produce the basaltic magmas which erupt though oceanic lithosphere and form oceanic plateaus. The contributions in the four major chapters in this dissertation (described below) address some of the enduring questions about the origin of oceanic plateaus. PREVIOUS RESEARCH In the 1 970s, Jones and co-workers (1977) defined the fault-bound blocks of crust that contain diagnostic Triassic flood basalts in BC, Yukon, and Alaska as Wrangellia, named after the type section in the Wrangell Mountains of Alaska. Early paleomagnetic and paleontological studies of Wrangellia indicated long-distance displacement of the basalts from equatorial latitudes (Hillhouse, 1977) and similar Daonella bivalves were found in sediments directly beneath the flood basalts on Vancouver Island and in the Wrangell Mountains (Jones et aL, 1977). A back-arc setting was initially proposed for the formation of Karmutsen basalts on Vancouver and Queen Charlotte Islands based on major- and trace-element geochemistry of 12 samples (Barker et al., 1989). Richards and co-workers (1991) proposed a plume initiation model for the Wrangellia flood basalts based on evidence of rapid uplift prior to volcanism, lack of evidence of rifting associated with volcanism (few dikes and abundant sills), and the short duration and high eruption rate of volcanism. A geochemical study of 36 samples of Wrangellia flood basalts, 29 samples from Buttle Lake on Vancouver Island and 9 samples from the Wrangell Mountains in Alaska, was undertaken by Lassiter and co-workers (1995) as part of the only modem geochemical and isotopic study of Wrangellia flood basalts until the initiation of this project. Lassiter and co-workers (1995) suggested mixing of a plume type source with low Nb/Th arc-type mantle could reproduce variations in the Wrangellia flood basalts. 11 TUE IMPORTANCE OF LIPS AND MANTLE PLUMES Since the recognition of oceanic plateaus in the early 1 970s (Edgar et al., 1971; Donnelly, 1973; Kroenke, 1974), there have been significant advances in our understanding of oceanic plateaus. A wide range of scientific approaches have been used to study oceanic plateaus; however, we only partially understand the physical and chemical processes that lead to the formation of oceanic plateaus. Oceanic plateaus have formed for at least the last several hundred million years and likely well back into Earth history. Archean and Proterozoic greenstone belts have been interpreted to be remnants of oceanic plateaus (e.g. Kerr, 2003) and komatiites (>18 wt % MgO, with spinifex-textured olivines) may be the high-temperature melting products of ancient mantle plumes (e.g. Arndt & Nesbet, 1982) and some Phanerozoic plume heads (e.g. Storey et aL, 1991). Phanerozoic oceanic plateaus may form from mantle plumes that originate at the core-mantle boundary (Campbell, 2005). The large temperature and density contrasts that exist between the outer core and mantle are expected to produce an unstable boundary layer above the core that episodically leads to the formation of mantle plumes (Jellinek & Manga, 2004). Oceanic plateaus are thus the surface manifestations of a major mode of heat loss from the interior of the Earth. The geochemical study of flood basalts is one important way by which we gain information about mantle plumes rising from their origin to the eruption of their melting products at the Earth\u00E2\u0080\u0099s surface. Although mantle plumes are considered the strongest hypothesis for the origin of LIPs, there are a number of alternative interpretations: meteorite impacts (Rogers, 1982), enhanced convection at the edge of continental cratons (King & Anderson, 1998), delamination of lithosphere causing rapid mantle upwelling (Elkins-Tanton, 2005), and melting of fertile eclogitic mantle (Korenaga, 2005). One example of evidence that mantle plumes are responsible for the formation of most LIPs is the high melt production rates that form LIPs, which make them difficult to explain by an origin related to plate tectonic processes (Jellinek & Manga, 2004). Perhaps the best evidence for the high source temperatures required to explain the melting that produces LIPs is the presence of picrites (e.g. Saunders, 2005), with MgO between 12 and 18 wt % and 1-2 wt % Na2O + K20 (Le Bas, 2000). In this study, high-Mg picritic lavas have been discovered for the 12 first time within the Wrangellia oceanic plateau on northern Vancouver Island and in Alaska and modeling of their compositions indicate high source temperatures and high degrees of melting. The eruption ofbasalts in LIPs has the potential to significantly affect the composition of the atmosphere and oceans by releasing large amounts of gas (primarily CO2 and SO2) and aerosols that may trigger heating via runaway greenhouse effect or cooling via the spread of stratospheric sulphate aerosols that backscatter and absorbe the sun\u00E2\u0080\u0099s radiation (e.g. Rampino & Stothers, 1988; Wignall, 2001). Although CO2 emissions in LIPs are small compared to the amount of CO2 present in the atmosphere and the land- ocean-atmosphere flux of C02, the gradual buildup of CO2 from LIP eruptions may tip the balance enough to initiate release of other greenhouse gases, such as methane hydrates on the seafloor (Saunders, 2005). The release of SO2 in LIP eruptions is incomparable to inputs at any other time during the Phanerozoic (Self et a!., 2006). The eruption of a single <2400 km3 flow field is estimated to release as much as 6,500 Tg a1 for a 10-year duration, which is enormous compared to the background amount of S in the atmosphere (<1 Tg) (Self et a!., 2006). In the oceans, a series of variables that are consequences of large releases of CO2 lead to warmer polar waters, decreased solubility of CO2and 02, and increased biogenic productivity in surface waters (e.g. Kerr, 2005). These effects can potentially lead to a shutdown of the ocean circulation system and anoxic conditions. It is likely that the eruption of Wrangellia flood basalts would have significantly affected the chemical composition of the ocean and atmosphere. The close correlation between major mass extinctions in Earth history and the formation of most transient LIPs suggests a causal link between LIP eruptions and environmental change (Rampino & Stothers, 1988; Fig. 1.6). The three most recent large diversity depletion events in the Phanerozoic (Permian-Triassic, Triassic-Jurassic, Cretaceous-Tertiary) coincide with the timing of peak volcanic activity in the formation of a continental flood basalt province (Siberia, Central Atlantic Magmatic Province (CAMP), and Deccan, respectively) (Courtillot & Renne, 2003). The eruption of the Siberian traps may have played a key role in the End Permian extinction event because of the high proportion ofpyroclastic eruptions and intrusion of magmas into carbon- and methane-bearing strata (Wignall, 2001). Recent studies of the CAMP and the Deccan 13 Age Difference (Myr) 10 10 0 . . . \u00E2\u0080\u00A2 . I Large igneous province stratigraphic boundary Ethiopian Traps (30 Ma) 0i2 event 50 . End Early Palaeocene End PalaeoceneNorth Atlantic Province (56 Ma) Deccan Traps (65.5 Ma) Caribbean (89 Ma) End Cen End Cretaceous Madagascar (88 Ma) omanian 100 Early AptianOntong Java (122 MaLIP Age Kerguelen/Rajmahal (118 MaFd_1d EarlyAptian (Ma) Paran\u00C3\u00A1-Etendeka (133 Ma)\u00E2\u0080\u0094\u00E2\u0080\u0094\u00E2\u0080\u0094\u00E2\u0080\u0094\u00E2\u0080\u0094 End Valanginian 150 Karoo-Ferrar (183 Ma)\u00E2\u0080\u0094 i\u00E2\u0080\u0094 End Pliensbachian 200 Central Atlantic Magmatic Province (201 Ma) \u00E2\u0080\u0094\u00E2\u0080\u00A2\u00E2\u0080\u0094 End Triassic Wrangellia (230 Ma) ? \u00E2\u0080\u00941 i\u00E2\u0080\u0094? End Camian 250 Siberian Traps (250 Ma) End Permian Emeishan Traps (259 Ma)\u00E2\u0080\u0094 \u00E2\u0080\u0094 End Guadalupian LIP pre-dates boundary LIP post-dates boundary 300 stratigraphic boundary Figure 1.6 Age difference between peak eruption ages of LIPs from the last 260 Myr versus the age of a stratigraphic boundary associated with major sudden environmental changes. Diagram modified from Kelley (2007). Ages are mostly from the compilation of Courtillot and Renne (2003). The possible connection between Wrangellia and a climatic and biotic crisis recognized in the Carnian is hypothetical and needs to be explored further (Furin eta!., 2006). 14 Traps support a close temporal relationship between the End Triassic and End Cretaceous extinctions, respectively. A recent study by Courtillot and Olsen (2007) suggests there may be a connection between magnetic superchrons, where the Earth\u00E2\u0080\u0099s magnetic field does not reverse polarity for abnormally long periods of time (30-50 Myr), and the timing of mantle plumes. The -\u00E2\u0080\u009850 Myr long Kaiman Long Reverse Superchron (310 to 260 Ma) ended 10 Myr before eruption of the Siberian Traps and the 35 Myr long Cretaceous Long Normal Superchron (118 to 83 Ma) ended -l 8 Myr before eruption of the Deccan Traps (Courtillot & Olsen, 2007). Courtillot and Olsen (2007) suggest a mechanism whereby a major change in heat flow at the core-mantle boundary may lead to the end of a superchron and cause initiation of \u00E2\u0080\u009Ckiller\u00E2\u0080\u009D mantle plumes. Along with aerially-extensive flood basalt sequences, radiating dike swarms are impressive aspects of transient LIPs which extend >2800 km and are a major mode of lateral transport of LIP magmas in the crust. Radiating dike swarms are diagnostic of major magmatic events linked to mantle plumes and associated continental break-up and serve as indicators of plume centers and magma transport (Ernst et al., 2001). Dike swarms have been identified in every Mesozoic and Cenozoic CFB on Earth and are expected to form in oceanic plateaus (Ernst & Buchan, 2003). Where the flood basalts are largely eroded, such as the CAMP, dike swarms can be used to reconstruct plate motions and identify plume events through Earth\u00E2\u0080\u0099s history (Ernst & Buchan, 1997). There are many important and fascinating aspects of LIPs. The study of LIPs holds great relevance in a variety of large-scale Earth processes and this study provides constraints which help to further our understanding of these geological phenomena. AN OVERVIEW OF THE FOUR CHAPTERS IN THIS DISSERTATION AND ADDITIONAL REFERENCES The chapters in this dissertation were written in manuscript form for submission to major international geological journals. Three of the first four manuscripts are detailed studies of the Wrangellia flood basalts from different regions of western North America (Vancouver Island, Alaska, and Yukon). These three manuscripts parallel one another in that similar analytical techniques are used, but each study has distinct results and 15 interpretations. Each study involved extensive field work using different modes of transportation in remote areas of western North America (Fig. 1.7). Additional contributions from this project that are not included in the dissertation are four government geological survey papers (Greene et a!., 2005b, 2005c, 20060; Nixon eta!., 2008b) and eight abstracts to international geological conferences (Greene et a!., 2005a, 2006a, 2006b, 2007a, 2007b, 2007c, 2008a, 2008b). Contributions to other conference proceedings include studies of platinum-group elements (Scoates et a!., 2006, 2007, 2008) and stratigraphic successions of Wrangellia flood basalts (Nixon et a!., 2008a). The first manuscript in this dissertation is a comprehensive study of the Karmutsen Formation on Vancouver Island, which has major implications for the evolution of this large oceanic plateau. Karmutsen basalts on Vancouver Island comprise one of the thickest and most complete examples of the volcanic stratigraphy of an emergent oceanic plateau on Earth. My co-authors and I examine the field relationships, stratigraphy, petrography, and geochemical and isotopic compositions of the Karmutsen Formation to assess the nature of the mantle source and to evaluate the melting history and subsequent magmatic evolution ofbasalts involved in the construction of this major oceanic plateau. The second manuscript is a study of the Nikolai Formation in Alaska. This study integrates field relationships in widespread areas of Alaska and geochemistry of continuous sections of volcanic stratigraphy to understand the physical and chemical processes that occur as a mantle plume head impinges on the base of oceanic arc lithosphere. This information provides insights into the temporal and spatial variation of the magmas in a major oceanic plateau that result from plume-lithosphere interaction. The third manuscript is a field, petrographic, and geochemical study ofNikolai basalts in Yukon, where volcanic stratigraphy and bounding sedimentary sequences are similar to Alaska. Nikolai basalts in Yukon are more altered than their counterparts in Alaska and a comparative geochemical study of Nikolai basalts from Yukon and Alaska allows for analysis of the effects of alteration and regional differences in basalts erupted in the northern part of Wrangellia. This study also examines the geochemistry of the underlying Paleozoic arc sequences and their role in the generation of the Nikolai basalts. 16 Figure 1.7 Photographs showing the different modes of transportation used for field work in remote areas of Alaska, Yukon, and BC as part of this project. (a) Canoe access to the Amphitheater Mountains in the Alaska Range, Alaska. (b) Helicopter access in the Kluane Ranges, southwest Yukon. (c) Transportation by Super Cub in Wrangell-St. Elias National Park, Alaska. (d) Four-wheel drive vehicle access on logging roads on northern Vancouver Island. Photograph by Graham Nixon. 17 The fourth manuscript is a stratigraphic and geochronological study of Wrangellia flood basalts from throughout BC, Yukon, and Alaska. This synthesis integrates observations from field work and dozens of government geological survey reports and maps to provide detailed descriptions of the volcanic stratigraphy and the pre- and post- volcanic rock stratigraphy of Wrangellia. This overview is presented in the form of stratigraphic columns and descriptions, compiled geologic maps, photographic databases, interactive Google Earth files, and a review and compilation of previous research on Wrangellia. The maps, photographs, and archiving of information are presented differently than in the previous chapters and offer users interactive electronic tools to visualize and explore information about the Wrangellia oceanic plateau. This work, in combination with40Ar/39rgeochronology of flood basalts throughout Wrangellia, provides an over-arching contribution that brings together past and present research on Wrangellia in a new light. CONTRIBUTIONS TO THIS PROJECT This project benefited from the assistance of many people. Assistance with field work in Alaska, Yukon, and BC was provided by Bruno Kieffer, Frederico Henriques, and James Scoates. Advice for field work was provided by Graham Nixon (British Columbia Geological Survey), Nick Massey (British Columbia Geological Survey), Don Carlisle, Steve Israel (Yukon Geological Survey), Jeanine Schmidt (United States Geological Survey-Anchorage), David Brew (United States Geological Survey-Menlo Park), Danny Rosenkrans (Wrangell-St. Elias National Park), and Jeff Trop (Bucknell University). Isotopic and trace-element analyses for a suite of 24 samples on Vancouver Island were made by Bruno Kieffer and Jane Barling. Bruno Kieffer, Jane Barling, and Bert Mueller also provided training for analytical work for Yukon and Alaska samples at the Pacific Centre for Isotopic and Geochemical Research (PCIGR). The analyses for 40Ar/39rgeochronology were carried out by Tom Ulfrich from samples that were crushed, separated, picked, and leached by me. This project was made possible from funding provided by the BC & Yukon Chamber of Mines (Association of Mineral Exploration BC) from the 2004 Rocks to Riches Program, the BC Geological Survey, the Yukon Geological Survey, and by 18 NSERC Discovery Grants to James Scoates and Dominique Weis. The author was also supported by a University Graduate Fellowship at UBC. Major- and trace-element analyses for this project were performed by Activation Laboratories (ActLabs) in Ontario, Canada and at the Ronald B. Gilmore X-Ray Fluorescence Laboratory (XRF) at the University of Massachussetts. Claude Herzberg (Rutgers University), Julian Pearce (Cardiff University), and Haibo Zou (University of California Los Angeles) offered assistance with geochemical modeling. My supervisors, James Scoates and Dominique Weis, offered invaluable assistance and guidance with all aspects of this project. REFERENCES Arndt, N. T. & Nisbet, E. G. (1982). Geochemistry of Munro township basalt. In: Arndt, N. 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Emergent basalt and submergent carbonate-clastic sequences including the Upper Triassic Dilleri and Welleri zones on Vancouver Island. Canadian Journal ofEarth Sciences 11, 254-279. Condie, K. C. (2001). Mantle Plumes and Their Record in Earth History. Cambridge University Press: Cambridge, 306 p. Courtillot, V. & Olsen, P. E. (2007). Mantle plumes link magnetic superchrons to Phanerozoic mass depletion events. Earth and Planetary Science Letters 260, 495-504. Courtillot, V. E. & Renne, P. R. (2003). On the ages of flood basalt events. Comptes Rendus Geoscience 335, 113-140. Donnelly, T. W. (1973). Late Cretaceous basalts from the Caribbean: a possible flood basalt province of vast size. EOS 54, 1004. Edgar, N. T., Ewing, J. I. & Hennion, J. (1971). Seismic refraction and reflection in the Caribbean Sea. American Association ofPetroleum Geologists Bulletin 55, 833- 870. 19 Elkins-Tanton, L. (2005). Continental magmatism caused by lithospheric delamination. In: Foulger, G. 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Canadian Journal ofEarth Sciences 17, 1210-1228. 25 CHAPTER 2 Wrangellia Flood Basalts on Vancouver Island: Significance of Picritic and Tholeiitic Lavas for the Melting History and Magmatic Evolution of a Major Oceanic Plateau \u00E2\u0080\u0098A version of this chapter has been submitted for publication. 26 INTRODUCTION The largest magmatic events on Earth have led to the formation of oceanic plateaus. Oceanic plateaus are transient large igneous provinces (LIPs) that cover up to two million square kilometers of the ocean floor and form crustal emplacements 20-40 km thick with submarine or subaerial flood basalt sequences six or more kilometers thick (Coffin & Eldholm, 1994). LIPs typically form in one or more pulses lasting less than a few million years. Three key aspects of LIPs are that they form from unusually high melt production rates, they are predominantly basaltic in composition, and their formation is often not directly attributable to seafloor spreading processes (e.g. Saunders, 2005). The high melt production rates are best explained by high mantle source temperatures of rapidly upwelling mantle (i.e. mantle plumes) and direct evidence of high mantle temperatures is eruption of high-MgO, near-primary lavas (Kerr & Mahoney, 2007). Studies of oceanic plateaus may provide important information about their construction and growth history, the temperature and depth of melting of the mantle source, the volume of magma and melt production rates, and the composition of the mantle source. Combined, this information serves to constrain aspects of the mantle plume hypothesis from the origin of components in the source to emplacement of voluminous lava sequences. From a geochemical perspective, study of oceanic plateaus is important because the lavas erupted in oceanic plateaus are generally unaffected by continental contamination. Presently, one of the great challenges in studying oceanic plateaus in the ocean basins is the difficulty in sampling more than the uppermost few hundred meters of flows (e.g. Kerguelen, Ontong Java, etc.). Accreted sections of oceanic plateaus (e.g. Caribbean, Ontong Java) provide an opportunity to closely examine their volcanic stratigraphy. Wrangellia flood basalts in the Pacific Northwest ofNorth America are parts of an immense LIP that erupted in a marine setting and accreted to western North America in the Late Jurassic or Early Cretaceous (Richards et al., 1991). Although their original areal distribution was likely considerably larger, current exposures of the flood basalts extend in a thin belt over 2300 km in British Columbia (BC), Yukon, and Alaska and retain a large part of their original stratigraphic thickness (\u00E2\u0080\u0094\u00E2\u0080\u00986 km on Vancouver Island; \u00E2\u0080\u0094\u00E2\u0080\u00983.5 km in Alaska). The present extent of the Wrangellia oceanic plateau 27 remnants is primarily a result of transform fault motions that occurred along western North America after accretion, rather than an indication of the original size of the plateau. The flood basalts erupted in the Middle to Late Triassic (\u00E2\u0080\u009423 1-225 Ma; Richards et a!., 1991) atop different-aged Paleozoic arc volcanic and marine sedimentary sequences and are overlain by Late Triassic limestone. On Vancouver Island, a tripartite succession of flood basalts includes submarine, volcaniclastic, and subaerial flows formed as part of an enormous emergent oceanic plateau. Richards et a!. (1991) proposed a plume initiation model for the Wrangellia flood basalts based on evidence of rapid uplift prior to volcanism, lack of evidence of rifting associated with volcanism (i.e. few dikes and abundant sills), and the short duration and high eruption rate of volcanism. Wrangellia flood basalts are perhaps the most extensive aecreted remnants of an oceanic plateau in the world where parts of the entire volcanic stratigraphy are exposed, but they have been the focus of only one study in the last 20 years using multiple types of isotopic and geochemical data (Lassiter et al., 1995). In this study, we examine the field relationships, stratigraphy, petrography, major and trace elements, and Sr-Nd-Hf-Pb isotopic compositions of Wrangellia flood basalts from different areas of Vancouver Island to assess the nature of the mantle source and to evaluate the melting history and subsequent magmatic evolution of basalts involved in the construction of this major oceanic plateau. The geochemistry of picritic and tholeiitic basalts that form the volcanic stratigraphy of this oceanic plateau offers a view of the melting history of plume-derived magmas that does not involve continental lithosphere and where source heterogeneity does not have a major role. This study is one part of a large research project on the nature, origin and evolution of the Triassic Wrangellia flood basalts in British Columbia, Yukon and Alaska. GEOLOGIC SETTING Wrangeffia on Vancouver Island Wrangellia flood basalts form the core of the Wrangellia terrane, or Wrangellia, one of the largest outboard terranes accreted to western North America (Jones et a!., 1977). Middle to Late Triassic flood basalts extend in a discontinuous belt from Vancouver and Queen Charlotte Islands (Karmutsen Formation), through southeast 28 Alaska and southwest Yukon, and into the Wrangell Mountains, Alaska Range, and Talkeetna Mountains in east and central Alaska (Nikolai Formation) (Fig. 2.1). Wrangellia covers approximately 80% of Vancouver Island, which is 460 km long by 130 km wide (Fig. 2.1). Wrangellia is the uppermost sheet of a stack of southwest vergent thrust sheets that form the crust of Vancouver Island and has a cumulative thickness of>10 km (Monger & Journeay, 1994). Pre-Karmutsen units of Wrangellia on Vancouver Island are Devonian arc sequences of the Sicker Group and Mississippian to Early Permian siliciclastic and carbonate rocks of the Buttle Lake Group (Muller, 1980; Sutherland-Brown eta!., 1986). The Paleozoic formations have varied estimated thicknesses (\u00E2\u0080\u0094P2-5 km) (Muller, 1980; Massey, 1 995a) and are only exposed on central and southern Vancouver Island (Fig. 2.1). The Karmutsen Formation is overlain by the shallow-water Quatsino Limestone (30-750 m) and deeper-water Parson Bay Formation (-600 m), which is intercalated with and overlain by Bonanza arc volcanics (169-202 Ma; Nixon et a!., 2006b). Bonanza arc plutonic rocks (167-197 Ma) also intrude the Karmutsen Formation and are some of the youngest units of Wrangellia that formed prior to accretion with North America (Nixon et a!., 2006b). Following accretion, Wrangellia units were intruded by the predominantly Cretaceous Coast Plutonic Complex (Wheeler &McFeely, 1991). The Karmutsen Formation (19,142 km2,based on mapped areas from digital geologic maps) is composed ofbasal sediment-sill complexes, a lower member of pillowed and unpillowed submarine flows, a middle member of mostly pillow breccia and hyaloclastite, and an upper member of predominantly massive subaerial flows (Carlisle & Suzuki, 1974). The pillow basalts directly overlie thick sediment-sill complexes composed of mafic sills intruding Middle Triassic pelagic sediments and Late Paleozoic formations. The boundary between pillow breccialhyaloclastite and massive lava flows represents the transition from a submarine to a subaerial eruptive environment. The uppermost flows of the Karmutsen are intercalated and overlain by shallow-water limestone and local occurrences of submarine flows occur within the upper subaerial member. The Karmutsen Formation and Wrangellia on Vancouver Island were the focus of mapping efforts and stratigraphic descriptions by Carlisle (Carlisle, 1963; Carlisle, 1972; Carlisle & Suzuki, 1974) and Muller (Muller et a!., 1974; Muller, 1977). Recent 29 Figure 2.1 Simplified map of Vancouver Island showing the distribution of the Karmutsen Formation (gray) and underlying Paleozoic formations (black; after Massey eta?., 2005a, 2005b). The main areas of field study are indicated with boxes or circles with capital letters (see legend). Ocean is light gray. The inset shows the extent of the Wrangellia flood basalts (gray) in British Columbia, Yukon, and Alaska. -5ON 12W W Middle\u00E2\u0080\u0094Late Triassic Karmutsen Formation Paleozoic\u00E2\u0080\u0094Middle Triassic Sicker and Buttle Lake Groups Karmutsen Range (Alice\u00E2\u0080\u0094Nimpkish Lake> Schoen Lake Provincial Park area Mount Arrowsmith area L) Buttle Lake Holberg Inlet 30 descriptions of the Karmutsen Formation on northern Vancouver Island have been made during regional mapping studies (1:50,000 scale) on northern Vancouver Island by Nixon et al. (Nixon eta!., 2006b; Nixon & Orr, 2007; Nixon eta!., 2008). Age of the Karmutsen Formation The age and duration of Karmutsen volcanism are constrained by fossils in the underlying and overlying sedimentary units and by three U-Pb isotopic age determinations on intrusive rocks that are likely related to the Karmusten volcanics. Daone!la-bearing shale 100-200 m below the base of the pillow basalts on Schoen Mountain and Halobia-rich shale interlayered with flows in the upper part of the Karmutsen indicate eruption of the flood basalts in the Upper Ladinian (Middle Triassic) to Upper Carnian (Late Triassic; Carlisle and Suzuki, 1974). The only published U-Pb age is based on a single concordant analysis of a multi-grain baddeleyite fraction from a gabbro on southern Vancouver Island that yielded a206Pb/38Uage of 227.3 \u00C2\u00B1 2.6 Ma (Parrish & McNicoll, 1992). Two unpublished 206Pb/38Ubaddeleyite ages, also from a gabbro on southern Vancouver Island, are 226.8 \u00C2\u00B1 0.5 Ma (5 fractions) and 228.4 \u00C2\u00B1 2.5 (2 fractions; Sluggett, 2003). VOLCANIC STRATIGRAPHY AND PETROGRAPIIY Field studies undertaken on Vancouver Island in 2004-2006 explored the volcanic stratigraphy of the Karmutsen flood basalts in three main areas: the Karmutsen Range (between Alice and Nimpkish lakes), the area around Schoen Lake Provincial Park, and around Mount Arrowsmith (Greene eta!., 2005; Greene eta!., 2006), and also around Holberg Inlet, on northernmost Vancouver Island, and Buttle Lake (Fig. 2.1). The character and thickness of the flood basalt sequences vary locally, although the tripartite succession of the Karmutsen Formation appears to be present throughout Vancouver Island. The stratigraphic thicknesses for the pillow, pillow breccia, and massive flow members are estimated at \u00E2\u0080\u0094\u00E2\u0080\u00982600 m, 1100 m, and 2900 m, respectively, in the type area around Buttle Lake (Surdam, 1967); on northern Vancouver Island estimated thicknesses are >3000 m, 400-1500 m, and >1500 m, respectively (Nixon et al., 2008); on Mount 31 Arrowsmith and nearby areas on southern Vancouver Island estimates are 1100 m, 950 m, and 1200 m, respectively (Yorath et a!., 1999; Fig. 2.1). Picritic pillow lavas occur west of the Karmutsen Range on northern Vancouver Island, in a roughly triangular-shaped area (30 km across) bounded by Keogh, Maynard and Sara lakes (Figs 2.2 and 2.3; Greene et a!., 2006; Nixon et al., 2008). Excellent exposures ofpicritic pillow lavas occur in roaclcuts along the north shore of Keogh Lake, the type locality (Greene eta!., 2006). The Keogh Lake picrites mostly form pillowed flow units (<15 m thick), with pillows and tubes of varied dimensions (typically <1 m wide), and unpillowed flows. Numerous thermal contraction features in the pillows are filled with quartz-carbonate, such as drain-back ledges, tortoise-shell jointing, and interpillow voids containing spalled rims (Fig. 2.3; Greene eta!., 2006). The picritic pillow basalts are not readily distinguishable in the field from basalt, except by their density and non-magnetic character and minor interpillow quartz-carbonate. Recent fieldwork and mapping indicates that the picrites occur mostly near the transition between pillow lava emplacement and hyaloclastite deposition (Nixon et a!., 2008). In and around Schoen Lake Provincial Park, there is a well-preserved sediment- sill complex at the base of the Karmutsen Formation (Figs 2.2 and 2.4). Middle Triassic marine sedimentary rocks overlie Pennsylvanian to Permian limestone and siliceous sedimentary rocks, and both successions are intruded by mafic sills related to the overlying flood basalts (Carlisle, 1972). Carlisle (1972) estimated the sediment-sill complex to be approximately 600-900 m thick with a total thickness of 150-200 m ofpre intrusive sedimentary rocks. The Triassic sedimentary rocks range from thinly-bedded siliceous and calcareous shale to banded chert and finely-laminated, Daonella-bearing shale (Carlisle, 1972). The basal sediment-sill complex is immediately overlain by thick successions of submarine flows (Fig. 2.4). Basal sills and pre-Karmutsen sediments are also exposed around Buttle Lake. Exposures with large vertical relief around Mount Arrowsmith (Appendix A) and Buttle Lake preserve thick successions of pillow basalt, breccias, and subaerial flows and there are rarely sediments between flows or in interpillow voids. Unpillowed submarine flows interspersed with pillowed flows are locally recognizable by irregular, hackly columnar jointing. The massive subaerial flows form monotonous sequences marked 32 BONANZA VOLCANICS waterlain pyroclastics, subaerial basalt, rhyolite flows, tuff, and intrusive suites (Island Plutonic Suite) HARBLEDOWN FORMATION calcareous siltstone, feldspathic wacke PARSON BAY FORMATION 1J well\u00E2\u0080\u0094bedded siliceous limestone and] wacke, with minor volcanics QUATSINO FORMATION massive to well\u00E2\u0080\u0094bedded micritic and locally bioclastic limestone intra\u00E2\u0080\u0094Karmutsen limestone lenses KARMUTSEN FORMATION subaerial flows with minor pillow basalt and hyaloclastite (\u00E2\u0080\u00943000 m) - massive lava i pillow lava picrite jabz T... .._., Late Triassic-Middle Jurassic e to Late Triassic Alert Bay Volcanics ] Bonanza Group L.._. uatsino Formation Upper Cretaceous Parson Bay Formation Karmutsen Formation Nanaimo Group Early to Middle Jurassic / TJ Undivided Parson Bayequivalents and Bonanza sediments l5tsfld Plutonic Suite Figure 2.2 Geologic map and stratigraphy of the Schoen Lake Provincial Park and Karmutsen Range areas (locations shown in Figure 2.1, northern Vancouver Island). (a) Stratigraphic column depicts units exposed in the Schoen Lake area, derived from Carlisle (1972) and fieldwork. (b) Generalized geology for the Schoen Lake area with sample locations. Map derived from Massey et a!. (2005a). The exposures in the Schoen Lake area are the lower volcanic stratigraphy and base of the Kannutsen Formation. (c) Stratigraphic column for geology in the Alice-Nimpkish Lake area, derived from Nixon & Off (2007). (d) Geologic map from mapping of Nixon & Orr (2006a) and Nixon et a!. (2008). Sample sites and lithologies are denoted in the legend. The Keogh Lake picrites are exposed near Keogh, Sara, and Maynard lakes and areas to the east of Maynard Lake (Nixon eta!., 2008). pillowed and unpillowed flows, breccia and hyaloclastite (\u00E2\u0080\u00942500 m) silicified shale, chert and limestone with Daonella beds intruded by mafic sills BUTTLE LAKE GROUP massive bioclastic limestone (Mount Mark Fm.), variety of chert, thinly\u00E2\u0080\u0094 \u00E2\u0080\u0094 bedded shale, and limestone (Fourth Lake (a) Fm.) Early-MIddle Jurassic Island Plutonic Suite Late Triassic- Early Jurassic Parson Bay Formation MIddle-Lower Triasslc Quatsino Limestone Karmutsen Formation Shale-chert-limestone(b) LEMARE LAKE VOLcANIcS Mississlpplan-Permlan [] Buttle Lake Group(sedimentary rocks) massive lava / o sill I gabbro / fault \u00E2\u0080\u00A2 pillow lava o shale or chert river \u00E2\u0080\u0094 park boundary I subaerialbasalt and rhyolite flows, breccia, and tuff minor pillow lava, hyaloclastite, debris\u00E2\u0080\u0094flow and epiclastic deposits \u00E2\u0080\u0094 plag\u00E2\u0080\u0094megacrystic flows -interbedded volcaniclastic and sedimentary rocks \u00E2\u0080\u0098U a I It -J PARSON BAY FORMATION well\u00E2\u0080\u0094bedded shale, limestone, wacke, with minor volcanics c breccia, tuff, reworked cs, minor pillows and INOF N limestone lenses fKARMUTSEN FORMATION subaerial flows with minor pillow basalt and hyaloclastite I pillow breccia and hyaloclastite \u00E2\u0080\u0094 Keogh Lake picrite (mostly pillow lavas) pillowed and unpillowed flows (c) 33 34 - 2.3 P1 - - of picritic and tholeiitic pillow basalts from the Karmutsen Range area (Alice and Nimpkish Lake area), northern Vancouver Island. (a) Stack of dense, closely packed, asymmetric picritic pillows with radial vesicle infillings (circled sledgehammer 8O cm long for scale). (b) Picritic pillow basalts in cross-section near Maynard Lake. Note vesicular upper margin to pillows. (c) Unpillowed submarine flow with coarse, hackly columns (marked with arrows) draped over high-MgO pillow basalt. (d) Spalled rims from pillows filling voids between tholeiitic pillow basalt (coin for scale). (e) Cross-section of a large picritic pillow lobe with infilling of quartz-carbonate in cooling-contraction cracks (sledgehammer for scale). (f) High-MgO pillow breccia stratigraphically between submarine and subaerial flows (lens cap for scale --7 cm diameter). Figure 2.4 Photographs showing field relations from the Schoen Lake Provincial Park area, northern Vancouver Island. (a) Sediment-sill complex at the base of the Karmutsen Formation on the north side of Mount Adam (see Fig. 2.2 for location). (b) Interbedded mafic sills and deformed, finely-banded chert and shale with calcareous horizons, from location between Mt. Adam and Mt. Schoen. 35 Arrowsmith and nearby areas on southern Vancouver Island estimates are 1100 m, 950 m, and 1200 m, respectively (Yorath et al., 1999; Fig. 2.1). Picritic pillow lavas occur west of the Karmutsen Range on northern Vancouver Island, in a roughly triangular-shaped area (--\u00E2\u0080\u009830 km across) bounded by Keogh, Maynard and Sara lakes (Figs 2.2 and 2.3; Greene et a!., 2006; Nixon et al., 2008). Excellent exposures ofpicritic pillow lavas occur in roadcuts along the north shore of Keogh Lake, the type locality (Greene eta!., 2006). The Keogh Lake picrites mostly form pillowed flow units (<15 m thick), with pillows and tubes of varied dimensions (typically <1 m wide), and unpillowed flows. Numerous thermal contraction features in the pillows are filled with quartz-carbonate, such as drain-back ledges, tortoise-shell jointing, and interpillow voids containing spalled rims (Fig. 2.3; Greene et a?., 2006). The picritic pillow basalts are not readily distinguishable in the field from basalt, except by their density and non-magnetic character and minor interpillow quartz-carbonate. Recent fieldwork and mapping indicates that the picrites occur mostly near the transition between pillow lava emplacement and hyaloclastite deposition (Nixon et aL, 2008). In and around Schoen Lake Provincial Park, there is a well-preserved sediment- sill complex at the base of the Karmutsen Formation (Figs 2.2 and 2.4). Middle Triassic marine sedimentary rocks overlie Pennsylvanian to Permian limestone and siliceous sedimentary rocks, and both successions are intruded by mafic sills related to the overlying flood basalts (Carlisle, 1972). Carlisle (1972) estimated the sediment-sill complex to be approximately 600-900 m thick with a total thickness of 150-200 m ofpre intrusive sedimentary rocks. The Triassic sedimentary rocks range from thinly-bedded siliceous and calcareous shale to banded chert and finely-laminated, Daonel!a-bearing shale (Carlisle, 1972). The basal sediment-sill complex is immediately overlain by thick successions of submarine flows (Fig. 2.4). Basal sills and pre-Karmutsen sediments are also exposed around Buttle Lake. Exposures with large vertical relief around Mount Arrowsmith (Appendix A) and Buttle Lake preserve thick successions of pillow basalt, breccias, and subaerial flows and there are rarely sediments between flows or in interpillow voids. Unpillowed submarine flows interspersed with pillowed flows are locally recognizable by irregular, hackly columnar jointing. The massive subaerial flows form monotonous sequences marked 36 mainly by amygdaloidal horizons and brecciated flow tops are rarely observed. A single locality with well-preserved pahoehoe flow features is exposed north of Holberg Inlet (Nixon & Orr, 2007; Nixon et al., 2008). There is no evidence of significant detrital material from a continental source in sediments associated with the flood basalts anywhere on Vancouver Island. The uppermost Karmutsen flows are interbedded with thin (>4 m) lenses of limestone, and rarely siliciclastic sedimentary rocks (Nixon et a!., 2006b). Plagioclase-phyric (>0.8 mm) trachytic-textured flows are also commonly found near the top of the Karmutsen Formation (Nixon et a!., 2006b). A total of 129 samples were collected from the Karmutsen Formation on Vancouver Island and 63 samples were selected for geochemical work based on the relative degree of alteration and geographic distribution of the samples. Fifty-six of these samples have been divided into four groups based on petrography (Table 2.1) and geochemistry, including tholeiitic basalt, picrite, high-MgO basalt, and coarse-grained mafic rocks. The tholeiitic basalts are dominantly glomeroporphyritic with an intersertal to intergranular groundmass (Table 2.1). Plagioclase forms most of the phenocrysts and glomerocrysts and the groundmass is fme-grained plagioclase microlites, clinopyroxene granules, small grains of Fe-Ti oxide, devitrified glass, and secondary minerals; there is no fresh glass. The picritic and high-MgO basaltic lavas exhibit plagioclase and clinopyroxene with spherulitic morphologies with abundant euhedral olivine phenocrysts (Fig. 2.5; Table 2.1). Only olivine is strongly altered and is completely pseudomorphically replaced by talc/tremolite. Plagioclase and clinopyroxene phenocrysts are not present in the high-MgO samples. The coarse-grained mafic rocks are characterized petrographically by subophitic texture with average grain size typically >1mm, and are generally non-glomeroporphyritic. The coarse-grained mafic rocks are mostly from the interiors of massive flows, although some may be from sills. SAMPLE PREPARATION AND ANALYTICAL METHODS Only the freshest rocks in the field were sampled and only the least altered samples were selected for chemical and isotopic analysis based on thorough petrographic inspection. Sixty-three of the 129 collected samples (including 7 non-Karmutsen 37 Figure 2.5 Photomicrographs of picritic pillow basalts, Karmutsen Formation, northern Vancouver Island. Scale bars are 1 mm long. (a) Picrite with abundant euhedral olivine pseudomorphs from Keogh Lake type locality (Fig. 2.2) in cross-polarized transmitted light (sample 4722A4; 19-19.8 wt % MgO, 4 analyses). Samples contain dense clusters (24-42 vol %) of olivine pseudomorphs (<2 mm) in a groundmass of curved and branching sheaves of acicular plagioclase and intergrown with clinopyroxene and altered glass. In many cases, plagioclase nucleated on the edges of the olivine phenocrysts. (b) Sheaves of intergrown plagioclase and clinopyroxene in aphyric picrite pillow lava from west of Maynard Lake (Fig. 2.2) in cross polarized transmitted light (sample 4723A2; 10.8 wt % MgO). 38 Table 2.1 Summary of oetroaraohic characteristics and ohenocrist orooortions of Karmutsen basalts on Vancouver Island. B.C. Sampie Areab FIOWC Groupd Texture voi% 011 PIag Cpx Ox Alteration9 Note\u00E2\u0080\u009D 4718A1 MA PIL THOL intersertal 20 5 3 few plag glcr <2 mm, cpx <1 mm 471 8A2 MA PIL THOL intersertal, glomero 15 1 plag glcr <2 mm, very fresh 471 8A5 MA PLO THOL intersertal, glomero 10 2 mottled, few plag glcr <4 mm 471 8A6 MA BRE THOL porphyritic 20 2 plag 5-6 mm, sericite alteration 4718A7 MA PIL THOL glomero 10 3 plag glcr<3 mm 4719A2 MA PIL THOL porphyritic, intersertal 5 3 plag glcr <2 mm 471 9A3 MA PIL THOL glomero 10 2 plag glcr <3 mm 4720A2 SL BRE THOL glomero 5 3 plag glcr <2 mm 4720A3 SL FLO THOL glomero 3 3 plag glcr <3 mm 4720A4 SL FLO THOL glomero 5 1 plag glcr <5 mm, slightly cg, very fresh 4720A5 SL PLO THOL senate 20 3 plag glcr <2 mm, plag needles and laths 4720A8 SL FLO THOL glomero 20 2 plag glcr <2 mm, plag needles and laths 4720A9 SL FLO THOL glomero 10 2 plag glcr<1.5 mm 4721A1 SL PIL THOL glomero 15 3 plagglcr<1.5mm, plag needles aligned 4721A2 SL FLO THOL glomero 10 1 plag glcr<3 mm, very fresh 4721A3 SL FLO THOL glomero 20 3 plag glcr <2 mm, plag needles 4721A4 SL FLO THOL glomero, ophimottled 20 5 1 plag glcr <1.5 mm, cpx <1 mm (oik), very fresh 4721A5 SL FLO THOL glomero, ophlmottled 5 15 3 plag glcr <1.5 mm, cpx <1 mm (01k) 4722A2 KR FLO THOL intengranular, intersertal 1 1 few plag phenos <2 mm 4723A10 KR PLO THOL intengranular 5 3 10 3 few plag glcr <1 mm, ox 0.5-1 mm 4724A5 SL PLO THOL porphyritic 25 3 plag 7-8mm 5614A10 KR FLO THOL intergranular, intersertal 2 5 3 ox <0.5mm 5614A11 KR FLO THOL intergranular, intersertal 5 7 3 ox 0.5-1 mm 5614A13 KR FLO THOL lntergranular, porphynitlc 5 5 3 ox 0.5-1 mm 561 5A1 KR PIL THOL aphyric, Intersertal 1 2 f.g., no phenos 5615A8 KR PIL THOL intengranular 3 1 f.g., abundant small ox, very fresh 5615A10 KR PIL THOL intengranular 3 3 2 plag glcr <4 mm 5616A2 KR FLO THOL intergranular, intersertal 5 7 3 ox 0.2-0.5mm 5618A1 Cl FLO THOL intergranular 3 2 10 3 ox 0.5-2 mm, c.g., plag laths >2 mm 93G171 KR PIL PlC subophitic 23 01<2.3 mm, cpx<1 .5 mm, partially enc plag 4722A4 KR PIL PlC cumulus, intergranular 35 10 3 01<1.5 mm, cpx<2 mm enc plag 4723A3 KR PIL PlC spherulitic 31 3 01<1 mm, swtl plag <1.5mm 4723A4 KR PIL PlC intergranular, intersertal 0 3 swtl plag <1 mm, no ci phenos 4723A13 KR PIL PlC spherulitic 24 3 swtl plag <1 mm 5614A1 KR PIL PlC spherulitic 24 1 01<1.5 mm, swtl plag <1 mm 5615A7 KR PIL PlC cumulus, intergranular 42 10 1 01<1.5 mm, cpx <1.5mm enc plag 5615A12 KR PIL PlC spherulltic 13 1 01 <2 mm, swtl plag <1 mm 5616A1 KR PIL PlC iritengranular, intersertal 25 1 01 <2mm 4723A2 KR PIL HI-MG spherulitic 0 3 swtl plag <2 mm, no ol phenos 5614A3 KR PIL HI-MG spherulitic, intrafasciculate 12 1 01<1.5 mm, swtl plag <2mm 561 4A5 KR BRE HI-MG porphyritic, ophimottled 13 2 ol <2 mm, cpx <2 mm enc plag 561 6A7 KR PIL HI-MG intersertal 2 2 ol <2 mm, plag needles <1 mm 4722A5 KR FLO OUTLIER intersertal 3 mottled, very f.g., v. small p1 needles 5615A11 KR PIL OUTLIER intersertal 3 mottled, very f.g., v. small p1 needles 4720A6 SL 4720A7 SL 4720A10 SL 4724A3 SL 5614A14 KR 5614A15 KR 5615A5 KR 561 5A6 KR 5616A3 KR 5617A1 SL 5617A5 SL 5617A4 SL SIL MIN intersertal 5 3 to. PLO CG FLO CG SIL CG FLO CG SIL CG SIL CG subophitic subophitic subophltic, Intrafasciculate ophimottled, subophitic intergranular subophitic GAB CG GAB CG GAB CG SIL CG SIL CG 15 1 plag glcr <3 mm, cpx <3 mm 25 1 plag chad <1.5 mm, cpx oik <2mm, very fresh 5 3 plag chad<2 mm, cpx oik <3 mm, ox <1 mm, c.g. 5 30 5 3 plag glcr 4-5 mm, cpx olk <2mm, ox <1 mm 10 5 3 plag52 wt %) are probably not Karmutsen flood basalts, but younger Bonanza basalts and andesites. Three pillowed flow samples (x) and a mineralized sill (+) are distinguished separately because of their anomalous chemistry. Coarse-grained samples are indicated separately and generally have subophitic texture. Three tholeiitic basalt samples (4724A5, 4718A5, 4718A6) with >16 wt % A1203 contain 10-25 modal % of large plagioclase phenocrysts (7-8 mm) or glomerocrysts (>4 mm). 45 4.0 3.5 -1102 + 3.0 2.5 2.0 1.5 1.0 Picnte O High-MgO basalt 0 Coarse-grained 9 Tholelitic basalt - ,. \u00E2\u0080\u00A2 X Pillowed flow + Mineralized sill ., * Karmutsen Form.. :. 0 0 \u00E2\u0080\u00A2 (compiled data) + .111.111.1.. I.... 40 45 II 50 Si02 (wt%) II 55 Ni (ppm) \u00E2\u0080\u0094(b) II liii 600 - 0.5 0.0 60 15 14 13 12 11 10 400 - 11111111111 I I IIIIIIIIIII - 0 -CaO 6* -(wt%) +\u00E2\u0080\u00A2. \u00E2\u0080\u00A2. \u00E2\u0080\u0094 0 A - \u00E2\u0080\u00A2 . 1: LO (wt oo0 0 Ocr. 0 200 0 16 15 14 13 12 11 10 9 8 7 6 5 10 15 20 MgO(wt%) 10 MgO (wt%)3 \u00E2\u0080\u0094 I I \u00E2\u0080\u00A2 i\u00E2\u0080\u0094I\u00E2\u0080\u0094I I I I I 2 FeO(T) \u00E2\u0080\u00A2 *(wt%) :fE (f) - 11111 5 20 25 5 10 15 MgO (wt LA A1203 (w \u00E2\u0080\u00A2\u00E2\u0080\u00A2 l. . e 25 0 \u00E2\u0080\u00A2\u00E2\u0080\u00A2 * 6* .. \u00E2\u0082\u00AC0 AA 0O*OA AA . \u00E2\u0080\u00A2* I \u00E2\u0080\u0094 .l. \u00E2\u0080\u0094 \u00E2\u0080\u0094 \u00E2\u0080\u0094 I \u00E2\u0080\u0094 20 16 15 14 13 12 11 - I.... I.... I 0 5 10 15 20 250 MgO (wt%) (g) 5 10 15 MgO (wt%) Table 2.2 Maior element (wt% oxide) and trace element (ppm) abundances in whole rock samples of Karmutsen basalts, Vancouver Island, B.C. Sample 4718A1 4718A2(1) 4718A2(2) 4718A5 4718A6 4718A7 4719A2 4719A3 4720A2 4720A3 Group THOL THOL THOL THOL TI-IOL THOL THOL THOL THOL ThOL Area MA MA MA MA MA MA MA MA SL SL Flow Pillow Pillow Pillow Flow Brenda Pillow Pillow Pillow Brenda Flow UTM EW 5455062 5455150 5455150 5455459 5455518 5455280 5454625 5454625 5567712 5567305 UTMNS 384113 384260 384260 383424 383237 382261 381761 381761 708686 707890 Unnormalized Major Element Oxides (Wefght %): Si02 47.93 48.65 49.07 48.41 48.69 48.44 49.68 49.08 49.24 48.47 Ti02 1.646 1.857 1.835 1.402 1.359 1.487 1.705 1.676 1.577 1.716 203 14.41 14.30 14.15 15.89 15.92 14.74 14.93 14.06 13.54 13.52 Fe3* 12.29 12.07 12.48 9.90 10.74 12.13 9.75 12.21 11.30 11.61 MnO 0.171 0.183 0.181 0.155 0.157 0.167 0.185 0.178 0.212 0.185 MgO 6.25 6.21 6.16 6.81 6.74 6.89 6.98 6.55 6.09 7.12 CaO 10.22 11.85 11.70 11.31 10.76 11.56 10.96 10.94 14.00 11.94 Na20 3.22 2.28 2.25 2.51 2.89 2.39 2.23 2.24 2.12 2.18 1(20 0.24 0.18 0.14 0.35 0.34 0.24 0.27 0.31 0.10 0.29 P205 0.15 0.14 0.15 0.10 0.12 0.12 0.08 0.13 0.12 0.12 LOl 2.72 1.38 1.22 2.17 2.28 1.64 1.97 1.63 1.15 1.95 Total 99.24 99.09 99.33 99.01 99.98 99.81 98.75 99.00 99.45 99.11 Trace Elements (ppm): La 7.88 9.03 8.72 6.50 6.58 6.10 7.17 7.02 720 7.88 Ce 19.0 22.2 21.6 15.5 16.2 15.4 18.0 17.7 17.3 19.0 Pr 2.71 3.10 3.02 2.15 2.20 2.23 2.62 2.52 2.42 2.72 Nd 13.0 15.2 14.5 10.7 11.2 11.4 13.2 12.5 12.2 13.5 Sm 3.57 4.11 4.05 3.03 3.13 3.42 3.67 3.66 3.43 3.85 Eu 1.40 1.57 1.48 1.18 1.20 1.25 1.47 1.39 1.33 1.47 Gd 4.39 4.77 4.87 3.59 3.64 3.90 4.32 4.24 4.12 4.48 Tb 0.76 0.82 0.82 0.63 0.64 0.68 0.76 0.74 0.71 0.78 Dy 4.55 5.02 4.79 3.80 3.87 4.12 4.68 4.41 4.28 4.78 I-to 0.90 0.99 0.96 0.77 0.78 0.84 0.94 0.89 0.87 0.95 Er 2.57 2.79 2.72 2.20 2.27 2.34 2.66 2.45 2.48 2.69 Tm 0.36 0.40 0.39 0.32 0.32 0.33 0.37 0.35 0.34 0.38 Yb 2.15 2.39 2.32 1.90 1.96 2.00 2.23 2.11 2.05 2.33 Lu 0.31 0.33 0.32 0.27 0.27 0.28 0.30 0.30 0.29 0.33 Sc 40.7 41.3 40.4 38.9 38.3 40.2 42.8 42.8 41.1 46.2 V 328 351 353 285 279 317 341 341 325 344 Cr 93 146 125 290 297 252 130 131 151 168 Co 48.7 49.9 48.9 46.1 45.4 51.4 53.1 53.4 50.5 56.2 Ni 87 90 88 125 122 115 94 103 83 93 Cu 177 201 198 167 158 159 180 168 167 184 Zn 83 91 91 77 73 82 86 87 79 88 Ga 19 19 20 17 19 18 16 18 19 17 Ge 1.6 1.5 1.4 1.1 1.5 1.5 0.7 1.5 1.9 1.1 Rb 4 2 2 6 6 4 5 5 6 Sr 418 231 228 282 332 263 281 286 192 214 Y 25 28 28 22 22 24 26 26 25 27 Zr 86 97 94 72 71 77 86 83 83 90 Nb 8.4 10.1 10.0 7.0 6.8 6.5 7.4 7.3 7.9 8.6 Cs 0.7 0.3 0.2 0.5 0.5 0.2 0.2 0.2 02 Ba 173 50 49 71 98 62 143 133 28 81 Hf 2.6 2.8 2.8 2.0 2.2 2.1 2.6 2.5 2.4 2.6 Ta 0.59 0.68 0.65 0.45 0.46 0.41 0.53 0.49 0.51 0.60 Pb 4 7 7 10 4 6 7 11 7 9 Th 0.65 0.74 0.72 0.52 0.53 0.54 0.61 0.61 0.57 0.64 U 0.20 0.23 0.24 0.17 0.16 0.17 0.22 0.19 0.18 0.20 Abbreviations for group are: THOL, tholeiilic baaalt; PlC, picrite; HI-MG, high MgO basalt; CG, coarse-grained (sill or gabbro); MIN SIL, mineralized sill; OUTLIER, anomalous pillowed flow in plots. Abbreviations for area are: MA, Mount Arrowsmith; SL, Schoen Lake; KR, Karmutsen Range; 01, Quadra Island. Sample locations are given using the Universal Transverse Mercator (UTM) coordinate system (NAD83; zones 9 and 10). Analyses were perfoiTned at Activation Laboratoiy (AcilLabs). Fe 203 is total ron expressed as Fe203.LOl is loss-on-ignition. AU major elements, &, V, andY were measured by CF quadrupole OES on solutions of fused samples; Cu, Ni, Pb, and Zn were measured by total dilution ICP; Cs, Ga, Ge, Hf, Nb, Rb, Ta, Th, U, Zr, and REE were measured by magnetic-sector ICP on solutions of fused samples; Co Cr, and Sc were measured by INAA. Blanks are below detection lwnit. Ni concentrations for these high-MgO samples by XRF. See Appendices B and C for complete XRF data and PCIGR trace element data, respectively. Major elements for sample 93G1 7 are normalized, anhydrous. Samples from Quadra Island are not shown in figures. 46 Sample 4720A4 4720A5 4720A5 4720A6 4720A7(1) 4720A7(2) 4720A8 4720A9 4720A10 4721A1 Group ThOL THOL THOL CG CG CG ThOL TI-IOL CG ThOL Area SL SL SL SL SL SL SL SL SL SL Flow Flow Flow Breccia Flow Flow Flow Flow Flow SIU PUlow UTM EW 5566984 5563304 5563304 5566161 5566422 5566422 5566800 5564002 5560585 5563843 UTM NS 707626 705978 705978 704411 703056 703056 700781 703739 702230 704932 Unnrxmalized Major Element Oxides (Weight %): 5102 49.16 50.25 48.88 48.90 49.60 49.53 48.02 48.06 47.38 48.17 riO2 1.776 1.736 1.781 1.799 1.811 1.809 1.768 1.767 0.829 1.890 A1203 14.00 13.43 13.84 14.21 14.34 14.36 14.34 14.11 14.42 14.96 Fe203* 13.30 13.03 13.46 11.78 13.37 13.39 13.18 13.73 10.58 11.40 MnO 0.194 0.175 0.179 0.195 0.199 0.199 0.195 0.185 0.161 0.180 MgO 6.39 5.93 6.12 6.40 5.93 5.94 6.73 6.35 7.69 6.39 CaO 11.85 11.05 11.43 12.08 11.69 11.68 11.37 11.61 11.66 11.75 Na20 1.86 2.12 2.16 1.82 1.97 1.97 1.97 2.08 2.77 1.93 1<20 0.14 0.12 0.14 0.31 0.15 0.13 0.11 0.11 0.12 0.10 P205 0.15 0.15 0.16 0.12 0.16 0.15 0.15 0.16 0.06 0.13 LOl 0.93 1.44 1.42 1.38 0.75 0.75 1.91 1.37 4.07 2.11 Total 99.75 99.42 99.56 99.00 99.97 99.90 99.73 99.52 99.75 99.01 Trace Elements (ppm): La 8.44 7.81 8.07 7.75 7.84 7.74 6.86 7.06 2.24 7.31 Ce 20.5 18.8 19.5 19.0 19.0 19.3 17.5 17.3 5.6 18.9 Pr 2.88 2.63 2.73 2.71 2.67 2.74 2.54 2.46 0.85 2.78 Nd 14.3 13.1 13.5 13.9 13.3 13.7 12.6 12.3 4.8 13.5 Sm 4.07 3.68 3.87 4.03 3.86 3.89 3.72 3.51 1.76 4.06 Eu 1.49 1.43 1.45 1.43 1.42 1.43 1.37 1.36 0.75 1.47 Gd 4.70 4.60 4.74 4.62 4.73 4.66 4.40 4.19 2.58 4.72 Th 0.81 0.79 0.83 0.81 0.81 0.82 0.77 0.75 0.50 0.81 Dy 4.90 4.77 4.92 5.01 4.95 4.97 4.86 4.62 3.38 4.99 Ho 0.99 0.96 0.98 1.01 1.01 1.01 0.98 0.93 0.76 1.03 Er 2.81 2.80 2.79 2.89 2.92 2.89 2.78 2.65 2.31 2.96 Tm 0.39 0.40 0.40 0.41 0.42 0.40 0.40 0.37 0.34 0.41 Yb 2.31 2.47 2.46 2.51 2.49 2.44 2.51 2.27 2.16 2.50 Lu 0.33 0.35 0.35 0.36 0.34 0.35 0.35 0.33 0.31 0.36 Sc 42.9 41.0 44.5 43.4 44.2 42.0 44.0 49.9 44.4 V 349 354 362 363 366 367 362 362 288 378 Cr 122 133 156 152 154 155 166 311 158 Co 53.7 50.8 54.8 52.5 52.6 54.5 55.5 49.5 54.8 Ni 92 99 105 107 97 85 116 112 121 106 Cu 202 174 185 188 212 210 218 187 134 208 Zn 92 90 93 96 93 94 109 90 64 86 Ga 20 19 19 18 19 19 19 18 14 18 Ge 1.6 1.6 1.6 1.2 1.6 1.7 1.8 1.4 1.4 1.3 Rb 2 1 1 10 2 2 1 2 2 Sr 179 183 188 155 173 174 174 196 130 174 Y 27 28 28 27 29 28 28 28 21 30 Zr 97 93 92 94 94 93 95 88 38 97 Nb 9.1 8.6 8.2 8.7 8.6 8.7 8.3 7.8 1.7 8.5 Cs 0.2 0.1 0.1 0.9 0.3 0.3 0.2 0.1 0.3 0.6 Ba 34 37 38 39 45 43 33 35 68 36 Hf 2.7 2.7 2.6 2.7 2.7 2.8 2.7 2.6 1.2 2.9 Ta 0.60 0.57 0.56 0.57 0.56 0.55 0.55 0.53 0.09 0.58 Pb 7 8 8 7 14 6 8 8 14 Th 0.68 0.63 0.61 0.64 0.61 0.61 0.62 0.61 0.24 0.68 U 0.20 0.21 0.19 0.21 0.19 0.19 0.19 0.18 0.11 022 47 Sample 4721A2 4721A3 4721A4 4721A5 4722A2(1)* 4722A2(2)* 472(3)* 4722(4)* 4722A4(1)* 4722A4(2)* Group THOL THOL THOL THOL THOL THOL THOL THOL PlC PlC Area SL SL SL SL KR KR KR KR KR KR Flow Flow Flow Flow Flow Flow Flow Flow Flow Pillow Pillow UTM EW 5563936 5564229 5564285 5564343 5590769 5590769 5590769 5590769 5595528 5595528 IJTM NS 704941 704928 704896 705008 634318 634318 634318 634318 629490 629490 Unnormalized Major Element Oxides (Weight %): Si02 49.56 48.17 49.49 49.37 48.33 48.67 47.23 48.16 43.85 43.84 1102 1.770 1.768 1.791 1.806 1.799 1.816 1.783 1.782 0.425 0.425 Al203 13.92 14.16 13.98 13.98 13.59 13.74 13.77 13.62 11.56 11.74 Fe203* 13.07 13.24 13.13 13.53 13.75 13.05 14.13 13.76 10.11 9.65 MnO 0.174 0.183 0.176 0.201 0.185 0.187 0.187 0.187 0.161 0.158 MgO 6.06 6.61 6.46 5.98 6.97 7.02 7.09 7.09 17.74 17.51 GaO 12.15 11.30 11.76 11.59 11.42 11.42 11.43 11.48 9.43 9.36 Na20 2.01 2.03 1.92 1.92 2.03 2.05 2.02 2.08 0.53 0.53 K20 0.09 0.15 0.12 0.15 0.14 0.13 0.06 0.14 0.10 0.13 P205 0.16 0.14 0.14 0.15 0.15 0.14 0.14 0.12 0.04 0.00 LOl 0.76 1.78 0.87 1.04 1.48 1.34 1.38 1.37 5.45 5.33 Total 99.73 99.53 99.85 99.71 99.84 99.56 99.22 99.79 99.39 98.66 Trace Elements (ppm): La 7.86 7.42 7.21 7.62 6.95 7.34 7.65 6.95 1.06 0.96 Ce 19.2 18.3 17.9 18.8 17.8 18.5 19.3 17.8 2.6 2.5 Pr 2.70 2.66 2.57 2.73 2.53 2.64 2.94 2.67 0.41 0.38 Nd 13.5 13.3 13.0 13.9 12.9 12.7 13.6 13.3 2.6 2.3 Sm 3.84 3.84 3.81 3.91 3.72 3.66 4.05 3.77 0.92 0.84 Eu 1.46 1.40 1.38 1.47 1.41 1.42 1.55 1.42 0.38 0.37 Gd 4.47 4.59 4.28 4.73 4.42 4.62 4.76 4.59 1.40 1.42 Tb 0.79 0.79 0.77 0.83 0.77 0.77 0.83 0.80 0.30 0.30 Dy 4.75 4.88 4.77 5.12 4.61 4.60 4.92 4.77 2.16 2.17 Ho 0.96 0.98 0.97 1.04 0.94 0.94 0.92 0.95 0.49 0.50 Er 2.72 2.82 2.80 2.99 2.65 2.70 2.64 2.69 1.53 1.54 Tm 0.38 0.39 0.40 0.42 0.38 0.38 0.389 0.37 0.24 0.23 Yb 2.35 2.42 2.40 2.55 2.32 2.27 2.41 2.28 1.58 1.52 Lu 0.32 0.35 0.34 0.35 0.32 0.34 0.349 0.34 0.23 0.23 Sc 42.1 42.3 42.2 39.7 39.2 29.2 41.6 41.5 38.3 40.1 V 351 364 364 367 365 365 362 362 201 189 Cr 118 164 151 122 143 98 162 165 1710 1830 Co 51.6 54.9 53.3 46.5 50.0 37.7 55.7 53.5 80.3 84.6 Ni 91 115 108 90 107 105 93 93 755 755 Cu 195 206 211 210 189 189 178 174 92 83 Zn 90 87 92 92 104 103 98 94 77 55 Ga 19 19 19 19 19 19 20 17 10 9 Ge 1.7 1.3 1.7 1.6 1.4 1.5 1.5 1.2 1.0 0.7 Rb I 1 1 1 2 2 2 2 5 4 Sr 187 162 160 171 190 193 178 190 100 97 Y 26 28 29 29 27 28 26 27 16 13 Zr 97 95 95 96 96 87 94 102 16 19 Nb 9.0 8.5 8.6 8.5 8.3 8.4 8.5 8.5 0.7 0.9 Cs 0.2 0.4 0.3 0.2 0.2 0.2 0.4 0.2 2.8 2.7 Ba 39 34 28 34 34 34 28 33 19 18 Hf 2.8 2.8 2.7 2.9 2.8 2.6 2.8 2.8 0.5 0.6 Ta 0.60 0.57 0.55 0.59 0.55 0.54 0.6 0.57 0.09 0.03 Pb 8 9 9 5 11 12 91 7 4 Th 0.69 0.62 0.62 0.67 0.66 0.64 0.59 0.68 0.10 0.11 U 0.21 0.20 0.19 0.20 0.19 0.20 0.23 0.15 0.05 0.03 48 Sample 4722A4(3)* 4722A5(1)* 4722A5(2r 4723A2* 4723A3* -- 4723A10 4723A1 3(1 )* 4723A13(2)* 4724A3 Group PlC OUTLIER OUTLIER HI-MG PlC PlC THOL PlC PlC CG Area KR KR KR KR KR KR KR KR KR SL Flow Pillow Flow Flow Pillow Pillow Pillow Flow Pillow Pillow Flow UTMEW 5595528 5595029 5595029 5588266 5588274 5586081 5578863 5599233 5599233 5581870 UTMNS 629490 627605 627605 626698 626641 626835 630940 616507 616507 704472 Unnormalized Major Element Oxides (Weight %): Si02 42.94 48.95 48.45 46.73 44.41 44.39 50.03 44.62 44.71 49.08 1102 0.42 2.295 2.333 0.611 0.539 0.663 2.083 0.443 0.442 1.457 A1203 11.26 13.61 12.51 15.24 12.75 14.93 12.31 13.71 13.75 14.77 Fe203 10.82 12.56 15.24 10.26 10.33 10.11 15.21 10.38 10.37 11.60 MnO 0.161 0.188 0.191 0.158 0.148 0.139 0.223 0.138 0.138 0.128 MgO 18.28 6.18 5.99 10.27 15.42 13.02 5.66 14.47 14.48 8.73 GaO 8.98 9.33 8.99 9.93 8.73 9.73 9.19 9.37 9.36 11.02 Na20 0.54 3.26 3.30 2.26 0.78 1.56 2.94 0.86 0.86 1.99 K20 0.02 0.30 0.30 0.38 0.07 0.07 0.70 0.06 0.07 0.15 0.03 0.13 0.19 0.06 0.05 0.06 0.18 0.05 0.05 0.12 LOl 5.7 2.00 1.95 3.70 5.61 4.91 1.55 5.59 5.59 0.98 Total 99.16 98.80 99.45 99.60 98.83 99.58 100.07 99.68 99.81 100.02 Trace Elements (ppm): La 1.08 8.77 9.46 1.94 1.80 1.78 8.93 1.40 1.36 5.57 Ce 2.6 23.0 25.2 5.0 4.5 4.5 21.5 3.4 3.4 14.3 Pr 0.43 3.37 3.68 0.74 0.67 0.71 3.10 0.52 0.51 2.09 Nd 2.5 17.2 17.9 4.2 3.8 4.1 15.5 3.0 2.9 10.4 Sm 0.87 5.20 5.21 1.41 1.27 1.48 4.20 1.00 1.01 2.98 Eu 0.405 1.74 1.79 0.52 0.51 0.63 1.62 0.46 0.44 1.21 Gd 1.45 6.39 6.63 2.09 1.86 2.16 5.21 1.71 1.69 3.85 Th 0.3 1.11 1.20 0.42 0.37 0.42 0.90 0.36 0.35 0.65 Dy 2.11 6.59 7.08 2.91 2.60 2.79 5.40 2.54 2.50 4.01 Ho 0.47 1.34 1.39 0.67 0.59 0.60 1.11 0.59 0.56 0.81 Er 1.54 3.85 3.94 2.10 1.86 1.79 3.23 1.89 1.79 2.27 Tm 0.242 0.55 0.57 0.32 0.29 0.26 0.45 0.29 0.28 0.33 Yb 1.62 3.31 3.48 2.06 1.85 1.67 2.71 1.89 1.84 1.98 Lu 0.244 0.45 0.51 0.31 0.28 0.25 0.39 0.29 0.28 0.28 Sc 36.4 38.7 33.1 47.2 41.0 38.1 38.3 43.5 35.1 V 194 481 495 261 218 235 496 219 222 287 Cr 1750 79.7 59.0 358 1570 725 34.2 1370 352 Co 80 45.6 40.7 48.8 72.9 60.4 47.1 72.2 52.4 Ni 755 59 59 163 656 339 61 583 583 209 Cu 83 116 114 111 110 106 306 142 98 208 Zn 55 106 103 61 60 63 107 106 62 83 Ga 11 17 20 13 12 13 20 12 12 19 Ge 1.2 0.7 1.3 1.2 1.1 1.1 1.6 1.2 1.3 1.6 Rb 6 6 6 10 2 2 12 2 1 2 Sr 93 225 229 271 64 132 299 73 73 167 Y 15 39 39 20 17 18 32 16 17 23 Zr 16 127 126 33 29 36 110 24 22 73 Nb 0.7 10.0 10.6 1.5 1.3 1.1 9.7 0.8 0.7 6.5 Cs 5.8 0.3 0.4 6.5 0.9 0.8 0.4 0.7 0.7 0.3 Ba 27 87 88 84 15 20 152 13 13 55 Hf 0.6 3.7 3.7 1.0 0.9 1.0 3.1 0.7 0.7 2.1 Ta 0.67 0.70 0.08 0.06 0.05 0.65 0.04 0.04 0.42 Pb 22 6 7 4 5 8 7 Th 0.09 1.08 1.08 0.23 0.20 0.10 0.77 0.15 0.14 0.43 [I 007 0R oq 010 009 005 027 005 007 015 49 Sample 4724A5 5614AV 5614A3\u00E2\u0080\u0099 5614A5 5I4A1Q 5614A11 5614A13 5614A14 5614A15 5615A1 Group THOL PlC HI-MG HI-MG THOL THOL THOL CG CG THOL Area SL KR KR KR KR KR KR KR KR KR Flow Flow Pillow Pillow Breccia Flow Flow Flow Sill Flow Pillow UTMEW 5580653 5599183 5599183 5599192 5595261 5593546 5588018 5588246 5589935 5599424 UTMNS 704736 616472 616472 614756 615253 615098 618867 618183 615917 620187 Unnormalized Major Element Oxides (Weight %): Sb2 48.08 46.24 48.43 47.1 46.69 48.06 47.78 46.17 46.2 47.61 1102 1.729 0.471 0.477 0.512 1.761 1.741 1.86 1.773 1.605 1.873 2O3 16.39 14.57 14.9 14,52 13.32 13.35 12.91 13.8 14.77 13.55 Fe203* 11.13 8.57 7.59 9.12 12.72 12.96 14.23 13.53 10.89 14.34 MnO 0.196 0.145 0.134 0.135 0.168 0.187 0.211 0.136 0.123 0.188 MgO 5.54 12.11 10.66 10,92 7.09 6.38 6.47 8.25 9.37 7.42 CaO 12.38 10.81 10.68 10.78 10.62 10.51 10.24 1041 10.22 9.86 Na20 2.01 1.3 1.51 2.1 3.49 3.23 2.91 2.01 1.55 2.87 K20 0.10 0.01 0.06 0.01 0.01 0.44 0.44 0.21 0.08 0.21 p205 0.15 0.04 0.04 0.05 0.14 0.13 0.15 0.14 0.13 0.15 LOI 1.29 4.91 4.13 4.67 3.53 2.39 2.02 2.87 4.07 1.92 Total 99.01 99.14 98.6 99.89 99.43 99.37 99.21 99.3 99.03 100 Trace Elements (ppm): La 7.98 1.79 1.71 0.91 8.04 7.23 8.51 7.31 6.88 8.17 Ce 19.4 4.3 4.0 2.5 19.7 18.0 20.9 18.7 17.6 20.2 Pr 2.72 0.65 0.59 0.43 2.96 2.68 3.08 2.81 2.68 3,01 Nd 13.3 3.4 3.2 2.7 13.7 12.3 14.7 13.2 12.5 14.3 Sm 3.69 1.11 1.09 1,02 3.88 3.6 4.2 3.89 3.43 4,12 Eu 1.42 0.501 0.483 0.526 1.44 1,39 1.59 1.47 1.34 1.58 Gd 4.58 1.74 1.71 1.6 4.47 4.24 4.79 4.61 4.03 4.88 Th 0.75 0.37 0.37 0.34 0.79 0.77 0.87 0.81 0.7 0.85 Dy 4.53 2.67 2.66 2.35 4.59 4.57 5.12 4.76 4.11 5.01 Ho 0.89 0.6 0.59 0.52 0.85 0.89 0.98 0.9 0.76 0.94 Er 2.54 1.87 1.87 1,67 2.47 2.53 2.83 2.55 2.18 2.7 Tm 0.36 0.297 0.292 0.27 0.37 0.36 0.413 0.375 0.324 0.404 Yb 2.26 2.01 1,99 1.8 2.27 2.26 2.54 2.35 2 2.51 Lu 0.32 0.308 0.312 0.267 0.325 0.329 0.375 0.336 0.29 0.365 Sc 34.4 45 48.8 43.7 43.8 43.8 424 39.3 32.3 44.5 V 299 222 217 206 354 362 372 332 271 380 Cr 207 1370 1420 797 195 76 71.9 328 400 172 Co 43.1 68.7 76.1 68.6 48.9 53.6 51.5 58.9 56.8 57.4 NI 98 564 551 315 93 67 65 147 213 86 Cu 198 104 111 96 134 181 165 83 111 182 Zn 76 53 51 48 80 93 102 86 72 84 Ga 21 11 12 13 20 17 20 20 19 19 Ge 1.5 0.9 0.8 0.9 1.4 0.9 1.6 1.3 1.1 1.3 Rb 9 8 3 5 Sr 272 120 130 141 86 381 274 283 183 287 Y 25 18 17 16 26 27 29 28 23 29 Zr 92 22 24 19 90 86 98 91 86 98 Nb 8.6 0.7 0.8 0.5 8.1 8.0 9.0 8.2 7.7 8.6 Cs 0.1 0.4 0.4 0.8 0.4 0.1 0.1 0.2 0.9 Ba 32 18 20 9 11 80 85 62 32 39 Hf 2.7 0.7 0.7 0.6 2.7 2.5 2.9 2.8 2.6 3.0 Ta 0.60 0.6 0.5 0.7 0.5 0.5 0.6 Pb 26 28 29 97 97 98 94 82 97 Th 0.69 0.15 0.17 0.08 0.75 0.62 0.66 0.54 0.52 0.64 ii fl9 nil nii nn n n4 n\u00E2\u0080\u0099i n92 (127 50 Sample 5615A5 5615A6 5615A7(1) 5615A7(2)* 5815A8 5615A10 5615A11 5615A1 5616A1* 5616A2 Group CG CG PlC PlC ThOL ThOL OUTLIER PIG PlC ThOL Area KR KR KR KR KR KR KR KR KR KR Flow Flow Sill Pillow Pillow Pillow Pillow Pillow Pillow Pillow Flow UTM EW 5601095 5601095 5595569 5595569 5595513 5595376 5595029 5586126 5598448 5585731 UTM NS 624103 624103 629573 629573 629434 629069 627605 626824 616507 623077 Unnomialized Major Element Oxides (Weight %): Si02 47.76 46.53 47.16 45.73 48.31 47.29 49.08 45.35 43.7 46.72 riO2 2.125 2.119 0.466 0.442 1.745 1.807 2.304 0.643 0.439 1.939 AJ2O 14.41 14.06 11.84 11.48 13.45 13.7 11.86 14.07 13.66 13.21 Fe203* 12.5 13.01 11.54 11.22 13.48 14.23 15.2 10.4 10.47 14.11 MnO 0.191 0.177 0.172 0.166 0.196 0.195 0.215 0.142 0.158 0.235 MgO 5.71 5.7 18.59 18.19 6.78 6.77 5.73 12.56 15.55 6.29 CaO 8.81 10 9.45 9.14 11.77 10.65 9.84 9 8.71 11.19 Na20 4.16 3.64 0.42 0.41 1.8 2.58 3.25 2.11 0.98 2.49 K2O 0.2 0.09 0.01 0.16 0.03 0.21 0.04 0.02 0.04 0.01 P205 0.18 0.19 0.03 0.05 0.14 0.14 0.22 0.06 0.04 0.16 LOI 3.17 345 2.86 1.74 1.57 2.07 4.68 6.05 3.38 Total 99.2 98.96 99.67 99.84 99.43 99.14 99.81 99.02 99.8 99.66 Trace Elements (ppm): La 11.6 11.7 1.09 1.02 7.32 7.34 6.41 1.73 1.52 8.87 Ce 28.4 26.9 2.7 2.6 18.4 18.5 17.3 4.7 3.6 21.0 Pr 4.01 3.95 0.44 0.41 2.69 2.84 2.89 0.77 0.55 3.02 Nd 17.7 17.7 2.5 2.4 12.9 13.2 15.6 4.4 3.0 14.4 Sm 4.74 4.76 0.88 0.85 3.78 3.92 5.06 1.47 1.02 4.16 Eu 1.78 1.73 0.381 0.361 1.47 1.48 1.88 0.607 0.454 1.59 Gd 5.49 5.32 1.42 1.35 4.4 4.65 6.19 2.13 1.57 4.81 Th 0.93 0.93 0.3 0.3 0.79 0.81 1.13 0.42 0.34 0.86 Dy 5.49 5.31 2.16 2.12 4.65 4.79 6.65 2.78 2.44 5.07 Ho 1.02 0.98 0.49 0.46 0.89 0.91 124 0.59 0.55 0.92 Er 2.91 2.86 1.55 1.46 2.58 2.58 3.67 1,78 1.73 2.72 Tm 0.441 0.43 0.253 0.236 0.379 0.38 0.547 0.267 0268 0.413 Yb 2.75 2.73 1.71 1.58 2.34 2.42 3.44 1.75 1.77 2.62 Lu 0.392 0.391 0247 0.245 0.339 0.348 0.493 0.269 0.277 0.378 Sc 35.7 38.5 39.2 39.5 40.4 39.8 43.8 38.3 37.5 41.6 V 381 376 224 222 353 363 520 215 204 401 Cr 127 164 1910 1850 173 166 107 906 3000 76 Co 39.4 43.7 87.1 89.3 53 52,4 53.4 67.2 67.9 50.8 Ni 73 85 729 680 96 94 56 368 559 58 Cu 105 148 86 80 175 185 208 83 77 210 Zn 86 102 57 54 88 86 94 50 53 85 Ga 21 21 10 10 20 20 18 14 12 22 Ge 1.4 1.4 1.1 0.9 1.4 1.3 0.7 1.1 1.1 1.6 Rb 2 6 6 2 9 2 Sr 128 170 124 120 194 279 143 189 112 67 Y 32 31 15 15 28 28 39 16 16 30 Zr 116 121 16 15 91 96 124 35 22 100 Nb 12.7 13.0 0.9 0.7 8.0 8.5 9.9 1.4 0.9 9.2 Cs 0.2 4.4 4.5 0.6 1.3 0.1 0.6 0.8 Ba 36 34 27 24 39 61 20 15 20 8 Hf 3.4 3.5 0.6 0.5 2.7 2.8 3.8 1.1 0.7 2.9 Ta 0.9 0.9 0.6 0.6 0.7 0.6 Pb 110 118 24 22 91 92 113 34 22 96 Th 0.92 0.93 0.1 0.08 0.59 0.61 1.01 0.09 0.14 0.63 U 0.34 0.33 0.07 0.07 - 0,22 0.24 0.41 - 0.07 0.1 0.25 51 Sample 5616A3 5616A7 5617A1 5617A4 5617A5(1)* 5617A5(2)* 5618A1 5618A3 5618A4 93G171 Group CG HI-MG CG MINSIL CG CG THOL THOL THOL PlC Area KR KR SL SL SL SL 01 QI QI KR Flow Flow Pillow Sill Sill Sill Sill Flow Pillow Breccia Pillow UTM EW 5584647 5589833 5560375 5557712 5557712 5557712 5557892 5552258 5552258 5599395 UTMNS 623236 626879 702240 700905 700905 700905 338923 341690 341690 616613 Unnormalized Major Element Oxides (Weight %): Si02 46.77 46.75 49.1 48.06 49.31 47.4 48.61 47.9 40.29 47.93 1102 1.814 0.732 0.901 3.505 1.713 1.71 1.749 1.362 2.159 0.461 Al203 14.23 15.92 13.72 12.79 13.39 13.33 12.82 13.79 16.27 14.02 Fe203* 12.85 10.88 11.34 16.78 12.39 12.71 13.81 11.62 18.1 9.9 MnO 0.185 0.153 0.16 0.164 0.174 0.174 0.211 0.158 0.28 0.19 MgO 7.28 8.7 6.99 5.25 7.5 7.46 6.96 6.57 9.7 15.8 CeO 10.92 11.69 10.96 648 12.01 11.93 10.54 12.08 6.78 10.42 Na20 1.86 2.12 3.46 2.31 1.83 1.8 2.76 3.13 0.58 1.02 K20 0.16 0.07 0.03 0.69 0.22 0.17 0.5 0.12 0.52 0.11 P205 0.13 0.04 0.08 0.35 0.13 0.14 0.14 0.1 0.18 0.08 LOl 2.92 2.86 3.23 3.22 1.28 1.38 1.8 3.17 4.77 Total 99.14 99.91 99.97 99.6 99.95 98.21 99.9 99.99 99.63 100.00 Trace Elements (ppm): La 7.14 1.51 2.84 22.3 8.13 7.94 7.72 6.57 10.7 1.4 Ce 17.9 3.8 6.9 51.5 19.7 19.2 19.1 15.4 24.3 3.6 Pr 2.72 0.59 1.1 7.19 2.89 2.85 2.85 2.22 3.48 0.56 Nd 12.9 3.5 6.1 32.7 13.3 13.3 13.5 10.5 15.9 2.9 Sm 3.82 1.36 2.14 8.86 3.83 3.77 3.98 2.98 4.53 1.23 Eu 1.47 0.638 0.888 3.33 1.45 1.43 1.47 1.21 2.04 0.47 Gd 4.49 2.05 3.05 9.84 4.36 4.32 4.56 3.56 5.64 2.10 Tb 0.79 0.43 0.63 1.64 0.75 0.75 0.79 0.63 0.99 0.32 Dy 4.66 2.9 4.19 9.58 4.35 4.25 4.71 3.84 5.95 2.38 Ho 0.9 0.61 0.86 1.78 0.83 0.81 0.88 0.75 1.15 0.64 Er 2.6 1.91 2.65 5.06 2.37 2.31 2.56 2.16 3.36 2.11 Tm 0.373 0.305 0.415 0.73 0.341 0.342 0.375 0.331 0.495 0.297 Yb 2.35 2.02 2.66 4.56 2.13 2.14 2.35 2.12 3.11 1.97 Lu 0.34 0.302 0.401 0.637 0.304 0.298 0.342 0296 0.455 0.282 Sc 40 50.7 54.5 44.2 40.8 37.7 42.9 41.8 54.2 V 349 270 327 517 342 338 367 313 499 140 Cr 317 346 210 64.4 274 255 114 194 159 606 Co 52.6 56.7 46.8 38.3 51.8 47.6 53 46 62 Ni 110 122 70 39 98 97 79 67 68 491 Cu 164 111 160 232 161 160 25 125 109 91 Zn 87 51 69 160 77 76 89 92 75 57 Ga 20 15 16 27 19 19 19 18 24 Ge 1.3 1 1.3 2 1.1 0.7 1.5 1.5 1.3 Rb 3 21 7 7 8 3 10 Sr 237 181 58 197 255 254 227 170 220 Y 25 19 27 52 24 23 27 21 38 13 Zr 91 26 45 229 89 86 89 67 112 24 Nb 8.1 1.1 2.1 19.7 8.9 8.5 8.1 6.6 9.8 0.9 Ca 1.2 0.3 0.2 1.2 0.2 0.2 0.4 0.6 Ba 39 23 12 771 38 37 103 17 109 20 Hf 2.7 0.8 1.4 6.4 2.6 2.5 2.7 2 3.3 0.8 Ta 0.6 0.1 1.4 0.6 0.6 0.6 0.5 0.7 0.1 Pb 93 37 48 151 85 83 83 66 65 Th 0.52 0.14 0.28 2.09 0.61 0.57 0.57 0.45 0.71 0.15 U 0.23 0.09 0.16 0.88 0.23 0.25 0.22 0.22 0.34 52 (rooiuopspux)qjudniIUo!IApiojo&iu.iiuaijjippu\u00E2\u0080\u0098(q>jpu;ossojAjjids)\u00E2\u0080\u0098-jj\u00E2\u0080\u0099-j ijuouoi.ijosoojjp\u00E2\u0080\u0098suoidpuiiioiipQJ1u-33fluoquo3uSip ijo\u00E2\u0080\u00A2(c661)mispunouoaowoijStLJUAUO!z!1UUUOUliv\u00E2\u0080\u00A2SUiUpjTJUTUW 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0L30L qWSMoJJUflOfiJP!WSMOIJVUflOjAJ till11111111111111 ,0L \u00E2\u0080\u0098I1111111 \u00E2\u0080\u009400L fl1cVJ3AoHXaqIpDn31;H,zwsJspNJda)e1eLqN)1flqLeaqus)flCIJWI13OH1(53(Ii.p9fl3W5WdPN143)3 I\u00E2\u0080\u00A2p\u00E2\u0080\u00A2ipi\u00E2\u0080\u00A2\u00E2\u0080\u00A2\u00E2\u0080\u00A2IIIIII I I(D) If, 3 3 __________________________________ liii 01(\u00E2\u0080\u00A2t 001 \u00E2\u0080\u00A2ee,uq)S liii111111 ____________________________________________ OOL fl53),13AoHXoqIp9flJ1.JHJZU.JSiSPNd3)l1qN)IflqjqSDfl(CI),Wj13OH1(53(Ijp9fl3WWdPN143)s, \u00E2\u0080\u00A2,\u00E2\u0080\u00A2\u00E2\u0080\u00A2,\u00E2\u0080\u00A2\u00E2\u0080\u00A2,1111111liii111110\u00E2\u0080\u00A2p\u00E2\u0080\u00A2p\u00E2\u0080\u00A2i\u00E2\u0080\u00A2\u00E2\u0080\u00A2\u00E2\u0080\u00A2\u00E2\u0080\u00A2\u00E2\u0080\u00A2\u00E2\u0080\u00A2\u00E2\u0080\u00A2\u00E2\u0080\u00A2 (oo\u00E2\u0080\u00993psirSSSIeS) e6eJ3Aeoowpesidea \u00E2\u0080\u00A2 001 usnw 111111111I ______________ ___________________________________________________________ IIIIIIIIIIIII 001 sediment-sill complex at Schoen Lake, are distinguished by higher Ti02 and FeOT than the main group of tholeiitic basalts. The tholeiitic basalts are similar in major-element composition to previously published results for samples from the Karmutsen Formation; however, the Keogh Lake picrites, which encompass the picritic and high-MgO basalt pillow lavas, extend to \u00E2\u0080\u009420 wt % MgO (Fig. 2.6, and references therein). The tholeiitic basalts from the Karmutsen Range form a tight range of parallel light rare earth element (LREE)-enriched patterns (La/YbcN=2.1-2.4; mean 2.2 \u00C2\u00B1 0.2), whereas the picrites and high-MgO basalts from the Karmutsen Range form a range of parallel LREE-depleted patterns (LaJYbcN=0.3-0.7; mean 0.6 \u00C2\u00B1 0.2) with lower REE abundances than the tholeiitic basalts (Fig. 2.7). Tholeiitic basalts from the Schoen Lake and Mount Arrowsmith areas have similar REE patterns to samples from the Karmutsen Range (Schoen Lake- LaJYbCN=2.l-2.6; mean 2.3 \u00C2\u00B1 0.3; Mount Arrowsmith LaJYbCN=1.9-2.5; mean 2.2 \u00C2\u00B1 0.4), and the coarse-grained mafic rocks have similar REE patterns to the tholeiitic basalts (LaJYbCN=2.l-2.9; mean 2.5 \u00C2\u00B1 0.8). Two LREE-depleted coarse-grained mafic rocks from the Schoen Lake area have similar REE patterns (LaJYbCN=0.7) to the picrites from the Keogh Lake area, indicating that this distinctive suite of rocks is exposed as far south as Schoen Lake (Fig. 2.7). The anomalous pillowed flow from the Karmutsen Range has lower La/YbCN (1.3-1.8) than the main group of tholeiitic basalts from the Karmutsen Range (Fig. 2.7). The mineralized sill from the Schoen Lake area is distinctly LREE-enriched (La/YbCN=3.3) with the highest REE abundances (La=94.0) of all Karmutsen samples, which may reflect contamination by adjacent sediment. The primitive mantle-normalized trace-element patterns (Fig. 2.7), and trace- element variations and ratios (Fig. 2.8), highlight the differences in trace-element concentrations between the four main groups of Karmutsen flood basalts on Vancouver Island. The tholeiitic basalts from all three areas have relatively smooth, parallel trace element patterns; some samples have small positive Sr anomalies, relative to Nd and Sm, and many samples have prominent negative K anomalies relative to U and Nb, reflecting K-loss during alteration (Fig. 2.7). The large ion lithophile elements (LILE) in the tholeiitic basalts (mainly Rb and K, not Cs) are depleted relative to HFSE and LREE, and LILE segments, especially for the Schoen Lake area (Fig. 2.7d), are remarkably parallel. 