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Early Paleozoic Post-Breakup Magmatism

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Early Paleozoic Post-Breakup Magmatism EARLY PALEOZOIC POST-BREAKUP MAGMATISM ALONG THE CORDILLERAN MARGIN OF WESTERN NORTH AMERICA: NEW GEOCHRONOLOGICAL AND GEOCHEMICAL RESULTS FROM THE KECHIKA GROUP, YUKON, CANADA By © Roderick William Campbell A Thesis submitted to the School of Graduate Studies in partial fulfilment of the requirements for the degree of Master of Science, Department of Earth Sciences Memorial University of Newfoundland August 2018 St. John’s Newfoundland and Labrador i Abstract Lower Paleozoic igneous rocks were emplaced along the Cordilleran margin of western North America following lithospheric breakup. Several competing rift models have been proposed to explain the significance of this magmatism to Cordilleran passive margin evolution. New field and laboratory studies of the Kechika group, south-central Yukon, were conducted to test these rift models and constrain the timing and tectonic setting of post- breakup magmatism. The Kechika group contains vent-proximal and sediment-sill facies that developed outboard of continental margin shelf and trough basins. Zircon U-Pb dates indicate that Kechika group rocks crystallized during the late Cambrian (488-483 Ma) and Early Ordovician (473 Ma). Whole-rock trace element and Nd-Hf isotope results are consistent with the low-degree partial melting of a lithospheric mantle source during margin-scale extension. Post-breakup rocks emplaced along the magma-poor North Atlantic margins, including the Orphan Knoll and Galicia Bank crustal blocks, are likely modern analogues for the Kechika group. ii Acknowledgments Firstly, I would like to thank my supervisor Dr Luke P. Beranek. He provided me with the opportunity to work on fascinating outstanding scientific questions in the beautiful Pelly Mountains, to present at conferences with those whom wrote the book on Cordilleran geology, and provided constant support, help, and advice throughout the course of the project. I frequently took advantage of an open-door policy and was always greeted with enthusiastic discussion. I would also like to thank Dr Stephen J. Piercey for providing valuable insight during fieldwork, in discussions, for being on my committee, and reviewing this manuscript. Thank you to Dr. Richard Friedman for providing the U-Pb analysis for this work. I would also like to thank various people at the Yukon Geological Survey, for their assistance in the field and providing such a welcoming experience in the Yukon. In particular thank you to Maurice Colpron for putting me up on my first trip to Yukon, and to Rosie Cobbett for assistance with the YEG manuscript, in the field and being a welcoming host in Whitehorse. Thank you to Will Tompson of TransNorth Turbo Air for providing excellent helicopter support. Thank you to the graduate students and friends whom have made living in a Newfoundland such a welcoming and memorable experience. Finally thank you to my family whom have provided unending support. This project was supported by research grants from the Geo-mapping for Energy and Minerals program (Natural Resources Canada) to Beranek and NSERC Discovery Grants to Beranek and Piercey. The Yukon Geological Survey generously supported field logistics and helicopter support. We are grateful to Dominique Weis, Corey Wall, and Bruno Kieffer for their assistance at the Pacific Centre for Isotopic and Geochemical Research. iii Table of Contents Abstract ...................................................................................................................... ii Acknowledgments .................................................................................................... iii List of Tables ............................................................................................................ vii List of Figures ........................................................................................................ viii List of Abbreviations ................................................................................................. xi List of Appendices .................................................................................................. xiv Chapter 1 Introduction and purpose of study ................................................................. 1 1.1 Introduction .......................................................................................................... 1 1.2 Geological Background ........................................................................................ 6 1.2.1 Cordilleran margin development .................................................................. 6 1.2.2 Pure-shear rift models ................................................................................... 7 1.2.3 Simple-shear rift models ............................................................................... 7 1.2.4 Magma-poor rift models ............................................................................... 9 1.2.5 Cambrian-Ordovician margin architecture ................................................. 11 1.2.6 Cambrian to Ordovician extension ............................................................. 13 1.2.7 Cambrian-Ordovician magmatism .............................................................. 14 1.2.8 Geology of the Pelly Mountains, south-central Yukon .............................. 15 1.3 Thesis Objectives ............................................................................................... 21 1.4 Methods .............................................................................................................. 22 1.4.1 Fieldwork and sample collection ................................................................ 22 1.4.2 CA-TIMS zircon U-Pb geochronology ....................................................... 22 1.4.3 Major, trace and Nd isotope geochemistry ................................................. 23 1.5 Co-Authorship Statement ................................................................................... 24 1.6 Thesis presentation ............................................................................................. 25 Chapter 2 Volcanic stratigraphy of the Cambrian-Ordovician Kechika group, Pelly Mountains, south-central Yukon* ............................................................................................ 27 2.1 Abstract .............................................................................................................. 27 2.2 Introduction ........................................................................................................ 28 2.3 Geological framework ........................................................................................ 35 2.3.1 Ketza group ................................................................................................. 35 2.3.2 Kechika group ............................................................................................. 38 2.3.3 Askin group ................................................................................................. 40 iv 2.4 Field studies........................................................................................................ 40 2.4.1 Groundhog formation stratigraphy ............................................................. 43 2.4.2 Cloutier formation stratigraphy .................................................................. 45 2.5 Preliminary analytical results ............................................................................. 50 2.6 Stratigraphic interpretation ................................................................................. 52 2.6.1 Basaltic facies association .......................................................................... 52 2.6.2 Volcanogenic sedimentary facies association ............................................. 52 2.6.3 Limestone-argillite facies association ......................................................... 54 2.6.4 Intrusive rocks ............................................................................................. 54 2.7 Regional correlations and future work ............................................................... 55 2.8 Acknowledgements ............................................................................................ 56 2.9 References .......................................................................................................... 56 Chapter 3 Early Paleozoic post-breakup magmatism along the Cordilleran margin of western North America: new geochronological and geochemical results from the Kechika group, Yukon, Canada* ........................................................................................................... 72 3.1 Abstract .............................................................................................................. 72 3.2 Introduction ........................................................................................................ 74 3.3 Lower Paleozoic Stratigraphy of the Northern Cordillera ................................. 81 3.3.1 Overview ..................................................................................................... 81 3.3.2 Pelly Mountains, south-central Yukon ....................................................... 87 3.4 Methods .............................................................................................................. 93 3.4.1 CA-TIMS zircon U-Pb geochronology ....................................................... 93 3.4.2 Whole-rock lithogeochemistry...................................................................
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