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Wrangellia flood basalts in Alaska, Yukon, and British Columbia : exploring the growth and magmatic history.. 2008

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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 ©Andrew 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 (—‘380-355 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 (‘—1500°C), which is consistent with an origin from a mantle plume head. Wrangellia basalts extend —-‘450 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 ‘-.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” 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’s interior 4 Figure 1.3 “Pin-pricking” 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 —1000 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’s 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 Paraná-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±1 Ma) (201±1 Ma) (56±1 Ma; 61±2 Ma) wrangellia Columbia River • , 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±1 Ma) (65.5±0.5 Ma) (259±3 Ma) (250±1 Ma) Ethiopia Deccan Emeishan Siberia a Ontong Java-H (122±1 Ma) Caribbean Paraná-Etendeka (123±1.5 Ma) (89±1 Ma) (133±1 Ma) 3 Figure 1.2 Schematic diagrams of mantle plumes in the Earth’s interior. (a) A view of the deep-mantle complexities beneath the central Pacific Ocean showing the Hawaiian hotspot, a D” 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’s 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 “pin-pricking the elephant” (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 “Pin-pricking” 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 ‘eye structure’ (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; Révillon 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’s 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’s 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 . . . • . 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) Paraná-Etendeka (133 Ma)————— End Valanginian 150 Karoo-Ferrar (183 Ma)— i— End Pliensbachian 200 Central Atlantic Magmatic Province (201 Ma) —•— End Triassic Wrangellia (230 Ma) ? —1 i—? End Camian 250 Siberian Traps (250 Ma) End Permian Emeishan Traps (259 Ma)— — 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’s magnetic field does not reverse polarity for abnormally long periods of time (30-50 Myr), and the timing of mantle plumes. The -‘50 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 “killer” 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’s 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. 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Wrangellia Flood Basalts: Exploring the architecture and composition of an accreted oceanic plateau. Geological Association ofCanada 2006 Annual Meeting, Montreal, Quebec. Greene, A. R., Scoates, J. S., Nixon, G. T. & Weis, D. (2006c). 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. 20 Greene, A. R., Scoates, J. S. & Weis, D. (2005a). Wrangellia Terrane in B.C. and Yukon: Where are its Ni-Cu-PGE deposits? Cordilleran Mineral Roundup Expo, Vancouver, BC. Greene, A. R., Scoates, J. S. & Weis, D. (2005b). 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. (2008a). Wrangellia flood basalts in Alaska: A record of plume-lithosphere interaction in a Late Triassic accreted oceanic plateau. Goldschmidt 2008 Conference, Vancouver, BC. Greene, A. R., Scoates, J. S., Weis, D. & Israel, S. (2005c). 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., Weis, D., Israel, S., Nixon, G. T. & Kieffer, B. (2007a). Geochemistry and geochronology of the Wrangellia Flood Basalts in British Columbia, Yukon, and Alaska. Cordilleran Mineral Roundup Expo, Vancouver, BC. Greene, A. R., Scoates, J. S., Weis, D. & Nixon, G. T. (200Th). Significance of picritic and tholeiitic lavas within Wrangellia Flood Basalts on Vancouver Island for the melting history and magmatic evolution of a major oceanic plateau. AGU 2007 Fall Meeting San Francisco, CA. Abstract V33A-1 153 Greene, A. R., Scoates, J. S., Weis, D. & Nixon, G. T. (2008b). Picritic and