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Magmatism and Metamorphism in the Leech River Complex

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Magmatism and Metamorphism in the Leech River Complex MAGMATISM AND METAMORPHISM IN THE LEECH RIVER COMPLEX, SOUTHERN VANCOUVER ISLAND, BRITISH COLUMBIA, CANADA - IMPLICATIONS FOR EOCENE TECTONICS OF THE PAClFlC NORTHWEST by Wesley Glen Groome, B.Sc. .University of Alberta, 1998 A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE in the OEPARTMENT OF EARTH SCIENCES, SIMON FRASER UNIVERSITY, Bumaby, BC Q Wesley Glen Groome 2000 SIMON FRASER UNIVERSITY May 2000 All rights reserved. This work may not be reproduced in Mole or in part, by photocopy or other means, without the permission of the author. The aidhar has granted a ncm- L'auteur a accordé une licence non exciusive licence allowing the exclusive permettant B la National hiof Canada to Bbiiothéque nationaie du Canada de reproduce, loan, distribute or seiî reproduire, prêter, distribuer ou copies of this thesis in microfonn, vendce des copies de cette thèse sous paper or electrdc farmats. la fmede microfichelfilm, de reproduction sur papier ou sur foxmat électronique. The author retains ownership of the L'auteur comme la propriété du copyright in this thesis. Neither the droit d'auteur qui protège cette thèse. thesis nor subsîantial extracts fbm it Ni la îhèse ni des extraits substantiels may be printed or othdse de celle-ci ne doivent être imprimés reproduced without the auîhor's ou autrement reprodrnts sans son permission. autorisation. ABSTRACT The Leech River Complex of southem Vancouver Island consists of the Leech River Schist and Mesozoic and Eocene deformed igneous rocks. In the study area, the igneous rocks consist of the Walker Creek intrusions, and two previously unrecognized intrusive un&, the Tnpp Creek metabasite and the Jordan River metagranodiorite. The Walker Creek intrusions occur as peraluminous, tonalitic dykes that are interfoliated with the Leech River Schist, and range in width from several centimetres to several metres. The Tripp Creek metabasite occurs as between 5 and 12 actinolite-chlorite-biotite-garnet schist and metagabbro interfoliated with the pelitic Leech River Schist. The Jordan River metagranodiorite consists of a large, homogenous, foliated to massive biotite granodiorite and several related dykes. The study area is bounded on the south by the Leech River Fault, a 500 m-wide sinistral oblique shear zone. Metamorphic grade in the study area ranges from zeolite-facies in the Metchosin lgneous Complex rocks to amphibolite-facies in the southem Leeh River Complex. Within the Leech River Complex, two metamorphic regimes are apparent: a regional greenschist-facies regime and an amphibolite-facies regime which is interpreted as a contact metarnorphic event. Gamet-biotite exchange geothennometric results for a number of samples from the amphibolite-facies rocks, combined with published petrogenetic grids, indicate a pressure- temperature range between 2.5 and 4 kbar at 500 to 550 OC. New U-Pb and 'OArpAr age deteminations from within the study area reveal that the Walket Creek intrusions were emplaced between 50 and 51 Ma (50.9 +/- 0.6 Ma WG98-1-1-1C and 50.7 +/- 0.9 Ma WG98-5-44 C). Magmatism was followed by rapid exhumation of the Leech River Complex between 45.2 +/- 0.2 Ma ('OArPgAr muscovite cooling age) and 42.5 +/- 0.2 Ma (40Arf39Arbiotite cooling age). Age determinations from the Jordan River metagranodiorite are more problematic. A definitive crystallization age was not determined, although a titanite cooling age of 88 Ma is considered to be a minimum age. Petrogenetic modeling using trace elements indicates that the Walker Creek intmsions can be generated by a three-stage process involving the partial melting of the Leech River Schist, mixing the resultant magma with the Tripp Creek metabasite, and finally by fractional crystallization. A tectonic mode1 is proposed by which the subduction of the Kula-Faraflon ridge near the latitude crf the Leech River Complex between 51 and 50 Ma is the root cause of the Walker Creek magmatisrn, Following this magmatism, the Metchosin lgneous Complex was juxtaposed against the Leech River Complex along the Leech River Fauk which caused the rapid uplift of the Leech River Cornplex. ACKNOWLEDGEMENTS Many people have inspired, guided and grounded me during my time at Simon Fraser University. First and foremost I would like to acknowledge the efforts of rny senior supervisor, Oerek Thatkelson, for providing me the oppoitunity to hone my craft and the guidance to become a better geologist. I would alsa like to express my gratitude ta the rest of my cornmittee, who have provided valuable insight at various stages of this project. Nick Massey (BCGS) for his insight into Tertiary tectonics and for providing the catalyst for getting this project