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Microstructural Geochronology of Zircon Across the Central Uplift of the Vredefort Impact Structure, South Africa

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Microstructural Geochronology of Zircon Across the Central Uplift of the Vredefort Impact Structure, South Africa Western University Scholarship@Western Electronic Thesis and Dissertation Repository 9-15-2016 12:00 AM Microstructural Geochronology of Zircon Across the Central Uplift of the Vredefort Impact Structure, South Africa Connor Lawrence Davis The University of Western Ontario Supervisor Dr. Desmond Moser The University of Western Ontario Graduate Program in Geology A thesis submitted in partial fulfillment of the equirr ements for the degree in Master of Science © Connor Lawrence Davis 2016 Follow this and additional works at: https://ir.lib.uwo.ca/etd Part of the Geology Commons Recommended Citation Davis, Connor Lawrence, "Microstructural Geochronology of Zircon Across the Central Uplift of the Vredefort Impact Structure, South Africa" (2016). Electronic Thesis and Dissertation Repository. 4185. https://ir.lib.uwo.ca/etd/4185 This Dissertation/Thesis is brought to you for free and open access by Scholarship@Western. It has been accepted for inclusion in Electronic Thesis and Dissertation Repository by an authorized administrator of Scholarship@Western. For more information, please contact [email protected]. Abstract The Vredefort central uplift, or ‘dome,’ represents the erosional remnant of one of the largest (~300 km diameter) and oldest (~2.020 Ga) terrestrial impact structures. This investigation was performed to help elucidate the complexity of the shock process on zircon, incorporating various electron beam methods, including BSE and SE imaging, EBSD, CL, and EDS (mapping and semi-quantitative compositional analysis). A new shock microstructural progression in terrestrial zircon is suggested, as well as a complete structure-wide analysis of impact melt inclusions in zircon. Regional trends in the effects of shock on zircon are included. Impact melt glass inclusion compositions vary widely, from felsic to mafic inclusions, and are related to the partial melting of local minerals. This study is significant for the future study of similar impact structures on other rocky planets, and the quantification and qualification of shock conditions as recorded in zircon. Keywords Vredefort impact structure, zircon, impact melting, shock microstructures, shock metamorphism, impact melt glass inclusions. Acknowledgments First and foremost, I would like to acknowledge Desmond Moser for his incredible support and guidance over the past two years. His passion and insight into the geology of Vredefort has been invaluable to the success of my thesis. I sincerely thank Des for providing me the opportunity to do field work in South Africa, and for all the other opportunities to attend conferences, field camps, and extracurricular courses. Thank you also for the help in editing and tying this thesis together. I would also like to thank Ivan Barker, who’s technical assistance in the ZAPLab was instrumental to my success. Thank you for putting up with my questions, and always being there to help me figure out the numerous issues we encountered. I appreciate the help of the ASTRO-CSA group for funding some of my conference travel, such as the ASTRO short course in Edmonton, AB. I am also thankful for all the other field trips I was a part of, including Oman, southwest U.S., and across Canada. It was the opportunities like this that truly made my graduate experience. I thank the members of my thesis defense committee, Sean Shieh, Robert Klassen, and Kim Tait, for their efforts in critiquing my thesis. Thank you also to the defense chair, Bill Church. I could not have gotten through the last two years without the support of my friends and family. Thank you to my family, Rita, Jim, and Corrin, whose support has been so important to me. Thank you to all my fellow graduated (and soon to be) Western friends, specifically Marcus, Taylor, and Diego, as well as many others. The practical jokes will never be forgotten. And finally, thanks, Obama, for providing the inspiration to rise above and prove your harshest critics wrong (Diego). ii Table of Contents Abstract ................................................................................................................................ i Acknowledgments............................................................................................................... ii Table of Contents ............................................................................................................... iii List of Tables .................................................................................................................... vii List of Figures .................................................................................................................... ix List of Appendices ........................................................................................................... xvi 1 Introduction .................................................................................................................... 1 1.1 Introduction ............................................................................................................. 1 1.2 The importance of impact cratering ........................................................................ 3 1.3 The impact cratering process .................................................................................. 4 1.3.1 The shock wave........................................................................................... 4 1.3.2 Stage 1: Contact and Compression ............................................................. 5 1.3.3 Stage 2: Excavation..................................................................................... 6 1.3.4 Stage 3: Modification and collapse ............................................................. 7 1.3.5 Types of impact craters ............................................................................... 7 1.3.6 Impact Melting .......................................................................................... 10 1.4 The Vredefort impact structure, South Africa ...................................................... 11 1.4.1 Geology of the Vredefort central uplift or ‘dome’.................................... 13 1.4.2 Evidence of an impact origin for Vredefort .............................................. 16 1.4.3 Shock microstructures in quartz and other minerals ................................. 18 1.5 Zircon .................................................................................................................... 20 1.5.1 Shock microstructures in zircon................................................................ 21 iii 1.5.2 Melt inclusions in zircon ........................................................................... 25 1.5.3 Alkali mobility in glasses under an electron beam ................................... 26 1.5.4 Geochronology of zircon .......................................................................... 27 1.6 Sample Suites ........................................................................................................ 28 1.7 Overview of thesis ................................................................................................ 29 1.8 References ............................................................................................................. 31 2 Microstructural evolution of in-situ zircon across the central uplift of highly shocked Archean crust at the Vredefort impact structure, South Africa .................................... 41 2.1 Introduction ........................................................................................................... 41 2.1.1 Shock metamorphic studies of quartz and other rock-forming minerals .. 42 2.1.2 Zircon shock microstructures .................................................................... 44 2.1.3 Purpose of this study ................................................................................. 46 2.2 Geological Setting ................................................................................................. 47 2.3 Methods & Sample Locations ............................................................................... 48 2.3.1 Methods..................................................................................................... 48 2.3.2 Sample Locations ...................................................................................... 49 2.4 Results ................................................................................................................... 50 2.4.1 Shock microstructures across the core-collar transect .............................. 50 2.4.2 Proportions of pre-impact and shock-related features in zircon populations ................................................................................................................... 67 2.4.3 Types of shock-related features in zircon populations ............................. 70 2.5 Discussion ............................................................................................................. 73 2.5.1 Proportion of pre-impact and shocked zircon features in impacted crust . 73 2.5.2 Microstructural evolution.......................................................................... 75 2.6 Conclusion & Scientific Implications ................................................................... 86 2.7 References ............................................................................................................. 89 iv 3 Impact melt glass inclusions in zircon from the central uplift of the Vredefort impact structure, South Africa ................................................................................................
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