tholeiitic lavas within Wrangellia Flood Basalts on Vancouver Island for the melting history and magmatic evolution of a major oceanic plateau. Cordilleran Tectonics Workshop, Vancouver, BC. Greene, A. R., Weis, D. & Scoates, J. S. (2007c). Wrangellia Flood Basalts: Exploring the architecture and composition of an accreted oceanic plateau. Geological Association ofCanada, NUNA 2007 Conference: The Pulse ofthe Earth and Planetary Evolution, Sudbury, Ontario. Griffiths, R. W. & Campbell, I. H. (1990). Stirring and structure in mantle starting plumes. Earth and Planetary Science Letters 99, 66-78. Hilihouse, J. W. (1977). 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G. (2002). Basement geochemistry and geochronology of Central Malaita, Solomon Islands, with implications for the origin and evolution of the Ontong Java Plateau. Journal ofPefrology 43(3), 449-484. 24 Weis, D. & Frey, F. A. (2002). Submarine basalts of the northern Kerguelen plateau: interaction between the Kerguelen plume and the southeast Indian Ridge revealed at ODP Site 1140. Journal ofPetrology 43(7), 1287-1309. Wignall, P. B. (2001). Large igneous provinces and mass extinctions. Earth-Science Reviews 53, 1-33. 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-al. 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/. Yole, R. W. & Irving, E. (1980). Displacement of Vancouver Island, paleomagnetic evidence from the Karmutsen Formation. 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 ‘A 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 (—‘6 km on Vancouver Island; —‘3.5 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 (—23 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 (—P2-5 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—Late Triassic Karmutsen Formation Paleozoic—Middle Triassic Sicker and Buttle Lake Groups Karmutsen Range (Alice—Nimpkish 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 ± 2.6 Ma (Parrish & McNicoll, 1992). Two unpublished 206Pb/38Ubaddeleyite ages, also from a gabbro on southern Vancouver Island, are 226.8 ± 0.5 Ma (5 fractions) and 228.4 ± 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 —‘2600 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—bedded siliceous limestone and] wacke, with minor volcanics QUATSINO FORMATION massive to well—bedded micritic and locally bioclastic limestone intra—Karmutsen limestone lenses KARMUTSEN FORMATION subaerial flows with minor pillow basalt and hyaloclastite (—3000 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 (—2500 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— — 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 • pillow lava o shale or chert river — park boundary I subaerialbasalt and rhyolite flows, breccia, and tuff minor pillow lava, hyaloclastite, debris—flow and epiclastic deposits — plag—megacrystic flows -interbedded volcaniclastic and sedimentary rocks ‘U a I It -J PARSON BAY FORMATION well—bedded 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 — 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 (--‘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 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” 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 plag<lmm,cpx<2mm 25 3 1 plag<1 mm, CPX 01k <3 mm, ox <0.2mm intergranular, plag-phynic intergranular, plag-phynic subophitic intergranular, lntergrowths intergranular, intersertal 30 5 3 ox <0.5 mm, plag laths <4 mm 15 10 3 plag<3mm,oxo.5-1.5mm 25 5 1 plag laths <1 mm, cpx 01k <2mm, ox 0.5-1 mm 30 3 plag <3 mm, cpx <5 mm, cpx-plag intergrowths 1 1 plag <1 mm, f.g., mottled asample number: last digit year, month, day, initial, sample station (except 93G171).bMA, Mount Arrowsmith; SL, Schoen Lake; KR, Karmutsen Range. °PIL, pillow; BRE, breccia; PLO, flow; SIL,sill; GAB, gabbro.dTHOL, tholeiitic basalt; PlC, picrite; HI-MG, high-MgO basalt; CG,coarse-grained; MIN, mineralized sill. °glomero, glomeroporphyrltic.OIivine modes for picrites based on area calculations from scans (see Olivine accumulation in picritic lavas section and Figure 2.13).9Visual alteration index based