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Complex P-T-t-d history of supracrustal rocks of the Metamorphic Internal zone of the southern Wopmay… Smar, Leanne Marie 2015

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  COMPLEX P-T-T-D HISTORY OF SUPRACRUSTAL ROCKS OF THE METAMORPHIC  INTERNAL ZONE OF THE SOUTHERN WOPMAY OROGEN, NT  by LEANNE MARIE SMAR B.Sc. Hons, Carleton University, 2008   A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF  THE REQUIREMENTS FOR THE DEGREE OF  MASTER OF SCIENCE in  THE FACULTY OF GRADUATE AND POSTDOCTORAL STUDIES (Geological Sciences)   THE UNIVERSITY OF BRITISH COLUMBIA (Vancouver)   April 2015 © Leanne Marie Smar, 2015  ii  ABSTRACT An abundance of Paleoproterozoic orogenic belts are distributed globally, as peripheral terranes accreted against the margins of Archean cratons. The abundance of these terranes indicates a period of tectonic activity between 2.1 and 1.8 Ga. The study of such belts has offered insight into the nature and rate of tectonic activity in the Paleoproterozoic. Analysis of thermo-tectonic events, paired with the collection of geochronological data has led to the discovery of a supercontinent that existed before the formation of Rodinia, called Columbia. The 1.9 Ga Wopmay Orogen in northern Canada is a Paleoproterozoic belt, comprising a complex amalgamation of tectonic elements, which is thought to have formed during the assembly of Columbia. The advent of new geochronological data has demanded that the current P-T-t-d (Pressure-Temperature-time-deformation) model of the Wopmay Orogen be reconsidered. To reconstruct the evolution of an orogenic belt, structural analysis of overprinting kinetic fabrics, thermodynamic analysis of metamorphic mineral assemblages, and the determination of absolute timing of thermo-tectonic events are required. Two map areas were chosen to conduct an analysis of the kinetic and metamorphic evolution of the southern Wopmay Orogen, a region of the belt that has historically been under-studied. The Brown Water and the Little Crapeau Lake areas are situated south of 65˚N in the Metamorphic Internal zone of the orogen, and are bounded to the east and west by the Archean Slave craton and the ca. 1850 Ma Great Bear Magmatic zone, respectively. The areas comprise a sequence of pelitic to semi-pelitic schists and gneisses that overlie parautochthonous Archean basement. Rocks in both areas show evidence for five generations of overprinting fabrics, S 1C-S5C at Little Crapeau Lake, and S 1B-S5B at Brown Water Lake. Conditions of peak metamorphism were attained synchronous with the intrusion of the ca. 1877 Ma Little Crapeau Sill, and the regionally expansive ca. 1850 Ma Rodrigues granite. At Brown Water Lake peak metamorphic conditions were reached syn-S 5B at 3.1-4.2 kbars and 570- 670˚C, and syn-S2C at 3.1-3.9 kbars and 570- 630˚C at Little Crapeau Lake. Chemical and isotopic geochronological dating of monazite provided metamorphic age constraints that concurred with ages of local intrusions.    iii  PREFACE This dissertation is original, unpublished, independent work by the author, L. Smar .    iv TABLE OF CONTENTS ABSTRACT ........................................................................................................................................................ii PREFACE .......................................................................................................................................................... iii TABLE OF CONTENTS ................................................................................................................................. iv LIST OF TABLES ............................................................................................................................................ ix LIST OF FIGURES ........................................................................................................................................... x ACKNOWLEDGEMENTS .......................................................................................................................... xiii 1 INTRODUCTION.......................................................................................................................................... 1 1.1 Wopmay Orogen .................................................................................................................................................. 3 1.2 Geology of the Metamorphic Internal Zone ............................................................................................. 5 1.2.1 Geology of the Northern Metamorphic Internal Zone .................................................................... 6 1.2.2 Structure and Conditions of Metamorphism ...................................................................................... 7 1.3 Geology of the Southern Metamorphic Internal Zone ......................................................................... 9 1.3.1 Lithology, Structure and Metamorphism ............................................................................................ 9 1.3.2 Relevant Regional Geochronological Data ....................................................................................... 11 1.4 Map Areas for this Study ................................................................................................................................ 12 2 RESEARCH PLAN & METHODS .......................................................................................................... 18 2.1 Field Data Collection ........................................................................................................................................ 18 2.2 Structural and Metamorphic Analysis ...................................................................................................... 19 2.3 Whole  Rock and Mineral Geochemistry ................................................................................................... 19 2.4 Pres sure-Temperature Determinations .................................................................................................. 21 2.5 Geochronological Studies .............................................................................................................................. 