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Origin and Geochemical Evolution of Mafic Magmas from Mount Baker in the Northern Cascade Arc, Washington: Probes Into the Mantle and Crustal Processes

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Origin and Geochemical Evolution of Mafic Magmas from Mount Baker in the Northern Cascade Arc, Washington: Probes Into the Mantle and Crustal Processes Western Washington University Western CEDAR WWU Graduate School Collection WWU Graduate and Undergraduate Scholarship 2010 Origin and geochemical evolution of mafic magmas from Mount Baker in the northern Cascade arc, Washington: probes into the mantle and crustal processes Nicole E. Moore Western Washington University Follow this and additional works at: https://cedar.wwu.edu/wwuet Part of the Geology Commons Recommended Citation Moore, Nicole E., "Origin and geochemical evolution of mafic magmas from Mount Baker in the northern Cascade arc, Washington: probes into the mantle and crustal processes" (2010). WWU Graduate School Collection. 44. https://cedar.wwu.edu/wwuet/44 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]. ORIGIN AND GEOCHEMICAL EVOLUTION OF MAFIC MAGMAS FROM MOUNT BAKER IN THE NORTHERN CASCADE ARC, WASHINGTON: PROBES INTO MANTLE AND CRUSTAL PROCESSES By Nicole E. Moore Accepted in Partial Completion of the Requirements for the Degree Master of Science Moheb A. Ghali, Dean of the Graduate School ADVISORY COMMITTEE Chair, Dr. Susan M. DeBari Dr. Elizabeth R. Schermer Dr. Pete Stelling 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. Nicole E. Moore May 3, 2010 ii ORIGIN AND GEOCHEMICAL EVOLUTION OF MAFIC MAGMAS FROM MOUNT BAKER IN THE NORTHERN CASCADE ARC, WASHINGTON: PROBES INTO MANTLE AND CRUSTAL PROCESSES A Thesis Presented to the Faculty of Western Washington University In Partial Fulfillment of the Requirements for the Degree Master of Science By Nicole E. Moore May 2010 iii ABSTRACT Five mafic lava flows located on the southern flank of Mount Baker are among the most primitive in the volcanic field. A comprehensive whole rock and mineral chemistry dataset (including major, trace, REE and isotopic abundances as well as petrography and mineral chemistry) reveals the diversity between these mafic lavas, which come from distinct sources and have been variably affected by ascent through the crust. Disequilibrium textures present in all of the lavas indicate that crustal processes have affected the magmas. Despite this evidence, mantle source characteristics have been retained, demonstrated by a lack of variation in the most highly incompatible elements, insignificant addition of crustal material, lack of an obvious crustal radiogenic source, and minimal chemical differences between lavas that are the result of mixing and their mafic endmembers. In evaluating mantle sources, four endmember lava types are represented. These include modified low-K tholeiitic basalt (LKOT-like), typical calc-alkaline (CA) lavas, high- Mg basaltic andesite (HMBA), and a second but distinct basaltic andesite. The first type, the basalt of Park Butte (49.3-50.3 wt.% SiO2, Mg# 64-65), is classified as low-K, and has major element chemistry similar to LKOT found elsewhere in the Cascades. Park Butte also has the lowest overall abundances of trace elements (with the exception of the HREE), indicating it is either derived from the most depleted mantle source or has undergone the largest degree of partial melting. A second lava type is represented by the basalts of Lake Shannon (50.7- 52.6 wt.% SiO2, Mg# 58-62) and Sulphur Creek (51.2-54.6 wt.% SiO2, Mg# 56-57). These two lavas are comparable to calc-alkaline rocks found in arcs worldwide, and have similar trace element patterns; however, they differ in abundances of REE, indicating variation in iv degree of partial melting or fractionation. The third lava type is represented by the basaltic andesite of Tarn Plateau (51.8-54.0 wt.% SiO2, Mg# 68-70), which has characteristics similar to high-Mg andesite worldwide. The second basaltic andesite unit, Cathedral Crag (52.2- 52.6 wt.