STRUCTURAL, COMPOSITIONAL, AND EMPLACEMENT ANALYSIS OF THE HORSE BASIN RHYOLITE UNIT NEAR BRUNEAU, IDAHO

Erupted roughly 9 Ma, the 10-20 km³ Horse Basin Rhyolite (HBR) is one of several large volume (>1 km³), high-silica, volcanic units in southern Idaho. It is the youngest of five units associated with the Jacks Creek Volcanic Center (JCVC) and is associated with extension of the western Snake River Plain (WSRP) graben coeval with the formation of the Bruneau-Jarbidge Eruptive Center (BJEC). While the HBR is a remarkably large and geochemically evolved unit, its mode of emplacement remains a matter of debate. Units of similar volumes and silica contents, such as the Cougar Point Tuff, are typically emplaced explosively via large ignimbrites. However, many (e.g. Long Draw Rhyolite and Indian Batt Rhyolite also in southwestern Idaho) exhibit similar characteristics and have been interpreted to have emplaced as large volume lava flows despite their high viscosity.

The HBR is characterized by several facies. Basal breccia, 1-2 m thick and ranging from (vitrophyre) clast-supported to matrix-supported, is common in places where the base of the HBR is observable. The HBR contains distinct basal and upper vitrophyres (10-25% phenocrysts – primarily subhedral to euhedral plagioclase) encasing a lithoidal interior (with similar phenocryst content and morphology). Red and black jostle breccia (RBJB) outcrops as resistant ridges at the upper surface and margins of the HBR, sometimes intermingled with the upper vitrophyre in one outcrop. Despite being visually distinct, preliminary geochemistry of 10 samples taken from the various HBR facies (from top to bottom) at the southwest end of the HBR demonstrate that the unit is remarkably homogeneous: with 72-73 wt% SiO2, 12.5-13 wt% Al2O3, 2.6-3.2 wt% Na2O, and 4.6-5.3 wt.% K2O.

The presence of a basal breccia, and red and black jostle-breccia in the HBR is consistent with emplacement as a lava flow, as is the absence of pervasive eutaxitic textures, pumice wall shards, or lithoidal fragments.