A NEWLY DOCUMENTED MAJOR MID-MIOCENE RHYOLITE FIELD ASSOCIATED WITH COLUMBIA RIVER BASALT MAGMATISM, EASTERN OREGON, USA: THE STRAWBERRY RHYOLITES

Mid-Miocene rhyolites are part of the Strawberry Volcanics located within the Malheur National Forest near the town of John Day and were first mentioned in the 1960-70s. Still, neither their distribution, composition, nor ages were studied in detail until we started to work in the area. Building on our initial reconnaissance data, the main efforts to map rhyolites took place in the context of three Edmap projects from 2019-2021 aided by PSU field study students and covering the area of three 7.5 min quadrangles (Jump-off Joe Mountain, Big Canyon, 1/2 of Logan Valley West, 1/2 of Magpie Table quads).

Our study has revealed a rhyolite field that is among the largest in Oregon. A minimum of 10 distinct effusive rhyolite units erupted over a 2-million-year period in addition to one mixed, rhyolite-andesite pyroclastic deposit from ~16.2 to 14.4 Ma with most ⁴⁰Ar/³⁹Ar ages falling in the 15.4-14.4 Ma bracket, but stratigraphically highest undated rhyolite units could be slightly younger. The mapped distribution of rhyolites covers an area of 186 km^2, but the estimated original distribution area is likely ~400 km² with an estimated volume on the order of 100 km³. Rhyolites range from low-silica to high-silica and from phenocryst rich containing >20% phenocryst to those that are aphyric. All units display glassy-to-devitrified lithologies. Phenocryst assemblages are dominated by plagioclase, some units contain quartz, and mafic silicates often are amphibole, biotite, or both. Orthopyroxene occurs in some units in addition to or instead of biotite and amphibole.

Our data suggest the following age-volume-composition-lithology relationships. Lower silica, crystal-rich units containing complexly zoned plagioclase erupted early, while crystal poorer and silica-rich units are more prevalent later. Units with slight A-type affinities erupted the latest. This sequence is compatible with basalt intruding into crust initiating partial melting to produce rhyolites. This is followed by increased heat input from basalt to support near liquidus rhyolite magmas. Interaction of tholeiitic magmas with rhyolites culminates in the production of A-type-like rhyolites. Evidence for this stage is found in the commingling of Fe-rich andesite and rhyolitic magmas to produce the late tuff of Milk Spring.