21 3 BRO WN WATER LAKE .......................................................................................................................... 22 3.1 Lithological Units .............................................................................................................................................. 22  v  3.1.1 Pelitic to Semi-pelitic Schists ................................................................................................................. 22 3.1.2 Garnet Schists .............................................................................................................................................. 24 3.1.3 Coticule Rocks .............................................................................................................................................. 25 3.1.4 Calc-silicates, Marbles .............................................................................................................................. 26 3.1.5 Gneissic Rocks .............................................................................................................................................. 26 3.1.6 Intrusive Rocks ............................................................................................................................................ 27 3.1.7 Archean Gneiss Domes.............................................................................................................................. 27 3.2 S tructural Fabrics.............................................................................................................................................. 28 3.2.1 S0B ..................................................................................................................................................................... 29 3.2.2 S1B ..................................................................................................................................................................... 29 3.2.3 S2B ..................................................................................................................................................................... 30 3.2.4 S3B ..................................................................................................................................................................... 30 3.2.5 S4B ..................................................................................................................................................................... 31 3.2.6 S5B and Late Crenulations ....................................................................................................................... 31 3.3 Radi al Analysis of Two Phase Garnet Growth ....................................................................................... 32 3.4 Me soscopic and Macroscopic Structure .................................................................................................. 32 3.5 Meta morphism ................................................................................................................................................... 343.5.1 Isograds ......................................................................................................................................................... 35 4  LITTLE CRAPEAU LAKE ....................................................................................................................... 58 4.1 Lithological Units .............................................................................................................................................. 59 4.1.1 Pelitic Schists ............................................................................................................................................... 59  vi  4.1.2 Intrusive Rocks ............................................................................................................................................ 61 4.1.3 Carbonate ..................................................................................................................................................... 62 4.1.4 Archean Gneiss Dome ............................................................................................................................... 62 4.2 S tructural Fabrics.............................................................................................................................................. 62 4.2.1 S0C ..................................................................................................................................................................... 634.2.2 S1C ..................................................................................................................................................................... 634.2.3 S2C ..................................................................................................................................................................... 634.2.4 S3C ..................................................................................................................................................................... 644.2.5 S4C ..................................................................................................................................................................... 644.2.6 S5C ..................................................................................................................................................................... 644.3 Me soscopic and Macroscopic Structure .................................................................................................. 644.4 Meta morphism ................................................................................................................................................... 664.4.1 Isograds ......................................................................................................................................................... 675 P-T CONDITIONS OF METAMORPHISM & GEOCHRONOLOGY ............................................. 82 5.1 Whole Rock Geochemistry Results ............................................................................................................ 