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THESIS APPROVAL the Abstract and Thesis of Christy L. Lee for The

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THESIS APPROVAL the Abstract and Thesis of Christy L. Lee for The THESIS APPROVAL The abstract and thesis of Christy L. Lee for the Master of Science in Geology presented November 9, 2000, and accepted by the thesis committee and the department. COMMITTEE APPROVALS: _________________________________________ Michael L. Cummings, Chair _________________________________________ Kenneth M. Cruikshank _________________________________________ Martin J. Streck _________________________________________ Virginia L. Butler Representative of the Office of Graduate Studies DEPARTMENTAL APPROVAL: _______________________________________ Ansel G. Johnson, Chair Department of Geology Abstract An abstract of the thesis of Christy L. Lee for the Master of Science in Geology presented November 9, 2000. Title: Magmatic processes operating during the middle to late Pliocene and Pleistocene along the Cascades - Basin and Range transition zone near 43º North. A geochemical and structural transition zone exists between the Cascades and Basin and Range provinces in Oregon. Geologic mapping in this area has allowed the definition of six volcanic rock sequences: 1) interfingered alkali-rich basaltic trachyandesite to trachydacite lava flows and low-alkali basaltic andesite lava flows, 2) HAOT lava flows, 3) basaltic andesite lava flows, 4) andesite lava flows and vent deposits with minor basaltic andesite lava flows, 5) HAB lava flows, and 6) interfingered basaltic andesite, andesite, and trachyandesite lava flows. Geochemical models of sequence 1 alkali-rich rocks suggest the fractionation of the parent assemblage plag (An55)+magn+ol±aug at relatively dry (H2O < 0.4 weight percent) and shallow (P = 1.5 to 3 kbar (< 9 km)) initial conditions. As water content increases, augite and olivine disappear to form the daughter assemblage plag (An29)+opx+magn+amph+ap. Approximately 50 percent fractionation combined with contamination by a LIL and LREE enriched and HFSE and HREE depleted contaminant account for the observed geochemical trends in these rocks. Sequence 2 HAOT have similar geochemistry to N-MORB and are LIL, HFSE, LREE and incompatible element enriched when compared to the composition of primitive mantle. In addition, sequence 2 HAOT geochemical trends are more similar to HAOT erupted along the Cascades than HAOT from the Basin and Range. Andesitic lavas of sequences 1 (low-alkali), 3, 4, and 6 show evidence of batch melting and mixing in their scatter on geochemical diagrams and in their textures (e.g., complex plagioclase zoning patterns, abundant inclusions in plagioclase phenocrysts, and multiple plagioclase phenocryst populations). The variety of lava types produced reflect multiple magmatic processes that may be distinctive of the Cascades - Basin and Range transition zone. HAOT and HAB lavas were able to rise to the surface with little to no ponding in the crust. Magmas were able to rise to shallow levels where they underwent fractionation and contamination to produce alkali-rich trachytic suites. Multiple andesitic lavas are most likely the product of batch melting and combined fractionation, contamination and magma mixing. These sequences are consistent with extension in the transition zone during the middle to late Pliocene and Pleistocene. 2 MAGMATIC PROCESSES OPERATING DURING THE MIDDLE TO LATE PLIOCENE AND PLEISTOCENE ALONG THE CASCADES - BASIN AND RANGE TRANSITION ZONE NEAR 43º NORTH by CHRISTY L. LEE A thesis submitted in partial fulfillment of the requirements for the degree of MASTER OF SCIENCE in GEOLOGY Portland State University 2000 Acknowledgments Funding for this study was provided by the U.S. Geological Survey Educational Mapping Program Award Number 98HQAG2102 and the Oregon Department of Geology and Mineral Industries (DOGAMI). This project would never have reached completion without the advice and support of several individuals. Because there are so many, I have chosen to expand on only three people here. I would like to acknowledge my advisor Mike Cummings for the hours of advising during scheduled meetings or just passing in the hall. He taught me that one must produce something before it can be revised. I would also like to acknowledge Martin Streck for his superior teaching skills. I owe much of my knowledge of petrology to his teachings. Finally, I would like to acknowledge my husband, Thomas Confar, who financially supported me and pushed me to complete this project. I would also like to commend the PSU geology department faculty. I not only leave this school with a much increased knowledge of geology, I also leave with a better sense of myself as a geologist. I now feel that I may converse on an intelligent level with other geologists whether they are in the academic or the