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A Study of Pseudotachylyte Associated with the Sudbury Structure, Ontario, Canada

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A Study of Pseudotachylyte Associated with the Sudbury Structure, Ontario, Canada National Library Bibliothèque nationale du Canada Acquisitions and Acquisitions et Bibliographie Services sewices bibliographiques 395 Wellington Street 395, rue Wellington Onawa ON KlA ON4 ûttawaON KIAON4 Canada Canada Your nlr Votm diafonce Our Ers ~otrenlfdrence The author has granted a non- L'auteur a accordé une licence non exclusive Licence aiiowing the exclusive permettant à la National Library of Canada to Bibliothèque nationale du Canada de reproduce, loan, distribute or seli reproduire, prêter, distribuer ou copies of this thesis in microform, vendre des copies de cette thèse sous paper or electronic formats. la forme de rnicrofiche/nlm, de reproduction sur papier ou sur format électronique. The author retains ownership of the L'auteur conserve la propriété du copyright in this thesis. Neither the droit d'auteur qui protège cette thèse. thesis nor substantial extracts fiom it Ni la thèse ni des extraits substantiels may be printed or otherwise de celle-ci ne doivent être imprimes reproduced without the author's ou autrement reproduits sans son permission. autorisation. ABSTRACï Pseudotachylyte is ubiquitous throughout the Archean and Proterozoic basement of the 1.85 Ga Sudbury impact structure. At the small scale pseudotachylyte is pervasive and occurs as thin, mm-thick veinlets. At the large scale, it occurs as dyke-like bodies up to 500 m thick, associated with steeply dipping planar surfaces. There is a range of sizes between these extremes. Four subconcentric zones of enhanced deformation and pseudotachylyte development have been identified: 0-13, 25-35,42048 and 78-80 km north of the impact melt sheet. The maximum individual pseudotachylyte thickness developed within each of the zones decreases outwards, concomitant with a progressive decrease in overall zone thickness. Four main pseudotachylyte variants are identified in the field: (1) sharp-maagined, which is (a) massive; or (b) flow-foliated; (2) mwmscopically ductile- ordiffuse-mwined; (3) injection; and (4) ca-tuclusiic. Cataclastic pseudotachylytes are more comrnon towards the periphery of the Sudbury Structure and may also occur at the margins of certain pseudotachylyte melts. Gradation exists between al1 four of these end-members. Geochemical, mineralogical and rnicrotextural results show that the pseudotachylytes were produced in high-speed slip zones by the frictional comminution and selective melting of host rocks. Preferential assimilation of hydrous ferromagnesian phases produced relatively basic matrices, leaving more resistant quartz and feldspar rock and minera1 clasts. The pseudotachylytes produced by comminution and frictional melting comprise three distinct assemblages: (1) N.85 Ga inherited rock and minera1 clasts derived from the hosts; (2) 1.85 Ga neo-igneous, rapidly cooled, quartz + sanidine + labradorite + phlogopitic biotite matrix assemblage, formed from a melt at 800-900°C; and (c) a 4.85 Ga retrograde assemblage which overprints both clasts and matrices, which is the result of several post-impact resetting events, spanning 1.85-0.90 Ga. Al1 pseudotachylytes show evidence of recrystallization within -1 km of the impact melt sheet. Field evidence indicates that the majority of the pseudotachylytes fomed in large- displacement fault systems during collapse of the impact-generated transient cavity after passage of the shock wave. These fault systems constitute the rings of the Sudbury multi-ring impact basin. The thinner, mm-scale pseudotachylytes were probably fomed in response to stress and fracturing during, or imrnediately succeeding, the passage of the shock wave. The difference between endogenic pseudotachylytes and the majority of the Sudbury pseudotachylytes is one of scale. Large displacements (e.g. up to 6 km on the moon) on impact ring faults can generate vast volumes of friction melt resulting in spectacularly large bodies as seen at Sudbury. ACKNOWLEDGEMEN'IS This work was funded by NSERC, EMR and LITHOPROBE grants to John Spray, my supervisor. 1 also acknowledge the support of the Ontario Geological Survey and INCO. In particular, 1 would like to thank Walter Peredery of INCO for introducing me to the geology of Sudbury. Discussions and field work with Ken Card and Mike Dence of the GSC regarding Sudbury and impact geology proved invaluable. Amy Prebble, Alastair Still and Susan Tingley ably assisted in the field and are also thanked for their good Company. The technical staff at UNB Department of Geology are acknowledged for their assistance in thin section preparation, drafting, operation of XRF and XRD equipment and photography. The staff of the Electron Microscopy Unit were also invaluable. This work would not have been possible without the support of my supervisor, John Spray, and the many enthusiastic discussions we shared on Sudbury and impact geology. Also, Heather Gibson, Chris Hawkes and James Whitehead are thanked for making the office a pleasant place to be and, in particular, Heather and James for their friendship. The many other friends made here at UNB also made my stay and work here enjoyable. While living in Fredericton 1 have thoroughly enjoyed living with Mary Blathenvick and John L'Aventure. 