Distribution and Characterization of Rhyolites of the Strawberry Volcanics -- Evolution of a Major Rhyolite Field Associated with Columbia River Basalt Magmatism, Eastern Oregon, USA

The Strawberry Rhyolites constitute a significant rhyolite field among the largest in Oregon. Aerial coverage of approximately 386 km2 and an overall estimated volume of ~67 km3 using a median thickness for each unit. On the other hand, the total volume could be greater than 100 km3 if greater thicknesses apply. The Strawberry Rhyolites, a largely unknown mid-Miocene silicic volcanic rocks, crop out amongst voluminous flood basalt flows of the Columbia River Basalt Group (CRBG) in eastern Oregon. The eruptive activity of the Strawberry Rhyolites is currently constrained to span a period of about 2 million years, ranging from around ~16.2 to 14.3 Ma. Mid-Miocene rhyolites are part of the Strawberry Volcanics located within the Malheur National Forest near the town of John Day and were first mentioned in the 1960-70s. Still, neither their distribution, composition nor ages were studied in detail until we started to work in the area. Building on our initial reconnaissance data, the main efforts to map rhyolites took place in the context of three Edmap projects from 2019-2021 aided by PSU field study students and covering the area of three 7.5 min quadrangles (Jump-off Joe Mountain, Big Canyon, 1/2 of Logan Valley West, 1/2 of Magpie Table quads) in the NE part of the Harney Basin north of Burns, have shed light on the existence of a diverse set of volcanic units previously lumped into broad single mapping units by earlier regional reconnaissance maps. Hence, there is a much better understanding of the geologic history of the entire northeast margin of the Harney Basin, which plays a significant role in the regional volcanic framework due to this mapping. The focus is to detail the distribution of rhyolite units used for volume estimates, determine the composition and petrography, and obtain additional age dates to constrain better the eruptive history and generate a geologic map covering the study area. This thesis summarizes fieldwork, XRF/ICP-MS geochemistry, thin section petrography, and age date analysis by the 40Ar/39Ar method spanning 2017 to 2020. Nine distinct rhyolite units make up the Strawberry Rhyolites. These are distinguished by their lithology, petrography, and compositions. This study does include a brief overview of other non-Strawberry Rhyolite units found in the area. Our study has revealed a rhyolite field that is among the largest in Oregon. A minimum of 10 distinct effusive rhyolite units erupted over a 2-million-year period in addition to one mixed, rhyolite-andesite