primarily on degree of plagioclase alteration and presence of secondary minerals (1, least altered; 3, most altered). Plagioclase phenocrysts commonly altered to albite, pumpellyite, and chlorite; divine is altered to talc, tremolite, and clinochiore (determined using the Rietveld method of X-ray powder diffraction); clinopyroxene is unaltered; Fe-Ti oxide commonly replaced by sphene.’glcr, glomerocrysts; f.g., fine-grained; c.g., coarse grained, oik, oikocryst; chad, chadacryst; enc, enclosing, swtl, swallow-tail. Mineral abbreviations: 01, olivine pseudomorphs; plag, plagioclase; cpx, clinopyroxene; ox, oxides (includes ilmenite + titanomagnetite). 39 samples) were crushed (400 g) into small pieces <2 mm in diameter in a Rocklabs hydraulic piston crusher between WC-plates to minimize contamination. 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. ActLabs Analytical Methods The major- and trace-element compositions of the whole rock powders were determined at Activation Laboratories Ltd. (Actlabs) in Ancaster, Ontario. Analytical techniques and detection limits are also available from Actlabs (http://www.actlabs.com/methsub_code4ere.htm). The particular analytical method for each of the elements analyzed is indicated in Table 2.2. 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 (30 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- 1, 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 1NAA analyses, 1.5-2.5 g of sample was weighed into small polyethylene vials and irradiated with control international reference material CANIVIET WMS- 1 and NiCr flux wires at a thermal neutron flux of 7 x 1012 n cm2s 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 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. In-lab standards or certified reference materials 40 (e.g. W-2, BIR- 1, DNC- 1) were used for quality control. A total of 15 blind duplicates were analyzed to assess reproducibility and the results were within analytical error. University ofMassachussetts XRF Analytical Methods Fourteen sample duplicate powders (high-MgO basalts and picrites) were also 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 using a Rh X-ray tube. Loss-on-ignition (LOl) and ferrous iron measurements were made as described by Rhodes and younger (2004). Precision and accuracy estimates for the data are described by Rhodes (1996) and Rhodes and younger (2004). Results for each sample are the average of two separate analyses (shown in Appendix B). PCIGR Analytical Methods A subset of 19 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 (UBC). Samples were selected from the 63 samples analyzed for whole-rock chemistry at ActLabs, based on major- and trace-element chemistry, alteration (low LOT and petrographic alteration index), and sample location. 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 (—100 mg) were weighed in 7 mL screw-top Savillex® beakers and dissolved in 1 mL --‘l4N HNO3 and 5 mL 48% HF on a hotplate for 48 hours at 130°C 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 al. (2006) within 24 hours of 41 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% HNO3between 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. Results for PCIGR trace- element analyses are shown in Appendix C. Sample digestion for purification of Sr, Nd , Hf, and Pb for column chemistry began by weighing each sample powder (400-500 mg) prior to leaching. All samples were leached with 6N HC1 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 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 l4N HNO3 and 10 mL 48% HF on a hotplate for 48 hours at 130°C 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 other 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 a!. (2007). Sr 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 (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‘46NdJ1d= 0.7219. Each sample was then normalized using the barrel average of the reference material relative to the values of ‘43Nd/’4d= 0.511858 and87Sr/6r=0.710248 (Weis et al., 2006). During the course of 42 the Vancouver Island analyses, the La Jolla Nd standard gave an average value of 0.511856 ± 6 (n=7) and NBS987 standard gave an average of 0.710240 ± 8 (n=1 1) (2a error is reported as times 106).