82 5.2 Metamorphic Mineral Chemistry Results ............................................................................................... 82 5.3 T hermodynamic Modeling of the KFMASH System ............................................................................ 83 5.4 Co mparative Chemical and Isotopic Geochronology .......................................................................... 86 5.4.1 U-Th Electron Microprobe In-Situ Geochronology ........................................................................ 87 5.4.2 U-Th Results ................................................................................................................................................. 89 5.4.2.1 Little Crapeau Lake ........................................................................................................................... 90  vii  5.4.2.2 Brown Water Lake ............................................................................................................................ 91 5.4.3 SHRIMP II U-Pb Geochronology Methods .......................................................................................... 92 5.4.4 U-Pb Results ................................................................................................................................................. 935.4.4.1 Brown Water Lake ............................................................................................................................ 935.4.4.2 Little Crapeau Lake ........................................................................................................................... 935.4.5 Advantages and Disadvantages to Dating Techniques, and Sources of Error ..................... 945.4.6 Discussion of Intrusive Ages ................................................................................................................... 95 6  INTERPRETATION AND DISCUSSION ........................................................................................... 104  6.1 P-T-t-d Summary  ............................................................................................................................................ 104  6.1.2 Regional and Orogen-Scale Interpretations and Questions .................................................... 108 6.2 Summary/Conclusions ................................................................................................................................ 112 6.2.1 Recommendations for Future Work ................................................................................................. 113  REFERENCES .............................................................................................................................................. 117  APPENDICES ............................................................................................................................................... 122 APPENDIX A ................................................................................................................................................ 123  Plate 1: Geological Map of Brown Water Lake Area ............................................................................ 124  Plate 2: Geological Map of Little Crapeau Lake Area ........................................................................... 125 APPENDIX B ................................................................................................................................................ 126  Compiled Field Structural Measurements ................................................................................................ 127  APPENDIX C ................................................................................................................................................ 147  Whole Rock Geochemistry and Calculations ............................................................................................ 148  APPENDIX D ................................................................................................................................................ 150 Compiled Mineral Chemistry Microprobe Results .................................................................................. 151 APPENDIX E ................................................................................................................................................ 153   viii  Geochronological U-Th Results, Isoplots ................................................................................................... 154  Compiled Grain Maps with Domain Outline Traces .............................................................................. 158 Conditions for Monazite Analysis, Multipoint Background Counting Times ............................... 167  U-Pb SHRIMP Mount Photo ............................................................................................................................ 169  APPENDIX F ................................................................................................................................................ 170  Mineral Abbreviations and Ideal Chemistries ......................................................................................... 171      ix   LIST OF TABLES Table 3.1 Summary of metamorphic mineral paragenesis ......................................................................... 36Table 4.1 Summary of metamorphic mineral paragenesis ......................................................................... 67Table 6.1 Summary table of overprinting fabrics and metamorphic timing .................................... 116      x  LIST OF FIGURES Figure 1.1 Maps showing locations of study areas ........................................................................................ 13 Figure 1.2 Main tectonic elements of the Wopmay Orogen ....................................................................... 14 Figure 1.3 Simplified geology of the Metamorphic Internal zone of the Southern Wopmay orogen with regional intrusive ages ...................................................................................................................... 15 Figure 1.4 Geology of Brown Water Lake .......................................................................................................... 