Initiation and Evolution of Subduction: T-T-D History of the Easton Metamorphic Suite, Northwest Washington State
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Western Washington University Western CEDAR WWU Graduate School Collection WWU Graduate and Undergraduate Scholarship Fall 2017 Initiation and Evolution of Subduction: T-t-D History of the Easton Metamorphic Suite, Northwest Washington State Jeremy Cordova Western Washington University, [email protected] Follow this and additional works at: https://cedar.wwu.edu/wwuet Part of the Geology Commons Recommended Citation Cordova, Jeremy, "Initiation and Evolution of Subduction: T-t-D History of the Easton Metamorphic Suite, Northwest Washington State" (2017). WWU Graduate School Collection. 626. https://cedar.wwu.edu/wwuet/626 This Masters Thesis is brought to you for free and open access by the WWU Graduate and Undergraduate Scholarship at Western CEDAR. It has been accepted for inclusion in WWU Graduate School Collection by an authorized administrator of Western CEDAR. For more information, please contact [email protected]. INITIATION AND EVOLUTION OF SUBDUCTION: T-t-D HISTORY OF THE EASTON METAMORPHIC SUITE, NORTHWEST WASHINGTON STATE By Jeremy Cordova Accepted in Partial Completion of the Requirements for the Degree Master of Science in Geology ADVISORY COMMITTEE Co-Chair, Dr. Elizabeth Schermer Co-Chair, Dr. Sean Mulcahy Dr. Susan DeBari GRADUATE SCHOOL Dr. Gautam Pillay, Dean Master’s Thesis In presenting this thesis in partial fulfillment of the requirements for a master’s degree at Western Washington University, I grant to Western Washington University the non- exclusive royalty-free right to archive, reproduce, distribute, and display the thesis in any and all forms, including electronic format, via any digital library mechanisms maintained by WWU. I represent and warrant this is my original work, and does not infringe or violate any rights of others. I warrant that I have obtained written permissions from the owner of any third party copyrighted material included in these files. 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Jeremy Cordova November 18th, 2017 INITIATION AND EVOLUTION OF SUBDUCTION: T-t-D HISTORY OF THE EASTON METAMORPHIC SUITE, NORTHWEST WASHINGTON STATE A Thesis Presented to The Faculty of Western Washington University In Partial Fulfillment Of the Requirements for the Degree Master of Science by Jeremy Cordova November, 2017 Abstract In the Northwest Cascades, results from previous workers show the Easton Metamorphic Suite contains an inverted metamorphic sequence with ultramafic rocks underlain by amphibolite and high-temperature blueschist that are juxtaposed above high-pressure low-temperature Shuksan Greenschist and Darrington Phyllite. In this study, amphibole and white mica 40Ar/39Ar geochronology is combined with cation exchange thermometry to constrain the temperature-time-deformation history of the suite. Amphibolite facies fabrics record two deformation events at 10 kbar. The oldest fabric formed at ~760 C̊ prior to 167.4 Ma (2σ). A second fabric formed at ~590 ̊C prior to 164.4 Ma. Na-amphibole schist associated with the amphibolite crystallized at ~530 ̊C and yields a white mica cooling age of 165.3 Ma. A blueschist overprint on amphibolite occurred at 162.5 ± 0.9 Ma. Post-tectonic mica in metasomatized amphibolite crystallized at 160.1 Ma at ~350 ̊C. Assemblages in the S2 fabric of Shuksan Greenschist and Darrington Phyllite crystallized at ~325 ̊C at 7 kbar, and white mica ages span 148-136 Ma. Results demonstrate subduction began prior to 167 Ma, as recorded by amphibolite facies metamorphism of mafic protoliths underthrust beneath ultramafic rocks. Subduction initiation was followed by cooling of amphibolite below ~500˚C by 167 Ma, and formation of high-temperature blueschist by 165 Ma. Early high- grade rocks were exhumed from ~10 to ~7 kbar and retrograded to blueschist between ~162 and 160 Ma. Temperature decreased to ≤350 ̊C by ~162 Ma and stayed approximately constant from 148-136 Ma during underthrusting and exhumation of Shuksan Greenschist and Darrington Phyllite. iv Acknowledgments Incredible thanks are due to my thesis advisors. Dr. Elizabeth Schermer, who accepted me into Western, and introduced me to the Pacific Northwest’s incredible geology, and provides students with knowledge and inspiration. Dr. Sean Mulcahy, my co-adviser, who guided me through geothermometry, petrography, and the incredible world of mafic metamorphic rocks with a completely unique and insightful style. Dr. Susan DeBari, whose