‘47SmJ’Nd ratio errors are approximately —1.5%, or —0.006. United States Geological Survey (USGS) reference material BHVO-2 was processed with the samples and yielded Sr and Nd isotopic ratios of 0.703460 ± 7 and 0.512978 ± 6, respectively. These are in agreement with the published values of 0.703479 ± 20 and 0.5 12984 ± 11, 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 Plasma at the PCIGR is described in Weis et al. (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, repectively. All sample solutions were analyzed with approximately the same Pb/Tl ratio (-.4) as the reference material NIST 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 samples were run, analyses of the SRM 981 Pb reference material gave values of206Pb/4=16.9403 ± 22, 207Pb/4=15.4958 ± 23, and208Pb/4b=36.7131 ± 64 (n=6 1; 2a error is reported as times 1 0); these values are in excellent agreement with reported TIMS triple-spike values of Galer & Abouchami (1998). Fractionation-corrected Pb isotopic ratios were further corrected by the sample- standard bracketing method or the in-In correction method described by White et a!. (2000) and Blichert-Toft eta!. (2003). Leached powders of USGS reference material BHVO-2 yielded Pb isotopic ratios of206Pb/4= 18.6454 ± 8, 207Pb/4= 15.4910 ± 5, and 208Pb/4= 38.2225 ± 14 and BCR-2 yielded206Pb/4= 18.8046 ± 6, 207Pb/4 15.6251 ± 8, and208Pb/4= 38.8349 ± 6 (2a error is reported as times 10). These values are in agreement with leached residues of BITVO-2 and BCR-2 from Weis et a!., (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 43 mass 172 to allow for interference correction to masses 174 and 176. Hf isotopic ratios were normalized internally for mass fractionation to a‘79Hf/’7fratio 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‘76Hf/’7fratio for JMC 475 of 0.282160. During the course of analyses, the Hf standard JMC-475 gave an average value 0.282 167 ± 9 (n=l30). USGS reference materials BCR-2 and BHVO-2 were processed with the samples and yielded Hf isotopic ratios of 0.282867 ± 5 and 0.283100± 5, respectively. Published values for BCR-2 and BHVO-2 are 0.28287 1 ± 7 and 0.283104 ± 8, respectively (Weis et a!., 2007). WHOLE-ROCK CHEMISTRY Major- and trace-element compositions The most abundant type of lava in the Karmutsen Formation is tholeiitic basalt with a restricted range of major- and trace-element compositions. The tholeiitic basalts have similar compositions to the coarse-grained mafic rocks, and both groups are distinct from the picrites and high-MgO basalts (Fig. 2.6). The tholeiitic basalts have lower MgO (5.7-7.7 wt % MgO) and higher Ti02 (1.4-2.2 wt % Ti02)than the picrites (13.0-19.8 wt % MgO, 0.5-0.7 wt % Ti02)and high-MgO basalts (9.1-11.6 wt % MgO, 0.5-0.8 wt % Ti02) (Fig. 2.6; Table 2.2). Almost all data plot within the tholeiitic field in a total alkalis versus silica plot, although there has been substantial K-loss in most samples from the Karmutsen Formation (Fig. 2.7), and the tholeiitic basalts generally have higher Si02, Na20+K,CaO, and FeOT (total iron expressed as FeO) than the picrites. The tholeiitic basalts also have noticeably lower LOl (mean 1.72 ± 1.3 wt %) than the picrites (mean 5.39 ± 0.8 wt %) and high-MgO basalts (mean 3.84 ± 1.5 wt %; Fig. 2.6) reflecting the presence of abundant altered olivine phenocrysts in the latter groups. Ni concentrations are significantly higher for the picrites (339-755 ppm) and high-MgO basalts (122-551 ppm) than the tholeiitic basalts (58-125 ppm, except for one anomalous sample) and coarse-grained rocks (70-213 ppm) (Fig. 2.6; Table 2.2). An anomalous tholeiitic pillowed flow (3 samples; outlier in Tables 2.1 and 2.2), from near the picrite type locality at Keogh Lake, and a mineralized sill (disseminated sulfide), from the basal 44 00 06 (C) :00 I I I I I 14 5 5 10 15 20 MgO (wt%) 0 x . ‘cc • • . + .