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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

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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 2) with uniform plume-type basalts are LREE-enriched high-Ti basalt (1.6-2.4 wt% Ti0 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) 2) with pronounced negative-HFSE anomalies depleted low-Ti basalt (0.4-1.2 wt% Ti0 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 4O,39Information about samples analyzed by 40Ar/39Ar in this study 235 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/39Ar analytical 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) 40Ar/39Ar samples (.xls file) SD 3- Geochemistry for SD 4- 40Ar/39Ar 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
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