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Three Sisters Volcanic Complex, andesite, plagioclase, pyroxene


The Three Sisters Volcanic Complex in central Oregon is a cluster of three primary stratovolcanoes and a mafic periphery which lies at the intersection of the Cascade volcanic arc and the Basin and Range extensional province. This project focuses on the two youngest stratovolcanoes, Middle Sister and South Sister. Middle Sister experienced a period of frequent eruptive activity from approximately 27 ka to 14 ka, during which it erupted dacites, andesites, and basaltic andesites. South Sister experienced an intense period of eruptive activity from approximately 36 ka to 22 ka, during which it erupted a diverse suite of magmas ranging from rhyolites to andesites, as well as a single basaltic andesite. During the period of overlapping eruptive activity from 27 ka to 22 ka, both volcanos erupted compositionally similar units. This study aims to examine a set of similarly aged units to determine if the two adjacent volcanoes could draw on the same magmatic reservoirs. In order to accomplish this, this thesis examines the textures and compositions of major mineral phases in two compositionally similar units that erupted on the eastern flanks of their respective source volcanoes ca. 24 ka, during the period in which eruptive activity was decreasing at South Sister and increasing at Middle Sister. These units are the andesite of Demaris Lake from Middle Sister and the andesite of West Fork Park Creek from South Sister. Analysis of chemical and textural data of plagioclase crystals suggests that the most primitive crystals in both units may have shared origins from a deeper crustal source. However, the units diverged at some point, resulting in distinct populations of intermediate-composition crystals.


This paper is James Peale's Senior Thesis for the Department of Geology, Western Washington University. James' advisor was Dr. Mai Sas.






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