started. Dan Marshall (SFU) for providing and helping to interpret microprobe data for geothennometry. Jirn Mortensen (WC) for the use of his facilities for geochronology, and for input in that arena. In addition to these individuals, several other geologists have provided insight and guidance throughout this project. I wuld like to thank Richard Friedman (WC) for his many insightful discussions about geochronology, and for enthusiasticaliy running the many samples that were submitted. Steve Johnston (UVic) and John Moore (SFU) provided views into the structural geology that had eluded me - thank you. Finally, 1 would like to thank Paul Layer and Jeff Drake (LM) for providing me with the aArPAr geochronology data. Without the comptent assistance of my field crew(s) I would never have been &le to complete the fieldwork portion of this project. Pat Johnstone assisted me for one week of reconnaissance mapping at the beginning of my first field season, and helped me to pick my base camp location for the rernainder of the season - a place he affectionately called the "post-apocalyptic den of sin." Sheryl Beaudoin assisted me for 6 weeks of mapping, during which she was always enthusiastic, especially when everything she had got wet. Katrin Breitsprecher, who assisted me for one week in the field, was also enthusiastic, even though everything she had got wet. Finally Danette Schwab assisted me for ten days in the field, during which she was exposed to the Sun, snow and beer that makes field work mernorable. I would also like to Say thank you to my fellow graduate students at Simon Fraser University (Chris, Dam, Brian, Deanne, Michelle, Katrin, Scott, Dan, Kevin, Lorraine, Michele, Jane, Selina, Jeff, Quinn), who seemed to know just when to suggest going for refreshment. In particular I would like to thank my office mate Katrin for the many enlightening discussions we had about Tertiary tectonics. Finally, I would like to thank Allison Morling for her patience and understandingI during my time at SFU. You're the one who kept me focussed on my final goal. Ithink the imrnortal words of Robert Plant and Jimmy Page can sum up my time at Simon Fraser University: %ood times, bad times You know i've had rny sharen TABLE OF CONTENTS TABLE OF CONTENTS............................ .... ....... .. ....... ....... vii 1.1 Study Area Location and Access ............ ,....................... 8 2 SELECTED ASPECTS OF REGIONAL GEOLOGY ..................... ... 12 . 2.1 Pacific Rim Terrane.....,,..,,,.... .. ..................... ............ ...... 13 2.1 .1 Leech River Complex ....................... ... ...,........... 13 2.1.2 Pacific Rim Complex .............................-.... .. 15 2.1.3 Pandora Peak Unit ....................................................... 16 2.2 Metchosin lgneous Complex and Crescent Terrane ... ... ... .. .. .. .... .... .. .. .. .. .. .. .. 18 2.3 Carmanah Group ....................................................... ......... 21 2.4 Eady fertiary intrusive rocks of Vancouver Island .............. ....... .. ................... ............ .......... 21 3 UNIT DESCRIPTIONS AND TYPE LOCAtmES..m......m.. .. ....... 23 3.1 Metasedimentary Rocks..................................................... 23 3.1 .1 Greenschist Facies................................................. 24 3.1.2 Amphibolite Facies.................................................... 28 3.2 Metaigneous Rocks .............................................................. 33 3.2.1 Jordan River metagranodiorite ..................................... 33 3.2.2 Walker Creek intrusions.............................................. 41 3.2.3 Tdpp Craek metabasite......................................... 57 3.2.4 Metchosin lgneous Complex ...................................... 62 3.3 Bear Creek mylonite .................................. ............................ 64 4 STRUCTURAL GEOLOGYmmmmm..mm*mm.w.mm.m*m~.m.m.m.m.mm.mmm..m.mmmm.mmmmmm..m~m m. 67 4.1 Di Stwdures.. ...................................................................... 68 4.2 D2Stnictures.................................................................... 70 4.3 Timing of Magmatism Relative to Deformation...................... 72 4.4 Leech River Fault and the Bear Creek shear zone ................ 74 4.5 Implications of the Bear Creek shear zone ............................ 76 5 METAMORPHISM IN THE STUDY AREA. .................. .. .......II.... 80 5.1 Metamorphism in the Metchosin Igneous Complex ............... 80 5.2 Greenschist Grade Regional Metamorphism......................... 81 5.3 Amphiborie Grade Contact Metamorphism........................... 82 5.4 Geothermometry................................................................ 85 5.5 Relationship between rnetamorphism and deformation......... 87 6 WHOLE ROCK GEOCHEMISTRY......
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