()....I....I....I Figure 2.6 Whole-rock major-element, Ni, and LOT variation diagrams for the Karmutsen Formation. New samples from this study (Table 2.2) are shown by symbols with black outlines (see legend). Previously- published analyses are shown in small gray symbols without black outlines. 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. References for the 322 compiled analyses for the Karmutsen Formation are G. Nixon (unpublished data), Barker et a!. (1989), Kuniyoshi (1972), Lassiter eta!. (1995), Massey (1995a; 1995b), Muller eta!. (1974); Surdam (1967), and Yorath et a!. (1999). Note that the compiled data set has not been filtered; many of the samples with high Si02 (>52 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 - ,. • X Pillowed flow + Mineralized sill ., * Karmutsen Form.. :. 0 0 • (compiled data) + .111.111.1.. I.... 40 45 II 50 Si02 (wt%) II 55 Ni (ppm) —(b) II liii 600 - 0.5 0.0 60 15 14 13 12 11 10 400 - 11111111111 I I IIIIIIIIIII - 0 -CaO 6* -(wt%) +•. •. — 0 A - • . 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 — I I • i—I—I I I I I 2 FeO(T) • *(wt%) :fE (f) - 11111 5 20 25 5 10 15 MgO (wt LA A1203 (w •• l. . e 25 0 •• * 6* .. €0 AA 0O*OA AA . •* I — .l. — — — I — 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’ 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’i 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‘(q>jpu;ossojAjjids)‘-jj’-j ijuouoi.ijosoojjp‘suoidpuiiioiipQJ1u-33fluoquo3uSip ijo•(c661)mispunouoaowoijStLJUAUO!z!1UUUOUliv•SUiUpjTJUTUW jotpwosjdmUsiojstuUd3uwp-ooI3pzquuou-uwA!w!.Idai(j)pu‘(p)‘(q)•pusjJA1LOOUEA uosaropjgWUW,jotpwogsjdwUsio;sudpzquuou-upuotpai()pu‘(a) ‘to)uouuousnuujiojsuoirnIouoo1um-jqdmoouipOpu3J3pOJ-OuL1a.In,I rn13AOH1(53qlpD03‘1IHJZWSISPHd3)1ELqN)flqjq5)flWL13OH1(53qIp9553W5WdPN143), ••II•••••••••,•,,•,•I•••.••••••••••1 (1)() II!SP2Z!TeJ3Ui+ MOp3MOfldX ieseq!w3Ior1e 3)OJiJewp3u1e16-3s13o)0 eseqOw-1 ’H03 !J’IdV 0L30L qWSMoJJUflOfiJP!WSMOIJVUflOjAJ till11111111111111 ,0L ‘I1111111 —00L fl1cVJ3AoHXaqIpDn31;H,zwsJspNJda)e1eLqN)1flqLeaqus)flCIJWI13OH1(53(Ii.p9fl3W5WdPN143)3 I•p•ipi•••IIIIII I I(D) If, 3 3 __________________________________ liii 01(•t 001 •ee,uq)S liii111111 ____________________________________________ OOL fl53),13AoHXoqIp9flJ1.JHJZU.JSiSPNd3)l1qN)IflqjqSDfl(CI),Wj13OH1(53(Ijp9fl3WWdPN143)s, •,••,••,1111111liii111110•p•p•i•••••••• (oo’3psirSSSIeS) e6eJ3Aeoowpesidea • 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 —20 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 ± 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 ± 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 ± 0.3; Mount Arrowsmith LaJYbCN=1.9-2.5; mean 2.2 ± 0.4), and the coarse-grained mafic rocks have similar REE patterns to the tholeiitic basalts (LaJYbCN=2.l-2.9; mean 2.5 ± 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 • • . . , 1.6 Nb(ppm) Nb/La 1.2- 8o 10 . - 0.8 - oo ‘ 5 Picrite • High-MgO basalt 04 - 0 Coarse-grained • 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 • I • I • 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— +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 ± 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 ‘ 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 ‘ • i • 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 • 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)‘43NW1dvs. ‘47Sm/’Nd. (b) 176Hf/7fvs. 176LuJ’77Hf. The slope of the best-fit line for all samples corresponds to an age of 241 ± 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•1J• 143Nd/ a A Picrite High-MgO basalt X 0 Coarse-grained • tholeütic basalt X Pillowed flow (a) i L_ I I I I 176Hf/7f - Age=241 ± l5Ma - (0=+9.90± 0 —I I I I i LOl (wt 96) t AA 4723A2 dD (d) ê “Sri Sr ‘ I ‘ I I I ‘ E (230Ma) 0 Hf 0 • 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 • ‘ i ‘ i • i ‘ i 39.0 111111111111111 4723A2 - (230 Ma) 39.2 - 38.6 — _____________ 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 ‘ ‘ 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/°4. (b) Initial 207Pb/4 vs. 206PbP°4b. Age correction to 230 Ma. (c) Measured 208Pb/4 vs. 206PbP°b. (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 • I • I • I•I•1•I•I• 4723A2 • 207pb/4 0(230 Ma) • - — 4723A13 — — _‘><•s4723A3 4722A4 ‘ Picrite <>High-MgO basalt 0 Coarse-grained Tholeiitic basalt (b) Pillowed flow LOl (wt %) A 4723A2 0 0 0 (d) • 20Pb’04 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 /° 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 ° 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’76 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’ U Th Pb 2° P b fl ° 4P b 2m 2°7P b / 0 4b 2m 2w P b / o4P b 2m 2U / ° 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 ° 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 (Révillon 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® 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’Hara, 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 — 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’Hara, 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’Hara, 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 — Dashed fines = initial melting pressure — 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(°C) 1354 1375 1397 1369 1374 1382 1415 1422 Potential T(°C) 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 (—‘1 490°C) indicate melting from anomalously hot mantle (100-200°C) compared to ambient mantle that produces mid-ocean ridge basalt (MORB; ‘--1280-1400°C; 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 —17 wt % MgO and —l0 wt % CaO (Table 2.6), and these magmas would have formed by 27% melting with mantle potential temperatures of —1500°C, and first melting occurring at —-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°4b. 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 •S 14 12 10 8 6 4 2 0 I. I • • ft-’ 4• .1 . -2 0.702 II I liii 16 Indian__. EHfinitiah • Indian MORB 6 .F • • East Pacific Rise • — Caribbean Plateau 4 £ • Ontong Java Hawaii 2 I • Karmutsen tholeiitic basalt 0 iOlB array • Karmutsen high-MgO lava i\ 2(b) I I I I I I I I I I I I I E Nd(t1ao) • 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 •• I 0 • I.. • I • East Pacific Rise Juan de Fuca/Gorda - • Explorer Ridge • Caribbean Plateau • Ontong Java • Hawaii o Karmutsen Formation (measured) — • Karmutsen Formation (initial) • 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 £Hf 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 £Hf 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 —l55 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 — • Tholelitic basalt 11 - - -15 - —155 m.y.—+I j.-’ / II - Hf 10 hypothetical I actual 230 Ma 0 • • ‘ 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!’7Hffor 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’ 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 (—0.9-2.5 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 • I • I • 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 ‘ ‘ ‘ ‘ ‘ o’Ie,it’ic lva 14 13 12 xgtlherz+4xspiherz -— DyIYb 1.1Spinel lherzolite + garnet lherzolite melt 1.0 .e(O.7DM÷3P 0.9 (C) L Ce Pr Nd P4Sm Eu Gd Tb Dy • ‘ • os Ho Er TmYb Lu 0. 1.5 Tholeiitic lavas 1 1 I 1 ‘ x gnt lherz ÷ 3x sp lherz - - 1% melting 1.4 . 25% 20% 10% ____ 13• !eltin - Spinellherzolite+ 25 .....J_ 12 .garnetlherzolitemelts %meltinp ratio Dy/Yb 1.1 xgntlherz÷4xsplherz 1.0 • A Picrite 0 High-MgO basalt 0.7DM+O.3PM) 0.9 - 0 Coarse-g rained(e) • • I I I I I I • • . • Tholeiitic basaltA. •0.8 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°C) 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 —25-30 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 —P15-20 wt % of the liquid mass has fractionated, and the residual magma contains 9-10 wt % MgO (Fig. 2.16). At 1235-1225°C, plagioclase begins crystallizing and between 1235 and 1190°C 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 (‘-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 —6 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,’isiusisnoip(qusiojuod-puS11.OZH %O!1I!IP5UOJP1(11I!