First Advisor

Martin J. Streck

Term of Graduation

Spring 2020

Date of Publication


Document Type


Degree Name

Master of Science (M.S.) in Geology






Rhyolite -- Research -- Oregon -- Lake Owyhee, Rhyolite -- Oregon -- Lake Owyhee -- Classification, Geochemistry -- Oregon -- Lake Owyhee, Petrology -- Oregon -- Lake Owyhee, Igneous rocks -- Oregon -- Lake Owyhee



Physical Description

1 online resource (x, 148 pages)


The Lake Owyhee Volcanic Field (LOVF) in eastern Oregon experienced several major eruptive events during the mid-Miocene (16.8 to 15.5 Ma), resulting in the emplacement of 3,900 km3 of rhyolitic lava flows and ash-flow tuffs. Eighteen samples from eleven different silicic centers in the LOVF were selected for this study. While all samples of this study are classified as rhyolite, their compositions vary greatly in terms of trace and major elements. Using trace elements, these rhyolite samples are classified as "A-type", described as resulting from "hot and dry" magmas and associated with hot-spot activity, or as "I-type" or "calc-alkaline", described as rhyolites from "cool and wet" magmas and associated with subduction zones. Seven samples from four units (Buchanan, Circle Bar, Unity, and Dam rhyolites) are classified as I-type, eight samples from five units (Dinner Creek Tuff, Littlefield, Jump Creek, Mahogany Mountain, and Three Fingers rhyolites) are classified as A-type, and three samples from two units (Cottonwood and Dooley Mountain rhyolites) are classified as "borderline", plotting between A- and I-type.

Mineral assemblages were determined for each sample. Similarities in these assemblages can be seen among samples from different centers of the same "type" (A- or I-). I-type samples contain plagioclase feldspar with high An, sanidine with high Or, orthopyroxene, apatite, biotite, and/or amphibole. A-type rhyolites, on the other hand, typically contain lower-An plagioclase, low-Or alkali feldspar, Fe-rich clinopyroxene, and/or zircon.

Temperatures were estimated using geothermometers specific to the mineral assemblages that were recorded for each sample. The temperature range for samples that are classified as A-type is 783 to 984 °C, whereas the I-type rhyolites give a much larger range of 737 to 1043 °C when including every geothermometer used. When excluding geothermometers that yield estimates that are greatly influenced by composition (i.e. high An feldspar will yield anomalously high temperatures), the estimate range for I-type rhyolites is 737 to 900 °C.

The temperatures estimated for the A-type rhyolites in this study are consistent with published A-type rhyolite temperatures, such as those found in the Snake River Plain. The I-type rhyolites are slightly more complicated. When excluding the geothermometers that appear greatly influenced by composition, the I-type rhyolites align with published values, such as calc-alkaline rhyolites found at South Sister in Oregon and Okareka center in New Zealand. The appearance of hydrous minerals, biotite and/or amphibole, in several of these I-type rhyolites correlates with much lower average temperatures than samples that do not contain these phases. Similarly, the I-type samples that contain pyroxene tend to yield much higher average temperatures than other I-type rhyolites.


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