54 15 \u00E2\u0080\u00A2 \u00E2\u0080\u00A2 . . , 1.6 Nb(ppm) Nb/La 1.2- 8o 10 . - 0.8 - oo \u00E2\u0080\u0098 5 Picrite \u00E2\u0080\u00A2 High-MgO basalt 04 - 0 Coarse-grained \u00E2\u0080\u00A2 Q 0 Tholeiltic basalt (a) X Pillowed flow (b) 0 I I I I I I I I I I 00 0 50 100 150 0 5 10 15 20 25 Zr (ppm) MgO (wt%) 1.25 ....i....i.,..i....i,.,. 1.2 i i i i i i i Th (ppm) Th (ppm) >X :: 0.25 0.2 (d) (C) 561 5A1 2 _________________ 0.00 I I \u00E2\u0080\u00A2 I \u00E2\u0080\u00A2 I \u00E2\u0080\u00A2 0.0 I I I I I I I I I 0 1 2 3 4 5 0.0 0.1 0.2 0.3 0.4 0.5 Hf(ppm) U(ppm) Figure 2.8 Whole-rock trace-element concentrations and ratios for the Karmutsen Formation (except panel b is versus MgO). (a) Nb vs. Zr. (b) Nb/La vs. MgO. (c) Th vs. Hf. (d) Th vs. U. There is a clear distinction between the tholeiitic basalts and picrites in Nb and Zr, both in concentration and the slope of each trend. 55 The picrites and high-MgO basalts form a tight band of parallel trace-element patterns and are depleted in HFSE and LREE with positive Sr anomalies and relatively low Rb. The coarse-grained mafic rocks from the Karmutsen Range have identical trace-element patterns to the tholeiitic basalts. Samples from the Schoen Lake area have similar trace- element patterns to the Karmutsen Range, except for the two LREE-depleted coarse grained mafic rocks and the mineralized sill (Fig. 2.7d). Sr-Nd-Hf-Pb isotopic compositions The nineteen samples of the Karmutsen Formation from the four main groups, selected on the basis of major and trace-element variation, stratigraphic position, and location to cover the range of these parameters, have indistinguishable age-corrected Hf and Nd isotope ratios and distinct ranges of Sr isotopic compositions (Fig. 2.9). The tholeiitic basalts, picrites, high-MgO basalt, and coarse-grained mafic rocks have initial 8Hf\u00E2\u0080\u0094 +8.7 to +12.6 and 5Nd= +6.6 to +8.8 corrected for in situ radioactive decay since 230 Ma (Fig. 2.9; Tables 2.3 and 2.4). All the Karmutsen samples form a well-defined linear array in a Lu-Hf isochron diagram corresponding to an age of 241 \u00C2\u00B1 15 Ma (Fig. 2.9). This is within error of the accepted age of the Karmutsen Formation and indicates that the Hf isotope systematies have behaved as a closed system since ca. 230 Ma. The anomalous pillowed flow has similar initial Epj (+9.8) and slightly lower initial ENd (+6.2) than the other Karmutsen samples. The picrites and high-MgO basalt have higher initial 87Sr/6r (0.70398-0.705 18) than the tholeiitic basalts (initial 87Sr/6r=0.70306-0.70381) and the coarse-grained mafic rocks (initial 87Sr/6r=0.70265-0.70428) overlap the ranges of the picrites and tholeiitic basalts (Fig. 2.9; Table 2.3). The measured Pb isotopic compositions of the tholeiitic basalts are more radiogenic than those of the picrites and the most magnesian Keogh Lake picrites have the least radiogenic Pb isotopic compositions. The range of initial Pb isotope ratios for the picrites is 206Pb/4 = 18.142-18.580, 207Pb/4 = 15.547-15.562, and 208Pb/4 = 37.873-38.257 and the range for the tholeiitic basalts is 206Pb/4= 18.782-19.098, 207Pb/4 = 15.570-15.584, and 208Pb/4 = 37.312-38.587 (Fig. 2.10; Table 2.5). The coarse-grained mafic rocks overlap the range of initial Pb isotopic compositions for the tholeiitic basalts with 206Pb/4= 18.652-19.155, 207Pb/4 = 15.568-15.588, and 56 0.5132 - - 0.2834 0.2833 0.5131 0.2832 0.5130 0.2831 0.5129 - 0.2830 0.5128 \u00E2\u0080\u0098 0.2829 0.15 0.17 0.19 0.21 0.23 0.25 0.00 0.02 0.04 0.06 0.08 0.10 1475m/4Nd 176Lu/7Hf 10 \u00E2\u0080\u0098 \u00E2\u0080\u00A2 i \u00E2\u0080\u00A2 14 (230 Ma) Nd 12 8 0 - 0 6 10 6 X4 8 (c) i 0.702 0.703 0.704 0.705 0.7064 \u00E2\u0080\u00A2 6 0.702 0.703 0.704 0.705 0.706 5 6 7 8 9 10 87Sr/ 86Sr (230 Ma) Nd Ma) Figure 2.9 Whole-rock Sr, Nd, and Hf isotopic compositions for the Karmutsen Formation. (a)\u00E2\u0080\u009843NW1dvs. \u00E2\u0080\u009847Sm/\u00E2\u0080\u0099Nd. (b) 176Hf/7fvs. 176LuJ\u00E2\u0080\u009977Hf. The slope of the best-fit line for all samples corresponds to an age of 241 \u00C2\u00B1 15 Ma. (c) Initial ENd VS. 87Sr/6. Age correction to 230 Ma. (d) LOT vs. 87Sr/6. (e) Initial CHf VS. 6Nd Average 2a error bars are shown in a corner of each panel. Complete chemical duplicates, shown in Tables 2.3 and 2.4 (samples 4720A7 and 4722A4), are circled in each plot. 11\u00E2\u0080\u00A21J\u00E2\u0080\u00A2 143Nd/ a A Picrite High-MgO basalt X 0 Coarse-grained \u00E2\u0080\u00A2 thole\u00C3\u00BCtic basalt X Pillowed flow (a) i L_ I I I I 176Hf/7f - Age=241 \u00C2\u00B1 l5Ma - (0=+9.90\u00C2\u00B1 0 \u00E2\u0080\u0094I I I I i LOl (wt 96) t AA 4723A2 dD (d) \u00C3\u00AA \u00E2\u0080\u009CSri Sr \u00E2\u0080\u0098 I \u00E2\u0080\u0098 I I I \u00E2\u0080\u0098 E (230Ma) 0 Hf 0 \u00E2\u0080\u00A2 x -(e) I I i I i I I I i 57 15.68 15.60 15.66 15.58 - 15.64 15.56 - 15.62 15.54 - 15.60 15.52 - 15,58 4723A4 15.56 1530 18.6 19.0 194 19.8 202 20.6 21.0 17.8 18.0 18.2 18.4 18.6 18.8 19.0 19.2 19.4 206pb/4P 206Pb/4 (230 Ma) 39.6 \u00E2\u0080\u00A2 \u00E2\u0080\u0098 i \u00E2\u0080\u0098 i \u00E2\u0080\u00A2 i \u00E2\u0080\u0098 i 39.0 111111111111111 4723A2 - (230 Ma) 39.2 - 38.6 \u00E2\u0080\u0094 _____________ X 6 38.8 - 5 38.2 384 2 37.8 4723 - (c) c186 19.0 19A 19.8 20.2 20.6 21.0 (e) 38.0 \u00E2\u0080\u0098 \u00E2\u0080\u0098 I I I I I I I I I ililililili liii 18.6 19.0 194 19.8 20.2 20.6 21.0 17.8 18.0 18.2 18.4 18.6 18.8 19.0 19.2 19.4 206pb/4 (230 Ma) Figure 2.10 Pb isotopic compositions of leached whole-rock samples measured by MC-TCP-MS for the Karmutsen Formation. Error bars are smaller than symbols. (a) Measured 207Pb/4 vs. 206Pb/\u00C2\u00B04. (b) Initial 207Pb/4 vs. 206PbP\u00C2\u00B04b. Age correction to 230 Ma. (c) Measured 208Pb/4 vs. 206PbP\u00C2\u00B0b. (d) LOT vs. 206Pb/4. (e) Initial 208Pb/4 vs. 206Pb/4. Complete chemical duplicates, shown in Table 2.5 (samples 4720A7 and 4722A4), are circled in each plot. The dashed lines in panels b and e show the differences in age- corrections for two picrites (4723A4, 4722A4) using the measured U, Th, and Pb concentrations for each sample (black triangles) and the age-corrections when concentrations are used from the two picrites (4723A3, 4723A13) that appear to be least affected by alteration. 58 I I I I I I I I I I I 4723A2 (a) I I I I I I I I I I I \u00E2\u0080\u00A2 I \u00E2\u0080\u00A2 I \u00E2\u0080\u00A2 I\u00E2\u0080\u00A2I\u00E2\u0080\u00A21\u00E2\u0080\u00A2I\u00E2\u0080\u00A2I\u00E2\u0080\u00A2 4723A2 \u00E2\u0080\u00A2 207pb/4 0(230 Ma) \u00E2\u0080\u00A2 - \u00E2\u0080\u0094 4723A13 \u00E2\u0080\u0094 \u00E2\u0080\u0094 _\u00E2\u0080\u0098><\u00E2\u0080\u00A2s4723A3 4722A4 \u00E2\u0080\u0098 Picrite <>High-MgO basalt 0 Coarse-grained Tholeiitic basalt (b) Pillowed flow LOl (wt %) A 4723A2 0 0 0 (d) \u00E2\u0080\u00A2 20Pb\u00E2\u0080\u009904 0 0 Ta bl e 2. 3 Sr a n dN d iso to pi cg eo ch em is tiy o fK ar m ut se n ba sa lts , Va nc ou ve rI sla nd ,B .C . 47 22 A 5 OU TL IE R KR 4. 46 19 1 0. 70 38 77 7 0. 06 78 0. 70 36 6 6. 10 21 .0 0. 51 29 22 6 5.5 0. 17 56 0. 51 26 6 6. 2 47 23 A 2 HI -M G KR 8. 23 26 9 0. 70 54 66 7 0. 08 85 0. 70 51 8 1. 99 5. 79 0. 51 30 59 8 8. 2 0. 20 81 0. 51 27 5 7. 9 Sa m pl e 47 18 A2 47 18 A7 47 19 A2 47 19 A 3 47 20 A 4 47 21 A 2 47 21 A 4 G ro up s Ar ea b Rb Sr 8 7 S rI S r 2am 8 7 R b /S r 8 1 5 r /\u00C2\u00B0 6S r 1 Sm Nd 1 4 3 N W d 2am 6N d 1 4 7 S m / N d 1 4 3 N d / d (pp m) (pp m) 23 0 M a (pp m) (pp m) 23 0 M a TH OL TH OL TH OL TH OL TH OL TH OL TH OI M A 1. 28 M A 3. 78 M A 3. 27 M A 3. 49 SL 2. 54 SL 1. 58 SL 1. 65 20 2 0. 70 32 47 6 28 3 0. 70 37 02 8 24 4 0. 70 38 11 9 22 9 0. 70 37 57 6 21 5 0. 70 30 66 8 20 9 0. 70 30 76 7 17 2 0. 70 30 62 7 0. 01 83 0. 03 87 0. 03 87 0. 04 41 0. 03 41 0. 02 20 0. 02 77 CN d(t ) 0. 70 31 9 4. 60 16 .8 0. 51 30 04 6 0. 70 35 8 4. 79 16 .6 0.5 12 99 8 6 0. 70 36 8 4. 48 16 .1 0. 51 29 72 6 0. 70 36 1 4. 22 14 .8 0. 51 29 72 6 0. 70 29 5 5. 77 21 .0 0. 51 29 84 7 0. 70 30 0 4. 05 14 .6 0. 51 29 65 7 0. 70 29 7 5. 48 19 .6 0. 51 29 33 6 7.1 0. 16 56 7. 0 0. 17 42 6. 5 0. 16 88 6. 5 0. 17 18 6. 7 0. 16 62 6. 4 0. 16 75 5.8 0. 16 91 0. 51 27 5 0. 51 27 4 0. 51 27 2 0. 51 27 1 0. 51 27 3 0. 51 27 1 0. 51 26 8 8.1 7. 7 7. 3 7. 2 7. 6 7. 2 6. 6 47 20 A 6 CG 5L 6. 11 12 3 0. 70 31 21 9 0. 14 38 0. 70 26 5 4. 75 16 .8 0. 51 29 76 7 6. 6 0. 17 11 0. 51 27 2 7. 3 47 20 A 7 CG SL 2. 18 15 7 0. 70 30 78 6 0. 04 01 0. 70 29 5 4. 96 17 .3 0. 51 30 12 6 7. 3 0. 17 35 0. 51 27 5 8. 0 47 20 A 7(d up ) CG SL 1. 79 14 0 0. 70 30 67 9 0. 03 69 0. 70 29 5 4. 56 16 .0 0. 51 29 94 6 6. 9 0. 17 22 0. 51 27 3 7. 7 47 20 A 10 CG SL 0. 11 60 0. 70 42 94 7 0. 00 55 0. 70 42 8 2. 05 5. 7 0. 51 31 20 7 9. 4 0. 21 91 0. 51 27 9 8. 7 47 24 A 3 CG SL 1. 44 13 5 0. 70 34 21 8 0. 03 10 0. 70 33 2 3. 84 13 .2 0. 51 29 61 6 6. 3 0. 17 57 0. 51 27 0 6. 9 56 16 A3 CG KR 3. 00 23 7 0. 70 38 49 7 0. 03 66 0. 70 37 3 3. 82 12 .9 0. 51 29 83 6 6. 7 0. 17 90 0. 51 27 1 7. 2 56 17 A1 CG SL 2. 00 58 0. 70 42 41 7 0. 09 98 0. 70 39 1 2. 14 6. 06 0. 51 30 97 7 9. 0 0. 21 35 0. 51 27 8 8. 5 47 22 A 4 Pl C KR 3. 16 77 0. 70 52 20 8 0. 11 84 0. 70 48 3 1. 07 2. 70 0. 51 31 15 6 9. 3 0. 23 84 0. 51 27 6 8.1 47 22 A 4(d up ) Pl C KR 3. 16 77 0. 70 52 14 7 0.1 18 4 0. 70 48 3 0. 97 2. 48 0. 51 31 22 6 9. 4 0. 23 61 0. 51 27 7 8. 3 47 23 A 3 Pl C KR 1. 65 69 0. 70 42 09 9 0. 06 96 0. 70 39 8 2. 09 5.9 1 0. 51 30 98 7 9. 0 0. 21 42 0. 51 27 8 8. 5 47 23 A 4 Pl C KR 1. 43 11 0 0. 70 43 52 8 0. 03 75 0. 70 42 3 2. 14 5. 90 0. 51 30 60 10 8. 2 0. 21 92 0. 51 27 3 7. 6 47 23 A 13 Pl C KR 0. 67 63 0. 70 41 32 8 0. 03 12 0. 70 40 3 1. 33 3. 54 0. 51 31 18 7 9. 4 0. 22 68 0. 51 27 8 8. 5 \u00C2\u00B0 TH OL ,t ho len tic ba sa lt, CG ,c o ar se -g ra in ed rn af ic ro ck ,P lC ,p icr ite , H I-M G, hi gh -M gO ba sa lt; OU TL IE R, an o m al ou s pi llo we d flo w in pl ot s. b A bb re vi ati on sf or a re a ar e: KR , K ar m ut se n R an ge ,S L, Sc ho en La ke ,M A, M ou nt Ar ro ws m ith .( du p) in di ca te s co m pl et e ch em ist ly du pl ica te. Al l i so to pi ca n d el em en ta la n al ys es ca rr ie d o u ta tt he PC IG R th e co m pl et e tr ac e el em en ta n al ys es a re sh ow n in A pp en di x C. Table 2.4 Hfisotopic compositions ofKarmutsen basalts, Vancouver Island, B.C. Sample Groupa Areab Lu Hf 177Hff\u00E2\u0080\u009976 2m 45 176Lu/Hf 177Hf/6f 45(t) (ppm) (ppm) 230 Ma 4718A2 THOL MA 0.38 2.53 0.283007 4 8.3 0.0211 0.28291 10.2 4718A7 THOL MA 0.46 2.41 0.283001 7 8.1 0.0271 0.28288 9.1 4719A2 THOL MA 0.40 2.16 0.283079 5 10.9 0.0262 0.28296 12.0 4719A3 THOL MA 0.37 2.12 0.283024 9 8.9 0.0246 0.28291 10.2 4720A4 THOL SL 0.53 3.04 0.283002 7 8.1 0.0247 0.28289 9.5 4721A2 THOL SL 0.36 2.99 0.283004 10 8.2 0.0173 0.28293 10.7 4721A4 THOL SL 0.53 2.79 0.283006 6 8.3 0.0268 0.28289 9.3 4722A5 OUTLIER KR 0.56 3.15 0.283015 6 8.6 0.0254 0.28290 9.8 4720A6 CG SL 0.45 2.38 0.283012 5 8.5 0.0269 0.28289 9.5 4720A7 CG SL 0.46 2.23 0.283012 6 8.5 0.0291 0.28288 9.1 4720A7(dup) CG SL 0.43 2.35 0.283012 4 8.5 0.0260 0.28290 9.6 4720A10 CG SL 0.36 1.00 0.283179 7 14.4 0.0515 0.28295 11.5 4724A3 CG SL 0.34 1.41 0.283035 8 9.3 0.0339 0.28288 9.2 5616A3 CG KR 0.34 2.70 0.283024 7 8.9 0.0179 0.28294 11.3 5617A1 CG SL 0.40 140 0.283161 4 13.8 0.0407 0.28298 12.6 4722A4 PlC KR 0.29 0.48 0283321 10 194 0.0857 0.28294 11.2 4722A4(dup) PlC KR 0.29 0.45 0.283350 11 20.4 0.0915 0.28294 11.3 4723A3 PIG KR 0.45 0.98 0.283197 6 15.0 0.0649 0.28291 10.1 4723A4 PIG KR 0.35 0.90 0.283121 26 12.3 0.0561 0.28287 8.7 4723A13 PlC KR 0.35 0.63 0.283250 7 16.9 0.0791 0.28290 9.7 4723A2 HI-MG KR 0.43 1.06 0.283186 7 14.7 0.0579 0.28293 10.8 a THOL, tholeiitic basalt, CG, coarse-grained mafic rock, PlC, picrite, HI-MG, high-MgO basalt; OUTLIER, anomalous pillowed flow in plots. b Abbreviations for area are: KR, Karmutsen Range, SL, Schoen Lake, MA, Mount Arrowsmith. (dup) indicates complete chemistry duplicate. All isotopic and elemental analyses carried out at the PCIGR; the complete trace element analyses are shown in Appendix C. 60 Ta bl e 2. 5 Pb iso to pi c co m po sit io ns o fK ar m ut se n ba sa lts , V an co uv er Is la nd ,B .C . Sa m pl e Gr ou ps ke a\u00E2\u0080\u0099 U Th Pb 2\u00C2\u00B0 P b fl \u00C2\u00B0 4P b 2m 2\u00C2\u00B07P b / 0 4b 2m 2w P b / o4P b 2m 2U / \u00C2\u00B0 P b 5U / 2M P b 2 3 2 T h l 0 4 P b 2 0 6 P b /4b 2 0 7 P b /4b 2 0 8 P b l4b (pp m) (pp m) (pp m) 23 0 M a 23 0 M a 23 0 M a 47 18 A 2 TH OL M A 0. 18 0. 56 0.7 1 19 .5 36 9 0. 00 21 15 .6 10 5 0. 00 17 39 .0 68 4 0. 00 35 16 .0 0. 11 6 53 .2 18 .9 57 15 .58 1 38 .4 55 47 18 A 7 TH OL M A 0. 17 0. 52 0. 54 19 .5 13 3 0. 00 16 15 .6 07 0 0. 00 15 39 .0 69 7 0. 00 42 20 .1 0. 14 6 65 .7 18 .7 82 15 .5 70 38 .3 12 47 19 A 2 TH OL M A 0. 18 0. 47 0. 68 19 .5 78 0 0. 00 15 15 .6 13 0 0. 00 12 39 .0 35 9 0. 00 32 17 .6 0. 12 7 46 .4 18 .9 40 15 .58 1 38 .5 01 47 19 A 3 TH OL . M A 0. 14 0. 45 0. 52 19 .6 61 4 0. 00 17 15 .6 06 6 0. 00 15 39 20 27 0. 00 42 18 .0 0. 13 1 58 .8 19 .0 07 15 .5 73 38 .5 24 47 20 A 4 TH OL SL 0.2 1 0. 72 0. 65 19 .8 16 9 0. 00 12 15 .6 21 8 0. 00 10 39 .3 99 7 0. 00 26 21 .0 0. 15 3 75 .7 19 .0 53 15 .5 83 38 .5 27 47 21 A 2 Th O L SL 0. 25 0. 72 0.9 1 19 .4 97 2 0. 00 19 15 .6 08 4 0. 00 15 39 .0 66 1 0. 00 39 17 .6 0. 12 7 53 .1 18 .8 59 15 .5 76 38 .4 54 47 21 A 4 Th O L SL 0. 17 0.6 1 0. 54 19 .8 55 2 0. 00 09 15 .6 22 9 0. 00 08 39 .4 70 9 0. 00 20 20 .9 0. 15 1 76 .7 19 .0 98 15 .5 84 38 .5 87 47 22 A 5 OU TL IE R KR 0. 33 0. 82 0.8 1 19 .6 27 2 0. 00 16 15 .6 16 4 0. 00 12 39 .2 32 2 0. 00 34 26 .3 0.1 91 68 .0 18 .6 73 15 .5 68 38 .4 48 47 20 A 6 GG SL 0. 13 0. 45 0. 56 19 .6 20 9 0. 00 17 15 .6 12 4 0. 00 13 39 .2 30 7 0. 00 39 15 .4 0. 11 2 54 .4 19 .0 62 15 .5 84 38 .6 03 47 20 A 7 CG SL 0. 13 0. 48 0.6 1 19 .6 64 0 0. 00 09 15 .6 13 8 0. 00 06 39 .2 54 7 0. 00 19 14 .2 0. 10 3 53 .5 19 .1 49 15 .5 88 38 .6 38 47 20 A 7(d up ) CG SL 0. 14 0. 44 0.6 1 19 .6 95 1 0. 00 06 15 .6 15 7 0. 00 06 39 .2 97 5 0. 00 22 14 .9 0. 10 8 48 .8 19 .1 54 15 .5 88 38 .7 35 47 20 M 0 CG SL 0. 05 0.1 1 0.5 1 18 .8 74 5 0. 00 18 15 .5 78 8 0. 00 16 38 .3 17 6 0. 00 42 6.1 0. 04 4 14 .3 18 .6 52 15 .5 68 38 .1 53 47 24 A 3 CG SL 0. 09 0. 30 0. 60 19 .2 52 0 0. 00 14 15 .5 88 0 0. 00 13 38 .9 07 2 0. 00 32 9. 7 0. 07 0 33 .6 18 .9 00 15 .5 70 38 .5 19 56 16 A3 CG KR 19 .4 84 5 0. 00 07 15 .5 98 6 0. 00 06 39 .0 67 9 0. 00 15 56 17 A1 CG SL 19 .1 20 2 0. 00 07 15 .5 91 0 0. 00 06 38 .4 40 1 0. 00 16 47 22 A 4 PI G KR 0. 05 0. 07 0. 17 18 .9 51 4 0. 00 17 15 .5 85 2 0. 00 17 38 .4 06 3 0. 00 41 20 .8 0.1 51 28 .8 18 .1 97 15 .5 47 38 .0 74 47 22 A 4(d up ) Pl C KR 0. 05 0. 03 0. 16 18 .9 53 9 0. 00 13 15 .5 89 0 0. 00 12 38 .4 18 6 0. 00 29 22 .4 0. 16 2 14 .1 18 .1 42 15 .5 48 38 .2 57 47 23 A 3 PI G KR 0. 06 0. 19 0. 32 18 .9 87 3 0. 00 12 15 .5 82 0 0. 00 11 38 .4 43 4 0. 00 27 11 .4 0. 08 3 38 .8 18 .5 73 15 .5 61 37 .9 96 47 23 A 4 Pl C KR 0. 05 0. 09 19 .1 73 3 0. 00 13 15 .5 83 2 0. 00 12 38 49 84 0. 00 33 35 .9 0. 26 1 55 .5 17 .8 68 15 .5 17 37 .8 58 47 23 A 13 Pl C KR 0. 03 0.1 1 0. 18 18 .9 13 1 0. 00 11 15 .5 78 4 0. 00 11 38 .3 51 8 0. 00 30 9. 2 0. 06 7 41 .5 18 .5 80 15 .56 1 37 .8 73 47 23 A 2 HI -M G KR 0. 06 0. 19 0. 09 20 .7 32 1 0. 00 37 15 .6 65 6 0. 00 30 39 .2 29 5 0. 00 73 41 .0 0. 29 8 13 4. 7 19 .2 42 15 .5 90 37 .6 76 \u00C2\u00B0 TH OL ,t ho lel iti c ba sa lt, CG ,c o ar se -g ra in ed m af ic ro ck ,P lC ,p icr ite ,H I-M G, hi gh -M gO ba sa lt; OU TL IE R, an o m al ou s pi llo we d flo w in pl ot s. b A bb re vi ati on s f or ar ea ar e: KR ,K ar m ut se n R an ge ,S L, Sc ho en La ke ,M A, M ou nt Ar ro ws m lth .( du p) in di ca te sc o m pl et e ch em ist ry du pl ic at e. Al li so to pi ca n d el em en ta la n al ys es ca rr ie d o u ta tt he PC IG R; th e co m pl et e tr ac e el em en ta n al ys es sh ow n in A pp en di x C. 208Pb/4 = 38.153-38.735. One high-MgO basalt has the highest initial 206Pb/4 (19.242) and 207Pb/4 (15.590), and the lowest initial 208Pb/4 (37.676). The Pb isotopic ratios for Karmutsen samples define broadly linear relationships in Pb isotope plots. The age-corrected Pb isotopic compositions of several picrites have been affected by U and Pb (andlor Th) mobility during alteration (Fig. 2.10). ALTERATION The Karmutsen basalts have retained most of their original igneous structures and textures; however, secondary alteration and low-grade metamorphism generated zeolitic and prehnite-pumpellyite-bearing mineral assemblages (Table 2.1) (Cho et a?., 1987), which have primarily affected the distribution of the LILE and the Sr isotopic systematics. The Keogh Lake picrites show the strongest effects of alteration and have higher LOT (up to 5.5 wt %), variable LILE, and higher measured Sr, Nd, and Hf and lower measured Pb isotope ratios than tholeiitic basalts. As a result, Sr and Pb isotopes show a relationship with LOl (Figs 2.9 and 2.10), whereas Nd and Hf isotopic compositions do not correlate with LOI (not shown). The relatively high initial Sr isotopic compositions for high-MgO lavas (up to 0.7052) likely resulted from an increase in8Srir through addition of seawater Sr (e.g. Hauffet a?., 2003). High-MgO lavas from the Caribbean plateau have similarly high 87Sr/6r compared to basalts (R\u00C3\u00A9villon et a?., 2002). The correction for in situ decay on initial Pb isotopic ratios has been affected by mobilization of U and Pb (andlor Th) in whole rocks since their formation. A thorough acid leaching during sample preparation was used that has been shown to effectively remove alteration phases (Weis et a?., 2006; Nobre Silva et a?., submitted). The HFSE abundances for tholeiitic and picritic basalts exhibit clear linear correlations in binary diagrams (Fig. 2.8), whereas plots of Lll.E versus HFSE are highly scattered (not shown) due to the mobility of some of the alkali and alkaline earth LILE (Cs, Rb, Ba, K) and Sr for most samples during alteration. The degree of alteration in the Karmutsen samples does not appear to be related to eruption environment or depth of burial. Pillow rims are compositionally different from pillow cores (Surdam, 1967; Kuniyoshi, 1972), and aquagene tuffs are chemically different from isolated pillows within pillow breccia, but submarine and subaerial basalts 62 exhibit a similar degree of alteration. There is no clear correlation between the submarine and subaerial basalts and some commonly used chemical alteration indices [e.g. Ba/Rb vs.K20/P5(Huang & Frey, 2005)]. There is also no definitive correlation between the petrographic alteration index (Table 2.1; 1-least altered, 3-intensely altered) and chemical alteration indices, although 10 of 13 tholeiitic basalts with the highest K and LILE abundances have a petrographic alteration index of 3. OLIVINE ACCUMULATION IN PICRITIC LAVAS Geochemical trends and petrographic characteristics indicate that accumulation of olivine played an important role in the formation of the Keogh Lake picrites on northern Vancouver Island. The Keogh Lake picrites show a strong linear correlation in plots of Al203,Ti02, Sc, Yb, and Ni versus MgO and many of the picrites have abundant clusters of olivine pseudomorphs (Table 2.1; Fig. 2.11). There is a strong linear correlation between modal percent olivine and whole-rock magnesium content for the Keogh Lake picrites (Fig. 2.11). Magnesium contents range between 10 and 19 wt % MgO and the proportion of olivine phenocrysts in these samples varies from 0 to 42 vol % (Fig. 2.11). With a few exceptions, the size range and median size of the olivine phenocrysts in most samples is comparable. The clear correlation between proportion of olivine phenocrysts and whole-rock magnesium contents indicates that accumulation of olivine was directly responsible for the high MgO contents (>10 wt %) for most of the picritic lavas. DISCUSSION The Wrangellia oceanic plateau on Vancouver Island was constructed in a three- layered structure of mostly tholeiitic basalt with a restricted range of composition. The plateau formed rapidly during a single phase (ca. 230 Ma) and hiatuses between eruptions were not long-lasting so there was very little accumulation of sediments. The pillowed and unpillowed flows that built up the submarine volcanic edifice from the deep seafloor resulted primarily from different effusive rates and local topography. Some of the unpillowed flows represent master tubes for the delivery of lava to distal parts of submarine flow fields. Abundant picritic pillow lavas erupted during the middle and latter stages of submarine growth (Nixon et aL, 2008), in areas of the plateau exposed on 63 Figure 2.11 Relationship between abundance of olivine phenocrysts and whole-rock MgO contents for Keogh Lake picrites. (a) Tracings of olivine grains (gray) in six high-MgO samples made from high resolution (2400 dpi) scans of petrographic thin-sections. Scale bars are 10 mm in length and sample numbers are indicated in upper left. (b) Modal % olivine vs. whole-rock MgO. Black line is the best-fit line for ten high MgO samples. The areal proportion of olivine was calculated from raster images of olivine grains using ImageJ\u00C2\u00AE image analysis software, which provides an acceptable estimation of the modal abundance (e.g. Chayes, 1954). MgO (wt %) 20 64 northern Vancouver Island. The overlying volcaniclastic units formed as a consequence of eruption in shallow water. A broad subaerial platform was constructed of well-layered sheet flows and, as volcanism waned, local interfiow carbonate deposits developed, along with plagioclase megacrystic flows and local volcaniclastic deposits. The geochemical and stratigraphic relationships observed in the Karmutsen Formation on Vancouver Island provide constraints on the construction of oceanic plateaus, the source of magmas for a plume head impinging on oceanic lithosphere, and the conditions of melting and subsequent magmatic evolution of basaltic magmas involved in the formation of an oceanic plateau. Studies of the formation of this oceanic plateau in the following sections examine the (1) temperature and extent of melting of picritic lavas; (2) composition of the mantle source; (3) the depth of melting and residual mineralogy in the source region; and (4) low-pressure evolution of the Karmutsen basalts. Melting conditions and major-element composition of primary magmas The primary melts to most flood basalt provinces are believed to be picritic (e.g. Cox, 1980) and they have been found in many continental and oceanic flood basalt provinces worldwide [e.g. Siberia; Karoo; Parana-Etendeka; Caribbean; Deccan (Saunders, 2005)). Near-primary picritic lavas with low total alkali abundances require high-degree partial melting of the mantle source (e.g. Herzberg & O\u00E2\u0080\u0099Hara, 2002). The Keogh Lake picrites from northern Vancouver Island are the best candidates for least- modified partial melts of the mantle plume source, despite having accumulated olivine phenocrysts, and can be used to estimate conditions of melting and the composition of primary magmas for the Karmutsen Formation. Herzberg et al. (2007) have provided a thorough description of a technique for inverse and forward modeling for estimating mantle temperature, melt fraction, primary magma composition, and source residue composition using major elements. The estimated melting conditions and primary magma compositions for the picritic lavas of the Karmutsen Formation, and a brief summary of the modeling method, are presented in Figure 2.12 and Table 2.6. If Karmutsen picritic lavas were derived from accumulated fractional melting of fertile peridotite, the primary magmas would have contained 15-17 wt % MgO and -40 wt % CaO (Fig. 2.12; Table 2.6), formed from 23- 65 14 12 10 8 6 4 14 12 10 30 MgO (wt%) 8 Karmutsen flood basalts A Picrite 6 D High-MgO basalt e Tholeiitic basalt \u00E2\u0080\u0094 OIMne addition model 4 + Olivine addition (5% increment) $ Pnmary magma Figure 2.12 Estimated primary magma compositions for three Keogh Lake picrites (samples 4723A4, 4723A13, 5616A7) using the forward and inverse modeling technique of Herzberg et a!. (2007). Karmutsen compositions and modeling results are overlain on diagrams provided by C. Herzberg. (a) Whole-rock FeO vs. MgO for Karmutsen samples from this study. Total iron estimated to be FeO is 0.90. Gray lines show olivine compositions that would precipitate from liquid of a given MgO-FeO composition. Black lines with crosses show results from olivine addition (inverse model) using PRIMELT1 (Herzberg et al., 2007). (b) Si02 vs. MgO with Kanntusen lavas and model results. (c) FeO vs. MgO with Karmutsen lava compositions and results of forward model for accumulated fractional melting of fertile peridotite. (d) CaO vs. MgO with Karmtusen lava compositions and model results. To briefly summarize the technique [see Herzberg et a!. (2007) for complete description], potential parental magma compositions for the high-MgO lava series were selected (highest MgO and appropriate CaO) and, using PRIMELTI software, olivine was incrementally added to the selected compositions to show an array of potential primary magma compositions (inverse model). Then, using PRIMELT 1, the results from the inverse model were compared to a range of accumulated fractional melts for fertile peridotite, derived from parameterization of the experimental results of Walter (1998) (forward model; Herzberg & O\u00E2\u0080\u0099Hara, 2002). A melt fraction was sought that was unique to both the inverse and forward models (Herzberg et al., 2007). A unique solution was found when there was a common melt fraction for both models in FeO-MgO and CaO-MgO-A13-Si0 (CMAS) projection space. This modeling assumes olivine was the only phase crystallizing and ignores chromite precipitation, and possible augite fractionation in the mantle (Herzberg & O\u00E2\u0080\u0099Hara, 2002). Results are best for a residue of spinel lherzolite (not pyroxenite). The presence of accumulated olivine in samples of Keogh Lake picrites used as starting compositions does not significantly affect the results because we are modeling addition of olivine. The tholeiitic basalts cannot be used for modeling because they are all plag + cpx + ol saturated. Si02 (wt%) Melting model \u00E2\u0080\u0094 Dashed fines = initial melting pressure \u00E2\u0080\u0094 Solidus 7 KR-4G03(b) Garnet Peridosite 40 Liquid compositions Fertile peridotite source Accumulated Fractional Melting model Thick black lines = initial melting pressure Gray lines = final melting pressure 66 Table 2.6 Estimatedprimary magma compositions for Karmutsen basalts and other oceanic plateaus/islands Sample 4723A4 4723A13 561 6A7 93G171 Average OJPa Mauna Keab Gorgona (Weight %): Si02 47.0 47.6 46.9 47.8 47.3 48.0 46.3 46.1 Ti02 0.68 0.46 0.60 0.46 0.55 0.62 1.93 0.56 A1203 15.3 14.3 13.0 13.9 14.1 12.3 9.6 11.7 Cr203 0.10 0.20 0.04 0.09 0.11 0.07 0.26 0.16 Fe203 1.03 1.08 0.89 1.09 1.02 0.90 1.08 1.18 FeO 8.7 9.0 9.5 8.9 9.0 9.2 10.3 10.1 MnO 0.15 0.15 0.16 0.19 0.16 0.17 0.18 0.18 MgO 15.3 16.3 17.4 16.1 16.3 16.8 18.3 18.8 CaO 9.9 9.8 9.6 10.3 9.9 10.3 10.1 10.0 Na20 1.59 0.90 1.74 1.01 1.31 1.36 1.67 1.04 K20 0.07 0.06 0.05 0.11 0.07 0.08 0.41 0.03 NO 0.05 0.08 0.08 0.07 0.07 0.10 0.08 0.11 Eruption T(\u00C2\u00B0C) 1354 1375 1397 1369 1374 1382 1415 1422 Potential T(\u00C2\u00B0C) 1467 1491 1517 1486 1490 1500 1606 Fo content (olMne) 91.2 91.2 91.6 91.2 91.3 90.5 91.4 90.6 Melt fraction 0.23 0.27 0.26 0.27 0.26 0.27 0.28 %ol addition 4.2 2.5 24.3 0.8 7.9 18 Ontong Java primary magma composition for accumulated fraction melting (AFM) from Herzberg (2004). Mauna Kea primary magma composition is average of 4 samples in Table I of Herzberg (2006). Gorgona primary magma composition for AFM for 1 F, fertile source, in Table 4 of Herzberg and OHara (2002). Total iron estimated to be FeO is 0.90. 67 27% partial melting, and would have crystallized olivine with a Fo content of --91 (Fig. 2.12; Table 2.6). Calculated mantle temperatures for the Karmutsen picrites (\u00E2\u0080\u0094\u00E2\u0080\u00981 490\u00C2\u00B0C) indicate melting from anomalously hot mantle (100-200\u00C2\u00B0C) compared to ambient mantle that produces mid-ocean ridge basalt (MORB; \u00E2\u0080\u0098--1280-1400\u00C2\u00B0C; Herzberg et al., 2007). Several picrites are near-primary melts and required very little addition of olivine (e.g. sample 93G1 71, with measured 15.8 wt % MgO). These temperature and melting estimates of the Karmutsen picrites in this study are consistent with the decompression of hot mantle peridotite in an actively convecting plume head (i.e. plume initiation model). The estimated primary magma compositions and melting conditions for some other LIPs (e.g. the Ontong Java Plateau) are similar to estimates for the Karmutsen Formation (Table 2.6). Herzberg (2004) found that primary magma compositions for Ontong Java basalts, assuming a peridotite source, would contain \u00E2\u0080\u009417 wt % MgO and \u00E2\u0080\u0094l0 wt % CaO (Table 2.6), and these magmas would have formed by 27% melting with mantle potential temperatures of \u00E2\u0080\u00941500\u00C2\u00B0C, and first melting occurring at \u00E2\u0080\u0094-3.6 GPa. Fitton & Godard (2004) estimated 30% partial melting of the Ontong Java lavas based on the Zr contents of primary magmas calculated by incremental addition of equilibrium olivine to analyses of Kroenke-type basalt. If a considerable amount of eclogite was involved in the formation of the Ontong Java Plateau, the excess temperatures estimated by Herzberg et a!. (2007) may not be required (Korenaga, 2005). Estimated primary magmas for Mauna Kea and Gorgona have higher MgO (18-19 wt % MgO) and comparable CaO (10 wt % CaO) (Table 2.6; Herzberg (2006)). Source of Karmutsen lavas Karmutsen samples have isotopic compositions belonging to the field of ocean island basalts (OIB) and provide a sampling of the Pacific mantle Ca. 230 Ma (Fig. 2.13). The Karmutsen tholeiitic basalts have initial ENd and 87Sr/6r that fall within the range of basalts from the Caribbean Plateau (e.g. Kerr et a!., 1996; Kerr et al., 1997; Hauff et a!., 2000; Kerr, 2003), a range of Hawaiian isotope data (e.g. Frey eta!., 2005), and lie within and just above the range for the Ontong Java Plateau (Tejada et a!., 2004; Fig 2.13). Karmutsen tholeiitic basalts with the highest initial 8Nd and lowest initial 87Sr/65 lie just below the field for northern East Pacific Rise (EPR) MORB (e.g. Niu eta!., 1999; 68 0.704 87SrI 86Sr (initial) 206pb/4 Figure 2.13 Comparison of age-corrected (230 Ma) Sr-Nd-Hf-Pb isotopic compositions for Karmutsen flood basalts on Vancouver Island to age-corrected OIB and MORB. (a) Initial 8Nd vs. 87Sr/6. (b) Initial 8Hf vs. 8Nd Both fields with dashed lines are Indian MORE. (c) Measured and initial 207Pb/4 vs. 206Pb/4. (d) Measured and initial 208Pb/4 vs. 206PbP\u00C2\u00B04b. Most of the compiled data was extracted from the GEOROC database (http://georoc.mpch mainz.gwdg.de/georocl). Data for Ontong Java from Mahoney et a?. (1993), Babbs (1997), and Tejada et a?. (2004); Indian MORE from Salters (1996), Kempton et a?. (2002), and Janney et a?. (2005); Pacific MORE from Mahoney et a?. (1992, 1994), Nowell et a?. (1998), Salters and White (1998), and Chauvel and Blichert-Toft (2001); Explorer Ridge data from Cousens and Weis (pers. conmi., 2007); OTB array line from Vervoort et a?. (1999). EPR is East Pacific Rise. Several high-MgO samples affected by secondary alteration were not plotted for clarity in Pb isotope plots. Dashed lines indicate Bulk Silicate Earth (B SE). An extended reference list is available upon request. _ t \u00E2\u0080\u00A2S 14 12 10 8 6 4 2 0 I. I \u00E2\u0080\u00A2 \u00E2\u0080\u00A2 ft-\u00E2\u0080\u0099 4\u00E2\u0080\u00A2 .1 . -2 0.702 II I liii 16 Indian__. EHfinitiah \u00E2\u0080\u00A2 Indian MORB 6 .F \u00E2\u0080\u00A2 \u00E2\u0080\u00A2 East Pacific Rise \u00E2\u0080\u00A2 \u00E2\u0080\u0094 Caribbean Plateau 4 \u00C2\u00A3 \u00E2\u0080\u00A2 Ontong Java Hawaii 2 I \u00E2\u0080\u00A2 Karmutsen tholeiitic basalt 0 iOlB array \u00E2\u0080\u00A2 Karmutsen high-MgO lava i\ 2(b) I I I I I I I I I I I I I E Nd(t1ao) \u00E2\u0080\u00A2 0.703 0.705 0.706 -4 -2 0 2 4 6 8 10 12 14 15.60 15.55 15.50 15.45 I1F[ I I I I I I I I I I F F FI1 I 1 I I \u00E2\u0080\u00A2\u00E2\u0080\u00A2 I 0 \u00E2\u0080\u00A2 I.. \u00E2\u0080\u00A2 I \u00E2\u0080\u00A2 East Pacific Rise Juan de Fuca/Gorda - \u00E2\u0080\u00A2 Explorer Ridge \u00E2\u0080\u00A2 Caribbean Plateau \u00E2\u0080\u00A2 Ontong Java \u00E2\u0080\u00A2 Hawaii o Karmutsen Formation (measured) \u00E2\u0080\u0094 \u00E2\u0080\u00A2 Karmutsen Formation (initial) \u00E2\u0080\u00A2 Karmutsen Formation (different age-correction for 3 picrites) IIIIIIIIIIIII II I I I I I I I I 17.5 18.0 18.5 19.0 19.5 20.0 17.5 18.0 18.5 19.0 19.5 20.0 69 Regelous et a!., 1999). Initial Hf and Nd isotopic compositions place the Karmutsen basalts on the edge of the field of OIB (age-corrected), with slightly higher initial 8Nd and similar initial 8Hf to Ontong Java, and slightly lower initial \u00C2\u00A3Hf and similar initial ENd to the range of compositions for Hawaii and the Caribbean Plateau (Fig. 2.13). Karmutsen samples are slightly offset to the low side of the OIB array (Vervoort et a!., 1999), slightly overlap the low 8Hf end of a field for Indian MORB (Kempton et a!., 2002; Janney et aL, 2005), and samples with the highest initial 8Hf and 8Nd lie just at the edge of the presently defined field for Pacific MORB (Mahoney et aL, 1992, 1994; Nowell et al., 1998; Chauvel & Blichert-Toft, 2001). Karmutsen Pb isotope ratios overlap a broad range for the Caribbean Plateau and the linear trends for Karmutsen samples intersect fields for EPR MORB, Hawaii, and Ontong Java in 208Pb-6 space, but do not intersect these fields in207Pb-6 space (Fig. 2.13). The more radiogenic207Pb/4 for a given 206Pb/4 of Karmutsen basalts indicate they are isotopically enriched in comparison to EPR MORB, Hawaii, and Ontong Java. The Hf-Nd isotopic geochemistry of volcanic rocks of the Karmutsen Formation on Vancouver Island indicates an isotopically rather uniform mantle source. The Hf and Nd isotopic compositions of Karmutsen basalts indicate long-term depletion of the more highly incompatible elements in the mantle source and are distinct from MORE; they are less depleted than the source of MORE and there is no clear evidence of involvement of MORE-type mantle. The limited variation of Hf and Nd isotopic compositions is slightly greater than in the isotopically very uniform mantle source of Ontong Java. Small-scale heterogeneities in the mantle source of Karmutsen lavas may have been diluted by the high degrees of partial melting, similar to Ontong Java (Tejada et al., 2004). Lassiter et a!. (1995) suggested that mixing of a plume-type source with 8Nd +6 to +7 with arc material with low Nb/Th could reproduce variations in the Karmutsen basalts, but the absence of low Nb/La ratios in most of the basalts restricts the amount of arc lithospheric involvement (based on major and trace elements and Sr, Nd, and Pb isotopic compositions for a suite of 29 samples from Buttle Lake in Strathcona Provincial Park on central Vancouver Island; Fig. 2.1). The isotopic composition and trace-element ratios of Karmutsen basalts in this study do not indicate significant involvement of Paleozoic arc 70 lithosphere on Vancouver Island. There are no clear HFSE-depletions in Karmutsen tholeiitic basalts. The Sr-Nd-Hf-Pb isotope systematics for Karmutsen basalts distinguish the source of Karmutsen basalts from MORB and some OIB. The Hf and Nd isotope systematics provide the firmest constraints on the character of the source for Karmutsen lavas. The LU/Hf ratios of the picritic lavas (mean 0.08) are considerably higher than in tholeiitic basalts (mean 0.02), but the small range of initial \u00C2\u00A3Hf for the two lava suites suggests that these differences in trace-element compositions were not long-lived, and do not correspond to intrinsic differences in the mantle plume source. Evolution of Hf isotopes with time shows that if the picritic and tholeiitic lavas originally had a similar range of8Hf it would take \u00E2\u0080\u0094l55 m.y. for Hf isotope ratios to evolve so there would be no overlap in 8Hf (Fig. 2.14); this is similar to a concept emphasized by Arndt et a!. (1997) using Nd isotopes of lavas from Gorgona Island. Therefore, it is unlikely that the picrites had high LU/Hf ratios long before ascent of the plume and the different Lu/Hf ratios likely developed during the melting process within the plume during ascent. The similar initial Hf and Nd isotopic ratios of the picrites and tholeiitic basalts preclude the limited isotopic range being simply the result of mixing of magmas. The slightly different Pb isotope ratios of the picrites and tholeiitic basalts are at least partly a result of alteration- related effects. As observed in the Caribbean Plateau, the radiogenic Pb in Karmutsen basalts is systematically different than in MORB, Ontong Java, and OIB from Hawaii, but Hf and Nd isotope compositions indicate a homogeneous, OIB-type enriched mantle source. REE modeling: Dynamic melting and source mineralogy The combination of isotopic and trace-element geochemistry of picritic and tholeiitic Karmutsen lavas indicates that differences in trace elements may have originated from different melting histories within a predominantly homogeneous, depleted (but not MORE) mantle source. Dynamic melting models simulate progressive decompression melting where some of the melt fraction is retained by the residue and only when the degree ofpartial melting is greater than the critical mass porosity, and the source becomes permeable, is excess melt extracted from the residue (Zou, 1998). For the 71 25 Picrite High-MgO basalt 20 \u00E2\u0080\u0094 \u00E2\u0080\u00A2 Tholelitic basalt 11 - - -15 - \u00E2\u0080\u0094155 m.y.\u00E2\u0080\u0094+I j.-\u00E2\u0080\u0099 / II - Hf 10 hypothetical I actual 230 Ma 0 \u00E2\u0080\u00A2 \u00E2\u0080\u00A2 \u00E2\u0080\u0098 I a I I I I 500 400 300 200 100 0 Age (Ma) Figure 2.14 Evolution of EHf with time for picritic and tholeiitic lavas for the Karmutsen Formation. The different Lu/Hf ratios of the Karmutsen lavas likely originated during melting processes within the plume. For example, if the picritic and tholeiitic lavas possessed the Lu/Hf ratios that they did at 230 Ma more than 155 m.y. before their formation, they would have evolved to have a different range of Hf isotopic compositions by 230 Ma (shown by the gray and white vertical boxes for tholeiitic and picritic lavas, respectively). This is shown by hypothetical compositions at 400 Ma (using EHf(23O Ma) for each sample) and evolution trends using 176Lu!\u00E2\u0080\u00997Hffor each sample. Picritic lavas have high Lu! Hf ratios so they accumulate radiogenic Hf within a relatively short geologic timespan. Decay constant of 1.87 x 101yr\u00E2\u0080\u0099 used from Scherer et a!. (2001). Error bars are smaller than symbols. 72 Karmutsen lavas, evolution of trace element concentrations was simulated using the incongruent dynamic melting model developed by Zou & Reid (2001). Melting of the mantle at pressures greater than -4 GPa involves incongruent melting (e.g. Longhi, 2002) with olivine being produced during reaction of cpx + opx + sp for spinel peridotites. Modeling was separated into two intervals of melting, for a garnet therzolite (gt lherz) and spine! lherzolite (sp lherz) source, using coefficients for incongruent melting reactions based on experiments on lherzolite melting (e.g. Kinzler & Grove, 1992; Longhi, 2002). The model solutions are non-unique and a range in degree of melting, partition coefficients, melt reaction coefficients, and proportion of mixed source components is possible to achieve acceptable solutions. The LREE-depleted high-MgO lavas require melting of a depleted spinel lherzolite source (Fig. 2.15). The modeling results indicate a high degree of melting (22- 25%), similar to results from major-element modeling using PRIMELT1 (discussed above), from a LREE-depleted source, and melting of garnet was probably not involved in formation of the high-MgO lavas. The enriched tholeiitic basalts involved melting of both garnet and spinel lherzolite and represent aggregate melts produced from continuous melting throughout the depth of the melting column (Fig. 2.15), which were subsequently homogenized and fractionated in magma chambers at low pressure. The modeling results indicate that these aggregate melts would have involved lesser proportions of enriched, small-degree melts (1-6% melting) generated at depth and greater amounts of depleted, high-degree melts (12-25%) generated at lower pressure (ratio of gt lherz to sp lherz melt would have been approximately ito 3, or 1 to 4; see Fig. 2.15). The degree of melting was high (23-27%), similar to the degree of partial melting of primary melts that erupted as the Keogh Lake picrites, from a source that was likely depleted in LREE. Although the modeling results indicate that the depleted high-MgO Karmutsen lavas were formed from high-degree partial melting within the spinel therzolite stability field (\u00E2\u0080\u00940.9-2.5 GPa), the source was not necessarily more depleted in incompatible trace elements than the source of the tholeiitic lavas. It is possible that early high-pressure, low-degree melting left a residue within parts of the plume (possibly the hotter, interior portion) that was depleted in trace elements (e.g. Elliott et al., 1991); further decompression and high-degree melting of such depleted regions could then have 73 100 La Ce Pr Nd Pm Sm Eu Gd Tb Dy Ho Er Tm Yb Lu 0.0 03 1.0 La/SmCN Figure 2.15 Trace-element modeling results for incongruent dynamic mantle melting for picritic and tholeiitic lavas from the Karmutsen Formation. Three steps of modeling for tholeiitic lavas are shown (order of modeling is shown going upwards from panel e to c to a, following the arrows): (e) Melting of garnet lherzolite, (c) Mixing of garnet and spinel therzolite melts, and (a) Mixing of enriched tholeiitic and depleted picritic melts. (b) Melting of spinel therzolite for high-MgO lavas. Shaded field is the range of REE patterns for Karmutsen tholeiitic basalts in panels a, c, and e and for picrites in panel b. Patterns with symbols in panels a, b, c, and e are modeling results (patterns are 5% melting increments in panels a and c and 1% increments in panel e. Abbreviations are: PM, primitive mantle; DM, depleted MORB mantle; gt therz, garnet therzolite; sp lherz, spinel lherzolite. The ratios of percent melting for garnet and spinel therzolite are indicated in panel c. A range of proportion of source components (0.7DM:0.3PM to 0.9DM:0.1PM) can reproduce the variation in high-MgO lavas. (d) Dy!YbCN vs. La! SmcN for results of spinel Iherzolite melting modeling and Karmutsen samples. Tickmarks are 5% increments and proportions of mantle source components are labelled next to curves. (f) Dy/YbCN vs. La/SmCN for results of spinel lherzolite + garnet therzolite melting modeling and Karmutsen samples. Melting modeling uses the formulation of Zou & Reid (2001), an example calculation is shown in their Appendix. PM from McDonough & Sun (1995) and DM from Salters & Stracke (2004). Melt reaction coefficients of spinel lherzolite from Kinzler & Grove (1992) and garnet lherzolite from Walter (1998). Partition coefficients from Salters & Stracke (2004) and Shaw (2000) were kept constant. Source mineralogy for spinel lherzolite (0.l8cpx:0.27opx:0.52o1:0.O3sp) from Kinzler (1997) and for garnet therzolite (0.34cpx:0.O8opx:0.53o1:0.OSgt) from Salters & Stracke (2004). A source composition with mantle components up to 0.3 PM and 0.7 depleted mantle can reproduce REE patterns similar to those of the high-MgO lavas, with best fits for a melt fraction of 0.22-0.25. Concentrations of tholeiitic basalts cannot easily be reproduced from an entirely primitive or depleted source. A range of source compositions (0.7DM:0.3PM to 0.3DM:0.7PM) and garnet and spinel lherzolite melt mixtures generate similar results (x gtlherz + 3x splherz to x gtlherz + 4x splherz; where x is percent melting in the interval of modeling; the ratio of the respective melt proportions in the aggregate melt was kept constant). hoiclavas 0.8 enriched melt + 0.2 depleted high-MgO melt Spinel lherzolite + Spirl lherzolite melt garnet lherzolite melt (from panel B) (from panel C) (a) . . A. C .210 U E High-MgO lavas Melting of spinel lherzolite 20% melting 30% melting urce(O.7DM+O3PM C 0 -C U 0. E C, C, C 0 L) C, 0. E C, 0 C 0 -C U C, 0. E C, U, La Ce Pr Nd Pm Sm Eu Gd Tb Dy Ho Er Tm Yb Lu 10 100 10 100 10 I \u00E2\u0080\u00A2 I \u00E2\u0080\u00A2 I \u00E2\u0080\u00A2 Melting of spinel lherzolite melting 0.7DM +0.3PM melting 30% melting (d) La/Srn 0 0.5 1.0 1.5 2.( % melting ratio, La Ce Pr Nd Pn-fSm Eu Gd Tb Dy[lo Er Tm Yb Lu 1.5 \u00E2\u0080\u0098 \u00E2\u0080\u0098 \u00E2\u0080\u0098 \u00E2\u0080\u0098 \u00E2\u0080\u0098 o\u00E2\u0080\u0099Ie,it\u00E2\u0080\u0099ic lva 14 13 12 xgtlherz+4xspiherz -\u00E2\u0080\u0094 DyIYb 1.1Spinel lherzolite + garnet lherzolite melt 1.0 .e(O.7DM\u00C3\u00B73P 0.9 (C) L Ce Pr Nd P4Sm Eu Gd Tb Dy \u00E2\u0080\u00A2 \u00E2\u0080\u0098 \u00E2\u0080\u00A2 os Ho Er TmYb Lu 0. 1.5 Tholeiitic lavas 1 1 I 1 \u00E2\u0080\u0098 x gnt lherz \u00C3\u00B7 3x sp lherz - - 1% melting 1.4 . 25% 20% 10% ____ 13\u00E2\u0080\u00A2 !eltin - Spinellherzolite+ 25 .....J_ 12 .garnetlherzolitemelts %meltinp ratio Dy/Yb 1.1 xgntlherz\u00C3\u00B74xsplherz 1.0 \u00E2\u0080\u00A2 A Picrite 0 High-MgO basalt 0.7DM+O.3PM) 0.9 - 0 Coarse-g rained(e) \u00E2\u0080\u00A2 \u00E2\u0080\u00A2 I I I I I I \u00E2\u0080\u00A2 \u00E2\u0080\u00A2 . \u00E2\u0080\u00A2 Tholeiitic basaltA. \u00E2\u0080\u00A20.8 1.5 2.0 74 generated the depleted high-MgO Karmutsen lavas. However, the modeling results indicate that this earlier melting was not necessary for their formation, depending on the original trace-element composition of the source. Shallow, high-degree melting would preferentially sample depleted regions, whereas deeper, low-degree melting may not sample more refractory, depleted regions (e.g. Caribbean; Escuder-Viruete et al., 2007). Decompression melting within the mantle plume initiated within the garnet stability field at high mantle potential temperature (T>l450\u00C2\u00B0C) and proceeded beneath oceanic arc lithosphere within the spinel field where more extensive degrees of melting could occur. Magmatic evolution of Karmutsen tholelitic basalts Primary magmas in LIPs leave extensive coarse-grained residues within or below the crust and these mafic and ultramafic plutonic sequences represent a significant proportion of the original magma that partially crystallized at depth (e.g. Farnetani et al., 1996). For example, seismic and petrological studies of Ontong Java (e.g. Farnetani et al., 1996), combined with the use of MELTS (Ghiorso & Sack, 1995), indicate that the volcanic sequence may have been only -40% of the total magma reaching the Moho. Neal et al. (1997) estimated between 7 and 14 km of cumulate thickness corresponding with 11 to 22 km of flood basalt, depending on the presence ofpre-existing oceanic crust and the total crustal thickness (25-3 5 km), and that 3 0-45% fractional crystallization took place for Ontong Java magmas. Interpretations of seismic velocity measurements suggest the presence of pyroxene and gabbroic cumulates --9-16 km thick beneath the Ontong Java Plateau (e.g. Hussong et al., 1979). Wrangellia is characterized by high crustal velocities in seismic refraction lines, which is indicative of mafic plutonic rocks extending to depth beneath Vancouver Island (Clowes et al., 1995). Wrangellia crust is \u00E2\u0080\u009425-30 km thick beneath Vancouver Island and is underlain by a strongly reflective zone of high velocity and density that has been interpreted as a major shear zone where lower Wrangellia lithosphere was detached (Clowes et a!., 1995). The vast majority of Karmutsen flows are evolved tholeiitic lavas (e.g. low MgO, high FeOIMgO) indicating an important role for partial crystallization of melts at low pressure, and a significant portion of the crust beneath Vancouver Island has seismic properties that are consistent with crystalline residues from partially crystallized 75 Karmutsen magmas. To test the proportion of the primary magma that fractionated within the crust, MELTS (Ohiorso & Sack, 1995) was used to simulate fractional crystallization using several estimated primary magma compositions from modeling results using P1UMELT1. The major-element composition of primary magmas could not be estimated for the tholeiitic basalts due to extensive plag + cpx + ol fractionation, and thus the MELTS modeling of the picrites is used as a proxy for the evolution of major elements in the volcanic sequence. A pressure of 1 kbar was used with variable water contents (aithydrous and 0.2 wt % H20), calculated at the quartz-fayalite-magnetite (QFM) oxygen buffer. Experimental results from Ontong Java indicate that most crystallization occurs in magma chambers shallower than 6 km deep (<2 kbar; Sano & Yamashita, 2004); however, some crystallization may take place at greater depths (3-5 kbar; Farnetani et aL, 1996). A selection of the MELTS results are shown in Figure 2.16. Olivine and spinel crystallize at high temperature until \u00E2\u0080\u0094P15-20 wt % of the liquid mass has fractionated, and the residual magma contains 9-10 wt % MgO (Fig. 2.16). At 1235-1225\u00C2\u00B0C, plagioclase begins crystallizing and between 1235 and 1190\u00C2\u00B0C olivine ceases crystallizing and clinopyroxene saturates (Fig. 2.16). As expected, the addition of clinopyroxene and plagioclase to the crystallization sequence causes substantial changes in the composition of the residual liquid; FeO and Ti02 increase and A1203and CaO decrease, while the decrease in MgO lessens considerably (Fig. 2.16). Tn general, the compositions of the predicted residual liquids follow trends for compositions of Karmutsen lavas, but miss some of the low MgO, high CaO basalts (some of which are due to plag accumulation). Also, trends in Al203and CaO of Karmutsen basalts are not clearly decreasing with decreasing MgO. Increasing the crystallization pressure slightly and water content of the melts does not systematically improve the match to the observed data. The MELTS results indicate that a significant proportion of crystallization takes place before plagioclase (\u00E2\u0080\u0098-15-20% crystallization) and clinopyroxene (-35-45% crystallization) join the crystallization sequence. These results suggest >45% crystallization of primary magmas occurred (estimated from residual liquid % in Fig. 2.16) and, if the original flood basalt stratigraphy on Vancouver Island was \u00E2\u0080\u00946 1cm, at least an equivalent amount of complementary plutonic rocks should have crystallized. 76 LL EL Vt EL 9L LL St 6t 9 L 8 6 OL LL EL Vt EL 9L LL ssduizqjs&I josiutpjqjinrndwSApmbijTP!so1uaoj().OZHouq(ETVZLI7jduius)urnjJji uowo.sssdPz!TImsjosuoTijodoidTuI!N()coiipuuuoruuonJj%LsiOH%iotpA .iojiuod-puspucpuuotuoT.go,\u00E2\u0080\u0099isiusisnoip(qusiojuod-puS11.OZH %O!1I!IP5UOJP1(11I!\u00E2\u0080\u0098II)1\u00C2\u00B01N1OPIt1O1\u00E2\u0080\u0098OH%opuOHOU1{IAp5511SuA iqjjjoainsssidvsjnsaz\u00E2\u0080\u0098W\u00E2\u0080\u0099!piIL10E6pu\u00C2\u00A3VEZLssidwusiOjswuwi(iuiudpsuwsssqj ijutuos\u00E2\u0080\u0098EIvlLtsjdwsoj(/f)ivissqz.Is-{JOsnbimps&IiJspouIsiwoi,jssussuuj stio;uwui&Iuuiudpswmtssjouonisodwoosqusn\u00E2\u0080\u0098(sjaipi)uoiiisodmos&iiissuoiojuMo1s smsijnsaisi\u00E2\u0080\u0098i(iupiosuornsoduioouuussnuujopaidwoosprnbJJrnlpsa1josuornsodwoousuisjs -iofwsi(p)puu\u00E2\u0080\u0098(o)\u00E2\u0080\u0098(q)\u00E2\u0080\u0098(s)uouuojussnuustpwoSA1siiiisJopuo!.Is!dopsidwoo(c661 \u00E2\u0080\u0098i\u00C2\u00B0s\u00E2\u0080\u0098oslon{9)S1iNWOSU5UI5S1018WJOjSflUSSJU!j5pOwRO13uZqSAJO1\u00E2\u0080\u0099\u00C2\u00B0!3\u00C2\u00B0pJAUO{91Z\u00E2\u0080\u0098flLI (%M)06W(%IM)OW OE8L9LVLLOL89V0O8L9tVtLOL89V0 (p) V V V V 9 L 8 6 OL LL EL (%IM)Qe3, III (3) VV V \u00E2\u0080\u00A2!1!!IOq.1.@ seq06W-L1 !H(%M)S0Z1y 1!J)!dV IIIIIIIII (%M)06W OZ8L9tVLtOL89VZ0 (%IM)06W 0St9LVLLOL891\u00E2\u0080\u00990 (q) 0\u00E2\u0080\u00A20 06W c.0 0\u00E2\u0080\u00A2 (%)p!nb!Iienpsi OLOOSO0009OL c\u00E2\u0080\u00A2 (8) 0906OOL 0ZH%1MO(J) / 0611 0HOU\u00E2\u0080\u0099 0ZL ed6uIIsAjpue\u00E2\u0080\u00A2\u00E2\u0080\u00A2 PiflbIIIeflP!S9\u00C2\u00B0 c. -i 0OLL0.E 0056 (%)501j.SE I IO\u00E2\u0080\u00A2V The Karmutsen basalts and their plutonic residues thus represent a significant addition of crust (perhaps >12 km thickness), previously thickened by Paleozoic arc activity. CONCLUSION The Karmutsen Formation covers \u00E2\u0080\u009420,000 km2 of Vancouver Island, British Columbia, and was constructed as a large oceanic plateau during a single phase over a geologically short interval (ca. 230 Ma). The tripartite volcanic stratigraphy on Vancouver Island is upwards of 6 km of submarine flows, volcaniclastic deposits, and massive sheet flows; these volcanological differences are primarily related to the eruption environment (deep-water, shallow-water, subaerial). The rapid growth of the plateau prevented intervening sediments from accumulating, except in the uppermost stratigraphy where isolated limestone lenses commonly associated with pillowed and volcaniclastic basalts preserve a record of the subsidence history of the plateau. The Wrangellia plateau on Vancouver Island was constructed dominantly of tholeiitic basalt with a restricted major- and trace-element, and isotopic, composition. Picritic pillow basalts erupted in the middle to latter portion of the submarine phase. Modeling results of Karmutsen picrites indicate melting of anomalously hot mantle (\u00E2\u0080\u00944500\u00C2\u00B0C) and extensive degrees of partial melting (23-27%) and are consistent with a plume initiation model. The lavas that built the volcanic edifice were derived from an isotopically relatively uniform depleted mantle source distinct from the source of MORE. There are compositional similarities between the source of Karmutsen basalts on Vancouver Island and the sources of ocean islands (e.g. Hawaii) and plateaus (e.g. Ontong Java) in the Pacific Ocean. The Karmutsen basalts erupted atop Paleozoic arc volcanic sequences, but there is no clear evidence from our work on Vancouver Island of significant involvement of arc or continental material in formation of the basalts. Picrites formed from melting of spinel lherzolite with a depleted mantle isotopic composition similar to the source of the tholeiitic basalt. The tholeiitic basalts underwent extensive low-pressure fractionation (<2-3 kbar) and seismic work indicates some of the Wrangellia crust beneath Vancouver Island may correspond to the plutonic residues of the Karmutsen Formation. 