1 also thank Gretchen Johnston for friendship and support. Lastly, 1 would like to thank my family back in the U.K., in particular, my parents for their constant support in everything 1 do, including coming to Canada to do my PhD. TABLE OF CONTEN'IS CHAPTER 1: INTRODUCTION .................................... 1 Geological setting ......................................... 1 Pseudotachylyte .......................................... 6 Previous Work ...................................... 7 Aim .................................................. 12 CHAPTER 2: FIELD RELATIONS AND DISTRIBUTION ................ 13 Field Observations ........................................ 13 Pseudotachylyte Variants .............................. 13 (1) Sharpmargined. aphanitic- to crystailine-matixed .... 13 (2) Ductile- and diffuse-margined ................... 27 (3) Injections ................................. 30 (4) Cataclastic ................................ 35 Relationship of Pseudotachylyte Types .................... 35 Pseudotachylyte Distribution ................................. 41 CHAPTER3: CHEMISTRY ....................................... 50 Introduction ............................................ 50 Ssimple Locations and Descriptions ............................ 51 Site1 ........................................... 51 Site2 ........................................... 53 Site3 ........................................... 53 Analytical Procedure ...................................... 54 X-Ray Fluorescence ................................. 54 Analytical Scanning Electron Microscopy .................. 55 Chemistry .............................................. 56 Discussion ............................................. 64 CHAPTER 4: MNERALOGY AND PETROLOGY ..................... 67 Introduction ............................................ 67 Anaiytical Techniques ..................................... 67 Host Rock Mineralogy ..................................... 68 Pseudotachylytes ......................................... 77 Matrix Textures .................................... 79 (1) Inherited mineralogy .............................. 83 (2) Neo-igneous mineralogy ............................ 88 (3) Post-impact retrograde mineralogy ..................... 93 Discussion ............................................. 95 CHAPTER 5: GEOCHRONOLOGY ................................ 97 Introduction ............................................ 97 Previous Age Data ........................................ 100 Sample Selection and Description ............................. 102 Analyticd Technique ...................................... 105 Resdts ................................................ 107 Interpretation/Discussion ................................... 112 Conclusion ............................................. 117 CHAPTER 6: ORIGIN OF TKE SUDBURY PSEUDOTACHYLYTES ........ 119 Mode1 for Pseudotachylyte Formation .......................... 129 REFERENCES ............................................... 134 AppendixI .................................................. 150 Appendix II ................................................. 159 AppendixIII ................................................. 172 AppendixN ................................................ 195 AppendixV ................................................. 199 vii LIST OF TABLES Table 3.1: Major element analyses of pseudotachylytes and host rocks from Site1 ................................................. 57 Table 3.2: Major elernent analyses of pseudotachylytes and host rocks from Site2 ................................................. 58 Table 3.3: Major element analyses of pseudotachylytes and host rocks from Site3 ................................................. 59 Table 4.1: Selected ASEM analyses of plagioclases from host rocks and pseudotachylytes ......................................... 70 Table 4.2: Selected ASEM analyses of K-feldspars from host rocks and pseudotachylytes ........................................ 72 Table 4.3: Selected ASEM and electron microprobe analyses of biotites from host rocks and pseudotachylytes .............................. 73 Table 4.4: Selected ASEM analyses of retrograde minerals in pseudotachylytes . 95 Table 5.1: Argon isotope and age data for ten pseudotachylyte samples ........ 109 viii LIST OF FIGURES Figure 1.1 Simplified
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