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Felsic Magmas from Mt. Baker in the Northern Cascade Arc: Origin and Role in Andesite Production

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Western Washington University Western CEDAR WWU Graduate School Collection WWU Graduate and Undergraduate Scholarship 2012 Felsic magmas from Mt. Baker in the northern Cascade arc: origin and role in andesite production Julie A. (Julie Angela) Gross Western Washington University Follow this and additional works at: https://cedar.wwu.edu/wwuet Part of the Geology Commons Recommended Citation Gross, Julie A. (Julie Angela), "Felsic magmas from Mt. Baker in the northern Cascade arc: origin and role in andesite production" (2012). WWU Graduate School Collection. 239. https://cedar.wwu.edu/wwuet/239 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]. Felsic magmas from Mt. Baker in the northern Cascade arc: origin and role in andesite production By Julie Gross Accepted in Partial Completion Of the Requirements for the Degree Master of Science Kathleen L. Kitto, Dean of the Graduate School ADVISORY COMMITTEE Chair, Dr. Susan DeBari Dr. Michael Clynne Dr. Scott Linneman 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. I acknowledge that I retain ownership rights to the copyright of this work, including but not limited to the right to use all or part of this work in future works, such as articles or books. Library users are granted permission for individual, research and non‐commercial reproduction of this work for educational purposes only. Any further digital posting of this document requires specific permission from the author. Any copying or publication of this thesis for commercial purposes, or for financial gain, is not allowed without my written permission. Julie Gross October 29, 2012 Felsic magmas from Mt. Baker in the northern Cascade arc: origin and role in andesite production A Thesis Presented to The Faculty of Western Washington University In Partial Fulfillment Of the Requirements for the Degree Master of Science By Julie Gross October 2012 Abstract Dacitic magmas in volcanic arcs play a critical role in the growth and development of felsic continental crust through mixing to form andesite, or to a lesser extent, by directly adding new crustal material through fractionation of mantle derived basalts. Though dacitic erupted lavas are scarce on Mt. Baker, this study discusses their importance in subsurface processes such as mixing with more mafic magmas, and their potential to add directly to the volume of continental crust. A comprehensive data set (including major, trace, and rare earth element abundances, as well as petrography and mineral chemistry) reveals that the most Si- rich, Mg-poor dacitic compositions analyzed in this study (dacite of Mazama Lake) can be modeled as liquids derived by crystal fractionation from Mt. Baker high-Mg andesites. These Si-rich compositions are in turn back-mixed with mafic magmas to produce more Si- poor dacites (dacite of Cougar Divide) and andesites (andesite of Mazama Lake). The origin of one enigmatic hornblende-bearing dacite unit (dacite of Nooksack Falls) is unconstrained. None of the dacitic units have geochemical signatures that suggest an origin by melting of a garnet-bearing source such as the subducting slab or the lower crust. The dacite of Mazama Lake (plagioclase, clinopyroxene, orthopyroxene, Fe-Ti oxides) represents a near end-member fractionated composition with only minor contamination from xenocrystic material. Mineral populations commonly lack disequilibrium textures, and exhibit normal zoning. Plagioclase and pyroxene chemistry suggests the majority of the crystal population is original to the dacite of Mazama Lake. Sparse resorbed olivine grains (<1% total crystal population) and weak reverse zoning in some plagioclase and pyroxene grains indicates a minor addition of xenocrystic material. The majority of the Mazama Lake compositions can be reproduced after 44% fractionation iv (55% remaining liquid) of a high-Mg andesite (the andesite of Glacier Creek), with fractionating phases of 69% plagioclase, 16% orthopyroxene, 11% clinopyroxene, 3% ilmenite, and 1% apatite. Excellent fits of major elements, most trace elements are provided by this model. The dacite of Cougar Divide (plagioclase, clinopyroxene, orthopyroxene, Fe-Ti oxides, olivine) and the andesite of Mazama Lake (plagioclase, clinopyroxene, orthopyroxene, Fe-Ti oxides, olivine) are more Si-poor, and exhibit evidence for magma mixing. The Cougar Divide unit exhibits mingling textures in hand sample and both Si-poor units exhibit mixing textures in thin section, such as calcic normal and sodic reverse zoned plagioclase populations and pyroxene grains with abrupt Mg-rich rims. This suggests that their primary geochemical characteristics come from mixing between more mafic and more felsic magmas. The dacite of Mazama Lake can be used to reasonably reproduce compositions observed in the mixed magmas. Mixing between the high-Mg andesite of Glacier Creek and dacite of Mazama Lake can reproduce an average major and trace element composition from the Cougar Divide unit in mixing proportions of ~60% andesite and ~40% dacite. Major and trace element compositions from the andesite of Mazama Lake can be reproduced by mixing ~30% the high-Mg basaltic andesite Tarn Plateau (a less fractionated parent magma of the andesite of Glacier Creek) and ~70% Mazama Lake dacite. The dacite of Nooksack Falls (plagioclase, hornblende, clinopyroxene, orthopyroxene, Fe-Ti oxides) appears to represent a near-endmember composition, but cannot be reproduced by fractional crystallization of any known parental composition at Mt. Baker. A distinct set of minerals with compositions expected from a basaltic source (such as calcic plagioclase grains, and Mg-rich clinopyroxene grains with high Cr concentrations) v suggests the dacite of Nooksack Falls acquired some xenocrystic material. However, removal of this contamination does not permit a fractionation origin from known mafic compositions. One possibility is that the dacite of Nooksack Falls was derived from more mafic magmas that are not currently observed or erupted. These dacites are unlikely to be crustal melts given their high H2O contents. Ultimately, these hypotheses cannot be reconciled without isotopic analysis. The role of dacitic magmas at Mt. Baker is clear; (1) they have the potential to directly contribute to the continental crust through fractionation, and (2) they have a role in mixing, in which andesitic compositions (a common composition at arcs worldwide) are formed. vi Acknowledgements I would like to recognize the numerous individuals and organizations whose time and support helped make this project a success. First and foremost, I would like to extend a heartfelt thank you to my advisor, Sue DeBari. Your guidance, encouragement, and knowledge have helped me develop scientific and critical thinking skills I never knew I had. In addition, I would like to thank my committee members Mike Clynne, and Scott Linneman, whose thoughtful comments and many e-mails aided immensely in polishing this work. A deep thanks to the organizations that made this research possible: The Jack Kleinman Memorial Grant from Cascade Volcanic Observatory, the Graduate Student Research Grant from the Geologic Society of America, a research grant from the Mount Baker Volcanic Research Center, Western Washington University RSP Fund for the Enhancement of Graduate Research, and funds from our own distinguished WWU Geology department. I would like to extend a warm thanks to George Mustoe for instilling in me all the knowledge of sample preparation (of every kind) he has collected over the years. In addition, George was miraculously available when the equipment seemed to be behaving the most ornery. To Dave Tucker, who was a priceless asset in the field with regards to locating the Nooksack Falls dacite. To Nikki Moore, and Steve Shaw, my predecessors who provided tips and valuable advice for my thesis as well as for basic grad student existence. To my fellow graduate students, in particular Rose Bloom and Angela Cota. Finally I would like to thank my husband Michael, who supported me on all fronts, mentally, physically, and emotionally. In spite of all the support around me, this would have been a wearisome and lonely journey without you. vii Contents Abstract .................................................................................................................................................................iv Acknowledgements ............................................................................................................................................. vii List of Tables .........................................................................................................................................................ix List of Figures .......................................................................................................................................................
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