78 ACKNOWLEDGEMENTS We would like to thank Nick Arndt for helping us get this project started and Nick Massey for insights into Vancouver Island geology. We would also like to thank Claude Herzberg for his kind help with modeling techniques. We are grateful to Mikkel Schau for his insight and enthusiasm during fieldwork. Jane Barling assisted with analyses by MC-ICP-MS. Funding was generously provided by the Rocks to Riches Program administered by the BC & Yukon Chamber of Mines in 2004, by the BC Geological Survey in 2005, and by NSERC Discovery Grants to J. Scoates and D. Weis. A. 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Geochimica et Cosmochimica Acta 65(1), 153-162. 85 CHAPTER 3 Wrangellia Flood Basalts in Alaska: A Record of Plume-Lithosphere Interaction in a Late Triassic Accreted Oceanic Plateau \u00E2\u0080\u0098A version of this chapter has been submitted for publication. 86 INTRODUCTION Oceanic plateaus and continental flood basalts (CFBs) are produced from the largest melting events recorded on Earth. Oceanic plateaus and CFBs are transient large igneous provinces (LIPs) that form from unusually high magmatic fluxes over several million years or less (Saunders, 2005). The presence of transient LIPs in a variety of tectonic settings attests to large thermal anomalies that are not directly attributable to seafloor spreading processes. A longstanding controversy in many transient LIPs worldwide is the role of the mantle lithosphere in generation of the basaltic magmas. CFBs have compositions that indicate involvement of subcontinental lithospheric mantle and continental crust (e.g. Peate & Hawkesworth, 1996). Compositional evidence of plume-lithosphere interaction in oceanic plateaus, however, remains elusive because oceanic plateaus are less accessible. Oceanic plateaus are enormous volcanic edifices (2-4 km high) that are isolated from continents and form upon mid-ocean ridges, extinct arcs, detached or submerged continental fragments, or intraplate settings (Coffin et a!., 2006). The basalts that form oceanic plateaus have a better chance of avoiding interaction with continental lithosphere than basalts erupted along the margins or in the interiors of continents (Kerr & Mahoney, 2007). A significant issue in the geochemistry of flood basalt provinces has been the origin of high- and low-titanium basalts within the flood basalt stratigraphy (e.g. Arndt et a?., 1993). Numerous flood basalt provinces have been found to possess two or more distinguishable groups of basalts based on titanium contents [e.g. Siberia (Wooden et a?., 1993); Emeishan (Xu eta?., 2001); Karoo (Cox eta?., 1967); Ferrar (Hornig, 1993); Paran\u00C3\u00A1-Etendeka (Peate, 1997); Deccan (Melluso et a?., 1995); Ethiopia (Pik et a?., 1998); Columbia River Basalts (looper & Hawkesworth, 1993)]. In several of these provinces, the high- and low-titanium basalts are geographically distributed, and in several provinces these different lava types have a distinct stratigraphic distribution. However, all of these LIPs formed upon continental crust and are thought to have involved interaction with metasomatized lithospheric mantle or continental crust during parts of their eruptive history. The Wrangellia flood basalts in Alaska erupted in the eastern Panthalassic Ocean over <5 Myr in the Middle to Late Triassic, with accretion to western North America 87 occurring in the Late Jurassic or Early Cretaceous (Jones et aL, 1977). The Wrangellia flood basalts form thick successions of flood basalts bounded by marine sediments that extend over widespread areas of Alaska, Yukon, and British Columbia (>2300 km in length). In south-central Alaska, parts of the complete flood basalt stratigraphy overlie Late Paleozoic oceanic arc crust and marine sediments and are overlain by Late Triassic limestone. Most of the 3.5-4 km of flood basalts in Alaska erupted subaerially, however, in areas of Alaska there are pillowed and volcaniclastic basalts in the lower part of the volcanic stratigraphy. The Wrangellia flood basalts in Alaska provide an exceptional opportunity to examine the volcanic stratigraphy of an accreted oceanic plateau. While Ocean Drilling Program (ODP) and Deep Sea Drilling Program (DSDP) legs have drilled extant oceanic plateaus in the ocean basins, the vast majority of the stratigraphic sequence of oceanic plateaus remains generally unsampled and undescribed. This study examines volcanic stratigraphy, petrography, and geochemistry of flood basalts in the Wrangell Mountains and Alaska Range in south-central Alaska to determine the source and origin of high- and low-titanium basalts in the accreted Wrangellia oceanic plateau and the role of the pre existing Paleozoic oceanic arc lithosphere. A mantle plume origin was proposed for Wrangellia flood basalts by Richards et aL (1991) based on the large volume of flood basalts erupted in a short duration, the absence of evidence of rifting, and evidence of uplift prior to eruption of the flood basalts. The only previous modem analytical study of the Wrangellia flood basalts in Alaska involved major- and trace-element chemistry, and Sr, Nd, and Pb isotopic analyses, of 9 basalts from the Wrangell Mountains (Lassiter et al., 1995). This present study is part of a larger research project on the origin and evolution of the Triassic Wrangellia flood basalts in British Columbia, Yukon, and Alaska (Greene et a!., 2008, submitted-b). The generation of compositionally distinct basalts in part of the Wrangellia oceanic plateau has implications for the interaction of mantle plumes and oceanic mantle modified by subduction. 88 GEOLOGIC SETTING Wrangeffia in Alaska The Wrangellia Terrane, or Wrangellia, was defined by Jones et a!. (1977) as a set of fault-bound crustal blocks with similar stratigraphy along the margin of western North America (Fig. 3.1). The Wrangellia flood basalts have been mapped as the Nikolai Formation in Alaska and Yukon (Northern Wrangellia) and the Karmutsen Formation on Vancouver and Queen Charlotte Islands (Southern Wrangellia). Three key aspects of Wrangellia led Jones and co-workers (1977) to suggest these crustal blocks formed as part of a once-contiguous terrane: thick sections of tholeiitic flood basalts directly overlie shale with Middle to Late Ladinian Daonella; similar-aged Late Triassic limestones overlie the flood basalts; and paleomagnetic evidence indicated that eruption of the flood basalts occurred at low latitude. Wrangellia may have joined with parts of the Alexander Terrane, primarily in southeast Alaska, as early as the Late Pennsylvanian (Gardner et aL, 1988) and may have been in close proximity to the Peninsular Terrane of southern Alaska by the Late Triassic (Rioux et al., 2007). Wrangellia extends 450 km in an arcuate belt in the Wrangell Mountains, Alaska Range, and Tailceetna Mountains in southern Alaska (Fig. 3.1). The northwest margin of Wrangellia is one of the most prominent geophysical features in south-central Alaska and is exposed along the Talkeetna Suture Zone (Glen eta!., 2007). The suture between Wrangellia and transitional crust to the northwest is well-defined geophysically by a series of narrow gravity and magnetic highs along the Talkeetna Suture Zone, between dense, strongly magnetic Wrangellia crust and less dense, weakly magnetic crust beneath flysch basins to the northwest (Glen et a!., 2007). This area lies directly in the axis of the major orocline of southern Alaska, where structures curve from northwest- to northeast- trending (e.g. Plafker eta!., 1994; Fig. 3.1). The Wrangellia terrane is bounded by the Denali Fault to the northeast and extends more than 300 km to the southeast in Yukon where Wrangellia stratigraphy is very similar to Alaska (Fig. 3.1). Wrangell Mountains Wrangellia stratigraphy is well-exposed in a northwest-trending belt extending l00 km along the southern flank of the Wrangell Mountains in Wrangell-St. Elias 89 Figure 3.1 Simplified map of south-central Alaska showing the distribution of the Nikolai Formation (black), derived from the GIS-based digital map compilations of Wilson et a!. (1998, 2005) and Schmidt (pers. comm, 2006). The three main areas that were studied are outlined with boxes and indicated in the legend. The inset shows the extent of the Wrangellia flood basalts (green) in Alaska, Yukon, and British Columbia. The red lines are faults. 90 National Park (Fig. 3.2). The Nikolai Formation disconformably overlies the Skolai Group, which comprises Pennsylvanian to Early Permian volcanic arc sequences and marine sediments of the Station Creek and Hasen Creek Formations, respectively (Smith and MacKevett, 1970; Fig. 3.2). In most areas, Nikolai basalts unconfomably overlie parts of the Hasen Creek Formation called the Golden Horn Limestone Lentil (<250 m thick), which forms prominent yellow- and red-stained cliffs of bioclastic Early Permian limestone. Isolated lenses of Daonella-bearing Middle Triassic argillite (<100 m thick) lie between the top of the Golden Horn Limestone and the base of the Nikolai Formation (Smith and MacKevett, 1970). The Skolai Group is intruded by mafic and ultramafic intrusive bodies related to the Nikolai basalts. The Nikolai basalts cover l057 km2 (4.2% of all Wrangellia flood basalts from Vancouver Island to central Alaska) within the McCarthy and Nabesna Quadrangles in Wrangell-St. Elias National Park and are approximately 3.5-4 km in total thickness. A cumulative thickness of over 3.5 km of marine sedimentary rocks, ranging in age from Late Triassic to Late Jurassic, overlies the Nikolai Formation in the Wrangell Mountains (MacKevett et al., 1997). Volcanic successions of the Miocene to Holocene Wrangell volcanic field unconformably overlie Jurassic and Cretaceous sedimentary sequences (MacKevett, 1978; Richter et al., 1990). Eastern Alaska Range The Nikolai Formation in the eastern Alaska Range and small areas of the Tailceetna Mountains covers 666 km2 (2.6% of all Wrangellia flood basalts) mostly in the Mount Hayes and Healy Quadrangles, and is 3.5-4 km thick in the Amphitheater and Clearwater Mountains (Fig. 3.1). Volcanic and marine sedimentary sequences similar to the Late Paleozoic successions in the Wrangell Mountains underlie the Nikolai basalts in the Alaska Range (Nokleberg eta?., 1994). In the Amphitheater Mountains, a major feeder system for the Nikolai Formation stratigraphically underlies flood basalt stratigraphy within a well-preserved synform that contains exposures of mafic and ultramafic intrusive units. These are the most significant occurrence of plutonic rocks associated with flood basalts within Wrangellia. The upper part of the volcanic stratigraphy contains interbedded volcanic and sedimentary horizons (argillite and 91 Figure 3.2 Geologic map and stratigraphy of the Wrangell Mountains, Alaska (location shown in Figure 1). (a) Stratigraphic column depicts Late Paleozoic to Jurassic units on the south side of the Wrangell Mountains, derived from Smith and MacKevett (1970) and MacKevett (1978). (b) Simplified map showing the distribution of the Nikolai Formation (green) in the Wrangell Mountains, derived from the GIS-based digital map compilation of Wilson et al. (2005). The four areas of field study are outlined with labelled boxes. The red lines are faults. 92 limestone) that give way to fine-grained sedimentary strata, which are poorly exposed in the Amphitheater Mountains. Age of the Nikolai Formation Biostratigraphy and geochronology provide bounds on the age and duration of emplacement of the Nikolai basalts. Fossil assemblages in finely laminated shale immediately beneath the Nikolai basalts in the Wrangell Mountains indicate a Middle to Late Ladinian age (McRoberts, pers. comm. 2007; Jones etal., 1977) and fossils in limestone disconformably overlying the Nikolai Formation are Late Carnian to Early Norian (Armstrong & MacKevett, 1977; Plaficer et al., 1989). Five 40Ar/39rplateau ages for hornblende and biotite from intrusive rocks in the Amphitheater Mountains in the Alaska Range interpreted to be comagmatic with Nikolai basalts indicate formation of these rocks at 23 1-225 Ma (Bittenbender et al., 2007; Schmidt & Rogers, 2007). Three Nikolai basalt samples from the Wrangell Mountains yielded40Ar/39r step-heating ages of 228.3 \u00C2\u00B1 5.2, 232.8 \u00C2\u00B1 11.5, and 232.4 \u00C2\u00B1 11.9 Ma (Lassiter, 1995). VOLCANIC STRATIGRAPHY AND PETROGRAPHY Field studies in Alaska focussed in three general areas where parts of the entire flood basalt stratigraphy are well-exposed: the southern flank of the Wrangell Mountains, and the Amphitheater and Clearwater Mountains in the southern part of the eastern Alaska Range (Fig. 3.1). In the Wrangell Mountains, field studies focussed in 4 areas: Skolai Creek, Glacier Creek, Hidden Lake Creek, and Nugget Creek (Fig. 3.2). The base of the Nikolai Formation is well-exposed near Skolai Creek, the type section for the underlying Paleozoic Skolai Group. At Skolai Creek, the base of the Nikolai Formation is basalt flow-conglomerate, pillow breccia, and minor pillow basalt. (Figs 3.2 and 3.3). Pebbles and cobbles comprise >30% of the basal flow-conglomerate and all the clasts appear to be derived from the underlying Skolai Group (Fig. 3.3). Middle to upper portions of the flood basalt stratigraphy are well-exposed above Glacier Creek, where massive maroon- and green-colored flows form monotonous sequences with amygdaloidal-rich horizons and no discernible erosional surfaces or sediments between flows (Fig. 3.4). The top of 93 Figure 3.3 Photographs of the base of the Nikolai Formation in the Wrangell Mountains, Alaska. (a) Westward-dipping Paleozoic arc volcanic rocks of the Station Creek Formation overlain by Early Permian shale and limestone (Phc-Hasen Creek Formation; Pgh-Golden Horn Limestone Lentil), isolated lenses of Middle Triassic \u00E2\u0080\u0098Daonella-beds\u00E2\u0080\u0099 (TRd), basalt flow-conglomerate with local pillows, and massive subaerial flows on the north side of Skolai Creek. Photograph by Ed MacKevett, Jr. (b) Close-up photograph of area b in photo a. (c) Close-up photograph of area c in photo b showing basal flow-conglomerate with clasts of rounded cobbles of white limestone (<20 cm) derived from Golden Horn Limestone Lentil and red basalt (<40 cm) from Station Creek Formation. Pen (14 cm) in middle ofphoto for scale. S 94 Figure 3.4 Photograph of \u00E2\u0080\u00944000 m of continuous flood basalt stratigraphy at the top of the Nikolai Formation along Glacier Creek in the Wrangell Mountains, Alaska. The yellow line marks the contact between Nikolai basalts and the overlying Chitistone Limestone. 95 the flood basalts are best exposed around Hidden Lake Creek where a sharp contact between Nikolai basalts and overlying Chitistone Limestone is mostly a smooth surface with minimal evidence of weathering (Fig. 3.5; Armstrong & MacKevett, 1982). Several occurrences of a thin zone (<1 m) of highly-oxidized subangular cobble-sized clasts of Nikolai basalt and minor thinly-bedded siltstone occur along the contact (Fig. 3.5). In the Amphitheater Mountains, fieldwork concentrated in five areas: Glacier Gap Lake, Landmark Gap Lake, Tangle Lakes (West), Sugarloaf Mountain, and Rainy Creek (Fig. 3.6). The Amphitheater Mountains are formed of exceptionally well-preserved flood basalt sequences flanked by associated underlying mafic and ultramafic plutonic rocks. South of the Eureka Creek fault is a broad synform with approximately 3.5 km of basaltic flows with basal sill complexes exposed along the outer margins (Fig. 3.6). The lower \u00E2\u0080\u0094\u00E2\u0080\u0098500 m of volcanic stratigraphy are submarine flows and subaerial flows comprise most of the remainder of the volcanic stratigraphy. Within the synform, several north-south- trending, U-shaped glacial valleys (e.g. Lower Tangle Lakes) provide excellent cross- sectional exposures of sediment-sill complexes and the base of the flood basalt stratigraphy (Figs 3.6 and 3.7). The lowest part of over 1000 m of continuous volcanic stratigraphy consists of unfossiliferous shale and siliceous argillite (<4 m thick) interbedded with massive mafic sills (2-30 mthick), in turn overlain by pillow basalt (Fig. 3.7). The basal pillowed flow is 13 m thick and pillows are typically <1 m in diameter with sediment between pillows along the base of the flow. Sills interbedded with thinly bedded basaltic sandstone and minor hyaloclastite also occur slightly higher in the stratigraphy, within the submarine stratigraphy (Figs 3.6 and 3.7). Middle and upper sections of the Nikolai Formation are massive amygdaloidal flows (mostly <15 m thick) with no discernible erosional surfaces or sediments between flows. A small segment of Wrangellia consisting of a heterogeneous assemblage of mafic and ultramafic plutonic and volcanic rocks forms a wedge between the Broxson Gulch Thrust and the Eureka Creek fault in the northern part of the Amphitheater Mountains (Fig. 3.6). A complex steeply-dipping sequence of picritic tuff and volcaniclastic rocks, mafic and ultramafic intrusives and dikes, and limestone occurs within several ridges near Rainy Creek. These units have distinct lithologic character from the volcanic stratigraphy of the Nikolai Formation south of the Eureka Creek Fault 96 :Figure 3.5 Photographs showing the top of the Nikolai Formation in the Wrangell Mountains, Alaska. (a) Chitistone Limestone overlying the Nikolai Formation above Hidden Creek in the Wrangell Mountains. Faulting has offset the contact. (b) Close-up photograph of uppermost Nikolai flow and base of the Chitistone Limestone from where photo a was taken. (c) Cobbles <10 cm long along the contact between the Chitistone Limestone and Nikolai Formation. The oxidized cobbles are subangular, closely packed, aligned along their long axis, and are glomeroporphyritic basalt identical to the uppermost flows of the Nikolai Formation. Sledgehammer handle (4 cm wide) for scale. 97 Amphitheater Mountains Figure 3.6 Simplified geologic map and stratigraphy of the Amphitheater Mountains, Alaska (location shown in Figure 3.1). (a) Stratigraphic column with sample lithologies and estimated vesicularity for flood basalts from the lower part of the volcanic stratigraphy, derived from three traverses marked by red lines in b. Vesicularity estimated visually from thin-sections. (b) Generalized geology of the Nikolai Formation and related plutonic rocks in the Amphitheater Mountains. Five main field areas are outlined with numbered boxes (see legend). Map derived Nokleberg et a!. (1992) and digital compilation of Wilson et aL (1998). (c) Schematic cross-section of Amphitheater Mountains from A to A? in panel b, adapted from Nokleberg et aL (1985). 98 neater Mountains, Alaska Range Figure 3.7 Photographs of the base of the Nikolai Formation in the Amphitheater Mountains, east-central Alaska Range (Tangle Lakes, West), Alaska. (a) Basal sill and sediments beneath submarine flows. Letters denote locations of other photographs. (b) Fissile shale (\u00E2\u0080\u00944 m thick) and a mafic sill from the lowermost exposure of shale. (c) Pillow basalt with shale between pillows lying directly above shale similar to photograph b. (d) Pillow basalt (pillow tubes are <1 m diameter in cross-section) in the lowermost flow (13 m thick) in the Tangle section. Photograph c is from the base of this flow. Sledgehammer (80 cm long) for scale. (e) Sequence of at least 4 massive sills (<8 m thick) interbedded with fine-grained tuff (<2 m thick). Tuff layers interbedded with sills contain plagioclase crystals (<0.5 mm), curvilinear shards, and local areas of volcanic breccia containing basaltic clasts with abundant small acicular plagioclase (<0.5 mm). 99 within the broad synform. A small suite of eight samples, including several highly altered olivine-bearing picritic tuffs, were collected for comparison to Nikolai basalts within the synform and are referred to as Rainy Creek picrites. In the Clearwater Mountains, a small mountain range 40 km west of the Amphitheater Mountains, the lowest level of exposure is pelagic sedimentary sequences interbedded with mafic sills, very similar to units in the Amphitheater Mountains (Fig. 3.8). The lowest flows of the Nikolai Formation are pillow basalt directly overlying thin beds of shale and argillite (<3 m thick) with sediment commonly filling interpillow voids in the lowermost pillows. Picritic pillow lavas have been found in the submarine stratigraphy in the Clearwater Mountains. Similar to the Amphitheater Mountains, the lower <400 m of the volcanic stratigraphy is submarine flows and the remainder of the stratigraphy is primarily subaerial flows. Upper parts of the volcanic stratigraphy contain subaerial flows (or sills) with columnar jointing, minor occurrences of tuff and volcanic breccia, and limestone and argillite lenses interbedded with flows are overlain by fine grained sediments with diagnostic index fossils (bivalve Halobia and ammonoid Tropite; Smith, 1981). A total of 111 samples of the Nikolai Formation and several Paleozoic, Late Mesozoic, and Cenozoic volcanic and sedimentary rocks were collected for petrography and geochemical analysis. Fifty-three of these samples were selected for geochemistry based on the visual degree of alteration (Table 3.1; 1-least altered, 3-intensely altered) and are grouped into high- and low-titanium basalts, sills, and picrites based on geochemistry. The high- and low-titanium basalts have similar petrographic textures with simple mineralogy and aphyric or glomeroporphyritic texture (Fig. 3.9; Table 3.1). Half of the 26 high-titanium basalts are glomeroporphyritic and about half of the low-titanium basalts are aphyric (Table 3.1). Phenocrysts and glomerocrysts in the Nikolai Formation are almost exclusively plagioclase and the primary minerals in the groundmass are plagioclase, clinopyroxene, and Fe-Ti oxide, and olivine is rarely present (Fig. 3.9; Table 3.1). High-titanium basalts have a higher proportion of Fe-Ti oxide than low-titanium basalts (Table 3.1). Several of the uppermost flows in the Wrangell Mountains are plagioclase-rich (>50% plagioclase laths \u00E2\u0080\u00941mm long with \u00E2\u0080\u00945:1 aspect ratio) with 100 Figure 3.8 Geologic map and stratigraphy of the Clearwater Mountains, Alaska. (a) Stratigraphic column with sample lithologies and estimated vesicularity for flood basalts from the lower part of the stratigraphy, derived from three traverses and maps of Smith (1973) and Silberling et a!. (1981). Vesicularity estimated visually from thin-sections. (b) Generalized geology of the Nikolai Formation and related plutonic rocks in the Clearwater Mountains, adapted from Silberling et a!. (1981). Sample locations are shown by red dots. Location of map shown in Figure 3.1. (c) Schematic cross-section of Clearwater Mountains from A to A\u00E2\u0080\u0099 in panel b, adapted from Silberling eta!. (1981). 101 Table 3.1 Summaiyofpefrographic characteristics and phenoc,yst proportions of Nikolai basalts in Alaska Sample\u00E2\u0080\u0099 Areab Flow\u00E2\u0080\u0099 Groupd Texture\u00E2\u0080\u0099 5710A2 WM FLO high-Ti glomero 5801A2 CL PLO high-Ti glomero 5806A3 GG PLO high-Ti aphyric, recrystallized 5708A2 WM PLO high-Ti trachylic, glomero 5801A9 CL PIL high-Ti aphyric, intersertal 5725A2 TA SIL?? high-Ti sill glomero 5716A2 WM PLO high-Ti intergranular, aphyric 5806A5 GG PLO high-Ti aphyric, intersertal 5716A3 WM PLO high-Ti plag-phyric, intersertal 5707A3 WM PLO high-Ti intergranular, intersertal 5710A3 WM PLO high-Ti glomero, intergranular 5719A6 WM PLO high-Ti glomero, intergranular 5719A5 WM PLO high-Ti glomero, intergranular 5801A5 CL PLO high-Ti glomero, intergranular 5806A6 GG PLO high-TI aphyric, intersertal 5712A2 WM PLO high-Ti glomero 5726A1 TA PIL high-Ti aphyric 5726A6 TA PIL high-Ti glomero, intersertal 5725A4 TA SIL?? high-Ti sill aphylic, ophimottled 5726A3 TA PLO high-Ti intersertal 5810AtO TA PIL high-Ti aphyiic, Intersertal 57\u00E2\u0080\u009915A1 WM PLO high-Ti trachylic, glomero 5714A1 WM PLO high-Ti trachylic, glomero 5716A1 WM PLO high-Ti plag-phyric 5726A2 TA PLO high-Ti aphyric, intersertal 5714A3 WM PLO high-Ti intergranular, glomero 5808A3 RC TUP RCPIC tuffaceous 5808A8 RC SIL RC tuffaceous 5808A2 RC TUP RCPIC tuffaceous 5808A1 RC TUP RC tutfaceous 5808A6 RC DIK RC interaranular 20 3 plag glcr <3 mm 20 1 abundant ox 10 3 abundant ox, few relict phenos 3 15 2 abundant ox, plag glcr <2 mm 3 aphyric 15 10 2 plag glcr <2mm, cpx <0.5mm 10 2 plag laths <1.5 mm, plag-rich, secondary mm 15 3 abundantox,aphyric 30 3 abundant plag phenos and glcr <4 mm 3 10 3 fewplag<2mm 10 15 3 plag glcr<3 mm, plag laths <1 mm, ox-rich 10 15 2 abundant ox, plag glcr <3 mm, plag-rich 10 15 1 abundant ox, plag glcr <3 mm, plag-rich 15 20 2 abundant ox, plag glcr <2 mm 3 plag <0.5 mm, cpx <0.5 mm 5 15 2 plag glcr <3 mm, plag laths <0.5 mm 2 vesicles 3%, very f.g., I plag glcr <3 mm 20 2 plag glcr <2 mm, plag needles <0.5 mm 7 2 plag <0.5 mm, cpx <0.5 mm 1 10 3 plag phenos <2 mm 5 1 plag <0.5 mm, cpx <0.5 mm 20 10 1 plag glcr <3 mm, aligned p1 <0.5 mm 10 10 2 plag glcr <3mm, aligned plag laths <0.5mm 5 5 3 plag <6 mm, very c.g., plag laths <1 mm 3 2 vesicles <0.5 mm, plag needles <0.5mm 10 3 3 c.g., abundant sec mi plag glcr <3 mm vnl \u00C3\u00B4i Plan Cnx Os Alferatlnna Notch 5715A5 WM PLO low-Ti relict glomero 5801A8 CL PLO low-Ti intergranular, aphyric 5802A5 CL PIL low-Ti intergranular, ophimotfied 5731A5 CL PIL low-Ti intergranular, ophimottied 5802A6 CL SIL low-Ti intergranular 5810A4 TA SIL low-Ti sill glomero, intersertal 5802A1 CL PIL low-Ti interaertal 5731A6 CL PIL low-Ti intergranular, intersertal 5810A6 TA PIL low-Ti glomero, intersertal 5727A3 TA SIL low-Ti sill subophitic 5810A1 TA PIL low-Ti aphyric, intersertal 5727A5 TA PIL low-Ti aphyric, intersertal 5727A7 TA PIL low-Ti aphyric, intersertal 5810A2 TA SIL low-Ti sill aphyiic, intersertal 5727A2 TA SIL low-Ti sill intergranular 5731A3 CL PIL low-Ti intergranular, intersertal 5802A3 CL PIL low-Ti aphyric, intersertal 5727A6 TA SIL low-Ti sill subophilic, ophimottied 5802A2 CL PIL low-Ti variolitic 5731A4 CL PIL low-Ti subophitic, ophimotfied 5811A1 TA GAB low-Ti sill intergranular 571 SAl WM BRE basal trachytic 5802A4 CL PIL CWPIC spherulitic 3 5 3 5 2 2 15 1 1 25 1 2 2 3 10 1 5 3 3 2 1 2 2 2 1 2 5 3 2 2 20 1 2 3 1 3 15 2 2 2 3 5 3 3 3 2 5 2 30 altered, abundant sec mm, plag glcr <3 mm blocky ox <0.5 mm aphyric aphyric plag laths <1 mm, cpx <1.5mm plag glcr <3 mm plag needles <0.5 mm, cpx <1.5mm plag <0.5 mm 10% vesicles, plag glcr <3 mm plag 8 wt% MgO have higher concentrations ofNi than the high-titanium basalts and the three picrite samples have noticeably higher Ni concentrations (525-620 ppm) than all basalts (Fig. 3.10). Both the high- and low-titanium basalts have a large range in CaO, which appears to be independent of MgO variation. A single Clearwater picrite (13.6 wt % MgO) and two Rainy Creek picrites (15.5-16.2 wt % MgO) have higher MgO with similar Ti02,FeOT, and alkali contents to the low-titanium basalts, however, the Rainy Creek picrites have notably lower A1203(Fig. 3.10). The basal flow-conglomerate from the Wrangell Mountains has distinct major-element chemistry compared to the other Nikolai basalts. 104 0(f) 5102 (%) D o QD oVoLc W70( i 600 500 .0000 0 \u00E2\u0080\u00A2_-8 dj:l . 0 2 4 6 8 10 12 4 6 8 10 12 MgO(wt%) 12 11 10 9 8 7 0 14 16 18 0 2 4 \u00E2\u0080\u0094 lillIllIll 111111111 \u00E2\u0080\u0094.\u00E2\u0080\u0094 . \u00E2\u0080\u00A2 x \u00E2\u0080\u00A2 high-titanium basalt 7 Na20 - T102 \u00E2\u0080\u00A2 high-titanium sill \u00E2\u0080\u00A2 + . 2.5 D low-titanium basalt 6 K20 alkalic - 0 low-titanium sill (wt%) 0 2.0 o 0 A ClearwaterpicriteRainy Creek picrite5 cP X basalfiow0 0 0 0 Yukon high-Ti basalt4. D tholeiitic 1.5 0 o Yukon low-Ti basalt 3 0 \u00E2\u0080\u00A200 i.o 2- 1- 0 a) \u00E2\u0080\u0098(b) 0 III I I I 1111111111 III 0.0 40 45 50 55 60 0 2 I I I I I I I I I I I I I I 4 6 8 10 12 14 16 18 MgO (wt%) I I CaO \u00E2\u0080\u00A2 (wt%) \u00E2\u0080\u00A2 (C) II 20 19 18 17 16 15 14 A1203 x \u00E2\u0080\u00A2 (wt%) x \u00E2\u0080\u0098III\u00E2\u0080\u0099 11111 III 0 0 C C o 000 \u00E2\u0080\u00A2 \u00E2\u0080\u00A2o \u00C2\u00B0 (e?I I i I I I 400 15 14 13 12 11 10 9 8 7 6 5 0 6 5 4 3 2 0 2 4 6 8 300 MgO (wt%) I I \u00E2\u0080\u00A2 i 200 0.Ni(ppm) A o C ;(d) \u00E2\u0080\u00A2J 100 6 8 10 12 14 16 18 MgO (wt%) Na20 X (wt%) 0 , . 1\u00E2\u0080\u00A21111\u00E2\u0080\u00A2 14 16 FeO fr) (wt%) 18 0 00 DC 0 00 I I I - I -- I - C C I I I I 6 8 10 12 14 16 18 MgO (wt%) MgO (wt%) Figure 3.10 Whole-rock major-element and Ni variation diagrams vs. MgO for the Nikolai Formation in Alaska with data for the Nikolai Formation in Yukon (see chapter 4). The boundary of the alkaline and tholeiitic fields is that of MacDonald and Katsura (1964). Total iron expressed as FeO, LOT is loss-on-ignition, and oxides are plotted on an anhydrous, normalized basis. Note the clear distinction between the high- and low-titanium basalts in panel b and the difference in alkali contents between the Nikolai basalts in Alaska and Yukon in panel a. 105 Table 3.2 Major element (wt% oxide) and trace element (ppm) abundances in whole rock samples of Nikolai basalts, Alaska. SAMPLE 5707A3 5708A2 5710A2 5710A3 5712A2 5714A1 5714A3 5715A1 (1) 5715A1 (2) 5715A5 Group HI-TI HI-TI HI-TI HI-TI HI-TI HI-TI HI-TI HI-TI HI-TI LOW-TI Area WM WM WM WM WM WM WM WM WM WM Flow FLO PLO FLO FLO PLO FLO FLO FLO PLO FLO UTM EW 6816521 6813830 6838676 6836850 6838178 6826371 6826390 6827794 6827794 6828050 UTM NS 430777 428595 356035 356266 353987 384783 384806 383399 383399 383641 Unnormalized Major Element Oxides (Weight %): Si02 49.74 48.40 48.81 50.02 49.90 49.31 50.21 49.61 49.41 50.25 TiO2 1.96 2.36 2.4 1.95 1.89 1.66 1.6 1.69 1.68 1.14 A12O3 13.37 13.49 15.21 14.45 13.76 15.58 16.01 15.56 15.75 16.69 Fe2O3* 14.05 16.14 12.94 13.39 13.76 13.21 10.99 13 12.97 10.39 MnO 0.27 0.27 0.23 0.18 0.24 0.17 0.16 0.2 0.2 0.18 MgO 6.69 5.89 6.85 6.56 6.74 6.6 6.64 6.63 6.62 7.94 CaO 10.61 10.09 9.16 8.56 11.35 10.8 9.97 11.09 11.04 9.48 Na2O 2.64 2.47 3.15 3.56 1.87 2.08 3.77 1.84 1.82 3.36 1<20 0.29 0.61 0.58 1.03 0.22 0.42 0.28 0.2 0.2 0.67 0.16 0.23 0.21 0.18 0.16 0.15 0.14 0.15 0.14 0.09 LOI 1.68 1.41 2.37 2.47 1.28 2.39 3.12 0.84 1.84 3.57 Total 99.78 99.95 99.54 99.88 99.89 99.98 99.77 99.97 99.83 100.19 Trace elements (ppm): La 10.32 8.08 6.56 6.56 3.74 Ce 24.25 21.79 17.07 17.07 10.69 Pr 3.66 2.90 2.35 2.35 1.49 Nd 18.83 14.73 12.20 12.20 7.68 Sm 5.22 4.19 3.51 3.51 2.32 Eu 1.73 1.43 1.25 1.25 0.90 Gd 5.57 4.17 3.80 3.80 2.55 Tb 1.02 0.77 0.72 0.72 0.50 Dy 6.84 5.04 4.77 4.77 3.34 Ho 1.35 0.99 0.98 0.98 0.69 Er 4.02 2.85 2.86 2.86 1.98 Tm 0.51 0.33 0.36 0.36 0.25 Yb 3.58 2.37 2.59 2.59 1.77 Lu 0.53 0.36 0.40 0.40 0.28 Sc 44.06 42.52 41.17 41.17 38.50 V 362 411 349 337 346 329 276 354 352 249 Cr 165 62 167 119 133 156 282 165 167 460 Co 47 45 44 44 39 Ni 75 57 89 69 73 82 81 85 85 116 Cu 19 1067 126 126 64 Zn 121 176 126 108 113 109 88 110 111 68 Ga 19 20 21 19 19 19 17 20 19 15 Rb 3.4 9.7 6.1 15 2.2 5.2 2.6 1 1 8.5 Sr 237 162 263 481 184 334 394 184 184 437 Y 28.1 36.1 31.6 27.8 25.8 26.2 22.6 26 26.2 18.3 Zr 113 148 145 115 110 99 93 98 98 60 Nb 10.1 12.5 12.2 9.7 9.9 7.7 7.3 7.7 7.7 4.4 Cs 0.08 0.14 0.28 0.28 0.83 Ba 79 108 147 246 70 63 56 55 56 129 Hf 4.10 3.15 2.78 2.78 1.79 Ta 0.74 0.67 0.48 0.48 0.30 Pb 1.59 0.81 0.52 0.52 0.44 Th 0.98 0.72 0.64 0.64 0.34 U 0.31 0.20 0.19 0.19 0.09 La(XRF) 6 7 8 7 6 6 4 6 6 2 Ce(XRF) 21 27 24 22 21 16 13 17 16 8 Abbreviations for group are: HI-TI, hIgh-titanium; LOW-TI, low-titanium; RC, Rainy Creek; RCPIC, Rainy Creek picnte; CWPIC, Clearwater picrite. Abbreviations for flow are: PLO, massive flow; PIL, pillow basalt; FLO-BRE, flow-conglomerate-pillow breccia; SIL, sill; TUF, tuff. Abbreviations for area are: WM, Wrangell Mountains; TANGLE, Tangle Lakes; GLAC, Glacier Gap Lake; CLEAR, Clearwater Mountains; RAINY, Rainy Creek. Sample locations are given using the Universal Transverse Mercator (UTM) coordinate system (NAD83; zones 6 and 7). XRF analyses were performed at University of Massachusetts Ronald B. Gilmore XRF Laboratory. Fe2O3*is total iron expressed as Fe2O3.LOI is loss-on-ignition. Elements by XRF: Sc, V, Cr, Ni, Zn, Ga, Rb, Sr, Y, Zr, Nb, Ba. Elements by ICP-MS: REE, Co, Cu, Cs, Hf, Ta, Pb, Th, U. 106 SAMPLE 5716A1 5716A2 5716A3 5719A1 5719A5 (1) 5719A5 (2) Group HI-TI HI-TI HI-TI LOW-TI HI-TI HI-TI Area WM WM WM WM WM WM Flow FLO FLO FLO FLO-BRE FLO FLO UTM EW 6824771 6824809 6825071 6840607 6840632 6840632 UTM NS 384884 384800 384853 430357 430204 430204 Unnormalized Major Element Oxides (Weight %): SIO2 50.89 50.52 50.76 55.72 49.54 49.62 TiO2 1.66 2.12 1.96 0.67 1.92 1.93 A1203 15.82 13.56 15.57 18.59 14.57 14.55 FC2O3* 12.03 14.22 13.37 7.84 12.84 12.86 MnO 0.18 0.19 0.18 0.16 0.21 0.21 MgO 6.13 7.83 4.8 3.37 6.52 6.55 CaO 7.94 7.01 8.1 6.07 11.47 11.5 Na20 4.26 4.52 4.71 5.14 2.01 2.02 K20 0.58 0.08 0.23 2.18 0.29 0.3 0.15 0.18 0.19 0.38 0.17 0.17 LOI 2.96 3.57 3.54 5.14 1.05 1.09 Total 99.64 100.23 99.87 100.12 99.54 99.71 Trace elements (ppm): La 18.10 9.13 9.13 Ce 36.95 21.43 21.43 Pr 4.56 3.05 3.05 Nd 20.65 16.26 16.26 Sm 4.26 4.45 4.45 Eu 1.31 1.48 1.48 Gd 3.96 4.58 4.58 Tb 0.52 0.79 0.79 Dy 3.40 5.31 5.31 Ho 0.66 0.97 0.97 Er 2.12 2.94 2.94 Tm 0.26 0.35 0.35 Yb 1.94 2.46 2.46 Lu 0.31 0.36 0.36 Sc 11.02 41.37 41.37 V 289 393 319 170 366 367 Cr 157 133 70 0 162 160 Co 16 42 42 Ni 66 61 47 1 77 76 Cu 25 167 167 Zn 94 97 82 83 104 104 Ga 15 17 21 17 19 19 Rb 5.8 0.6 2 25.4 5.8 5.9 Sr 321 56 97 158 200 200 V 26.4 30.8 28.5 19.5 26.3 26.1 Zr 98 120 118 81 113 113 Nb 7.6 9.7 10.2 6.3 10.1 10 Cs 0.06 0.09 0.09 Ba 115 14 47 1277 94 91 Hf 2.30 3.23 3.23 Ta 0.36 0.63 0.63 Pb 2.96 0.85 0.85 Th 4.18 0.87 0.87 U 1.91 0.25 0.25 La(XRF) 4 4 7 17 7 8 Ce(XRF) 16 18 18 34 23 23 5719A6 5725A2 5725A4 5726A1 HI-TI HI-TI HI-TI HI-TI WM TANGLE TANGLE TANGLE FLO SIL SIL PIL 6840632 7001750 7002024 7002334 430204 556168 555867 553245 49.76 50.53 50.43 49.01 1.92 2.19 1.82 1.88 14.45 14.98 14.31 13.96 12.79 11.73 12.36 13.13 0.21 0.2 0.2 0.23 6.65 5.99 6.66 7.07 11.45 11.74 12.13 11.74 2.12 1.99 1.78 2.26 0.24 0.46 042 0.25 0.17 0.21 0.16 0.16 2.66 1.26 1.03 1.88 99.76 100.02 100.27 99.69 8.57 7.52 21.35 19.80 3.00 2.74 15.16 14.35 4.30 4.07 1.43 1.08 4.32 4.15 0.77 0.74 4.89 4.92 0.94 0.91 2.69 2.59 0.32 0.30 2.17 2.03 0.30 0.27 38.05 41.02 340 358 325 359 164 140 127 217 42 42 78 68 80 90 160 138 104 96 101 110 19 20 20 17 4.1 16.2 16.5 2.1 275 213 207 239 25.6 29.6 24.3 24.3 112 140 107 110 9.9 12.1 9.2 9.4 0.53 0.07 85 245 83 191 3.08 2.66 0.57 0.52 0.66 0.95 0.83 0.86 0.25 0.23 8 8 5 5 21 23 19 18 107 SAMPLE 5726A2 5726A3 5726A6 5727A2 5727A3 5727A5 5727A6 (1) 5727A6 (2) 5727A7 5731A3 Group HI-TI HI-TI I-Il-TI LOW-TI LOW-TI LOW-TI LOW-TI LOW-TI LOW-TI LOW-TI Area TANGLE TANGLE TANGLE TANGLE TANGLE TANGLE TANGLE TANGLE TANGLE CLEAR Flow FLO FLO PIL SIL SIL PIL SIL SIL PIL PIL UTM EW 7002466 7002475 7002270 6999661 6999663 6999658 6999646 6999646 6999646 6993085 UTM NS 553936 553826 553495 550468 550377 550351 550324 550324 550270 483499 Unnarmalized Major Element Oxides (Weight %): S1O2 51.41 49.90 50.11 50.70 50.97 47.43 48.89 48.89 46.13 50.94 T1O2 1.63 1.81 1.85 0.54 0.58 0.57 0.48 0.48 0.56 0.53 A12O3 13.83 14.73 14.74 14.49 15.85 15.81 15.06 15.06 17.97 15.51 FC2O3* 12.16 12.34 11.71 11.21 10.79 11.04 10.51 10.51 11.84 9.75 MnO 0.19 0.21 0.19 0.19 0.19 0.19 0.18 0.18 0.17 0.17 MgO 6.96 6.72 6.75 9.41 7.75 10.35 11.51 11.51 9.09 10.52 CaO 11.25 11.96 12.7 12.3 11.96 13.12 12.43 12.43 12.78 10.68 Na2O 2.2 1.95 1.94 1.09 1.76 1.06 0.79 0.79 1.19 1.22 K2O 0.11 0.09 0.13 0.06 0.1 0.09 0.05 0.05 0.1 0.22 P2O5 0.14 0.16 0.16 0.04 0.05 0.07 0.06 0.06 0.07 0.1 LOI 1.6 1.35 1.29 2.18 2.3 2.87 3 3 2.75 3.15 Total 99.88 99.87 100.28 100.03 100.00 99.73 99.96 99.96 99.90 99.64 Trace elements (ppm): La 1.13 1.13 1.25 1.27 2.56 Ce 3.19 3.22 3.56 3.58 5.66 Pr 0.49 0.51 0.54 0.56 0.80 Nd 2.92 2.92 3.05 3.09 3.86 Sm 1.22 1.23 1.11 1.13 1.30 Eu 0.56 0.49 0.37 0.36 0.67 Gd 1.47 1.47 1.33 1.37 1.63 Tb 0.36 0.34 0.34 0.32 0.38 Dy 2.62 2.60 2.35 2.37 2.80 Ho 0.56 0.60 0.55 0.53 0.64 Er 1.67 1.70 1.63 1.58 1.93 Tm 0.22 0.22 0.21 0.21 0.25 Yb 1.61 1.61 1.53 1.53 1.91 Lu 0.27 0.26 0.25 0.24 0.33 Sc 44.90 43.02 45.67 49.00 50.33 V 324 332 333 242 254 257 233 233 268 217 Cr 118 148 136 555 269 455 521 521 199 605 Co 51 44 54 54 57 Ni 79 84 85 178 106 157 214 214 163 191 Cu 96 116 98 96 131 Zn 102 100 84 81 77 84 74 74 92 68 Ga 18 20 19 14 14 13 12 12 15 12 Rb 0.7 0.4 3.4 1.7 1.8 1.2 0.6 0.6 1.5 4.6 Sr 195 178 198 76 117 146 123 123 155 205 Y 22.2 24.6 24.7 15.1 15.5 15.8 14 14 17.2 17.2 Zr 94 106 109 23 24 17 15 15 20 38 Nb 8 92 9.4 0.9 0.9 1 0.8 0.8 0.8 2.2 Cs 0.36 0.28 0.12 0.12 0.23 Ba 45 47 48 98 76 130 46 46 54 125 Hf 0.80 0.87 0.57 0.57 0.70 Ta 0.04 0.05 0.04 0.04 0.05 Pb 0.55 0.50 0.20 0.18 1.21 Th 0.17 0.17 0.07 0.07 0.29 U 0.07 0.07 0.03 0.03 0.10 La(XRF) 6 7 6 0 1 1 1 1 2 4 Ce(XRF) 17 18 19 2 2 2 3 3 3 11 108 Trace elements (ppm): La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Sc V Cr Co Ni Cu Zn Ga Rb Sr V Zr Nb Cs Ba Hf Ta Pb Th U La(XRF) Ce(XRF) 49.75 1.91 15.17 13.13 0.21 5.76 11.84 1.69 0.28 0.17 1.25 99.91 8.65 21.65 3.07 15.51 4.25 1.46 4.40 0.80 5.22 1.01 2.93 0.35 2.50 0.36 38.99 358 58 41 56 182 111 20 5.9 187 26.2 113 10.3 0.09 70 3.17 0.61 0.72 0.84 0.24 7 20 50.82 49.97 49.18 1.09 2.33 0.71 15.06 13.95 16.27 12.51 12.28 10.82 0.22 0.19 0.19 7.16 6.69 9.41 10.44 11.77 10 1.99 1.85 2.82 0.27 0.38 0.63 0.15 0.22 0.17 2.13 1.5 3.64 99.71 99.63 100.20 3.00 6.92 1.00 4.04 1.31 0.45 1.57 0.34 2.53 0.57 1.76 0.23 1.75 0.39 50.28 285 330 265 247 69 129 360 364 44 127 89 68 11 2.1 303 15.3 23 1.3 0.30 118 73 186 44 0.78 0.08 0.21 0.56 0.20 6 5 6 3 13 21 13 6 242 346 151 70 13 16.9 260 14.9 29 1.7 198 4 8 5801A5 HI-TI CLEAR FLO 6992750 480328 5801A8 5801A9 5802A1 LOW-TI HI-TI LOW-TI CLEAR CLEAR CLEAR FLO PIL PIL 6992452 6992443 6993172 480516 480528 483888 5802A2 LOW-TI CLEAR PIL 6993140 483867 49.94 0.48 15.33 10.44 0.19 8.85 12.67 1.88 0.11 0.06 3.43 99.95 5802A3 LOW-TI CLEAR PIL 6993104 483847 47.76 0.52 16.76 10.42 0.18 9.85 12.31 1.2 0.59 0.06 2.7 99.65 SAMPLE 5731A4 5731A5 5731A6 5801A2 Group LOW-TI LOW-TI LOW-TI HI-TI Area CLEAR CLEAR CLEAR CLEAR Flow PIL PIL PIL FLO UTMEW 6993131 6993230 6993320 6992862 UTM NS 483495 483531 483566 480105 Unnormalized Major Element Oxides (Weight %): Sb2 48.61 48.12 49.13 50.00 TiO2 0.48 0.92 0.64 2.37 A12O3 15.89 16.1 15.9 14.02 Fe203* 10.32 11.5 9.69 13.68 MnO 0.19 0.19 0.16 0.21 MgO 9.63 9.67 8.37 6.08 CaO 12 11.71 13.54 11.77 Na2O 1.87 1.35 2.13 1.74 1<20 0.63 0.28 0.29 0.13 0.06 0.1 0.16 0.22 LOI 3.1 3.34 3.77 1.26 Total 99.68 99.94 100.01 100.22 12.70 28.04 4.14 21.10 5.55 1.79 5.49 0.93 6.21 1.13 3.25 0.37 2.66 0.38 37.45 232 256 237 386 373 578 412 211 43 141 232 137 80 207 67 72 62 115 12 15 13 20 15.9 6.5 3.3 1.7 392 272 484 225 13.8 20.4 16.2 29.4 26 45 35 146 1.4 1.6 2.8 13.1 0.12 173 108 190 57 4.03 0.76 1.01 1.24 0.36 3 2 6 9 7 9 14 31 56 81 142 118 95 73 16 19 12 4.6 5 16.3 148 190 485 26.7 24.8 18.1 59 109 40 3.9 9.4 3.1 109 SAMPLE 5802A4 (1) 5802A4 (2) 5802A5 5802A6 5806A3 5806A5 5806A6 5808A1 5808A2 5808A3 Group LOW-TI CWPIC LOW-TI LOW-TI HI-TI HI-TI HI-TI RC RCPIC RCPIC Area CLEAR CLEAR CLEAR CLEAR GLAC GLAC GLAC RAINY RAINY RAINY Flow PIL PIL PIL SIL? FLO FLO FLO TUF TUF TLJF UTMEW 6993057 6993057 6993004 6992950 7000695 7000232 6999795 7021484 7021213 7021298 UTM NS 483801 483801 483762 483733 538926 538787 538622 556534 556314 556392 Unnormalized Major Element Oxides (Weight %): Si02 48.26 48.28 46.68 51.77 49.67 49.86 50.25 49.93 46.44 46.89 Ti02 0.5 0.5 0.93 0.9 2.37 1.97 1.89 1.01 1.06 1.18 A1203 15.81 15.83 16.19 14.82 13.98 14.58 13.96 14.09 10.72 10.72 Fe203* 10.73 10.75 12.21 11 13.53 12.98 12.79 13.4 10.72 10.72 MnO 0.19 0.19 0.2 0.19 0.21 0.21 0.21 0.22 0.15 0.18 MgO 13.49 13.47 1021 7.95 6.4 6.6 6.62 7.31 15.42 14.78 CaO 9.2 9.19 12.16 11.72 10.36 10.83 12.24 11.9 9.59 9.42 Na20 1.48 1.51 1.23 147 2.14 2.25 1.59 1.61 1.34 0.88 1<20 0.05 0.05 0.12 0.3 0.68 0.57 0.29 0.31 0.35 1.12 P205 0.09 0.09 0.1 0.09 0.22 0.17 0.17 0.1 0.09 0.11 LOl 4.51 4.53 3.32 1.56 1.82 1.62 1.67 1.98 3.2 2.4 Total 99.80 99.86 100.03 100.21 99.56 100.02 100.01 99.88 99.86 100.01 Trace elements (ppm): La 5.18 5.18 3.58 5.99 6.52 Ce 12.40 12.40 9.28 15.32 18.08 Pr 148 148 1.32 1.96 2.33 Nd 6.61 6.61 6.68 9.85 11.57 Sm 1.69 1.69 2.19 2.69 3.01 Eu 0.51 0.51 0.84 0.94 1.09 Gd 1.84 1.84 2.53 2.77 3.02 Tb 0.40 0.40 0.54 0.51 0.54 Dy 2.62 2.62 3.76 3.25 3.23 Ho 0.56 0.56 0.79 0.66 0.63 Er 1.76 1.76 2.30 1.79 1.79 Tm 0.23 0.23 0.29 0.21 0.22 Yb 1.69 1.69 2.12 1.48 1.45 Lu 0.27 0.27 0.33 0.22 0.25 Sc 40.80 40.80 45.91 2729 29.39 V 202 203 257 254 369 339 327 313 285.54 297.75 Cr 1596 1577 633 448 213 141 123 138 1327 1250 Co 60 60 58 67 63 Ni 539 538 244 128 89 87 78 76 620 525 Cu 78 78 103 123 79 Zn 74 74 82 85 119 112 101 111 60 63 Ga 13 13 16 16 21 20 19 16 13 13 Rb 0.4 0.5 3 8.8 13.9 11.8 4.9 5.8 6.11 2643 Sr 183 184 252 188 215 208 205 185 126.86 92.53 Y 15.4 15.2 20.7 18.5 29.1 25.4 25.4 28.5 16.01 15.81 Zr 35 35 46 49 144 116 112 41 58.73 65.10 Nb 2 2.1 1.5 3 12.9 10.4 9.6 5.3 6.00 8.11 Cs 0.50 0.50 0.30 0.43 1.10 Ba 39 37 40 92 182 118 108 140 72 191 Hf 1.05 1.05 1.51 1.71 1.88 Ta 0.11 0.11 0.08 0.35 048 Pb 0.13 0.13 0.50 0.76 0.65 Th 0.89 0.89 0.34 0.70 1.06 U 0.32 0.32 0.11 0.26 0.38 La(XRF) 4 5 2 2 9 7 6 3 6 7 Ce(XRF) 7 11 4 4 27 21 19 7 15 18 110 SAMPLE 5808A6 5808A8 5810A1 (1) 581 OAI (2) 5810A2 5810A4 58106(1) 5810A6(2) 5810A10 5811A1 Group RC RC LOW-TI LOW-TI LOW-TI LOW-TI LOW-TI LOW-TI HI-TI LOW-TI Area RAINY RAINY TANGLE TANGLE TANGLE TANGLE TANGLE TANGLE TANGLE TANGLE Flow DIK SIL PIL PIL SIL SIL PIL PIL PIL SIL UTMEW 7021445 7021631 6999686 6999686 6999760 6999733 6999748 6999748 7000019 6999515 UTMNS 556560 557160 550380 550380 550367 550310 550268 550268 550092 550374 Unnormalized Major Element Oxides (Weight %): SiO2 49.27 50.04 47.02 46.82 48.39 48.45 48.75 48.69 50.27 49.18 Ti02 0.93 1.14 0.57 0.57 0.54 0.73 0.64 0.64 1.74 0.35 A1203 14.71 13.39 15.84 15.8 16.2 17.2 16.2 16.1 14.61 15.84 Fe203* 10.76 15.35 10.94 10.96 11.32 11.81 12.01 12.03 12.04 9.09 MnO 0.2 0.25 0.18 0.18 0.2 0.2 0.24 0.24 0.19 0.17 MgO 8.88 6.57 11.23 11.16 9.96 7.32 6.34 6.26 6.71 11.93 CaO 11.31 11.27 13.14 13.09 12.21 12.5 13.95 13.98 12.81 12.38 Na2O 1.73 1.52 1.19 1.04 1.22 1.25 1.86 1.77 1.3 0.88 K2O 1.57 0.06 0.06 0.07 0.07 0.06 0.15 0.15 0.14 0.07 0.27 0.11 0.07 0.07 0.06 0.12 0.08 0.08 0.15 0.05 LOI 2.16 1.96 2.68 2.65 2.62 2.19 1.73 1.72 1.49 3.27 Total 99.63 99.70 100.24 99.76 100.17 99.64 100.22 99.94 99.96 99.94 Trace elements (ppm): La 1.16 1.16 1.94 3.45 2.30 2.30 7.81 Ce 3.40 3.40 4.53 8.87 5.88 5.88 20.71 Pr 0.57 0.57 0.61 1.20 0.83 0.83 2.83 Nd 3.31 3.31 3.21 6.12 4.52 4.52 13.98 Sm 1.26 1.26 1.16 1.96 1.55 1.55 3.95 Eu 0.43 0.43 0.46 0.73 0.51 0.51 1.45 Gd 1.51 1.51 1.46 2.15 1.79 1.79 3.94 Tb 0.36 0.36 0.35 0.45 0.42 0.42 0.76 Dy 2.59 2.59 2.53 3.28 2.94 2.94 4.63 Ho 0.57 0.57 0.56 0.72 0.64 0.64 0.95 Er 1.74 1.74 1.78 2.13 1.95 1.95 2.50 Tm 0.24 0.24 0.23 0.28 0.26 0.26 0.29 Yb 1.70 1.70 1.75 1.99 1.88 1.88 2.05 Lu 0.27 0.27 0.27 0.32 0.33 0.33 0.32 Sc 50.28 50.28 47.06 41.96 43.29 43.29 35.80 V 289 363 261 260 253 271 261 262 308 182 Cr 629 58 493 491 182 81 131 129 133 380 Co 53 53 50 45 43 43 41 Ni 137 56 208 207 163 100 91 92 88 274 Cu 98 98 110 116 85 85 151 Zn 102 127 74 73 78 88 98 98 100 64 Ga 18 17 13 13 13 15 14 14 19 10 Rb 18.3 0.5 0.9 0.6 0.8 0.4 2 1.9 2.6 0.9 Sr 801 58 151 150 141 198 170 170 231 114 Y 16.7 32.6 15.5 15.5 15.5 18.5 17.2 17.2 23 11.8 Zr 65 43 18 17 19 28 26 26 102 19 Nb 4 4.8 1.1 1 1 1.4 1.3 1.3 8.8 1.3 Cs 0.17 0.17 0.12 0.12 0.18 0.18 0.09 Ba 674 39 62 62 62 33 75 78 59 102 Hf 0.66 0.66 0.66 0.96 0.88 0.88 2.84 Ta 0.04 0.04 0.05 0.06 0.10 0.10 0.59 Pb 0.23 0.23 0.44 0.26 0.63 0.63 0.76 Th 0.09 0.09 0.27 0.30 0.28 0.28 0.78 U 0.04 0.04 0.10 0.09 0.12 0.12 0.24 La(XRF) 8 3 1 0 2 3 3 0 6 2 Ce(XRF) 20 7 3 1 3 7 2 4 19 3 111 The Nikolai Formation in Alaska has similar major- and trace-element chemistry to Nikolai basalts from Yukon, which also have high- and low-titanium basalts (Fig. 3.10). Nikolai basalts in Alaska are notably lower in total alkalis than basalts from Yukon due to alkali metasomatism during alteration of the Yukon basalts (see chapter 4). The low-titanium basalts are characterized by a range of flat and slightly light rare earth element (LREE)-depleted chondrite-normalized REE patterns (mean La/SmcN=0.8 \u00C2\u00B1 0.5, except one LREE-enriched sample) with flat, parallel heavy REE (HREE) segments (mean Dy/YbCN=l.0 \u00C2\u00B1 0.2; Fig. 3.11). The high-titanium basalts form a tight range of parallel LREE-enriched patterns (mean LaJYbCN=2.3 \u00C2\u00B1 0.9) with higher REE abundances (mean YbCN= 16.1 \u00C2\u00B1 6.4) than the low-titanium basalts (mean Ybc=1 1.0 \u00C2\u00B1 2.2; Fig. 3.11). Several low-titanium basalts from the Amphitheater Mountains have positive Eu anomalies (Fig. 3.11). Several high-titanium basalts from the Wrangell Mountains have patterns with flatter HREE segments. The basal flow-conglomerate from the Wrangell Mountains has a distinct LREE-enriched pattern (LaJSmcN= 2.7) with a flat HREE segment (DyJYbCN=1.2). The Rainy Creek picrites are LREE-enriched (LaJYbcN=2.8-3.l) and the REE pattern of the Clearwater picrite is LREE-enriched (LaJSmCN= 1.9) with a flat HREE segment (Dy/YbCN=l.0). The low-titanium basalts have mostly parallel, primitive mantle-normalized trace- element patterns with pronounced HFSE depletions, especially for Nb, Ta, and Zr, relative to LILE and REE (Fig. 3.11), and HFSE (Nb and Zr) form linear trends (Fig. 3.12). The large negative Zr anomalies in the low-titanium basalts are not accompanied by comparably low Hf (Fig. 3.1 id). The LILE segments of trace-element patterns for low-titanium basalts are parallel and each of the patterns has a pronounced positive Sr anomaly, relative to Nd and Sm. The high-titanium basalts form a tight range of parallel, concave-downward trace-element patterns with negative Pb anomalies, relative to Ce and Pr, and small negative K anomalies, relative to U and Nb (Fig. 3.11). The LILE in the high-titanium basalts are slightly depleted relative to the HFSE and LREE. The Clearwater picrite is depleted in HFSE with a positive Sr anomaly, similar to the low- titanium basalts, whereas Rainy Creek picrites have gently negative-sloping patterns with negative Sr anomalies (Fig. 3.11). 112 4? C 0 10 4? a Ce vs titanium basalts 9000 0 Wrangell Mountains a Amphitheater Mountains A clearwater Mountains (a) 100 4? C : 0. 4? a a 4? vs High-titanium basalts 000 Cs Rb8a ThU K NbTa La CePb Pr NdSrSmZr H 0 EuGdThDyHoY ErYbLuLa Ce Pr Nd PmSm Eu 0dm Dy Ho Er Tm Yb Lu 4? 0 C 0 10 a a 4? vs 0 e basal flow Low-titanium basalts o0 / 0 0 (c) 4? C Ce a 0. 4? a E Ce vs 100 15 La Ce Pr Nd Pm Sm Eu Gd Th Dy Ho & Tm Yb Lu 100 4? C 0 U 4? a aCe vi CsRb8aThU KNbTuLaCePbPrNdSrSmZrFfTiEuGdThDyHoY &YbLu a? C a 0. 4? a a Ce vs 10 La Ce Pr Nd Pm Sm Eu Gd Tb Dy Ho Er Tm Yb Lu CsRbBaThU K NbTaL.aCePbPrNdSrSmZr HfT1 EuGdThDyHoY ErYbLu Figure 3.11 Whole-rock REE and other incompatible-element concentrations for the Nikolai Formation in Alaska. (a), (c), and (e) are chondrite-normalized REE patterns for high-titanium basalts, low-titanium basalts, and picrites, respectively, from field areas in southern Alaska. (b), (d), and (f) are primitive mantle-normalized trace-element patterns for high-titanium basalts, low-titanium basalts, and picrites, respectively. All normalization values are from McDonough and Sun (1995). Depleted MORB average from Salters and Stracke (2004). Note the clear distinction between LREE-enriched high-titanium basalt and mostly LREE-depleted low-titanium basalts. Trace-element patterns for the basal flow-conglomerate and one Clearwater sample are not shown in panel d for clarity. 113 IIT I I-I- I I I I I I I \u00E2\u0080\u0094I--\u00E2\u0080\u0094 3.0 150 \u00E2\u0080\u0098 0 2 4 6 8 10 12 14 16 18 Figure 3.12 Whole-rock trace-element concentration variations and ratios for the Nikolai Formation in Alaska (except panel b is versus MgO), with data for the Nikolai Formation in Yukon. (a) Nb vs. Zr. (b) Nb/La vs. MgO. (c) Th vs. Hf. (d) Sr vs. Y/Nb. Note the clear distinction between the high- and low-titanium basalt in each of the plots. j 12 Nb (ppm) 10 8 6 C jD 4. C Co 2 0 50 2.5 2.0 1.5 1.0 0,5 \u00E2\u0080\u00A2 high-titanium basalt Nb/La \u00E2\u0080\u00A2 high-titanium sill \u00E2\u0080\u00A2 D low-titanium basalt 0 low-titanium sill A Clearwaterpicrite 0 RainyCreekpicrite \u00E2\u0080\u00A2 \u00E2\u0080\u00A2Ij X basal flow 0 Yukon high-Ti basalt Q C Yukon low-Ti basalt 0 -0-- \u00E2\u0080\u0094 0 x \u00E2\u0080\u00A2 (b) I I I I I I I I I I I I I I I I 0 2.0 100 Zr (ppm) 1.5 MgO(wt%) 1.0 0.5 I, IQ I I I Sr (ppm) 400 \u00E2\u0080\u00A2 C I 300 C \u00E2\u0080\u00A2 C C 200 4. QD 0C C o $ 0 \u00E2\u0080\u00A2 C I I I . 0.0 0 100 1 2 3 4 5 Hf (ppm) (d) 0 0 5 10 Y/Nb 15 20 114 Sr-Nd-Hf-Pb isotopic compositions The high- and low-titanium Wrangellia basalts in Alaska have distinct Hf and Sr isotope ratios (Fig. 3.13). The low-titanium basalts have higher initial EHf (+ 13.7 to + 18.4) and 875r/6Sr(0.70422-0.70554) than the high-titanium basalts (initial 8H= +9.7 to +10.7, initial87Sr/6r= 0.70302-0.70376), except for sample 571 5A5, which has anomalous chemistry (Fig. 3.13; Tables 3.3 and 3.4). The low-titanium basalts have a narrower range in initial 8Nd (+4.6 to +5.4) than the high-titanium basalts (initial8Nd= +6.0 to +8.1) and a wider range in initial 8Hf, except for three low-titanium samples with higher initial 8Nd (+7.0 to +7.6). The basal flow-conglomerate in the Wrangell Mountains has similar initial Sr, Nd, and Hf isotopic compositions to the high-titanium basalts. The Clearwater picrite lies between the high- and low-titanium basalts in initial 8Hf, and has the lowest initial 6Nd and initial 875r/6r at the upper end of the range of all samples. The two Rainy Creek picrites have similar Sr, Nd, and Hf isotope ratios to high-titanium basalts with slightly lower intial 8Nd (Fig. 3.13). The high- and low-titanium basalts have indistinguishable age-corrected Pb isotopic compositions, although the high-titanium basalts show a narrower range (Fig. 3.14). The range of initial Pb isotopic compositions for low-titanium basalts is 206Pb/4 18.421-19.418, 207Pb/4 = 15.568-15.609, and 208Pb/4 = 37.962- 38.481 and the high-titanium basalts have 206Pb/4= 18.504-18.888, 207Pb/4 = 15.556-15.587, and 208Pb/4 = 38.008-38.451 (Fig. 3.14; Table 3.5). The basal flow- conglomerate has a slightly lower initial Pb isotopic composition than the high- and low- titanium basalts. The Clearwater picrite has a lower initial Pb isotope ratio than the basalts and the Rainy Creek picrites have noticeably higher initial Pb isotope ratios than the basalts (Fig. 3.14). ALTERATION The Nikolai basalts generally preserve primary mineralogical, textural, and volcanological features and have retained most of their primary magmatic composition. Secondary minerals have replaced variable, but generally small, proportions of primary minerals in the Nikolai Formation and the basalts contain zeolite to prehnite-pumpellyite facies alteration minerals (prehnite + pumpellyite + epiclote + chlorite + quartz \u00C2\u00B1 115 0.2835 9 , ,\u00E2\u0080\u0098 I I I I I I 571 5A5 0,2834 \u00E2\u0080\u009C6Hf/177f 5727A3 5727A2 8 727A2 5727A3 0.2833 a ENd 230 Ma) 5802A55802A5 7 0.2832 6 05132 I I I I I I 727A3 143Nd/ 5727A20.2831 5715A5 05131 5802A2 \ 5715A5 C 0.2830 05130 ,.5802A5 C 0.5129 x 0.2829 + (a) 05128 (c) 0.2828 ____________________________ A I i \u00E2\u0080\u00A2 a I i 0.00 0.02 0.04 05127 0.703 0.704 0.705 0.706 -4- 176Lu/7Hf (b) \u00E2\u0080\u00A2 87SrI 8tSr (230 Ma)0512C 20 0.1 0.15 0.2 0.25 03 _________________________________________________ I I I 4 \u00E2\u0080\u009847Sm/\u00E2\u0080\u0099Nd I I 5802A5 -18 D18 Ma) 5727A3\u00E2\u0080\u00940 E (230 Ma) 5727A2\ E\u00E2\u0080\u00995727A2 Hf 16 - 16 5802A2 5810A4 14- . 14 \ 12- 12\u00E2\u0080\u0099 ,5715A5 ,5715A5 10 10 \u00E2\u0080\u00A2 hi9h-titaniumbasalt \u00E2\u0080\u00A2 high-titanium sill Q low-titanium basalt 0 low-titanium sill 8 - 8 A Clearwaterpicrite Rainy Creek picrite(d) \u00E2\u0080\u0098 (e) X basal flow 6 I I I I I I I I 2 4 6 8 10 0.702 0.703 0.704 0.705 0.706 ENd23\u00C2\u00B0Ma) 87SrI 86Sr (230 Ma) Figure 3.13 Whole-rock Sr, Nd, and Hf isotopic compositions for the Nikolai Formation in Alaska. (a)\u00E2\u0080\u009876Hf\u00E2\u0080\u0099177f vs.\u00E2\u0080\u009876LuJ\u00E2\u0080\u00997Hf. (b) 143Nd/vs. 147Sm/Nd. (c) Initial 8Nd vs. t7SrI6. Age correction to 230 Ma. (d) Initial CHf vs. 8Nd\u00E2\u0080\u00A2 (e) Initial vs. 87SrJ6. Average 2a error bars are shown in a corner of each panel. 116 15.85 \u00E2\u0080\u0098 i \u00E2\u0080\u0098 i \u00E2\u0080\u0098 i i \u00E2\u0080\u0098 i \u00E2\u0080\u0098 15.68 207Pb/4 207Pb/4 (230 Ma) 15.80 15.64 - 15.75 5802A2 .58O2A2 15.60 15.70 \u00E2\u0080\u00A2 high-titanium basalt \u00E2\u0080\u00A2 high-titanium sill 15.56 15.65 D low-titanium basalt Q low-titanium sill X 15.60 . 15.52 X basalfiow(a) (b) 15.55 \u00E2\u0080\u00A2 I I I I I I 15.48 I I I I I 18 19 20 21 22 23 24 17 18 19 20 21 206Pb/4 206Pb/4b (230 Ma) 43.0 40.0 i \u00E2\u0080\u0098 i A 208nkIO4nL.. Z.1 395 nj! ni (230 Ma) 42.0\u00E2\u0080\u0099 39.0 41.0 38.5 \u00C3\u00B6 5802A2 0 \u00E2\u0080\u00945802A2 40.0 38.0 x 37-5 39.0 X 37.0 (c) (d) 38.0 \u00E2\u0080\u0098 I 1 I 36.5 18 19 20 21 22 23 24 17 18 19 20 21 206Pb/4 (230 Ma) Figure 3.14 Pb isotopic compositions of leached whole-rock samples by MC-ICP-MS for the Nikolai Formation in Alaska. Error bars are smaller than symbols. (a) Measured 207PbP\u00C2\u00B04b vs. 206Pb/4. (b) Initial 207Pb/4 vs. 206Pb/4. Age-correction to 230 Ma. (c) Measured 208Pb/4 vs. 206Pb/4. (d) Initial 208PbP\u00C2\u00B04b vs. 206pb/4P 117 Ta bl e 3. 3 Sr a n d N d iso to pi cg eo ch em ist ry o fw ho le ro ck sa m pl es o fN ik ol ai ba sa lis .A la sk a Sa m pl e Gr ou ps Ar ea t) Rb Sr 8 7 S rl S r 2m 8 7 R b / 6 S r a7Sr lw Sr t Sm Nd N d lN d 2a m EN d 1 4 7 5 m / \u00E2\u0080\u009D N d 1 4 3 N d / \u00E2\u0080\u0099 t d CN d(t ) (nn mt (o nm 23 0 M a (n nm (o nm 23 0 M a 57 19 A1 B as al W M 25 .4 15 8 0. 70 51 38 7 0. 46 52 0. 70 36 2 4. 26 20 .6 5 0. 51 28 92 6 5.0 0. 12 48 0. 51 27 0 57 25 A4 Si ll (H i-T i) TA 16 .5 20 7 0. 70 37 75 8 0. 23 06 0. 70 30 2 4. 30 15 .16 0. 51 29 16 7 5.4 0. 17 15 0. 51 26 6 6.2 GO 58 02 A4 Pi cr ite CL 04 18 3 0. 70 55 30 9 0. 00 63 0. 70 55 1 1. 69 6.6 1 0. 51 27 46 6 2.1 0. 51 25 1 57 08 A2 Hi -T i W M 9.7 16 2 0. 70 37 76 7 0. 17 33 0. 70 32 1 5.2 2 18 .8 3 0. 51 30 11 5 7. 3 0. 16 77 0. 51 27 6 8.1 58 01 A2 Hi -T i CL 1.7 22 5 0. 70 33 65 8 0. 02 19 0. 70 32 9 5. 55 21 .1 0 0. 51 29 34 7 5. 8 0. 15 92 0. 51 26 9 6. 9 57 19 A5 HI -T I W M 5.9 20 0 0. 70 33 44 8 0. 08 54 0. 70 30 6 4. 45 16 .26 0. 51 29 88 6 6. 8 0. 16 53 0. 51 27 4 7. 7 58 01 .8. 5 Hi -T i CL 5. 9 18 7 0. 70 33 89 8 0. 09 13 0. 70 30 9 4. 25 15 .51 0. 51 29 37 6 5. 8 0. 16 59 0. 51 26 9 6.7 57 12 A2 Hi -T i W M 2. 2 18 4 0. 70 31 79 8 0. 03 46 0. 70 30 7 4. 19 14 .7 3 0. 51 29 97 6 7. 0 0. 17 20 0. 51 27 4 7. 7 57 26 A1 H i-l i TA 2.1 23 9 0. 70 38 44 7 0. 02 54 0. 70 37 6 4. 07 14 .3 5 0. 51 29 08 7 5.3 0. 17 13 0. 51 26 5 6.0 58 10 A1 0 Hi -T i TA 2. 6 23 1 0. 70 34 48 9 0. 03 26 0. 70 33 4 3. 95 13 .98 0. 51 29 18 6 5. 5 0. 17 07 0. 51 26 6 6.2 57 15 A1 Hi -T i W M 1.0 18 4 0. 70 36 19 7 0. 01 57 0. 70 35 7 3.5 1 12 .2 0 0. 51 30 17 7 7. 4 0. 17 38 0. 51 27 6 8.1 57 15 A5 Lo w- Ti W M 8. 5 43 7 0. 70 36 02 8 0. 05 63 0. 70 34 2 2. 32 7. 68 0. 51 30 30 5 7. 6 0. 18 29 0. 51 27 5 8.1 58 02 A5 Lo w- Ti CL 3.0 25 2 0. 70 49 42 6 0. 03 45 0. 70 48 3 2. 19 6. 68 0. 51 30 00 6 7.1 0. 19 80 0. 51 27 0 7. 0 58 10 A6 Lo w- Ti TA 2. 0 17 0 0. 70 47 04 7 0. 03 40 0. 70 45 9 1. 55 4. 52 0. 51 29 24 6 5.6 0. 20 70 0. 51 26 1 5.3 58 10 A1 Lo w- Ti TA 0. 9 15 1 0. 70 47 21 7 0. 01 72 0. 70 46 6 1. 26 3.3 1 0. 51 29 63 5 6.3 0. 23 03 0. 51 26 2 5.4 57 27 A7 Lo w- Ti TA 1.5 15 5 0. 70 48 65 9 0. 02 80 0. 70 47 7 1. 30 3. 86 0. 51 29 06 7 5. 2 0. 20 38 0. 51 26 0 5. 0 58 02 A2 Lo w- Ti CL 2.1 30 3 0. 70 56 04 8 0. 02 01 0. 70 55 4 1.3 1 4. 04 0. 51 28 72 7 4. 6 0. 19 59 0. 51 25 8 4. 6 7.1 57 27 .8. 2 Si ll (L ow -T i) TA 1.7 76 0. 70 45 32 8 0. 06 47 0. 70 43 2 1. 22 2. 92 0. 51 30 98 6 9. 0 0. 25 31 0. 51 27 2 7. 3 57 27 A3 Si ll (Lo w- TI) TA 1.8 11 7 0. 70 48 72 8 0. 94 45 0. 70 47 3 1. 23 2. 92 0. 51 31 15 8 9. 3 0. 25 53 0. 51 27 3 7. 6 57 27 A6 Si ll (L ow -T i) TA 0. 6 12 3 0. 70 48 57 10 0. 01 41 0. 70 48 1 1.1 1 3. 05 0. 51 29 30 7 5.7 0. 22 03 0. 51 26 0 5.0 57 27 A6 (du p) Si ll (L ow -T i) TA 0. 6 12 3 0. 70 48 51 8 0. 01 41 0. 70 48 0 1. 13 3. 09 0. 51 29 28 10 5.7 0. 22 02 0. 51 26 0 5.0 58 10 A2 Si ll (L ow -T i) TA 0. 8 14 1 0. 70 48 46 7 0. 01 64 0. 70 47 9 1. 16 3.2 1 0. 51 29 17 6 5.4 0. 21 87 0. 51 25 9 4. 8 58 10 A4 Si ll (L ow -T i) TA 0.4 19 8 0. 70 42 35 8 0. 00 58 0. 70 42 2 1. 96 6. 12 0. 51 28 86 6 4. 8 0. 19 37 0. 51 25 9 4. 9 0. 15 44 3. 3 58 08 A2 Pi cr ite RC 6.1 12 7 0. 70 41 24 10 0. 13 95 0. 70 36 7 2. 69 9. 85 0. 51 28 80 6 4. 7 0. 16 49 0. 51 26 3 5.7 58 08 .8. 3 Pl cr ite RC 26 .4 93 0. 70 63 07 9 0. 82 65 0. 70 36 0 3.0 1 11 .57 0. 51 28 11 6 3. 4 0. 15 76 0. 51 25 7 4. 5 H i-T i, hi gh -ti tan iu m ba sa lt; Lo w- Ti ,l ow -ti tan iu m ba sa lt; Ba sa l, ba sa lf Io w -c on gI om er at e.A bb re vi at io ns fo ra re a ar e: W M ,W ra ng ell M ou nt ain s; TA ,T an gl e La ke ;G G, G la ci er G ap La ke ; C L, Cl ea rw at er M ou nt ain s; RC ,R ain y Cr ee k. (du p) in di ca te s co m pl et e ch em ist ry du pl ica te. Al lt ra ce -e le m en ta n d iso to pi ca n al ys es w er e ca rr ie d o u ta tt he PC IG R. Th e an aly tic al m et ho ds ar e de sc rib ed in A pp en di x D. Table 3.4 Hfisotopic compositions ofwhole rock samples ofNikolai basalts, Alaska 571 9A1 Basal WM 0.31 2.30 0.28301 6 8.3 0.0188 0.28292 9.9 5802A4 Picrite CL 0.27 1.05 0.28315 6 13.2 0.0365 0.28298 12.0 Hi-Ti, high-titanium basalt; Low-TI, low-titanium basalt; Basal, basal flow-conglomerate. Abbreviations for area are: WM, Wrangell Mountains; TA, Tangle Lake; GG, Glacier Gap Lake; CL, Clearwater Mountains; RC, Rainy Creek. All trace-element and isotopic analyses were carried out at the PCIGR. The analytical methods are described in Appendix D. Sample Groups Areab Lu Hf 177HfP76f 2m 8ijt 176Lu/7Hf 177HfP76f (ppm) (ppm) 230 Ma 5708A2 Hi-Ti WM 0.53 4.10 0.28302 5 8.6 0.0184 0.28293 10.3 5801A2 Hi-Ti CL 0.38 4.03 0.28299 6 7.6 0.0134 0.28293 10.0 5719A5 Hi-Ti WM 0.36 3.23 0.28301 6 8.4 0.01 59 0.28294 10.5 5801A5 Hi-Ti CL 0.36 3.17 0.28302 7 8.7 0.0161 0.28295 10.7 5712A2 HI-Ti WM 0.36 3.15 0.28300 6 8.2 0.0162 0.28293 10.2 5726A1 Hi-Ti TA 0.27 2.66 0.28300 6 8.0 0.0146 0.28293 10.3 5810A10 Hi-Ti TA 0.32 2.84 0.28299 6 7.6 0.0160 0.28292 9.7 5715A1 Hi-Ti WM 0.40 2.78 0.28301 5 8.5 0.0202 0.28292 9.9 5715A5 Low-Ti WM 0.28 1.79 0.28304 6 9.6 0.0223 0.28294 10.6 5802A5 Low-Ti CL 0.33 1.51 0.28330 14 18.6 0.0309 0.28316 18.4 5810A6 Low-TI TA 0.33 0.88 0.28330 6 18.7 0.0537 0.28306 14.9 5810A1 Low-TI TA 0.27 0.66 0.28340 5 22.1 0.0581 0.28314 17.5 5727A7 Low-Ti TA 0.33 0.70 0.28339 7 21.9 0.0670 0.28309 15.9 5802A2 Low-Ti CL 0.39 0.78 0.28336 7 20.7 0.0717 0.28304 14.0 5725A4 Sill (HI-TI) TA 0.30 3.08 0.28299 5 7.7 0.0141 0.28293 10.1 5727A2 Sill (Low-Ti) TA 0.27 0.80 0.28333 7 19.7 0.0489 0.28311 16.5 5727A3 Sill (Low-Ti) TA 0.26 0.87 0.28333 9 19.8 0.0425 0.28314 17.7 5727A6 Sill (Low-Ti) TA 0.24 0.57 0.28335 8 20.6 0.0602 028309 15.7 5810A2 Sill (Low-Ti) TA 0.27 0.66 0.28337 6 21.3 0.0581 0.28311 16.7 581 0A4 Sill (Low-Ti) TA 0.32 0.96 0.28325 8 16.7 0.0481 0.28303 13.7 5808A2 Picrite RC 0.22 1.71 0.28302 7 8.7 0.0184 0.28294 10.3 5808A3 Picrite RC 0.25 1.88 0.28298 6 7.2 0.0189 0.28289 8.8 119 Ta bl e 3. 5 Pb iso to pi c co m po sit io ns o fw ho le ro ck sa m pl es o fN ik ol ai ba sa lts ,A la sk a Sa m pl e Gr ou ps Ar ea \u00E2\u0080\u009D U Th Pb 2o6P b l m P b 2m 2 0 7 P b l 2 0 4 P b 2m 2 0 8 P b / 2 0 4 P b 2\u00C2\u00B0m 2U / M P b 2U / M P b 2 3 2 T h l 2 0 4 P b 2 0 6 P b / 2 0 4 P b , 2 0 7 P b / 2 0 4 P b , 2 0 8 P b / 2 0 4 P b , (pp m) (pp m) (pp m) 23 0 M a 23 0 M a 23 0 M a 57 08 A2 Hi -T i W M 0.3 1 0. 98 1. 59 19 .2 55 7 0. 00 06 15 .6 04 0 0. 00 05 38 .7 61 3 0. 00 14 12 .3 9 0. 09 40 .9 5 18 .8 06 15 .58 1 38 .2 89 58 01 A2 HI -T I CL 0. 36 1. 24 1.0 1 19 .4 40 4 0. 00 08 15 .6 10 0 0. 00 07 39 .0 21 7 0. 00 19 23 .11 0. 17 82 .1 7 18 .60 1 15 .56 7 38 .0 74 57 19 A5 Hi -T i W M 0. 25 0. 87 0. 85 19 .3 19 8 0. 00 07 15 .6 02 3 0. 00 06 38 .9 18 3 0. 00 13 18 .64 0. 14 67 .8 6 18 .6 43 15 .5 68 38 .1 36 58 01 A5 HI -T I CL 0. 24 0. 84 0. 72 19 .6 02 0 0. 00 08 15 .6 19 7 0. 00 07 39 .1 95 5 0. 00 19 21 .5 8 0. 16 78 .2 9 18 .8 18 15 .5 80 38 .2 93 57 12 A2 Hi -T i W M 0. 20 0. 72 0.8 1 19 .3 83 4 0. 00 11 15 .5 97 4 0. 00 09 39 .0 09 3 0. 00 24 16 .00 0. 12 59 .61 18 .8 02 15 .5 68 38 .3 22 57 26 A1 HI -T I TA 0. 23 0. 86 0. 95 19 .4 59 4 0. 00 08 15 .6 15 8 0. 00 07 39 .1 50 6 0. 00 18 15 .73 0.1 1 60 .6 8 18 .8 88 15 .58 7 38 .4 51 58 10 A1 0 Hi -T i TA 0. 24 0. 78 0. 76 19 .45 51 0. 00 07 15 .61 41 0. 00 05 39 .0 57 7 0. 00 14 20 .2 0 0. 15 68 .7 4 18 .72 1 15 .5 77 38 .2 65 57 15 A1 HI -T I W M 0. 19 0. 64 0. 52 19 .3 81 5 0. 