RHYOLITES OF DOOLEY MOUNTAIN AND BUCHANAN, OREGON: SILICIC VOLCANISM AT THE NORTHERN AND WESTERN EXTENT OF THE COLUMBIA RIVER BASALT RHYOLITE FLARE-UP

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 350 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. Prior work on Dooley Mountain was done in connection with mapping in the 1980’s (e.g., Evans, 1992) as was done for the Buchanan complex. Limited geochemical data exist for Dooley Mountain and none for Buchanan and only one age date for either complex despite their importance for understanding co-genetic rhyolite volcanism of the flood basalts of the CRBG.

Our ongoing study of both complexes is targeted to establish eruptive units, to place units into an eruptive stratigraphy, to determine activity span, and to detail the petrology of rhyolite units by mapping or remapping key areas, 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% phenocyrsts 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 more A-type varieties, that are dominant in centrally located centers of the CRBG rhyolite flare-up, coincide in these peripheral rhyolite complexes with the peak of Grande Ronde Basalt activity that impacted the greater area (cf. Streck et al., 2015).

Evans, J.G., 1992, U.S.G.S. Map GQ-1694, scale 1:24000,

Streck et al., 2015, Geosphere, v. 11 no 2, doi:10.1130/GES01086.1