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Date of Award
Spring 2025
Document Type
Masters Thesis
Department or Program Affiliation
Geology
Degree Name
Master of Science (MS)
Department
Geology
First Advisor
Sas, Mai
Second Advisor
Andersen, Nathan L.
Third Advisor
Caplan-Auerbach, Jacqueline
Abstract
The Three Sisters Volcanic Complex (TSVC) is located in the central Oregon Cascades, at the intersection of the Cascadia Subduction Zone, the Basin and Range extensional province, and the High Lava Plains. The convergence of these tectonic regimes has resulted in a high volume of volcanism, producing the three main stratovolcanoes and many smaller edifices that make up the TSVC. Two of the main stratocones, South Sister (active 40-2 ka, largely intermediate to felsic) and Middle Sister (active 50 to 14 ka, largely mafic to intermediate), erupted a diverse suite of lava compositions ranging from basaltic andesite to true rhyolite. All but one of the rhyolites are associated with South Sister, including the rhyolite of Separation Creek (rsc) that erupted as a dome in the saddle dividing the sibling volcanoes. This rhyolite is unique in that it contains abundant mafic enclaves and crystal clots throughout the entire dome. This study focuses on the major mineral phases in rsc, both within the silicic host and in the mafic componentry, using clotting relationships, textural observations, and mineral and glass compositions to make inferences relating to the magmas that constitute rsc. Results reveal two magmatic endmembers, a voluminous rhyolitic host magma (plagioclase + amphibole + pyroxene) and a basaltic andesite mafic recharge magma (plagioclase + olivine), as well as a less abundant reaction product between the two. Magmatic parameters (temperature, pressure, water contents, oxygen fugacity) estimated using mineral populations were utilized to explore crystallization conditions, indicating the silicic reservoir was situated in the mid-to-shallow crust (ca. 3 kbar) at relatively high temperatures (ca. 930 ºC), and with somewhat low water contents (ca. 3.8 wt.% H2O), prior to injection of the hotter (ca. 1080 ºC) and slightly drier (ca. 3.2 wt.% H2O) mafic magma. More oxidizing conditions conveyed by ilmenite and magnetite pairs support the association of the silicic component with South Sister. In contrast, olivine compositions are high Ni, suggesting association of the mafic component with Middle Sister. Taken together, rsc is the first studied lava to suggest interaction between the South Sister and Middle Sister magmatic systems.
Type
Text
Keywords
South Sister, Middle Sister, petrogenesis, trace elements, thermobarometry
Publisher
Western Washington University
OCLC Number
1522762936
Subject – LCSH
Petrogenesis--Oregon--Three Sisters; Rhyolite--Oregon--Three Sisters; Minerals--Oregon--Three Sisters; South Sister Volcano (Or.)
Geographic Coverage
Three Sisters (Or.); South Sister Volcano (Or.)
Format
application/pdf
Genre/Form
masters theses
Language
English
Rights
Copying of this document in whole or in part is allowable only for scholarly purposes. It is understood, however, that any copying or publication of this document for commercial purposes, or for financial gain, shall not be allowed without the author’s written permission.
Recommended Citation
Cook, Madelyn, "Using mineral records to unveil the magmatic history of the rhyolite of Separation Creek, Three Sisters, Oregon" (2025). WWU Graduate School Collection. 1377.
https://cedar.wwu.edu/wwuet/1377
Appendix data tables