professional realm. Others who played a role in the mapping project associated with this study are Robin Johnston (PSU), Ken Cruikshank (PSU), David Percy (PSU), Jeff Conaway (PSU), Charlie Palmer (PSU), Tom Wiley and George Priest (DOGAMI), Andy Peavy and Jim Dorr (Winema National Forest), W. P. Layer at the University of Alaska, Fairbanks, Dave Sherrod (USGS), and the rangers of Collier State Park. i Table of Contents Acknowledgments ______________________________________________________ i Table of Contents ______________________________________________________ ii List of Tables __________________________________________________________ v List of Figures ________________________________________________________ vii Introduction __________________________________________________________ 1 Analytical Methods _____________________________________________________ 2 Geology ______________________________________________________________ 4 Regional Geology ____________________________________________________ 4 High Cascades magmatic province _____________________________________ 4 Northern Basin and Range magmatic province ____________________________ 6 Devil’s Garden lava field ___________________________________________ 6 Volcanics of the Cascades-Basin and Range transition zone _________________ 8 Yamsay Mountain volcanic complex _________________________________ 8 Newberry Volcano _______________________________________________ 9 Medicine Lake Volcano __________________________________________ 12 Geology of the study area ____________________________________________ 14 Location _________________________________________________________ 14 Geologic Framework _______________________________________________ 14 Volcanic Stratigraphy ______________________________________________ 18 Petrology ____________________________________________________________ 23 Petrography _______________________________________________________ 23 Geochemistry ______________________________________________________ 23 Sequence 1 _______________________________________________________ 26 Classification ___________________________________________________ 26 Low-alkali subdivision geochemical characteristics _____________________ 29 ______________________________________ Error! Bookmark not defined. Alkali-rich subdivision geochemical characteristics _____________________ 31 Sequence 2 _______________________________________________________ 33 Classification ___________________________________________________ 33 Geochemical characteristics _______________________________________ 34 Sequence 3 _______________________________________________________ 34 Classification ___________________________________________________ 34 Geochemical characteristics _______________________________________ 34 Sequence 4 _______________________________________________________ 35 Classification ___________________________________________________ 35 Geochemical characteristics _______________________________________ 37 Sequence 5 _______________________________________________________ 37 ii Classification ___________________________________________________ 37 Geochemical characteristics _______________________________________ 39 Sequence 6 _______________________________________________________ 39 Classification ___________________________________________________ 39 Geochemical Characteristics _______________________________________ 39 Summary of geochemistry ___________________________________________ 42 Geochemical Modeling _________________________________________________ 45 Sequence 1 alkali-rich subdivision _____________________________________ 46 Major element crystal fractionation model ______________________________ 47 COMAGMAT parameter selection __________________________________ 48 COMAGMAT results ____________________________________________ 49 COMAGMAT limitations _________________________________________ 49 Trace element crystal fractionation model ______________________________ 51 Magma mixing model ______________________________________________ 54 Contamination model ______________________________________________ 57 Sequence 2 ________________________________________________________ 58 Evaluation of primitive character _____________________________________ 58 Discussion ___________________________________________________________ 61 Sequence 1 alkali-rich rocks produced by dry, shallow crystal fractionation __ 61 Liquid line of descent for alkali-rich sequence 1 rocks _____________________ 61 Role of assimilation in the production of alkali-rich sequence 1 rocks _______ 63 Sequence 2 HAOT produced by partial melting of peridotite ______________ 64 Comparison of sequence 2 HAOT to Cascades and Basin and Range HAOT ___ 64 LIL ___________________________________________________________ 65 REE __________________________________________________________
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