Geochronological and Geochemical Constraints on the Origin of the Paleoproterozoic Union Island Group Mafic Magmatism, East Arm Basin, N.W.T
Total Page：16
File Type：pdf, Size：1020Kb
Geochronological and geochemical constraints on the origin of the Paleoproterozoic Union Island Group mafic magmatism, East Arm Basin, N.W.T. by Alex I-Fan Sheen A thesis submitted in partial fulfillment of the requirements for the degree of Master of Science Department of Earth and Atmospheric Sciences University of Alberta © Alex I-Fan Sheen, 2017 ABSTRACT The Slave craton underwent widespread extension during the Paleoproterozoic. In the southern Slave craton, this extension is manifest in multiple periods of continental mafic magmatism emplaced between 2.3–2.2 Ga and prior to 2.0–1.9 Ga calc-alkaline magmatism of the Taltson and Thelon zones along the Slave-Rae boundary. During this time the East Arm basin of Great Slave Lake was formed and now preserves a protracted sedimentary and volcanic record along the southeastern margin of the Slave craton. Within the East Arm basin package, the Union Island Group represents voluminous mafic volcanism with subordinate interbedded carbonate/shale sedimentary strata. Previous stratigraphic interpretations posit that the ca. 1928 Ma Wilson Island Group is the base of the East Arm basin stratigraphy, and that the Union Island Group is younger. This relationship is however equivocal; the Wilson Island Group is structurally isolated, and new field observations indicate that the Union Island Group was deposited unconformably on Archean granitic basement, the latter considered to be derived from the nearby Slave craton. We report the first U–Pb baddeleyite crystallization age, 2042.7±3.0 Ma, for a diabase body which intrudes volcaniclastic horizons belonging to the Union Island Group lower basalt unit; the diabase intrusion is geochemically identical to flows of the lower basalt unit and is therefore interpreted as a feeder to these flows. The diabase crystallization age demonstrates that the Union Island Group is the oldest identified supracrustal package in the East Arm basin and that the East Arm basin is ~115 Myr older than previously thought. Our new stratigraphic interpretation is supported by detrital zircon provenance age distribution in two Union Island Group sedimentary samples. These detrital zircons are dominated by 2.76–2.56 Ga ages reflecting prominent input from the Archean basement. The youngest detrital zircons have ages indistinguishable from the emplacement age of the diabase. Zircon ages from the 1.9–2.0 Ga Taltson and Thelon magmatism, are absent from the ii Union Island Group sedimentary rocks while being prominent in all other strata of the East Arm basin; further confirming that the Union Island Group is the oldest strata in the East Arm basin. The Union Island Group contains two stratigraphically and geochemically distinct mafic volcanic packages. The lower magmatism is predominantly alkaline, is characterized by high enrichment levels of incompatible elements and a large compositional range (108–438 ppm Zr, 13–62 ppm Nb), and preserves an OIB-like chemical signature. The upper magmatism, in contrast, is tholeiitic, displays much lower levels of incompatible elements with a uniform composition (83–101 ppm Zr, 2–4 ppm Nb), and has a DM-like chemical signature. Both packages display overlapping depleted time-integrated εNd(i) values (+1.1 – +3.2). Petrological modeling suggests that the lower magmatism originated from interaction between upwelling asthenosphere and a depleted mantle reservoir; the upper magmatism was produced by decompression melting of the shallow mantle consisting mostly of the depleted reservoir. The petrogenesis established in this study is consistent with a continental rift origin for the Union Island Group magmatism and is further supported by geochemical similarities with Proterozoic and Phanerozoic rift successions. Combing geochronological and geochemical findings, we propose that the Union Island Group represents an incipient rift sequence during the ca. 2043 Ma rifting at the southern margin of the Slave craton. The proposed tectonic model is consistent with the break-up of the Slave craton from a pre-Laurentia supercontinent during the 2.23–2.01 Ga period. Our conclusions further suggest an intraplate origin for the Taltson and Thelon magmatism, and that the Slave craton and the Rae domain were a contiguous crustal unit as early as ca. 2043 Ma. iii PREFACE Research presented in this study is part of a larger project investigating the origin and evolution of the East Arm basin, a collaboration between the University of Alberta and the Northwest Territories Geological Survey. The unconformity between the Union Island Group and the Archean basement, shown in Fig. 2.4b, was discovered by L. Ootes and A. Bekker during additional fieldwork