00 09 15 .6 00 2 0. 00 08 38 .9 55 7 0. 00 21 24 .1 5 0. 18 82 .2 2 18 .5 04 15 .5 56 38 .0 08 57 15 A5 Lo w- TI W M 0. 09 0. 34 0. 44 19 .2 52 0 0. 00 08 15 .5 90 8 0. 00 07 38 .8 09 6 0. 00 27 13 .8 8 0. 10 51 .3 3 18 .7 48 15 .5 65 38 .2 18 58 02 A5 Lo w- Ti CL 0.1 1 0. 34 0. 50 19 .1 70 8 0. 00 12 15 .6 01 9 0. 00 11 38 .68 61 0. 00 24 14 .8 0 0.1 1 45 .7 9 18 .6 33 15 .5 75 38 .1 58 58 10 A6 Lo w- Ti TA 0. 12 0. 28 0. 63 19 .0 17 8 0. 00 07 15 .5 98 0 0. 00 06 38 .5 42 8 0. 00 17 11 .8 4 0. 09 29 .8 7 18 .5 88 15 .5 76 38 .1 98 58 10 A1 Lo w- Ti TA 0. 04 0. 09 0. 23 19 .0 02 5 0. 00 08 15 .5 92 7 0. 00 07 38 .4 69 7 0. 00 18 11 .6 4 0. 08 24 .4 8 18 .5 80 15 .57 1 38 .1 88 57 27 A7 Lo w- Ti TA 0. 10 0. 29 1.2 1 19 .1 19 0 0. 00 07 15 .6 09 9 0. 00 05 38 .6 66 7 0. 00 14 5. 54 0. 04 16 .07 18 .9 18 15 .6 00 38 .4 81 58 02 A2 Lo w- Ti CL 0. 20 0. 56 0.2 1 21 .6 90 9 0. 00 09 15 .7 24 8 0. 00 09 40 .5 11 9 0. 00 25 62 .5 7 0. 45 18 5.2 1 19 .4 18 15 .6 09 38 .3 76 57 19 A1 B as al W M 1.9 1 4. 18 2. 96 19 .8 85 5 0. 00 07 15 .6 16 0 0. 00 06 38 .7 76 7 0. 00 20 41 .91 0. 30 94 .9 3 18 .3 63 15 .5 39 37 .6 82 57 25 A4 Si ll (H i-T i) TA 0. 25 0. 83 0. 66 19 .6 09 2 0. 00 10 15 .6 25 8 0. 00 09 39 .2 09 3 0. 00 18 24 .3 3 0. 18 84 .2 2 18 .7 25 15 .58 1 38 .2 38 57 27 A2 Si ll (L ow -T i) TA 0. 07 0. 17 0. 55 18 .8 62 5 0. 00 08 15 .60 21 0. 00 07 38 .4 71 1 0. 00 19 8. 05 0. 06 20 .11 18 .5 70 15 .5 87 38 .2 39 57 27 A3 Si ll (L ow -T i) TA 0. 07 0. 17 0. 50 18 .9 00 4 0. 00 09 15 .5 99 8 0. 00 08 38 .4 64 1 0. 00 24 8. 68 0. 06 22 .0 4 18 .5 85 15 .5 84 38 .2 10 57 27 A6 Si ll (L ow -T i) TA 0. 03 0. 07 0. 20 18 .9 07 6 0. 00 07 15 .5 96 7 0. 00 06 38 .4 96 0 0. 00 17 8. 37 0. 06 22 .3 6 18 .6 03 15 .58 1 38 .2 38 57 27 A6 (du p) Si ll (L ow -T i) TA 0. 03 0. 07 0. 18 18 .9 01 5 0. 00 05 15 .5 97 8 0. 00 05 38 .4 88 3 0. 00 14 9. 40 0. 07 25 .9 9 18 .5 60 15 .58 1 38 .1 89 58 10 A2 Si ll (L ow -T i) TA 0. 10 0. 27 0. 44 19 .1 96 8 0. 00 08 15 .6 08 9 0. 00 07 38 .7 77 6 0. 00 16 14 .21 0. 10 40 .9 7 18 .68 1 15 .5 83 38 .3 05 58 10 A4 Si ll (L ow -T i) TA 0. 09 0. 30 0. 26 19 .2 63 8 0. 00 08 15 .6 10 3 0. 00 07 38 .8 50 1 0. 00 18 23 .2 0 0. 17 77 .0 4 18 .42 1 15 .5 68 37 .9 62 58 02 A4 Pi cr ite CL 0. 32 0. 89 0. 13 23 .7 35 2 0. 00 18 15 .8 25 5 0. 00 13 42 .3 71 6 0. 00 43 16 8. 73 1. 22 49 1. 95 17 .6 06 15 .51 4 36 .6 99 58 08 A2 Pl cr lte RC 0. 26 0. 70 0. 76 21 .2 09 5 0. 00 14 15 .7 08 3 0. 00 12 40 .5 54 0 0. 00 30 23 .5 9 0. 17 64 .9 6 20 .3 53 15 .6 65 39 .8 05 58 08 A3 Pi cr ite RC 0. 38 1. 06 0. 65 21 .7 00 2 0. 00 09 15 .7 37 9 0. 00 08 40 .5 46 8 0. 00 24 39 .3 2 0. 29 11 4. 80 20 .2 72 15 .6 65 39 .2 23 H i-T i, hi gh -ti tan iu m ba sa lt; Lo w- Ti ,l ow -ti tan iu m ba sa lt; Ba sa l, ba sa lf lo w -c on gl om er at e\u00E2\u0080\u009D A bb re vi ati on sf or ar ea ar e: W M ,W ra ng ell M ou nt ain s; TA ,T an gl e La ke ;G G, G la ci er G ap La ke ;C L, Cl ea rw at er M ou nt ain s; RC ,R ain y Cr ee k. (du p) in di ca te sc o m pl et e ch em ist ry du pl ica te. Al lt ra ce -e le m en ta n d iso to pi ca n al ys es w er e ca rr ie d o u ta tt he PC IG R. Th e an aly tic al m et ho ds ar e de sc rib ed in A pp en di x D. t\u00E2\u0080\u0099 J C laumonite), primarily making up the amygdules (Stout, 1976; Smith, 1981; MacKevett et a!., 1997). Many of the Nikolai basalts in the synform in the Amphitheater Mountains are exceptionally unaltered compared to flows in the Wrangell and Clearwater Mountains. Vesicles and interpillow voids in the Amphitheater Mountains commonly remain unfilled and secondary minerals are less common. Seventeen of the 21 low-titanium basalts have LOl greater than 2.5 wt % and greater than 8 wt % MgO, whereas only four high-titanium basalts have greater than 2.5 wt % LOT and all have less than 8 wt % MgO (Fig. 3.15). Three of the four high-titanium basalts that lie within the alkalic field are plagioclase-rich, highly amygdaloidal, and were collected near a mineralized area at the top of the Nikolai Formation. Tight linear arrays are apparent on plots of HFSE and REE (not shown) indicating negligible affect of element mobility. Only a limited group of samples (5808A3, 5802A4, 5802A2, 5725A4, 5726Al) have LTLE concentrations outside the narrow range of most high- and low- titanium basalts (Fig. 3.11) and there is no correlation between LOT and L]LE. All of the low-titanium basalts have positive Sr anomalies that are complemented with small positive Eu anomalies in most samples, and none of the high-titanium basalts have Sr anomalies (Fig. 3.11), which indicates Sr concentrations probably represent primary values. U and Th show a linear relationship, whereas Pb and Th do not show a clear relationship (not shown), indicating some secondary mobility of Pb, especially in the low-titanium basalts. Almost all initial Nd and Hf, and to a lesser extent Sr and Pb isotopic compositions represent close to magmatic compositions. Several of the more altered samples were not selected for isotopic analyses and leaching effectively removed most of the secondary alteration products (Weis et a!., 2006; Nobre Silva et a!., in revision). A single exception is the Clearwater picrite (5802A4), which was significantly affected by Pb loss and has less radiogenic age-corrected Pb isotopic ratios (Figs 3.11 and 3.14). The correlation of LOT and875r/6,206Pb/4, and238U/04Pb in the low- and high-titanium basalts is a primary feature that is not apparent with age correction (Fig. 3.15). The rather small range of initial Pb and distinct initial Hf and Sr isotopic compositions for high- and low-titanium basalts clearly reflect the isotopic compositions of the sources of Wrangellia flood basalts in Alaska. 121 5 . i i \u00E2\u0080\u00A2 . i . I I I 4 3 2 0 5 4 3 2 0\u00E2\u0080\u0094 18.5 19 . \u00E2\u0080\u00A2 \u00E2\u0080\u00A2 .. . . 0.704 0.705 0.706 Figure 3.15 Loss-on-ignition versus MgO and isotopic ratios for the Nikolai Formation in Alaska. (a) LOT vs. MgO. (b) LOl vs. measured t7Sr/86. (c) LOT vs. measured2\u00C2\u00B06Pb/04. (d) LOT vs. measured 238U/06Pb. The two insets show the expanded x-axis for 206PbP\u00C2\u00B04b and 238U/06Pb. Note the generally higher LOT and MgO for the low-titanium basalts. The differences in measured t7Sr/6r and 206Pb/4 within the suites of high- and low- titanium basalts are mostly not apparent after age-correction. 0 \u00E2\u0080\u00A2 a LOl (wt %) (a) \u00E2\u0080\u00A2\u00E2\u0080\u00A2 . 0 0 I I I I I I I I 0 3 6 9 MgO (wt LOl (wt %) 4 5715A5 0 3 0 00 0 2 \u00E2\u0080\u00A2 \u00E2\u0080\u00A2 .. 1 \u00E2\u0080\u00A2 (b) I I I I I I I I I - I \u00E2\u0080\u00A2 high-titanium basalt \u00E2\u0080\u00A2 high-titanium sill 0 low-titanium basalt 0 low-titanium sill A Clearwater picrite 0 Rainy Creek picrite 0 12 15 18 0.703 87Sr/ 86Sr 5802A2 I I I LOl (wt %) 02 0 0 18 0 0 \u00E2\u0080\u00A2 I I 20 22 (c) \u00E2\u0080\u00A2 \u00E2\u0080\u00A2 19.5 206pb/4 20 5 10 15 20 25 238u/04Pb 122 FLOOD BASALT CHEMOSTRATIGRAPIIY Samples of the Nikolai Formation were collected going up or downsection through the volcanic stratigraphy to provide an estimate of the relative stratigraphic position of each sample and to determine the relationship between stratigraphic position and chemical composition. Figure 3.16 shows sample numbers, lithologies, relative stratigraphic height, and Ti02 and MgO contents for Nikolai basalts from the three main areas of Alaska where fieldwork was undertaken (Fig. 3.1). Each stratigraphic column is a combination of multiple traverses (separated by dashed lines in Figure 3.16). We are coafident in the relative position of each section of stratigraphy because of the continuous exposure and minimal disruption by faults in these areas. Since the trace-element and isotopic variation of the basalts generally correspond with variation in Ti02 (Figs 3.10- 3.13), only Ti02 and MgO are shown in Figure 3.16. In the Clearwater and Amphitheater Mountains, there is a clear relationship between stratigraphic position and chemical composition of the flood basalts (Fig. 3.16). The low-titanium basalts form the lowermost several hundred meters of flows (10-15% of stratigraphy) and the high-titanium basalts form the majority of the flows (\u00E2\u0080\u009485-90% of stratigraphy) above the lowest several hundred meters. More of the low-titanium basalts and sills were sampled, partly because lower sections of volcanic stratigraphy were more easily accessible and partly because there are more interesting relationships with pre Nikolai sediments and mafic sills and submarine units preserved lower in the stratigraphy. The transition from low- to high-titanium basalts does not appear to coincide with the transition from submarine to subaerial flows, but almost all of the low- titanium basalts that were sampled are submarine flows. In the Wrangell Mountains, there do not appear to be any low-titanium basalts, except for two anomalous samples (Fig. 3.16). A single sample of the basal flow conglomerate has a low titanium content (0.67 wt %), similar initial to the high titanium basalts, and anomalous LaJYbCN (6.4), Ba (1277 ppm), and Th (4.18 ppm; Figs 3.10 and 3.13; Tables 3.2 and 3.4). Field observations and several other geochemical characteristics indicate the chemistry of this basal flow-conglomerate is the result of considerable assimilation (-30 vol %) of material derived from underlying Paleozoic 123 Figure 3.16 Chemostratigraphy of the Nikolai Formation in three areas of Alaska (Clearwater, Amphitheater, and Wrangell Mountains). Each column shows lithology, sample numbers, relative stratigraphic height, and Ti02 and MgO contents (in wt %). Dashed lines in each column separate individual traverses. 124 \u00E2\u0080\u00A25801\u00E2\u0080\u00992 95801A5 Cleaiwater Mountains Amphitheater Mountains 5714M 5715A1 1794A3 5716A1 \u00E2\u0080\u00A25801A8 55715A5 Wrangeii Mountains 5716A2 571603 I I 55501A5 1573106 15802A1 15802A2 1580203 1880204 158025.5 1550206 *570*5.2 \u00E2\u0080\u00A25731A5 I?I I Il I Ill15707A3 I 5710*2 !571053 \u00E2\u0080\u00A25731A4 1573103 .sed*s I if5712*2 I I I I MgO I I I, I jO 15 wt% 1102 MgO I I I I I I I I I I I I 1 2 3 5 10 15 06% 06% 87190* 8719*5 \u00E2\u0080\u00A25759A1 1102 MgO 1 2 33 8 9 06% 06% sequences. The next sample going upsection, 2O m above the uppermost exposure of basal flow conglomerate in the Wrangell Mountains, does not have visible assimilated material and is high-titanium basalt with unexceptional chemistry. A single sample with a low titanium content (1.14 wt %Ti02;571 5A5) was collected from near the top of the stratigraphy in the Wrangell Mountains, but this sample has similar isotopic composition to the high-titanium basalts, atypical petrographic texture, and is at the upper range of Ti02 of low-titanium basalts. DISCUSSION Source of Nikolai basalts The Nikolai Formation in Alaska has two main lava types with distinct isotopic compositions. The high-titanium basalts in Alaska have depleted Hf and Nd isotopic compositions that are not as depleted as most Pacific and Indian mid-ocean ridge basalts (MORB) and are displaced just below the ocean island basalt (OIB) mantle array (Fig. 3.17). The high-titanium basalts have similar initial Sr and Nd isotopic compositions to Ontong Java Plateau, Hawaii, and Caribbean Plateau basalts and similar intial to Ontong Java, with slightly lower initial 8Hf than most Hawaii and Caribbean basalts (Fig. 3.17). In contrast, the low-titanium basalts are displaced well above the 0113 mantle array in a EHf(t)-ENd(t) correlation diagram and have a similar range of initial 8W to Pacific MORB, with initial 8Nd 2 to 5 epsilon units lower than Pacific MORB. The Hf isotopic compositions of the low-titanium basalts are 2 to 6 epsilon units higher than for most samples from Ontong Java, with slightly lower initial 8Nd. Sr isotopic compositions for low-titanium basalts extend to significantly higher initial 87Sr/6rthan Ontong Java and Hawaii. Three low-titanium basalts with particularly high initial ENd lie within a field for Indian MORB in EHf(t)-ENd(t) space. Two Rainy Creek picrite samples lie close to the EHf(t)-ENd(t) OIB mantle array with lower initial 6Nd than the high-titanium basalts (Fig. 3.17). The high- and low-titanium basalts have uniform initial Pb isotopic compositions that overlap a field for Caribbean basalts and have more radiogenic initial 207Pb/4 than Ontong Java, Hawaii, and a field for the East Pacific Rise (EPR; Fig. 3.18). Pb isotopic compositions form a linear trend in 208Pb-6 space which intersects the field of Pacific MORB compositions and is slightly offset towards lower 208Pb/4 from 125 0.702 0.703 0.704 87SrI Sr(1) E Nd Figure 3.17 Comparison of age-corrected (230 Ma) Sr-Nd-Hf isotopic compositions for Nikolai basalts in Alaska to age-corrected OIB and MORB. (a) Initial vs. 87Sr/6. (b) Initial vs. Nd Both fields with dashed lines are Indian MORB. All of the references for the compiled data are too numerous to cite here. Most of the compiled data was extracted from the GEOROC database (http://georoc.mpch-mainz.gwdg.de/ georoc/). Data for Ontong Java from Mahoney et al. (1993), Babbs (1997), and Tejada et a!. (2004); Indian MORE from Salters (1996), Kempton et a!. (2002), and Janney et a!. (2005); Pacific MORE from Mahoney et a!. (1992, 1994), Nowell et a!. (1998), Salters and White, and Chauvel and Blichert-Toft (2001); Explorer Ridge data from Cousens and Weis (pers. comm., 2007); O]B array line from Vervoort (1999). EPR is East Pacific Rise. Dashed lines indicate Bulk Silicate Earth (BSE). An extended reference list is available upon request. t I I \u00E2\u0080\u00A2 I I J 1 I I I \u00E2\u0080\u0094I I. . I I. \u00E2\u0080\u00A2 \u00E2\u0080\u00A2. I. I \u00E2\u0080\u00A2 S I . x \u00E2\u0080\u0098.i\u00E2\u0080\u00A2 I.. \u00E2\u0080\u00A2 East PadficRe \u00E2\u0080\u00A2 \u00E2\u0080\u00A2 Ontongiava \u00E2\u0080\u00A2 .1 \u00E2\u0080\u00A2 Hawaii \u00E2\u0080\u00A2 I \u00E2\u0080\u00A2 Nikolai high-titanium basaIt \u00E2\u0080\u00A2e \u00E2\u0080\u00A2 \u00E2\u0080\u00A2 \u00E2\u0080\u00A2 Nikolai low-titanium basalt %. I \u00E2\u0080\u00A2 Nikolai picrite . 1 XNikoiaibasalflow(WrangeilMountains) \u00E2\u0080\u0094 \u00E2\u0080\u0094 \u00E2\u0080\u0094I\u00E2\u0080\u0094 (a) \u00E2\u0080\u00A2 \u00E2\u0080\u00A2 . \u00E2\u0080\u00A2 I \u00E2\u0080\u00A2 \u00E2\u0080\u00A2 \u00E2\u0080\u00A2 I \u00E2\u0080\u00A2 . \u00E2\u0080\u00A2 \u00E2\u0080\u00A2 I . \u00E2\u0080\u00A2 14 12 10 8 (I) 6 Nd 4 2 0 -2 22 20 18 16 14 12 EHf 1: 6 4 2 0 -2 -4 0.705 0.706 -4 -2 0 4 6 8 10 12 14 126 15.65 I I I II II I I II II II I I I I p I I I I 15.60 15.55 15.50 15.45 15.40 17.5 19.5 20.0 20.0 Figure 3.18 Comparison of Pb isotopic compositions of the Nikolai Formation in Alaska to OIB and MORB. (a) Measured and initial 207Pb/4 vs. 206PbP\u00C2\u00B04b. (b) Measured and initial 208Pb/4 vs. 206Pb/4. See figure caption for Figure 3.17 for references for OIB and MORB data. : . . 60 \u00E2\u0080\u00A2 East Pacific Rise Juan de Fuca/Gorda \u00E2\u0080\u00A2 Explorer Ridge Caribbean Plateau \u00E2\u0080\u00A2 Ontong Java Hawaii \u00E2\u0080\u00A2 Nikolai Formation (initial) o Nikolai Formation (measured) - ii ii I I Ii I I ii II II Ii I I \u00E2\u0080\u00A2 p \u00E2\u0080\u0098a - \u00E2\u0080\u00A2 (a) 18.0 18.5 19.0 39.5 39.0 38.5 38.0 37.5 17.5 127 Ontong Java and Hawaii. The Pb in Nikolai basalts is more enriched in radiogenic Pb than MORB, Ontong Java, and OIB from Hawaii, but similar to basalts of the Caribbean Plateau. The initial Hf and Nd isotopic compositions of high-titanium basalts indicate a uniform OIB-type Pacific mantle source derived from a long-term depleted source, distinct from the source of MORB. In contrast, the low-titanium basalts have initial Hf isotopic compositions that are clearly distinct from OIB and initial Nd isotopic compositions that are distinct from the Pacific MORE source. The displacement of the low-titanium basalts well above the OIB array indicates involvement of a depleted component (mantle or crust), distinct from depleted MORE mantle, early in the formation of Nikolai basalts in Alaska. The origin of the distinct isotopic and geochemical signature of the low-titanium basalts is the focus of subsequent discussion sections. Lithospheric involvement in derivation of the low-titanium basalts The stratigraphic relationship of the two contrasting lava types in the Nikolai Formation preserve a record of a shift in composition of the source of Nikolai basalts in Alaska and provide a rare opportunity to evaluate the role of oceanic arc lithosphere in the formation of an oceanic plateau. Thus far, the low-titanium basalts have primarily been recognized in the lowermost part of the stratigraphy in the western part of Wrangellia in Alaska, where there is a substantial section of submarine flows (5OO m). The low-titanium basalts possess distinct negative-HFSE anomalies in normalized trace- element patterns and have high initial 8Hfand high initial 87Sr/6r compared to the high- titanium basalts. This compositional and stratigraphic evidence suggests that the underlying Paleozoic arc lithosphere may have played a significant role in the generation of early-erupted low-titanium basalts in the Wrangellia oceanic plateau of Alaska. Nature ofunderlying Paleozoic arc lithosphere The Paleozoic arc (320-285 Ma) and marine sedimentary sequences (Early Permian to Middle Triassic) exposed underlying the Nikolai basalts in Alaska are >2.5 km thick in areas. Recent geophysical studies in southern Alaska by Saltus et al. (2007) indicate Wrangellia crust is at least 50 km thick between the Denali and Border Ranges 128 Faults (Fig. 3.1). The arc crust that the Wrangellia oceanic plateau was built upon may have been 20-30 km thick and this would have included a substantial sub-arc mantle lithosphere that was metasomatized during arc activity. In the Alaska Range, the Nikolai basalts are underlain, in decreasing order of depth, by the Paleozoic Tetelna Volcanics, the Slana Spur and Eagle Creek Formations. Tetelna Volcanics (<1000 m) are anclesitic and dacitic flows, tuffs interbedded with volcaniclastic rocks, and debris-flow deposits; the Slana Spur Formation (P4400 m) is marine voicaniclastics, with lesser limestone and sandstone; and the Eagle Creek Formation (900 m) is Permian argillite and limestone (Nokieberg et a!., 1985). Numerous comagmatic intermediate to felsic piutonic rocks intrude Teteina Volcanics and the Sian Spur Formation (Nokleberg eta?., 1994). In the Wrangell Mountains, the Paleozoic sequences include the Station Creek Formation (\u00E2\u0080\u0094P1200 m of mostly basaltic and andesitic flows and 800 m of volcaniclastic sequences) and the sedimentary Hasen Creek Formation (500 m of chert, black shale, sandstone, bioclastic limestone, and conglomerate) (Smith & MacKevett, 1970; Fig. 3.2). Trace-element and isotopic source constraints ofthe low-titanium basalts The trace element and isotopic compositions of the early-erupted low-titanium basalts are not typical of OIB and indicate involvement of a HFSE-depleted component that was different than the plume-type source of the high-titanium basalts. The arc lithosphere is a key suspect for derivation of the low-titanium basalts because: 1) the geochemical and isotopic signature of the low-titanium basalts is similar to rocks formed in subduction settings (e.g. Kelemen et a?., 2003); 2) arc crust is exposed beneath the Nikolai basaits in Alaska; and 3) the low-titanium basalts only form -4 0-15% of the lowest part of the volcanic stratigraphy. Figure 3.19 highlights differences in trace elements and isotopic compositions between the high- and low-titanium basalts and indicates a strong arc lithospheric, or subduction-modified mantle, component in the low-titanium basalts. The high-titanium basaits form a concentrated cluster of points in each of the plots and show a remarkably small degree of variation, whereas the low-titanium basalts have a noticeably wider range of variation, which mostly does not overlap the range for the high-titanium basaits (Fig. 129 \u00E2\u0080\u0098b.o 0.2 0.4 0.6 0.8 Nb/La PM ci ciI1Jci (d) 4 8 12 Y/N b 10 20 30 40 Sr/Nd Figure 3.19 Trace-element ratios and isotopic compositions of the Nikolai Formation in Alaska. (a) 176Lu/\u00E2\u0080\u009977Hfvs. Nb/Th. (b) Initial 5Nd vs. NbJTh. (c) Initial s vs. Nd/Zr. (d) Zr/Nb vs. YINb. (e) Initial vs. Nb/La. (f) Initial EHf VS. Sr/Nd. Primitive mantle (PM) from McDonough and Sun (1995), depleted mantle (DM) trace-element and isotopic composition estimates from Salters and Stracke (2004). Talkeetna arc lower crust compositions from Greene et a!. (2006) and Tailceetna arc lavas from Clift et aL (2005). Dashed circle in each panel outlines samples 5727A2, 5727A3, and 5802A5. I I I I I I I0.08 176Lu/7Hf baIflow(Amph.Mtns) A z.0.00 \u00E2\u0080\u00A2 E (I) Nd Talkeetna arc lower crust 8 Talkeetna arc lavas :: : Nb/Th 10 I I I I I I I I I I I I I I I I basal flow (Amph. Mtns) Nb/Th 10 15 EHf : (c) PM: \u00E2\u0080\u00A2 \u00E2\u0080\u00A2 raiketnac \u00E2\u0080\u00A2\u00C3\u00A7 ) \u00E2\u0080\u00A2 \u00E2\u0080\u00A2 Z /Nb lavas ci ci ci A x I I I III III Talkeetna arci lower crust I ci 6 \u00E2\u0080\u0098\u00E2\u0080\u0094\u00E2\u0080\u0098 32 24 16 8 F 18 16 14 12 10 8 a1 20 18 16 14 12 10 8 )8 0.11 0.14 0.17 0.20 0.23 0. Nd/Zr ci ci \u00E2\u0080\u00A2 high-titanium basalt \u00E2\u0080\u00A2 high-titanium sill ci low-titanium basalt * low-titanium sill A Clearwater picrite G Rainy Creek picrite X basal flow (Wrangell Mtns) I I I I I I I I I 16 20 2 4 I I I I I I I I I I I I I : EHf(i) c:-- . ci ci \u00E2\u0080\u00A2A \u00E2\u0080\u00A2() I I I I I I I 1I I I r/Nd 114 \u00E2\u0080\u00A2 I I 18 EHf 16 14 12 1o (f) PM 6 1.4 0 I I I I I I 9ci \u00E2\u0080\u00A2 I I I I \u00E2\u0080\u00A2 I I I I Talkeetna arc lower crust \u00E2\u0080\u0094 5802A2, ci. ci I I I I 1.0 1.2 50 60 70 80 130 3.19). The low-titanium basalts have low Nb/Th and Nb/La relative to primitive mantle, which is characteristic of subduction-related rocks (Pearce, 1982). Except for three analyses from the basal flow in the Amphitheater Mountains, which has pelagic sediment between pillow tubes derived from the directly underlying strata and has similar Nb/Th to the high-titanium basalts, the low-titanium basalts have similar Nb/Th and Nb/La to accreted arc crust from the Early Jurassic Talkeetna arc exposed in southern Alaska \u00E2\u0080\u0098-\u00E2\u0080\u009850 km south of the Amphitheater Mountains (Fig. 3.19; Greene eta!., 2006). Low Nb/Th in arc magmas is commonly attributed to inheritance from subducted sediments (e.g. Kelemen eta!., 2003). Low Nb/La may be related to a process whereby migration of REE into magma takes place, but mobilization of Nb is inhibited, such as by reaction between magmas and metasomatized peridotite (e.g. Kelemen et al., 1990, 1993). The low- titanium basalts have high Sr/Nd and Nd/Zr relative to primitive mantle and the high- titanium basalts (Fig. 3.19). Elevated Sr relative to REE may indicate addition of Sr to arc lithosphere through aqueous fluids, since Sr is more soluble than REE at high pressure (e.g. Johnson & Plank, 1999) or addition from Sr-enriched cumulates from gabbroic lower crust (e.g. Kelemen et a!., 2003). The trace-element and isotopic compositions of the low-titanium basalts have an arc-type geochemical signature. Figure 3.20 utilizes proxies described by Pearce (2008) for identifying lithospheric input (Th-Nb) and assessing depth of melting (Ti-Yb). For the Th-Nb proxy, all the high-titanium basalts lie within a diagonal MORB-OIB array parallel to a melting vector, whereas most of the low-titanium basalts are displaced above the array, oblique to the melting vector. The low-titanium basalts follow a trend for lavas that have a subduction component, or have interacted with continental crust, and they are consistent with a small amount of assimilation (F<0.9; F is melt fraction) combined with fractional crystallization (AFC), shown by the modeling curve of Pearce (2008; Fig. 3.20). A Nb Th depleted source is indicated for the low-titanium basalts, which also have similar Th Nb to Talkeetna arc lavas and lower crust. For the Ti-Yb proxy, high Ti/Yb ratios for high-titanium basalts indicate residual garnet from melting at high pressure, within the OIB melting array, whereas low-titanium basalts lie along a complementary mantle melt depletion trend indicating shallow melting, similar to compositions of Talkeetna arc lavas (Pearce, 2008; Fig. 3.20). 131 Figure 3.20 Th-Nb and Ti-Yb proxies of the Nikolai Formation in Alaska with data compilation and modeling results from Pearce (2008). See Pearce (2008) for full reference list used to create MORB and OIB arrays, and OIB average in panel b. Primitive mantle (PM) estimate in panel a from McDonough & Sun (1995). (a) Th/Yb vs. Nb/Yb. MORB OIB array and assimilation-fractional crystallization (AFC) model from Pearce (2008). (b) Ti02/Yb vs. Nb/Yb. Talkeetna arc lower crust from Greene et a!. (2006) and Tailceetna arc lavas from Clift et a!. (2005). Mariana arc data from Pearce et a!. (2007) and Woodhead et a!. (2001). The low-titanium basalts indicate a depleted source and interaction with a subduction component combined with fractional crystallization, whereas the high-titanium basalts lie within an OIB array in panel b, parallel to a melting vector that indicates higher pressure melting. See Pearce (2008) for parameters of polybaric melting and assimilation-fractional crystallization (AFC) modeling. Blue line in panel A represents an AFC model following the modeling of DePaolo (1981). Red line in panel b illustrates a polybaric melting trend (with changing composition of pooled melt extracted from the mantle that undergoes decompression from the solidus to the pressure marked) for high and lower mantle potential temperatures which correspond to representative conditions for the generation of present-day MORE and OIB (Pearce, 2008). Nb/Yb Nb/Yb 132 Lassiter et a!. (1995) suggested a minor role for the arc lithosphere in formation of the Wrangellia flood basalts based on a suite of nine samples from the Wrangell Mountains in Alaska. They inferred that assimilation of low ENd, low Nb/Th arc material may have affected the composition of the Wrangellia basalts, but that mixing of MORB mantle with low 5Nd arc material did not reproduce the trends in the Wrangellia basalts. Rather, Lassiter et al. (1995) suggested mixing of a plume-type source, with 8Nd +6 to +7, with arc material with low Nb/Th could reproduce variations in the Wrangellia flood basalts. They noted that the absence of low Nb/La ratios in flood basalts from their dataset suggests a restricted amount of lithospheric involvement. The lower FeO content for most of the low-titanium basalts also may reflect melting generated from refractory arc lithophere (Lassiter & DePaolo, 1997). The widespread sampling of Nikolai basalts in Alaska in this study supports the interpretation of Lassiter et a!. (1995), that arc lithosphere was involved in formation of the Nikolai basalts in Alaska. The low-titanium basalts may have developed an arc-type signature by substantial melting of subduction-modified mantle, interaction of plume-derived melts with melts or material derived from the arc lithospheric, and/or reaction of magmas and metasomatized arc peridotite early in generation of the Nikolai basalts. All CFBs show compositional evidence of involvement of continental crust or lithospheric mantle in parts of their volcanic stratigraphy (e.g. Saunders et a!., 1992). Several CFBs and volcanic rifled margins show a transition in the eruptive sequence from a lithospheric to a plume-derived signature [e.g. Siberia (Wooden eta!., 1993); Parana (Peate & Hawkesworth, 1996); North Atlantic Igneous Province (Kerr, 1994); Ethiopia (Pik eta!., 1999)] and a number of influential studies have examined the role of plume-lithosphere interactions in the formation of flood basalt provinces (e.g. Arndt & Christensen, 1992; Menzies, 1992; Saunders eta!., 1992; Turner et a!., 1996; Lassiter & DePaolo, 1997). White and McKenzie (1995) presented geochemical evidence for continental lithospheric contribution to flood basalts, but indicated that the conduction of heat to the lithopshere from the plume is too slow to produce large volumes of magma in short timespans. Arndt and Christensen (1992) found that >96% of melt in CFBs comes from the asthenosphere and only minor amounts of melt (<5%) may originate in the lithosphere. Although there are conflicts with anhydrous melting models for the lithosphere, Lassiter and DePaolo 133 (1997) found evidence for lithospheric mantle melting and typically these melts are more abundant during early phases of flood volcanism, as they usually represent a minor volume (10-20%) of the eruptive sequences (e.g. Siberia). Pik et al. (1999) proposed melting of a shallow-level, depleted source for low-titanium basalts from Ethiopia, with a strong, but variable, lithospheric contribution. For certain conditions (e.g. lithospheric thickness, duration of heating, and temperature), modeling predicts that small volumes of lithosphere-derived basalts may be overlain by larger volumes of asthenospheric basalts (Turner et al., 1996). Turner et al. (1996) concluded that the lithospheric mantle can contribute melt if it is less than 100 km thick and if the solidus is lowered from addition of volatiles at some time in the past. Saunders et al. (1992) suggested that, although conduction alone may not cause melting of the lithosphere, rifting and decompression, the presence of hydrous phases (e.g. Gallagher & Hawkesworth, 1992), melt injection from the plume into the lithosphere, and thermal and mechanical erosion of the lithosphere may all facilitate melting. Numerical modeling of d\u00E2\u0080\u0099Acremont et al. (2003) involving plume head-lithosphere interaction and the formation of oceanic plateaus indicates thermal weakening may be less important than mechanical weakening at timescales of plume head flattening and related strain rates. Farnetani and Richards (1994) found from numerical modeling, partly applied to Triassic Wrangellia stratigraphy, that without extension, melting would likely be entirely sublithospheric, however, they note that they did not examine complexities of arc lithosphere and the presence of hydrous phases that would enhance melting. The combination of chemostratigraphy and trace-element and isotopic compositions suggests involvement of a subduction-modified mantle component early in the formation of the Nikolai basalts in Alaska. The compositions of Nikolai basalts in Yukon show the same characteristics and are interpreted as having formed from a similar degree of involvement of subduction-modified mantle, as part of the same oceanic plateau (Greene et al., 2008, submitted-a). Origin ofdecoupled Hfand Nd isotopic compositions oflow-titanium basalts The initial Hf isotopic compositions of the low-titanium basalts indicate involvement of a component that evolved with high LuJHf over time, but not 134 corresponding high Sm/Nd. There are only a few processes that are known to be able to fractionate the parent-daughter elements Lu and Hf, but not Sm and Nd, and the combined use of\u00E2\u0080\u009876Hf7\u00E2\u0080\u0099fand\u00E2\u0080\u009843NdJ\u00E2\u0080\u0099Nd is an indicator of these processes (Salters and White, 1998). Parent isotopes\u00E2\u0080\u009876Lu and\u00E2\u0080\u009847Sm are more compatible during melting than their daughter isotopes\u00E2\u0080\u009876Hfand\u00E2\u0080\u009843Nd, respectively (Salters and White, 1998), and show a close coupling in the crust-mantle system; when plotting Hf versus Nd isotopes they form the \u00E2\u0080\u009Cterrestrial array\u00E2\u0080\u009D (Vervoort & Blichert-Toft, 1999; van de Flierdt et a!., 2004; Fig. 3.21). Decoupling of the Hf and Nd isotope system has been attributed to processes that involve zircon or garnet, which result in larger fractionation of LU/Hf than of Sm/Nd (Patchett et a!., 1984; Vervoort et a!., 2000). These processes may involve pelagic sediment, ancient melt extraction, or oceanic lithosphere modified by subduction (e.g. Geidmacher et al., 2003). Pelagic sediment can acquire high Lu/Hf compared to Sm/Nd because of the sedimentary fractionation of zircon (Patchett et a!., 1984). Ancient melt extraction can result in high Lu/Hf relative to Sm/Nd from a source with a high garnetlclinopyroxene ratio (Geldmacher et a!., 2003). The arc mantle wedge can develop high Lu/Hf, compared to Sm/Nd, that will evolve over time to high\u00E2\u0080\u009876Hf7\u00E2\u0080\u0099frelative to \u00E2\u0080\u009843Nd/\u00E2\u0080\u0099Nd and displace compositions above the OIB array (e.g. Barry eta!., 2006). Kempton et a!. (2002) and Janney et a!. (2005) have presented models that involve subduction-modified mantle to explain high 8Hf for Indian MORB. Subduction-modified mantle may lead to greater decoupling of Hf and Nf isotopic compositions than pelagic sediment alone, and this may initially lead to a more significant decrease in\u00E2\u0080\u009843NdJ\u00E2\u0080\u0099Nd than 176Hf7\u00E2\u0080\u00997f, which then evolve along a similar path to depleted compositions well above the mantle array (Kempton et a!., 2002; Janney et a!., 2005). The high sHf and HFSE-depleted characterisitics of the low-titanium basalts were probably not intrinsic to the plume source of Wrangellia flood basalts in Alaska; the isotopic and trace element compositions indicate involvement of subduction-modified lithospheric mantle. Along with high Hf isotope ratios, high primary Sr isotopic compositions of the low-titanium basalts suggest a degree of seawater influence (Hauff et aL, 2003) or influence from high Rb/Sr arc mantle. Figure 3.22 shows Sr, Nd, and Hf isotopic compositions of the Nikolai basalts compared to modem Pacific arcs and sediments. The low-titanium basalts have comparable initial 8Hf to modem Pacific arcs 135 30 I I I I I I I I I I I I I I I I I I I I I I I I I I I I 20 - 10 - -10 - -20 - -30 \u00E2\u0080\u0094 -20 -15 -10 -5 ENdW 5 10 15 Figure 3.21 Global Hf-Nd isotope systematics with age-corrected data of the Nikolai Formation in Alaska. Data for terrestrial array are from Vervoort and Blichert-Toft (1999) and references therein. Data for the seawater array, mostly from Fe-Mn crusts, and are too numerous to cite here. Pelagic sediment data from DSDP 595/596 provided courtesy of J. Vervoort. The mixing curve shown is for mixing of pelagic sediment with arc basalt. Average pelagic sediment composition used for mixing curve is initial = +5.0 \u00C2\u00B1 4.0 and initial eNd= -4.5 \u00C2\u00B1 2.4. Average arc basalt from Pearce et al. (2007) and Woodhead et a!. (2001). Pacific MORB from Nowell et a!. (1998), Salters and White (1998), and Chauvel and Blichert-Toft (2001). An extended reference list is available upon request. Mixing curve low-Ti basalts Vervoort, unpublished data pelagic clays DSDP site 595/596(-\u00E2\u0080\u00981000 km east of Tonga trench) Fe-Mn crusts (seawater array) Al IA \u00E2\u0080\u009Cacific MORB A A ,J-. I \u00E2\u0080\u00A2 high-Ti basalts A A \u00E2\u0080\u0094Aterrestrial array Ar I AA AA A A A A \u00E2\u0080\u00A2 oceanic rocks \u00E2\u0080\u00A2 continental rocks A A A A sedimentary rocks A A A \u00E2\u0080\u00A2 Wrangellia flood basalts (Alaska) \u00E2\u0080\u00A2 Pacific arcs \u00E2\u0080\u00A2 Pacific MORB \u00E2\u0080\u00A2 Manus Basin MORB (New Britain arc) \u00E2\u0080\u00A2 Pacific Fe-Mn crusts 0 Alaska/Kamchatka Fe-Mn crusts Eolian dust from Asia (Pacific Ocean) I I I I I i i i i I i i i i I i i i I I I I I I I I I I I I I I I I 136 20 18 16 14 12 10 8 6 4 2 0 -2 -4 -2 0 2 4 6 8 10 12 14 0.702 0.703 0.704 0.705 0.706 E NdW 87SrI Sr()) Figure 3.22 Comparison of initial Hf, Nd, and Sr isotopic compositions of the Nikolai Formation in Alaska to Pacific arcs (Tonga-Kermadec, Mariana, Vanuatu, New Britain) and Pacific MORB. Compiled Pacific arc and sediment data from Pearce et a!. (2007) and Woodhead et a!. (2001). See figure caption for Figure 3.17 for references for Pacific MORB. Ontong Java data from Tejada et a!. (2004). Hawaii data from numerous references. Dashed lines indicate Bulk Silicate Earth (BSE). 137 EHfo > () \u00E2\u0080\u00A2 Ontong Java H\u00C3\u00A7 \u00E2\u0080\u00A2 , BE .EHfo a MORB adf3 a, \u00E2\u0080\u00A2 Java 20 18 16 14 12 10 8 6 4 2 0 -2 .. \u00E2\u0080\u00A2 sedimentr X Nikolai basal flow \u00E2\u0080\u00A2 Nikolai Formation high-titanium basalt. \u00E2\u0080\u00A2 Nikolai Formation low-titanium basalt \u00E2\u0080\u00A2 Nikolai Formation picrite <)sediments Ipacific arcs) I I I I I I I (b) Pacific arcs measured calculated at 230 Ma Tonga-Kermadec Marlana Vanuatu New Britain I I BSE (and Pacific MORB) with lower initial 8Nd and higher initial 87Sr/6. High Sr isotope ratios, in combination with high Hf ratios, indicate a significant degree of Rb/Sr and Lu/Hf fractionation, which are characteristic of fractionations produced in the mantle wedge. The trace-element compositions of the low-titanium basalts indicate they probably did not originate from a source with a high gamet/clinopyroxene ratio. A binary mixing curve between average Pacific arc basalt composition and pelagic sediments, from -.-l000 km east of the Tonga trench (DSDP site 595/596; Vervoort, pers. comm.), in a plot of initial 8Hf versus 8Nd suggests involvement of a pelagic sediment component with high Lu/Hf could generate high initial Hf isotopic compositions similar to those of the low-titanium basalts (Fig. 3.21). This does not preclude the role of subduction-modified mantle. The addition of a small amount of a pelagic component that underwent radiogenic ingrowth from high Lu/Hf could explain the displacement of low-titanium basalts above the OIB mantle array from a source more depleted than that of the high-titanium basalts (Fig. 3.21). The pelagic sediment component is different than the local contamination of the basal pillowed flows in the Alaska Range which contain sediment filling interpillow voids. The trace-element and isotopic geochemistry of the low-titanium basalts is consistent with involvement of a subduction-modified mantle component, possibly including a pelagic sediment component, which may have evolved with high\u00E2\u0080\u009876Hf\u00E2\u0080\u009D7f(and87Sr/6) relative to \u00E2\u0080\u009843Nd!\u00E2\u0080\u0099I\u00E2\u0080\u0099d. Melting conditions and estimated major-element composition of primary low-Ti magma The conditions of partial melting and composition of primary melts can be estimated using a modeling technique of Herzberg et a!. (2007). All of the high-titanium basalts have MgO contents that are too low to represent direct partial melts of mantle peridotite and underwent significant fractional crystallization. The low-titanium basalts extend to higher MgO contents, but several high-MgO basalts and picrites have been sampled in the Alaska Range and are the best candidates for least-modified partial melts of the mantle source. In addition to several high-MgO, low-titanium basalts (samples 5727A6, 58l0A2, 5802A3), we use one Rainy Creek picrite (sample 5808A2; Fig. 3.9) to 138 estimate melting conditions and major-element primary magma composition because it is available. However, this sample is from outside the main flood basalt stratigraphy and has a distinct LREE-enriched trace-element composition, and a slightly offset isotopic composition. Several high-MgO, low-titanium basalts are more typical of the volcanic stratigraphy. Herzberg et a!. (2007) have provided a thorough description of a technique for inverse and forward modeling for estimating mantle temperature, melt fraction, primary magma composition, and source residue composition using major elements. The estimated melting conditions and primary magma compositions for several high-MgO lavas from the Nikolai Formation, and a brief summary of the modeling method, are presented in Figure 3.23 and Table 3.6. Calculated mantle potential temperatures for several Nikolai picrites (\u00E2\u0080\u0094\u00E2\u0080\u00981495\u00C2\u00B0C) are significantly higher than ambient mantle that produces MORB (\u00E2\u0080\u0094\u00E2\u0080\u00981 280-1400\u00C2\u00B0C; Herzberg et al., 2007). If the Nikolai basalts were derived from accumulated fractional melting of fertile peridotite, the primary magmas would have 15-17 wt % MgO and 10-11 wt % CaO, formed from 25-29% partial melting, and would have crystallized olivine with a Fo content of\u00E2\u0080\u0094\u00E2\u0080\u00999l (Fig. 3.23; Table 3.6). A picrite from the Rainy Creek area requires very little addition of olivine, however, it has a trace-element and isotopic composition that is distinct from the basalts (e.g. sample 5808A2; Table 3.6). The estimated primary magma compositions and melting conditions for Nikolai picrites are similar to estimates for Ontong Java, where Herzberg (2004) found that primary magmas would have formed by 27% partial melting with mantle potential temperatures of \u00E2\u0080\u0094\u00E2\u0080\u00981 500\u00C2\u00B0C (Table 3.6). The estimated melting conditons of several Nikolai picrites indicate high-degree melting of unusually hot mantle peridotite, which is consistent with a plume initiation model for the Nikolai basalts. The geochemical data for the low-titanium basalts reflects significant involvement of subduction-modified lithospheric mantle, or even has its source in the lithospheric mantle; however, this would lead to the formation of comparatively small-degree melts since lithospheric mantle is cold relative to plume-derived magmas at the same pressure. This raises the question for future studies as to whether the low-titanium basalts may have involved mixing of plume-derived melts and low-degree partial melts of the arc lithospheric mantle. 139 14 12 10 8 6 4 14 12 10 0 10 20 30 MgO (wt%) 8 Nikolai Formation Clearwaterpicrite G Rainy Creek picrite 6 0 Low-Ti basalt \u00E2\u0080\u00A2 High-Ti basalt \u00E2\u0080\u0094 Olivine addition model 4 + Olivine addition (5% increment) o Primary magma Figure 3.23 Estimated primary magma compositions for two picrites from the Nikolai Formation (samples 5808A2, 5727A6) using the forward and inverse modeling technique of Herzberg et al. (2007). Nikolai basalt and picrite compositions and modeling results are overlain on diagrams provided by C. Herzberg. (a) Whole-rock FeO vs. MgO for Nikolai samples from this study. Total iron estimated to be FeO is 0.90. Green lines show olivine compositions that would precipitate from liquid of a given MgO-FeO composition. Black lines with crosses show results from olivine addition (inverse model) using PRIMELT1 (Herzberg et al., 2007). (b) Si02 vs. MgO with Nikolai basalt and model results. (c) FeO vs. MgO with Nikolai lava compositions and results of forward model for accumulated fractional melting of fertile peridotite. (d) CaO vs. MgO with Nikolai basalt compositions and model results. See Herzberg et a!. (2007) for complete description of the modeling technique. Potential parental magma compositions for the high-MgO lava series were selected (highest MgO and appropriate CaO) and, using PRIMELT1 software, olivine was incrementally added to the selected compositions to show an array of potential primary magma compositions (inverse model). Then, using PRIMELT1, the results from the inverse model were compared to a range of accumulated fractional melts for fertile peridotite, derived from parameterization of the experimental results of Walter (1998) (forward model; Herzberg & O\u00E2\u0080\u0099Hara, 2002). A melt fraction was sought that was unique to both the inverse and forward models (Herzberg et a!., 2007). A unique solution was found when there was a common melt fraction for both models in FeO-MgO and CaO-MgO-Al3 Si02 (CMAS) projection space. This modeling assumes olivine was the only phase crystallizing and ignores chromite precipitation, and possible augite fractionation in the mantle (Herzberg & O\u00E2\u0080\u0099Hara, 2002). Results are best for a residue of spinel lherzolite (not pyroxenite). The high- and low-titanium basalts cannot be used for modeling because they are all plag + cpx + ol saturated. 0 10 20 30 40 MgO (wt%) IlillilulIll 1111111 CaO (w/o) (d) liii\u00E2\u0080\u0099 0 00 :\u00C2\u00B0 0idotitePjalelts \u00E2\u0080\u0094 \u00E2\u0080\u00A2 Liquid compositions Fertile peridotite source \u00E2\u0080\u00A2 Accumulated Fractional Melting model Red lines = initial melting pressure Rlue lines = final melting pressure illuluiuiuu .111.1,, multi 140 Table 3.6 Estimatedprimaly magma compositionsfor Nikolai basalts and other oceanic plateaus/islands Sample 5727A6 5808A2 5810A2 5802A3 Average OJP Mauna Keab Gorgonaa Area Tangle Rainy Creek Tangle Clearwater (Weight %): SiC2 48.4 48.7 47.1 47.2 47.8 48.0 46.3 46.1 Ti02 0.43 1.09 0.44 0.45 0.60 0.62 1.93 0.56 A1203 13.5 11.1 13.1 14.5 13.0 12.3 9.6 11.7 Cr203 0.07 0.20 0.05 0.06 0.09 0.07 0.26 0.16 Fe203 0.94 1.11 0.91 0.90 0.97 0.90 1.08 1.18 FeC 8.7 9.1 9.5 8.8 9.0 9.2 10.3 10.1 MnO 0.18 0.16 0.20 0.18 0.18 0.17 0.18 0.18 MgO 15.7 16.8 17.6 15.7 16.4 16.8 18.3 18.8 CaO 11.2 9.9 9.9 10.7 10.4 10.3 10.1 10.0 Na20 0.71 1.39 0.99 1.03 1.03 1.36 1.67 1.04 1<20 0.04 0.36 0.06 0.51 0.24 0.08 0.41 0.03 NiC 0.06 0.09 0.08 0.03 0.07 0.10 0.08 0.11 Eruption T(C) 1361 1385 1401 1361 1377 1382 1415 1422 Potential T(\u00C2\u00B0C) 1476 1503 1521 1476 1494 1500 1606 Focontent(olivine) 91.1 91.3 91.5 91.2 91.3 90.5 91.38 90.6 Melt fraction 0.27 0.29 0.28 0.25 0.27 0.27 0.28 %ol addition 11.7 1.8 22.2 16.1 13.0 18 a Ontong Java primary magma composition for accumulated fraction melting (AFM) from Herzberg (2004). Mauna Kea primary magma composition is average of 4 samples in Table 1 of Herzberg (2006). Gorgona primary magma composition for AFM for iF, fertile source, in Table 4 of Herzberg and OHara (2002). Area: Tangle, Tangle Lakes area; Rainy Creek, Rainy Creek area (Fig. 6); Clearwater, Clearwater Mountains (Fig. 3.8) 141 CONCLUSION The Nikolai Formation in southern Alaska forms an arcuate belt 45O km long that extends eastward into southwest Yukon. The volcanic stratigraphy of the Nikolai Formation in the Wrangell Mountains and Alaska Range (Amphitheater and Clearwater Mountains) is approximately 3.5-4 km thick and formed as part of an extensive oceanic plateau in the Middle to Late Triassic during a single, short-lived phase of volcanism lasting <5 Myr. The Nikolai Formation is bounded by marine sedimentary sequences and uncomformably overlies Late Paleozoic volcanic arc sequences. The volcanic stratigraphy is predominantly subaerial flows in Alaska, but consists of\u00E2\u0080\u00945OO m of submarine flows and basal sills intruding pre-existing shale in the southern Alaska Range. The Nikolai Formation is composed of high- and low-titanium basalts that record a change in the source of magmas that constructed the Wrangellia oceanic plateau in Alaska. The low-titanium basalts form the lowest --4OO m of volcanic stratigraphy in the Alaska Range, and the remainder of the volcanic stratigraphy in the Alaska Range and all of the sampled stratigraphy in the Wrangell Mountains is high-titanium basalt. The geochemistry of the erupted sequences of the Wrangellia oceanic plateau in Alaska provide a way of detecting the different contributions from the crust and lithospheric mantle, and plume-type mantle. The high-titanium basalts were derived from a uniform O]13 plume-type Pacific mantle source, with similar initial Hf and Nd isotopic ratios to Ontong Java. The low-titanium basalts require involvement of a HFSE-depleted, high eHf source component that is distinct from OIB and MORE and was only involved during the early phase of this major melting event. There is no indication as yet discovered of a transitional lava type, nor do the high-titanium basalts indicate significant assimilation or interaction of lithospheric material so as to be detectable with geochemistry relative to the volume of the magmas erupted. Whereas almost all CFBs, and at least one oceanic plateau (e.g. Kerguelen), record involvement of continental lithosphere, Wrangellia flood basalts in Alaska do not indicate involvement of low sNd, low sHf continental material. However, the low titanium basalts have compositions that indicate melting and/or interaction with subduction-modified mantle was involved in their formation. 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Petrologic and geochemical constraints on the petrogenesis of Permian-Triassic Emeishan flood basalts in southwestern China. Lithos 58(3-4), 145. 150 CHAPTER 4 Geochemistry of Flood Basalts from the Yukon (Canada) Segment of the Accreted Wrangellia Oceanic Plateau 1A version of this chapter has been submitted for publication. 151 INTRODUCTION Oceanic plateaus cover 10% of the seafloor (Ben-Avraham et a!., 1981) and form from melting events that are distinct from melting at mid-ocean ridges. The largest Phanerozoic examples of oceanic plateaus largely formed below sea-level (Ontong Java Plateau), or subsided below sea-level after their formation (Kerguelen Plateau), and the basement rock and direct evidence of the tectonic setting they formed in remains hidden beneath the surface of the ocean and the thick sequence of flood basalts. Oceanic plateaus and flood basalts are constructed on rifted margins, extinct mid-ocean ridges, old oceanic lithosphere, rifted parts of continental crust, or extinct or active oceanic arcs. Geochemical and stratigraphical studies of accreted oceanic plateaus where the base of the volcanic stratigraphy is exposed provide a means of discerning the tectonic setting during formation and its effect on the generation of magmas and their geochemical and isotopic characteristics. The erupted flood basalts preserve a record of the interaction between a plume and the oceanic lithosphere, without complications from thicker and geochemically distinct continental lithosphere (Saunders et a!., 1992). The Wrangellia flood basalts in the Pacific Northwest ofNorth America are part of an enormous oceanic plateau that formed in the Middle to Late Triassic and accreted to western North America in the Late Jurassic or Early Cretaceous (Jones et a!., 1977). Parts of the volcanic stratigraphy of this oceanic plateau extend over 2300 km in British Columbia (BC), Yukon, and Alaska where the flood basalts overlie Paleozoic arc volcanic and marine sedimentary sequences and are overlain by Late Triassic limestone. In Yukon, Canada, a significant part of the Wrangellia flood basalts (Nikolai Formation) are exposed in a belt that extends more than 500 km westward into Alaska and consists predominantly of well-layered subaerial flows. A companion study in Alaska has revealed two geochemically distinct groups of lavas that record a shift in the source region that produced the erupted basalts (see Chapter 3). In this study, we characterize the geochemical and isotopic variations of the Nikolai Formation in Yukon to provide insights about the construction and tectonic setting of this part of the Wrangellia oceanic plateau and to assess the source regions that were involved in the generation of the magmas. The geochemistry of underlying 152 Paleozoic volcanic arc sequences is also examined to evaluate whether the lower arc mantle lithosphere may have had a role in the formation of the Nikolai basalts. This work is used to support interpretations made about geochemical variations of the Nikolai Formation in Alaska, and also describes some of the key differences between these two areas. There are no previously published geochemical and Sr-Nd-Hf-Pb isotopic data of Wrangellia flood basalts in Yukon and this work substantially adds to our dataset as part of a larger project on the Wrangellia oceanic plateau in BC, Yukon, and Alaska. Previous work on Wrangellia flood basalts in Alaska and Vancouver Island by Richards et al. (1991) proposed a plume initiation model for the origin of Wrangellia flood basalts based on the large volumetric output of lavas over a short timespan, evidence of uplift preceding volcanism, and an absence of evidence of rifting. GEOLOGIC SETTING AND AGE CONSTRAINTS In the southwest corner of Yukon, Wrangellia forms a southeast- to northwest- trending linear belt (<20 km wide and 300 km long), bordered by Alaska to the west and British Columbia to the south (Fig. 4.1). In the Kiuane Ranges, Wrangellia stratigraphy lies adjacent to older units of the Alexander terrane, in a southeast-tapering wedge between the Duke River and Denali Faults (Fig. 4.1). Late Mesozoic and Cenozoic strike- slip faulting that juxtaposed parts of the Wrangellia and Alexander Terranes resulted in extensive folding, faulting, and low-grade metamorphism of Wrangellia stratigraphy (Israel et a!., 2006). The Denali Fault is a major terrane boundary between Wrangellia and the Nisling and Windy-McKinley Terranes to the northeast, where there has been \u00E2\u0080\u0094370 km of right-lateral displacement since the mid-Cretaceous (Lowey, 1998). The oldest definitive units ofWrangellia in Yukon are Late Paleozoic arc volcanic sequences and marine sediments of the Skolai Group (Station Creek and Hasen Creek Formations; Fig. 4.2). The Nikolai Formation unconfomably overlies the Hasen Creek Formation in most areas in Yukon and there are isolated occurrences of Middle Triassic argillite-chert-siltstone-limestone (<100 m thick; Hoge Creek succession) between the Skolai Group and Nikolai Formation (Read & Monger, 1976; Israel et al., 2006). The Paleozoic formations are intruded by mafic and ultramafic sills related to the overlying Nikolai Formation (Hulbert, 1997). The Nikolai Formation covers \u00E2\u0080\u0094700 km2 (<3% of all 153 ,,,,,_,/ 14b\u00C2\u00B0 WI ALASKA Alaska Range 62\u00C2\u00B0N uane Ranges T< Vngefl> YUKON - BRmSH ,\u00E2\u0080\u0098 1L%/\ %%%%., aI1otte \ 2\u00E2\u0080\u0099 - I Vancouver \u00E2\u0080\u00A2 \u00E2\u0080\u0098: - \u00E2\u0080\u0098 1 h M Island -1 \u00C2\u00B0N eem N - 4 \u00E2\u0080\u0098 /j?\u00E2\u0080\u0099 - -- - \u00E2\u0080\u00A2 uncton o - u , 1 \u00E2\u0080\u0098S - \u00E2\u0080\u00A2 - S \u00E2\u0080\u00A2 - \u00E2\u0080\u00944- 1 S \u00E2\u0080\u00A2, r S 55 55 1 4 \ 2\SEWARD GLACIER 1 VUKOf4 /fr - I\u00E2\u0080\u009D2- C \u00E2\u0080\u00982 \u00E2\u0080\u0098 ALAS \u00E2\u0080\u00A2$?) S S ) \u00E2\u0080\u0094 \u00E2\u0080\u00A2 S-S S - / N ,\u00E2\u0080\u00A25 Y 0 1400 WI 50 km - S - \u00C3\u00A7 iiS CUMBIA Figure 4.1 Simplified map of southwest Yukon showing the distribution of the Nikolai Formation (black; after Israel, 2004; Israel & Van Zeyl, 2004; Israel et a!., 2005). The main area of field study in the Kluane Ranges is specified with a black box. The inset shows the extent of the Wrangellia flood basalts (green) in BC, Yukon, and Alaska. 154 Figure 4.2 Geologic map and stratigraphy of the northern part of the Kluane Ranges, Yukon (location of map shown in Fig. 4.1). (a) Stratigraphic column of Wrangellia units exposed in the Kluane Ranges, derived from the works of (Read & Monger, 1976; Israel & Van Zeyl, 2004; Israel eta!., 2005). (b) Simplified geologic map of the Kluane Ranges (1:50,000 scale) between the Donjek River and Burwash Creek, derived from (Israel & Van Zeyl, 2004; Israel et a!., 2005). Nikolai Formation PENNSYLVANIAN AND I Skolai Group Hasen Creek Formation El Station Creek Formation 155 Wrangellia flood basalts from Vancouver Island to central Alaska) from the Mush Lake area to the Nutzotin Mountains and is best preserved in the Kluane Ranges where the total estimated thickness is \u00E2\u0080\u00944000 m (accurate estimation of the thickness is not possible because of extensive faulting and folding) (Figs 4.1 and 4.2). The Nikolai Formation is capped by shallow-water to deep-water limestone and shale of the Upper Triassic to Lower Jurassic Chitistone and Nizina Limestones and McCarthy Formation (Read & Monger, 1976; Armstrong & MacKevett, 1977; MacKevett, 1978) (Fig. 4.2). Thick deep marine fan deposits of the Jurassic Dezadeash Formation overlap the Wrangellia and Alexander Terranes (Read & Monger, 1976) and mid-Cretaceous arc plutonic rocks of the Kluane Ranges suite intrude parts of Wrangellia (Plafker & Berg, 1994). Detailed regional mapping in southwest Yukon was accomplished by Muller (1967), Read and Monger (1976), and Dodds and Campbell (1992a, b, c) and recent mapping in the Kluane Ranges (1:50,000 scale) is by Israel et al. (2004; 2005; 2007a, b). Daonella imprints in shale immediately underlying the Nikolai Formation and conodonts in limestone overlying the Nikolai Formation, in Yukon and Alaska, indicate Late Ladinian to Late Carnian-Early Norian ages (ca 227-2 16 Ma; Ogg, 2004; Furin et al., 2006), respectively (Smith & MacKevett, 1970; Read & Monger, 1976; MacKevett, 1978; Israel et aL, 2006). U-Pb dating of zircon separated from a gabbro sill possibly related to the Nikolai Formation yielded a Ladinian age of 232.2 \u00C2\u00B1 1.0 Ma (average 207Pb/6 age of 3 discordant (1.6 to 2.4%) analyses from multi-grain zircon fractions) (Mortensen & Hulbert, 1991). Five40Ar/39rages of hornblende and biotite from intrusive rocks in the Alaska Range, interpreted to be comagmatic with Nikolai basalts, indicate formation of these rocks at ca. 231-225 Ma (Bittenbender et al., 2007; Schmidt & Rogers, 2007). In addition, three Nikolai basalt samples from the Wrangell Mountains yielded40Ar/39r step-heating ages of 228.3 \u00C2\u00B1 5.2, 232.8 \u00C2\u00B1 11.5, and 232.4 \u00C2\u00B1 11.9 Ma (Lassiter, 1995). FIELD RELATIONS AND PETROGRAPHY Fieldwork was undertaken from Mush Lake to the Donjek River to examine and sample the volcanic stratigraphy of the Nikolai Formation and underlying Paleozoic sequences (Fig. 4.1). Complete sections of the original volcanic stratigraphy are not 156 preserved in any one area because of the pervasive faulting in southwest Yukon. The basal flows of the Nikolai Formation in Yukon are composed of volcanic breccia and minor pillow lava less than 70 meters in total thickness (Fig. 4.3). Conglomerate with rounded clasts derived from the underlying Paleozoic formations also occurs along the base. The basal conglomerate is laterally discontinuous, and bound by faults in several localities (Read & Monger, 1976; Israel & Cobbett, 2008), and these structures are interpreted as grabens associated with uplift or rifting during the initial phase of Nikolai volcanism. Above these basal units, almost all the flood basalts (90-95%) are subaerial flows that form monotonous maroon- and green-colored sequences marked primarily by amygdaloidal-rich horizons (Fig. 4.3). No discernible erosional surfaces are preserved along flow contacts and rarely are any sediments found between flows (Fig. 4.3). Several occurrences of hematite-rich tuff and breccia are preserved between massive flows (Read and Monger, 1976). The massive lava flows are commonly <10 m thick and rarely exhibit columnar jointing. Close to the top of the flood basalt sequence, the basalt flows are commonly interbedded with thin lenticular beds of marine limestone (<1 meter thick). Several occurrences of the interbedded limestone reach close to 30 m in thickness (e.g. Read and Monger, 1976). There is no evidence of detrital material from a continental source anywhere associated with the flood basalts. The Nikolai Formation in Yukon is divided into high- and low-titanium basalts, based on geochemistry; the two types also have distinct petrographic differences (Table 1). Much of the Nikolai Formation in Yukon is intensely altered; eighteen of the 59 samples (16 of 42 low-titanium basalts; 2 of 17 high-titanium basalts) are pervasively altered with no remaining primary igneous texture or minerals (Table 1). Seventeen of the 41 low-titanium samples with discernible igneous texture exhibit glomeroporphyritic to porphyritic texture (plagioclase phenocrysts) and 8 samples are aphyric with intersertal to intergranular texture (Table 1; Fig. 4.4). Fourteen of the 17 high-titanium samples are coarse-grained with subophitic textures and contain plagioclase laths (1-2 mm long) and Fe-Ti oxides (>1 mm), which are more abundant in the high-titanium basalts (Table 1; Fig. 4.4). Plagioclase is commonly replaced by epidote, chlorite, sericite, clinozoisite, and clay minerals and clinopyroxene is mostly unaltered. Fe-Ti oxides are altered to sphene and leucoxene mineral. Hematitic staining is common throughout most subaerial flows 157 Figure 4.3 Photographs of the Nikolai Formation in the Kluane Ranges, Yukon. (a) Vertically-oriented subaerial basalt flows south of Quill Creek, above the headwaters of the Tatamagouche Creek. Dali sheep (white dots) in center of photograph for scale. (b) Pillow basalt at the base of the Nikolai Formation. Hammer (45 cm) for scale. (c) Massive amygdaloidal basalt typical of the Nikolai Formation in Yukon. Hammer (45 cm) for scale. 158 Figure 4.4 Photomicrographs of representative Nikolai basalts in southwest Yukon. (a) Glomeroporphyritic pillow basalt with abundant zeolite-filled amygdules in cross-polarized transmitted light (sample 2151; low-titanium basalt). (b) Thin-section scan of typical red-stained amygdaloidal basalt in plane-polarized transmitted light (sample 482 1A4; no chemistry). (c) Abundant plagioclase glomerocrysts in a fme-grained intersertal groundmass in cross-polarized transmitted light (sample 481 OAI 0; low-titanium basalt). (d) Aphyric basalt with abundant Fe-Ti oxides in cross-polarized transmitted light (sample 4812A6; high-titanium basalt). Scale bars are labeled in each of the photographs. Table 4.1 contains detailed descriptions of the petrographic characteristics ofNikolai samples. 