Western Washington University Western CEDAR WWU Graduate School Collection WWU Graduate and Undergraduate Scholarship 2014 A geochemical study of the Riddle Peaks gabbro, North Cascades: evidence for amphibole accumulation in the mid-crust of an arc Angela C. Cota Western Washington University Follow this and additional works at: https://cedar.wwu.edu/wwuet Part of the Geology Commons Recommended Citation Cota, Angela C., "A geochemical study of the Riddle Peaks gabbro, North Cascades: evidence for amphibole accumulation in the mid-crust of an arc" (2014). WWU Graduate School Collection. 362. https://cedar.wwu.edu/wwuet/362 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]. A GEOCHEMICAL STUDY OF THE RIDDLE PEAKS GABBRO, NORTH CASCADES: EVIDENCE FOR AMPHIBOLE ACCUMULATION IN THE MID CRUST OF AN ARC By Angela C. Cota Submitted for 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. Elizabeth Schermer Dr. Robert Miller 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 nonexclusive 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. Angela C. Cota July 22, 2014 A GEOCHEMICAL STUDY OF THE RIDDLE PEAKS GABBRO, NORTH CASCADES: EVIDENCE FOR AMPHIBOLE ACCUMULATION IN THE MID CRUST OF AN ARC A Thesis Presented to The Faculty of Western Washington University In Partial Fulfillment Of the Requirements for the Degree Master of Science By Angela Cota July 2014 Abstract Mid-crustal arc rocks are not commonly exposed, hampering our understanding of magma differentiation processes and mineral crystallization in the mid-crust of arc systems. This thesis presents results of the study of one exposed mid-crustal arc pluton, which is a unique laboratory to understand the geochemical effects of crystallization in this type of system. I report on the major and trace element characteristics of amphibole, plagioclase, and apatite in hornblendite and hornblende gabbro cumulates from the ~44 km2 Riddle Peaks pluton (~77 Ma) in the North Cascades Crystalline Core (NCCC), Washington. Electron microprobe and laser ablation-induced mass spectrometry (LA-ICP-MS), coupled with whole rock major and trace element data, show that the Riddle Peaks contains low Mg# cumulates 2+ with 40.7-47.2 wt.% SiO2; Mg# 33-67, where Mg# is defined as 100*[(Mg/(Mg+Fe )]. The two rock types present in the pluton are a rhythmically layered gabbro, consisting of hornblendite and hornblende gabbro layered with anorthite to plagioclase-rich gabbro, and a massive hornblende gabbro. The layered gabbro has higher Mg# amphibole (60-70, with the majority 66-70) than massive gabbro (60-63) and more anorthitic plagioclase (layered gabbro = An81-85; massive gabbro = An71-77), suggesting that it was formed by a more primitive liquid. This is supported by modeling that shows that equilibrium liquids from the massive gabbros could have been produced by 40% crystallization of a hornblende gabbro lithology from the parent, calculated liquids in equilibrium with the layered gabbros. Equilibrium liquid calculations also allow for calculation of new apatite partition coefficients for 16 trace elements and REE in a mid-crustal, basaltic andesite system. This study finds that cumulate amphiboles crystallized from a basaltic andesite parent are responsible for increasing La/Yb iv ratios in derivative melts, such as arc magmas, continental crust and NCCC magmas (NCCC magmas approximated by liquid compositions from the Cardinal Peak and Tenpeak plutons). Amphibole crystallization decreases Dy/Yb in derivative melts; these results are in accordance with predictions from observed arc magmas. Other observed ratios in arc and crustal magmas, such as high Sr/Y (16-20), low Nb/Ta (10-17) and Ti/Zr (30) relative to primitive mantle/chondritic values (Sr/Y = 4.6; Nb/Ta = 18-20; Ti/Zr = 115) are not explained by amphibole crystallization. It has been suggested that amphibole-rich plutons could fractionate certain incompatible trace element pairs to explain the differing ratios in arc magmas and continental crust versus primitive mantle values. With the exception of REE, the Riddle Peaks pluton does not fractionate these ratios sufficiently to explain the differing ratios. If mineral fractionation is occurring, another mineral partitions these elements; or, another process occurs. v Acknowledgements This research would not have been possible without the support of the Geological Society of America, Sigma Xi and the Western Washington University Geology Department. Additionally I personally would not have been able to complete this project without the support of many individuals. Many, many thanks to Susan DeBari for her invaluable guidance in the realm of igneous petrology. Committee members Dr. Liz Schermer and Dr. Bob Miller provided comments and input throughout the process. La-ICP-MS analysis would not have been possible without Dr. Brian Rusk’s training and help with analyses and data interpretation. In the tradition of WWU graduate students, hearty thanks must go to George Mustoe for all his guidance in the sample preparation process and his friendship. Outside the department I thank the Whitman College Geology faculty for general support. Learning from my peers has been an important part of my graduate experience. To that end, I wish to thank all of my fellow graduate students. Julie Gross in particular was instrumental for her endless guidance and friendship and for obtaining my titanite analyses. Chelsea Mack provided help in the field. Zack McGuire also provided help in the field, and his personal support and contribution to this process went far beyond carrying rocks. And thank you to my ever- supportive family and friends, especially my wonderful and loving parents. vi Table of Contents Abstract ............................................................................................................................................ iv Acknowledgements ......................................................................................................................... vi List of Tables ................................................................................................................................. viii List of Figures .................................................................................................................................. ix Introduction ...................................................................................................................................... 1 Geologic Setting ............................................................................................................................... 5 Regional Geology ................................................................................................................ 5 Riddle Peaks Geology .......................................................................................................... 6 Field Relations and Rock Descriptions ............................................................................... 7 Layered gabbros (hornblendite to hornblende gabbro to leucogabbro) ................. 8 Massive gabbros (hornblende gabbro) ................................................................ 10 Methods .......................................................................................................................................... 12 Whole Rock Chemical Analysis ......................................................................................... 12 Major Element Analysis .................................................................................................... 13 Trace Element Analysis ..................................................................................................... 14 Mineral Chemistry ............................................................................................................. 15 Results ............................................................................................................................................ 17 Whole Rock Major Element Data ...................................................................................... 17 Whole Rock Trace Element Data ....................................................................................