FACIES AND EMPLACEMENT MECHANISMS OF THE HORSE BASIN RHYOLITE IN SOUTHERN IDAHO: IMPLICATIONS OF DISTAL BRECCIA FORMATION

Erupting roughly 9 Ma, the 10-20 km³ Horse Basin Rhyolite (HBR) is one of several large-volume (>1 km³), high-silica (>70 wt. %) volcanic units in southern Idaho. It is the youngest of the five units of the Jacks Creek Volcanic Center (JCVC), associated with the extension of the western Snake River Plain (WSRP) graben, coeval with the formation of the Bruneau-Jarbidge Eruptive Center (BJEC). The HBR exhibits multiple facies: upper and lower vitrophyre, a central lithoidal zone, red and black jostle breccia (RBJB) throughout the upper unit, and basal breccia. Vitrophyres contain 10-25% phenocrysts, primarily 1-3 mm subhedral to euhedral plagioclase. They also contain quartz, augite, pigeonite, and opaque oxide phenocrysts. The lithoidal interior displays comparable phenocryst content and morphology, often characterized by sheeting joints where exposed. Basal breccia is observed in a couple of different locations within the unit, notably in Big Horse Basin Gap, where 1-2 m thick clast-supported breccia contains clasts of vitrophyre ranging from >0.5 m to around 1 m. Underlying the basal breccia here is ~14-20 cm of fine-grained, laminated white ignimbrite, on top of orange silt-sized Miocene sediment. RBJB forms resistant ridges at the upper surfaces of the unit, often intermingled with upper vitrophyre, and is observed from proximal to distal margins of the unit. RBJB outcrops in the HBR range from ~1-6 m tall and are matrix-supported, encasing 0.01-0.5 m vitrophyre clasts. Approximately from 9 until 2 Ma, the WSRP graben was occupied by the former Lake Idaho. The distal margins of HBR are marked by brecciation suggested to result from the interaction of the HBR unit with the lake. Along the distal margin of the HBR, cemented, angular brecciation, and lithoidal rhyolite are observed. Distal breccia is interspersed with more cohesive units, potentially indicating that parts of the unit broke off and fell into the ancient lake during its flowage. Distal breccia samples are distinctly different from basal breccia samples due to the distal breccia being vesicle-bearing and containing light blue, laminated chert within the clasts.