159 Table 4.1 Summary ofpeirographic characteristics andphenocryst proporlions ofNikolai basalis in Yukon OC PLO low-li glomero, amygdaloidal DJ PLO low-TI intergranular DJ PIL low-li relict glomero, perv alt DJ PLO low-li glomero, amygdaloidal 01 PLO low-Ti porphyritic CC PLO low-Ti relict porphyritic CC PLO low-Ti pervaalve alteration DJ PLO low-Ti glomero CC PLO low-Ti pervaaive alteration CC PLO low-Ti perv alt, relict glomero DJ PLO low-li c.g. intergranular CC PLO low-li perv alt, relict glomero DJ PLO low-li aphyric, intergranular 01 PLO low-li glomero, amygdaloidal CC PLO low-li relict glomero CC PIL low-li relict intersertal, aphyric CC PLO low-li relict giomero CC PLO low-li lotersertal, intergranular OC PLO low-li aphyric wl chiled margin CC PLO low-li porphyritic, interaertal OC PLO low-li perv alt 01 PLO low-li intergranular, glomero CC PLO low-li pervasive alteration CC PLO low-li pervasive alteration 01 PLO low-li porphyritic, pteg-phyric ML PLO low-li pervasive alteration CC PIL low-li glomero, \u00E2\u0080\u0098mteraertal Dl PLO low-li glomero, intergranular ML PLO low-Ti pervasive alteration CC PLO low-li aphyric DJ PLO low-li subophitic, ophimottied ML PLO low-li pervasive alteration OC PLO low-TI pervasive alteration CC PLO low-Ti intergranular, aphyric OC PLO low-Ti pervasive alteration ML PLO low-Ti pervasive alteration Dl PLO low-Ti glomero, intersertal CC PLO low-Ti pervasive alteration CC PLO low-li aphyric ML PLO low-li pervasive alteration Dl PLO low-li aphyric, interaertal 1031 CC PBRE basal aphyric, intersertal 3 amyg <5 mm, few plsg glcr <2 mm I Pe-oxy alt, patchy zones alt, few plag phenos <2 mm 5 1 relict plag glcr <2 mm 2 plag glcr <2 mm 2 tabular plag phenos <2 mm, p1 laths <1 mm in gm 3 outiine of plag phenos <2 mm, f.g. plag needles 5 faint outilne of plag needles <0.5 mm 2 abund plag glcr <2 mm, alt felty f.g. gm 5 no primary texture or minerals 5 faint sign of plag glcr <3 mm, f.g. gm 3 few alt plag phenos <2 mm, patchy alt 5 relict deformed plag phenos <2 mm 10 2 ox-rich, plag laths <1mm, patchy zones alt 1 \u00E2\u0080\u009425% amyg, plag gic and phenos <2mm 4 fresh cpa <0.5 mm, alt plag glcr <3mm 4 plag needles <0.5 nan, patchy zones alt 3 pteg replaced, cpx <0.5 mm, relict pteg glcr <3 mm 2 pteg needles <0.5 mm, few plag glcr <2 mm 3 chilled margin wl microphenoa <0.5 mm 2 plagphenoa<2mm,f.g.gm 5 hint of plag glomerocryata 2 plaglatha S 0. 0) 0. E Cu 0 10 CsRbBaThU K NbTaLaCePbPr NdSrSmZr Hf Ti EuGdTbDyHoY ErYbLu 11111111 111111111 1111111111 Low-titanium basalts (d) I 11111111111111111111111111 Cs RbBa Th U K NbTa La CePb Pr Nd Sr SmZr I-sf Ti Eu GdTb Dy Ho Y Er Yb Lu 10 100 10 100 10 La Ce Pr Nd Pm Sm Eu Gd Tb Dy Ho Er Tm Yb Lu 0) C Cu S 0 8, 0. S Cu 0 La Ce Pr Nd Pm Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Cs RbBa Th U K Nb Ta La CePb Pr Nd Sr SmZr Hf TI Eu GdTh Dy Ho Y Er Yb Lu degree of variability of LILE (Cs, Rb, Ba, K). The high- and low-titanium basalts form two distinct trends on plots ofNb vs. Zr and Th vs. Hf (Fig. 4.7). Sr-Nd-Hf-Pb isotopic compositions The low-titanium basalts have mostly higher initial EHf (+11.1 to +15.8) and lower initial 8Nd (+2.3 to +6.8) than the high-titanium basalts (initial 8Hf= +10.4 to +12.0; initial ENd= +6.6 to +9.0). One low-titanium basalt sample (4808A2) lies within the range of initial EHf for the high-titanium basalts and a separate sample (481 6A5a) from outside the Kluane Ranges has particularly low initial 8Hf (+9.3) and initial 8Nd (+2.3) (Fig. 4.8; Tables 3 and 4). The high- and low-titanium basalts have wide ranges of overlapping initial Sr isotopic compositions, which is likely due to the effects of secondary alteration. Two Paleozoic arc samples overlap the range of Sr and Nd isotopic compositions of the high- and low-titanium basalts and have similar 176Hf7\u00E2\u0080\u00997 fand\u00E2\u0080\u009876Lu/\u00E2\u0080\u00997Hfto the high- titanium basalts (Fig. 4.8). A single sample of the basal pillow breccia lies at the low end of the ranges of high- and low-titanium basalts in Hf and Nd, and close to the upper end of the ranges in Sr. The range of present-day Pb isotope ratios are similar for high and low-titanium basalts, with low-titanium basalts extending to slightly lower Pb isotope ratios (high-titanium basalt: 206Pb/4= 19.008-20.297, 207Pb/4 = 15.579-15.627, 208Pb/4 = 38.453-39.090; low-titanium basalt: 206Pb/4= 18.561-19.836, = 15.559-15.646, 208Pb/4 = 38.118-38.755 ) (Fig. 4.8; Table 5). Both the basal pillow breccia and Paleozoic arc samples lie within the range of Pb isotopic compositions for the high- and low-titanium basalts (Fig. 4.8). DISCUSSION Effects of alteration and comparison to Nikolai basalts in Alaska Since eruption of the Nikolai Formation at ca. 230 Ma, the flood basalts have undergone burial metamorphism during subsidence of the Wrangellia plateau (Late Triassic-Early Jurassic), a pre-mid-Cretaceous contraction event related to accretion of Wrangellia (Late Jurassic-Early Cretaceous), and post-mid-Cretaceous strike-slip faulting during northward transport of Wrangellia along the continental margin (MacKevett et a!., 1997; Israel et a!., 2006). More than 3600 m of marine sediments accumulated on top of 171 15 (\u00E2\u0080\u0098I\u00E2\u0080\u0099 ) 2 4 6 8 10 12 14 16 1 5 0.0 0.2 0.4 0.6 0.8 1.0 Hf (ppm) U (ppm) Figure 4.7 Whole-rock trace-element concentrations for the Nikolai Formation in Yukon, with data for the Nikolai Formation in Alaska (Greene et a!., submitted). (a) Nb vs. Zr. (b) Ni vs. MgO. (c) Th vs. Hf. (d) Th vs. U. 300 250 10 5 0 200 150 \u00E2\u0080\u00A2 I1II\u00E2\u0080\u0099 N (ppm) 0 1741 4816A5aQj \u00E2\u0080\u00A2 i::i b) \u00E2\u0080\u00A2 n. . \u00E2\u0080\u00A2 \u00E2\u0080\u00A2 \u00E2\u0080\u00A2 100 0 50 100 50 Zr (ppm) 0 150 if,1 I I MgO(wt%) 2.0 1.5 8 1.0 I I I I I I I I I I I I I I I I I Th (ppm) 1141 4816A5a x oA A A IDDP \u00E2\u0080\u00A2 ooo 1451 A (d) I I FI I I I I I I I I I I I I 0.5 0.0 0 1 2 3 4 172 \u00C3\u0094.00 0.02 0.04 0.08 0.58 \u00E2\u0080\u00A2 12 0.706 2 4 6 8 \u00C2\u00A3Nd(230 Ma) 18.0 18.5 19.0 19.5 20.0 20.5 21.0 18.0 18.5 19.0 19.5 20.0 20.5 21.0 206Pb/4 206Pb/4 Figure 4.8 Whole-rock Sr, Nd, and Hf isotopic compositions for the Nikolai Formation in Yukon, with fields for high- and low-titanium basalts of the Nikolai Formation in Alaska (Greene et a!., 2008, submitted). (a) Initial CNd vs. 87SrI6. Age-correction to 230 Ma. (b) Measured\u00E2\u0080\u009876IJf/177Hf vs. 176Lu/7Hf. (c) Initial c vs. 8Nd\u00E2\u0080\u00A2 (d) Measured 207PbP\u00C2\u00B04b vs. 206PbJ4. (e) Measured 208Pb/4 vs. 206Pb/4. Average 2a error bars shown in lower right corner of panels; they are smaller than symbols in the Pb isotope ratio plots. Results from a complete chemistry duplicate, shown in Tables 3, 4, and 5 (sample 482 1AI), are circled in each plot (except panel b). \u00E2\u0080\u00A2 I \u00E2\u0080\u00A2 I 0.2834 4821A1 1561 .e ENC Alaska high-Ti 9 8 7 6 5 4 3 176HfI7f Alaska low-li QD 9 D\u00E2\u0080\u00944810A0 l\u00E2\u0080\u0094 4808A2 \u00E2\u0080\u009C4818A5a Alaska hh-Ti \u00E2\u0080\u009876Lu/\u00E2\u0080\u00997Hf +(b) \u00E2\u0080\u0098 I I 0.2033 0.2832 0.2831 A \u00E2\u0080\u00A2 \u00E2\u0080\u00A20.2830LI 0.2829 A\u00E2\u0080\u0099aska low-Ti / A 4816A5a / .LII EHf(23 Ma) Alaska low-Ti/ LI LI LI 4810A80 \u00E2\u0080\u00A2 < 4808A2 Alaska high-Ti (a) 0.702 10 8 0.703 18 16 14 1561 12. 4821A1 8 4816A5a \u00E2\u0080\u00A21 LI (c) 0.704 0.705 87SrI 86Sr(230 Ma) I I I I I I i 15.64 15.62 15.60 15.58 LI4816I8o 1451 high-TiLI 10 / Alaska low-Ti f-\u00E2\u0080\u00944821A1 4810A8\u00E2\u0080\u0094 \u00E2\u0080\u00A2\u00E2\u0080\u00941561 AlasIa high-Ti\u00E2\u0080\u0099 1451 39.2 208PbI4 39.0 38.8 38.6 Alaska low-Ti 4821A1 \u00E2\u0080\u0094 1561 -38.4 38.2 (e) 441\u00E2\u0080\u0094LI 15.56 2191{] (d) \u00E2\u0080\u00A2 high-titanium basalt D low-titanium basalt X basal flow A Paleozoic arc 173 10 31 B as al QC 3. 94 29 6 0. 70 50 96 6 0. 03 8 0. 70 49 7 2. 74 11 .5 7 0. 51 27 94 6 3. 0 0. 14 29 0. 51 25 8 4. 6 48 07 A 4 Pa le oz oi c QC 10 .8 0 18 2 0. 70 46 25 11 91 Pa le oz ol c QC 24 .4 8 26 5 0. 70 49 94 0. 17 33 0. 51 27 1 7.1 0. 18 14 0. 51 26 0 5.1 Ta bl e4 .3 Sr an dN d iso to pi cg eo ch em ist ry o fN ik ol ai ba sa lts ,Y uk on Sa m pl e Gr ou pa Ar ea b Rb Sr o lS r/w Sr 2m 8TR b/ w Sr oT Sr /w Sr t Sm Nd 1 4 3 N dP \u00E2\u0080\u009DN d 2m 8N d 1 4 7 S 1 T 1 / 1\u00E2\u0080\u009D N d \u00E2\u0080\u00984 3 N d lN d 1 EN d( t) (pp m) (pp m) 23 0 M a (pp m) (pp m) 23 0 M a 48 20 A1 Hi -T I HC 5. 93 26 6 0. 70 45 96 6 0. 06 4 0. 70 43 9 4. 17 14 .5 5 0. 51 29 79 5 6. 7 0. 17 33 0. 51 27 2 7. 3 16 91 Hi -T i QC 4. 27 29 8 0. 70 53 70 7 0.0 41 0. 70 52 3 4. 22 14 .2 9 0. 51 29 78 6 6. 6 0. 17 85 0. 51 27 1 7. 2 14 51 Hi -T i QC 6. 70 31 7 0. 70 36 92 8 0.0 61 0. 70 34 9 3. 83 13 .1 5 0. 51 29 45 7 6. 0 0. 17 60 0. 51 26 8 6. 6 48 16 A 2 Hi -T i M L N/ A 55 0. 70 39 88 8 3. 44 12 .0 4 0. 51 29 78 9 6. 6 0. 17 26 0. 51 27 2 7. 3 15 61 Hi -T i QC 13 .91 11 5 0. 70 40 85 8 0. 34 9 0. 70 29 4 3. 06 8. 35 0. 51 31 37 8 9. 7 0. 22 18 0. 51 28 0 9. 0 48 21 A1 H i-l i BC 13 .3 5 10 4 0. 70 35 05 7 0.3 71 0. 70 22 9 2. 56 6. 81 0. 51 31 41 7 9. 8 0. 22 77 0. 51 28 0 8. 9 48 21 A1 (du p) Hi -T i BC 13 .3 5 10 4 0. 70 35 25 7 0.3 71 0. 70 23 1 2. 56 6. 81 0. 51 31 43 7 9. 9 0. 22 77 0. 51 28 0 8. 9 48 08 A 2 Lo w -T i QC 13 .6 0 45 1 0. 70 40 41 7 0. 08 7 0. 70 37 6 3. 07 13 .0 7 0. 51 28 85 6 4. 8 0. 14 21 0. 51 26 7 6A 48 16 A 5A Lo w -T i M L 21 .7 9 22 0 0. 70 56 77 8 0. 28 7 0. 70 47 4 2. 27 8. 76 0. 51 26 95 8 1.1 0. 15 65 0. 51 24 6 2. 3 13 01 Lo w -T i QC 3. 78 10 3 0. 70 40 01 7 0. 10 6 0. 70 36 5 1. 56 4. 67 0. 51 29 34 6 5.8 0. 20 20 0. 51 26 3 5.6 44 1 Lo w -T i QC 37 .7 2 17 4 0. 70 55 10 7 0. 62 9 0. 70 34 5 1. 14 2. 73 0. 51 30 47 7 8. 0 0. 25 34 0. 51 26 7 6. 3 88 1 Lo w -T i QC 12 .7 5 10 2 0. 70 49 44 9 0. 36 3 0. 70 37 6 1. 27 3. 22 0. 51 30 37 7 7. 8 0. 23 76 0. 51 26 8 6. 6 21 91 Lo w -T i QC 10 .0 0 37 0 0. 70 56 77 8 0. 07 8 0. 70 54 2 1.4 1 4. 18 0. 51 29 20 6 5. 5 0. 20 48 0. 51 26 1 5.3 48 10 A 8 Lo w -T i DJ 22 .6 2 37 6 0. 70 51 06 6 0. 17 4 0. 70 45 4 1. 58 5. 46 0. 51 29 55 6 6. 2 0. 17 48 0. 51 26 9 6. 8 7 0. 17 2 0. 70 40 6 6. 52 22 .7 4 0. 51 29 66 6 6. 4 8 0. 26 7 0. 70 41 2 2. 69 8. 98 0. 51 28 74 7 4. 6 8H i-T i, hi gh -ti tan iu m ba sa lt; Lo w- Ti ,l ow -ti tan iu m ba sa lt; B as al ,b as al pil low br ec ci a, Pa le oz oi c, Pa le oz oi c ar c (St ati on Cr ee k Fo rm at io n). bA bb re vi ati on s f or ar ea ar e: QC ,Q uil lC re ek ;M L, M us h La ke ;D J, D on jek Ri ve r BC ,B uJ w as h Cr ee k; HC ,H alf br ee d Cr ee k. (du p) in di ca te sc o m pl et e ch em ist ry du pl ic at e. N/ A is n o an al ys is. Al li so to pi c an al ys es ca rr ie d o u ta tt he PC IG R. Tr ac e el em en t a n al ys es w er e pe rfo rm ed at Ac tiv ati on La bo ra to rie s( Ac tLa bs) .A na ly tic al m et ho ds gi ve n in A pp en di x E. Table 4.4 Hf isotopic compositions ofNikolai basalts, Yukon Sample Groups Areab Lu Hf 1Hf/76f 2m 4it 176Lu/7Hf 1Hf/T6f (ppm) (ppm) 230 Ma 4820A1 Hi-Ti HC 041 3.28 0.283044 6 9.6 0.0179 0.28296 12.0 1691 Hi-Ti QC 0.33 2.89 0.283006 5 8.3 0.0161 0.28293 11.0 1451 Hi-Ti QC 0.26 2.89 0.282984 5 7.5 0.0129 0.28293 10.7 4816A2 Hi-Ti ML 0.31 2.54 0.282995 5 7.9 0.0172 0.28292 10.4 1561 Hi-Ti QC 0.35 1.93 0.283065 5 10.4 0.0260 0.28295 11.5 4821A1 Hi-Ti BC 0.34 1.86 0.283067 7 10.4 0.0261 0.28295 11.5 4821A1 (dup) Hi-Ti BC 0.34 1.86 0.283063 4 10.3 0.0261 0.28295 11.4 4808A2 Low-Ti QC 0.40 1.66 0.283090 6 11.2 0.0341 0.28294 11.1 4816A5A Low-Ti ML 0.35 1.84 0.283008 5 8.3 0.0273 0.28289 9.3 1301 Low-Ti QC 0.23 0.73 0.283270 4 17.6 0.0446 0.28307 15.8 441 Low-TI QC 0.24 0.66 0.283253 6 17.0 0.0527 0.28302 13.9 881 Low-TI QC 0.26 0.77 0.283280 5 18.0 0.0481 0.28307 15.6 2191 Low-TI QC 0.26 0.69 0.283285 6 18.1 0.0535 0.28305 15.0 481 0A8 Low-Ti DJ 0.24 0.82 0.283162 6 13.8 0.0409 0.28298 12.6 1031 Basal QC 0.25 1.52 0.282991 5 7.7 0.0234 0.28289 9.3 4807A4 Paleozolc QC 0.57 4.84 0.282995 6 7.9 0.0168 0.28292 10.5 1191 Paleozoic QC 0.25 1.79 0.282993 5 7.8 0.0200 0.28290 9.9 aHi..Tl high-titanium basalt; Low-Ti, low-titanium basalt; Basal,basal pillow breccia, Paleozoic, Paleozoic arc (Station Creek Formation). bpjbreviaons for area are: QC, Quill Creek; ML, Mush Lake; DJ, Donjek River; BC, Burwash Creek; HC, Halfbreed Creek. (dup) indicates complete chemistry duplicate. All isotopic analyses carried out at PCIGR. Trace element analyses were performed at Activation Laboratories (ActLabs). Analytical methods given in Appendix E. 175 Table 4.5 Pb isotopic compositions ofNikolai basalts, Yukon Sample Groups Areab U Th 2wPb/b 2m 207PbFPb 2m 208PbPwPb 2m (oom (oom 4820A1 Hi-Ti 1691 Hi-Ti 1451 Hi-Ti 4816A2 Hi-Ti 1561 HI-TI 4821A1 Hi-TI 4821A1 (dup) HI-TI HC 0.24 0.79 QC 0.23 0.64 QC 0.83 0.65 ML 0.16 0.58 QC 0.12 0.28 BC 0.06 0.27 BC 0.06 0.27 19.6386 0.0010 15.6144 0.0007 19.6362 0.0008 15.6103 0.0007 20.2967 0.0009 15.6272 0.0007 19.5320 0.0008 15.6073 0.0007 19.0263 0.0009 15.5790 0.0008 19.0089 0.0011 15.5831 0.0010 19.0075 0.0007 15.5833 0.0006 39.0899 0.0019 39.0232 0.0021 38.6070 0.0017 39.0425 0.0017 38.4525 0.0022 38.5045 0.0019 38.5049 0.0018 4808A2 Low-Ti 481 6A5A Low-Ti 1301 Low-Ti 441 Low-Ti 881 Low-Ti 2191 Low-Ti 481 0A8 Low-Ti 1031 Basal QC 1.44 1.07 ML 0.71 1.57 QC 0.14 0.36 QC 0.07 0.14 QC 0.15 0.30 QC 0.11 0.20 DJ 0.19 0.52 QC 0.44 1.39 19.8356 0.0006 15.6419 0.0007 19.3641 0.0011 15.6456 0.0008 19.1146 0.0006 15.6184 0.0007 18.9507 0.0013 15.5698 0.0011 19.1467 0.0007 15.5979 0.0006 18.5606 0.0007 15.5588 0.0008 18.8847 0.0006 15.5825 0.0005 19.3089 0.0008 15.6133 0.0007 38.4723 0.0018 38.7554 0.0022 38.7403 0.0022 38.3631 0.0029 38.4415 0.0014 38.1117 0.0019 38.3364 0.0015 38.7445 0.0017 4807A4 Paleozolc QC 0.40 1.25 1191 Paleozoic QC 0.38 1.14 19.5791 0.0009 15.6160 0.0008 19.2205 0.0010 15.6084 0.0009 39.0046 0.0021 38.7223 0.0024 Hi-Ti, high-titanium basalt; Low-Ti, low-titanium basalt; Basal,basal pillow breccia, Paleozoic, Paleozoic arc (Station Creek Formation). bAbbreviations for area are: QC, Quill Creek; ML, Mush Lake; DJ, Donjek River; BC, Burwash Creek; NC, Halfbreed Creek. (dup) indicates complete chemistry duplicate. All isotopic analyses carried out at PCIGR. Trace element analyses were performed at Activation Laboratories (ActLabs). Analytical methods given in Appendix E. 176 the Nikolai Formation (MacKevett et al., 1997), at least 2 km by the end of the Triassic (Armstrong & MacKevett, 1977). This systematic decrease in the accumulation rate of marine sediments during the Late Triassic (-200 m/Myr to -20 m/Myr) may have been controlled by cooling and deflation of the lithosphere following eruption of the flood basalts (Saltus et aL, 2007) or related to rifting. Seven whole rock K-Ar ages of the Nikolai Formation in the Wrangell Mountains yield an isochron age of 112 \u00C2\u00B1 11 Ma and indicate resetting of K-Ar systematics during tectonism related to northward transport of Wrangellia (MacKevett, 1978; Plafker et al., 1989). The least-altered samples were selected for chemistry and, although alteration and low-grade metamorphism has dramatically affected the primary igneous texture and mineralogy of the sampled Nikolai basalts, many of the analyzed basalt samples appear to have retained a large part of their primary geochemistry. There is no clear correlation between petrographic alteration index and chemical alteration indices (e.g. Ba/Rb vs. K20/P5)or LOl and any of the measured or age-corrected isotopic ratios; compositions of more mobile elements (Cs, Rb, Ba, and K) show some coherence with compositions of immobile elements (Nb, Zr, HREE). The Nikolai Formation in Yukon and Alaska formed as part of the same contiguous flood basalt province and the chemistry of most of the basalt flows in Alaska has not been significantly affected by alteration and metamorphism (see chapter 3). The similarity in volcanic stratigraphy, underlying and overlying formations, age, petrography, and chemistry between these two areas provides an excellent opportunity to compare the flood basalts that have undergone different degrees of alteration (Fig. 4.9). Figure 4.9 shows trace element concentrations of high- and low-titanium basalts in Yukon normalized to a mean for high- and low-titanium basalts from Alaska to help distinguish the general differences in compositions of the Nikolai basalts from these two areas. LILE show a higher degree of scatter in normalized trace-element patterns than HFSE and REE, especially for the low-titanium basalts (Figs 4.6 and 4.9). The high- titanium basalts in Yukon have similar concentrations to Alaska high-titanium basalts, with the exception of the LILE (Fig. 4.9). The differences in low-titanium basalt trace- element concentrations between Yukon and Alaska increase systematically from moderately incompatible to highly incompatible trace elements with almost all trace 177 Figure 4.9 Comparison of trace-element compositions of the Nikolai basalts in Yukon to averages for high- and low-titanium basalts from Alaska (Greene eta!., submitted). (a) High-titanium basalts from Yukon divided by the mean of high-titanium basalts from Alaska. (b) Low-titanium basalts from Yukon divided by the mean of low-titanium basalts from Alaska. Averages for all of the normalized patterns (except two indicated in panel a) are shown by the patterns with symbols in each of the two panels. Note the close similarity of the trace-element concentrations of high-titanium basalts from Yukon and Alaska (panel a), except for LILE (Cs, Rb, Ba, K). The trace-element concentrations of low-titanium basalts in Yukon are systematically higher than the average concentration of low-titanium basalts in Alaska for the more incompatible elements, with the most significant differences in Rb and K relative to adjacent elements. liii,,, I 11111 Co Rb Eu Th U K Nb Ta La Ca Pr Nd Sr Sm Zr Hf Ti Eu Gd Th Dy Ho Y Er Yb Lu E 10Co m 0) E Co Co C > 178 elements, except the HREE, having higher concentrations in Yukon basalts (Fig. 4.9). The low-titanium basalts in Yukon and Alaska both have HFSE depletions, but on average the Yukon low-titanium basalts have higher concentrations of HFSE. Similar to the high-titanium basalts, the LILE (mostly Rb and K) show the largest differences in low-titanium basalts from Yukon and Alaska and Sr shows considerable scatter (Fig. 4.9). The systematic differences between the low-titanium basalts in Yukon and Alaska are not simply the result of alteration. The low-titanium basalts in Yukon on average are more enriched in incompatible elements, which reflects differences in lava compositions erupted in Yukon and Alaska. Sr isotopic compositions are highly sensitive to alteration and metamorphism in oceanic basalts and the relatively high initial Sr isotopic compositions for the Nikolai basalts in Yukon (up to 0.7054) are probably due to an increase in 87Sr/6r through addition of seawater Sr andlor an increase in87Rb/6Srthrough preferential addition of Rb relative to Sr during low-temperature alteration (Hauff et al., 2003). The Sr isotopic compositions of the Nikolai basalts in Yukon cannot be used to assess magmatic processes and clear differences are apparent between Sr isotopic compositions from Yukon and Alaska (Fig. 4.8a). Several measured Pb isotopic compositions lie well outside the expected range of variation (samples 4808A2, 481 6A5a, 2191, and 1451) and cover a greater range, but several of the present-day Pb isotopic compositions from Yukon are similar to those of Nikolai basalts from Alaska. Relationship between chemistry and stratigraphic position In Yukon, unless volcanic stratigraphy is in unfaulted contact with underlying or overlying units, it is extremely challenging to be confident of the exact stratigraphic position of flows. The relative stratigraphic positions of 31 of the 60 analyzed basalt samples were estimated using a ranking system (1-near base; 5-near top). Six analyzed basalts from two traverses from the base of the Nikolai Formation several hundred meters upward into the volcanic stratigraphy were all low-titanium basalts. A total of 13 basalts appear to be located near the base of the volcanic stratigraphy and all of these samples are low-titanium basalts. Only three samples were estimated to come from near the upper contact with overlying Chitistone Limestone and each of these samples is high-titanium 179 basalt. Several samples from the vertically-oriented subaerial stratigraphy above Tatamagouche Creek (Fig. 4.3a) are high-titanium basalt; this section of stratigraphy does not appear to be close to the base of the Nikolai Fonnation and several interfiow limestones are present, which are typically found between flows in the upper part of the stratigraphy. Lastly, each traverse contained either high-titanium basalt or low-titanium basalt; both lava types were not collected in the same traverse. In Alaska, low-titanium basalts comprise the lower part of the volcanic stratigraphy (5O0 m) in the Alaska Range and high-titanium flows form most of the remainder of the 3 km of stratigraphy (see manuscript 2). Given these constraints on the chemostratigraphy, and the connection to stratigraphically distributed high- and low-titanium basalts in Alaska, a similar transition from low-titanium to high-titanium basalt is inferred in Yukon. Primarily the lower parts of the stratigraphy appear to be preserved in the Kluane Ranges. However, as seen in Figures 4.6 and 4.9, the trace-element concentrations of low-titanium basalts in Yukon extend to higher abundances than the low-titanium basalts in Alaska. Source characteristics of Nikolai basalts in Yukon The trace-element and isotopic compositions of the Nikolai basalts in Yukon indicate a Pacific ocean-island type mantle source for the high-titanium basalts and the involvement of a different component for the low-titanium basalts. The high-titanium basalts have depleted Hf and Nd isotopic compositions that are distinct from mid-ocean ridge basalts (MORB) and there is no indication of involvement of low-eNd, low-eHf continental material (Fig. 4.10). The high-titanium basalts from Yukon and Alaska are displaced just below the OIB mantle array and broadly overlap fields for the Ontong Java Plateau, Hawaii, and the Caribbean Plateau (Fig. 4.lOb). Two high-titanium basalts (samples 4821 Al, 1561) exhibit a mid-ocean ridge basalt (MORB)-like composition (Fig. 4.10). These two LREE-depleted basalts have similar trace-element concentrations to a depleted MORB average (Fig. 4.6; Salters & Stracke, 2004) and Sr-Nd-Hf isotopic compositions that place them at the edge of the Pacific MORB field (Fig. 4.10). Five of seven low-titanium basalts from Yukon with initial 8Hf values that are 1 to 4 epsilon units higher than those of the high-titanium basalts are displaced above the OIB mantle array and indicate involvement of a source that evolved with high LuJHf, but not 180 \u00E2\u0080\u0098IA . \u00E2\u0080\u00A2 20 I Indian MORB 12 18 E) 10 16. 14 8 12. 6 8 Nikolai Formation 4 6 \u00E2\u0080\u0094 AI\u00C3\u0094RB 0 Yukon high-titanium basalt 4 . . \u00E2\u0080\u0094 Q Yukon low-titanium basalt 2 XYukonbasalflow2 . I \u00E2\u0080\u00A2 Alaska high-titanium basalt \u00E2\u0080\u0094 \u00E2\u0080\u0094 _i QIB array \u00E2\u0080\u0094 \u00E2\u0080\u0094 \u00E2\u0080\u00A2Alaska low-titanHjm basalt 0 \u00E2\u0080\u0094 I \u00E2\u0080\u00A2 Alaska picnte -2 . (b) I A Paleozoic arc XASI -2 - I I I I I I I I I I I I I I I I 0.702 0.703 0.704 0.705 0.706 -4 -2 0 2 4 6 8 10 12 14 57SrI86Sr(iflitial) (initial) Nd Figure 4.10 Comparison of age-corrected (230 Ma) Sr-Nd-Hf isotopic compositions of the Nikolai Formation in Yukon and Alaska to age-corrected OIB and MORB. (a) Initial 6Nd vs. 87Sr/6. (b) Initial e vs. 8Nd\u00E2\u0080\u00A2 The complete references for the compiled data are too numerous to cite here; most of the compiled data was extracted from the GEOROC database (http://georoc.mpch-mainz.gwdg.de/georocl). Data for Ontong Java from Mahoney eta!, (1993), Babbs (1997), and Tejada eta!. (2004); Indian MORB from Salters (1996), Kempton eta!. (2002), and Jamiey et a!. (2005); Pacific MORB from Mahoney et a!. (1992, 1994), Nowell et a?. (1998), Salters and White (1998), and Chauvel and Blichert-Tofi (2001); OIB array line from Vervoort (1999). EPR is East Pacific Rise. Dashed lines indicate Bulk Silicate Earth (BSE). Note the two LREE-depleted high-titanium basalts (4821A1, 1561) that lie at the edge of the field of Pacific MORB. 181 (initial) a i.t\u00E2\u0080\u00A2I\u00E2\u0080\u00A2 I \u00E2\u0080\u00A2\u00E2\u0080\u00A2 \u00E2\u0080\u00A2 \u00E2\u0080\u00A2\u00E2\u0080\u00A2\u00E2\u0080\u00A2\u00E2\u0080\u00A2 I \u00E2\u0080\u00A2 I .\u00E2\u0080\u00A2 \u00E2\u0080\u00A2A1j\u00E2\u0080\u0099II D . ..4. . l.,x \u00E2\u0080\u00A2Hawaii \u00E2\u0080\u00A2, \u00E2\u0080\u00A2i \u00E2\u0080\u00A2 Ontong Java \u00E2\u0080\u00A2 \u00E2\u0080\u00A2 East Pacific Rise l Caribbean Plateau \u00E2\u0080\u00A2 .1 (a) \u00E2\u0080\u00A2 \u00E2\u0080\u00A2 \u00E2\u0080\u00A2 .1.. \u00E2\u0080\u00A2 .1.... I.... correspondingly high Sm/Nd (Fig. 4.lOb). The Hf and Nd isotopic compositions of the low-titanium basalts from Yukon generally follow the trend of the low-titanium basalts from Alaska, which are 4 to 8 epsilon-Hf units higher than the high-titanium basalts from Alaska and Yukon and (Fig. 4.1 Ob). The displacement of low-titanium basalts above the OIB mantle array could have involved the addition of a small amount of a pelagic component that underwent radiogenic ingrowth from high Lu/Hf because of the sedimentary fractionation of zircon (Patchett et aL, 1984). Several low-titanium basalts (samples 4808A2 and 481 0A8) have initial 8Hf and ENd that lie between the high- and low- titanium basalt fields (Fig 4.8c), and, together with some low-titanium basalts with high La/Yb, may indicate a transitional lava type. Overall, the low-titanium basalts are mostly distinct from OIB and the source of MORB and require a different depleted source than the source of the high-titanium basalts, early in the formation of Nikolai basalts. The negative HFSE anomalies in the low-titanium basalts are also markedly different from basalts produced from OIB- or MORB-type mantle. Proxies of mantle-crust interaction (Th-Nb) and melting depth (TiO2-Nb) of Pearce (2008) suggest involvement of arc mantle or crust in generation of the low-titanium basalts (Fig. 4.11). Additional trace- element ratios also distinguish the low-titanium basalts from OIB and N-MORB-type mantle (Fig. 4.12). The following section examines the origin of the low-titanium basalts as indicated by their chemistry. Melting of arc mantle in formation of the low-titanium basalts A shift in the source region that produced the Nikolai basalts is apparent in the major- and trace-element and isotopic compositions. The chemistry of the low-titanium basalts indicates involvement of an arc-type source (or subduction-modified mantle) during the early part of the eruptive history of the Nikolai Formation (Fig. 4.13). The Nikolai basalts erupted through a thick succession of Late Paleozoic arc volcanics, presumably consisting of complementary sub-arc mantle lithosphere, which may have played a key role in the development of the geochemical signature of the low-titanium basalts (here, lithosphere indicates a region that was not involved in the convective flow of the mantle). The high-titanium basalts have compositions that suggest they originated 182 Figure 4.11 Th-Nb and Ti-Yb systematics for the Nikolai Formation in Yukon with data compilation and modeling results from Pearce (2008). (a) Tb/Yb vs. Nb/Yb. MORB-OIB array and assimilation-fractional crystallization (AFC) model trend from Pearce (2008). (b) Ti02/Yb vs. Nb/Yb. MORB-Offi array and assimilation-fractional crystallization (AFC) model from Pearce (2008). Talkeetna arc lower crust from Greene et a!. (2006) and Talkeetna arc lavas from Clift et a!. (2005). Mariana arc data from Pearce eta!. (2007) and Woodhead eta!. (2001). The low-titanium basalts indicate a depleted source and interaction with a subduction component combined with fractional crystallization, whereas the high-titanium basalts lie within an OJB array in panel b, parallel to a melting vector that indicates higher pressure melting. See Pearce (2008) for parameters of polybaric melting and assimilation-fractional crystallization (AFC) modeling. Blue line in panel A represents an AFC model following the modeling of DePaolo (1981). Red line in panel b illustrates a polybaric melting trend (with changing composition of pooled melt extracted from the mantle that undergoes decompression from the solidus to the pressure marked) for high and lower mantle potential temperatures which correspond to representative conditions for the generation of present-day MORB and OIB (Pearce, 2008). Nb/Yb Nb/Yb 183 10 0 0.4 0.8 1.2 1.6 Nb/La Figure 4.12 Trace-element ratios of high- and low-titanium basalts of the Nikolai Formation in Yukon with Paleozoic arc samples and fields for Nikolai basalts in Alaska. (a) Nb/Y and Zr/Y variation (after Fitton et aL, 1997). (b) Dy/Ybc1vs. Nb/La. The NMORB and OIB fields, not including Icelandic OIB, are from data compilations of Fitton et al. (1997; 2003). Note the two LREE-depleted high-titanium basalts (4821A1, 1561) that lie within the field for NMORB in panel a. The two parallel gray lines are the limits of data for Icelandic rocks. 1.4 1.2 \u00E2\u0080\u00A2 I I I flI Dy/YbCN Alaska\ high-ill A \u00E2\u0080\u0098AfJ A ,D Alaska low-il A \ 441 outside the(a) Kluane Ranges Zr/Y 184 Stage 2 - Figure 4.13 Schematic diagrams of two stages of melting of Nikolai basalts in Yukon and Alaska that produced the low- and high-titanium basalts in Yukon and Alaska. Parts of the diagrams were adapted from a drawing by J. Holden in Fodor (1987) and Farnetani and Samuel (2005). (a) Low-titanium basalts form the lower parts of the stratigraphy in parts of Yukon and Alaska. In this scenario, a plume head impinging on the base of arc lithosphere leads to the formation of small-degree, 1{FSE-depleted low-titanium melts from a source in the arc mantle. (b) High-titanium basalts, which dominate the remainder of the volcanic stratigraphy, were derived from an OIB plume-type source. White arrows indicate flattening of the plume head. Black arrows indicate heat conduction from the plume head into the base of the lithosphere. The dashed line represents the base of the mechanical boundary layer. Vertical exaggeration= \u00E2\u0080\u009410:1. 185 from an OIB plume-type source and followed the eruption of most of the low-titanium basalts (Fig. 4.13). The arc-type signature of the low-titanium basalts may have been generated from substantial melting of subduction-modified mantle, interaction of plume-derived melts with melts or material derived from the arc mantle, andlor reaction of magmas and metasomatized arc peridotite early in generation of the Nikolai basalts. Thus, we modelled melting of arc mantle compositions to determine if magma compositions similar to the low-titanium basalts can be produced by melting of ultramafic arc mantle rocks. The trace element concentrations of the low-titanium basalts were simulated using the incongruent dynamic melting model developed by Zou & Reid (2001) and source compositions of exposed arc mantle peridotite from the Early to Middle Jurassic Talkeetna arc in south-central Alaska (Kelemen et al., 2003). The modeling results indicate that small degrees of melting (<5%) of arc mantle can produce HFSE-depleted trace-element signatures similar to those of low-titanium basalts of the Nikolai Formation (Fig. 4.14). Negative Nb and Zr-Hf anomalies are generally not found in samples of metasomatized mantle peridotite (Arndt & Christensen, 1992), but they are present in mantle samples of exposed oceanic arc sections such as the Talkeetna arc. Small-degree melting of HFSE-depleted arc mantle offers one of the best explanations for the origin of the chemical signature of the low-titanium basalts. These modeling results require a fairly high proportion of cpx in the source (<30 vol. %). Modeling results depend highly on the average trace-element composition of the source, which caused the model compositions ofpartial melts of 10% or greater to be more LREE-depleted than most of the low-titanium basalts. Mixing of high- and low-titanium magmas would not significantly change the trace-element concentrations of either lava type unless >25 vol. % of the other lava type was mixed (Fig. 4.14). Assimilation of subduction-modified mantle is a less likely scenario because the amount of contaminant necessary to account for the trace-element and isotopic compositions of the low-titanium basalts would be unreasonably large. The compositions of the Late Paleozoic arc volcanic sequences are variable and partially overlap the compositions ofboth the high- and low titanium basalts; therefore, they are not ideally-suited as assimilants responsible for the geochemical variation of the low-titanium basalts. 186 0 0.4 0.8 1.2 \u00E2\u0080\u00A2 1.6 Nb/La \u00E2\u0080\u00A2 high-titanium basalt X degrees of melting Q low-titanium basalt shown in panel c . 10 a E ic average (low-titanium basalts) Tb U Nb La Ce Pr Nd SmZr Hf Eu GdTb Dy Ho Er Yb Lu average (Talkoetna uItrumafic) (c) I I I I I I I I I I I I Tb U Nb La Ce Pr Nd Sn, Zr Hf Eu Gd Tb Dy Ho Er Yb Lu Figure 4.14 Trace-element abundances of low-titanium basalts from Yukon compared to arc mantle compositions of the Early to Middle Jurassic Talkeetna arc and incongruent dynamic melting modeling results. (a) Primitive mantle- normalized trace-element patterns for low-titanium basalts from Yukon and Talkeetna arc ultramafic rocks (harzburgite, websterite, wherlite, and dunite) from south-central Alaska (Kelemen et al., 2003), with averages denoted in the legend. (b) Zr/Sm vs. Nb/La for high- and low-titanium basalts from Yukon with melting modeling results, shown in panel c. (c) Melting modeling of the average Talkeetna arc ultramafic composition using the formulation for incongruent dynamic melting of Zou and Reid (2001), an example calculation is shown in their Appendix. Melting increments are labelled next to patterns. (d) Mixing modeling of high- and low-titanium melts. A range of melt reaction coefficients were used from Kinzler and Grove (1992) and Kelemen et a!. (1990) and slight changes do not significantly affect the results. Partition coefficients from Salters and Stracke (2004) for 2 GPa were kept constant. Source mineralogy ranges from 0.28cpx:0.l5opx:0.55o1:0.O2sp to 0.l8cpx:0.27opx:0.52o1:0.O3sp. Higher proportions of cpx in the source yield better fits to the Yukon low-titanium basalt compositions. The results serve to illustrate that small degrees of melting (<5%) of arc mantle compositions may produce HFSE-depleted trace-element compositions similar to those of low-titanium basalts. a) a E a) > E 0 a) 0. E aCl) 26 20 .Zr/Sm ci ED .lb\u00E2\u0080\u00A2 ci . ci . DD bLtl3 DX1% (b) ci X5% 15 Tb U Nb La Ce Pr Nd Sm Zr Hf Eu Gd Tb Dy Ho Er Yb Lu I I I I I I I I I I I I I I I I I I _A.. ,1% Melting modeling of Talkeetna arc mantle Mixing of high- and low-titanium basalt \u00E2\u0080\u0094g- average (low-titanium basalts) \u00E2\u0080\u0094.\u00E2\u0080\u0094average (high-titanium basalts), 025 Ie-Ti: 0.75 high-Ti 10 . . 187 Studies of plume-lithosphere interaction suggest that, although conduction alone may not cause melting of the lithosphere, rifting and decompression, the presence of hydrous phases (e.g. Gallagher & Hawkesworth, 1992), melt injection from the plume into the lithosphere, and thermal and mechanical erosion of the lithosphere may all facilitate melting (Saunders et al., 1992). Lassiter and DePaolo (1997) found geochemical evidence indicating that lavas with a lithospheric geochemical signature are commonly erupted during the early stages of flood basalt sequences and are often followed by lavas with more plume-type signatures. Lassiter et al. (1995) also suggested that a minimal amount of involvement of arc material was indicated by geochemistry in the generation of Nikolai basalts in Alaska. Geochemistry indicates melting of subduction-moclified mantle in the Yukon and Alaska segments of the Wrangellia oceanic plateau, possibly from intrusion of the plume into the lithosphere or erosion and melting of cooler subduction-modified mantle along the edge of the plume head. CONCLUSION Parts of the volcanic stratigraphy of the accreted Middle to Late Triassic Wrangellia oceanic plateau are exposed in a linear belt (300 km long) in southwest Yukon, which extends westward over large areas of southern Alaska. Approximately 1000 m of predominantly massive subaerial basalt flows of the Nikolai Formation consist of pillow breccia and close-packed pillows (<70 m) along the base. The Nikolai Formation is bounded by Middle to Late Triassic marine sediments and unconformably overlies Paleozoic arc volcanic sequences. This northerly part of the Wrangellia oceanic plateau formed mostly above sea-level during a single short-lived magmatic phase (ca. 230 Ma). Importantly, these voluminous basaltic lavas erupted through a Late Paleozoic oceanic arc assemblage which presumably was underlain by a complementary region of metasomatized arc mantle. This study presents the first set of geochemical and Sr-Nd-Hf-Pb isotopic compositions of Wrangellia flood basalts and Paleozoic arc sequences in Yukon and, along with a comparable study in Alaska, has provided a clear understanding of the source and temporal evolution of magmas that constructed the Wrangellia oceanic plateau in Yukon and Alaska. Similar to Alaska, the Nikolai basalts in Yukon have two 188 fundamentally distinct lava types. The high-titanium basalts that generally form the upper parts of the volcanic stratigraphy have compositions that are characteristic of OIB and distinct from N-MORE. The low-titanium basalts that generally form the lower part of the volcanic stratigraphy are depleted in HFSE and have relatively high Hf isotopic compositions that strongly differ from OIB and MORE, and are similar to lavas produced in arc settings. The involvement of lithosphere created and modified by prolonged subduction-related magmatism, and possible incorporation of a high LU/Hf sediment component, offers the best explanation for derivation of the early-erupted low-titanium basalts. The low-titanium basalts in Yukon extend to more LREE-enriched compositions than in Alaska and may indicate a transitional lava type between the high- and low- titanium basalts. There is minor evidence of extension preserved in the underlying units directly beneath the Nikolai Formation and evidence of prolonged subsidence (>20 Myr) in sediments overlying the Nikolai Formation. 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Geochimica et Cosmochimica Acta 65(1), 153-162. 194 CHAPTER 5 The Age and Volcanic Stratigraphy of the Accreted Wrangellia Oceanic Plateau in Alaska, Yukon, and British Columbia 195 INTRODUCTION Approximately 10% of the ocean floor is covered with oceanic plateaus or flood basalts, mostly in the western Pacific and Indian Oceans (Fig. 5.1; Ben-Avraham et a!., 1981). These regions can rise thousand of meters above the ocean floor and rarely exhibit magnetic lineations like the surrounding seafloor (Ben-Avraham et a!., 1981). Oceanic plateaus are produced from high volumetric output rates generated by high-degree melting events which are distinct from melting beneath mid-ocean ridges. Most of what we know about the architecture of oceanic plateaus is based on obducted portions of oceanic plateaus, drilling of the volcanic sequences of extant oceanic plateaus, and geophysical studies of the seismic properties of oceanic plateaus. The study of oceanic plateaus furthers our understanding of the relationship between large igneous provinces (LIPs) and mantle plumes, mass extinctions, and continental growth. Oceanic plateaus have formed near spreading ridges, extinct arcs, fragments of continental crust, and in intraplate settings. They form crustal emplacements 20-40 km thick with flood basalt sequences up to 6 km thick that cover up to 2 million km2 of the ocean floor (Coffm & Eldholm, 1994). Detailed studies of the volcanic stratigraphy of oceanic plateaus are rare because of the inaccessibility of oceanic plateaus in the ocean basins. Some of the best evidence for understanding the construction of oceanic plateaus, other than studies of the age, composition, and stratigraphy of the volcanic rocks themselves, is to examine the eruption record in conjunction with observations of inter relationships between sedimentation, erosion, and magmatism (Saunders et a!., 2007). Stratigraphic and geochronological studies of an obducted oceanic plateau, where the base and top of the volcanic stratigraphy are exposed as well as the underlying and overlying sediments, provide a means for understanding the construction of an oceanic plateau (e.g. the emplacement of flows, eruption environment, the tectonic setting during formation, the timescale of volcanism, the palaeoenvironment directly preceding and following the eruptions, and the uplift and subsidence history). Wrangellia flood basalts are the remnants of one of the best exposed accreted oceanic plateaus on Earth. Dissected sections of extensive Triassic flood basalts form a major part of Wrangellia, one of the largest terranes accreted to western North America. Wrangellia contains a rare example of an accreted oceanic plateau where parts of the 196 Ethiopia Deccan Emeishan Siberia - _- Karoo Madagascar Ontong Java Hikurangi Manihiki Caribbean Paran\u00C3\u00A1 Figure 5.1 Map showing the distribution of Phanerozoic large igneous provinces in red (Mahoney & Coffin, 1997). Oceanic plateaus and flood basalts are mostly concentrated in the western Pacific and Indian Oceans. Map modified from base map by A. Goodlife and F. Martinez in Mahoney and Coffin (1997). 197 entire volcanic stratigraphy as well as the pre- and post-volcanic stratigraphy are preserved. The stratigraphy of this oceanic plateau is exposed in numerous fault-bound blocks in a belt extending from Vancouver Island, British Columbia (BC) to south-central Alaska. Detailed descriptions of the volcanic and pre- and post-volcanic stratigraphy of Wrangellia are presently dispersed in hundreds of geologic maps, geologic survey reports in BC, Yukon, and Alaska. In this contribution, we integrate new observations on the volcanic stratigraphy and pre- and post-volcanic stratigraphy of Wrangellia, including geochronology and biostratigraphy with previously published data, to evaluate the construction and age of the Wrangellia oceanic plateau. This material is presented in the form of detailed descriptions of Wrangellia stratigraphy, compiled geologic maps, photographic databases, interactive Google Earth files, and a review and compilation of previous research on Wrangellia. The maps, photographs, and archiving of information offer tools to visualize and fully explore the large body of information about Wrangellia, bringing together past and present research on Wrangellia to provide an overview of the origin and evolution of the Wrangellia oceanic plateau (Greene et al., 2008a, 2008b, 2008c, submitted). WRANGELLIA FLOOD BASALTS: THE VOLCANIC STRATIGRAPHY OF AN OCEANIC PLATEAU A large part of Wrangellia formed as an oceanic plateau, or transient LIP, that accreted to western North America in the Late Jurassic to Early Cretaceous (e.g. Trop & Ridgway, 2007). Wrangellia flood basalts are the defining unit of Wrangellia, in conjunction with underlying and overlying sediments with age-diagnostic fossils (Jones et aL, 1977). The flood basalts are defmed as the Karmutsen Formation on Vancouver and Queen Charlotte Islands, and the Nikolai Formation in southwest Yukon and south- central Alaska (Fig. 5.2). Smaller elements of Middle to Late Triassic basalt stratigraphy in southeast Alaska are also believed to correlate with the Wrangellia flood basalts (Plafker & Hudson, 1980). 198 \u00C2\u00AE Kluane Ranges \u00C2\u00AE Chilkat Peninsula Figure 5.2 Simplified map showing the distribution of Wrangellia flood basalts in Alaska, Yukon, and British Columbia. Map derived from (Wilson et a!., 1998; Israel, 2004; Massey et a!., 2005a, b; Wilson et a!., 2005; Brew, 2007, written comm.). Inset shows northwest North America with Wrangeffia flood basalts, and outlines for the Peninsular (orange) and Alexander (blue) Terranes. Purple lines are faults in Alaska and parts of Yukon. Circled numbers indicated in the legend refer to areas mentioned in the text. Basalts indicated in southeast Alaska are mostly part of the Alexander Terrane. 199 - - Alaska - - 0 Talkeetna Mountains \u00C2\u00AE Clearwater Mountains \u00C2\u00AE Amphitheater Mountains \u00C2\u00AE Wrangell Mountains \u00C2\u00AE Chichagof & Baranof Is lands \u00C2\u00AE Southeast Alaska (within Alexander Terrane) \u00C2\u00AE Queen Charlotte Islands Vancouver Island = 0 British Columbia Scale 500 km Geographic distribution and aerial extent of the Wrangeffia flood basalts The Wrangellia flood basalts in BC, Yukon, and Alaska have been identified by geologic mapping and regional geophysical surveys. Exposures of Wrangellia flood basalts extend over 2300 km from Vancouver Island to south-central Alaska (Fig. 5.2; Supplementary Google Earth files). Exposures of the Karmutsen Formation cover \u00E2\u0080\u009458% of Vancouver Island, BC. In southern Alaska, elements of Wrangellia have a well defmed northern boundary along the Denali fault and extend southwards to the outboard Peninsular Terrane (Fig. 5.2). In southeast Alaska and southwest Yukon, Wrangellia is mostly limited to slivers of intensely dissected crustal fragments with minimal aerial extent (Fig. 5.2). From recently compiled digital geologic maps, the aerial exposure of the Wrangellia flood basalts is \u00E2\u0080\u0094P20,000 km2 on Vancouver Island, \u00E2\u0080\u0094800 km2 in southwest Yukon and southeast Alaska, and \u00E2\u0080\u00942,000 km2 across southern Alaska (Table 5.1). The original areal distribution was considerably greater and these estimates of outcrop extent do not consider areas of flood basalt covered by younger strata and surficial deposits. The boundaries and crustal structure of Wrangellia in southern Alaska, as defmed by recent magnetic and gravity surveys, indicate a distinct magnetic high interpreted as due to the presence of thick, dense crust from Triassic mafic magmatism (Glen et aL, 2007a, 2007b; Saltus et a!., 2007). Wrangellia crust beneath Vancouver Island (\u00E2\u0080\u009425-3 0 km thick) has seismic properties corresponding to mafic plutonic rocks extending to depth that are underlain by a strongly reflective zone of high velocity and density, which has been interpreted as a major shear zone where lower Wrangellia lithosphere was detached (Clowes eta!., 1995). Geologic history of Wrangellia A note about the terminology used in this contribution. Wrangellia (or the Wrangellia Terrane) refers to fault-bound sections of the upper crust that contain successions of diagnostic Middle to Late Triassic flood basalts and Paleozoic formations that underlie the flood basalts (as originally defined by Jones et a!. (1977)). The areas of Wrangellia in Alaska and Yukon are referred to as Northern Wrangellia and areas in British Columbia are referred to as Southern Wrangellia. The Wrangellia Composite 200 Table 5.1 Area! extent and volumetric estimatesfor the Wrangelliaflood basalts Quadrangle Area (km2) % Area Estimated thickness (km) Estimated volume (km3) % Volume mm max mm max British Columbia 4 6 42,323 63,485 45.2 4 6 34,245 51,368 36.6 3 4.5 5,319 7,979 5.7 3 4.5 5,481 8,221 5.9 1 3 86 257 0.2 1 3 705 2,115 1.5 McCarthy 720 2.9 Nabesna 275 1.1 Valdez 55 0.2 Gulkana 7 0.0 Mt Hayes 356 1.4 Healy 205 0.8 Talkeetna Mtns 105 0.4 25,256 93,204 140,525 Estimates for British Columbia are calculated from Massey ef at. (2005). Estimates for Yukon are calculated from Israel et al. (2004). Estimates for Alaska are calculated from Wilson at al. (2005) and Wilson et al. (1998). Southern VI (NM1O) 10,581 41.9 Northern VI (NM9) 8,561 33.9 Queen Charlotte Is (NN8) 1,773 7.0 Queen Charlotte Is (NN9) 1,827 7.2 Northern BC (N08) 86 0.3 Yukon Kluane South Alaska 705 2.8 Totals 3 4 2,161 2.882 2.1 3 4 824 1,098 0.8 1 2 55 110 0.1 1 2 7 14 0.0 3 4.5 1,067 1,601 1.1 3 4.5 616 924 0.7 3 4.5 314 471 0.3 201 Terrane refers to three distinct terranes (Wrangellia, Alexander, Peninsular; Fig. 5.2) that share similar elements or have a linked geologic history (as defmed by Plafker et al., 1989b; Nokleberg et aL, 1994; Plafker & Berg, 1994; Plafker et al., 1994). The connections between the Wrangellia, Alexander, and Peninsular Terranes are not well- established, although the age from a single pluton in Alaska is proposed to link the Proterozoic to Triassic Alexander Terrane to Wrangellia by late Pennsylvanian time (Gardner et aL, 1988). This paper specifically focuses on Wrangellia and does not examine the relationship of Wrangellia to the Alexander and Peninsular terranes. Wrangellia has a geologic history spanning a large part of the Phanerozoic prior to its accretion with western North America. The geologic record beneath the flood basalts contains Paleozoic oceanic arc and sedimentary sequences with a rich marine fossil assemblage. Paleontological studies indicate that Wrangellia was located in cool- temperate northern paleolatitudes (-25\u00C2\u00B0N) during the Permian and not far from the North American continent (Katvala & Henderson, 2002). The geologic record is sparse to absent in the Middle Permian to Early Triassic throughout Wrangellia. A major, short- lived phase of tholeiitic flood volcanism occurred in the Middle to Late Triassic in submarine and subaerial environments. The Wrangellia flood basalts subsided during and after their emplacement. Paleomagnetic studies of Wrangellia flood basalts indicate eruption in equatorial latitudes (Irving & Yole, 1972; Hilthouse, 1977; Hillhouse & Gromme, 1984) and Late Triassic bivalves indicate an eastern Panthalassan position in the Late Triassic (Newton, 1983). Previous geochronological and biostratigraphic studies indicate magmatism occurred between \u00E2\u0080\u0094230 and 225 Ma (e.g. Parrish and McNicoll, 1992; Sluggett, 2003; Bittenbender et al., 2007). A mantle plume origin for the Wrangellia flood basalts was initially proposed by Richards et al. (1991) and is supported by ongoing geochemical and petrological studies (Greene et al., 2008, submitted-c). Late Triassic to Early Jurassic arc magmatism is preserved as intrusions within and volcanic sequences overlying Wrangellia flood basalts throughout areas on Vancouver Island. Paleobiogeographic studies indicate Wrangellia was located in the northeast Pacific Ocean during the Early Jurassic (Smith, 2006) and probably accreted to western North America in the Late Jurassic to Early Cretaceous (Csejtey et al., 1982; McClelland et al., 1992; Nokleberg et al., 1994; Umhoefer & Blakey, 2006; Trop & Ridgway, 2007). 202 The following overview of the stratigraphy of Wrangellia covers the Paleozoic and early Mesozoic formations pre-dating Triassic flood volcanism, but focuses primarily on the volcanic stratigraphy of the Wrangellia flood basalts and their relationship with the immediately underlying and overlying sedimentary formations. Descriptions below are presented from northwest (Alaska) to southeast (BC) and generally from the oldest to the youngest strata in each area. A summary of previous research related to Wrangellia is given in Appendix G and Supplementary data files 1 and 2 contain a compilation of \u00E2\u0080\u0094500 references related to Wrangellia. STRATIGRAPHY OF WRANGELLIA Wrangeffia in southern Alaska The accretion and northward migration of parts of Wrangellia, followed by the oroclinal bending of Alaska, has left Wrangellia flood basalts exposed in an arcuate belt extending \u00E2\u0080\u0094450 km across south-central Alaska (Fig. 5.3). Wrangellia stratigraphy underlies most of the Wrangell Mountains in the eastern part of southern Alaska and extends westward in a wide belt immediately south of the Denali Fault, along the southern flank of the eastern Alaska Range and in the northern Talkeetna Mountains (Fig. 5.3). Recent mapping and geophysical studies have recognized Wrangellia basalts (Nikolai Formation) southwest of previously mapped exposures, in the Talkeetna Mountains (Schmidt et al., 2003b; Glen et al., 2007a). Triassic flood basalt stratigraphy may extend several hundred kilometers to the southwest of the Talkeetna Mountains on the Alaska Peninsula (Cottonwood Bay Greenstone of Detterman and Reed, 1980), however, the relationship between these basalts and the Nikolai Formation has not been explored. Geophysical studies and detailed work on sedimentary basins along the margins of Wrangellia have helped define the boundaries of Wrangellia and led to the development of a model for the accretionary history of southern Alaska (Glen et aL, 200Th; Trop & Ridgway, 2007). The northwestern boundary of Wrangellia corresponds with a prominent steeply-dipping structure (Talkeetna Suture Zone), which may represent the original suture between Wrangellia and the former continental margin (Glen et a!., 2007). 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(19 98 )a n d W ils on et a t (20 05 ), w ith u n pu bl ish ed m ap pi ng fro m Sc hm id t ( pe rs. co m n i, 20 06 ). St ra tig ra ph ic co lu m ns de pi ct La te Pa le oz oi c to Ju ra ss ic u n its o n th e so u th sid e o f t he W ra ng el l M ou nt ai ns , de riv ed fro m Sm ith an d M ac K ev et t (19 70 ) a n d M ac K ev et t (19 78 ), an d th e ea st -c en tra l A la sk a Ra ng e, de riv ed pa rtl y fro m N ok le be rg et a t (19 92 ). Wrangellia, adjacent to and overlying the Talkeetna Suture Zone and Denali Fault, record the uplift and collisional history of Wrangellia and the former continental margin, north of the Denali Fault (Trop et a!., 2002; Ridgeway et al., 2002; Trop and Ridgeway, 2007). The southern boundary of Wrangellia in Alaska is interpreted to lie along a prominent northeast-trending geophysical gradient separating Wrangellia units to the north from parts of the Peninsular Terrane to the south (Glen et a!., 2007b). Talkeetnu Mountains and Eastern Alaska Range The westernmost Wrangellia stratigraphy in the Talkeetna Mountains consists of Paleozoic strata that are intruded by mafic sills associated with Nikolai basalts. The Wrangellia basement consists of siliceous argillite, siltstone, and chert interbedded with limestone, which are similar to Paleozoic sedimentary units elsewhere in Alaska (Schmidt & Rogers, 2007). The arc sequences that are typical of Wrangellia basement in the Alaska Range and Wrangell Mountains are absent or unexposed in the Talkeetna Mountains. A rare quartz-pebble conglomerate has been recognized in the Talkeetna Mountains that has not been reported elsewhere beneath the flood basalts (Schmidt et a!., 2003a). The overall thickness of Nikolai basalts is limited in the Talkeetna Mountains (\u00E2\u0080\u0094300 m) and only minor occurrences of overlying sedimentary rocks have been reported (Schmidt & Rogers, 2007). From southwest of the Clearwater Mountains eastward to the Amphitheater Mountains, volcanic stratigraphy of the Nikolai Formation approaching 4 km thick is exposed in a discontinuous belt (Fig. 5.3). There are limited exposures of underlying Paleozoic units in this area. In the Clearwater Mountains, the base of the volcanic stratigraphy consists of pillowed flows overlying interlayered shale and mafic sills. The pillow basalt contains sediment derived from directly underlying beds filling voids between small-diameter pillows (<1 m). The total thickness of the pillowed flows is less than several hundred meters and they are overlain by predominantly massive subaerial flows. Picritic pillow lavas with abundant olivine pseudomorphs occur within the basal pillowed flows (Greene eta!., 2008, submitted-c). In the Clearwater Mountains, the upper parts of the volcanic stratigraphy is primarily subaerial flows (and rarely sills) with columnar jointing preserved in several of 205 the thicker flows (<25 m). Smith (1981) described minor occurrences of tuff, flow breccia, and limestone and argillite lenses within the volcanic stratigraphy. These rare interbedded horizons are mainly found in the lower parts or at the top of the volcanic stratigraphy (Smith, 1981). Tuffaceous layers (or redistributed hyaloclastite) occur within rhythmically layered carbonaceous argillite or are a mixture of volcanic clasts and carbonaceous matter (Smith, 1981). Diagnostic index fossils (the bivalve Halobia and the ammonoid Tropites) have been found in intervolcanic sedimentary lenses at the top of the volcanic stratigraphy and in strata overlying the basalts (Smith, 1981). The complex interbedding of volcanic and sedimentary horizons gives way to fine-grained marine sedimentary strata above the flood basalts. In the eastern Alaska Range, Paleozoic volcanic and volcaniclastic rocks of the Tetelna Volcanics form isolated exposures east of the Amphitheater Mountains, where they are intruded by plutonic rocks related to the Nikolai basalts (Nokleberg et a!., 1992). Pre-flood basalt sedimentary units are mostly non-fossiliferous carbonaceous black shale and siliceous argillite (Blodgett, 2002). Some of the best exposures of Wrangellia flood basalt stratigraphy are preserved as part of an east-west-trending synform underlain by mafic and ultramafic plutonic rocks in the Amphitheater Mountains (Fig. 5.4). The synform is dissected by several north-south-trending U-shaped glacial valleys filled with elongate lakes that provide access to kilometer-high exposures of volcanic stratigraphy. The Amphitheater Mountains synform is largely intact and much of the stratigraphy lacks pervasive faulting. Five large sheet-like mafic-ultramafic intrusions, which occur within argillites underlying the flood basalts, are exposed around the Amphitheater Mountains (Schmidt & Rogers, 2007). The volcanic stratigraphy in the Amphitheater Mountains is very similar to the Clearwater Mountains. The lowest exposures are argillite and shale underlying the pillowed flows and intruded by numerous massive sills (Fig. 5.5). The lowermost pillowed flows engulf sediment of underlying strata and only occur in the lowest several hundred meters of stratigraphy (1 0-15% of the total section). Tuffs and breccia are preserved in the submarine part of the stratigraphic section below the transition from submarine to subaerial flows. Mafic sills form parts of the lower volcanic stratigraphy and are locally interbedded with tuff and breccia (Fig. 5.5). There are approximately 3 206 M ap le ge nd Fi gu re 5. 4 G eo lo gy an d m ag ne tic m ap o ft he A m ph ith ea te rM ou nt ai ns , A la sk a (lo ca tio n sh ow n in Fi gu re 5. 3). (a) Co lo r sh ad ow to ta lm ag ne tic fie ld m ap o fp ar to ft he A m ph ith ea te r M ou nt ai ns .A su m m ar y o ft he m ag ne tic su rv ey is pr ov id ed in Bu rn s an d Cl au tic e (20 03 )a n d th e fu ll co lo rs ha do w m ap is pr es en te d in Bu rn s et aL (20 03 ). D et ai ls o ft he da ta ac qu isi tio n, in te ip re ta tio n, pu bl ic at io ns ,a n d da ta fo rm at s ar e pr ov id ed in Pr ic ha rd (20 03 ). Th e su rv ey s (19 95 an d 20 02 ) m ap pe d th e m ag ne tic an d co n du ct iv e pr op er tie s o ft he ar ea to de te ct co n du ct iv e m in er al iz at io n. Th is w as ac co m pl ish ed by u sin g a D IG IT EM (V )m u lti -c oi l, m u lti -fr eq ue nc y el ec tro m ag ne tic sy ste m , su pp le m en te d by a hi gh se n sit iv ity ce siu m m ag ne to m et er an d G PS sy ste m (B um s& Cl au tic e, 20 03 ). Th e ro ck s w ith th e hi gh es tm ag ne tic su sc ep tib ili ty (F e-r ich m ag ne tic m in er al s; m af ic an d u ltr am af ic ro ck s) da m pe n th e m ag ne tic sig na la n d pr od uc e hi gh s an d lo w s, m ea su re d in n an o Te sla s (nT ). Th e hi gh n T v al ue s ar e pu rp le an d o ra n ge in co lo r, in di ca tin g m ag ne tic ro ck s, an d th e lo w v al ue s ar e bl ue an d gr ee n. Th e pu rp le an d re d ar ea s ar e as su m ed to be co in ci de nt w ith m af ic an d u ltr am af ic ro ck s, an d th e bl ue ar ea s ar e in fe rre d to be Pa le oz oi c se di m en ts. (b) G en er al iz ed ge ol og y o ft he N ik ol ai Fo rm at io n an d re la te d pl ut on ic ro ck s in th e A m ph ith ea te rM ou nt ai ns . F iv e m ai n fie ld ar ea s ar e o u tli ne d w ith n u m be re d bo xe s (de no ted in m ap ). M ap de riv ed fro m N ok le be rg et aL (19 92 )a n d di gi ta lc o m pi la tio n o fW ils on e ta !. (19 98 ). In fe rre d u n its fro m Bu rn s et aL (20 03 ). (c) Sc he m at ic cr o ss -s ec tio n o fA m ph ith ea te rM ou nt ai ns fro m A to A\u00E2\u0080\u0099 (sh ow ni n pa ne ls a an d b), ad ap te d fro m N ok le be rg et aL (19 85 ). ult ram afi c & m afi cr oc ks M ap pe d u n its [ m as si ve ba sa lt p1 00 w ba sa lt ba sa lt u n di ffe re nt ia te d ga bb ro m af lc lu ltr am af ic In fe rr ed u n its U ltr am af ic ro ck s G ab br oi c ro ck s Pa leo zo ic se di m en ts (I) G la ci er G ap La ke (iJ La nd m ar k G ap La ke () Ta ng le La ke s (W est ) \u00C2\u00AE Su ga ilo af M ou nt ain (33 Ra in y Cr ee k 0 10 2f3 km 14 6W 0 \u00E2\u0080\u0094 10 km g d , Fi gu re 5.5 Ph ot og ra ph s o fb as e o f N ik ol ai Fo rm at io n in Ta ng le La ke s ar ea o ft he A m ph ith ea te rM ou nt ai ns (lo ca tio ns ho w n in Fi gu re 5. 