16 Figure 1.5 Geology of Little Crapeau Lake ......................................................................................................... 17 Figure 3.1 AFM Compositional diagram for pelitic schists and gneisses from Brown Water Lake and Little Crapeau Lake .............................................................................................................................................. 37Figure 3.2 AKF Diagrams for pelitic schists and gneisses of Brown Water Lake and Little Crapeau Lake; Mn vs Ca scatter plots for major element geochemistry ................................................ 38 Figure 3.3 Character of bedding at Brown Water Lake ............................................................................... 39Figure 3.4 Fabrics and porphyroblasts in schists at Brown Water Lake .............................................. 40 Figure 3.5 Tightly folded metapelites south of Brown Water Lake ........................................................ 41 Figure 3.6 Interbedded quartzite, calc-silicate, amphibolites, marble and conglomerate units 42 Figure 3.7 Cordierite and andalusite porphyroblasts from sample 2512C and 2513 ..................... 43Figure 3.8 Photomicrographs show overprinting fabrics in biotite cordierite andalusite schists from the high strain belt at Brown Water Lake ................................................................................................ 44Figure 3.9 Stereonets depicting fabrics per structural zone at Brown Water Lake ......................... 45 Figure 3.10 Garnet porphyroblasts preserve two generations of deformation, from radially cut sample 2512A ................................................................................................................................................................. 46Figure 3.11 Ptygmatic folding of coticule layers in sillimanite zone pelites ....................................... 47Figure 3.12 Boudinaged and metamorphosed calc-silicate beds ............................................................ 48 Figure 3.13 Brown Water Lake rocks coarsen approaching Archean gneiss domes and Rodrigues granite .......................................................................................................................................................... 49 xi   Figure 3.14 Coarse rocks in around and between Archean gneiss domes .......................................... 50 Figure 3.15 Migmatite grade pelites and the Rodrigues granite .............................................................. 51 Figure 3.16 Fabrics preserved differently between Archean gneiss domes ....................................... 52 Figure 3.17 Structural domains at Brown Water Lake ................................................................................ 53 Figure 3.18 Spot imagery for the southern extent of the Brown Water Lake map area ................ 54 Figure 3.19 Brown Water Lake fold patterns .................................................................................................. 55 Figure 3.20 Boudinage due to rhealogical differences ................................................................................. 56 Figure 3.21 Cross-section at Brown Water Lake ............................................................................................ 57 Figure 4.1 Magnetic maps covering the western Metamorphic Internal zone at the boundary of the Wopmay Fault zone .............................................................................................................................................. 68 Figure 4.2 Spot imagery for the Little Crapeau Lake map area ................................................................ 69Figure 4.3 Low grade pelitic schists from Little Crapeau Lake ................................................................. 70 Figure 4.4 Andalusite porphyroblasts in muscovite biotite pelitic schists at Little Crapeau Lake ............................................................................................................................................................................................... 71 Figure 4.5 Andalusite and cordierite biotite muscovite schists from Little Crapeau Lake ........... 72 Figure 4.6 Section 2562D shows andalusite porphyroblasts overgrowing early fabrics .............. 73Figure 4.7 Textural garnet relationships in biotite muscovite schist .................................................... 74Figure 4.8 Folds, refolded folds, and the Little Crapeau sill ....................................................................... 75 Figure 4.9 Outcrop photo from station 1534 ................................................................................................... 76Figure 4.10 Structural domains of Little Crapeau Lake ............................................................................... 77Figure 4.11 Stereonet zones for Little Crapeau Lake .................................................................................... 78 Figure 4.12 Equal area stereographic projection of structural field data from Little Crapeau Lake ..................................................................................................................................................................................... 79Figure 4.13 Detailed island sketches from Little Crapeau Lake ............................................................... 80 Figure 4.14 Cross-section for Little Crapeau Lake ......................................................................................... 81  xii  Figure 5.1 Petrogenetic grids show reactions approximated for pelitic schists ............................... 96Figure 5.2 Thorium chemical maps outlining zones analysed for monazite geochronology ....... 97Figure 5.3 Thorium chemical maps outlining zones analysed for monazite geochronology (cont’d) .............................................................................................................................................................................. 98 Figure 5.4 Scatter plot showing U-Th ages calculated for all analysed monazite grains ............... 99Figure 5.5 Thorium vs Uranium scatter plots for chemical domain analyses from monazite grains in high grade metamorphic rocks from Little Crapeau Lake ...................................................... 