in the Union Island area. The photograph of the unconformity was taken by L. Ootes. Geochemical studies presented in this thesis include re-analysis of samples originally presented in the late Stephen Goff’s 1984 Ph.D. thesis, entitled The magmatic and metamorphic history of the East Arm of the Great Slave Lake, N.W.T. Standard materials WGB-1, TDB-1, and SY-2 were provided by Bruce Kjarsgaard at the Geological Survey of Canada. U–Pb purification of baddeleyite fractions #1 and #2 of sample LH14-27 was done by Dr. Larry Heaman, with ID-TIMS analysis conducted by James LeBlanc. U–Pb detrital zircon LA-ICPMS study of sample LH15-EA51 was conducted by Madisen Janzen as a research assistant under Dr. Larry Heaman; the detrital zircon study is included in this thesis with her consent. All ideas presented in this thesis are my own, although they have been developed through discussions with my supervisor, Dr. Larry Heaman. All writing and figures presented here are my own work. iv ACKNOWLEDGEMENTS Funding for this project was supported by the Polar Continental Shelf Program, the Natural Sciences and Engineering Research Council of Canada (NSERC) Discovery Grants Northern Research Supplements Program (RGPNS; Grant #444185, to L. Heaman). Additional funding was supported by the Canada Graduate Scholarship-Master's (CGS M) from NSERC, the Walter H. Johns Graduate Fellowship and Eugene Brody Graduate Scholarship from the University of Alberta, and the Graduate Student Scholarship from Student Aid Alberta. First and foremost, I would like to thank my supervisor Dr. Larry Heaman for the incomparable opportunity to explore the Precambrian puzzle and for his unfailing guidance and support over the past three years. His encouragement motivated me to explore beyond what was envisioned at the start of the project, learning with excitement the numerous facets of research in the process; the abundance of information included in this thesis—big and small—is a testament to this. Through relentless edits, comments, and discussions Larry has immensely helped me develop the skills to communicate academic research. I am equally grateful for his guidance during fieldwork and for the maxim that good research is one that uncovers more questions. Secondly, I would like to thank Luke Ootes for his fabulous mentorship in the field and for his far- reaching support in every aspect of the fieldwork and other parts of the project. Luke's wealth of knowledge in everything Slave (and beyond) helped me grasp the significance of our findings and realize, at the same time, how one craton alone holds far more wonders and mysteries than one could possibly ever comprehend. Luke has also provided valuable feedback and suggestions regarding figures in chapters 1 and 2. Also, we owe Luke a big one for locating the Union Island Group basal unconformity. I would also like to thank the Heaman geochronology research group for training me, formally or informally, on the various sample preparation and processing protocols: Barry Herchuk, for teaching me to keep things clean at all times and without whose reminder to rinse thoroughly I would never had recovered enough baddeleyite for dating; James LeBlanc, for helping me overcome the anxiety of remotely operating the large and delicate TIMS instrument; Madisen Janzen and Barry Shaulis, for everything zircon-related (and for being wonderful field v companions); Ilona Ranger and Stephanie Nichols, for heartwarming moral support and the odd mantle-related chit-chats. Robert Creaser and Chiranjeeb Sarkar provided laboratory training for the tracer isotope studies. Thank you also to the following people for other various training, analysis, and logistics associated with this project: Lisa Budney, Diane Caird, Andy DuFrane, Nathan Gerein, Marilyn Huff, Mark Labbe, Andrew Locock, Martin von Dollen, and Barbara Ziger. Extra shoutout to Shyra Craig for never being annoyed whenever I came to sign out the mill room keys. Bruce Kjarsgaard of the GSC provided valuable mentorship during fieldwork and in subsequent geochemical discussions. Other members of the field parties deserve a special thank you as well: Andrey Bekker, Jordan Burke, Murray Gingras, and Jesse Reimink. Thank you to my family for always being there for me. Many friends have also poured out unending moral support in high times and in low. As it would take another thesis just to thank you all, I will give special shoutouts to the following few: Ricky, for being the best ever roommate and my moral compass; Devon, for your perpetual optimism and humour; Mike, for your candid sincerity that I can only aspire to possess; and Megan and Joel, for all our musical escapades. And lastly—cheesy as it may sound—thank you to all the wonderful people I have met in Edmonton over the last three years. As a good friend puts it, hibernation is common during the long