ANALYSIS OF THE EMPLACEMENT, COMPOSITION, AND ERUPTIVE HISTORY OF THE HORSE BASIN RHYOLITE NEAR BRUNEAU, IDAHO

Previously estimated at 10-20 km³ in size, the Horse Basin Rhyolite in Idaho is one of several large volume (>1 km³) volcanic units associated with extension along the western Snake River Plain (WSRP) and the passage of the North American plate over the Yellowstone hotspot. Erupted ~9-11 Ma, the Horse Basin Rhyolite is the youngest of five units associated with the Jacks Creek Volcanic Center (JCVC). Along with many similar large-volume rhyolites in southern Idaho and around the world, the Horse Basin Rhyolite is the subject of controversy and confusion: existing observations and measurements offer conflicting interpretations regarding emplacement style. The Horse Basin Rhyolite has been considered a lava flow in some works and a heavily rheomorphic ignimbrite in others. Initial geochemistry results from 33 samples across the Horse Basin unit indicate an average composition of 72.3 wt. % SiO₂ and suggest an extremely viscous rhyolite magma. Preliminary fieldwork noted several features consistent with heavily rheomorphic ignimbrites including: the presence of local fiammé, minor foreign lithics, fragmented phenocrysts within rhyolite samples, and alternating lithoidal and vitrophyric layers. While foreign lithics are a common indicator of ignimbrites, preliminary sampling indicated that lithics are rarely found in the Horse Basin Rhyolite. However, there are also observations consistent with a lava flow emplacement interpretation: seemingly homogenous geochemistry, flow banding on the surface of outcrops, and the blunt, cliff-like margin of the southern outcrops. Here we describe evidence from recent fieldwork and subsequent laboratory analyses that offer insight into whether the Horse Basin Rhyolite emplaced as a lava flow or as a rheomorphic ignimbrite. Study of this and similar units aids in describing how magma chamber conditions influence large continental silicic systems. Continuing work related to the Horse Basin Rhyolite includes improved mapping of vitrophyric and lithoidal outcrops and additional sampling for geochemical and petrographic analyses to clarify the eruptive history of the WSRP.