100 Figure 5.6 Thorium vs Uranium scatter plots for chemical domain analyses from monazite grains from Brown Water Lake ............................................................................................................................ 101 Figure 5.7 Thorium vs Uranium scatter plots for chemical domain analyses from monazite grains in high grade metamorphic rocks from Brown Water Lake ....................................................... 102 Figure 5.8 Concordia diagrams for U-Pb metamorphic monazite ages .............................................. 103  Figure 6.1 Schematic summary of overprinting fabrics at Little Crapeau Lake and Brown Water Lake .................................................................................................................................................................................. 114  Figure 6.2 Timeline and porphyroblast snapshots ..................................................................................... 115     xiii  ACKNOWLEDGEMENTS First and foremost, I would like to give special thanks to my main supervisors, Valerie Jackson at the Northwest Territories Geoscience Office (NTGO), and Ken Hickey at the University of British Columbia (UBC), for their time, guidance, and patience throughout this project . Their invaluable experience in and out of the field, thoughtful discussions, and overall thoroughness and dedication to scientific research came together to help create a final product that I am proud of. Luke Ootes of the NTGO also made significant contributions in and out of the field, particularly when it came to big picture ideas and the tectonic history of the Wopmay Orogen. Also much gratitude is given to the field assistants who were part of a great team during the 2009 -10 field seasons. Special thanks to Duncan Mackay for his expert assistance in the field over two summers , for t en o’clock snacks, and for Pirate Days on our Lonely Island 2010 fly camp. External help was also provided by Dave Hirsch at the University of Western Washington, by Mike Williams, Mike Jercinovic and Julien Allaz at the University of Massachusetts at Amherst, and by Bill Davis, Nicole Rainer and Tom Pestaj at the GSC SHRIMP lab in Ottawa. Much appreciation to them. And  last but not least, sincere and enormous thanks goes to my family and friends, officemates and others, for their lovin g support over the years.     1 INTRODUCTION Paleoproterozoic orogenic belts are distributed globally as peripheral terranes accreted against Archean cratonic shields ( Zhao et al., 2002 ). Peripheral orogenies are belts of orogenic rocks that occur at the exterior margin of a supercontinent, of Archean age in this case ( Murphy and Nance, 1991 ). They are often preserved in the middle of continents, frozen in time and space during crustal assembly. They can be complex in nature, comprising many tectonic elements, and having been affected by a long tectonic history which can include extension , collision, and thickening, with thrust and fold components ( Hoffman et al., 1988 ; Thompson et al., 1987 ). The detailed reconstruction of such complex histories into a relative sequence of deformational and metamorphic events is critical in the understanding of Proterozoic tectonic activity and its relationship to Archean cratons. An abundance of 2.1-1.8 Ga orogenic belts worldwide indicates a period of abundant tectonic activity whereby Archean and Paleoproterozoic cratonic blocks were welded together during collisional orogeny (Zhao et al., 2002 ; Zhao et al., 2004 ). This provides evidence for the formation of a Paleo-mesoproterozoic supercontinent that preceded Rodinia, named Columbia or Nuna (Zhao et al., 2002 ). Study of these early Proterozoic belts helps to further our understanding of earth’s early tectonic history.  The Wopmay orogen, which borders the Archean Slave craton in the Northwest Territories, Canada, is a Paleoproterozoic orogen that has had a long history of study. Its southern extent is located approximately 140 km northwest of Yellowknife, NT (Fig. 1.1 a). The orogen is exposed over >450 km north to south, and is variable in width from ~250 km tapering to a point at its southern extent (Fig. 1.1 b) . Over several decades, numerous mapping and geochronological studies across the orogen have defined its main tectonic components and have attempted to unravel its tectonic and thermal history ( Bowring, 1985 ; Davis et al., 2011; Hildebrand and 1  Bowring, 1984 ; Hildebrand et al., 1987 ; Hildebrand et al., 1991 ; Hoffman, 1980 , 1984 ; Hoffman and Bowring, 1984 ; Hoffman and Hall, 1993 ; Janots et al., 2009 ; King, 1986 ). These regional studies have however focused largely on the northern half of the orogen, above 65 °N, where stratigraphic sequences are well exposed, and where workers have been able to reconstruct a fold and thrust belt ( Grotzinger and Hoffman, 1983 ; Tirrul, 1983 ). Crustal-scale imaging studies, such as the SNORCLE profile produced by Canada's National Lithoprobe Geoscience Project, as well as regional magnetic surveys have shed light on the geometry of the deep Archean and Proterozoic crust; however these provide little insight into the processes and deformation phases that lead to their current geometry. This can be resolved through highly detailed microstructural and metamorphic studies. A regional geological mapping project by the Northwest Territories Geoscience Office (NTGO) has gathered widespread geochronological data to reconstruct the timing and relationships of many crustal scale intrusive suites (Jackson et al., 2013 ) (Fig. 1.1 c). This study has yielded geochronological data that has provided evidence for the presence of more intrusive suites than were previously recognized, therefore requiring reassessment of the tectonic evolution of the Wopmay Orogen. Whereas heating in the orogen was once attributed to two distinct intrusive phases, the Hepburn and Bishop intrusive suites , it is now evident that there was a longer period of magmatic activity spanning nearly 40 million years. The new mapping initiatives across the southern Wopmay orogen cover both the Metamorphic Internal zone (MIz) and the Great Bear Magmatic zone (GBMz) (Fig. 1.2). With the advent of this new data, redefining of previous orogenic models has been necessary, requiring the need for detailed structural and metamorphic analysis to rebuild a more accurate model to fit current knowledge.  