‘II)1°1N1OPIt1O1‘OH%opuOHOU1{IAp5511SuA iqjjjoainsssidvsjnsaz‘W’!piIL10E6pu£VEZLssidwusiOjswuwi(iuiudpsuwsssqj ijutuos‘EIvlLtsjdwsoj(/f)ivissqz.Is-{JOsnbimps&IiJspouIsiwoi,jssussuuj stio;uwui&Iuuiudpswmtssjouonisodwoosqusn‘(sjaipi)uoiiisodmos&iiissuoiojuMo1s smsijnsaisi‘i(iupiosuornsoduioouuussnuujopaidwoosprnbJJrnlpsa1josuornsodwoousuisjs -iofwsi(p)puu‘(o)‘(q)‘(s)uouuojussnuustpwoSA1siiiisJopuo!.Is!dopsidwoo(c661 ‘i°s‘oslon{9)S1iNWOSU5UI5S1018WJOjSflUSSJU!j5pOwRO13uZqSAJO1’°!3°pJAUO{91Z‘flLI (%M)06W(%IM)OW OE8L9LVLLOL89V0O8L9tVtLOL89V0 (p) V V V V 9 L 8 6 OL LL EL (%IM)Qe3, III (3) VV V •!1!!IOq.1.@ seq06W-L1 !H(%M)S0Z1y 1!J)!dV IIIIIIIII (%M)06W OZ8L9tVLtOL89VZ0 (%IM)06W 0St9LVLLOL891’0 (q) 0•0 06W c.0 0• (%)p!nb!Iienpsi OLOOSO0009OL c• (8) 0906OOL 0ZH%1MO(J) / 0611 0HOU’ 0ZL ed6uIIsAjpue•• PiflbIIIeflP!S9° c. -i 0OLL0.E 0056 (%)501j.SE I IO•V 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 —20,000 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 (—4500°C) 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. Greene was supported by a University Graduate Fellowship at UBC. REFERENCES Babbs, T. L. (1997). 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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 ‘A 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); Paraná-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 ± 5.2, 232.8 ± 11.5, and 232.4 ± 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 ‘Daonella-beds’ (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 —4000 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 —‘500 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 (—4 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 —1mm long with —5:1 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’ in panel b, adapted from Silberling eta!. (1981). 101 Table 3.1 Summaiyofpefrographic characteristics and phenoc,yst proportions of Nikolai basalts in Alaska Sample’ Areab Flow’ Groupd Texture’ 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 57’15A1 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 ôi 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 <I mm, cpx <I mm, ox <0.5mm 15% vesicles, tiny plag needles <0.3 mm plag needles <0.5 mm vesicular, plag needles <0.5 mm, cpx<0.5 mm f.g., plag needles <0.5mm plag laths <1 mm, cpx <1 mm few ol<1.5 mm few oI<1 mm cpx <2 mm enc plag <1 mm highly altered, glomero, aligned plag cpx <1 mm enc tiny plag needles cpx <2 mm filling Interstices calcite and qtz <3 mm, plag needles <1 mm 01 <2 mm, swfl plag <1 mm blocky ox <0.5 mm elonoate hbl (<1.5 mmt with atz cement ‘Sample number: last digit year, month, day, initial, sample station (except 93G171). ‘WM, Wrangell Mountains; TA, Tangle Lake; GG, Glacier Gap Lake; CL, Clearwater Mountains; RC, Rainy Creek; °PIL, pillow; BRE, breccia; FLO, flow; SIL,sill; GAB, gabbro; TUP, tuft; DIK, dike. dbasal basal flow-conglomerate; RC, Rainy Creek; RCPIC, Rainy Creek picrite; CWPIC, Cleaiwater picrite. °glomero, glomeroporphyritic.Modal proportions were visually estimated. alteration index based primarily on degree of plagioclase alteration and presence of secondary minerals (1, least altered; 3, most altered). Plagioclase is commonly replaced by epidote, chlorite, sericite, clmnozoisite, and clay minerals and zoning is obscured in many samples due to albitization; clinopyroxene is mosty unaltered compared to plagiodase; Pe-Ti oxides are altered to sphene and leucoxene minerals; amygdules (<20 volume %) are prevalent throughout most subaerial flows and are usually filled with quartz, calcite, epidote, prehnite and pumpellyite. hglcr, glomerocrysts; f.g., fine-grained; c.g., coarse-grained, oik, swtl, swallow-tail. Mineral abbreviations: 01, olivine pseudomorphs; plag, plagioclase; cpx, clmnopyroxene; ox, oxides (includes ilmenite + titanomagnetite). High- and low-titanium basalte are listed from highest (top) to lowest (bottom) TiO2 contents. 