POST-CALDERA VOLCANISM IN YELLOWSTONE NATIONAL PARK: PETROGENESIS OF THE INTRACALDERA UPPER BASIN MEMBER OF THE PLATEAU RHYOLITE AND EXTRA-CALDERA MINGLED MAGMA COMPLEXES
Pritchard, Chad J.
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The Plateau Rhyolite erupted following the last large Yellowstone caldera collapse and may have erupted from a rejuvenating magmatic system. Our preferred model for rhyolite genesis is by assimilation of continental crust by mafic magmas, followed by fractional crystallization. This model focuses on the extra-caldera mingled magma complexes and the eastern Upper Basin Member. Radiogenetic isotope trends of the mingled magma complexes show that the rhyolite and basalt portions are isotopically distinct and that cryptic mixing between rhyolite and basalt may have occurred in a number of other extra-caldera rhyolites. Rhyolites of the eastern Upper Basin Member contain petrologic textures and increased radiogenic 87Sr/86Sr in plagioclase phenocrysts (0.7134 to 0.7185) when compared to the groundmass and whole rock radiogenic values (0.7099 to 0.7161). δ18O depletions on the order of 5 / found in the Tuff of Sulphur Creek and Canyon flow indicate at least a two-stage petrogenesis involving assimilation and fractional crystallization. The presence of a low-δ18O rhyolite outside the caldera indicates that country rock was hydrothermally altered at depth and then assimilated to form the Dunraven Road flow.