The research presented herein represents part of such a metamorphic-structural study of the Wopmay Orogen. The overall goal was to decipher the P-T-t-d history of the Metamorphic 2  Internal zone of the southern Wopmay Orogen and assess the implications this may have for its tectonic evolution. Points addressed are: 1) The geometric and kinematic evolution of ductile deformation structures, especially foliations, 2) The timing of metamorphic mineral growth relative to foliations, 3) The pressure and temperature conditions that accompanied metamorphic mineral growth, and 4) Absolute age of these metamorphism and deformation. The study has regional-scale implications for the new orogenic model, and speculates on the nature and importance of Paleoproterozoic belts worldwide. For this study, two areas were chosen to highlight differences and similarities in deformation and metamorphic history in the Metamorphic Internal zone of the southern Wopmay Orogen (Fig. 1.3). The field areas are approximately 250 km to the nor th -northwest of Yellowknife, and are approximately 40 km apart from one another (Fig. 1.3). The Brown Water Lake area abuts the Archean Slave craton to the southeast (Fig. 1.4). The Little Crapeau Lake area is bordered by the Wopmay Fault zone to the northwest (Fig. 1.5). The Brown Water Lake area was the main focus for study, while Little Crapeau Lake served as a source for structural, metamorphic and chronological comparison. 1.1 Wopmay Orogen The Wopmay Orogen is a Paleoproterozoic orogenic belt that straddles the western margin of the Archean Slave craton in the Northwest Territories, Canada (Fig. 1.2). It is exposed over >500 km in north-south strike length, and is up to 200 km in width. It is overlain by Paleozoic cover to the west. From east to west, the orogen is comprised of several main tectonic elements: the Coronation Margin (~1. 92-.188 Ga), the Coronation Supergroup (~1.92 Ga), the Great Bear Magmatic zone (<1.85 Ga), and the Hottah Terrane (>2.0 Ga). The tectonic history and evolution of the orogen is under debate and is currently being remodeled, however the current understanding is that rifting on the western margin of the Slave craton was initiated around 3  2014.32 ± 0.89 Ma ( Hoffman et al., 2011 ). Following this, a sequence, several hundred kilometres thick, of rift, passive margin and collisional foredeep sediments and rift-related volcanic was deposited on the margin between ca. 2015-1882 Ma ( Hildebrand et al., 2010 ; Hoffman et al., 2011 ). These sediments comprise the Coronation margin (Fig. 1.2). They are interpreted to represent the sedimentary infill of a rift basin , and it has been proposed that the sediments are derived from an allochthonous source based on detrital zircon ages ( Bowring and Grotzinger, 1992 ). The origin of the sedimentary and plutonic rocks however remains unresolved ( Bennett and Rivers, 2006 ; Bowring and Grotzinger, 1992 ; Jackson et al., 2013 ; Lalonde, 1989 ; St-Onge et al., 1987 ). A maximum deposition age for these sediments has been given at 1.92 Ga ( Bennett and Rivers, 2006 ). During the ca. 1885 Calderian orogeny, these rocks were shortened, deformed and translated eastward over the Slave craton ( Tirrul, 1982 , 1983 ). Thin-skinned thrusting and folding of the Coronation margin resulted in the formation of the Asiak Thrust and Fold Belt (Fig. 1.1 b, 1.2). The Metamorphic Internal zone is thought to have formed as a result of the closing of the rift basin (King, 1986 ; Lalonde, 1989 ). It is uncertain whether and if so, how far the rocks of the Coronation Supergroup were transported during basin closure. The crustal-scale 1890-1875 Ma Hepburn Intrusive suite (Fig. 1.2 b ) intrudes rocks of the MIz and is thought to have been emplaced synchronous with ductile deformation and eastward translation of the Coronation margin ( Bowring and Grotzinger, 1992 ). Related plutons are not found to intrude Archean basemen t (St-Onge et al., 1984 ). The post-orogenic Bishop Intrusive suite (Fig. 1.2 b) also intrudes rocks of the MIz, and is synchronous with initiation of magmatism in the Great Bear magmatic zone ( Lalonde, 1989 ). To the west of the Coronation Supergroup are rocks of the Great Bear Magmatic zone and the Hottah Terrane. The Great Bear magmatic zone is separated from the Metamorphic Internal 4  zone by the ~500 km long Wopmay Fault zone (Fig. 1.2, 1.3). It is comprised of arc-like volcanic rocks and extensive plutonic rocks that range in age from 1875 to 1850 Ma ( Gandhi et al., 2001 ; Hildebrand et al., 1987 ; Jackson et al., 2013 ). The Hottah Terrane is cryptic in nature and origin, and comprises >2000 Ma basement and overlying 2000-1880 Ma plutonic rocks ( Jackson et al., 2013). The present geometry in the southern Metamorphic Internal zone reveals the metasedimentary rocks to be underlain by gneissic culminations of Archean basement, which crop out from under the Paleoproterozoic rocks in several locations (Fig. 1.2, 1.3, 1.4, 1.5; Appendix A Plates 1 & 2). The Exmouth Massif is an example of such an occurrence, where superposed folding events have resulted in the exposure of Archean basement (Fig. 1.3). 1.2 Geology of the Metamorphic Internal Zone Detailed regional mapping projects conducted throughout the Metamorphic Internal zone have allowed many early workers to define the tectonic elements that comprise the Wopmay Orogen. Stratigraphy is defined and described much more extensively in the northern half of the orogen, however, as rocks are better exposed and thicker sections are abundant than in the south. To the south rocks are more ductilely deformed and, alongside structural, geochronological and geochemical studies, extrapolations and correlations have been made to define ambiguous units. Evolutionary models that were based on deformation and metamorphism observed in the north have been used to draw parallels with structures and thermal effects observed in the south ( Easton, 1981 ; Frith, 1990 , 1992; Jackson and Ootes, 2012 ; Nielsen, 1977 , 1986 ; Pehrsson, 2002 ; St-Onge et al., 1984 ).   