perspective and comments on my previous drafts were very helpful, and each time improved the clarity and interpretations of results. Special thanks must be given to Ned Brown. Without his previous work and collection of samples, maps, and thin sections this study would never have been possible for a master’s student. I would like to thank Dr. Laura Webb and Daniel Jones, from the UVM Noble Gas Laboratory, for agreeing to work with me and providing so much instruction and advice. Also, for teaching me to appreciate complex age spectra, and never be disappointed by the lack of a plateau age. My field assistants deserve incredible thanks: Kenny Frank, Kenny Rukavina, and Curtis Clement. This work was supported in part by funding from the Western Washington University graduate school and Department of Geology. The geochronology in this thesis is supported by the EarthScope program, based upon work supported by the National Science Foundation under Grant Nos. EAR-1358514, 1358554, 1358401, 1358443, and 1101100. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the authors and do not necessarily reflect the views of the National Science Foundation. v Table of Contents: ABSTRACT…………..………………..…….………………………………………………….iv ACKNOWLEDGEMENTS………..……….……………………………………………………v LIST OF FIGURES…………………………………………………………………………….vii LIST OF TABLES………………………..…….……………………………………………...vii LIST OF APPENDICES………………….……………………………………………………vii INTRODUCTION…………………………..……………………………………………………1 THE EASTON METAMORPHIC SUITE…..………………………………………………….4 Regional Geology……………………………………….…………………………….4 Lithologic Units and Structural Fabrics…………….…………………………….6 Amphibole……………………………………………….……………………………..6 Albite-Actinolite and Tremolite-bearing Rocks….………………………………8 Na-amphibole Schist………………………………………………………………..10 Shuksan Greenschist……………………………………………………………….11 Darrington Phyllite…………………………………………………………………..12 PETROLOGY AND GEOTHERMOMETRY………………………………………………..13 Methods……………………………………………………………………………..13 Mineral Chemistry and Metamorphic Temperatures………………………..15 Summary of Metamorphic Temperatures……………………………………..17 40Ar/39Ar GEOCHRONOLOGY………………………………………………………………18 Step-Heating Methods…………………………………………………………….18 40Ar/39Ar Results……………………………………………………………………19 DISCUSSION…………………………………………………………………………………..22 Temperature-time-Deformation History of the Easton Metamorphic Suite..23 Comparison to other subduction zones…………………………………………29 Implications for the Middle Jurassic to Early Cretaceous Cordilleran Margin…………………………………………………………………………….…….30 CONCLUSIONS……………………………………………………………………………….31 REFERENCES…………………………………………………………………………………32 vi List of Figures: 1. Map of the western North American margin, and the Northwest Cascade thrust System………………………………………………………………………………….37 2. Geologic map of the Iron Mountain area, Skagit County, WA……………………38 3. Geologic map of the Gee Point to Finney Creek area, Skagit County, WA…….39 4. Geologic map of the ridge west of Gee Point, WA………………………………...40 5. Field photographs……………………………………………………………………..41 6. Photomicrographs of high-grade rocks……….…………………………………….42 7. Photomicrographs of the Shuksan Greenschist and Darrington Phyllite………..43 8. Stereonets for folds in the Easton Metamorphic Suite…………………………….44 9. Composition plots for amphiboles and garnets…………………………………….45 10. Composition plots for white micas…………………………………………………...46 11. Pressure-Temperature plots for geochronology samples…………………………47 12. Age spectra for high-grade rocks…………………………………………………….48 13. Age spectra for Shuksan Greenschist and Darrington Phyllite…………………...49 14. Temperature-time plot for the Easton Suite………………………………………...50 15. Pressure-Temperature plot for the Easton Suite………………………………......51 16. Fabric versus time plot for the Easton Suite………………………………………..52 17. Tectonic history of the Easton Suite in schematic cross-sections……………….53 List of Tables: 1. Mineral assemblages in rocks of the Easton Suite………………………………...54 2. Mineral chemical analyses in high-grade rocks…………………………………….55 3. Mineral chemical analyses in greenschist and blueschist…………………………56 4. Mineral chemical analyses in the Darrington Phyllite……………………………...57 5. Summary of metamorphic temperatures from the Easton Suite………………….58 6. Geochronology results………………………………………………………………...59 List of Appendices: A. Mineral Chemical Analyses…………………..………………………………………60 B. Geochronology Data…………………………………………………………………..68 C. Photomicrographs of Geochronology Samples…………………………………….73 vii 1. INTRODUCTION Subduction complexes exhumed in orogenic belts provide an unparalleled opportunity for studies of the tectonic and metamorphic processes that