Low delta18 oxygen rhyolites of southwestern Idaho and vicinity

This dissertation is divided into three chapters, each to be submitted to peer-reviewed journals, addressing the volcanology, petrology, and petrogenesis of rhyolitic volcanics in southwestern Idaho.;Chapter 1: Two models for the origin of low-delta18O rhyolites are currently under debate. The first, "Magmatic Autophagy" involves a single caldera system (or multiple, near contemporaneous, overlapping systems) producing hydrothermal alteration of the earliest erupted products, which are subsequently melted to form low-delta18O rhyolites. The second model appeals to a pre-existing crustal magma source, which was hydrothermally altered in a significantly prior thermal event. We critically evaluate the plausibility of each and provide thermal, temporal, and volume constraints which must be included in any model for the petrogenesis of low-delta 18O rhyolites of the central Snake River Plain.;Chapter 2: Conflicting explanations have been advanced for the petrogenesis of basalts and low-delta18O rhyolites of the central Snake River Plain. We show that, when the Yellowstone hotspot track including the Columbia River and Steens basalts is treated as a single province derived ultimately from a common mantle source, the isotope geochemistry of these lavas and tuffs can be explained to a first order by simple binary mixing of asthenosphere-derived basalt and granitic crust of the Idaho batholith. This model is consistent with, but does not require, a stationary mantle plume as the origin of the province.;Chapter 3: Rhyolites of the western Snake River Plain erupted near contemporaneously with a number of the low-delta18O rhyolites of the central Snake River Plain, and aside from their oxygen isotope ratios, are similar in chemistry and morphology. Here, we describe the volcanology, petrology, and physical properties of these "normal" delta18O rhyolites and propose a model for their petrogenesis which involves the melting of the middle crust straddling the boundary between the ancient craton to the east, and the younger accreted terrains to the west.