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Geochemistry and Geochronology of Eocene Forearc Magmatism on Vancouver Island: Implications for Cenozoic to Recent Plate Configurations in the Pacific Basin

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Geochemistry and Geochronology of Eocene Forearc Magmatism on Vancouver Island: Implications for Cenozoic to Recent Plate Configurations in the Pacific Basin GEOCHEMISTRY AND GEOCHRONOLOGY OF EOCENE FOREARC MAGMATISM ON VANCOUVER ISLAND: IMPLICATIONS FOR CENOZOIC TO RECENT PLATE CONFIGURATIONS IN THE PACIFIC BASIN Julianne Kathleen Madsen BSc. Hons. Earth Sciences The University of Victoria, 200 1 THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE In the Department of Earth Sciences O Julianne Madsen 2004 SIMON FRASER UNIVERSITY Fall 2004 All rights reserved. This work may not be reproduced in whole or in part, by photocopy or other means, without permission of the author. APPROVAL Name: Julianne Kathleen Madsen Degree: M.Sc. Earth Sciences Geochemistry and geochronology of Eocene forearc magmatsim on Vancouver Island: Title of Thesis: implications for Cenozoic to Recent plate configurations in the Pacific Basin Examining Committee: Chair: Dr. Doug Stead Professor Simon Fraser University Dr. Derek J. Thorkelson Senior Supervisor Associate Professor Simon Fraser University Dr. Daniel Marshall Associate Professor Simon Fraser University Dr. Richard Friedman Research Associate (PCIGR) University of British Columbia Dr. Stephen T. Johnston External Examiner Associate Professor Department of Earth and Ocean Sciences, University of Victoria Date Approved: December 2,2004 SIMON FRASER UNIVERSITY PARTIAL COPYRIGHT LICENCE The author, whose copyright is declared on the title page of this work, has granted to Simon Fraser University the right to lend this thesis, project or extended essay to users of the Simon Fraser University Library, and to make partial or single copies only for such users or in response to a request from the library of any other university, or other educational institution, on its own behalf or for one of its users. The author has further granted permission to Simon Fraser University to keep or make a digital copy for use in its circulating collection. The author has further agreed that permission for multiple copying of this work for scholarly purposes may be granted by either the author or the Dean of Graduate Studies. It is understood that copying or publication of this work for financial gain shall not be allowed without the author's written permission.\ Permission for public performance, or limited permission for private scholarly use, of any multimedia materials forming part of this work, may have been granted by the author. This information may be found on the separately catalogued multimedia material and in the signed Partial Copyright Licence. The original Partial Copyright Licence attesting to these terms, and signed by this author, may be found in the original bound copy of this work, retained in the Simon Fraser University Archive. W. A. C. Bennett Library Simon Fraser University Burnaby, BC, Canada ABSTRACT The forearc area of the northern Cordillera preserves Tertiary magmatism in a semi-continuous belt from Alaska to Oregon. Vancouver Island is situated in a central position within this belt and contains five suites of Paleogene igneous rocks, three of which were studied for this thesis: the Mt. Washington intrusions, the Clayoquot intrusions and the Flores volcanic rocks. Eight new conventional ID-TIMS U-Pb zircon ages were obtained for the Mt. Washington and Clayoquot intrusions (5 1.2 +/- 0.4,48.8 +/- 0.5 Ma, 38.6 +/- 0.1, 38.6 +/- 0.2, 37.4 +/- 0.2, 36.9 +/- 0.2, 35.4 +/- 0.2 and 35.3+/- 0.3 Ma). The compositions of these three suites range from granite to tonalite. Some units display an adakitic trace element signature (commonly associated with slab melting) but have S-type character (indicative of sedimentary source rock) and are interpreted to be anatectic melts of subducted forearc sediment. Forward modelling of tectonic plate configurations in the Pacific basin from 53 Ma to the present was carried out to identify the most plausible plate configuration for forearc magmatism from Alaska to Oregon. Ridge-trench intersection and slab window formation were regarded as the most likely causes of the forearc magmatism, and were used to constrain the plate configurations. The model includes the recently proposed Resurrection plate in addition to the Kula, Farallon and Pacific plates. The modelling suggests that at -46 Ma, the Resurrection plate became segmented into two plates, the more northern of which is herein named the Eshamy plate. The plate model not only accounts for the majority of Tertiary forearc magmatism from Alaska to Oregon, but is also in close accord with Tertiary to Recent inboard magmatic and structural features. DEDICATION For Grandma and Grandpa Madsen ACKNOWLEDGEMENTS Firstly I would like to thank Derek Thorkelson, my senior supervisor, for providing guidance and unwavering encouragement when the project felt out of control. I would also like to thank Majid Al-Suwaidi for always being there, John Laughton for being a great lab mate, Ryan Ickert for all the geology chats and being a fountain of knowledge, Katrin Breitsprecher for being so enthusiastic and encouraging, Natalka Allan for enthusiastic field assistance and a great time surfing, and all the grads for creating such a dynamic working environment. I would like to thank the USGS Slab Window Project for funding, Nick Massey for discussions and additional geochemical data for the Flores volcanics, and Bohdan Podstawskyj at the Royal Ontario Museum for two U-Pb ages. TABLE OF CONTENTS .. Approval ............................................................................................................................ 11 ... Abstract .............................................................................................................................ill Dedication .......................................................................................................................iv Acknowledgements ............................................................................................................v Table of Contents .............................................................................................................vi List of Figures ..................................................................................................................ix List of Tables .................................................................................................................... xi CHAPTER 1- Overview ...................................................................................................1 Objectives .........................................................................................................................1 Study Area ........................................................................................................................2 Thesis Format ...................................................................................................................2 CHAPTER 2- CENOZOIC TO RECENT PLATE CONFIGURATIONS IN THE PACIFIC BASIN: RIDGE SUBDUCTION AND SLAB WINDOW MAGMATISM IN WESTERN NORTH AMERICA ....................................................4 Summary .........................................................................................................................4 Introduction ......................................................................................................................5 Geological effects of slab windows .................................................................................8 Synopsis of Cenozoic to Recent forearc magmatism, Alaska to Oregon ....................... 11 Forearc magmatism in Alaska ........................................................................................13 Paleocene to Middle Eocene ......................................................................................13 Late Eocene to Oligocene ...........................................................................................14 Forearc magmatism on Queen Charlotte Islands ...........................................................15 Middle Eocene to Miocene .........................................................................................15 Forearc magmatism on Vancouver Island ......................................................................16 Middle to Late Eocene ................................................................................................16 New U-Pb age determinations ....................................................................................20 Forearc magmatism in Washington and Oregon ............................................................27 Paleocene to Early Eocene .........................................................................................27 Late Eocene ...............................................................................................................-28 Tectonic Model ..............................................................................................................29 Necessity of a new tectonic model .............................................................................29 The Resurrection plate ................................................................................................30 Purpose and Methodology ..........................................................................................31 Complications to modeling: northward terrane transport ...........................................34 Regional tectonic synthesis ............................................................................................36 53-50 Ma ...................................................................................................................38
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