4). (A )S ed im en t-s ill co m pl ex an d ba se o f f lo od ba sa lts o n th e w es t sid e o fL ow er Ta ng le La ke .C irc le d le tte rs m ar k th e lo ca tio n o ft he o th er ph ot os . ( B) Si lls in th e lo w er pa rt o ft he su bm ar in e se ct io n. (C )P ill ow ba sa lt in th e lo w er m os tf lo w w ith fin e- gr ai ne d se di m en tf ill in g sp ac es be tw ee n pi llo w s (m ark er fo rs ca le) . ( D) Ca rb on ac eo us n o n -fo ss ili fe ro us bl ac k sh ale w ith pa ra lle ll am in at io ns o v er la in by m af ic sil l. (F )L ow er m os tf lo w di re ctl y o v er ly in g sh ale . ( F) Th in -s ec tio n sc an o ft uf fa ce ou s la ye rs jus tb el ow th e ba se o f s ill s in ph ot og ra ph B. (G )P ho to m ic ro gr ap h o ft uf ff ro m be tw ee n sil ls, lo ca tio n in di ca ted in ph ot og ra ph B. 00 km of subaerial flows overlying the submarine stratigraphy. Neither the top of the flood basalts or significant sections of Late Triassic or Jurassic formations are well exposed in the Amphitheater Mountains. North of the Eureka Creek Fault is a small area with mafic and ultramafic plutonic rocks and volcanic stratigraphy different from that south of the Eureka Creek Fault within the synform (Fig. 5.4). Although originally mapped as a separate subterrane from assemblages in the Amphitheater synform by Nokelberg et a!. (1985), age constraints indicate that it is in fact part of Wrangellia (Bittenbender et a!., 2007). This area contains a complex sequence of vertically-dipping limestone, shale, and picritic tuff cross-cut by numerous mafic and ultramafic dikes. Wrangell Mountains Wrangellia flood basalts form two northwest- to southeast-trending belts along the northeast and southwest margins of the Wrangell Mountains, between the Totschunda Fault and Chitina Thrust Belt (Fig. 5.3). Large Miocene to Recent volcanoes of the Wrangell Mountains volcanic field, Tertiary continental sedimentary rocks, and glaciers overlie most of the central portion of the Wrangell Mountains, covering most of the pre Cenozoic Wrangellia stratigraphy. The type Wrangellia Terrane lies in the area between the Totschunda Fault and the Chitina Thrust Belt, although similar assemblages are exposed to the south between the Chitina Thrust Belt and the Border Ranges Fault (Fig 5.3). Paleozoic sequences south of the Chitina Thrust Belt have different character than sequences north of the thrust belt. Plafker et a!. (1989b) interpreted this area between the Chitina Thrust Belt and the Border Ranges Fault, referred to as the Southern Wrangellia Terrane Margin, as a deeper, more metamorphosed equivalent of the type Wrangellia Terrane in the Wrangell Mountains. There are exposures ofNikolai basalts in this area, but these units are not addressed in this paper. Wrangellia stratigraphy is well-exposed in a shallow northwest-trending syncline along the south side of the Wrangell Mountains. The entire stratigraphy is not exposed in one area, but different areas contain sections of the base, middle, and top of Wrangellia flood basalt stratigraphy (Figs 5.6, 5.7, and 5.8). The base of the volcanic stratigraphy is 209 Fi gu re 5.6 Ph ot og ra ph s o ft he ba se o fN ik ol ai Fo rm at io n n o rt h o fS ko lai Cr ee k in W ra ng el l-S t. El ia s N at io na lP ar k. (A )P al eo zo ic ar c ro ck s an d m ar in e se di m en ta ry se qu en ce s u n de rly in g N ik ol ai ba sa lts . Ph ot og ra ph by Ed M ac K ev et t, Jr. (Ph c, H as en Cr ee k Fo rm at io n; Pg h, G ol de n H or n Li m es to ne Le nt il; TR d, M id dl e Tr ia ss ic \u00E2\u0080\u0098 D ao ne lla -b ed s\u00E2\u0080\u0099) .( B) Ba sa l pi llo w br ec ci a jus ta bo ve M id dl e Tr ia ss ic ar gi lli te an d sh ale w ith D ao ne lla . (C )B as al flo w -c on gl om er at e w ith ro u n de d cl as ts o f w hi te lim es to ne (<2 0c m )d er iv ed fro m G ol de n H or n Li m es to ne Le nt il an d re d ba sa lt (<4 0c m )f ro m St at io n Cr ee k Fo rm at io n. M ar ke r ( 14 cm )f or sc ale .( D) A rg ill ite o v er la in by pi llo w br ec ci a ab ov et he G ol de n H or n Li m es to ne .T he lo ca tio n o fp ho to gr ap hs B, C, an d D ar e fro m th e ar ea jus ta bo ve TR d in ph ot og ra ph A. Fi gu re 5. 7 Ph ot og ra ph s o ff lo od ba sa lts in th e G la ci er Cr ee k ar ea in W ra ng el l-S t. El ia s N at io na lP ar k an d m ap o ft he so u th er n pa rt o ft he v. l l M ou nt ai ns . (A )P ho to gr ap h o f\u00E2\u0080\u0094 10 00 m o fs u ba er ial ba sa lt flo w so v er la in by Ch iti sto ne Li m es to ne (da sh ed o ra n ge lin e in di ca tes th e co n ta ct ). Fl ow sa pp ea ra s a la ye r-c ak e st ra tig ra ph y bu tm ay v ar y in th ic lm es s an d lo ca lly ap pe ar to te rm in at e. (B )P ho to gr ap h o f\u00E2\u0080\u0094 - 500 m o ft hi n an d th ic k ba sa lt flo ws .( C) Si m pl ifi ed m ap sh ow in g th e di str ib ut io n o ft he N ik ol ai Fo rm at io n (gr een )i n th e W ra ng el lM ou nt ai ns ,d er iv ed fro m W ils on et a!. (20 05 ). Th e fo ur ar ea s o ff ie ld st ud y ar e o u tli ne d w ith la be le d bo xe s. (D )P ho to gr ap h o f H as en Cr ee k Fo rm at io n in tru de d an d de fo rm ed by ga bb ro ic ro ck sr el at ed to th e N ik ol ai Fo rm at io n. Ph ot og ra ph by Ed M ac K ev et t, Jr. G la ci er C re ek ,W ra ng el l Mo un tai rTs II st on e s tO ne Fi gu re 5. 8 Ph ot og ra ph s o ft he to p o ft he N ik ol ai Fo rm at io n ar o u n d H id de n La ke Cr ee k in W ra ng el l-S t. El ia s N at io na lP ar k. (A )P ho to gr ap h o ft he to p o ft he N ik ol ai Fo rm at io n an d o v er ly in g Ch iti sto ne Li m es to ne ,t ak en by Ed M ac K ev et t, Jr. Se ve ra l fa ul ts o ffs et th e co n ta ct . (B )C lo se -u p ph ot og ra ph o fm as siv e m ic rit ic lim es to ne o v er ly in g th e N ik ol ai ba sa lts .L oc at io n o fp ho to D sh ow n. (C )C lo se -u p ph ot og ra ph o fa co n ta ct be tw ee n su ba er ia lf lo w s in th e u pp er N ik ol ai Fo rm at io n. M ar ke r( 14 cm ) fo rs ca le. (D )C ob bl es (<1 0 cm lo ng )a lo ng th e co n ta ct be tw ee n th e Ch iti sto ne Li m es to ne an d N ik ol ai Fo rm at io n. Th e o x id iz ed co bb les ar e su ba ng ul ar ,c lo se ly pa ck ed , al ig ne d al on g th ei rl on g ax is, an d ar e gl om er op or ph yr iti c ba sa lt id en tic al to th eu pp er m os tf lo w so f t he N ik ol ai Fo rm at io n. Sl ed ge ha m m er ha nd le (4 cm w id e) fo r s ca le. exposed on the north side of Skolai Creek, where Paleozoic arc volcanic rocks of the Skolai Group form the lowest stratigraphic level exposed of Wrangellia in Alaska (MacKevett, 1978) (Figs 5.6 and 5.7). The type section of the Skolai Group north of Skolai Creek is \u00E2\u0080\u00942400 m thick (Smith and MacKevett, 1970; Fig. 5.6; Supplementary photo file). The Skolai Group consists of a basal volcanic flow member (\u00E2\u0080\u00941200 m) and volcaniclastic unit (\u00E2\u0080\u0094750 m) comprising the Station Creek Formation, and a sedimentary package that includes the Hasen Creek Formation (\u00E2\u0080\u0094300 m) and the Golden Horn Limestone Lentil (\u00E2\u0080\u0094250 m locally; Fig. 5.3). The transition between the lower flow and volcaniclastic members consists of interbedded lava flows and volcaniclastic beds (Smith and MacKevett, 1970). The Hasen Creek is a heterogeneous assemblage of chert, shale, sandstone, bioclastic limestone, and conglomerate, all with very little volcanic-derived material (Smith and MacKevett, 1970). The Golden Horn Limestone is bioclastic grainstone and packstone, locally with 75% of the clasts consisting of crinoid stems, and is also rich in bryozoans, brachiopods, foraminifera, and corals (Smith & MacKevett, 1970). In most places, the Golden Horn Limestone is unconformably overlain by Nikolai basalts with little discordance. In several areas, between the top of the Skolai Group and the base of the Nikolai, there is Middle Triassic argillite (<30 m thick) with fissile shale beds containing imprints ofDaonella bivalves. Sills and discordant gabbroic intrusions related to the Nikolai occur within the Skolai Group, and sedimentary units of the Hasen Creek may be significantly deformed around these intrusions (MacKevett, 1978; Fig. 5.7; Supplementary photo file 2). The Nikolai Formation in the Wrangell Mountains is estimated to be \u00E2\u0080\u00943.5 km in total thickness and is almost entirely subaerial flows (MacKevett, 1978). In several areas on the south side of the Wrangell Mountains a thin zone of interbedded flow- conglomerate, pillow breccia, and pillow basalt, typically less than 70 m thick, forms the lowest flow unit of the Nikolai (Fig. 5.6). The basal flow unit lies directly on shale and contains abundant subrounded pebble- and cobble-sized clasts derived from the Golden Horn Limestone and volcanic rocks of the underlying Skolai Group in a basalt-rich matrix (Fig. 5.6; Supplementary photo file 2). Contacts between the basal flow conglomerate and overlying flows are mostly sharp, but locally conglomerate-rich beds 213 and basaltic lava are interbedcled along the upper part of the basal flow unit (MacKevett, 1970). The flood basalt stratigraphy forms continuous sections over 1000 m thick of monotonous sequences of massive amygdaloidal flows with few discernible features (Fig. 5.7). There are no interfiow sediments or submarine volcanic features within the Nikolai in the Wrangell Mountains (except in the basal flow-conglomerate unit). Amygdules and clusters of plagioclase phenocrysts are typically the only distinguishing features within flows (Supplementary photo file 2). A cumulative thickness of over 3.5 km of marine sedimentary rocks, which range in age from Late Triassic to Late Jurassic, overlie the Nikolai basalts. These marine sedimentary strata form impressive cliff-forming sequences in the Wrangell Mountains and the contact between the strongly contrasting black Nikolai basalts and the overlying white Chitistone Limestone is featured along many of the cliffs (Fig. 5.8; Supplementary photo file 2). Late Triassic to Early Jurassic limestone and shale successions above the Nikolai have been divided into three formations in the Wrangell Mountains, from oldest to youngest: Chitistone Limestone (3 50-600 m), Nizina Limestone (100-375 m), and McCarthy Formation (\u00E2\u0080\u0094P900 m). The Chitistone clisconformably overlies the Nikolai Formation and is gradational into the overlying Nizina Limestone. Faults commonly offset the contact between the uppermost basalt flows and the lowermost Chitistone Limestone; however, the top of the Nikolai is mostly a smooth flat surface (Fig. 5.8; Supplementary photo files). There are local occurrences of regolith between the top of the Nikolai and the base of the Chitistone (Fig. 5.8; Armstrong et aL, 1969). The carbonates in the lowermost 130 m of the Chitistone indicate deposition in a supratidal to intertidal environment and the stratigraphically higher parts of the Chitistone, up into the Nizina and McCarthy, indicate progressively deeper water marine deposition (Armstrong & MacKevett, 1977). Wrangellia stratigraphy documented on the northern side of the Wrangell Mountains is similar to that in the southern Wrangell Mountains (Richter, 1976). The Eagle Creek Formation, which underlies the Nikolai basalts, is correlative with the Hasen Creek Formation on the south side of the Wrangell Mountains and has a richly fossiliferous Early Permian limestone member, similar to the Golden Horn Limestone 214 Lentil (Richter, 1976). Richter (1976) documented and mapped (Nabesna A3 and A4 quadrangles) extensive mafic sills intruding the Eagle Creek Formation which make up approximately 70 % of the section beneath the Nikolai basalts. Thin discontinuous lenses of shale and argillite containing imprints of Middle Triassic bivalve Daonella are found in several localities between the Eagle Creek and the base of the Nikolai. Richter (1976) describes a basal Nikolai flow unit that is discontinuous volcanic conglomerate-breccia containing fragments of basalt and sedimentary rock derived from the underlying Paleozoic formations. In one area on the north side of the Wrangell Mountains, Richter (1976) describes Permian limestone fragments incorporated in flows as large lenses and masses, up to hundreds of meters long, which may represent flow-rafted debris. The volcanic stratigraphy on the northern side of the Wrangell Mountains is also predominantly subaerial flows. Late Triassic limestone and shale overlying the Nikolai in the northern Wrangell Mountains are correlative with the Chitistone, Nizina, and McCarthy Formations on the south side of the Wrangell Mountains (Richter, 1976). Wrangeffia in southwest Yukon From the Nutzotin Mountains along the Alaska-Yukon border to southeast Alaska, Wrangellia forms a thin northwest- to southeast-trending belt in the southwest corner of Yukon (Figs 5.2 and 5.9). This belt is separated from several small discontinuous exposures of Wrangellia to the southwest by rocks of the Alexander Terrane (Figs 5.2 and 5.9). The best exposures of Wrangellia are in the Kluane Ranges, where the stratigraphy is similar in most aspects to stratigraphy in the Wrangell Mountains, and it has been described using the same nomenclature (Fig. 5.9; Muller, 1967; Read & Monger, 1976). The key difference between these two areas is that Wrangellia stratigraphy in Yukon is intensely folded and faulted, and the stratigraphic relationships are commonly difficult to determine. Discontinuous sections of Wrangellia stratigraphy exposed south of the Kluane Ranges have limited exposures ofNikolai basalts. Paleozoic arc volcanic and marine sedimentary rocks of the Skolai Group are the oldest units of Wrangellia in Yukon (Fig. 5.9). The Skolai Group consists of lower flow and upper volcaniclastic members of the Station Creek Formation conformably overlain 215 I. z M CC AR Th Y FM hO .. I. .o o n .l NI 7I NA II U Fc Tt lN E ki n\u00E2\u0080\u0094 be dd ed lim es to ne . c he rt, sh al e C H rF IS Tn N F c lu ec T oN E lim es to ne ,d ol om ite NI KO LA I FO RM A1 1O N \u00E2\u0080\u0094 su ba er iaI ba sa lti c flo w s (\u00E2\u0080\u0094 10 00 m ex po se d th ic kn es s) _ \u00E2\u0080\u0094 ex te ns iv ely fau lte d\u00E2\u0080\u0094 Ipillow br ec ci a, m in or pi llo w ba sa lt / (< 7t Im ) a n d al tra m af ic ro ck s k Fi gu re 5.9 Ph ot og ra ph s an d m ap o ft he N ik ol ai Fo rm at io n in so u th w es tY uk on .( A) V er tic al ly -o rie nt ed su ba er ia lf lo w s (< 10 m th ic k) in th e K lu an e Ra ng es . D al is he ep o (w hit ed ot s) in ce n te r o fp ho to gr ap h fo rs ca le. (B )S im pl ifi ed m ap o fs o u th w es tY uk on sh ow in g th e di str ib ut io n o ft he N ik ol ai Fo rm at io n (bl ack ;a fte rI sr ae l, 20 04 ; I sr ae l& V an Ze yl ,2 00 4; Is ra el et al ., 20 05 ). St ra tig ra ph ic co iu nm fo r K lu an e Ra ng es de riv ed fro m Re ad & M on ge r (19 76 )I sr ae l et aL (20 06 ), Is ra el & V an Ze yl (20 05 ), an d fie ld wo rk .( C) N ik ol ai Fo rm at io n o v er la in by Ch iti sto ne Li m es to ne w ith D on jek Ri ve rv al le y in fa rb ac kg ro un d. D av e V an Ze yl in ce n te r o fp ho to gr ap h fo rs ca le. (D )P ho to gr ap h o fb as e o ff lo od ba sa lts in W el lg re en ar ea , w ith H as en Cr ee k Fo rm at io n o v er la in by pi llo w br ec ci a an d su ba er ia lb as al tf lo w so ft he N ik ol ai Fo rm at io n. by sedimentary units of the Hasen Creek Formation, similar to those described by Smith and MacKevett (1970) in the Wrangell Mountains. Numerous mafic and ultramafic intrusions related to the Nikolai basalts intrude the Skolai Group in Yukon (Came, 2003). Several occurrences of thin lenses of alternating beds of argillite, siltstone, and sandstone containing Middle Triassic Daonella imprints underlie the Nikolai Formation (Read & Monger, 1976). The Middle Triassic unit is very similar to the underlying Hasen Creek Formation and the presence of the Daonella is the only discerning characteristic between the two units. Everywhere in the Kluane Ranges where Middle Triassic sedimentary rocks have been identified they are overlain by a conglomerate and breccia unit assigned to the base of the Nikolai Formation (Read & Monger, 1976; Israel & Cobbett, 2008). The lithological character of the conglomerate/breccia unit is highly variable, ranging from clast supported boulder and pebble conglomerate to matrix supported cobble and pebble breccia. The clasts are primarily derived from the underlying Skolai Group and include the whole spectrum of volcanic and sedimentary rocks observed in the underlying units. Rounded to sub-rounded boulders and pebbles of augite and plagioclase phyric basaltic andesite and lithic tuffs of the Station Creek Formation are common, as are subrounded to subangular mudstone, siltstone and cherty pebbles from the Hasen Creek Formation. The basal conglomerate/breccia and the underlying Middle Triassic sedimentary units are laterally discontinuous and they are bound by faults in several localities (Read & Monger, 1976; Israel & Cobbett, 2008). These structures are interpreted as laterally discontinuous grabens associated with uplift or rifling during the initial stage ofNikolai volcanism. The Nikolai Formation, in southwest Yukon, includes a marine basal flow unit overlain by a thick succession of dominantly subaerial flows capped by a shallow submarine flow unit. The basal flow unit is predominantly pillow breccia with occurrences ofpillow basalt, which is less than 100 m thick (Greene et a!., 2005; Israel et al., 2006). The volcanic stratigraphy in Yukon is -4 km in total thickness. The subaerial flows (1-10 m thick) are strongly amygdaloidal in zones and are dominantly maroon to olive green. Local thin tuffs and breccias have been reported within the volcanic stratigraphy. In numerous locations within the uppermost portions of the Nikolai Formation, limestone and argillite are interbedded with basalt flows. These thin, 217 discontinuous beds are usually less than 1 meter thick, but locally as thick as 30 m. Microfossils collected from the interbedded limestone units yield a Late Carnian to Late Norian age (Read and Monger, 1976; Israel et al., 2006). Above the highest stratigraphic flows, massive limestone, including horizons of gypsum, dominates the stratigraphy. The limestone and gypsum are correlatives to the Chitistone Limestone described by MacKevett (1971, 1978) in the Wrangell Mountains. The limestone is often brecciated near the base and includes large blocks of the underlying basalt. The Chitistone Limestone ranges from hundreds of meters thick to less than 10 m thick as discontinuous lenses. Macrofossils are extremely rare in the Chitistone Limestone; however, there are abundant microfossils that yield Late Norian ages (Read & Monger, 1976; Israel & Van Zeyl, 2005). Conformably overlying the Chitistone Limestone is a Late Triassic to Early Jurassic, thinly-bedded argillite and limestone unit assigned to the McCarthy Formation. The McCarthy Formation is easily identifiable in outcrop by alternating light and dark grey calcareous and carbonaceous beds. Abundant macrofossils from the calcareous beds give Late Norian ages (Read & Monger, 1976). An Early Jurassic age for the uppermost McCarthy Formation is suggested by the presence of ammonite-bearing horizons. Wrangeffia in southeast Alaska Triassic basaltic and sedimentary rocks with similarities to Wrangellia sequences in southern Alaska are exposed in several areas of southeast Alaska (Fig. 5.10). Most of these sequences occur as elongate fault-bound slivers within a large and complex fault system. Not all of these sequences are definitively established as part of Wrangellia. Most of these sections occur within the Alexander Terrane. Two areas in the northern part of southeast Alaska with thick Triassic basalt sequences (Chilkat Peninsula, Chichagofand Baranof Islands) are probably correlative with Wrangellia flood basalts in southern Alaska (Fig. 5.10). The Chilkat Peninsula area contains Triassic metavolcanic sequences that form a thin (<5 km wide) belt northwest and southeast of Haines, Alaska, along the east side of the Chilkat River and on the Chilkat Peninsula (Plafker & Hudson, 1980) (Fig. 5.10). These Triassic basaltic sequences are steeply dipping and are approximately 3000 m in total thickness (Plaficer 218 ,, - \u00C2\u00A3 Figure 5.10 Simplified map of southeast Alaska showing the distribution of Triassic basalts which may be correlative with Wrangellia flood basalts. Most of these basalts are in the Alexander Terrane. Orange line is Alaska-BC border. Purple lines are faults. See text for description of units. Map adapted from a map provided courtesy of Brew (written comm., 2007). Inset shows location of map in northwestern North America. Wrangellia Q Chilkat Peninsula \u00C2\u00AE Chichagof and Baranof Islands Perseverance succession \u00C2\u00AE Juneau area Hyd Group \u00C2\u00AE Admirality Island \u00C2\u00AE Kupreanof Island/Keku Strait \u00C2\u00AE Gravina Island 55C Scale 0 00 krr 131\u00C2\u00B0 219 and Hudson, 1980). The metabasalt flows are primarily massive layered arnygdaloidal flows with local occurrences of pillow basalt, pillow breccia, and tuff (Plafker and Hudson, 1980). The base of the basaltic sequence is not exposed, but a 300-400 rn-thick section in the upper part of the stratigraphy contains abundant pillowed flows and breccia, locally with thin (<1 rn) lenses of limestone (Plafker et a!., 1 989a). Diagnostic Late Camian rnacrofossils (Ammonites and Bivalves) occur just above the top of the basalt sequence in overlying limestone (Plafker et aL, 1 989a). The basalt is overlain by -4 700 m of mostly limestone, carbonaceous argillite, siltstone, and volcanic sandstone (Plaficer and Hudson, 1980). These sedimentary sequences are slightly different than the inner-platform carbonate rocks overlying Nikolai basalts in the Wrangell Mountains (Plaficer and Hudson, 1980). Late Triassic radiolarians from the overlying sequences on Chilkat Peninsula are identical to species found above Wrangellia flood basalts in the Queen Charlotte Islands (Plaficer et a!., 1 989b). A thin belt of rnetabasalt northwest and southeast of Juneau, Alaska, is also lithologically and geochernically similar to Wrangellia flood basalts and may be a continuation of the belt on the Chilkat Peninsula (Fig. 5.10; Gehrels & Barker, 1992). The stratigraphy on Chichagof and Baranof Islands, which is similar to Wrangellia, may also extend northward along the west side of Tarr Inlet (Decker, 1981; Fig. 5.10). The rnetabasalt in this area, called the Goon Dip Greenstone, is mostly massive subaerial flows (<1000 m in total thickness) with local pillows (Decker, 1981). The Goon Dip is lithologically and geochernically similar to the Nikolai basalts in the Wrangell Mountains (Decker, 1981). The Goon Dip is overlain by Whitestripe Marble, similar to the Chitistone Limestone in the Wrangell Mountains, but no age-diagnostic fossils have been found in the uppermost Goon Dip or Whitestripe Marble (Decker, 1981). Metabasaltic sequences of Triassic age, with underlying Paleozoic and overlying Late Triassic sedimentary sequences, are found in several other areas of southeast Alaska (Brew, 2007, written comm.; Fig. 5.10). These areas are believed to be within the Alexander Terrane and they may not be correlative with Wrangellia stratigraphy. These units belong to the Hyd Group, mainly exposed on Admirality, Kupreanof Islands and other assorted islands, and the Perseverance succession of Brew (written comm., 2007) 220 (Fig. 5.10). The Hyd and Perseverance volcanic rocks contain Triassic basalt ranging from pillow lava, breccia, and tuff to massive flows (Brew, 2007, written comm.), but most of the Hyd Group is non-volcanic (Loney, 1964; Muffler, 1967). Basalt sequences do not exceed 600-700 m in thickness and are more variable and heterogeneous in composition than flood basalt stratigraphy in the main portions of Wrangellia. The basalts are Late Triassic in age and the only fossils associated with the basalts are Early and Middle Norian; Early Norian rocks underlie the basalts and Late Norian rocks overlie them (Muffler, 1967; Katvala & Stanley, 2008, in press). A variety of thin (<200 m) limestone and calcareous clastic sedimentary rocks with Late Carnian and Norian fossils have been found associated with some of the metabasalts in southeast Alaska (Brew, 2007, written comm.). Wrangeffia in the Queen Charlotte Islands (Haida Gwail) Wrangellia stratigraphy forms a large part of the southern Queen Charlotte Islands and is very similar to Wrangellia stratigraphy on Vancouver Island (Fig. 5.2). Permian chert, carbonate, and volcaniclastic rocks form the deepest level of exposure of Wrangellia stratigraphy (Hesthammer et al., 1991; Lewis et al., 1991). The Paleozoic marine sequences are overlain by flood basalts of the Karmutsen Formation, but the base of the Karmutsen is not exposed in the Queen Charlotte Islands. The Karmutsen Formation in the Queen Charlotte Islands is similar to basalt stratigraphy on Vancouver Island; however, a section of \u00E2\u0080\u0094\u00E2\u0080\u00984300 m of Karmtusen basalts measured by Sutherland- Brown (1968) consisted of 95 % submarine flows, with a ratio of pillow basalt to fragmental basalt of--\u00E2\u0080\u00998:2. Rare occurrences of shale and lenses of tuffaceous crinoidal limestone (<30 m thick), grading between limestone and lapilli tuff, occur in the lowest exposed part of the Karmutsen Formation (Sutherland-Brown, 1968). The Karmutsen Formation is overlain by Late Triassic and Early Jurassic marine limestone and sedimentary rocks of the Kunga and Maude groups (Lewis et aL, 1991). Limestone lenses (1-60 m thick), similar to the overlying Kunga limestone, are locally overlain by tuff in the upper part of the Karmutsen Formation along discontinuous horizons (Sutherland-Brown, 1968). Local interfmgering of flows and limestone also occurs in the lowest part of the overlying Kunga Group of Sutherland-Brown (1968). The 221 Kunga Group contains identical fossils and is lithologically indistinguishable from the micritic Quatsino limestone which overlies Karmutsen basalts on Vancouver Island (Sutherland-Brown, 1968). Wrangeffia on Vancouver Island Northern and central Vancouver Island is underlain by Wrangellia stratigraphy which forms the uppermost sheet of a thick sequence of northeast-dipping thrust sheets that constitute the upper crust of Vancouver Island (Fig. 5.11; Monger & Journeay, 1994; Yorath et al., 1999). The cumulative thickness of Wrangellia stratigraphy exposed on Vancouver Island is more than 10 km (Yorath et al., 1999). Wrangellia lies in fault contact with the Pacific Rim Terrane and West Coast Crystalline Complex to the west, and is intruded by the predominantly Cretaceous Coast Plutonic Complex to the east (Wheeler & McFeely, 1991). Most of the structures and contacts between units on Vancouver Island are northwest-trending. Two prominent northwest- to southeast- trending anticlinoria (Buttle Lake and Cowichan anticlinoria) are cored by Paleozoic rocks, which are not exposed on northern Vancouver Island (Fig. 5.11; Brandon et al., 1986; Yorath et a!., 1999). Jurassic and Cretaceous sedimentary strata overlap Wrangellia stratigraphy on parts of Vancouver Island (Muller et al., 1974). Central and Southern Vancouver Island The deepest levels of Wrangellia stratigraphy, mostly exposed in the Buttle Lake and Cowichan anticlinoria, comprise the lower to middle Paleozoic Sicker Group and the upper Paleozoic Buttle Lake Group (Fig. 5.11). The Devonian to Mississippian Sicker Group consists of volcanics, volcaniclastics, and minor chert (Brandon et al., 1986; Massey & Friday, 1988; Yorath et al., 1999). The overlying Mississippian Buttle Lake Group comprises chert, argillite, and limestone, and Pennsylvanian to Permian limestone, argillite, and chert overlain by minor clastics (Yole, 1969; Brandon et a!., 1986; Massey & Friday, 1988; Yorath et al., 1999). The Buttle Lake Group overlies the Sicker Group with some conformable contacts and is unconformably overlain by Triassic strata of the Vancouver Group (Karmutsen and Quatsino formations; Massey and Friday, 1988; Yorath et a!., 1999; Nixon and Off, 2007). The combined total thickness of the Sicker 222 pillowed and unpillowed flows (\u00E2\u0080\u00943000 m) Figure 5.11 Simplified map of Vancouver Island showing the distribution of the Karmutsen Formation (green) and underlying Paleozoic formations (black; after Massey et a!., 2005 a, b). The main areas of field study are indicated with boxes or circles with capital letters (see legend). Stratigraphic columns are shown for northern and central Vancouver Island. Column for northern Vancouver Island adapted from Nixon & Off (2007). Column for central Vancouver Island is derived from Carlisle (1972), Juras (1987), Massey (1995), and from fieldwork. volcanic breccia, volcanics, minor QUATEORMATJQN massive to well-bedded micritic and locally bioclastic limestone \u00E2\u0080\u0094 intra\u00E2\u0080\u0094Karmutsen limestone lenses KARMIITSFN FORMATION subaerial flows with minor (>1 500 m) pillow basalt and hyaloclastite pillow breccia and hyaloclastite (\u00E2\u0080\u0098\u00E2\u0080\u00941 500 m) Keogh Lake picrite Ipillow lavas> 223 and Buttle Lake Groups is estimated to be --5O00 m (Massey, 1995; Yorath et a!., 1999; Fig. 5.11). The oldest known rocks on Vancouver Island are pillowed and massive basalt flows of the Devonian Duck Lake Formation (Sicker Group). The Duck Lake Formation is conformably overlain by Upper Devonian interbedded andesitic volcanic and volcaniclastic strata of the Nitinat and McLaughlin Ridge Formations (Massey, 1995). The Buttle Lake Group contains three units (Fourth Lake, Mount Mark, and St. Mary Lake formations) which crop out across Vancouver Island in association with the Sicker Group (Fig. 5.11). The Fourth Lake Formation (Cameron River Formation of Massey & Friday, 1988) is the lowest and contains mostly thin-bedded, commonly cherty sediments including chert, argillite, possible tuffs, siltstones, volcanic sandstones and minor breccia at the base (Massey & Friday, 1988; Yorath et al., 1999). The overlying Mount Mark Formation either conformably overlies and laterally interfingers with the Fourth Lake Formation or unconformably overlies the Sicker Group (Massey & Friday, 1988; Yorath et a!., 1999). This formation comprises massive bioclastic limestone beds dominated by crinoid clasts (<2 cm across), with minor argillite and chert interbeds (Yole, 1969; Massey & Friday, 1988; Yorath eta!., 1999; Katvala & Henderson, 2002). The Mount Mark and Fourth Lake Formations may alternate in vertical sections as laterally adjacent facies throughout the entire Pennsylvanian to Early Permian succession (Katvala & Henderson, 2002), although the traditional definitions (Massey and Friday, 1988; Yorath et a!., 1999) are maintained in this paper for consistency. The St. Mary Lake Formation comprises sandstone, argillite, conglomerates, and chert that conformably overlie the Mount Mark Formation (Massey & Friday, 1988; Yorath eta!., 1999). Exposures of the St. Mary Lake Formation are rare and generally removed by the sub-Triassic unconformity (Massey & Friday, 1988). Conodonts indicate Mississippian to Permian ages in the Buttle Lake Group (Orchard fide Brandon eta!., 1986; Henderson & Orchard, 1991; Katvala & Henderson, 2002). Paleontologic age determinations can be found in Supplemental data tableS. The upper parts of the Buttle Lake Group (Mount Mark Formation) are commonly intruded by mafic sills related to the Karmutsen basalts (Massey, 1995; Yorath eta!., 1999). 224 The Karmutsen Formation covers large areas of central Vancouver Island, where it forms an emergent oceanic plateau sequence (6 1cm). On central Vancouver Island, the base and lower parts of the volcanic stratigraphy are well-exposed in three general areas (Buttle Lake and Cowichan anticlinoria, and the Schoen Lake area; Fig. 5.11). In each of these areas, thick sediment-sill complexes occur at the base of the Karmutsen basalts, which unconformably overlie Buttle Lake Group limestone and Middle Triassic sedimentary strata. In the Schoen Lake area, a sediment-sill complex and each of the three subdivisions of the Karmutsen Formation (pillow basalt, pillow breccia and hyaloclastite, and subaerial flows) are preserved (Fig. 5.12). Massive mafic sills intrude siltstone, chert, and interbedded limestone with Middle Triassic Daonella occurring near the top of the unit. The sediment-sill complex is approximately 1000 m thick and sedimentary layers between the sills range from 1 to 60 m thick. The sills commonly deform and envelop sediments along contacts (Carlisle, 1972; Fig. 5.12; Supplementary photo file 5). The sediment-sill complex is overlain by a thick succession (2000 m) of pillow basalt (Supplementary photo file 5). Some of the lowest pillowed flows contain sediment in pillow interstices, which is absent higher in the volcanic stratigraphy (Carlisle, 1972). The volcanic stratigraphy around Buttle Lake is proposed to be the type section for the Karmutsen Formation because close to a complete stratigraphic section (-\u00E2\u0080\u00986000 m thick) is preserved (Yorath et a!., 1999; Fig. 5.13; Supplementary photo file 5). Basal sills and lower pillowed flows are well-exposed and accessible on the east side of Buttle Lake (Fig. 5.13). Permian limestone of the Buttle Lake Group (90-120 m thick) on the west side of Buttle Lake is intruded by Karmutsen sills and overlain by pillow lavas (Fig. 5.13). The lower part of the submarine strata at Buttle Lake is intruded by mafic sills 30- 40 m thick (Surdam, 1967). Pillowed flows contain large-diameter pillows (1-4 m) and sediments are rarely present between flows. Above the submarine flows at Buttle Lake are <1500 m of pillow breccia and hyaloclastite and over 2000 m of massive subaerial flows (Surdam, 1967). Marine fossils were found at one locality within the pillow breccia in the lower part of the Karmutsen (Surdam, 1967). The lower part of the subaerial flow member contains thinner flows than the upper part. The upper parts of the Karmutsen Formation around Buttle Lake contain 225 Fi el d n o te s fr om D. Ca rli sle sil l - - i - - - - I - - - - : I - - - \u00E2\u0080\u0094 - - - . \u00E2\u0080\u0094 \u00E2\u0080\u00A21\u00E2\u0080\u0099. \u00E2\u0080\u0099 \u00E2\u0080\u00A2 5-- - \u00E2\u0080\u0094 \u00E2\u0080\u0099 c - - - : - - D ao ne Il a be ds \u00E2\u0080\u0099 41 \u00E2\u0080\u0099 - A - - \u00E2\u0080\u0094 - \u00E2\u0080\u00A2 :- - \u00E2\u0080\u0094 \u00E2\u0080\u0094 - - \u00C3\u00A7. sr f \u00E2\u0080\u0094 - - - 0 \u00E2\u0080\u0094 a t \u00E2\u0080\u0094 - \u00E2\u0080\u0098 sil l - . Sk et ch o f \u00E2\u0080\u0098 D ao ne lla be ds \u00E2\u0080\u0099 _ - V : l - i F -- 2 id dl e Ju ra ss ic isI Pl at on ic Su ite La te T ria ss ic - Ea rly Ju ra ss ic [] Pa rs on Ba y Fo rm at io n M id dl e- Lo w er T ha ss ic Qa atn ino Li m es to ne K ar m ut se n Fo rm at io n r ii St ra Ie -c he nt -ti m es to ne a D uo ne fta be ds M is si ss ip pi an -P en m an Bu ttl e La ke Gr ou p (se di m en tar y ro ck s) $ m a ss iv e la va / o sil l / ga bb ro /fa ul t \u00E2\u0080\u00A2 pi llo w la va o sh al e o r ch er t o v er \u00E2\u0080\u0094 pa rk bo un da ry (Ju ly 24 , 19 72 ) D ao ne lla lo ca lit y , , , rL \u00E2\u0080\u0099f l- I 5\u00E2\u0080\u0099 \u00E2\u0080\u0094 - - 1 ,, te ,- ,f. , I- - h. , W s U C N + L\u00E2\u0080\u0099- ) I Fi gu re 5. 12 Fi el d n o te s, ph ot og ra ph s, an d ge ol og ic m ap fo rt he Sc ho en La ke ar ea , V an co uv er Is la nd (lo ca tio n sh ow n in Fi gu re 5.1 1). (A )F ie ld n o te s co u rt es y o f D on Ca rli sle ,t ak en at th e D ao ne lla fo ss il lo ca lit y w ith in th e se di m en tsi ll co m pl ex on th e n o rt h sid e o fM ou nt Sc ho en .( B) Ph ot og ra ph o fD ao ne !la lo ca lit y de pi ct ed by fie ld n o te s in A. (C )S ili ci fle d sh ale an d ch er tw ith ca rb on ac eo us la ye rs in te rb ed de d w ith m af ic sil ls. (D )D ef or m ed ch er tw ith ca rb on ac eo us la ye rs (lo ca tio n o fp ho to sh ow n in C) .C oi n fo rs ca le. (E )C on ta ct be tw ee n sil la n d se di m en ta ry se qu en ce s (lo ca tio n o fp ho to sh ow n in C) . ( F) G en er al iz ed ge ol og y fo rt he Sc ho en La ke ar ea w ith ph ot og ra ph an d sa m pl e lo ca tio ns .M ap de riv ed fro m M as se y et aL (20 05 a). B ut tle La ke a re a , c e n tr al V an co uv er Is la nd . 4 . 4 . _ 49 , / S M ou nt H all G ab br o Bu ttl e La ke G ro up ! M ou nt Ha ll Ga bb ro M is si ss ip pi an -P er m ia n B ut tle La ke G ro up (li m es to ne ,a rg ilr ite ) D ev on ia n Si ck er G ro up K ar m ul se n Fo rm at io n \u00E2\u0080\u00A2 m as siv e sil l Q pil low lav a Fi gu re 5. 13 G en er al iz ed ge ol og y an d ph ot og ra ph so fB ut tle La ke ar ea , V an co uv er Is la nd .( A) Ph ot og ra ph o fc o n ta ct be tw ee n lim es to ne o ft he u pp er Pa le oz oi c Bu ttl e La ke Gr ou p in tru de d by m af ic sil ls, re la te d to K ar m ut se n ba sa lts ,a n d o v er la in by pi llo w ba sa lt o ft he K ar m ut se n Fo rm at io n. (B )G eo lo gi c m ap o ft he Bu ttl e La ke ar ea sh ow in g lo ca tio ns o fp ho to gr ap hs .M ap de riv ed fro m M as se y e ta !. (20 05 a). (C )P ho to gr ap h o fc o n ta ct be tw ee n tw o pi llo w ed flo w sw ith th in se di m en tl ay er al on g th e co n ta ct . (D )P ho to gr ap h o fa n u n pi llo w ed flo w in th e su bm ar in e se ct io n w ith ra di al ly -o rie nt ed co lu nm ar joi nti ng an d sh ale (<1 m th ic k) al on g th e ba se . C re ta ce ou s L IN an ai m o G r o u p (\u00C2\u00B0 \u00E2\u0080\u0099 Ea rly -M id dl e Ju ra ss ic B on an za G ro up v o lc an ic s iL lI sl an d Pl ut on ic Su ite M id dl e- La te Tr la ss lc Qu ats ino Fo rm at io n discontinuous alternations of pillow basalt, pillow breccia, and hyaloclastite typically <30 m thick, but a single, more widespread subaqueous unit is 1-120 m thick (Surdam, 1967). This subaqueous section overlies limestone and tuff up to 30 m thick (Surdam, 1967). The overlying Quatsino limestone at Buttle Lake lies directly on a Karmutsen basalt flow (Surdam, 1967); however, occurrences of paleosols between the Karmutsen and Quatsino Formations have been reported elsewhere on central Vancouver Island (Yorath et a!., 1999). Evidence of molding of limestone around basalt and disaggregation limestone lenses during interaction with basalt flows is described by Surdam (1967). The basal part of the Quatsino Formation west of Buttle Lake is intercalated with pillow basalt in several areas (Surdam, 1967). Northern Vancouver Island Stratigraphy on northern Vancouver Island extends from Middle to Late Triassic flood basalts (lower part of Karmutsen Formation) up through Middle Jurassic arc volcanics (Fig. 5.11). This sequence of lithostratigraphic units is divided into the Vancouver Group and the Bonanza Group (Parson Bay Formation, Volcaniclastic sedimentary unit, and LeMare Lake volcanics; Muller et al, 1981; Nixon et al., 2006c; Nixon and Orr, 2007; Fig. 5.11). The base of the Karmutsen basalts is not exposed on northern Vancouver Island. Recent mapping on part of northern Vancouver Island has established a three-part volcanic stratigraphy of pillowed lava sequences, hyaloclastite, and subaerial flows, of the stratigraphy has established a structural and stratigraphic framework for volcanic stratigraphy (Nixon et al., 2008; Fig. 5.14). The lower pillowed lava sequence is approximately 3000 m thick and contains closely-packed pillowed flows and unpillowed flows. The unpillowed flows typically have lensoidal geometry, convolute lower contacts with underlying pillowed flows, and rarely exhibit irregular, hackly columnar jointing (Fig. 5.14). Interpillow voids are rarely filled with non-volcanic sediment and display a variety of submarine volcanogenic features (Fig. 5.14). The hyaloclastite unit is estimated to vary in thickness from approximately 1550 mill the west to less than 400 m in the Maynard Lake area (Nixon et a!., 2008). The hyaloclastite is predominantly poorly-sorted clasts of pillow basalt in a fmely-comminuted matrix of basaltic shards, fragments, and 228 O H ig h- M g pi llo w la va (M gO ol Ow t% ) Su sp ec te d hi gh -M g pi llo w la va (no ge oc tie m ist ry ) \u00E2\u0080\u00A2 O liv in e- be an nq flo w _ _ _ _ _ _ _ _ _ (M gO < 10 w t Io) \u00E2\u0080\u00A2 O liv in e- be ar in g pi llo w la va (M gO l Ow t% ) \ A tti tu de o fb ed di ng an d \ flo w c o n ta ct s _ _ _ _ _ _ _ V ol ca ni c la m in at io n \u00E2\u0080\u0094 (am yg du len ,d ra in ag e fe at ur es ) E ar ly to M id dl e Ju ra ss ic - Sy nc lin al fo ld ax is _ _ _ _ _ _ _ Is la nd Pl ut on ic Su ite R oa d Fi gu re 5. 14 G en er al iz ed ge ol og y o ft he K ar m ut se n Fo rm at io n on n o rt he rn V an co uv er Is la nd in th e Po rt A lic e- Ro bs on Bi gh ta re a an d ph ot og ra ph o f m as siv e su bm ar in e flo w. (A )G eo lo gi c m ap sh ow in g th e di str ib ut io n o f t he th re e- pa rt v o lc an ic st ra tig ra ph y o ft he K ar m ut se n Fo rm at io n. M ap de riv ed fro m N ix on et al. (20 08 ). Sa m pl e lo ca tio ns an d u n its sh ow n in th e leg en d. St ra tig ra ph ic co lu m n fo rt hi s ar ea is sh ow n in Fi gu re 5.1 1. (B )P ho to gr ap h o f an u n pi llo w ed su bm ar in e flo w su rr o u n de d by pi llo w ba sa lt in th e su bm ar in e se ct io n o ft he K an nu ts en Fo rm at io n in th e Sc ho en La ke ar ea (lo ca tio n sh ow n in Fi gu re s5 .11 an d 5. 12 ). V ol ca ni c st ra tig ra ph y th e K ar m ut se n Fo rm at io n Su ba en al flo w s I I Pill ow ed an d _ _ _ _ _ _ _ u n pi llo w ed flo w s Fl oo d ba sa lts , _ _ _ _ _ _ u n di ffe re nt ia te d pillow rinds. The hyaloclastite and pillow breccia is composed of a matrix of dark brown, variably palagonatized shards of devitrified basaltic glass with clasts ofpillow fragments. Well-bedded basaltic sandstone occurs locally and exhibits soft-sediment deformation features (Nixon et a!., 2008). High-Mg picritic pillow lavas have been identified in the lower part of the Karmutsen Formation (Greene et a!., 2006; Nixon et al., 2008). Recent mapping and geochemistry of the internal stratigraphy place the high-Mg lavas in the upper part of the submarine section, near the transition between the pillowed lavas and the overlying hyaloclastite unit (Nixon et a!., 2008). The subaerial flow unit appears conformable with the underlying hyaloclastite unit and is approximately 1500 m thick (Nixon et al., 2008). The subaerial flow sequences have a well-layered, sheet-like appearance. The subaerial flows vary in flow thickness, grain size (:5 mm), and proportion of amygdules, and they rarely exhibit columnar jointing and paleosols are absent between flows. Interfiow sedimentary lenses are found mostly near the top of the subaerial flow unit and are commonly associated with pillowed flows and hyaloclastite. Most of these interflow lenses consist of limestone, but rarely comprise siliciclastic sediments (Carlisle and Suzulci, 1974; Nixon et a!., 2006; Fig. 5.15). Carlisle and Suzuki (1974) describe small-scale emergent sequences (<30 m thick) of pillow basalt, pillow breccia and hyaloclastite, and massive flows which overlie some limestone lenses. Some of these lenses contain Late Carnian fossil assemblages identical those found in the overlying Quatsino limestone (Carlisle and Suzuki, 1974). Some subaerial flows near the top of the Karmtusen Formation contain abundant (20-40 %) aligned plagioclase megacrysts (1-2 cm laths), and these can be used as stratigraphic markers (Nixon eta!., 2006, 2007; Fig. 5.13). The relationship between the top of the Karmutsen flood basalts and the base of the overlying Quatsino limestone is a sharp, flat-lying contact with minimal evidence of erosion (Fig. 5.16; Supplementary photo file 5). Locally, a thin (<25 cm) layer of brownish-orange basaltic siltstone and sandstone is preserved along the contact (Fig. 5.16). The thin beds of calcareous siltstone grade laterally into limestone. The overlying Quatsino limestone varies in thickness (40-500 m) and age (Carnian to Early Norian) across northern Vancouver Island (Muller et a!., 1981; Nixon eta!., 2006, 2007, 2008). The Quatsino Formation immediately above the Karmutsen is 230 Fi gu re 5. 15 Ph ot og ra ph s o fi nt ra -K ar m ut se n se di m en ta ry len sn ea r th e to p o ft he K ar m ut se n so u th w es to fN im pk ish La ke ,n o rt he rn V an co uv er Is la nd (lo ca tio n s ho w n in Fi gu re 5. 11 ). (A )P an or am ic ph ot og ra ph o fl en s o fs ha le be tw ee n tw o u n pi llo w ed flo ws .C irc le d sle dg eh am m er fo rs ca le. (B )D ef or m ed sh ale w ith de w at er in g st ru ct ur es al on g th e to p o ft he le ns (lo ca tio ns ho w n in ph ot o A) .( C) Ph ot og ra ph o fr ig ht sid e o fl en s, jus tto th e rig ht ed ge o fA .L oc al pi llo w ba sa lt o cc u rs al on g th e ba se . Fi gu re 5. 16 Ph ot og ra ph s fro m n ea r H ol be rg In let ,n o rt he rn V an co uv er Is la nd (lo ca tio n sh ow n in Fi gu re 5.1 1). (A )P an or am ic ph ot og ra ph o fc o n ta ct be tw ee n to p o f t he K ar m ut se n Fo rm at io n an d o v er ly in g Qu ats ino lim es to ne .( B) Th in la ye rs o fs ilt sto ne be tw ee n th e K ar m ut se n ba sa lts an d Qu ats ino lim es to ne (pe nf or sc ale ;l oc at io n sh ow n in A ). (C )B as al t-l im es to ne co n ta ct w ith po ss ib le re go lit h al on g th e co n ta ct (lo g\u00E2\u0080\u0094 1.5 m lo ng fo r sc al e). (D ) C lo se -u p o fr o py fe sto on s (1) an d th e ed ge o f a pa ho eh oe lo be (2) w ith in K ar m ut se n Fo rm at io n. K ni fe fo r sc ale . (E )P ho to gr ap h o fm ar gi n o fp ah oe ho e lo be fro m sa m e lo ca tio n as D. (F) Ph ot om ic ro gr ap h o fp la gi oc la se -ri ch , tr ac hy tic te xt ur ed ba sa lt fro m C. (G ).P ho to gr ap h o fc ut sla b o fp la gi oc la se m eg ac ry sti cb as al tf lo w co m m o n in u pp er pa rt o ft he K ar m ut se n Fo rm at io n. V * \u00E2\u0080\u00A2 \u00E2\u0080\u00A2 - - V . iim es td ne j - mostly a non-fossiliferous massive micritic limestone. The upper part of the Quatsino locally contains a diverse fossil assemblage (Muller et al., 1974; Jeletzky, 1976) and is intercalated and overlain by carbonate clastic sediments, as well as volcanic flows and volcaniclastic deposits of the Parson Bay Formation (Nixon et al., 2006, 2007). Overlying and intercalated with the Parson Bay Formation on northern Vancouver Island are volcanics of the Early to Middle Jurassic Bonanza arc (LeMare Lake volcanics), and equivalent plutonic rocks (Island Plutonic Suite) intrude Wrangellia stratigraphy (Nixon et al., 2006; Nixon and Orr, 2007). GEOCKRONOLOGY OF WRANGELLIA Previous geochronology for Wrangellia flood basalts and related plutonic rocks Samples of Wrangellia flood basalts and related plutonic rocks from BC, Yukon, and Alaska were previously dated in 8 separate studies and these results are summarized in Table 5.2. All ages below are quoted with 2a uncertainty. A total of 15 ages (4 U-Pb, 9 40Ar/39r and 2 K-Ar) of variable quality include 4 basalts and 11 plutonic rocks. In Southern Wrangellia, three U-Pb ages from gabbroic rocks on southern Vancouver Island are available based on (1) a single concordant analysis of a multi-grain baddeleyite fraction that yielded a 206Pb/38Uage of 227.3 \u00C2\u00B1 2.6 Ma (Parrish & McNicoll, 1992), and (2) two unpublished 206Pb/38Ubaddeleyite ages of 226.8 \u00C2\u00B1 0.5 Ma (5 fractions) and 228.4 \u00C2\u00B1 2.5 (2 fractions) (Table 5.2; Sluggett, 2003). A single whole rock40Ar/39r age of Karmutsen basalt from Buttle Lake on Vancouver Island yielded a plateau age of 224.9 \u00C2\u00B1 13.2 Ma (Lassiter, 1995). In Northern Wrangellia, zircon separated from a gabbro sill possibly related to the Nikolai basalts in southwest Yukon yielded an age of 232.2 \u00C2\u00B1 1.0 Ma (average 207Pb/6 age of 3 discordant (1.6 to 2.4%) analyses from multi-grain zircon fractions) (Table 5.2; Mortensen & Hulbert, 1991). K-Ar analyses of two biotite separates from peridotite in the Kluane mafic-ultramafic complex provided ages of 224\u00C2\u00B18 and 225\u00C2\u00B17 Ma (Campbell, 1981). In Alaska, three samples of Wrangellia flood basalts from the Wrangell Mountains yielded whole rock40Ar/39rplateau ages of 228.3 \u00C2\u00B1 5.2, 232.8 \u00C2\u00B1 11.5, and 232.4 \u00C2\u00B1 11.9 Ma (Lassiter, 1995). Five40Ar/39rplateau and isochron ages of variable precision have been determined for hornblende and biotite separates from mafic 233 Ta bl e 5. 2 Co m pi la tio n o fp re vi ou s ge oc hr on ol og y o fW ra ng el fia flo od ba sa lta a n d a ss o c ia le dp lu fo ni c ro ck s Sa m pl e N o. A ge Er ro r, \u00C2\u00B1 M et ho d M at er ia l R ef er en ce R oc k de ac rlp tlo nl lo ca tio n C om m en t A la sk a R an ge (2o ) G ab br o; Ta ng le La ke s ar ea , SW To ta lf us io n, pl at ea u ag e. No an al yt ic al in fo nn at io n av ai la bl e. Ar ra ly aia by L Sn ee (U SG S) . 00 A G 02 2 23 1. 1 11 Ar /A r t- lo m bl en de Sc hm id ta n d R og er s, 20 07 G ri zy Ri dg e 62 .7 \u00E2\u0080\u0099N ,1 4S .5 \u00E2\u0080\u0099W . PB 1 23 0. 4 2. 3 Ar /A r Bi ot ile B itt en be nd er at at ., 20 03 2 G ab br o; Ta ng le La ke s a re a No an al yt ic al in fo rm at io n av ai la bl e. AK 2S S1 S 22 8. 3 2. 2 Ar /A r Bi ot ite B itt en be nd er at at ., 20 07 2 G ab br o w ith po ik ili lic bi ot ite ,i nc lu di ng Pl at ea u ag e, S fra ct io ns ,8 1% m Ar re le as ed ,I nt eg ra te d ag e: 22 6. 0 \u00C2\u00B1 1.1 M a; In te rp re te d a s ol iv in e; Ra in y Cr ee k Co m pl ex a ge o fm ag m at ism . H BD -2 00 3- 29 22 5. 7 4 A r/ k No nm bt en de B itt en be nd ar at a t, 20 07 2 o th in e ba sa lt Ra in y Cr ee k C O m b Pl at ea u ag e, 8 fra ct io ns ,9 7% 3 9 A r re le as ed ,I nt eg ra te d ag e: 22 6. 4 \u00C2\u00B1 2. 0 M a; In te rp re te d a s ag e of m ag m at am . 10 83 0 22 5. 2 13 A r/ k ti on tl en de m B itt en be nd er at a t, 20 07 2 Ot iv in e pe r-d r-t m , Ra in y Cr ee k lso ch ro n ag e; In te rp re te d a s an a te ra lio n ag e, m ax im um a ge fo ra re se te v e n t Im pu re Co m pl ex se pa ra te ,e x c e ss Ar . W ra ng ef lM ou nt ai ns Ni ko lai ba sa lt flo w fro m th e u pp er th ird 92 tJ9 G -1 9 22 8. 3 5. 2 k /k W ho le ro ck La as ite r, ig g s 3 o ft he v o lc an ic st ra tig m ph y; G la ci er de Ar /m N st ep -h ea tin g te ch ni qu e; lso ch ro n ag e: 22 4. 8 * 52 M a; 58 % m Ar re le as ed ;n = 5f 7 Cm ek Ni ko lai ba sa lt flo w, - . 10 0 m ab ov e th e g2 LN G- 42 23 2. 8 11 .5 Ar /A r W ho le ro ck La ss ita r, 19 9& ba se o ft he ba sa lt st ra tig ra ph y; Sk ot ai m Ax /va Ax st ep -h ea tin g te ch ni qu e; tso ch ro n ag e: 23 0. 5\u00C2\u00B1 27 .8 M a; 57 % m Ar re le as ed ;n = 4/ 7 Cr ee k Ni ko lai ba sa lt flo w, fro m th e lo w er pa rt 92 LN G- 47 23 2. 4 1i S Ar /A r W ho le ro ck La ss its r, 19 95 o f b5 5a ll Sk ol al C ra Ar /m Ar st ep -h ea tin g te ch ni qu e; ts oc hr cn ag e: 22 8. 1 \u00C2\u00B1 11 .8 M a; 69 % m Ar re le as ed ; n = 5f 7 Y uk on H D B8 S- TA T2 2 23 2. 2 1 U /P b Zi rc on M or ta ns en et at ., 19 92 M ap le Cr ee k ga bb ro ;K tu an e R an ge s A ve ra ge u v P b / 2 2 6 P b a ge o f3 di sc or da nt (1. 6t o 2. 4% )a n a ly se s fro m m u lti -g ra in zi rc on fra ct io ns . Pe rid ot ite ;T at am ag ou ch e Cm ek , K- Ar ag e. No an al yt ic al in fo rm at io n av ai la bl e. A na ly sis at U ni ve rs ity o fB rit ish Co lu m bi a by . 1. SC 1 22 4 8 K/ Ar B io tte Ca m pb el l, 19 81 K tu an e R an ge s E. N ar ak al . Pe rid ot ite ;W hi te Ri ve r, K tu an e 5C 2 22 5 7 K/ Ar Bi ot ite Ca m pb el l, 19 81 K- Ar ag e. No an al yt ic al in fo rm at io n av ai la bl e. A na ly sis by M .L an ph am (U SG S) . R an ge s V an co uv er Is la nd 2m P b / a s U a ge fo r2 co n co rd an tf ra ct io ns yi el de d a ge s o f2 29 .4 \u00C2\u00B1 2. 5 an d 22 8. 4 \u00C2\u00B1 t6 M a. 02 -C S- 14 22 84 2. 5 U /P b Zi rc on Sl ug ge tt, 20 03 M ou nt Tu ar e G ab br o; Sa tts pr in g Is la nd Cr ys ta lli za tio n a ge in te rp re te d to be 22 8. 4 \u00C2\u00B1 2. 5 M a. Po rp hy dt ic ga bb ro mc ro ck ;C ro fto n, zr eP bf lm U a ge fro m a sin gl e co n co rd an ta n al ys is o f a m u lti -g ra in ba dd et ey ite fra ct io n. 84 82 2- iC 22 7. 3 2. 6 U /P b B ad de te yi te Pa rr ish an d M cN ico lt, 19 92 so u th er n V an co uv er Is la nd 2w P b / 2 3 s U a ge fo r1 of 5 fra ct io ns in te rp re te d a s th e cr ys ta lli za tio n ag e. 2 0 6 P bP m U a ge s of 02 -C S- li 22 8. 8 0. 5 U /P b Zi rc on Sl ug ge tt, 20 03 M ou nt Tu am G ab br o; Sa lts pr in g Is la nd o th er fra ct io ns a re 21 9. 8 \u00C2\u00B1 0. 7, 22 3. 3 \u00C2\u00B1 0. 5, 22 1. 9 \u00C2\u00B1 0. 6, an d 23 3. 9 \u00C2\u00B1 1. 4 M a, w hi ch w er e in te rp re te d to in di ca te po st- cr ys ta lli za tio n Pb -lo ss . 91 LK B- 46 22 4. g 13 2 k /k W ho le ro ck La sa fta ,I 2 K ar m ut se n ba sa lt 11 0w ,B ut tle La ke , m Ar /m Ar st ep -h ea tin g te ch ni qu e; ts oc hr on a ga 21 3. 7 \u00C2\u00B1 20 .5 ;6 9% m Ag n= 51 7 ce n tr al V an co uv er Is la nd t m R ep ta ce s c tn op yr ox en e. 2 A na ly se s pr od de d co u rt es y o fP .B itt en be nd er .A na ly se s pr ef or m ed at th e G eo di ro no to gy La bo ra to ry at th e U ni ve rs ity o fA la sk a. Fa hb an ks .M on ito rm in er al M M hb -l (S am so n an d A le xa nd er , 19 87 )w ith an a ge of 51 39 M a (L an ph ere an d D al ry m pt e, 20 00 )w as u se d to m o n ito rn eu tr on flu x. 3A na ty sa s pr ov id ed co u rt es y o fR .D un ca n (O reg on St at e U ni ve rs ity ), o rig in al y in te rp re te d in La ss ite r( 19 95 ). w Ar /m Ax a ge s a re re po rte d re ta bv e to bi ot ite st an da rd FC T- 3 (28 .03 \u00C2\u00B1 0. 16 M a, R en ne a /a l., 19 98 ), ca lc ul at ed w ith A rA rC aic (K op pe rs, 20 02 ). n = n u m be ro fh ea tin g st ep s u se d/ to ta l. Er ro rs m e 2o . 1% ) and ultramafic plutonic rocks in the Amphitheater Mountains in the Alaska Range (Table 5.2). Three samples with ages of 225.2 \u00C2\u00B1 6.5, 225.7 \u00C2\u00B1 2, and 228.3 \u00C2\u00B1 1.1 Ma are from the Rainy Creek area, which lies to the north across a major fault from typical volcanic stratigraphy of Wrangellia flood basalts (Bittenbender et a!., 2007). The Rainy Creek area is a steeply-dipping sequence of picritic tuff and volcaniclastic rocks, mafic and ultramafic intrusives and dikes, and limestone that is distinct from the volcanic stratigraphy of the Nikolai Formation and these units may be older than the Wrangellia flood basalts (Bittenbender eta!., 2003) or may be younger intrusions (Nokleberg et al., 1992). Analytical information is not available for two40Ar/39ranalyses of gabbro related to Wrangellia flood basalts in the Tangle Lakes area of the Amphitheater Mountains with reported ages of 230.4 \u00C2\u00B1 2.3 and 231.1 \u00C2\u00B1 11 Ma (Bittenbender et a!., 2003; Schmidt & Rogers, 2007). Information about samples dated by40Ar/39r in this study A total of 20 mineral separates, including 14 plagioclase, 5 hornblende, and 1 biotite mineral separates, were processed from 19 samples from throughout Wrangellia for40Ar/39rdating. Thirteen samples are Wrangellia flood basalts or intrusive equivalents and six are from younger, cross-cutting intrusive rocks. Of the 13 Wrangellia flood basalts or intrusive samples, nine are basalt flows and four are mafic sills or gabbroic rocks (Supplementary data file 3 and 4). One biotite separate is from an ultramafic plutonic rock from the Kiuane Ranges, Yukon (sample 05-SIS-751). The five homblende separates are all from younger dikes and intrusions that cross-cut Wrangellia basalts and were selected because they provide minimum ages for eruption. All ages indicated below are cooling ages that correspond to the bulk closure temperature (Tcb) of the different minerals to Ar diffusion (\u00E2\u0080\u0094200\u00C2\u00B0C plag; \u00E2\u0080\u0094550\u00C2\u00B0C hbl; \u00E2\u0080\u0094350\u00C2\u00B0C biotite; as summarized in Hodges, 2003). Petrographic textures and major- and trace-element chemistry for all of the geochronological samples are listed in Supplementary data file 3 and described in detail in chapters 2, 3, and 4. The samples that are younger, cross-cutting intrusive units are clearly distinguishable from the Wrangellia flood basalts by their different textures and whole-rock chemistry (Supplementary data file 3). 235 40Ar/39r geochronological results Analytical methods for40Ar/39rdating are described in Appendix H. Age spectra are shown in Figures 5.17 to 5.20 and the analytical results are summarized in Table 5.3. The analytical data are available in Supplementary data file 4. Plagioclase separates from three basalt flows (two submarine and one subaerial flow) and one gabbro from Vancouver Island were analyzed. The incremental heating data of the three basalt flows form plateaus over 67-96% of the Ar released with ages of 72.5 \u00C2\u00B1 1.2, 180.9 \u00C2\u00B1 3.1, and 161.1 \u00C2\u00B1 7.3 Ma (Fig. 5.17; Table 5.3); inverse correlation diagrams(39Ar/\u00E2\u0080\u0099\u00C2\u00B0Ar vs. 36Ar/40) yield isochron ages that are concordant with the plateau ages. The gabbro from the Alice-Nimpkish Lake area displays a disturbed saddle- shaped age spectrum, which may have been affected by excess 40Ar (e.g. Lanphere & Dalrymple, 1976; Harrison & McDougall, 1981). The cross-cutting mafic dike (sample 4718A3) with 54% of the 39Ar released had an age of 193 \u00C2\u00B1 26 Ma and a broadly concordant isochron age of 186 \u00C2\u00B1 13 Ma (Fig. 5.17; Table 5.3). The biotite separate from a peridotite in Yukon yielded a well-defmed plateau over \u00E2\u0080\u0094P60% of the Ar released with an age of 227.5 \u00C2\u00B1 1.2 Ma and a concordant isochron age of 226.1 \u00C2\u00B1 3 Ma (Fig. 5.18; Table 5.3). Plagioclase separates from two basalt flows from different levels of the volcanic stratigraphy of the Nikolai Formation in Yukon display disturbed age spectra (Fig. 5.18). Plagioclase separates from five basalt flows and one mafic sill in Alaska (two Wrangell Mountains, four Alaska Range) yield age spectra with a total of four interpretable ages with 62-99% of the 39Ar released and plateau or integrated ages of 191 \u00C2\u00B1 11, 160.7 \u00C2\u00B1 1.3, 160.57 \u00C2\u00B1 8.64, and 169.0 \u00C2\u00B1 2.4 Ma (Fig. 5.19; Table 5.3). Three of these samples (samples 571 9A5, 5802A6, 581 0A6) form well-defmed plateaus that correspond with isochron ages. Two samples of basalt flows (samples 5715A1, 5810A10) showed disturbed40Ar/Ar systematics. Incremental heating data of homblende separates from four younger intrusive samples from Alaska (two Wrangell Mountains, two Alaska Range) yielded well-defmed plateau ages of 29.7 \u00C2\u00B1 1.1, 123.13 \u00C2\u00B1 0.77, 148.80 \u00C2\u00B1 0.83, and 149.86 \u00C2\u00B1 0.93 Ma characterized by 54-95% of the Ar released (Fig. 5.20; Table 5.3); these plateau ages 236 4723A1 3* Alice-Nimpkish Lake picritic pillow basalt lsochron age: 160.5 \u00C2\u00B1 8.7 I I I I I 20 40 60 80 100 Figure 5.17 \u00E2\u0080\u0098Ar!39 age spectra for six analyses of plagioclase separates from Vancouver Island. Errors on plateaus are 2a. Samples with red plateau steps are Wrangellia flood basalts. The sample with blue plateau steps is not a Wrangellia flood basalt. Plateau steps included in age calculation are colored red. Asterisks after the sample number indicates Wrangellia flood basalt or associated intrusive. Samples with all white steps do not meet the criteria for a plateau age, defined in Appendix H. Analytical data are presented in Supplementary data file 4 and summarized in Table 5.3. 72.5\u00C2\u00B11.2 Ma 100 500 75 400 193\u00C2\u00B126Ma 50 4718A2* ::: Mount Arrowsmith 4718A3 25 . pillow basalt Mount Arrowsmith \u00E2\u0080\u00A2 lsochron age: 69\u00C2\u00B1 18 100 maficdike lsochron age: 186\u00C2\u00B1 13 I I I0 \u00E2\u0080\u00A2 \u00E2\u0080\u00A2 I I I I I I - 0 20 40 60 80 100 300 20 40 60 80 100 w a a ci 4-.. C G) a a 400 360 320 L) a240 a200 160 1 (\u00E2\u0080\u0098 200 161.1 \u00C2\u00B17.3 Ma 0 100 0- 20 40 60 80 100 0 \u00E2\u0080\u0094F 561 5A6* Alice-Nimpkish Lake gabbro % 39Ar Released 0 I I I I I I 20 40 60 80 100 % 39Ar Released 237 a) 4-, C a) 0 a 80 40 0 280 240 200 < 160 4-, C 120 a a cv a) I 227.5\u00C2\u00B11.2 Ma 05SI751* Biotite Kluane Ranges ultramafic intrusion Isochron age: 226.1 \u00C2\u00B13 I I I I I I I 0 20 40 60 80 100 Tatamagouche Creek, Kluane Ranges \u00E2\u0080\u00A2 massive basalt I I I I I I I 0 20 40 60 80 100 150 130 110 90 70 40 60 % 39Ar Released Figure 5.18 40Ar!39r age spectra of one biotite and two plagioclase separates from Yukon. Errors on plateaus are 2a. Sample 05S1-75-l is an ultramafic plutonic rock and samples 4808A8 and 481 IAI are Nikolai basalts. Plateau steps included in age calculation are colored red. Asterisks after the sample number indicates Wrangellia flood basalt or associated intrusive. Samples with all white steps do not meet the criteria for a plateau age, defined in Appendix H. Analytical data is presented are Supplementary data file 4 and summarized in Table 5.3. 238 200 L. 0 20 40 60 80 280 160.7 \u00C2\u00B1 1.3 Ma 240 5802A6* Tangle Lake, Amphitheater Mountains 200 pillow basalt lsochron age: 150.3 \u00C2\u00B1 2.1 . 1 6C 120 gn I I I I I I I 320 280 ) 240 200 a) I c 160 < 120 80 40 0 20 40 60 80 100 % 9Ar Released Figure 5.19 40Ar/39r age spectra for six analyses of plagioclase separates from Alaska. Errors on plateaus are 2a. Plateau steps included in age calculation are colored red. Asterisks after the sample number indicates Wrangellia flood basalt or associated intrusive. Samples with all white steps do not meet the criteria for a plateau age, defined in Appendix H. Analytical data is presented are Supplementary data file 4 and summarized in Table 5.3. 