Silicic Volcanism at the Northern and Western Extent of the Columbia River Basalt Rhyolite Flare-up: Rhyolites of Dooley Mountain and Buchanan, Oregon

Abstract:

Mid-Miocene (16.5-15 Ma) rhyolite volcanism associated with the eruption of lavas of the Columbia River Basalt Group (CRBG) stretches from NE Nevada to SW Idaho to NE Oregon over an area with about a 400 km diameter. Dooley Mountain and Buchanan rhyolite complexes are located along the northern termination near Baker City and along the western periphery near Burns of this rhyolite flare-up, respectively. Despite their importance for understanding co-genetic rhyolite volcanism to CRBG magmatism prior data on both complexes are sparse. Limited geochemical data exist only for Dooley Mountain and there is only one age date for either complex.

Our ongoing study of both complexes is targeted to establish eruptive units and stratigraphy, to determine activity span, and to detail the petrology of rhyolite units by mapping or remapping key areas building on prior work (e.g., Evans, J. G., 1992, U.S.G.S. Map GQ-1694), by petrographic analysis and whole rock geochemistry, and by Ar-Ar geochronology.

Both complexes are multi-phase, apparently long-lived centers that erupted mostly rhyolite lava flows and domes but locally derived pyroclastic units occur as well but are subordinate in volume. Rhyolite activity is currently pinned at 15.5-14.7 Ma at Dooley Mountain and at 16.1 Ma at Buchanan but is expected to be extending with more dating. Rhyolites range from aphyric to containing ~ 15% phenocrysts dominated by feldspar or quartz. Rhyolites range from low- to high-silica varieties and indicate variable degrees of differentiation as established by the following ranges for Ba, Sr, and Eu/Eu*, respectively: 1500-500, 200-75, 0.6-0.15 for Dooley Mountain and 1700-100, 250-8, 0.88-0.19 for Buchanan. In addition, preliminary data indicate rhyolites range in their affinity from calc-alkaline to more A-type rhyolites. We speculate that A-type varieties coincide in these peripheral rhyolite complexes with the peak of Grande Ronde Basalt activity that impacted the greater area.