% SiO2, Mg# 55-58), can be distinguished from the HMBA endmember because it has much lower MgO, Mg#, Ni and Cr, and is the least primitive of all the lavas in this study. The strongly depleted HREE nature of both basaltic andesite units suggests fractionation from a high-Mg basaltic parent (to a greater extent in the less primitive Cathedral Crag lavas) derived from a source with residual garnet is responsible for their generation. Assessment of the relative enrichment of a slab component in the lavas reveals that the calc-alkaline units are least enriched, the HMBA and basaltic andesite are intermediate, and the LKOT-like unit is most enriched in slab fluids. Petrogenetic indicators point to sediment as the primary slab fluid contributor, with a small role for altered oceanic crust fluids in the LKOT-like unit. Modeled fluid compositions support this assertion. Melt modeling indicates the LKOT-like and calc-alkaline lavas were generated by partial melting of a depleted spinel harzburgite, while the results for modeling HMBA and basaltic andesite units suggest a depleted garnet harzburgite source. Differences in primitiveness (i.e. MgO, Ni and Cr content) among the units are the result of differences in depth and degree of fractionation in the crust. A summary model is presented where 1) varying degrees of a primarily sediment-derived slab component infiltrate the mantle wedge; 2) slab flux assists in melting two distinct mantle sources; 3) melting occurs at various fractions, and the resulting melts stall at different levels in the crust; and 4) differentiation at a range of pressures to different degrees produces mafic magmas with diverse chemistry. v ACKNOWLEDGEMENTS I would like to recognize the many individuals and organizations whose support and efforts made successful completion of this research possible. First and foremost, I extend my deepest thanks to my advisor Sue DeBari, for countless discussions, unwavering encouragement, and limitless enthusiasm. My knowledge and confidence as a geologist has been dramatically increased due to your influence, and I am very fortunate to have had the opportunity to work with you. I would also like to thank my committee members, Liz Schermer and Pete Stelling, for their careful reviews, insightful comments and encouragement. Sincere thanks go to the organizations whose financial assistance made this project possible: the Geological Society of America Parke D. Snavely, Jr. Cascadia Research Award, the USGS Cascades Volcano Observatory Kleinman Grant for Volcano Research, the Western Washington University Department of Geology Advance for Fieldwork, and the Western Washington University RSP Fund for the Enhancement of Graduate Research. I would like to extend heartfelt thanks to Dave Tucker, for showing me the ropes out in the field, providing samples, and always being a consummate supporter. To my field assistants, Jen Wright, Jeremy Andrews, and Kaylen Andrews: thank you for hauling countless pounds of basalt down the mountain and enjoying the amazing beauty of Mount Baker with me. Thanks also go to George Mustoe, who had a knack for appearing in the rock or geochemistry lab just when I needed him. My fellow Mount Baker researcher, Troy Baggerman, provided assistance in innumerable ways whenever asked. My family, friends and fellow graduate students (particularly Julia Labadie and Jen Wright) have been a constant source of encouragement and support. Thanks to you all. vi TABLE OF CONTENTS ABSTRACT……………………………………………………………………………….. iv ACKNOWLEDGMENTS………………………………………………………………… vi LIST OF TABLES………………………………………………………………………... viii LIST OF FIGURES………………………………………………………………………... ix INTRODUCTION…………………………………………………………………………. 1 TECTONIC AND GEOLOGIC SETTING………………………………………………. 4 ANALYTICAL METHODS……………………………………………………………… 8 Whole Rock Analysis……………………………………………………………...... 9 Electron Microprobe……………………………………………………………….. 11 Isotopes…………………………………………………………………………….. 12 PETROGRAPHY AND MINERAL CHEMISTRY……………………………………. 13 Park Butte Basalt………………………………………………………………….. 14 Cathedral Crag Basaltic Andesite…………………………………………………. 16 Tarn Plateau Basaltic Andesite……………………………………………………. 19 Lake Shannon Basalt………………………………………………………………. 21 Sulphur Creek Basalt………………………………………………………………. 23 WHOLE ROCK CHEMISTRY………………………………………………………….. 26 Major Elements…………………………………………………………………….. 26 Trace Elements…………………………………………………………………….. 29 Isotopic Composition………………………………………………………………. 32 DISCUSSION………………………………………………………………………………33 Intensive Parameters………………………………………………………………..34
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