191 \u00C2\u00B111 Ma 160 120 A1*ewrangellMountains80 Hidden Cre massive basaft40 240 200 160 120 a) a) I 571 9A5* massive basalt Skolai Creek, Wrangell Mountains lsochron age: 189 \u00C2\u00B1 29 I I I I I I 0 \u00E2\u0080\u009C0 20 40 60 80 100 300 200 100 137.3 \u00C2\u00B1 8.5 Ma 5810A4* Tangle Lake, Amphitheater Mountains mafic sill lsochron age: 121 \u00C2\u00B189 I . I I \u00E2\u0080\u00A2 0 20 40 60 80 100 20 40 60 80 100 169.0 \u00C2\u00B1 2.4 Ma 581 0A6* Tangle Lake, Amphitheater Mountains pillow basalt lsochron age: 166.6 \u00C2\u00B1 3.5 I I I I I I 0 20 40 60 80 % 9Ar Released 100 239 280 120 4-, 40 ci) 4-, ci) 1\u00E2\u0080\u0099. Figure 5.20 40Ar/39r age spectra for six analyses of homblende separates from plutonic rocks that are younger than Wrangellia basalts in Alaska. Errors on plateaus are 2a. Samples with blue plateau steps are not Wrangellia flood basalts and do meet the criteria for a plateau age. Samples with all white steps do not meet the criteria for a plateau age, defined in Appendix H. Analytical data are presented in Supplementary data file 4 and summarized in Table 5.3. ci) 4-, ci) 148.80 \u00C2\u00B1 0.83 Ma 300 240 149.86\u00C2\u00B10.93 Ma 200 200 - 160 _ 120 5712A1 100 Nugget Creek,Wrangell Mountains 80 mafic dike lsochron age: 149 \u00C2\u00B1 1.4 40 0 \u00E2\u0080\u00A2 I I I I I 0 0 20 40 60 80 100 0 60 5712A4 Nugget Creek, Wrangell Mountains mafic dike lsochron age: 147.4\u00C2\u00B1 2.1 160 20 40 60 80 100 80 123.13 \u00C2\u00B1 0.77 Ma 5725A5 Tangle Lake, Amphitheater Mountains felsic intrusion lsochron age: 122.0 \u00C2\u00B1 1.8 I I I 40 20 0 020 40 60 80 100 5729A1 Glacier Gap Lake, Amphitheater Mountains 20 40 60 80 100 % 9Ar Released 0( 20C 5808A6 160 Rainy Creek, Amphitheater Mountains amphibolite 120 4O I I I I I I I I I \u00E2\u0080\u009C0 20 40 60 80 100 % 9Ar Released 240 Table 5.3 Ar/wAr dating results for 13 samples of W,angellia flood basalts and 6 samples from the Wrangellia Terrane Sample Mineral b Integrated Age (Ma) Plateau Age (Ma)\u00E2\u0080\u0099 Plateau Inlormationd laochron Note Vancouver Island (2a) (2a) 5 steps 69\u00C2\u00B118 471 8A2* Plag 76.31 \u00C2\u00B1 1.32 72.5 \u00C2\u00B1 1.2 67.6% mAr release CoArImAr =307\u00C2\u00B165 tholeiitic, pillowed flow MSWD=1.3 MSWD = 1.14. n=5 3 steps 186\u00C2\u00B113 4718A3 Plag (1) 255.99 \u00C2\u00B1 10.86 193 \u00C2\u00B1 26 54.3% 39Ar release lAr/mAr =322.4\u00C2\u00B19.6 mafic dike MSWD=5.1 MSWD = 2.6. n=7 177.1\u00C2\u00B13.1 471 8A3 Plag (2) 224.51 C 10.33 NP roArimAr) =327.5\u00C2\u00B16.2 last step: 231 \u00C2\u00B113 Ms MSWD = 1.02, n6 177.6\u00C2\u00B13.0 4718A3 Plag (C) 255.99\u00C2\u00B110.86 NP (Ar,mAr =325.3\u00C2\u00B14.5 MSWD = 1.4. n=9 6 steps 180.3\u00C2\u00B15.4 4720A6\u00E2\u0080\u0099 Ptag 182.31 \u00C2\u00B13.20 180.9 \u00C2\u00B13.1 95.6% mAr release (4oArImAr) =307\u00C2\u00B116 thdeiitic, massive flow MSWD=O.93 MSWD = 0.39, n=7 4723A13\u00E2\u0080\u0099 PIag 150.24 \u00C2\u00B1 8.50 161.1 \u00C2\u00B1 7.3 79.1%Areiease (4oA,mAF)280\u00C2\u00B115 slightly upstepping plateau, picritc MSWD=1.3 MSWD = 1.5. n=7 5615A6\u00E2\u0080\u0099 Plag 220.98 \u00C2\u00B1 2.15 NP NI coarse-grained gabbroic rock Yukon 9 steps 226.1\u00C2\u00B13 0551.75.1* Biotite 226.08 \u00C2\u00B1 0.53 227.5\u00C2\u00B1 1.2 60.1% mAr release (40k/mAr)i =462\u00C2\u00B1380 nine steps in platesu, peridotite MSWD1.6 MSWD = 1.7, n=9 48O8Ar PIag 114.88\u00C2\u00B12.60 NP NI high-fl massivefiow 4811A1* Plag 361.37\u00C2\u00B111.01 NP NI low-Timsssiveflow Wrangell Mountains 6 steps 146.5\u00C2\u00B12.1 5712M Plag 144.23 \u00C2\u00B1 1.08 145.6 \u00C2\u00B1 1.2 84.6% mAr release (Ar/mAr) 267\u00C2\u00B148 hbl-phyric hypebysssl rock MSWDO.91 MSWD = 0.72, n6 6 steps 149.0\u00C2\u00B11.4 571 2A1 Hbl 150.71 \u00C2\u00B1 0.53 149.86 \u00C2\u00B10.93 53.5% 35k release (\u00E2\u0080\u0098\u00C2\u00B0ArI%\u00E2\u0080\u0099 =359\u00C2\u00B123 hbl-phyric msflc intnjsive rock MSWD=0.56 MSWD = 1.3, n=9 6 steps 147.4\u00C2\u00B12.1 571 2A4 Hbl 152.51 * 0.63 148.80 \u00C2\u00B1 0.83 61.4% mAr release (4oArImAr) =378\u00C2\u00B196 hbl-ptlyric mafic intrusive rock MSWD=0.67 MSWD = 2.9, n=12 5715A1* Plag 131.21 \u00C2\u00B1 1.92 NP NI high-Ti msssiveflow 6 steps 189\u00C2\u00B129 571 9A5\u00E2\u0080\u0099 Plag 187.20\u00C2\u00B110.44 191 \u00C2\u00B1 11 82.4% mA! release (*oA,/mAF =296\u00C2\u00B121 high-Ti massive flow MSWD\u00E2\u0080\u00940.48 MSWD = 0.16, n=6 Alaska Range 6 steps 122.0\u00C2\u00B11.8 5725A5 Hbl 124.38 \u00C2\u00B10.69 123.13 \u00C2\u00B1 0.77 95.2% mAr release (4oArfmAr)1=334\u00C2\u00B118 hbl-phyric felsic intrusive rock MSWD1 .5 MSWD = 0.51. n=9 5 steps 26.3\u00C2\u00B12.2 U-Pb simon age 5729A1 Hbl 37.59 \u00C2\u00B1 2.41 29.7\u00C2\u00B1 1.1 82.8% mA release roAr/mArY =362\u00C2\u00B120 31.2\u00C2\u00B10.2 Ma, hbl-phyric felsic MSWD=1 .9 MSWD = 0.46, n=8 intrusive rock 7 steps 160.3\u00C2\u00B12.1 5802A6* Plag 161.86 \u00C2\u00B1 1.31 160.7\u00C2\u00B1 1.3 96.6% mAr release roAr/mAr) 299\u00C2\u00B116 low-Ti sill MSWD=1 .13 MSWD = 0.88, n7 5508A6 Hbl (1) 92.77 \u00C2\u00B1 0.88 NP NI low-Ti dike, Rainy Creak 5808A6 Hbl (2) 87.14 \u00C2\u00B1 0.6 NP NI low-Ti dike, Rainy Creek 5810A4* Rag 160.57 \u00C2\u00B1 8.64 137.3 \u00C2\u00B1 8.5 68.3% tmAr release 40Ar/\u00C2\u00B06r>=379C470 aitegrated age hipreferred, low-Ti MSWD=1.7 MSWD = 0.17, n=3 5 steps 166.6\u00C2\u00B13.5 5810W Plag 180.30 \u00C2\u00B1 3.65 169.0 \u00C2\u00B1 2.4 80.3% tmAx release (4Ar/mAr) =32l\u00C2\u00B121 low-Ti pillow basalt MSWD1.3 MSWD = 0.80. n=8 5810A10\u00E2\u0080\u0099 Plag 325.39\u00C2\u00B126.68 NP NI high-Ti pillow basalt \u00E2\u0080\u0098Wrangellia flood basalt or intrusive equivalent, based on field relations, petrography, and geochemistry.Sample number last digit year, month, day, initial, sample, except sample 05S1-75- bMineral separate and run # in parentheses, or (C) for combined result of multiple runs. Mineral abbreviations: hbl, homblende; plag, plagioclase?Bold italicized age is preferred age, error reported for \u00C2\u00B1 2o. NP, no plateau. dNumber of steps used for calculating age. Criteria for plateau age are described in the analytical methods in Appendix H.\u00C2\u00B0Inverse isochron (mArfAr vs. mAr/wAr) age, initial4oAr/mAr ratio, and MSWD. n is number of points included in isoctiron, MSWD refers to mean sum of the weighted deviates, which is a measure of the scatter compared to that which is expected from analytical uncertainties. NI, no isochron. See Supplementary data file 3 for petrographic snd geochemical data for all samples, and coordinates for sample locations. Supplementary data file 4 presents the complete analytical resultsfor40Ar/\u00C2\u00B0\u00C2\u00B0Ar analyses as an Excel workbook. 241 correspond with isochron ages. Two analyses of an amphibolite dike in the Rainy Creek area of the Amphitheater Mountains showed disturbed\u00E2\u0080\u009840Ar/39r systematics (Fig. 5.20). Among the 13 samples of Wrangellia flood basalt that were analyzed in this study, eight analyses satisfy the age spectra and isochron criteria to be geologically interpretable ages (see Appendix H). The plateau age of 227.5 \u00C2\u00B1 1.2 Ma from biotite in peridotite from Yukon is the only sample inferred to have retained a magmatic age corresponding to cooling of the ultramafic intrusion through the Tb of biotite (\u00E2\u0080\u0094350\u00C2\u00B0C), and is thus a minimum age of crystallization. The remaining seven ages from the basalts range from 191 to 73 Ma, indicating open-system behavior of the40Ar/Ar systematics (Table 5.3). The low closure temperature of plagioclase to 40Ar (<200\u00C2\u00B0C for Ar diffusion (Cohen, 2004)) resulted in these samples degassing 40Ar under lower greenschist facies metamorphic conditions and they record a reset age of metamorphism. Karmutsen basalts on Vancouver Island experienced prehnite-pumpellyite facies metamorphic conditions (1.7 kbar; \u00E2\u0080\u0094300\u00C2\u00B0C) and higher metamorphic gradients where proximal to granitoid intrusions increases (Cho & Liou, 1987). The mineralogy ofNikolai basalts in Alaska indicates a similar degree of metamorphism. Despite modification of the original trapped Ar, the seven reset plagioclase 40Ar/39r ages are geologically meaningful and provide important information about the geologic history of the Wrangellia Terrane. Briefly, three of the reset40Ar/39rages of Karmutsen basalts from Vancouver Island (191, 181, and 161 Ma) are within the age range of Bonanza arc intrusions and volcanic sequences (197-167 Ma), that intrude and overlie the Karmtusen basalts on Vancouver Island (Table 5.3; Fig. 5.14; Nixon & Orr, 2007). The three reset ages of Nikolai basalts from the Amphitheater Mountains (169, 161, 161 Ma) are similar to ages of felsic plutonic rocks in close proximity (<30 km) to the south (168-150 Ma). Schmidt and co-workers (2003a; written comm., 2006) found that reset plagioclase40Ar/39rages (174-152 Ma) for eight Nikolai basalts and gabbroic rocks in the Amphitheater and Talkeetna Mountains are similar to ages for these felsic plutonic rocks assigned to the Peninsular Terrane. Seven whole rock K-Ar ages of Nikolai basalts in the Wrangell Mountains yielded an isochron age of 112 \u00C2\u00B1 11 Ma and indicated resetting of K-Ar systematics during tectonism related to northward transport of Wrangellia (MacKevett, 1978; Plaficer et a!., 1 989b). The homblende40Ar/39rages are 242 coincident with regional magmatic events reported from other studies in the areas in which these samples were collected. Two Late Jurassic ages of 148.8 \u00C2\u00B1 0.83 and 149.86\u00C2\u00B1 0.93 Ma from mafic dikes in the Wrangell Mountains correspond with ages of Late Jurassic plutons of the Chitina arc (140-160 Ma, with most ages between 145-150 Ma), which are synchronous with a major regional orogeny related to subduction (Grantz et al., 1966; MacKevett, 1978; Hudson, 1983; Dodds & Campbell, 1988; Plaficer et al., 1989b; Roeske et al., 2003). Summary of isotopic age determinations for Wrangellia flood basalts The ages of Wrangellia flood basalts and intrusives with errors <10 Myr (n=9) range from 225.7 \u00C2\u00B1 2 to 232.2 \u00C2\u00B1 1 Ma (Fig. 5.21; Tables 5.2 and 5.3). The three U-Pb ages from mafic sills from Vancouver Island are within error of each of the four40Ar/39r ages from Alaska, as well as the4\u00C2\u00B0ArI39biotite plateau age of a peridotite of 227.5 \u00C2\u00B1 1.2 Ma from this study. The slightly older age of 232.2 \u00C2\u00B1 1 Ma from Yukon (average 207Pb/6 age based on results from 3 discordant fractions; Mortensen and Hulbert (1991)) is the only age which falls outside the range for the Vancouver Island samples. This gabbroic sill in Yukon may represent an earlier phase of magmatism. The relatively narrow range of ages for Wrangellia flood basalts and associated intrusive rocks indicates that the duration of the majority of the volcanism was likely less than 5 Myr, occurring between ca. 230 and 225 Ma. Paleontological studies Fossils in sedimentary strata directly underlying and overlying the Wrangellia flood basalts also provide constraints on the age and duration of volcanism. In 1964, MacKevett and co-workers (1964) reported that shale directly beneath Nikolai basalts on Golden Horn Peak in the Wrangell Mountains contained abundant Middle Triassic index fossils identified as Daonellaframi kittl. Tn 1971, a mineral exploration party discovered a sedimentary layer (<2 m thick) of fissile black shale between sills of Karmutsen basalts on Mount Schoen on Vancouver Island that contained imprints ofDaonella tyrolensis (Carlisle, 1972). In this study, samples ofDaonella were collected from Golden Horn Peak in Alaska and on Mount Schoen on Vancouver Island. The Daonella from both 243 \u00E2\u0080\u00A2 III \u00E2\u0080\u00A2 . I I Ar-Ar \u00E2\u0080\u00A2 Alaska Range I I I Ar-Ar Wrangell Mountains I Ar-Ar Q Yukon I I Vancouver Island I I Ar-Ar I \u00E2\u0080\u0094D\u00E2\u0080\u0094i U-Pb hD\u00E2\u0080\u0094H Ar-Ar I I I I U-PbI \u00C3\u0098\u00E2\u0080\u0094 lu-Pb I IOI U-Pb I Norian Carnian I (Furin et aL, 2006) Norian Camian Ladinian Anisian (Ogg, 2004) I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I 210 220 230 240 250 Age (Ma) Figure 5.21 Summary diagram showing40Ar/39r and U/Pb ages of Wrangellia flood basalts and plutonic rocks with analytical uncertainty <10 Myr. The results are shown from north (top) to south (bottom) and are distinguished by region. Analytical details are presented in Tables 5.2 and 5.3. Two schemes are shown for the boundaries of part of the Triassic, from Furin eta!. (2006) and Ogg (2004). Errors are 2a. 244 localities appear to have similar forms and are closely related to Daonellaframi kittl (C. A. McRoberts, 2006, pers. comm.). The Daonella appear older than Upper Ladinian forms and are likely of middle Ladinian age (Poseidon Zone, ca. 235-232 Ma; C. A. McRoberts, 2006, pers. comm.). Conodont Neospathodus in the Daonella beds from Vancouver Island confirms this age. DISCUSSION Overview of the geology and age of Northern and Southern Wrangeffia The stratigraphy and age of different areas of the Wrangellia oceanic plateau provide constraints on the construction of the volcanic stratigraphy, the paleoenvironments existing at the time of eruption, and the duration of volcanism. A summary of observed and previously reported field relationships of Wrangellia flood basalts, and the pre- and post-volcanic rock record, is presented in Table 5.4. A compilation of ages and biostratigraphy for Paleozoic through Triassic rocks of Wrangellia is presented in Figure 5.22 and Supplementary data file 5. The basement of Wrangellia has different age strata in Alaska and Yukon than on Vancouver Island (Fig. 5.22). The basement of Wrangellia was originally defined as a Pennsylvanian to Permian volcanic arc sequence that may have been deposited on oceanic crust (Jones et aL, 1977) and this is maintained by numerous authors (Smith & MacKevett, 1970; MacKevett, 1978; Coney et al., 1980; Monger et al., 1982; Saleeby, 1983; Beard & Barker, 1989). However, while Pennsylvanian to Permian volcanic arcs are preserved in Alaska (Smith & MacKevett, 1970; MacKevett, 1978; Beard & Barker, 1989), older Paleozoic rocks make up much of Vancouver Island (Muller, 1977; Brandon et al., 1986), and there is no arc volcanism in the Pennsylvanian to Permian of Vancouver Island (Fig. 5.22; Massey & Friday, 1988; Yorath et a!., 1999). The Paleozoic volcanic sequences on Vancouver Island are considerably older (380-355 Ma) than presently- dated volcanic sequences in Alaska (\u00E2\u0080\u00943 12-280 Ma, mostly from the Wrangell Mountains; Fig. 5.22). Paleozoic limestones beneath Wrangellia basalts in Alaska contain Early Permian bryozoans, brachiopods, foraminifera, and corals (Smith & MacKevett, 1970). On Vancouver Island, conodonts in the Buttle Lake Group indicate Mississippian to 245 Table 5.4 Cornoadson of geology and ages of Northern and Southern IMangeflla Ij,,.44,.., W..IU dastic sedimentary sequences above Nikolal span Triassic-Jurassic boundary Late Carnian to Early Norian fossils In overlyIng limestone overlying limestone grades spward from shallow wster to deeper water fades Nikolai basalta interbedded with limestone end argillite (Yukon) limestone is commonly breccisted nesr the bsse thin layer of siltatone in several locationa along Nikolai-Chitistone contact occurrences of regolith between the top of the Nikolai and the base of the Chitintone Nikolai Formation 4 U-Pb and Ar-k ages (error <10 Myr) of 225-230 Ma (with one Yukon outlier) Late Carnian to Early Norian fossils In limestone lenses in Yukon trachytic-textured plag-rich flown near top of Nikolai in Wrangell MIne ram dikes predominantly massive subaerial flows Alaeka Range (Amphitheater and Clearwater Mountains) \u00E2\u0080\u00947% of lowest part of utratigraphy Is pillow basalt (<500 m), and \u00E2\u0080\u00943000 m subaerial flown pillows tend to have mostly small diameter (<1 m) pillows have abundant vesidea (2040 <01%) indicaling eruption in shallow water (<800 m) vesicles in pillows am spherical (<1 mm) and evenly dislributed throughout pillows megapillowa ram in Alaska minor amounts of breccie and luff within submarine section interbedded limeatonelclastic near top of Nikolai In Clearwater MIna and in Yukon thick sediment-sill complex large area of complementary plutonic rocks represent feeder system for flood basalta no sheeted dike complexes, sill-dominated fender system black shale within sediment-sill complex, non-fossiliferous (starved, anoxic environment) picrltic pillow lava near base of submarine section In Clearwater Mtns pillow basalt engulfs fine-grained sediment In lowermost pitlowed flows picritic tuff found outside the main ama of flood basalts, but likely mIsted to Nikolai basalts Wrangell Mountains and Kluane Ranges almost no pillow basalt in basalt slratigmphy (only in lowermost \u00E2\u0080\u009470 m) pillow breccia and polymictic flow-conglomerate in lowermost \u00E2\u0080\u009470 m basal flow-congolmemte sits dimctty on argillite wfih Senile Daonella beds abundant rounded dasts derived from underlying Paleozoic formations larger pendanta (>100 m long ) of Pateozoic rocks In lowermost flown occurrences of regolith between top of Nikolai end overlying Chitistone limestone mm thin arllite (<10 cm) on top of uppermost flow intraflow limestone lenses (1-30 m tick) near top formed in shallow submarine areas plagioclase megacryatic flown near the top of the basalt succession -3500 m of subaerial street flows; columnar jointing is mm very ram occurrences of inlerfiow sediments, other than tenses in upper part quarts-pebble conglomerate in sediments beneath Nikdal In Talkeetna Mtna Paleozdc arc sequences not exposed in Talkeetna Mtes beneath Nikolai clastic sedimentary sequences above Karmutsen span Triasnic-Jurasnic borardary Late Camian to Early Nodan fossils In overlying limestone overlyIng limestone grades upward from shallow water to deeper water fades limestone and sedimentary rocks of the Kunga and Maude Groups on Haida Gwaii Kunga Formation contains identical fossils to Quatsino limestone Karmutsen basalt, or younger basalt, locally intrudes the Quatsino limestone moally micritic limestone directly overlying the basalts thIn (<25cm) layer of ailtetone immediately overlying basalt in several locallonu no evidence of erosion between Karmutsen and Quatsino limestone Karmutaen Fonnallon 3 U-Pb ages (error <10 Myr) of 226-228 Ma regolith above uppemront flow thin (<25 cm) ailtstone layem between top of Knrmtuaen and Quateino Intraflow llmestone/claalic lenses near top formed In shallow submarine areas Late Camian to Early Nurian fossils In lenses within uppermost Karmutsen pingioclane megecryatic flows near the top of the basalt succession Vancouver Island flood basaltn form emergent basalt sequence subaerial stage-subaerial flown (>2500 m on CVI, <1500 mon NVI) shallower-water stage-pillow bmccialhyaloclasllte (40-1500 mon NVI) deeper-water stage-pillowed nnd unpillowed basalt flown (>2500 m) pillow banalts erupted In deeper marine netting than in Northem Wrangellia most pillows am commonly large diameter (>1 m) pillows are more commonly non-vesIcular or have mdlally-oriented pipe vesides megapillowu (54 m) within pillowed flown on VI volcaniclastic rocks formed primarily via cooling-contraction granulation volcanlclasllcs mostly between pillow basalt and subaerial flow units pepertten In submarine section subaerial flow unit is conformable with the underlying hyalocinstite unit interfiow sedimentary lenses are found near the top of the subaerial flow unrl interfiow lenses contain Late Camian to Early Norian fossil rare occurmnces of Interfiow sediments , besides lenses between upper flows columnar jointing is mm mm dikes, no sheeted dike complexes thick sediment-sill complenes where bane is exposed on VI incmase in vesicularity end proportion of volcaniclastics upwards in submarine unit pshoehoe utructsms (e.g. ropy festoons), within flown and )ust below Quatsino pidritc pillow leves in upper pail of the nubmsrtne section on Norttiem VI marine fossils in pillow brecda, Butte Lake area pillow beset engulfs fne-grained sediment in lowermost p1 bowed flows submarine besets fonn volumetrically minor component of the subaerial utratigraphy no indication of upilft prior to eruption on VI Queen Charlotte Islands (Halda GwaN) one measured section (-4300 m) 95% submarine, pillow to fragmental ratio (82) local tsfteceous crindolat limestone lenses in lowermost flows Permien chert, carbonste, end volcanldantic rocks form deepest level of exposure Rocks undertvlng the Nlkolsl Fonnallon Wmngell Mountains and Kluane Ranges e\u00E2\u0080\u0099eidence of erosional surface beneath Nikolai, rounded dusts from underlying sequences rounded pebble- lo cobble-size dusts ledicats subaerial or shallow submarine reworldng Indication of upllt prior to eruption In Wrangetl Mountains c.g. mafic rocks intrude and deform underlying sedimentary sequences Decnefla imprints in fissle ntrale underiytirg lowermost pillow breccis erosional unconformity between Deonelfa beds end underlying Puleozcic limestone formation of laterally discontinuous gmbenn along base in Kluene Ranges Alaska Range (Amphitheater and Clearwater MountaIns) Pre-Nikotei sedmests grade upward from siliceous argililte to carbonaceous black shale Pre-Nikolsi black shale deposited in starved, anosic mertire setting no bioturbation (parallel laminations) end no bingenic structures (truce fossils) coerse-grained mafic rocks intrude end deform undentying sedimentary sequences limestone (with Early Permian fossils), chert, srgitlts underlying Nikolai basattn Peleozolc arc sequences underlying Nikolai ages 312-280 Ma no rocks dated older than Pennsylvanian in Wmngellia in Alaska mferences not induded in this table, see lest for references VI, Vancouver Inland; CVI, Central Vancouver Island; NVI, Northem Vancouver Island Rocks underlying the Karmutsen Formation Dsonells imprints in shale within sediment-sill complex on Mount Sclroen, VI Daonelta imply dysoxic bottom waters typical of mud-dwalleru on uoupy sediments coame-gralned mefic rocks intrude aed deform undertying sedimentary sequences mefic sills intrude siliceous argiflite, shale, died, and limestone Paleozdc sequences mostly exposed rn two antidinorte on central and southern Vt Conodonts indicate Mississippian to Permian ages hr the Butte Lake Group underlying Karmutsen flood basails Peleozoic arc sequences underiying Kermutsen ages 380455 Ms oldest known rockn on Vancouver Inland am the Devonian Duck Luke Formation 246 Le ge nd E Z 3 tu ff be d 70 sh al e, ar gi lli te flo od ba aa lta fl: : a rc v o lc an ic a [1 de ep er -w at er \u00C2\u00B0 j.. .. 2g .. J L... ...... ...J lim es to ne 30 0 ch er t-a ha le - sh al lo w -w at er \u00C2\u00B0 \u00E2\u0080\u0098 lim es to ne lim es to ne \u00C2\u00B0 \u00C2\u00B0 a rc v o lc an ic a [{] a0 5 v a Fi gu re 5. 22 Su m m ar y o fa ge sa n d bi os tra tig ra ph y fo rW ra ng el lia ,d iv id ed in to 5 ar ea s. R ad io rn et ric ag es ar e w hi te ci rc le s. Fo ss il ag es ar e co lo re d ac co rd in g to fo nn at io n in st ra tig ra ph ic co lu m n, u n le ss co lo re d gr ay .I nf or m at io n fo r a ge da ta an d bi os tra tig ra ph y ar e pr es en te d in Su pp le m en ta ry da ta fil e 5. A ge pr ob ab ili ty de ns ity di str ib ut io n pl ot s fo re ac h ar ea ar e ca lc ul at ed fro m th e pl ot te d ag es u sin g A ge D isp la y (S irc om be ,2 00 4). Th e ag es fo rt he pe rio d bo un da rie s ar e fro m G ra ds te in et a!. (20 04 ). A ge s fo re po ch bo un da rie so ft he Tr ia ss ic ar e ad jus ted u sin g Fu rin et aL (20 06 ). \u00E2\u0080\u0094 \u00E2\u0080\u0094 W ran ge ll M tns . 20 9. 9 \u00C2\u00B1 0. 07 t. )T rop ,p ar s. co m m .) \u00E2\u0080\u0094 \u00E2\u0080\u0094 K lu an e R an ge s H ; \u00E2\u0080\u0098 te o n e ll a D ao ne lla \ A m m on ite N or th er n V an co uv er Is la nd 22 0 I ra di om et dc a ge bi oa tra tig m ph ic a ge ra n ge Permian ages (Orchardfide Brandon et a!., 1986; Orchard, 1986; Fig. 5.22; Supplementary data file 5). The volcanic stratigraphy on Vancouver Island consists of a tripartite succession of submarine flows (50-60%), volcaniclastics, and subaerial flows, whereas volcanic stratigraphy in Alaska and Yukon is predominantly subaerial flows (>90%; Table 5.4; Fig. 5.22). In the Alaska Range, the lowest \u00E2\u0080\u0094\u00E2\u0080\u0098500 m of stratigraphy is submarine flows that were emplaced on non-fossiliferous black shale. In the Wrangell Mountains, the basal flow-conglomerate lies directly on shale with Daonella beds and contains erosional remnants from the underlying Paleozoic units. Laterally discontinuous zones of basal conglomerate along the base of the Nikolai in Yukon are interpreted as syntectonic deposits and flows related to the formation of grabens. On Vancouver Island, the lowest pillow basalts were emplaced on unconsolidated fine-grained sediments and mafic sills intrude marine strata with Daonella beds. However, the submarine section on Vancouver Island (\u00E2\u0080\u00943000 m) is substantially thicker than in the Alaska Range (\u00E2\u0080\u0098-SOO m). The overlying limestone and interfiow sedimentary lenses in Northern and Southern Wrangellia are lithologically similar and have a similar range of fossil ages (Table 5.4; Fig. 5.22). In Southern Wrangellia, interfiow sedimentary lenses are common in upper parts of the Karmutsen Formation and the overlying limestone contains Late Carnian to Norian fossils. Within upper Nikolai stratigraphy in Northern Wrangellia, interfiow sedimentary lenses occur in southwest Yukon and the Clearwater Mountains and Nikolai basalts are overlain by limestone with age-diagnostic Late Carnian to Early Norian fossils. Sedimentary strata extend up through the Triassic-Jurassic boundary in Northern and Southern Wrangellia. Eruption environment for Wrangeffia flood basalts Northern Wrangellia The Nikolai basalts in Alaska were emplaced as quiet effusive eruptions in a shallow marine and subaerial environment. Sediments directly beneath the flood basalts in the Alaska Range are mostly siliceous argillites, carbonaceous black shales, and mudstones, with the upper part of the 200-250 m sequence having a higher proportion of black carbonaceous shale and carbonate (Blodgett, 2002). In this area, Blodgett (2002) 248 interpreted the total absence of fossils and biogenic sedimentary structures (trace fossils), and even, parallel laminations (indicating lack of bioturbation), as indicative of deposition in a starved, anoxic shallow submarine environment. The higher proportion of black carbonaceous shale and calcareous component for sediments higher in the sequence may indicate a shallower depositional environment for the younger sediments than for the older sediments, possibly due to uplift prior to eruption (Blodgett, 2002). The pillow basalts in the Alaska Range (500 m) are highly vesicular, consistent with eruption in shallow water (Jones, 1969; Kokelaar, 1986). Volcaniclastic flows intercalated with pillow basalt also indicate eruption in shallow water; this transition usually occurs in <200 m water depth for tholeiitic magmas (Kokelaar, 1986). The subaerial flows (>3000 m) were emplaced as inflated compound pahoehoe flow fields during prolonged, episodic eruptions similar to those in most continental flood basalts (Self et al., 1997). In the Wrangell Mountains and Yukon, the basal flow-conglomerate and pillow breccia (<100 m thick) erupted in shallow water (<200 m). Rounded clasts in the basal flow-conglomerate indicate an area of relief near sea-level in the Wrangell Mountains. Above the basal flow unit in the Wrangell Mountains is 3500 m of subaerial sheet flows that lack features of submarine emplacement. The proportion of amygdules in the massive flows is variable, but generally high, similar to many continental flood basalts. Submarine sheet flows exposed in accreted portions of the Ontong Java Plateau in the Solomon Islands rarely have significant proportions of amygdules and pillowed and unpillowed flows are preserved throughout the stratigraphy (Petterson, 2004). Individual flow structures are indicative of emplacement by the inflation of effusive subaerial flows. Southern Wrangellia The tripartite Karmutsen stratigraphy on Vancouver Island formed as an emergent basalt sequence during a deeper-water, shallow-water, and subaerial stage, similar to those described in formation of emergent seamounts (Schmidt & Schmicke, 2000) and Hawaiian volcanoes (Garcia et al., 2007). Sills obscure relationships at the sediment basalt interface at the base of the Karinutsen, and there may have been sediments on top that are no longer preserved, but most of the fme-grained strata underlying the Karmutsen were deposited below storm-wave base. Carlisle (1972) reported that pre-Karmutsen 249 sediments show a progressive change from coarse bioclastic limestone to laminated and silicified shale, indicating transition from an organically rich, shallow-water environment to a starved, pelagic deeper-water depositional environment prior to initiation of volcanism. Daonella in fme black shale from near the top of this unit imply dysoxic bottom waters typical of mud-dwellers that float on soupy sediments (Schatz, 2005). The deeper-water stage (>200 m water depth) of the Karmutsen was dominated by quiescent effusive activity that formed pillowed and unpillowed flows. The pillowed flows are interconnected tubes and lobes that contain large-diameter pillows (>2 m) and have low abundances of amygdules. The unpillowed flows may be some of the master tubes for delivery to distal parts of flow fields or locally increased effusive rates, due to topography, as evinced by concave basal contacts. The basalts increase in vesicularity as does the proportion of volcaniclastics upwards in the submarine stratigraphy (Nixon et a!., 2008). The shallow-water stage of the Karmutsen preserves an increasing proportion of volcaniclastic units (pillow breccia and hyaloclastite) conformably overlying mostly close-packed pillows. The pillow breccias are commonly associated with pillowed flows and contain aquagene tuff (Carlisle, 1963), or redeposited hyaloclastite. The transition from close-packed pillowed flows to pillow breccia and hyaloclastite probably occurred in <500 m water depth; however, in certain areas on northern Vancouver Island the volcaniclastic unit is >1500 m thick (Nixon eta!., 2008). Sedimentary structures (e.g. graded bedding, fluidization structures) are present locally and indicate resedimentation processes. Pyroclastic deposits containing lapilli tuff and volcanic bombs do not appear to be common (Carlisle, 1963). The volcaniclastic rocks likely formed primarily via cooling-contraction granulation, magma-water-steam interaction, autobrecciation, and mass-wasting, rather than pyroclastic eruption. The emergent subaerial stage is marked by the relative absence of volcaniclastic and pillowed flow units and dominance of massive amygdaloidal sheet flows. The sheet flows were emplaced as inflated compound pahoehoe flow fields atop an enormous oceanic plateau. There are isolated sections of submarine flood basalts (<200 m thick) within the uppermost subaerial Karmutsen stratigraphy (Surdam, 1967; Carlisle and Suzuki, 1974); however, these units form a volumetrically minor component of the 250 subaerial stratigraphy. The intra-Karmtusen sedimentary lenses formed in isolated, low- lying areas in a predominantly subaerially-exposed plateau (Carlisle and Suzuki, 1974). There are similarities between stratigraphy of Karmutsen basalts and stratigraphy of Hawaii Scientific Drilling Project core (HSDP2; Garcia et al., 2007). The stratigraphies in both the Karmutsen basalts and HSDP2 are predominantly pillowed flows in the lower parts of the submarine stratigraphy and give way to increasing proportions of volcaniclastic units upsection, below the submarine-subaerial transition. Vesiculated pillows of the Karmutsen Formation on Northern Vancouver Island are more common near the top of the pillow unit and in the hyaloclastite unit. HSDP2 drill core shows an increase in vesicularity with decreasing depth in the submarine lava flows (Garcia et al., 2007). Intrusives more commonly intrude lower parts of the stratigraphy within both submarine sections and, although more difficult to identify, they appear to be less common within the subaerial sections. The accumulation and subsidence of the Wrangeffia basalts The geology, age and biostratigraphy of Wrangellia can be used to estimate the rate of accumulation of Wrangellia basalts and the subsidence of the Wrangellia oceanic plateau. Carlisle and Suzuki (1974) originally estimated an accumulation rate for the Karmutsen basalts of 0.17-0.27 cm/yr over 2.5-3.5 Myr. This yielded a total erupted volume ofbasalt of 3.7-4.0 x l0 km3 (they assumed an area 400 km by 150 km, roughly the size of Vancouver Island, or 60,000 km2, and a stratigraphic thickness of 6 km) and a volumetric output rate of 0.10-0.16 km3/yr (Carlisle & Suzuki, 1974). In this study, an estimate of volcanic output rate (0.03 km/yr) and total erupted volume of Karmutsen basalts (1.4-1.5 x 1 km3)are lower than the estimates of Carlisle and Suzuki (1974). The area of exposure of Karmutsen basalts in this study was calculated using digital geology maps for Vancouver Island and the Queen Charlotte Islands (shown in Figure 5.11), and thus represents a minimum estimate. The estimated total erupted volume and volumetric output rate were calculated using a stratigraphic thickness of 6 km and a duration of volcanism of 5 Myr. Even using this conservative estimate for the area of exposure, the volumetric output rate is comparable to recent estimates of long-term 251 volumetric eruption rates for ocean islands such as Iceland (0.02-0.04 km3/yr) and Hawaii (0.02-0.08 km3/yr) (White et aL, 2006). The subsidence of the Karmutsen basalts during volcanism was recorded by the deposition of interfiow sedimentary lenses between the upper flows during the waning stages of volcanism as low-lying areas of the plateau were submerged. The occurrence of interfiow lenses indicates that by the end of Karmutsen flood volcanism (5 Myr in duration) most of the top of the basalt plateau had subsided and submerged below sea- level. This implies that over the duration of subaerial volcanism, the rate of accumulation of basalt flows was comparable to the rate of subsidence. Carbonate deposition was occurring during the waning stages of volcanism, and there was no significant break after volcanism ceased. There are few signs of erosion between the Quatsino limestone and Karmutsen Formation (in places only a <25 cm thick siltstone/sandstone layer is preserved), and interfiow lenses have identical fossils to the lower part of the Quatsino limestone. Post-volcanic subsidence of the Wrangellia oceanic plateau is recorded by hundreds of meters to >1000 m of Late Triassic marine sedimentary rocks overlying the basalt stratigraphy. The Quatsino limestone was deposited on top of the plateau as it began to submerge beneath sea-level, and while volcanism waned. Deposition continued as the plateau became fhlly submerged and the sea transgressed over the entire plateau. Initially, intertidal to supratidal limestones were deposited in shallow-water, high-energy areas with some of the limestones reflecting quieter, subtidal conditions (Carlisle and Suzuki, 1974). Upsection, the Quatsino Formation reflects a slightly deeper-water depositional environment and abruptly grades into the overlying Parson Bay Formation (Carlisle & Suzuki, 1974; Nixon & Orr, 2007). In Alaska and Yukon, the carbonates overlying the Nikolai basalts (>1000 m; Chitistone and Nizina limestones) preserve a record of the gradual submergence of the extensive Nikolai basalt platform. There are only rare occurrences of thin (<0.5 m) weathered zones or discontinuous, intervening clastic deposits at the top of -3500 m of subaerial basalt flows in the Wrangell Mountains (Fig. 5.7; Armstrong et al., 1969). The absence of significant erosion or deposition of clastic sediment, and the age of the Chitistone Limestone (Fig. 5.22), indicates only a brief interval of non-deposition 252 between the end of volcanism and carbonate deposition. Following volcanism, several cycles of shaley to argillaceous limestone were deposited in a high-energy, intertidal to supratidal (sahbka) environment, similar to the modem Persian Gulf, and form the lowest 100 m of the Chitistone (Armstrong et al., 1969). Limey mudstone and wackestone with abundant disintegrated shelly material (\u00E2\u0080\u0098\u00E2\u0080\u0094\u00E2\u0080\u0098300 mthick) indicate gradual transition to low- energy shallow-water deposition with intermittent high-energy shoaling deposition. The upper part of the Chitistone and overlying Nizina Limestone reflect deeper-water deposition on a drowned carbonate platform (Armstrong et a!. 1969). Gray to black shale and chert of the overlying McCarthy Formation represent submergence of the carbonate platform below the carbonate compensation depth (CCD; Armstrong et a!. 1969). A thin tuff bed in the lower part of the McCarthy Formation has been dated at 209.9 \u00C2\u00B1 0.07 Ma (J. Trop, 2006, pers. comm.) and the Triassic-Jurassic boundary is preserved in the upper McCarthy Formation (Fig. 5.22). Between the end of basaltic volcanism and the end of the Triassic (\u00E2\u0080\u009425 Myr), approximately 2000 m of shallow- to deep-water marine sediments accumulated on top of the Nikolai basalts. Neglecting sediment compaction, this indicates a minimum subsidence rate of \u00E2\u0080\u009480 mlMyr. This subsidence decreased substantially, to less than 20 m/Myr, in the Early Jurassic (Saltus et a!., 2007). Several oceanic plateaus and ocean islands worldwide preserve evidence of rapid subsidence after their formation (Detrick et a!., 1977) and mantle plume models predict subsidence following the formation of oceanic plateaus (e.g. Campbell & Griffiths, 1990). Subsidence may result from: dispersion of the mantle buoyancy anomaly above a plume; decay of the thermal anomaly and cooling and contraction of the lithosphere; removal of magma from the plume source, causing deflation of the plume head; and/or depression of the surface from loading of volcanic and plutonic material on the lithosphere (e.g. Detrick et a!., 1977; Campbell & Griffiths, 1990). Hawaiian volcanoes undergo rapid subsidence during their growth due to the response from loading of intrusive and extrusive magmas on the lithosphere (Moore & Clague, 1992). In Hawaii, subsidence, rather than rising sea level, accounts for the submergence of the volcanoes (Clague & Dafrymphe, 1987). Pacific Cretaceous plateaus, such as Ontong Java, Manihiki and Shatsky, underwent significantly less subsidence than predicted by current models, with post-flood basalt subsidence comparable to that of normal ocean seafloor 253 (Ito & Clift, 1998; Roberge et aL, 2005). Drilling of the Kerguelen Plateau indicates subaerial flows, perhaps originally 1-2 km above sea level, subsided below sea level and paleoenvironments of the overlying sediments changed from intertidal to pelagic (Coffin, 1992; Frey et al., 2000; Wallace, 2002). Subsidence estimates for the Northern Kerguelen Plateau (Site 1140) indicate 1700 m of subsidence occurred since eruption of the basalts at 34 Ma (\u00E2\u0080\u0094\u00E2\u0080\u009850 mlMyr; Wallace, 2002). The subsidence history of Wrangellia may be more fully reconstructed by future studies that estimate the depth of sedimentation and age from microfossil data collected from sediments overlying the Karmutsen and Nikolai Formations. CONCLUSION The volcanic stratigraphy of the obducted Wrangellia flood basalts records the construction of a major oceanic plateau. The Wrangellia basalts cover \u00E2\u0080\u009427,000 km2 along the western edge ofNorth America and are --\u00E2\u0080\u00983.5 km thick in Alaska and \u00E2\u0080\u0094\u00E2\u0080\u00986 km thick on Vancouver Island. The Wrangellia basalts in Alaska and Yukon are bounded by Middle to Late Triassic marine sediments and unconformably overlie Pennsylvanian and Permian marine sediments and volcanic arc sequences (ca. 312-280 Ma). The earliest flows were emplaced in a shallow marine environment, but the main phase of volcanism consisted of compound pahoehoe flow fields that form a tabular, shingled architecture. Grabens formed along the base of Wrangellia basalts in Yukon. The flows erupted from a limited number of eruption sites that are rarely observed, except for a large eruptive center in the Amphitheater Mountains. Wrangellia basalts on Vancouver Island are bounded by Middle to Late Triassic marine sediments and unconformably overlie a basement of Devonian to Mississippian arc rocks and Mississippian to Permian marine sedimentary strata. Early growth of the volcanic stratigraphy on Vancouver Island was dominated by extrusion of pillow lavas and intrusion of sills into sedimentary strata. The plateau grew from the ocean floor and accumulated >3000 m of submarine flows, which were overlain by 400-1500 m of hyaloclastite and minor pillow basalt before the plateau breached sea level. The hyaloclastite formed primarily by quench fragmentation of effusive flows under low hydrostatic pressure. The plateau then grew above sea level as more than 1500 m of 254 subaerial flows were emplaced. The plateau subsided during its construction and intervolcanic sedimentary lenses formed in shallow water in local areas as eruptions waned. After volcanism ceased, the plateau continued to subside for more than 25 Myr and was overlain by hundreds to >1000 m of limestone and siliciclastic deposits. The absence of intervening sediments in the volcanic stratigraphy and magnetic reversals preserved in the basalts attests to the brief duration of volcanism. Middle Ladinian Daonella just beneath the basalts in Alaska and Vancouver Island, and conodonts in limestones overlying the basalts, restrict volcanism to a maximum duration of less than -10 Myr. The duration of volcanism in Northern and Southern Wrangellia was probably less than 5 Myr, and the ages of eruptions appear to overlap between ca. 230 and 225 Ma. ACKNOWLEDGEMENTS This manuscript was influenced from discussions with Travis Hudson and material that he provided. We are grateful to Jeff Trop for his advice. Jeanine Schmidt and Peter Bittenbender were very helpful with data and ideas about Alaskan geology. Don Carlisle thoughtfully provided maps and notes that helped with fieldwork on Vancouver Island. Andrew Caruthers and Chris Ruttan helped with fieldwork on Vancouver Island. David Brew was very helpful with information about southeast Alaska. Funding was provided by research grants from the BC Geological Survey and Yukon Geological Survey, the Rocks to Riches Program administered by the BC & Yukon Chamber of Mines, and NSERC Discovery Grants to James Scoates and Dominique Weis. A. Greene was supported by a University Graduate Fellowship at UBC. REFERENCES Armstrong, A. K. & MacKevett, E. M., Jr. (1977). The Triassic Chitistone Limestone, Wrangell Mountains, Alaska. U S. Geological Survey. Open-File Report 77-2 17, D49-D62 p. Armstrong, A. K., MacKevett, E. M., Jr. & Silberling, N. J. (1969). The Chitistone and Nizina limestones of part of the southern Wrangell Mountains-a preliminary report stressing carbonate petrography and depositional environments. U S. Geological Survey. 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Tectonic Growth ofa Collisional 264 Continental Margin: Crustal Evolution ofSouthern Alaska. Geological Society of America Special Paper 431, pp. 55-94. Umhoefer, P. J. & Blakey, R. C. (2006). Moderate (1600 1cm) northward translation of Baja British Columbia from southern California: An attempt at reconcilation of paleomagnetism and geology. In: Haggart, J. W., Enkin, R. J. & Monger, J. W. H. (eds.) Paleogeography ofthe North American Cordillera: Evidence For and Against Large-Scale Displacements. Geological Association of Canada. Special Paper 46, pp. 307-329. Wallace, P. J. (2002). Volatiles in submarine basaltic glasses from the Northern Kerguelen Plateau (ODP Site 1140): implications for source region compositions, magmatic proesses, and plateau subsidence. Journal ofPetrology 43, 1311-1326. Wheeler, J. 0. & McFeely, P. (1991). Tectonic assemblage map of the Canadian Cordillera and adjacent part of the United States of America. Geological Survey ofCanada Map l712A. White, S. 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LITHOPROBE, southern Vancouver Island, British Columbia Bulletin 498, 145 p. 265 991 SUOISflPUOD 9LLLJVI1 CONCLUSIONS AND DIRECTIONS FOR FUTURE RESEARCH There have been considerable gains made in understanding of the origin and growth of the Wrangellia oceanic plateau as a result of this project, yet a great deal continues to remain elusive and is ideally suited for prospective research. The obductecl parts of an accreted oceanic plateau offer the best way to observe and sample the volcanic stratigraphy of oceanic plateaus, in a similar way that ophiolites provide a way to examine rocks that form in mid-ocean ridges. The development of a complete understanding of the architecture, source and evolution of basaltic magmas, and timescale of formation of oceanic plateaus is essential for furthering our understanding of the origin and evolution of oceanic plateaus. The ideas presented in the four main chapters in this dissertation represent some of the more recent advances in understanding the formation of the Wrangellia oceanic plateau. The contributions in this study include: (1) extensive reconnaissance fieldwork in BC, Yukon, and Alaska; (2) geologic map compilations of the entire extent of Wrangellia flood basalts; (3) photographic database (>1000 photographs); (4) reference database on Wrangellia (>500 references); (5) archived samples of the volcanic stratigraphy (>300 samples and thin-sections); (6) major- and trace-element analyses of 175 samples; (7) Sr Nd-Hf-Pb isotopic analyses of 75 samples; (8) assessment of alteration of the basalts; (9) characterization of the composition of the source of Wrangellia basalts; (10) major- element modeling of melting conditions, primary melt compositions, and magmatic evolution; (11) trace-element dynamic melting modeling; (12) flood basalt chemostratigraphy in Alaska and studies of plume-lithosphere interaction; (13) detailed description and photographic illustration of volcanic stratigraphy and relationships with underlying and overlying units throughout Wrangellia; (14) Google Earth archiving of sample locations, mapped flood basalts, and field photographs. (15) Ar-Ar geochronology of 20 samples from throughout Wrangellia; (16) sampling of fossils and paleontological age determinations of bivalves in sediments beneath the flood basalts; (17) comprehensive summary of previous research on Wrangellia; (18) publication of 3 geological survey field reports (Greene et al. 2005a, 2005b, 2006); (19) abstracts and presentations at 8 scientific conferences; (20) co-author on 4 conference proceedings and 1 geological survey report; (21) preparationlsubmittal of 4 manuscripts for publication. 267 The four main chapters of this dissertation have each yielded an important set of interpretations about the source, generation, and emplacement of basalts of the Wrangellia oceanic plateau. The geochemistry of picritic and tholeiitic basalts that form the Karmutsen Formation provide constraints about the melting history of plume-derived magmas where continental lithosphere and significant source heterogeneity were not involved. The tripartite stratigraphy on Vancouver Island is constructed largely of tholeiitic basalt with a restricted major- and trace-element, and isotopic composition. The high-Mg picritic pillow lavas on northern Vancouver Island are depleted in LREE and have similar initial isotopic compositions to the tholeiitic basalts. Modeling results for the picrites, utilizing the technique of Herzberg et al. (2007), support that the Karmutsen Formation originated from high degrees of partial melting (25-30%) of unusually hot mantle (-4450\u00C2\u00B0C) during the start-up phase of a mantle plume. The picrites developed a depleted trace-element signature, likely during melting of the plume. The isotopic compositions of the tholeiitic basalts and picrites indicate a homogeneous, OIB-type enriched Pacific mantle source, less depleted than the source of MORB, and comparable to the source that produced basalts of the Caribbean oceanic plateau. Decompression melting within the mantle plume initiated within the garnet stability field (>2.7 GPa; 80 km) and proceeded beneath oceanic lithosphere where more extensive degrees of melting occurred within the spinel stability field (<60 km). The trace-element compositions of the Wrangellia flood basalts are best explained by a peridotite source rather than a source consisting of a high proportion of eclogite. The major-element compositions of Karmutsen tholeiitic basalts (low Mg#, MgO, etc.) indicate an important role for fractional crystallization of melts at low pressure and some of the crystalline residues from partially crystallized Karmutsen magmas may be present beneath Vancouver Island. The geochemistry of Wrangellia basalts in Alaska and Yukon has helped to improve our understanding of the relationship between mantle plumes and oceanic lithosphere through which the basaltic magmas erupted. The volcanic stratigraphy in Alaska and Yukon preserves a shift from low-titanium to high-titanium basalts and provides compositional evidence of plume-lithosphere interaction during the formation of the northern part of the Wrangellia oceanic plateau. The trace-element and isotopic compositions of the high- and low-titanium basalts are distinct and indicate involvement 268 of arc lithospheric mantle in the formation of the low-titanium basalts. In a similar way that CFBs acquire geochemical characteristics from the lithosphere through which they erupt, basalts erupted in oceanic plateaus can acquire geochemical properties of oceanic arc lithosphere when present. Results from geochemical modeling suggest that small- degree melting (1-5%) of arc mantle may explain the trace-element signature of the low- titanium basalts. Based on isotopic compositions, the high-titanium basalts in Alaska and Yukon originated from an OIB-type mantle source compositionally similar to Karmutsen basalts on Vancouver Island and the source of ocean islands (e.g. Hawaii) and plateaus (e.g. Ontong Java and Caribbean) in the Pacific Ocean. Basalts from the Wrangellia plateau, together with Hawaii and the Ontong Java and Caribbean Plateaus, provide a sampling of OIB-type mantle emanating from the Pacific mantle in the last 230 Myr. The volcanic stratigraphy of the Wrangellia flood basalts records the evolution of a major oceanic plateau (Nixon eta!., 2008). The duration of volcanism in Northern and Southern Wrangellia was probably less than 5 Myr, between Ca. 231 and 225 Ma (e.g. Parrish & McNicoll, 1992; Sluggett, 2003; Bittenbender eta!., 2007). The Wrangellia basalts in Alaska and Yukon overlie Late Pennsylvanian and Permian sediments and arc volcanic sequences (ca. 320-285 Ma), whereas basalts on Vancouver Island overlie a basement of Mississippian to Permian marine sedimentary strata and Devonian to Mississippian age (380-355 Ma) arc rocks (e.g. Muller et a!., 1974). The earliest flows in Alaska were emplaced in a shallow marine environment followed by accumulation of \u00E2\u0080\u00943.5 km of subaerial flows. On Vancouver Island, the early growth of the plateau was dominated by extrusion of pillow lavas, which accumulated more than 3000 m thick and were overlain by 40-1500 m of pillow basalt and hyaloclastite, before the plateau emerged above sea level. The accumulation of approximately 1500 m of sheet flows continued above sea level and intervolcanic sedimentary lenses formed as the plateau subsided and volcanism waned. The plateau continued to subside for more than 25 Myr following volcanism and is overlain with minimal disconformity by hundreds to thousands of meters of limestone and shale. The Wrangellia oceanic plateau is one of the best natural laboratories available on land to study and sample the entire volcanic stratigraphy of a well-preserved Phanerozoic oceanic plateau. A combined in-depth geochemical, geochronological, and 269 sedimentological study is needed to further elucidate the processes that led to volcanism, uplift and subsidence of the plateau, and the precise duration of volcanism. The following short list is a plan for prospective research of the Wrangellia oceanic plateau: \u00E2\u0080\u00A2 Widespread geochemical studies from sampling and portable XRF analysis to determine the spatial and stratigraphic distribution of high- and low-titanium basalts, as well as newly discovered picritic lavas, in Alaska and Yukon. \u00E2\u0080\u00A2 Widespread geochemical studies from sampling and portable XRF analysis to determine the spatial and stratigraphic distribution of picritic lavas on Vancouver Island and the Queen Charlotte Islands. \u00E2\u0080\u00A2 Identification of the tripartite stratigraphy throughout the entire Karmutsen Formation on Vancouver Island and the Queen Charlotte Islands (Haida Gwaii). \u00E2\u0080\u00A2 Fieldwork and sampling of correlative flood basalts in southeast Alaska, and examination of their relationship with underlying and overlying strata. \u00E2\u0080\u00A2 Studies of osmium and helium isotopic ratios to further determine the composition and origin of the source that produced Wrangellia basalts and whether lower mantle or core material was involved. \u00E2\u0080\u00A2 U-Pb geochronology of basal sills and coarse-grained thick flows to better constrain the duration of volcanism in Wrangellia. \u00E2\u0080\u00A2 Paleontological and sedimentological analysis of underlying and overlying sedimentary sequences to assess the environmental setting immediately preceding and following volcanism, the uplift and subsidence history of the Wrangellia plateau, and to provide additional age constraints. \u00E2\u0080\u00A2 Geochemical studies of underlying Paleozoic arc volcanic sequences on Vancouver Island and in Alaska to determine the role of arc lithosphere in the evolution of Wrangellia basalts erupted in these areas and the tectonic history of the Wrangellia Terrane. \u00E2\u0080\u00A2 Detailed mineralogical and whole-rock geochemical studies of the mafic and ultramafic plutonic rocks exposed within Wrangellia, particularly of the exposures in the Amphitheater Mountains, Alaska and the base of the Nikolai Formation in Yukon. \u00E2\u0080\u00A2 Studies of the environmental impact of eruption of the Wrangellia flood basalts in the Late Ladinian and Carnian stages of the Triassic. 270 The Wrangellia oceanic plateau is ripe for future research and may hold answers to some of the important questions about the development of oceanic plateaus that remain mostly hidden beneath the surface of the ocean. REFERENCES Bittenbender, P. E., Bean, K. W., Kurtak, J. M. & Deninger, J., Jr (2007). Mineral assessment of the Delta River Mining District area, East-central Alaska. U S. Bureau ofLand Management-Alaska. Technical Report 57, 676 p. http://www.blm.gov/pgdata/etc/medialib/blmlak!aktest/tr.Par.8641 2.File.dat/BLM _TR57.pdf. Greene, A. R., Scoates, J. S. & Weis, D. (2005a). Wrangellia Terrane on Vancouver Island: Distribution of flood basalts with implications for potential Ni-Cu-PGE mineralization. In: Grant, B. (ed.) Geological Fieldwork 2004. British Columbia Ministry of Energy, Mines and Petroleum Resources Paper 2005-1, pp. 209-220. Greene, A. R., Scoates, J. S., Weis, D. & Israel, S. (2005b). Flood basalts of the Wrangellia Terrane, southwest Yukon: Implications for the formation of oceanic plateaus, continental crust and Ni-Cu-PGE mineralization. In: Emond, D. S., Lewis, L. L. & Bradshaw, G. D. (eds.) Yukon Exploration and Geology 2004. Yukon Geological Survey, pp. 109-120. Greene, A. R., Scoates, J. S., Nixon, G. T. & Weis, D. (2006). Picritic lavas and basal sills in the Karmutsen flood basalt province, Wrangellia, northern Vancouver Island. In: Grant, B. (ed.) Geological Fieldwork 2005. British Columbia Ministry of Energy, Mines and Petroleum Resources Paper 2006-1, pp. 39-52. Herzberg, C., Asimow, P. D., Arndt, N., Niu, Y., Lesher, C. M., Fitton, J. G., Cheadle, M. J. & Saunders, A. D. (2007). Temperatures in ambient mantle and plumes: Constraints from basalts, picrites, and komatiites. Geochemistry Geophysics Geosystems 8(Q02006), doi: 10.1 029/2006GC00 1390. Muller, J. E., Northcote, K. E. & Carlisle, D. (1974). Geology and mineral deposits of Alert Bay - Cape Scott map area, Vancouver Island, British Columbia. Geological Survey ofCanada, Paper 74-8, 77 pp. Nixon, G. T., Laroque, J., Pals, A., Styan, J., Greene, A. R. & Scoates, J. S. (2008). High- Mg lavas in the Karmutsen flood basalts, northern Vancouver Island (NTS 092L): Stratigraphic setting and metallogenic significance. In: Grant, B. (ed.) Geological Fieldwork 2007. B.C. Ministry of Energy, Mines and Petroleum Resources Paper 2008-1. pp. 175-190. Parrish, R. R. & McNicoll, V. J. (1992). U-Pb age determinations from the southern Vancouver Island area, British Columbia. Geological Survey ofCanada. Radiogenic Age and Isotopic Studies: Report 5 Paper 9 1-2, 79-86 p. Sluggett, C. L. (2003). Uranium-lead age and geochemical constraints on Paleozoic and Early Mesozoic magmatism in Wrangellia Terrane, Saltspring Island, British Columbia. Unpublished B.Sc. thesis, University of British Columbia, 84 pp. 271 L7 SELIOMELJJV Mt. Washington Plutonic Suite Mississippian-Permian Nanaimo Group Buttle Lake Group(sedimentary rocks) Early-Middle Jurassic DevonianIsland Plutonic Suite Sicker Group (sed/volcanic rocks) Karmutsen Formation Mount Hall Gabbro Appendix A. Geologic map of the Mount Arrowsmith area with sample locations (location of map shown in Figure 2.1), south-central Vancouver Island. Map derived from Massey et al. (2005b). Cretaceous Middle-Lower Triassic + massive lava / fault river * pillow lava / 273 Appendix B. XRF whole-rock analyses of a subset of Karmutsen basalts, Vancouver Island, B.C. Sample 4722A2 4722A4 4722A5 4723A2 4723A3 4723A4 4723A13 5614A1 5614A3 5614A5 Group THOL PlC OUTLIER HI-MG PlC PlC PlC PlC HI-MG HI-MG Area KR KR KR KR KR KR KR KR KR KR Flow Flow Pillow Flow Pillow Pillow Pillow Pillow Pillow Pillow Breccia UTMEW 5590769 5595528 5595029 5588266 5588274 5586081 5599233 5599183 5599183 5599192 UTMNS 634318 629490 627605 626698 626641 626835 616507 616472 616472 614756 Unnorrnalized Major Element Oxides (Weight %): Si02 48.48 4605 48.20 48.47 47.43 46.38 46.91 48.79 51.03 49.84 1102 1.89 0.47 2.43 0.67 0.61 0.72 0.50 0.52 0.51 0.56 Al203 14.23 12.47 13.10 16.33 13.99 16.26 14.77 15.11 15.35 15.45 Fe203* 13.96 11.37 16.50 10.85 11.12 11.03 10.90 9.88 8.80 9.48 MnO 0.21 0.19 0.22 0.19 0.18 0.17 0.17 0.17 0.16 0.16 MgO 7.18 18.67 6.15 10.66 16.14 13.56 15.02 12.43 10.90 10.95 CaO 11.87 9.71 9.33 10.43 9.57 10.32 10.21 11.55 11.35 11.27 Na20 2.06 0.57 3.40 2.19 0.69 1.47 1.04 1.17 1.24 1.96 1(20 0.11 0.12 0.33 0.37 0.04 0.07 0.05 0.05 0.07 0.04 P205 0.16 0.04 0.22 0.06 0.06 0.07 0.05 0.05 0.05 0.04 LOl 1.49 5.28 1.71 3.52 5.46 4.77 5.21 4.04 3.63 4.00 Total 100.15 99.66 99.88 100.22 99.83 100.05 99.61 99.72 99.46 99.75 Trace Elements (ppm): La 5 2 6 1 3 1 2 1 1 0 Ce 16 1 21 3 3 4 3 2 3 3 Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Sc V 351 184 485 235 201 205 208 217 211 185 Cr 160 1784 100 396 1787 912 1541 1625 1549 814 Co Ni 95 755 59 163 656 339 583 564 551 315 Cu Zn 126 62 124 73 74 71 68 64 60 62 Ga 19 10 18 13 12 14 11 11 11 12 Ge Rb 1.4 5.4 5.4 10.3 1.6 1.7 1.5 0.7 1.5 0.3 Sr 182 95 226 276 65 134 74 122 133 144 Y 25.7 14.2 36.6 18.5 16.7 17.4 16.5 17.0 16.7 15.3 Zr 109 19 148 37 33 42 27 28 29 23 Nb 9.4 1.0 11.5 1.8 1.7 1.6 1.0 1.1 1.0 0.8 Cs Ba 39 37 95 98 21 25 20 30 28 18 Hf Ta Pb 0 1 2 0 1 1 1 1 1 1 Th 0 0 2 0 1 0 0 1 0 0 U I 1 1 1 1 1 1 1 1 1 Abbreviations for group are: ThOL, tholeiitic basalt; PlC, picrite; HI-MG, high MgO basalt; CG, coarse-grained (sill or gabbro); OUTLIER, anomalous pillowed flow in plots. Abbreviations for area are: SL, Schoen Lake; KR, Karmutsen Range. Sample locations are given using the Universal Transverse Mercator (UTM) coordinate system (NAD83; zones 9 and 10). Analyses were performed at University of Massachusetts Ronald B. Gilmore XRF Laboratory. Fe203*is total iron expressed as Fe203.LOl is loss-on-ignition. 274 Sample 5615A7 5615A12 5616A1 5617A5 Group PlC PlC PiC CG Area KR KR KR SL Flow Pillow Pillow Pillow Sill UTM EW 5595569 5586126 5598448 5557712 UTMNS 629573 626824 616507 700905 Unnormalized Major Element Oxides (Weight %): S102 46.28 48.46 46.89 49.10 Ti02 0.47 0.70 0.48 1.80 Al203 12.26 14.96 14.75 14.02 Fe203* 11.53 10.83 10.86 12.72 MnO 0.19 0.17 0.19 0.20 MgO 18.83 12.78 15.75 7.49 CaO 9.65 9.68 9.39 12.39 Na20 0.39 1.97 1.00 1.63 (20 0.12 0.07 0.05 0.25 0.04 0.06 0.05 0.15 LOl 5.23 4.58 5.84 1.42 Total 99.76 99.68 99.42 99.75 Trace Elements (ppm): La 7 0 3 7 Ce 18 4 4 18 Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Sc V 329 198 199 329 Cr 279 998 4059 279 Co Ni 100 368 559 100 Cu Zn 99 72 62 99 Ga 18 13 12 18 Ge Rb 6.6 2.1 1.1 6.6 Sr 258 193 114 258 Y 23.0 16.8 15.3 23.0 Zr 102 41 26 102 Nb 9.5 1.5 1.1 9.5 Cs Ba 49 24 29 49 Hf Ta Pb 1 0 0 1 Th 1 0 0 1 U I I 1 1 275 Anriendix C. PCh3R trace-element concentrations of Karmutsen basalts. Vancouver Island. B.C. Sample 4718A2(1) 4718A7 4719A2 4719A3 4720A4 4720A6 4720A7 4720A7dup 4720A10 4721A2 Group THOL THOL THOL THOL THOL CG CG Area MA MA MA MA SL SL SL Flow Pillow Pillow Pillow Pillow Flow Flow Flow UTM EW 5455150 5455280 5454625 5454625 5566984 5566161 5566422 UTMNS 384260 382261 381761 381761 707626 704411 703056 Trace Bements (ppm): CG CG THOL SL SL SL Flow Sill Flow 5566422 5560585 5563936 703056 702230 704941 Cr Co 41.0 43.9 Ni 55.9 77.3 Cu 156 142 Zn 77.8 76.6 Ga 15.1 15.5 Rb 1.28 3.78 Sr 202 283 Y 26.6 29.7 Zr 103 98 Nb 8.64 7.01 Cs 0.19 0.23 Ba 41.2 61.4 Hf 2.53 2.41 Ta 0.53 0.49 Pb 0.71 0.54 Th 0.56 0.52 Ii 0.18 0.17 9.12 8.36 12.48 10.02 10.13 23.70 22.00 31.35 25.17 26.05 3.41 3.17 4.46 3.56 3.68 16.06 14.84 20.99 16.80 17.28 4.48 4.22 5.77 4.75 4.96 1.65 1.57 2.05 1.67 1.76 5.35 4.98 6.87 5.65 5.81 0.87 0.80 1.14 0.96 0.99 5.52 5.12 6.87 6.03 6.17 1.14 1.06 1.46 1.25 1.29 3.16 2.85 4.01 3.50 3.58 0.43 0.40 0.55 0.48 049 2.69 2.46 3.42 2.93 3.10 0.40 0.37 0.53 0.45 0.46 45.2 39.3 58.0 44.5 50.9 359 353 472 357 384 44.1 43.1 51.5 41.7 40.6 62.9 66.7 67.2 57.1 53.0 147 138 193 162 164 73.5 74.5 94.7 72.0 77.7 14.7 14.5 18.0 14.0 14.8 3.27 3.49 2.54 6.11 2.18 244 229 215 123 157 27.6 25.1 35.2 29.8 31.8 88 90 126 97 93 5.76 5.75 10.23 5.54 5.50 0.13 0.15 0.23 0.55 0.24 125.6 115.7 37.3 34.1 37.8 2.16 2.12 3.04 2.38 2.23 0.37 0.38 0.70 0.36 0.36 0.68 0.52 0.65 0.56 0.61 047 0.45 0.72 0.45 0.48 0.18 0.14 0.21 0.13 0.13 9.39 2.68 8.64 23.93 7.16 21.94 3.38 1.10 3.07 16.02 5.67 14.60 4.56 2.05 4.05 1.63 0.87 1.44 5.31 3.08 4.77 0.91 0.58 0.78 5.63 3.91 4.90 1.20 0.88 0.99 3.28 2.57 2.78 0.45 0.37 0.39 2.86 2.38 2.47 0.43 0.36 0.36 43.8 26.5 42.8 376 313 330 40.7 37.6 48.4 51.5 78.4 79.5 164 111 182 75.2 48.6 93.6 14.4 9.8 19.0 1.79 0.11 1.58 140 60 209 28.6 19.9 24.1 98 40 118 6.00 1.32 7.36 0.20 0.02 0.26 37.8 55.5 41.6 2.35 1.00 2.99 0.41 0.09 0.49 0.61 0.51 0.91 0.44 0.11 0.72 0.14 0.05 0.25 La 10.32 Ce 26.11 Pr 3.66 Nd 16.79 Sm 4.60 Eu 1.63 Gd 5.25 Tb 0.86 Dy 5.39 Ho 1.11 Er 3.01 Tm 0.41 Yb 2.51 Lu 0.38 Sc 41.2 V 376 9.44 24.25 3.53 16.65 4.79 1.79 5.72 0.98 5.91 1.27 3.42 0.48 2.96 0.46 54.2 386 Abbreviations for group are: THOL, tholeiltic basalt; PlC, picrite; HI-MG, high MgO basalt; CG, coarse-grained (sill or gabbro); OUTLIER, anomalous pillowed flow in plots. Abbreviations for area are: MA, Mount Arrowsmith; SL, Schoen Lake; KR, Karmutsen Range. Sample locations are given using the Universal Transverse Mercator (UTM) coordinate system (NAD83; zones 9 and 10). All analyses were performed at the Pacific Centre for Isotopic and Geochemical Research (PCIGR) at UBC by HR-ICP-MS. Fe203*is total iron expressed as Fe203.*Samples marked with asterisk are not from the Karmutsen Formation, their age is unknown. 