INSIGHT INTO REGIONAL TECTONICS REVEALED THROUGH GEOCHEMICAL AND PETROGRAPHIC ANALYSIS OF THE RHYOLITES OF THE TUMALO VOLCANIC CENTER, CENTRAL OREGON

The Tumalo Volcanic Center (TVC) was a locus of silicic activity within central Oregon during the mid to late Pleistocene (≤0.7 Ma). The TVC is located at the intersection of two major volcanic regions: 1) the north-south trending modern High Cascades arc and 2) the southeast-northwest trending High Lava Plains. Although the TVC sits geographically between these two geologic regions, its petrogenetic relationship to either volcanic setting is poorly understood. As such, the goal of this study is to determine whether the TVC is related to subduction zone or intraplate volcanism by comparing similarities and differences between TVC’s petrography, geochemistry, and thermometry to High Lava Plain and High Cascades arc datasets. The High Lava Plains and modern Cascades produce rhyolites that are distinctive from one another. Silicic volcanism of the High Lava Plains has a “hot-dry” rhyolite signature (e.g., lacks hydrous phases, high FeO*) due to the lack of water present during melt formation, whereas Cascadian rhyolites are subduction-related and dominated by a “cold-wet” signature (e.g., hydrous phases are dominant, low FeO*). Petrographic analysis and BSE imaging reveals that clinopyroxene and orthopyroxene are the dominant mafic mineral phases in all TVC units, with hydrous phases being rare. Quartz is absent from most TVC units, which implies that residual liquid had not reached qtz-fs boundary curve and thus melt temperatures must have been high. TVC whole rock data plot consistently within High Lava Plain data fields, and exhibit high FeO*, FeO*/MgO, and Zr/Sr. Middle REE are not depleted for all TVC units, which is most consistent with average High Lava Plain REE concentrations and likely represents the lack of amphibole as a dominant fractionating phase. Lastly, two-pyroxene, plagioclase-melt, and clinopyroxene-only thermometers report high temperatures of >950°C. These temperatures are greater than those typically recorded from rhyolites within the modern Cascades (<900°C). Current results indicate that the TVC has a closer affinity to the “hot-dry” eruptive products of the High Lava Plains than to the rhyolites of the modern Cascade arc. Future work will include Ar-Ar dating of all eruptive units to determine how the TVC fits in with the timing of nearby regional volcanic activity.