AN OVERVIEW OF THE VOLCANOLOGY, PETROGENESIS, AND TECTONIC SETTING OF THE OWYHEE FRONT RHYOLITES IN THE WESTERN SNAKE RIVER PLAIN, IDAHO

Rhyolites of the western Snake River Plain (WSRP) erupted near contemporaneously with a number of the low-δ¹⁸O rhyolites of the central Snake River Plain, and aside from their oxygen isotope ratios, are similar in chemistry and morphology. They have received less attention than the more voluminous rhyolites of the main Yellowstone Hotspot Track, despite their location in the midst of a region of complex tectonic and volcanic history in the Inland Northwest. They represent a wide variety of eruptive activity, including extensive lava flows, smaller dome fields, and small ignimbrites. Although these rhyolites cannot be explicitly tied to hotspot volcanism, their location (near the Idaho/Oregon border) and age (11.7 - 11.0 Ma) provide insight about the timing of regional tectonic and magmatic activity. Their physical volcanology and structural features provide clear evidence that these units flowed downslope to the northeast into standing water, which indicates there was a relatively extensive lake present with a southwestern shoreline stretching for at least 100 km. We therefore postulate that the WSRP graben was already established by 11.7 Ma. Radiogenic isotopic data for these units also varies widely with location, and appears to represent the boundary between less radiogenic accreted oceanic terrains to the west, and more radiogenic cratonic material to the east. The boundary is quite sharp, and occurs near the Oregon/Idaho border over the distance of less than 5 km, though units on the cratonic side of the boundary get progressively more radiogenic to the east. Geochemical data from whole rock and insitu micro-analysis indicate that these rocks are high temperature, anhydrous, metaluminous, “A-type” rhyolites, which were likely generated at mid-crustal levels from the partial melting of pre-existing crust.