276 Sample 4721A4 4722A4 4722A4dup 4722A5 4723A2 4723A3 4723A4 4723A13 4724A3 4718A\u00E2\u0080\u009D Group THOL PlC PlC OUTLIER HI-MG PlC PlC PiC CG Area SL KR KR KR KR KR KR KR SL MA Flow Flow Pillow Pillow Flow Pillow Pillow Pillow Pillow Flow Flow UTMEW 5564285 5595528 5595528 5595029 5588266 5588274 5586081 5599233 5581870 5455174 UTMNS 704896 629490 629490 627605 626698 626641 626835 616507 704472 383960 Trace Elements (ppm): La 11.73 1.29 1.25 11.00 2.99 3.10 2.63 1.83 7.09 22.12 Ce 28.92 3.49 3.07 30.57 7.83 8.39 7.18 4.72 19.33 46.72 Pr 4.16 0.51 0.47 4.41 1.15 1.19 1.12 0.70 2.76 5.93 Nd 19.58 2.70 2.48 21.00 5.79 5.91 5.90 3.54 13.21 25.32 Sm 5.48 1.07 0.97 6.10 1.99 2.09 2.14 1.33 3.84 5.97 Eu 1.97 0.44 0.41 1.95 0.73 0.79 0.88 0.55 1.44 1.95 Gd 6.58 1.73 1.64 7.31 2.98 3.08 3.21 2.09 4.52 6.08 Tb 1.11 0.35 0.33 1.25 0.57 0.61 0.61 0.43 0.77 0.96 Dy 6.88 2.53 2.43 7.76 3.96 4.09 4.05 3.03 4.75 5.66 Ho 1.47 0.61 0.56 1.63 0.93 0.98 0.90 0.73 1.00 1.26 Er 4.03 1.88 1.77 4.44 2.75 2.87 2.59 2.26 2.71 3.59 Tm 0.57 0.27 0.26 0.60 0.42 0.43 0.38 0.34 0.37 0.52 Yb 3.52 1.83 1.79 3.75 2.70 2.89 2.40 2.19 2.32 3.43 Lu 0.53 0.29 0.29 0.56 0.43 0.45 0.35 0.35 0.34 0.56 Sc 55.6 38.0 41.9 45.2 57.5 59.3 43.3 42.6 38.5 37.3 V 446 211 212 475 371 295 244 237 301 343 Cr Co 46.6 63.9 67.7 42.4 52.3 74.9 52.2 59.7 44.6 24.3 Ni 70.6 475.0 522.6 41.8 141.5 567.1 227.7 382.0 136.3 1.7 Cu 178 71 82 92 119 116 82 75 171 50 Zn 81.7 50.2 50.1 84.9 67.2 68.4 50.1 49.7 69.1 84.8 Ga 15.7 7.1 6.1 14.4 12.5 10.8 10.5 8.4 13.7 17.2 Rb 1.65 3.16 0.00 4.46 8.23 1.65 1.43 0.67 1.44 13.94 Sr 172 77 50 191 269 69 110 63 135 356 Y 35.0 14.5 14.3 39.5 21.3 22.3 20.7 17.5 23.6 30.6 Zr 110 16 15 126 42 38 37 24 59 73 Nb 8.96 0.63 0.60 7.11 1.82 1.78 1.13 0.71 4.52 4.81 Cs 0.36 2.26 0.00 0.25 0.39 0.83 0.55 0.47 0.19 0.06 Ba 27.4 15.1 12.3 75.4 88.2 13.8 16.6 10.2 46.8 353.6 I-if 2.79 0.48 0.45 3.15 1.06 0.98 0.90 0.63 1.41 1.80 Ta 0.62 0.05 0.04 0.47 0.12 0.12 0.07 0.05 0.29 0.22 Pb 0.54 0.17 0.16 0.81 0.09 0.32 0.09 0.18 0.60 2.20 Th 0.61 0.07 0.03 0.82 0.19 0.19 0.07 0.11 0.30 1.51 U 0.17 0.05 0.05 0.33 0.06 0.06 0.05 0.03 0.09 0.86 277 Sample 471 8A4* 4723A12* Group Area MA KR Flow Flow Flow UTM EW 5455292 5579914 UTM NS 383790 628268 Trace Elements (ppm): La 17.59 7.58 Ce 37.73 21.00 Pr 4.79 2.93 Nd 20.26 13.69 Sm 4.74 3.88 Eu 1.54 1.34 Gd 4.66 4.57 Tb 0.73 0.80 Dy 4.52 5.04 Ho 0.96 1.11 Er 2.77 3.17 Tm 0.40 0.45 Yb 2.66 2.85 Lu 0.42 0.43 Sc 30.1 21.1 V 269 356 Cr Co 19.7 37.1 Ni 2.2 21.8 Cu 34 115 Zn 92.4 72.7 Ga 13.7 14.2 Rb 30.59 0.52 Sr 440 116 Y 24.6 21.6 Zr 64 113 Nb 3.75 3.11 Cs 0.23 0.00 Ba 711.9 106.5 Hf 1.44 2.49 Ta 0.17 0.20 Pb 1.95 0.78 Th 1.26 0.21 U 0.65 0.40 278 APPENDIX D. SAMPLE PREPARATION AN]) ANALYTICAL METHODS FOR ALASKA SAMPLES The least altered samples of the Nikolai Formation were selected for geochemical analysis based on thorough petrographic inspection. Thirty-seven of 68 samples from the Alaska Range and 16 of 36 samples from the Wrangell Mountains were crushed (400 g) into pieces <2 mm in diameter in a Rocklabs hydraulic piston crusher between WC plates. The coarse-crush was thoroughly mixed and 100 g was powdered in a planetary mill using agate jars and balls that were cleaned with quartz sand between samples. University ofMassachussetts XRF Analytical Methods Fifty-three sample powders and 6 duplicate powders were analyzed at the Ronald B. Gilmore X-Ray Fluorescence (XRF) Laboratory at the University of Massachussetts. Major elements were measured on a fused La-bearing lithium borate glass disc using a Siemens MRS-400 spectrometer with a Rh X-ray tube operating at 2700 W. Trace element concentrations (Rb, Sr, Ba, Ce, Nb, Zr, Y, Pb, Zn, Ga, Ni, Cr, V) were measured on a separate powder pellet using a Philips PW2400 sequential spectrometer with a Rh X ray tube. Loss on ignition (LOT) and ferrous iron measurements were made as described by Rhodes and Vollinger (2004). Precision and accuracy estimates for the data are described by Rhodes (1996) and Rhodes and Vollinger (2004). Results for each sample are the average of two separate analyses. A total of 4 complete duplicates were analyzed for Alaska samples. Eighteen sample duplicate powders of Wrangellia flood basalts were also analyzed at Activation Laboratories and the results for most elements were within analytical error (ActLabs; see Chapter 2 for analytical methods). PCJGR Trace Element and Isotopic Analytical Methods A subset of twenty-four samples was selected for high-precision trace-element analysis and Sr, Nd, Pb, and Hf isotopic analysis at the Pacific Centre for Isotopic and Geochemical Research (PCIGR) at the University of British Columbia (IJBC; Table 3.2). Samples were selected from the 53 samples analyzed by XRF, based on major- and trace element chemistry, alteration (low LOT and petrographic alteration index), sample 279 location, and stratigraphic position. Samples were prepared for trace-element analysis at the PCIGR by the technique described by Pretorius et al. (2006) on unleached rock powders. Sample powders (\u00E2\u0080\u0094400 mg) were weighed in 7 mL screw-top Savillex\u00C2\u00AE beakers and dissolved in 1 mL \u00E2\u0080\u009414N HNO3 and 5 mL 48% HF on a hotplate for 48 hours at 130\u00C2\u00B0C with periodic ultrasonication. Samples were dried and redissolved in 6 mL 6N HC1 on a hotplate for 24 hours and then dried and redissolved in 1 mL concentrated HNO3 for 24 hours before final drying. Trace element abundances were measured with a Thermo Finnigan Element2 High Resolution-Inductively Coupled Plasma-Mass Spectrometer (HR-ICP-MS) following the procedures described by Pretorius et a!. (2006) within 24 hours of redissolution. High field strength elements (HFSE) and large ion lithophile elements (LILE) were measured in medium resolution mode at 2000x dilution using a PFA teflon spray chamber washed with Aqua Regia for 3 minutes between samples. Rare earth elements (REE) were measured in high resolution mode, and U and Pb in low resolution mode, at 2000x dilution using a glass spray chamber washed with 2% HNO3 between samples. Total procedural blanks and reference materials (BCR-2, BHVO-2) were analyzed with the batch of samples. Indium was used as an internal standard in all samples and standard solutions. Background and standard solutions were analyzed after every 5 samples to detect memory effects and mass drift. Sample digestion for purification of Sr, Nd , Hf, and Pb for column chemistry involved weighing each sample powder. All samples were intially leached with 6N HCI and placed in an ultrasonic bath for 15 minutes. Samples were rinsed two times with 18 mega 2-cm H20between each leaching step (15 total) until the supernatant was clear (following the technique of Mahoney, 1987). Samples were then dried on a hotplate for 24 hours and weighed again. Sample solutions were then prepared by dissolving 400- 250 mg of the leached powder dissolved in 1 mL \u00E2\u0080\u009444N HNO3 and 10 mL 48% HF on a hotplate for 48 hours at 130\u00C2\u00B0C with periodic ultrasonication. Samples were dried and redissolved in 6 mL 6N HC1 on a hotplate for 24 hours and then dried. Pb was separated using anion exhange columns and the discard was used for Sr, REE, and Hf separation. Nd was separated from the REE and Hf required two additional purification steps. Detailed procedures for column chemistry for separating Sr, Nd, and Pb at the PCIGR are described in Weis eta!. (2006) and Hf purification is described in Weis et a!. (2007). Sr 280 and Nd isotope ratios were measured on a Thermo Finnigan Triton Thermal Ionization Mass Spectrometer (TIMS) in static mode with relay matrix rotation on a single Ta and double Re-Ta filament, respectively. Four to 5 filaments per barrel of 21 were occupied by standards (NIST SRM 987 for Sr and LaJolla for Nd) for each barrel where samples were run. Sample Sr and Nd isotopic compositions were corrected for mass fractionation using 86Sr/8r= 0.1194 and\u00E2\u0080\u009846NdJ\u00E2\u0080\u009DNd = 0.72 19. Each sample was then normalized using the barrel average of the reference material relative to the values of143NdJNd = 0.511858 and87Sr/6r=0.710248 (Weis et al., 2006). During the period when the Alaska samples were analyzed, the La Jolla Nd standard gave an average value of 0.511853 \u00C2\u00B1 11 (n=8) and NIST SRM standard gave an average of 0.710253 \u00C2\u00B1 11 (n=9; 2a error is reported as times 106). 147Sm!Nd ratio errors are approximately -1 .5%, or -0.006. Leached powder of United States Geological Survey (USGS) reference material BHVO-2 was processed with the samples and yielded Sr and Nd isotopic ratios of 0.703473 \u00C2\u00B1 8 and 0.512980\u00C2\u00B1 6, respectively. These are in agreement with the published values of 0.703479 \u00C2\u00B1 20 and 0.5 12984 \u00C2\u00B1 11, respectively (Weis et al., 2006). USGS reference material BCR-2 was processed with the samples and yielded Sr and Nd isotopic ratios of 0.705002 \u00C2\u00B1 9 and 0.5 12633\u00C2\u00B1 7, respectively. These are in agreement with the published values of 0.705013 \u00C2\u00B1 10 and 0.5 12637 \u00C2\u00B1 12, respectively (Weis et aL, 2006). Pb and Hf isotopic compositions were analyzed by static multi-collection on a Nu Plasma (Nu Instruments) Multiple Collector-Inductively Coupled Plasma-Mass Spectrometer (MC-ICP-MS). The detailed analytical procedure for Pb isotopic analyses on the Nu at the PCIGR is described in Weis et al. (2006). The configuration for Pb analyses allows for collection of Pb, Ti, and Hg together. Ti and Hg are used to monitor instrumental mass discrimination and isobaric overlap, repectively. All sample solutions were analyzed with approximately the same Pb/Tl ratio (\u00E2\u0080\u00944) as the reference material NEST SRM 981. To accomplish this, a small aliquot of each sampie soiution from the Pb columns was analyzed on the Eiement2 to determine the precise amount of Pb available for analysis on the Nu Plasma. The SRM 981 standard was run after every two samples on the Nu Plasma. During the time samples were run, analyses of the SRM 981 Pb reference material gave values of206Pb/416.9403 \u00C2\u00B1 19, 207Pb/4 15.4964 \u00C2\u00B1 20, and208Pb/4b=36.7142 \u00C2\u00B1 53 (n=1 24; 2a error is reported as times 1 0); these values are 281 in excellent agreement with reported TIMS triple-spike values of Galer and Abouchami (1998). Results were further corrected by the sample-standard bracketing method or the in-hi correction method described by White et al. (2000) and Blichert-Toft et al. (2003). Leached powder of USGS reference material BHVO-2 yielded Pb isotopic ratios of 206Pb/4 18.6500 \u00C2\u00B1 7, 207Pb/4= 15.5294 \u00C2\u00B1 7, and208Pb/4= 38.2380 \u00C2\u00B1 19. These values are in agreement with leached residues of BHVO-2 from Weis et al., (2006). Hf isotopic compositions were analyzed following the procedures detailed in Weis et al. (2007). The configuration for Hf analyses monitored Lu mass 175 and Yb mass 172 to allow for interference correction to masses 174 and 176. Hf isotopic ratios were normalized internally for mass fractionation to a\u00E2\u0080\u009879Hf\u00E2\u0080\u0099177fratio of 0.7325 using an exponential correction. Standards were run after every two samples and sample results were normalized to the ratio of the in-run daily average and a\u00E2\u0080\u009876Hf/\u00E2\u0080\u00997fratio for JMC 475 of 0.282160. During the course of analyses, the Hf standard JMC-475 gave an average value 0.282153 \u00C2\u00B1 3 (n=79). USGS reference materials BCR-2 and BHVO-2 were processed with the samples and yielded Hf isotopic ratios of 0.282874 \u00C2\u00B1 5 and 0.283114 \u00C2\u00B1 6, respectively. Published values for BCR-2 and BHVO-2 are 0.282871 \u00C2\u00B1 7 and 0.283104 \u00C2\u00B1 8, respectively (Weis et al., 2007). REFERENCES Blichert-Toft, J., Weis, D., Maerschalk, C., Agranier, A. & Albar\u00C3\u00A8de, F. (2003). Hawaiian hot spot dynamics as inferred from the Hf and Pb isotope evolution of Mauna Kea volcano. Geochemistry Geophysics Geosystems 4(2), 1-27, doi: 10. 1029/2002GC000340. Galer, S. J. G. & Abouchami, W. (1998). Practical application of lead triple spiking for correction of instrumental mass discrimination. Mineralogical Magazine 62A, 49 1-492. Mahoney, J. J. (1987). An isotopic survey of Pacific oceanic plateaus: implications for their nature and origin. In: Keating, B. H., Fryer, P., Batiza, R. & Boehlert, G. W. (eds.) Seamounts, Islands, andAtolls. American Geophysical Union: Washington, D.C. Geophysical Monograph 43, pp. 207-220. Pretorius, W., Weis, D., Williams, G., Hanano, D., Kieffer, B. & Scoates, J. S. (2006). Complete trace elemental characterization of granitoid (USGSG-2,GSP-2) reference materials by high resolution inductively coupled plasma-mass spectrometry. Geostandards and Geoanalytical Research 30(1), 39-54. 282 Rhodes, J. M. (1996). Geochemical stratigraphy of lava flows samples by the Hawaii Scientific Drilling Project. Journal of Geophysical Research 1O1(B5), 11,729- 11,746. Rhodes, J. M. & Vollinger, M. J. (2004). Composition of basaltic lavas sampled by phase-2 of the Hawaii Scientific Drilling Project: Geochemical stratigraphy and magma types. Geochemistry Geophysics Geosystems 5(3), doi: 10.1 029/2002GC000434. Weis, D., Kieffer, B., Hanano, D., Silva, I. N., Barling, J., Pretorius, W., Maerschallc, C. & Mattielli, N. (2007). Hf isotope compositions of U.S. Geological Survey reference materials. Geochemistry Geophysics Geosystems 8(Q06006), doi: 10.1 029/2006GC00 1473. Weis, D., Kieffer, B., Maerschalk, C., Barling, J., de Jong, J., Williams, G. A., Hanano, D., Mattielli, N., Scoates, J. S., Goolaerts, A., Friedman, R. A. & Mahoney, J. B. (2006). High-precision isotopic characterization of USGS reference materials by TIMS and MC-ICP-MS. Geochemistry Geophysics Geosystems 7(Q08006), doi: 10. l029/2006GC001283. White, W. M., Albar\u00C3\u00A8de, F. & T\u00C3\u00A9louk, P. (2000). High-precision analysis of Pb isotope ratios by multi-collector ICP-MS. Chemical Geology 167, 257-270. 283 APPENDIX E. SAMPLE PREPARATION AND ANALYTICAL METHODS FOR YUKON SAMPLES The freshest rocks possible were sampled in the field and only the least altered samples were selected for analysis based on thorough petrographic inspection. Thirty- four of the 85 Nikolai basalt samples and 3 Station Creek samples were crushed (400 g) into pieces <2 mm in diameter in a Rocklabs hydraulic piston crusher between WC plates. The coarse-crush was mixed and 100 g was powdered in a planetary mill using agate jars and balls cleaned with quartz sand between samples. The major- and trace-element compositions of the whole rock powders were determined at Activation Laboratories Ltd. (Actlabs) in Ancaster, Ontario. Twenty-six additional samples ofNikolai basalt and 8 Station Creek samples, provided by S. Israel, were crushed and powdered by Actlabs. Analytical techniques and detection limits are also available from Actiabs (http://www.actlabs.com/methsubcode4ere.htm). The particular analytical method for each of the elements analyzed is indicated in Table 4.2. For the major elements, a 0.2 g sample was mixed with a mixture of lithium metaborate/lithium tetraborate and fused in a graphite crucible. The molten mixture was poured into a 5% HNO3 solution and shaken until dissolved (\u00E2\u0080\u0094P30 minutes). The samples were analyzed for major oxides and selected trace elements on a combination simultaneous/sequential Thermo Jarrell-Ash Enviro II inductively coupled plasma optical emission spectrometer (ICP-OES). Internal calibration was achieved using a variety of international reference materials (e.g. W-2, BIR-l, DNC-1) and independent control samples. Additional trace elements were analyzed by both the INAA (instrumental neutron activation analysis) and ICP-MS (inductively couple plasma mass spectrometry) methods. For the INAA analyses, 1.5-2.5 g of sample was weighed into small polyethylene vials and irradiated with control international reference material CANMET WMS-1 and NiCr flux wires at a thermal neutron flux of 7 x 1012 n cm2s\u00E2\u0080\u0099 in the McMaster Nuclear Reactor. Following a 7-day waiting period, the samples were measured on an Ortec high-purity Ge detector linked to a Canberra Series 95 multichannel analyzer. Activities for each element were decay- and weight-corrected and compared to a detector calibration developed from multiple international certified 284 reference materials. For the ICP-MS analyses, 0.25 g of sample was digested in HF, followed by a mixture of HNO3 and HC1O4,heated and taken to dryness. The samples were brought back into solution with HC1. Samples were analyzed using a Perkin Elmer Optima 3000 ICP. Tn-lab standards or certified reference materials (e.g. W-2, BIR-l, DNC-l) were used for quality control. A total of 12 blind duplicates were analyzed to assess reproducibility. A subset of 18 samples was selected for Sr, Nd, Pb, and Hf isotopic analysis at the Pacific Centre for Isotopic and Geochemical Research (PCIGR) at the University of British Columbia (UBC). Samples were selected from the 60 samples analyzed for whole-rock chemistry at ActLabs on unleached rock powders, based on major- and trace- element chemistry, alteration (low LOT and petrographic alteration index), sample location, and stratigraphic position. Sample digestion for purification of Sr, Nd , Hf, and Pb for column chemistry involved weighing each sample powder. Column chemistry was performed at the same time as samples from Alaska (Greene et al., submitted manuscript). All samples were intially leached with 6N HCI and placed in an ultrasonic bath for 15 minutes. Samples were rinsed two times with 18 mega 2-cm H20 between each leaching step (15 total) until the supematant was clear (following the technique of Mahoney, 1987). Samples were then dried on a hotplate for 24 hours and weighed again. Sample solutions were then prepared by dissolving \u00E2\u0080\u0094100-250 mg of the leached powder dissolved in 1 mL 14N HNO3 and 10 mL 48% HF on a hotplate for 48 hours at 130\u00C2\u00B0C with periodic ultrasonication. Samples were dried and redissolved in 6 mL 6N HC1 on a hotplate for 24 hours and then dried. Pb was separated using anion exhange columns and the discard was used for Sr, REE, and Hf separation. Nd was separated from the REE and Hf required two additonal purification steps. Detailed procedures for column chemistry for separating Sr, Nd, and Pb at the PCIGR are described in Weis et al. (2006) and Hf purification is described in Weis et al. (2007). Sr and Nd isotope ratios were measured on a Thermo Finnigan Triton Thermal Tonization Mass Spectrometer (TIMS) in static mode with relay matrix rotation on a single Ta and double Re-Ta filament, respectively. Four to 5 filaments per barrel of2l were occupied by standards (NBS 987 for Sr and LaJolla for Nd) for each barrel where samples were run. Sample Sr and Nd isotopic compositions were corrected for mass fractionation using 86Sr/85 = 0.1194 and \u00E2\u0080\u0098NdJ\u00E2\u0080\u0099Nd = 0.72 19. 285 Each sample was then normalized using the barrel average of the reference material relative to the valuesof\u00E2\u0080\u009943NdJ\u00E2\u0080\u0099Nd = 0.511858 and87Sr/6r=0.710248 (Weis et al., 2006). While the Yukon samples were analyzed, the La Jolla Nd standard gave an average value of 0.511853 \u00C2\u00B1 11 (n=8) and the NBS987 standard gave an average of 0.7 10253 \u00C2\u00B1 11 (n=9; 2o error is reported as times 106).\u00E2\u0080\u009847Sm!\u00E2\u0080\u0099Nd ratio errors are approximately 1.5%, or \u00E2\u0080\u0094\u00E2\u0080\u00980.006. Leached powder of United States Geological Survey (USGS) reference material BHVO-2 was processed with the samples and yielded Sr and Nd isotopic ratios of 0.703473 \u00C2\u00B1 8 and 0.5 12980 \u00C2\u00B1 6, respectively. These are in agreement with the published values of 0.703479 \u00C2\u00B1 20 and 0.5 12984 \u00C2\u00B1 11, respectively (Weis et a!., 2006). USGS reference material BCR-2 was processed with the samples and yielded Sr and Nd isotopic ratios of 0.705002 \u00C2\u00B1 9 and 0.5 12633\u00C2\u00B1 7, respectively. These are in agreement with the published values of 0.705013 \u00C2\u00B1 10 and 0.512637 \u00C2\u00B1 12, respectively (Weis et a!., 2006). Pb and Hf isotopic compositions were analyzed by static multi-collection on a Nu Plasma (Nu Instruments) Multiple Collector-Inductively Coupled Plasma-Mass Spectrometer (MC-ICP-MS). The detailed analytical procedure for Pb isotopic analyses on the Nu at the PCIGR is described in Weis et a!. (2006). The configuration for Pb analyses allows for collection of Pb, Tl, and Hg together. TI and Hg are used to monitor instrumental mass discrimination and isobaric overlap, respectively. All sample solutions were analyzed with approximately the same Pb/Tl ratio (--\u00E2\u0080\u00984) as the reference material NEST SRM 981. To accomplish this, a small aliquot of each sample solution from the Pb columns was analyzed on the Element2 to determine the precise amount of Pb available for analysis on the Nu Plasma. The SRM 981 standard was run after every two samples on the Nu Plasma. During the time Yukon and Alaska samples were run, analyses of the SRM 981 Pb reference material gave values of206Pb/4b=l6.9 03 \u00C2\u00B1 19, 207Pb/4b=l5 4964 \u00C2\u00B1 20, and208Pb/4b=36.7142 \u00C2\u00B1 53 (n=124; 2a error is reported as times 10); these values are in excellent agreement with reported TIMS triple-spike values of Galer and Abouchami (1998). Results were further corrected by the sample- standard bracketing method or the in-in correction method described by White et a!. (2000) and Blichert-Toft et a!. (2003). Leached powder of USGS reference material BHVO-2 yielded Pb isotopic ratios of206Pb/4= 18.6500 \u00C2\u00B1 7, 207Pb/4 15.5294 \u00C2\u00B1 286 7, and208Pb/4= 38.2380 \u00C2\u00B1 19. These values are in agreement with leached residues of BHVO-2 from Weis et aL, (2006). Hf isotopic compositions were analyzed following the procedures detailed in Weis eta?. (2007). The configuration for Hf analyses monitored Lu mass 175 and Yb mass 172 to allow for interference correction to masses 174 and 176. Hf isotopic ratios were normalized internally for mass fractionation to a\u00E2\u0080\u009879HfY\u00E2\u0080\u00997fratio of 0.7325 using an exponential correction. Standards were run after every two samples and sample results were normalized to the ratio of the in-run daily average and a\u00E2\u0080\u009876HfY\u00E2\u0080\u00997fratio for IMC 475 of 0.282160. During the course of analyses of Yukon and Alaska samples, the Hf standard JMC-475 gave an average value 0.282 153 \u00C2\u00B1 3 (n=79). USGS reference materials BCR-2 and BHVO-2 were processed with the samples and yielded Hf isotopic ratios of 0.282874 \u00C2\u00B1 5 and 0.283114 \u00C2\u00B1 6, respectively. Published values for BCR-2 and BHVO-2 are 0.282871 \u00C2\u00B1 7 and 0.283 104 \u00C2\u00B1 8, respectively (Weis eta?., 2007). REFERENCES Blichert-Toft, J., Weis, D., Maerschalk, C., Agranier, A. & Albar\u00C3\u00A8de, F. (2003). Hawaiian hot spot dynamics as inferred from the Hf and Pb isotope evolution of Mauna Kea volcano. Geochemistry Geophysics Geosystems 4(2), 1-27, doi: 10.1 029/2002GC000340. Galer, S. J. G. & Abouchami, W. (1998). Practical application of lead triple spiking for correction of instrumental mass discrimination. Mineralogical Magazine 62A, 49 1-492. Mahoney, J. J. (1987). An isotopic survey of Pacific oceanic plateaus: implications for their nature and origin. In: Keating, B. H., Fryer, P., Batiza, R. & Boehlert, G. W. (eds.) Seamounts, Islands, and Atolls. American Geophysical Union: Washington, D.C. Geophysical Monograph 43, pp. 207-220. Weis, D., Kieffer, B., Hanano, D., Silva, I. N., Barling, J., Pretorius, W., Maerschalk, C. & Mattielli, N. (2007). Hf isotope compositions of U.S. Geological Survey reference materials. Geochemistiy Geophysics Geosystems 8(Q06006), doi: 10.1 029/2006GC00 1473. Weis, D., Kieffer, B., Maerschallc, C., Barling, 3., de Jong, J., Williams, G. A., Hanano, D., Mattielli, N., Scoates, J. S., Goolaerts, A., Friedman, R. A. & Mahoney, J. B. (2006). High-precision isotopic characterization of USGS reference materials by TIMS and MC-ICP-MS. Geochemist?y Geophysics Geosystems 7(Q08006), doi: 10.1 029/2006GC00 1283. White, W. M., Albar\u00C3\u00AAde, F. & T\u00C3\u00A9louk, P. (2000). High-precision analysis of Pb isotope ratios by multi-collector ICP-MS. Chemical Geology 167, 257-270. 287 Appendix F. Major element (wt% oxide) and trace element (ppm) abundances in whole rock samples ofLate Paleozoic Station Creek Formation, Yukon Sample 4812A\u00E2\u0080\u0099I 4807A3 4807A4 1442 2291 2053 2333 1191 6922 Group PALEO PALEO PALEO PALEO PALEQ PALEO PALEO PALEO PALEO Area DJ QC QC QC QC QC QC QC QC Flow VOLC VOLC VOLC VOLC VOLC VOLC VOLC VOLC VOLC UTM EW 6817620 6817581 6817559 6812321 6807024 6807279 6813893 6814658 6821358 UTM NS 575107 582090 581767 574584 580257 584933 579050 582008 570972 Unnormalized Major Element Oxides (Weight %): Si02 69.72 46.98 47.77 59.96 49.39 51.24 44.33 49.77 56.82 Ti02 0.27 1.489 3.015 0.61 0.5 1.03 1.39 1.11 0.72 A1203 15.51 14.91 11.86 17.48 15.28 14.11 15.29 15.91 19.12 Fe203* 2.09 11.25 15.59 6.44 10.21 10.66 14.4 9.89 6.27 MnO 0.032 0.155 0.192 0.134 0.199 0.225 0.192 0.155 0.142 MgO 0.88 7.33 6.06 2.31 7.59 8.04 8.14 6.67 3.18 GaO 2.72 10.54 7.54 4.17 11.22 7.22 10.39 11.21 2.59 Na20 5.55 2.47 3.94 6.74 4.24 3.99 2.23 1.87 3.9 K20 0.65 1.09 0.41 0.2 0.1 0.96 0.89 1.27 3.23 0.07 0.12 0.24 0.5 0.14 0.1 0.11 0.11 0.23 LOI 2.06 3.08 2.33 1.92 1.38 2.63 2.50 2.33 4.03 Total 99.54 99.41 98.94 100.46 100.25 100.20 99.86 100.29 100.23 Trace Elements (ppm): La 7.54 5.58 14.00 6.92 4.09 6.30 27.20 5.60 13.60 Ce 14.35 13.95 33.94 17.00 8.05 13.09 48.82 14.01 26.37 Pr 1.7 2.1 4.84 2.45 1.17 1.87 5.52 1.85 3.19 Nd 6.63 10.28 22.74 12.52 5.63 9.57 21.1 8.98 13.1 Sm 1.38 3.06 6.52 3.78 1.67 2.89 4.56 2.69 3.22 Eu 0.47 1.17 2.18 1.38 0.59 0.884 1.41 0.983 1.04 Gd 1.28 3.7 7.83 4.3 2.04 3.23 3.99 3.02 3.24 Th 0.2 0.63 1.37 0.75 0.37 0.58 0.69 0.52 0.56 Dy 1.07 3.82 8.12 4.67 2.4 3.59 3.98 3.23 3.34 Ho 0.21 0.76 1.59 0.93 0.55 0.74 0.83 0.63 0.72 Er 0.59 2.18 4.55 2.79 1.77 2.22 2.73 1.93 2.32 Tm 0.08 0.31 0.65 0.4 0.26 0.321 0.412 0.280 0.358 Yb 0.51 1.87 4.03 2.39 1.56 1.97 2.60 1.71 2.27 Lu 0.08 0.27 0.57 0.34 0.24 0.273 0.397 0.253 0.347 Sc 4.1 44.2 53.8 33.3 30.7 32.7 9.5 30.1 16.1 V 32 314 465 294 207 305 50 297 178 Cr 6.4 317 51.6 184 261 437 363 32 Co 4.5 47.1 50.4 38 33 43 9 38 16 Ni 4 111 45 64 79 134 111 Cu 11 22 98 39 66 115 126 Zn 49 69 97 65 66 84 77 82 90 Ga 16 16 20 20 16 Ge 0.7 1.3 1.3 1.2 1.4 Rb 11 17 11 19 1 17 5 24 93 Sr 389 257 182 269 59 169 435 265 216 Y 6 22 45 24.5 15.8 19.8 24.8 19.3 19.7 Zr 89 77 171 87 26 60 139 62 116 Nb 2.2 6.8 16.2 7.3 1.9 4.2 13.0 7.0 7.8 Cs 0.7 1.4 1.3 0.2 0.3 0.1 1.3 1.4 Ba 190 161 94 97 21 3730 66 336 668 Hf 2.5 2.2 4.8 2.5 0.8 1.7 3.2 1.8 2.9 Ta 0.17 0.45 1.18 0.45 0.06 0.23 0.67 0.27 0.59 Th 1.04 0.47 1.25 0.5 0.48 0.83 2.84 1.14 3.73 U 0.49 0.11 0.40 0.55 0.39 0.44 1.12 0.38 1.17 Abbreviations for group are: PALEO. Paleozoic Station Creek Formation. Abbreviations for area are: QC, Quill Creek; DJ, Donjek River. Abbreviations for flow are: VOLC, volcanic lava flow or tuff. Sample locations are given using the Universal Transverse Mercator (UTM) coordinate system (NAD83; zones 7 and 8). Analyses were performed at Activation Laboratories (ActLabs). Fe203* is total iron expressed as Fe203.LOl is loss-on-ignition. All major elements, Sr. V, and Y by Fused ICP quadrapole (ICP-OES); Cu, Ni, Pb, and Zn by Total dilution ICP; Cs, Ga, Ge, Hf, Nb, Rb. Ta, Th, U, Zr, and REE by Fused-magnetic-sector ICP; Co. Cr, and Sc by INM. Blanks are below detection limit. See Appendix E for sample preparation and analytical methods. 288 Sample 212 1571 561 11 2121 Group PALEO PALEO PALEO PALEO PALEO Area QC QC QC QC QC Flow VOLC VOLC VOLC VOLC VOLC UTMEW 6817725 6805380 6817757 6818536 6806109 UTMNS 582272 591287 575179 583527 584732 Unnormalized Major Element Oxides (Weight %): Si02 46.49 48.87 41.89 49.04 49.16 Ti02 1.28 0.87 1.74 1.28 0.56 A1203 19.8 19.5 15.49 12.72 14.84 Fe203* 9.07 9.91 8.98 14.04 10.42 MnO 0.138 0.135 0.14 0.203 0.172 MgO 5.44 5.89 5.05 6.14 7.07 CaO 8.14 6.85 10.79 10.12 8.69 Na20 4.03 3.04 3.51 2.28 4.61 1<20 0.77 1.59 1.09 0.05 0.18 P205 0.11 0.31 0.14 0.15 0.11 LOl 4.81 3.33 11.46 4.026 4.2 Total 100.08 100.29 100.28 100.05 100.01 Trace Elements (ppm): La 4.68 9.62 7.34 4.28 4.65 Ce 11.47 21.99 19.47 12.48 8.67 Pr 1.68 2.70 2.69 1.77 1.14 Nd 8.70 13.0 13.5 8.64 5.6 Sm 2.64 3.67 4.12 2.76 1.66 Eu 0.977 1.26 1.82 0.941 0.63 Gd 3.04 3.74 5.00 3.37 2.06 Tb 0.53 0.63 0.82 0.63 0.41 Dy 3.28 3.64 4.63 4.06 2.57 Ho 0.64 0.76 0.90 0.86 0.57 Er 2.03 2.36 2.65 2.86 1.84 Tm 0.291 0.347 0.374 0.439 0.28 Yb 1.76 2.13 2.21 2.78 1.68 Lu 0.257 0.323 0.316 0.418 0.26 Sc 23 22.6 36.6 38.5 39 V 215 241 421 405 274 Cr 79 54 347 38 323 Co 23 28 56 54 41 Ni 39 30 132 57 75 Cu 166 89 197 194 176 Zn 64 101 118 97 54 Ga 13 Ge 1 Rb 10 35 30 2 Sr 228 563 382 205 152 Y 18.4 23.0 27.8 26.4 15.9 Zr 59 67 96 77 27 Nb 5.0 2.8 8.4 5.9 1.4 Cs 2.7 0.5 1.0 0.3 Ba 114 530 278 25 89 Hf 1.6 1.8 2.6 2.1 0.8 Ta 0.31 0.13 0.50 0.34 0.05 Th 0.36 1.24 0.64 0.37 0.36 U 0.11 0.55 0.20 0.18 0.23 289 APPENDIX G. PREVIOUS RESEARCH ON WRANGELLIA This brief overview of previous research highlights some of the key references, grouped into three main areas (south-central Alaska, southwest Yukon and southeast Alaska, and British Columbia). Supplementary data files 1 and 2 (an Endnote library and .txt file, respectively) include over 500 references involving research related to Wrangellia. Early stratigraphic and paleomagnetic studies of Wrangellia significantly influenced the geological research community. One of the earliest modern works on Wrangellia, by Jones eta!. (1977), had a major influence in the usage of terranes to assist with tectonic reconstructions. Wrangellia was one of the first sets of allochthonous crustal fragments to be referred to as a terrane, and this was based on the distinct stratigraphy and paleomagnetism of Wrangellia flood basalts (Winlder, 2000). Paleomagnetic studies on the flood basalts in Alaska indicated long-distance displacement since their eruption at low latitude (Hillhouse, 1977). The landmark works of Jones et al. (1977) and Hilihouse (1977) laid part of the foundation for future research on Wrangellia and spawned paleogeographic reconstructions of other areas of western North America. Previous research on Wrangeffia in south-central Alaska There is a remarkable history of exploration and early geological work in southern Alaska. The earliest recorded geological work was conducted by pioneers exploring the expanse of unknown territory in Alaska in the late 1800s and early 1900s (Hunt, 1996). People native to the Wrangell Mountains area used copper implements, which were derived from copper deposits near the contact between the top of the Nikolai flood basalts and the overlying Chitistone Limestone (Wiulder, 2000) The search for these copper deposits spurred exploration of the Wrangell Mountains and eventually lead to mining of what were some of the highest-grade copper deposits in the world (Kennicott mill-site) (MacKevett eta!., 1997). The Wrangell and St. Elias Mountains are now part of a National Park and World Heritage Site, which is the largest internationally protected area 290 in the world (Hunt, 1996). Exceptional historical accounts from the Wrangell-St. Elias area are given in books by Winkler (2000), Hunt (1996), and Sherwood (1965). Modern geologic overviews of southern Alaska are presented by many authors [Plaficer et a!. (1 989b), Nokleberg et a!. (1994), Plafker et al. (1994), Winider (2000), and Trop and Ridgeway (2007)1. The geology of the Wrangell Mountains has been exceptionally well-mapped and described by E. M. MacKevett, Jr. (1978) and D. Richter (1976). The 1:250,000 scale mapping has been compiled in digital format by Wilson et a!. (2005). Major contributions describing the formations in the Wrangell Mountains include Armstrong eta!. (1969), Armstrong and MacKevett (1977, 1982), Smith and MacKevett (1970), MacKevett (1970; 1971) and Trop et a!. (2002). The paleomagnetic character of Wrangellia in Alaska is addressed by Hillhouse (1977), Hililiouse and Gromme (1984), Hillhouse and Coe (1994). On the southern flank of the east-central Alaska Range, geologic exploration and early mapping was carried out by Mendenhall (1900), Moffit (1912; 1954), Rose (1965, 1966a, b), Rose and Saunders (1965), Stout (1965; 1976), and Richter and Jones (1973). More recent mapping and a geologic overview of the Amphitheater Mountains and Mount Hayes quadrangle was undertaken by Nokleberg et a!. (1982; 1985; 1992). A digital version of this mapping is included in the work compiled by Wilson eta!. (1998). A nice summary of sedimentary sequences at the base of the Nikolai in the Amphitheater Mountains is given by Blodgett (2002). In the Clearwater Mountains, in the southeast corner of the Healy quadrangle, mapping was accomplished by Smith (1973), Silberling et a!. (1981), and Csejtey et a!. (1992). The geology of the Clearwater Mountains is summarized by Smith (1971; 1973) and Turner and Smith (1974). To the southwest, in the northern Talkeetna Mountains, recent mapping has revealed exposures ofNikolai basalts and sills (Werdon et a!., 2000a, b, 2002; Schmidt et a!., 2003a; Schmidt et a!., 2003b). The geophysical properties of Wrangellia in southern Alaska are described by Glen et a!. (2007a; 2007b) and Saltus et a!. (2007). The character and development of sedimentary basins in the east-central Alaska Range are described by Ridgway et a!. (2002). An overview of metallogenic prospects related to the Nikolai Formation in southern Alaska is presented by Schmidt and Rogers (2007). 291 Previous research on Wrangeffia in southwest Yukon and southeast Alaska The earliest geological mapping of the Kluane Ranges was by McConnell (1905), Cairnes (1915), Sharp (1943), Bostock (1952), and Kindle (1953). More recent regional mapping in southwest Yukon was accomplished by Muller (1967), Read and Monger (1976), and Docids and Campbell (1 992a, b, c). Muller (1967) first proposed the correlation between the flood basalts in Yukon and those underlying extensive areas of southern Alaska. Nickel and copper mineralization discovered in the Kluane Ranges in the early 1950\u00E2\u0080\u0099s has led to continuous exploration into the mineralization potential of the mafic and ultramafic intrusions related to the Nikolai basalts (Came, 2003). Campbell (1981) and Miller (1991) completed thesis projects on mineral deposits related to Nikolai basalts near Quill Creek. The Yukon Geological Survey (YGS) recently initiated a bedrock mapping project in the Kiuane Ranges (1:50,000 scale) led by Israel et al. (2004; 2005; 2006). Mapping and field studies in southeast Alaska spurred several reports on Triassic basalt sequences in different areas of southeast Alaska. Plaficer et al. (1976) first recognized Nikolai equivalents in southeast Alaska, and pursued this work with detailed studies of metabasalts on the Chilkat Peninsula area near Haines, Alaska (Plaficer & Hudson, 1980; Davis & Plafker, 1985; Plafker et al., 1 989a). Other studies that describe the lithologic character and geochemistry of isolated segments of Triassic basalts in southeast Alaska include those by Loney et el. (1975), Decker (1981), Ford and Brew (1987, 1992), and Gebrels and Barker (1992). Regional mapping (1:600,000 scale) and an overview of the geology of southeast Alaska are presented by Gebrels and Berg (1992, 1994). Previous research on Wrangeffia in British Columbia The earliest exploratory geological work in the Queen Charlotte Islands (Haida Gwaii) was made by Dawson (1880). The foundation for the modern geologic framework for the islands was accomplished in an exceptionally-detailed study by Sutherland-Brown (1968). These studies have been expanded upon with works on Mesozoic and younger 292 strata by Cameron (1988), Lewis (1991), Thompson (1988; 1991), and Woodsworth (1991). The earliest geologic studies on northern Vancouver Island were made by Selwyn (1872), Dawson (1887), Dolmage (1919), Gunning (1930; 1931; 1932), and Hoadley (1953). On central and southern Vancouver Island, early fieldwork was carried out by Clapp (1909, 1913), Clapp and Cooke (1917), Stevenson (1945), Jeletzky (1950), and Fyles (1955). Early regional mapping and stratigraphic studies on Vancouver Island were accomplished by Muller and co-workers (Muller & Carson, 1969; Muller & Rahmani, 1970; Muller eta!., 1974; Muller, 1977, 1980, 1981; Muller eta?., 1981) and Jeletzky (1970, 1976). Carlisle supervised four Ph.D. theses related to the Karmutsen Formation (Surdam, 1967; Asihene, 1970; Kuniyoshi, 1972; Lincoln, 1978) that spawned a number of papers on the low-grade metamorphism of the Karmutsen (Surdam, 1968a, b, 1969, 1970; Kuniyoshi & Liou, 1976; Lincoln, 1981; Lincoln, 1986). These studies were impressive in their thoroughness. Carlisle thoroughly documented the stratigraphic relationships and character of the Karmtusen Formation (Carlisle, 1963; Carlisle, 1972; Carlisle & Suzuki, 1974). Low-grade metamorphism of the Karmutsen Formation has also been studied by Cho eta?. (Cho & Liou, 1987; Cho eta?., 1987), Starkey and Frost (1990), Teraybayashi (1993), and Greenwood eta?. (1991). Wrangellia sequences have recently been described during regional mapping studies (1:50,000 scale) on northern Vancouver Island by Nixon and co-workers (1993b, a; 1994a, b; 1995a, b; 2006a; 2006b; 2006d; 2006c; 2006e; 2007; 2008) and on southern Vancouver Island by Massey and co-workers (Massey & Friday, 1987, 1988, 1989; Massey, 1995a, b, c). Paleozoic basement of Wrangellia is well-described in the works by Massey, as well as in studies by Juras (1987), Yole (1963, 1965, 1969), Sutherland- Brown and Yorath (1985), Brandon eta?. (1986), Yorath eta?. (1999) and Katvala and Henderson (2002). Irving and Yole (1972), Yole and Irving (1980), and Irving and Wynne (1990) studied the paleomagnetism of the Karmutsen basalts. A digital geologic map for British Columbia has been compiled by Massey et a?. (2005a, b). The most recent geochemical and isotopic studies of Karmutsen basalts were undertaken by Barker et a?. (1989), Andrew and Godwin (1989), Lassiter eta?. (1995), and Yorath eta?. 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Electron microprobe study of prehnite and pumpellyite from the Karmutsen Group, Vanouver Island, British Columbia. American Mineralogist 54, 256-266. Surdam, R. C. (1970). The petrology and chemistry of the Karmutsen group volcanic rocks. University of Wyoming, Contributions to Geology 9(1), 9-12. Sutherland Brown, A. (1968). Geology of the Queen Charlotte Islands, British Columbia. B.C. Department ofMines and Petroleum Resources, Bulletin 54, 226 p. Sutherland Brown, A. & Yorath, C. 3. (1985). Lithoprobe profile across southern Vancouver Island: Geology and Tectonics. In: Tempelman-Kluit, D. (ed.) Field 303 guides to geology and mineral deposits in the southern Canadian Cordillera. Geol. Soc. Amer. Cordilleran section annual meeting, Field Trip Guidebook 8: Vancouver, British Columbia, pp. 8.1-8.23. Terabayashi, M. (1993). Compositional evolution in Ca-amphibole in the Karmutsen metabasites, Vancouver Island, British Columbia, Canada. Journal of Metamorphic Petrology 11(5), 677-690. Thompson, R. I. (1988). Late Triassic through Cretaceous geological evolution, Queen Charlotte Islands, British Columbia. In: Current Research, part E. Geological Survey of Canada, Paper 88-1E, pp. Thompson, R. I., Haggart, J. W. & Lewis, P. D. (1991). Late Triassic through early Tertiary evolution of the Queen Charlotte Islands: a perspective on hydrocarbon potential. In: Woodsworth, G. J. (ed.) Evolution and Hydrocarbon Potential ofthe Queen Charlotte Basin, British Columbia. Geological Survey of Canada, Paper 90-10, pp. Trop, 3. M. & Ridgway, K. D. (2007). Mesozoic and Cenozoic tectonic growth of southern Alaska: A sedimentary basin perspective. In: Ridgway, K. D., Trop, 3. M., O\u00E2\u0080\u0099Neill, 3. M. & Glen, I. M. G. (eds.) Tectonic growth ofa collisional continental margin: Crustal evolution ofsouthern Alaska. Geological Society of America Special Paper 431, pp. 55-94. Trop, 3. M., Ridgway, K. D., Manuszak, 3. D. & Layer, P. W. (2002). Mesozoic sedimentary-basin development on the allochthonous Wrangeflia composite terrane, Wrangell Mountains basin, Alaska: A long-term record of terrane migration and arc construction. Geological Society ofAmerica Bulletin 114, 693- 717. Turner, D. L. & Smith, T. B. (1974). Geochronology and generalized geology of the central Alaska Range, Clearwater Mountains and northern Talkeetna Mountains. Alaska Division of Geological and Geophysical Surveys. Open-File Report AOF 72, lOp. Werdon, M. B., Riehle, J. R., Schmidt, 3. M., Newberry, R. J. & Pessel, G. H. (2000a). Major oxide, minor oxide, trace element, and geochemical data from rocks collected in the Iron Creek area, Tailceetna Mountains B-S Quadrangle. Alaska Division ofGeological & Geophysical Surveys. Major oxide, minor oxide, trace element, and geochemical data from rocks collected in the Iron Creek area, Tailceetna Mountains B-5 Quadrangle 2 sheets, scale 1:63,360., 31 p. Werdon, M. B., Riehie, J. R., Schmidt, 3. M., Newberry, R. J. & Pessel, G. H. (2000b). Preliminary geologic map of the Iron Creek area, Talkeetna Mountains B-5 Quadrangle, Alaska. Alaska Division ofGeological & Geophysical Surveys Preliminary Interpretive Report 2000-7, 1 sheet, scale 1:63,360. Werdon, M. B., Riehle, 3. R., Schmidt, 3. M., Newberry, R. 3. & Pessel, G. H. (2002). Geologic map of the Iron Creek area, Talkeetna Mountains B-5 Quadrangle, Alaska. Alaska Division of Geological & Geophysical Surveys Preliminary Interpretive Report 2002-4, 1 sheet, scale 1:63,360. Wilson, F. H., Dover, J. D., Bradley, D. C., Weber, F. R., Bundtzen, T. K. & Haeussler, P. J. (1998). Geologic map of Central (Interior) Alaska. U. S. Geological Survey Open-File Report 98-133-A http://wrgis.wr.usgs.gov/open-file/of98-133-a!. 304 Wilson, F. H., Labay, K. A., Shew, N. B., Preller, C. C., Mohadjer, S. & Richter, D. H. (2005). Digital Data for the Geology of Wrangell-Saint Elias National Park and Preserve, Alaska U S. Geological Survey Open-File Report 2005-1342 http://pubs.usgs.gov/of72005/l342/. Winider, G. R. (2000). A Geologic Guide to Wrangell-Saint Elias National Park and Preserve, Alaska: A tectonic collage of northbound terranes. US. Geological Survey. Professional Paper 1616, 166 p. Woodsworth, G. J. (ed.) (1991). Evolution and Hydrocarbon Potential ofthe Queen Charlotte Basin, British Columbia. Geological Survey of Canada, Paper 90-10. Yole, R. W. (1963). Early Permian fauna from Vancouver Island, British Columbia. Bulletin ofCanadian Petroleum Geology 11, 13 8-149. Yole, R. W. (1965). A faunal and stratigraphic study of Upper Paleozoic rocks of Vancouver Island, British Columbia. Ph.D., University of British Columbia, 254 pp. Yole, R. W. (1969). Upper Paleozoic stratigraphy of Vancouver Island, British Columbia. Proceedings ofthe Geological Association ofCanada 20, 30-40. Yole, R. W. & Irving, E. (1980). Displacement of Vancouver Island, paleomagnetic evidence from the Karmutsen Formation. Canadian Journal ofEarth Sciences 17, 1210-1228. Yorath, C. J., Sutherland Brown, A. & Massey, N. W. D. (1999). LITHOPROBE, southern Vancouver Island, British Columbia. Geological Survey ofCanada. LITHOPROBE, southern Vancouver Island, British Columbia Bulletin 498, 145 p. 305 APPENDIX H. 4Oj39 ANALYTICAL METHODS Twenty mineral separates (6 hornblende, 1 biotite, and 13 plagioclase) from 19 samples were processed for40Ar/39rgeochronology. The least altered samples were selected for40Ar/39rdating based on thorough petrographic inspection of minerals in the basaltic groundmass. For processing plagioclase separates, the samples were first crushed in a Rocklabs hydraulic piston crusher between WC-plates. During crushing, approximately 400 g of each sample was sieved to isolate the fraction of grains between 250 and 450 j.tm. These grains were then rinsed in deionized water (-10 times). After drying, a hand magnet was used to separate the most magnetic grains. A Frantz magnetic separator was then used to isolate the non-magnetic plagioclase from the groundmass. The remaining grains were then hand-picked to select the least altered, inclusion-free plagioclase grains. A 2 mm-diameter circle of the freshest plagioclase was leached with 3 N cold HC1 in an ultrasonic bath for 30 mm, followed by rinsing with deionized water. Samples were finally rinsed with 1 N HNO3 and deionized water and allowed to dry. For hornblende and biotite, samples were crushed, sieved, washed in deionized water and dried at room temperature. Mineral separates from the 0.25 mm to 0.15 mm size fraction were hand-picked, washed in acetone, and dried. Plagioclase, homblende, and biotite grains were wrapped in aluminum foil and stacked in an irradiation capsule with similar-aged samples and neutron flux monitors (Fish Canyon Tuff sanidine, 28.02 Ma (Renne et al., 1998)). The samples were irradiated at the McMaster Nuclear Reactor in Hamilton, Ontario, for 90 MWH, with a neutron flux of approximately 6x10\u00E2\u0080\u00993neutrons/cm2/s.Analyses (n=57) of 19 neutron flux monitor positions produced errors of <0.5% in the J value. Samples were analyzed at the Noble Gas Laboratory in the Pacific Centre for Isotopic and Geochemical Research at University of British Columbia. The mineral separates were step-heated at incrementally higher powers in the defocused beam of a lOW CO2 laser (New Wave Research MIR1O) until fused. The gas evolved from each step was analyzed by a VG5400 mass spectrometer equipped with an ion-counting electron multiplier. All measurements were corrected for total system blank, mass spectrometer sensitivity, mass discrimination, radioactive decay during and subsequent to 306 irradiation, as well as interfering Ar from atmospheric contamination and the irradiation of Ca, Cl and K (Isotope production ratios:(40Ar/39r)K=0.0302\u00C2\u00B10.00006, (37Ar/9r)ca=1416.4\u00C2\u00B10.5,(36Ar/9r)ca=0.3952\u00C2\u00B10.0004,CaIK=1 .83\u00C2\u00B10.01(37ArCa/9K). The analytical data are presented in Supplementary data file 4 and are summarized in Table 5.3. Age spectra and isochron diagrams for each of the samples are shown in Supplementary data file 4. The plateau and correlation ages were calculated using Isoplot ver.3.09 (Ludwig, 2003). Errors are quoted at the 95% confidence level (2a) and are propagated from all sources except mass spectrometer sensitivity and age of the flux monitor. Following Ludwig (2003), the best statistically-justified plateau and plateau age were picked based on: (1) a well-defined plateau with at least three or more contiguous steps comprising more than 50% of the 39Ar released; (2) probability of fit of the weighted mean age greater than 5%; (3) slope of the error-weighted line through the plateau ages equals zero at 95% confidence; (4) ages of the two outermost steps on a plateau are not significantly different from the weighted-mean plateau age (at 1 .8a, six or more steps only); (5) outermost two steps on either side of a plateau must not have nonzero slopes with the same sign (at 1 .8\u00E2\u0080\u00A2a, nine or more steps only). REFERENCES Ludwig, K. R. (2003). Isoplot 3.09, A Geochronological Toolkit for Microsoft Excel. Berkeley Geochronology Center. Special Publication No. 4. Renne, P. R., Swisher, C. C., ifi, Demo, A. L., Karner, D. B., Owens, T. & DePaolo, D. 3. (1998). Intercalibration of standards, absolute ages and uncertainties in40Ar/39r dating. Chemical Geology 145(1-2), 117-152. 307 Appendix I. Major element (wt\u00C2\u00B0h oxide) and trace element (ppm) abundances of reference materials DNC-1 and BIR-1 from ActLabs whole-rock analyses Sample DNC-1 Meas DNC-1 Meas DNC-1 Meas DNC-1 Meas DNC-1 AVE DNC-1 Cod %RSD Run VLI VL2 VL3 Yukon AVERAGE CERT. VALUE Rock dolerite dolerite dolerite dolerite dolerite dolerite Unnormalized Major Bement Oxides (Weight %): 5103 47.00 47.09 47.02 47.26 47.09 47.04 0.1 1102 0.470 0.48 0.48 0.479 0.48 0.48 0.4 M203 18.23 18.29 18.29 18.28 18.27 18.3 0.1 Fe203* 9.72 9.88 9.93 9.88 9.85 9.93 0.6 MnO 0.140 0.146 0.147 0.146 0.14 0.149 2.0 MgO 10.07 10.24 10.21 10.28 10.20 10.05 1.0 CaO 11.12 11.3 11.2 11.19 11.20 11.27 0.4 Na20 1.90 1.9 1.97 1.91 1.92 1.87 1.9 K20 0.22 0.15 0.11 021 0.17 0.23 20.2 0.07 0.07 0.08 0.05 0.07 0.07 2.6 LOl Total Trace Elements (ppm): La 3.83 3.66 3.93 3.69 3.78 3.80 0.4 Ce 8.2 7.9 8.8 8.0 8.2 11.0 20.3 Pr 1.06 1.08 1.14 1.06 1.08 1.3 12.8 Nd 4.8 4.8 5.0 4.8 4.8 4.9 1.1 Sm 1.40 1.35 1.48 1.35 1.40 1.38 0.8 Eu 0.605 0.628 0.621 0.607 0.62 0.59 3.0 Gd 2.05 2.05 2.06 2.02 2.04 2.00 1.5 Tb 0.4 0.4 0.4 0.4 OA 0.4 2.3 Dy 2.73 2.86 2.8 2.79 2.80 2.7 2.5 Ho 0.64 0.64 0.6 0.64 0.63 0.62 1.2 Er 1.97 1.96 1.94 2.00 1.97 2 1.2 Tm 0.318 0.325 0.318 0.318 0.32 0.38 12.2 Yb 1.96 2.01 2.00 2.00 1.99 2.01 0.7 Lu 0.286 0.294 0.303 0.296 0.29 0.32 5.8 Sc V 139 139 148 139 141 148 3.3 Cr Co Ni Cu Zn Ga 14 14 15 14 14 15 3.3 Ge 1.1 1.1 1.3 1.1 1.2 1.3 7.9 Rb 4 4 4 4 4 5 18.4 Sr 140 143 143 142 142 145 1.5 Y 18 18 18 17 18 18 1.0 Zr 35 31 33 30 32 41 16.9 Nb 1.3 1.4 1.4 1.4 1.4 3.0 52.5 Cs 02 0.2 02 0.2 0.20 0.3 28.4 Ba 104 106 106 105 105 114 5.6 Hf 0.9 0.9 1.0 0.9 0.9 1.0 4.4 Ta 0.07 0.08 0.08 0.1 20.0 Pb Th 0.24 025 0.22 0.24 024 0.20 12.0 U 0.06 0.06 0.05 0.06 0.10 9.6 Abbreviations are: VU, Vancouver Island; AVE. average; Cart., certified; Meas, measured. Analyses were performed at Activation Laboratory (ActLabs). Fe 203* is total iron expressed as Fe 203. AU major elements, Sr. V. and Y by Fused lOP quadrapole (ICP-OES); Cu, Ni, Pb, and Zn by Total dilution lOP; Cs, Ga, Ge, Hf, Nb, Rb, Ta, Th, U, Zr. and REE by Fused-magnetic-sector lOP; Co, Cr, and Sc by INAA. Blanks are below detection limit. See Chapter 2 and Appendix E for sample preparation and analytical methods at Actlabs. 308 aample lR-1 Meas BIR-1 Meas BIR-1 Meas BIR-1 Meas BIR-1 AVE BIR-1 Cert %RSD Run VLI VL2 VL3 Yukon AVERAGE CERT. VALUE Rock basalt basalt basalt basalt basalt basalt Unnorrnalized Major Element Oxides (Weight %): Si02 47.68 47.73 47.7 47.73 47.71 47.77 0.1 Tb2 0.958 0.955 0.959 0.958 0.96 0.96 0.2 \u00E2\u0080\u0098203 15.33 15.37 15.32 15.36 15.35 15.35 0.0Fe03* 11.23 11.24 11.14 11.25 11.22 11.26 0.3 MnO 0.167 0.17 0.167 0.169 0.17 0.171 1.1 MgO 9.64 9.64 9.6 9.67 9.64 9.68 0.3 CaO 13.14 13.21 13.09 13.18 13.16 13.24 0.5 Na20 1.83 1.8 1.82 1.82 1.82 1.75 2.7 <20 0.03 0.03 0.03 0.03 0.03 0.03 0.0 P206 0.03 0.04 0.02 0.05 0.04 0.05 25.0 LOb Total Trace Elements (ppm): La 0.78 0.78 0.7 0.74 0.75 0.62 13.5 Ce 2.0 2.0 2.0 2.0 2.0 2.0 0.0 Pr 0.40 0.4 0.38 0.38 0.39 0.38 1.4 Nd 2.4 2.5 2.4 2.4 2.4 2.5 2.1 Sm 1.11 1.09 1.11 1.08 1.10 1.1 0.1 Eu 0.544 0.576 0.564 0.549 0.56 0.54 2.3 Gd 1.89 1.92 1.84 1.89 1.89 1.85 1.4 Tb 0.4 0.4 0.4 04 0.4 0.4 8.7 Dy 2.66 2.72 2.63 2.66 2.67 2.5 4.6 Ho 0.61 0.62 0.56 0.59 0.59 0.57 3.0 Er 1.79 1.84 1.71 1.77 1.78 1.7 3.2 Tm 0.282 0.292 0.275 0.278 028 0.26 5.7 Yb 1.73 1.76 1.69 1.71 1.72 1.65 3.1 Lu 0.253 0.261 0.253 0.253 0.26 0.26 1.4 Sc V 322 322 315 322 320 313 1.6 Cr Co Ni Cu Zn Ga 17 17 16 16 16 16 1.8 Ge 1.5 1.4 1.5 1.4 1.4 1.5 2.8 Rb Sr 106 107 106 107 107 108 1.0 Y 16 17 16 16 16 16 1.1 Zr 16 13 13 15 14 16 82 Nb 0.4 0.5 0.5 0.5 0.5 0.6 15.6 Cs Ba 8 8 8 8 8 7 9.4 Hf 0.6 0.6 0.6 0.6 0.6 0.6 2.1 Ta 0.04 0.03 0.03 0.04 11.0 Pb Th U 0.02 0.01 001 n 01 0 R 309 APPENDIX J. DESCRIPTION OF SUPPLEMENTARY ELECTRONIC FILES ON CD-ROM *************************** Supplementary data ifies (SD) SD 1- Endnote database for Wrangellia (.enl file) SD 2- Reference list for Wrangellia (.doc file) SD 3- Geochemistry for40Ar/39r samples (.xls file) SD 4-40Ar/39ranalytical data (.xls file) SD 5- Wrangellia ages and biostratigraphy (.xls file) SD 1- Endnote database for Wrangellia (.enl file) This Endnote library is a compilation of approximately 500 references mostly related to research of Wrangellia, or parts of western North America. This .enl file was created with Endnote X1.0.1 (Bld 2682) and should be compatible with older versions of Endnote. SD 2- Reference list for Wrangellia (.doc file) This .doc file is an exported version of the Encinote file in Supplementary data file 1 (SD1) and was created with Microsoft Word 2003. SD 3- Geochemistry for40Ar/39r samples (.xls file) This Microsoft Excel file contains the analytical data for whole-rock analyses for the 19 samples dated by the40Ar/39rdating method. Analyses were performed by ActLabs and the analytical methods are summarized in Chapter 2 and Appendix E. Additional information in this table includes: UTM coordinates, geographic location, lithology, mineral proportion, and texture. This file was created with Microsoft Excel 2003. SD 4-40Ar/39ranalytical data (.xls file) This Microsoft Excel file contains the analytical data for the40Ar/39rdating method for the 20 mineral separates analyzed at the Noble Gas Laboratory in the Pacific Centre for Isotopic and Geochemical Research at University of British Columbia. Age spectra and isochron diagrams for each of the samples are shown along with all the analytical data from incremental step-heating. The analytical methods are summarized in Appendix H. Where multiple analyses of a single sample were made, all the results for each sample are included in a single worksheet in this workbook. This file was created with Microsoft Excel 2003. SD 5- Wrangellia ages and biostratigraphy (.xls file) This Microsoft Excel file contains 3 worksheets that are described separately below. Age (worksheet) The Age worksheet contains 750 isotopic ages for units assigned to Wrangellia. Most of this data was extracted from the CordAge 2004 database and was supplemented by -50 ages not in the CordAge database, mostly from Alaska. Samples of the Coast and Insular Belts are included, based on their location, and caution should be used in deciding 310 whether samples are actually located within Wrangellia. CordAge 2004 (a database of isotopic age determinations for rock units from the Canadian Cordillera) is an MS-Access based database, consisting of the merged datasets of the publically available products BCAge 2004A-1 (released October 2004; Breitsprecher and Mortensen (2004a)) and YukonAge 2004 (released July, 2004; Breitsprecher and Mortensen (2004b)). The compilation contains all reported non-proprietary isotopic age determinations for bedrock units from British Columbia and Yukon Territory respectively: 9321 age determinations from 5997 rock samples, summarizing 778 published articles, theses, reports or unpublished sources. Katrin Breitsprecher offered assistance with extracting this information. The user is referred to Breitsprecher and Mortensen (2004a, 2004b) for information about the rating system of ages (Rd_rating column; column H) and other details. The database should not be cited as the source of the age. Age determinations should be cited to the original source, which is provided in each record of the database. Age_refs (worksheet) This worksheet contains the information of the references for the age compilation listed in the Age worksheet (described above). The ref no column (column A) in the Age_refs worksheet refers to the Ref No column (column L) in the Age worksheet. Biostratigraphy (worksheet) This is a compilation of 75 fossil age determinations from published literature related to Wrangellia. The full reference where each determination is published is included in column B and the region is shown in column E. All of the age ranges are plotted in Figure 5.22 according to the region indicated in column E. The low and high ages for each age range use the age boundaries for epochs and stages from Gradstein et al. (2004). Age boundaries for the Triassic are from Ogg (2004) and revised based on Furin et al. (2006). This file was created with Microsoft Excel 2003. *************************** Supplementary Google Earth files (SGE) SGE 1- Mapped Wrangellia flood basalts (.kmz file) SGE 2- Major faults in Alaska and Yukon (.kml file) SGE 3- Major faults in southwest B.C. (.kml file) SGE 4- Alaska sample locations (.kml file) SGE 5- Yukon sample locations (.kml file) SGE 6- Vancouver Island sample locations (.kml file) SGE 7- Alaska Range photograph locations (.kmz file) SGE 8- Wrangell Mountains photograph locations (.kmz file) SGE 9- Yukon photograph locations (.kml file) SGE 10- Vancouver Island photograph locations (.kmz file) The .kmz and .kml files listed above consist of georeferenced information that are designed to be viewed with the satellite imagery in Google Earth. The Google Earth 311 application is available for free download at http://earth.google.com!. Information about Google Earth can be found at this address. SGE 1- Mapped Wrangellia flood basalts (.kniz file) This is a red transparent layer that shows the distribution of the Wrangellia flood basalts in Alaska, Yukon, and British Columbia. The map was derived from data in Wilson et al. (1998, 2005), Israel (2004), Massey eta!. (2005a, b), and Brew (2007, written comm.). See Chapter 5 for an overview of the Wrangellia flood basalts in these areas. SGE 2- Major faults in Alaska and Yukon (.kml file) SGE 3- Major faults in southwest BC (.kml file) These files show the location of faults in parts of Alaska, Yukon, and BC. These files were filtered from original files found at the following locations: Alaska from http://www.asgdc.state.ak.us/; Yukon from in Israel (2004); BC faults from Massey et a!. (2005a, b). SGE 4- Alaska sample locations (.kml file) SGE 5- Yukon sample locations (.kml file) SGE 6- Vancouver Island sample locations (.kml file) These files show the locations, sample numbers, and flow type of samples of Wrangellia flood basalts collected during this project. SGE 7- Alaska Range photograph locations (.kmz file) SGE 8- Wrangell Mountains photograph locations (.kmz file) SGE 9- Yukon photograph locations (.kml file) SGE 10- Vancouver Island photograph locations (.kmz file) These files contain small versions of georeferenced photographs. Multiple photographs are referenced to a single coordinate. Therefore, in order to view all of the photographs from a single coordinate it is necessary to open the folders for each region in the My Places menu so individual photographs can be selected and viewed. Zooming in also helps to distinguish photograph locations. These photographs and others can be viewed in higher resolution in the Supplementary photo files (described below). *************************** Supplementary photo ifies (SP) SP 1- Alaska Range photographs (.pdf file) SP 2- Wrangell Mountains photographs (.pdf file) SP 3- Ed MacKevett Jr. Wrangell Mountains photographs (.pdf file) SP 4- Yukon photographs (.pdf file) SP 5- Vancouver Island photos (.pdf file) These are .pdf files of field photographs from Alaska, Yukon, and British Columbia. SP3 is a file of photographs taken by Ed MacKevett Jr. The photographs by Ed were taken while mapping parts of the Wrangell Mountains in Alaska in the 1970\u00E2\u0080\u0099s. These photographs were provided by Travis Hudson. I would like to thank Travis Hudson for making these photographs available for the purposes of research. 312 *************************** Also included on the CD-ROM is an electronic file of this complete dissertation: Greene_2008_PhD_dissertation_UBC (.pdf file) REFERENCES Breitsprecher, K. and Mortensen, J.K., 2004a. BCAge 2004A-l - a database of isotopic age determinations for rock units from British Columbia. British Columbia Ministry ofEnergy and Mines, Geological Survey, Open File 2004-3 (Release 3.0), 7757 records, 9.3 Mb. Breitsprecher, K., and Mortensen, J.K. (compilers), 2004b. YukonAge 2004: A database of isotopic age determinations for rock units from Yukon Territory. Yukon Geological Survey, CD-ROM. Brew, D. A. C. (2007, written comm.). Unpublished map showing the distribution of the Late Triassic Wrangellia, Hyd Group, and Perserverance group rocks in southeastern Alaska, scale 1:600,000 (Based on Brew, D. A. (Compiler), Unpublished bedrock geologic map of southeastern Alaska.) Furin, S., Preto, N., Rigo, M., Roghi, G., Gianolla, P., Crowley, J. L. & Bowring, S. A. (2006). High-precision U-Pb zircon age from the Triassic of Italy: Implications for the Triassic time scale and the Camian origin of calcareous nannoplankton and dinosaurs. Geology 34(12), 1009-1012, 10.1 130/g22967a.1. Gradstein, F. M., Ogg, J. G. & Smith, A. G. (eds.) (2004). A Geologic Time Scale 2004. Cambridge University Press 610 pp. Israel, S. (2004). Geology of Southwestern Yukon (1:250 000 scale). Yukon Geological Survey Open File 2004-16. Massey, N. W. D., Maclntyre, D. G., Desjardins, P. J. & Cooney, R. T. (2005a). Digital Geology Map of British Columbia: Tile NM9 Mid Coast, B.C. B.C. Ministry of Energy and Mines Geofile 2005-2. Massey, N. W. D., Maclntyre, D. G., Desjardins, P. J. & Cooney, R. T. (2005b). Digital Geology Map of British Columbia: Tile NM1O Southwest B.C. B.C. Ministry of Energy and Mines Geofile 2005-3. Ogg, 3. G. (2004). The Triassic Period. In: Gradstein, F. M., Ogg, J. G. & Smith, A. G. (eds.) A Geologic Time Scale 2004. Cambridge University Press: Cambridge. pp. 271-306. Wilson, F. H., Dover, J. D., Bradley, D. C., Weber, F. R., Bundtzen, T. K. & Haeussler, P. J. (1998). Geologic map of Central (Interior) Alaska. U S. Geological Survey Open-File Report 98-133-A http://wrgis.wr.usgs.gov/open-file/of98-1 33-a!. Wilson, F. H., Labay, K. A., Shew, N. B., Preller, C. C., Mohadjer, S. & Richter, D. H. (2005). Digital Data for the Geology of Wrangell-Saint Elias National Park and Preserve, Alaska U S. Geological Survey Open-File Report 2005-1342, http ://pubs.usgs.gov/of/2005/1 342/. 313"@en . "Thesis/Dissertation"@en . "2008-11"@en . "10.14288/1.0052552"@en . "eng"@en . "Geological Sciences"@en . "Vancouver : University of British Columbia Library"@en . "University of British Columbia"@en . "Attribution-NonCommercial-NoDerivatives 4.0 International"@en . "http://creativecommons.org/licenses/by-nc-nd/4.0/"@en . "Graduate"@en . "Wrangellia flood basalts in Alaska, Yukon, and British Columbia : exploring the growth and magmatic history of a late Triassic oceanic plateau"@en . "Text"@en . "http://hdl.handle.net/2429/5282"@en .