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