102 Figure 3.9 Representative photomicrographs of Nikolai basalts, Alaska. (a) Aphyric pillow basalt from the Tangle Lake (West) in the Amphitheater Mountains in cross-polarized transmitted light (sample 5810A10). (b) Hyaloclastite from near the top of the submarine section at Tangle Lake (West) in plane-polarized transmitted light (sample 5810A9, no chemistry). (c) Hyaloclastite interbedded with sills from Tangle Lake (West) in plane-polarized transmitted light (sample 581 OA3b, no chemistry). (d) Glomeroporphyritic pillow basalt with abundant amygdules (—10 vol %) from Tangle Lake (West) in cross-polarized transmitted light (sample 5810A6). (e) Variolitic picritic pillow basalt with olivine pseudomorphs (<2 mm) and acicular plagioclase with swallow-tail terminations from the Clearwater Mountains, in plane-polarized transmitted light (sample 5802A4). (f) Rainy Creek picritic tuff with pseudomorphed olivine phenocyrsts, undeformed recrystallized angular and cuspate shards, and lithic particles in a fine-grained matrix, in plane-polarized transmitted light (sample 5808A2). Some clasts appear to be clusters of grains, some of which contain thin green rims. 103 trachytic-like texture and abundant oxides. Picrite from the Clearwater Mountains preserve spherulitic textures with bow-tie and fan-shaped bundles of acicular plagioclase, typically with swallow-tail terminations. Olivine (<2 mm) pseudomorphed by secondary minerals comprises —20 vol % of the picrite and plagioclase commonly radiates from the edges of olivine pseudomorphs and is intergrown with clinopyroxene. The sample preparation and analytical methods for whole-rock chemistry, major elements, trace elements, Sr, Nd, Hf, and Pb isotopes are described in Appendix D. WHOLE-ROCK CHEMISTRY Major- and trace-element compositions The most noteworthy feature of the major-element chemistry of the Nikolai Formation is two clearly distinguishable groups of high- and low-titanium basalt (Fig. 3.10). The low-titanium basalts range from 0.4 to 1.2 wt % Ti02 and the high-titanium basalts range from 1.6 to 2.4 wt % Ti02 (Fig. 3.10; Table 3.2). The high-titanium basalts have a limited range in MgO (5.7-7.9 wt % MgO, except for one plagioclase-rich flow with 4.8 wt % MgO) and Si02 (49.2-52.1 wt %), whereas the low-titanium basalts extend to higher MgO and have a significantly larger range in MgO (6.4-12.0 wt %) and Si02 (46.7-52.2 wt %). Almost all of the Nikolai basalts in Alaska fall within the tholeiitic field in a total alkalis versus silica plot, with low-titanium basalts generally having lower total alkalis than the high-titanium basalts. The low-titanium basalts exhibit broadly decreasing trends of Ti02,FeOT, and Na20with increasing MgO (Fig. 3.10) and have higher loss on ignition (LOl; mean LOI=2.7 ± 1.3 wt %) than the high-titanium basalts (mean LOI=1.9 ± 1.5 wt %; Table 3.2). Low-titanium basalts with >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 •_-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 — lillIllIll 111111111 —.— . • x • high-titanium basalt 7 Na20 - T102 • high-titanium sill • + . 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 •00 i.o 2- 1- 0 a) ‘(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 • (wt%) • (C) II 20 19 18 17 16 15 14 A1203 x • (wt%) x ‘III’ 11111 III 0 0 C C o 000 • •o ° (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 • i 200 0.Ni(ppm) A o C ;(d) •J 100 6 8 10 12 14 16 18 MgO (wt%) Na20 X (wt%) 0 , . 1•1111• 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 ± 0.5, except one LREE-enriched sample) with flat, parallel heavy REE (HREE) segments (mean Dy/YbCN=l.0 ± 0.2; Fig. 3.11). The high-titanium basalts form a tight range of parallel LREE-enriched patterns (mean LaJYbCN=2.3 ± 0.9) with higher REE abundances (mean YbCN= 16.1 ± 6.4) than the low-titanium basalts (mean Ybc=1 1.0 ± 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 —I--— 3.0 150 ‘ 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 • high-titanium basalt Nb/La • high-titanium sill • D low-titanium basalt 0 low-titanium sill A Clearwaterpicrite 0 RainyCreekpicrite • •Ij X basal flow 0 Yukon high-Ti basalt Q C Yukon low-Ti basalt 0 -0-- — 0 x • (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 • C I 300 C • C C 200 4. QD 0C C o $ 0 • 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 ± 115 0.2835 9 , ,‘ I I I I I I 571 5A5 0,2834 “6Hf/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 ________