First Advisor

Martin J. Streck

Date of Publication

Spring 6-19-2017

Document Type


Degree Name

Master of Science (M.S.) in Geology






Petrogenesis -- Umatilla National Forest (Or. and Wash.), Rhyolite -- Umatilla National Forest (Or. and Wash.)



Physical Description

1 online resource (vi, 305 pages)


Tower Mountain Caldera is the main feature of an Oligocene volcanic field located in the Umatilla National Forest, eastern Oregon. It is perfectly suited to investigate models of rhyolite petrogenesis as all of the important rock components for evaluating generation models are present in a single location and thus are presumably related; basalts, intermediate igneous rocks (which consist of older plutons and younger volcanic rocks, which are ~coeval with rhyolites), metamorphic basement rocks of significant grade, and rhyolites of varying composition. The formation of the caldera produced the Dale Tuff, which comprises the intra-caldera and outflow facies. 40Ar/39Ar dating places the age of the tuff at 32.66 ± 0.36 Ma. Post-caldera rhyolites erupted along apparent ring fractures and elsewhere. Radiometric U-Pb dating of zircons from three of these rhyolites yielded ages of 32.167 ± 0.020 Ma (#CH07a), 31.798 ± 0.012 Ma (#TM5), and 31.426 ± 0.016 Ma (#CH08a). All rhyolites at Tower Mountain range from low to high silica varieties. Some of the post-caldera rhyolites are chemically similar to the Dale Tuff, such as sample CH07a, and have compositions typical of rhyolites of calc-alkaline volcanic centers (I-type rhyolites), while others are similar to A-type rhyolites (CH08a and TM5). The ages indicate that the calc-alkaline rhyolites were followed by the A-type rhyolites. The petrogenetic relationships between the various rocks types were evaluated. Partial melt modeling based on experimental melts produced from crustal material indicates that batch partial melting of metamorphosed high silica crustal material modified by the addition of more primitive mafic material by assimilation/contamination is the most likely source for the Tower Mountain rhyolites.


In Copyright. URI: This Item is protected by copyright and/or related rights. You are free to use this Item in any way that is permitted by the copyright and related rights legislation that applies to your use. For other uses you need to obtain permission from the rights-holder(s).

Persistent Identifier

Included in

Geology Commons