Isotopic modeling and the formation of the post-caldera eastern Upper Basin Member rhyolites, Yellowstone, WY

An array of samples from the eastern Upper Basin Member of the Plateau Rhyolites in the Yellowstone Plateau were collected and analyzed to evaluate styles of deposition, geochemical variation, and plausible sources for these low δ18O rhyolites. The eastern Upper Basin Member is comprised of the Tuff of Sulphur Creek, Canyon flow, and Dunraven Road flow, all high silica rhyolites that were thought to have erupted from within the present Yellowstone Caldera between the northeastern resurgent dome and the northeastern portion of the ring fracture. These eruptions are some of the earliest intracaldera rhyolites and followed the last caldera collapse by approximately 160ka, and generally differ from most other intra-caldera rhyolites by low δ18O values (1.5 to 4.5 per mil) , higher radiogenic isotope ratios, increased plagioclase abundance, and less depleted/ more primitive geochemistry, e.g., higher Sc, Ti, Ba, and Sr. Petrologic textures, higher radiogenic 87Sr/86Sr values in plagioclase phenocrysts (0.7134 to 0.7185) than surrounding melt (0.7099 to 0.7161), whole rock radiogenic values, and δ18O depletions on the order of 5 per mil found in the Tuff of Sulphur Creek and Canyon flow indicate at least a two-stage petrogenesis. Assimilation and fractional crystallization processes may have occurred while the magmatic system was younger leaving un-erupted material at the margins. This magmatic rind may have been hydrothermally altered after cooling, subsided during caldera collapses, and then assimilated/melted to form the Tuff of Sulphur Creek and the Canyon flow. The youngest flow from the eastern Upper Basin Member, the Dunraven Road flow, is geochemically more similar to the extra-caldera Roaring Mountain Member and can be modeled using assimilation of hydrothermally altered wall rock from the ring fracture followed by fractional crystallization.