CHARACTERIZING SILICIC MAGMA GENERATION WITHIN CENTRAL OREGON: THE TUMALO VOLCANIC CENTER

The Tumalo Volcanic Center (TVC) of central Oregon represents a locus of silicic volcanic activity that occurred during the mid to late Pleistocene. This ~700 km² geologic region is composed of silicic to intermediate pyroclastic flows, tephra fallout deposits, volcanic domes and lava flows, totaling at least 30-50 km³ of erupted material over ~400-500 kyr, and is one of a limited number of places where rhyolites and other silicic volcanic rocks occur in the Cascade arc. In addition, the TVC is located in a tectonically complex region at the intersection of the modern Cascadian arc, the western edge of the High Lava Plains, and the northwest boundary of Basin and Range extension.

In this study we present new whole rock, mineral and melt, and thermobarometry data, with a goal of characterizing major TVC eruptive units and elucidating the processes that lead to formation of rhyolite and other silicic magmas in this region. Results thus far based on two pyroxene thermobarometry for the Desert Springs Tuff, Shevlin Park Tuff, Triangle Hill Volcano, Bearwallow Butte, and Todd Lake Volcano suggest relatively high temperatures (>950°C) and equilibration pressures focused between 3 and 8 kbar (mid-crustal depths). Whole rock geochemistry and petrography also indicate that many TVC magmas have affinities with intraplate rhyolites (i.e., high FeO* and FeO*/MgO, crystal-poor, lack hydrous phases, etc.), which is also consistent with the relatively high temperatures being recorded by CPX-OPX pairs. However, some units within the TVC are more similar in composition to arc-like rhyolitic compositions. These irregularities may be the product of variability in melt generation processes related to the complex tectonic region that the TVC resides within. As rhyolitic eruptions within the Cascade Arc are rare, the TVC offers an ideal location to study the processes related to highly silicic volcanism in arc settings.