5  1.2.1 Geology of the Northern Metamorphic Internal Zone Field work by St-Onge ( 1981 ) in and around the Hepburn and Wentzel batholiths described the presence of three “progressive” metamorphic suites in the northern Wopmay orogen. Two of these suites are related to metamorphism associated with the intrusion of the Hepburn batholith, and the third one associated with the Wentzel Batholith. Hepburn-related plutons are dated at ca. 1890 to 1875 Ma, and have fabrics that suggest they are syn-tectonic with the first major shortening event ( Jackson et al., 2013 ; St-Onge, 1984 ). The isograds mapped around the Hepburn and Wentzel Batholiths in these areas were shown to be both “inverted” and “normal” respectively (St-Onge, 1981 ). The dip of the isograds is attributed to the funnel-shape of the associated plutons. In the case of the Hepburn Batholith, it is thought that the present exposure represents the floor at approximately 12km depth, while the present exposure of the Wentzel Batholith shows its roof ( St-Onge, 1981). Structures and isograds are ‘nicely’ exposed in the north because of thermal culminations exposed in oblique section due to cross-folds ( St-Onge et al., 1984 ). Major NE-SW cross folds and minor NNE-SSW antiform/synform pairs throughout the region expose the lower levels of the Asiak Thrust and Fold b elt as well as the Archean basement. Hepburn plutons are coincident with the Calderian thermal culmination ( St-Onge et al., 1984 ). No Hepburn intrusions are found to intrude the Coronation margin basement. Three basement-cored fold structures are mapped north of 65 °, all of which are reported to preserve no Proterozoic fabrics, except at a regional scale (antiforms and synforms) (Fig. 1.2). One of three basement-cored fold structures exposed at the edge of the Slave craton at 65 ° N, overlain by rocks of the Coronation Supergroup, is reported to preserve Proterozoic fabric ( St-Onge et al., 1984 ). This schistosity was documented in the Exmouth Massif (Fig. 1.3). St-Onge et al. (1984 ) report “inverted isograds”, increasing in grade away from “cold” basement culminations. 6  In the northern half of the orogen many large fault and thrusts appear to have formed pre-metamorphic peak, as isograds cross the features but are not offset by them ( St-Onge, 1984 ). The metamorphic isograds are thought to be associated with large, older foliated granites, which may represent older phases of the batholiths ( St-Onge, 1981 ). It was once thought that intrusions associated with the Hepburn Intrusive Suite extend as far south as Arseno and Grant Lakes ( Frith, 1990 ), however it is now known that no associated intrusions exist south of 65° N (Bennett et al., 2012; Jackson and Ootes, 2012 ). The Rodrigues pluton, an expansive intrusion of largely undeformed k-feldspar porphyritic granite located between Little Crapeau Lake and Brown Water Lake has a U-Pb zircon crystallization ca. 1850 Ma ( Jackson et al., 2013 ). 1.2.2 Structure and Conditions of Metamorphism Conditions of metamorphism were studied at Hepburn Lake ( St-Onge, 1981 ), approximately 200 km north of the area studied in this project. Having documented the reaction quartz + plagioclase + biotit e + sillimanite ↔ garnet + cordierite + orthoclase + melt, the author calculated metamorphic P-T conditions around 440 °C (chlorite zone) up to 727 °C and 2.2-3.8 kbars. Garnet-biotite thermobarometry indicates that higher temperatures up to 560-765°C may have affected the rocks of the passive margin ( St-Onge, 1984 ). Additionally, the pressure culmination has been estimated at 2.6-5.2 kbars using the solid- solid reaction plagioclase ↔ grossular + aluminosilicate + quartz ( St-Onge, 1984 ). Further studies by St-Onge ( 1986) looking at PT paths through zoned poikiloblastic garnets through a 30 km composite transect of crust outline paths consistent with loading, uplift and erosion in “overthrust” terrains, with an average uplift rate of 1.5-2.7 mm/year. Variability in uplift rates throughout the area can be attributed to the cross-folding event noted in the northern part of the orogen ( St-Onge, 1986). In their 1987 descriptio n of a “rapidly evolving” thermo-tectonic regime, St-Onge and King (1987) describe crustal stretching and thinning, inducing the first ‘stage’ of metamorphism 7  which generated a High-T Low-P mineral assemblage. Approximately 5-10 My after stretching, and after deposition of west-facing sedimentary prism around 1.9-1.885 Ga, emplacement of a suite of plutons ca. 1896-1878 Ma occurred and was followed by subhorizontal shortening during the Calderian Orogeny. Plutons also intrude thrusted and folded syn-orogenic foredeep sedimentary rocks ( St-Onge et al., 1987 ). The 1st stage of metamorphism is attributed to crustal thinning and heating. The 2nd stage of metamorphism is recorded in high and intermediate structural levels in the form of syn-tectonic metamorphic mineral growths that are “spatially related” to the plutons. The 3rd stage is related to uplift and erosion of thickened and transported crust, recorded in P-T paths of post-tectonic garnets.  Hildebrand et al. ( 1990 ) mapped rocks of the Wopmay Fault zone east of Wopmay Lake (Fig. 1.3). Low-grade assemblages of metasedimentary rocks were observed here, including quartz arenite, semipelite and dolomite-argillite rhythmite, intruded by gabbro sills. They observed that most fold axes and mineral lineations in rocks in the vicinity of the Wopmay Fault zone displayed a moderate northward plunge, and dated these northerly-striking folds at 1843  Ma (Hildebrand et al., 1990 ). These folds postdate Great Bear magmatism, as cross-folding affects these rocks. It is thought that northwester ly-striking folds that rotate into northerly-striking folds along the fault zone are a result of strong strike-slip motion and dextral transpression during the collision of Hottah Terrane with the Slave Craton ( Hildebrand et al., 1990 ). Northerly-striking folds in the Wopmay Lake area are re-folded by easterly-striking cross-folds (Hildebrand et al., 1990 ).   