Date of Award
Bachelor of Science (B.S.) in Geology and University Honors
Rhyolite -- Classification, Rhyolite -- Eastern Oregon, Geochemistry -- Eastern Oregon
Eastern Oregon rhyolites are understood to be associated, spatially and temporally, with the main phase of the Columbia River flood basalt volcanism extending from 16.8 Ma to 15.9 Ma. This study focuses on rhyolitic units found between John Day and Burns, Oregon. While these rhyolites do not cover a wide extent and all erupted within a relatively short period of time, they vary widely in their chemical composition. Some of the rhyolites in this area are classified as A-type, according to the geochemical definition set forth by Whalen and others in 1989, while others are classified as Calc-alkaline (or I) types. Classifying the rhyolites in such a way implies that the chemical variance between the rhyolites is due to a difference in tectonic origin. However, the significantly small spatial and temporal distribution of their emplacement suggests otherwise. To investigate their source and evolution further, I estimate the temperature and oxygen fugacity of their magmas using the Fe-Ti Oxide thermobarometer by Ghiorso and Evans (2008). I compare the estimations between rhyolites designated as A-type, Calc-alkaline type, and a classification I created named 'Borderline'. Characterizing the physical conditions of each of these rhyolites aids in understanding the overall magmatic environment in the region. Using the compositions of titanomagnetite and ilmenite, I estimate that the A-type rhyolites in this region are on average slightly more reduced than Calc-alkaline types. However, all the rhyolites fall within a similar temperature and oxygen fugacity range of ~700-1100 oC and ±1 ∆NNO log fO2. Since the rhyolites in eastern Oregon are estimated to form in similar physical conditions, then I cannot support the presumption that the A-type and Calc-alkaline rhyolites are formed by distinctly different processes based on this method. The variation in fO2between the types of rhyolites may suggest differing magmatic histories.
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Orr, Aleysa A., "Physical Parameters Based on Fe-Ti Oxide Compositions and Their Correlation with Geochemical Signatures of Mid-Miocene Rhyolites, Eastern Oregon" (2019). University Honors Theses. Paper 744.