8  1.3 Geology of the Southern Metamorphic Internal Zone The Southern Metamorphic Internal zone is not as well understood in the literature, due to its poor exposure and disputed stratigraphic correlations. The areas encompassing Brown Water Lake and Little Crapeau Lake have been mapped and studied by Easton ( 1981 ), Frith ( 1990 , 1992), Nielsen ( 1977 , 1986 ) and Pehrsson ( 2002), while extensive new mapping initiatives are underway, led by Jackson and Ootes at the NTGO ( Jackson et al., 2013 ). Easton (1981 ) mapped the geology around the Grant Lake and Four Corners Lake areas at 1:50,000 scale, and described isograds at both Grant and Little Crapeau Lakes. He attributes metamorphism of the region’s Akaitcho sediments to intrusions of Hepburn and Wentzel batholiths ( Easton, 1981 ). 1.3.1 Lithology, Structure and Metamorphism Frith ( 1990 , 1992 ) produced two maps in the vicinity of Brown Water Lake. A 1:50,000 scale map at Arseno Lake placed emphasis on the structure and metamorphism around the area’s Archean gneiss domes, and another at 1:125,000 scale focussed mainly on the western margin of the Slave structural Province near Indin Lake, approximately 45 km southeast of Brown Water Lake. Metasedimentary rocks at Arseno Lake were subdivided into porphyroblastic to migmatitic schist, paragneiss and migmatite, and rusty coloured subgreywacke, arkosic, sandstone, mudstone, dolomite and calc-silicate rocks ( Frith, 1990 ). Mapping and geochronological work also confirmed the presence of the Archean cored gneiss domes throughout the map area, mantled by migmatitic paragneiss ( Frith, 1974 ; Frith et al., 1977 ). For the most part, metasediments in the map area were not specifically recognized as belonging to the Proterozoic Snare Group or the 2.67 Ga Yellowknife Supergroup, as Rb-Sr whole rock isochron studies remained ambiguous ( Frith, 1990 ; Frith and Loveridge, 1982 ; Nielsen, 1977 ). 9  Metamorphic isograds were documented as increasing westward, away from the Slave craton, from sub-biotite to above cordierite-garnet grade (Frith, 1990 ). Migmatitic banding, deformed by up to three phases of deformation, is preserved in paragneiss above the sillimanite isograd. These show thickening and thinning of compositional dif ferences in original bedding owing to metamorphic segregation ( Frith, 1990 ). In total, two phases of ductile deformation and two phases of brittle deformation were recognized in the area as a result of the Calderian Orogeny (Frith, 1990 ). The first two phases caused east-west shortening, subsequent thickening of the sedimentary prism, as well as large scale undulations which exposed various levels of the crust. The last two phases resulted in large scale dextral transpressive faulting and conjugate fault sets that crosscut local intrusions and the Slave Province, alike ( Frith, 1990 ).  Nielsen (1977 , 1986 ) mapped metamorphic mineral assemblages and isograds in the Proterozoic metasediments at Arseno Lake and described Abukuma facies type metamorphism. He defined six isograds with the first appearance of the following minerals: biotite; andalusite; cordierite (muscovite + chlorite out); sillimanite (andalusite out); sillimanite + K-feldspar (muscovite + quartz out); and almandine + K-feldspar ± cordierite (biotite + sillimanite + quartz out) (Nielsen, 1986 ). Nielsen distinguished bet ween “remobilized Archean paragneiss” and Proterozoic sediments in the Arseno Lake area on the basis of distinguishing characteristics such as polymetamorphism, presence of high P-T relic grains, and Rb/Sr whole rock data which indicated an age of >2100 Ma for some rocks ( Nielsen, 1977 ). Nielsen (1977 ) and McGlynn and Ross  (1963 ) also noted that rocks to the east of the cordierite isograd are deformed by tight, upright, westerly-dipping D 1 folds trending 020°. A second deformation event, D 2, overprints these upright folds with gentle, open folds that trend 080°. The authors attribute porphyroblast growth on axial planar structures to the “penetrative nature” of the D1 deformation ( Nielsen, 1977). 10  1.3.2 Relevant Regional Geochronological Data As part of the larger South Wopmay mapping initiative conducted by the NTGO, a large amount of geochronological data was gathered over a period of a decade to help develop an understanding of the orogen’s history (Jackson et al., 2013 ). Relevant to this study are the following U-Pb zircon crystallization dates (see Fig. 1.3 for selected sample locations; also see Jackson et al., 2013 for all sample locations): 1) Hepburn suite aged intrusions: Little Crapeau Sill (dated for this study): 1877 ± 2 Ma 2) Post-Hepburn aged intrusions: Deformed Zinto suite: 1867.4 ± 1 Ma Rebesca Lake granodiorite: 1867.7 ± 0.9 Ma Exmouth granodiorite: 1865.9 ± 0.8 Ma Blocky granite: 1862.83 ± 0.58 Ma 3)  Bishop suite aged intrusions and/or post-orogenic/Great Bear magmatism age: Rodrigues pluton: 1849.4 ± 1.4 Ma to 1851.8 ± 3.2 Ma Black Lichen: 1856.78 ± 0.95 Ma Peri Granite: 1858 ± 4.3 Ma While intrusions of groups 1) and 3) fall within the previously known age brackets of orogen-scale Hepburn and Bishop intrusive suites, rocks of group 2) fall somewhere in between. This indicates a separate phase of crustal heat circulation associated with a suite of intrusions that were previously unrecognized. Discussion about this is found in section 5.4. 6.    11  1.4 Map Areas for this Study The southern Metamorphic Internal zone is the focus of this study (Fig. 1.3). This area has been previously studied ( Easton, 1981 ; Frith, 1990 ; Hoffman and Hall, 1993 ; Nielsen, 1977 ; Pehrsson, 2002 ), however new mapping and geochronological data resulting from renewed study of the orogen require further investigation to reassess the kinetic and metamorphic history of the rocks.  Two map areas were chosen to perform this investigation for this project: the Brown Water Lake area, located on NTS map sheet 086B12, and the Little Crapeau Lake area, covering sheets 086C15 and 086C16 (Fig. 1.3, 1.4, 1.5; Appendix A Plates 1 & 2). These two areas sit within the 1:150,000 scale mapping project by the Northwest Territories Geoscience Office (Fig. 1.1 c). In both areas the Paleoproterozoic metasedimentary rocks were the focus of mapping. They are sequences of multiply deformed and metamorphosed pelitic schist, semi-pelitic schist, and pelitic gneiss. They are intruded by intrusions of various ages, and overlie parautochthonous Archean basement.   12Inset map B

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