Geology of the Mid-Miocene Rooster Comb Caldera and Lake Owyhee Volcanic Field, eastern Oregon: Silicic volcanism associated with Grande Ronde flood basalt

• Definition of Mid-Miocene Lake Owyhee Volcanic Field in SE Oregon is refined.
• These Mid-Miocene rhyolites are associated with Grande Ronde flood basalts.
• Rooster Comb Caldera collapsed at 15.8 Ma on eruption of Tuff of Leslie Gulch.
• Previously identified Tuff of Spring Creek is zeolitized Tuff of Leslie Gulch.
• Resurgence and erosion exposed feeder dikes for lavas that erupted in a caldera lake.

The Lake Owyhee Volcanic Field (LOVF) of eastern Oregon consists of rhyolitic caldera centers and lava fields contemporaneous with and spatially related to Mid-Miocene Columbia River flood basalt volcanism. Previous studies delineated two calderas in the southeastern part of LOVF near Owyhee Reservoir, the result of eruptions of two ignimbrites, the Tuff of Leslie Gulch and the Tuff of Spring Creek. Our new interpretation is that these two map units are differentially altered parts of a single ignimbrite produced in a major phreatomagmatic eruption at ~ 15.8 Ma. Areas previously mapped as Tuff of Spring Creek are locations where the ignimbrite contains abundant clinoptilolite ± mordenite, which made it susceptible to erosion. The resistant intracaldera Tuff of Leslie Gulch has an alteration assemblage of albite ± quartz, indicative of low-temperature hydrothermal alteration. Our new mapping of caldera lake sediments and pre- and post-caldera rhyolitic lavas and intrusions that are chemically similar to intracaldera Tuff of Leslie Gulch point to a single ~ 20 × 25 km caldera, which we name the Rooster Comb Caldera. Erosion of the resurgently uplifted southern half of the caldera created dramatic exposures of intracaldera Tuff of Leslie Gulch cut by post-caldera rhyolite dikes and intrusions that are the deeper-level equivalents of lava domes and flows that erupted into the caldera lake preserved in exposures to the northeast.The Rooster Comb Caldera has features in common with more southerly Mid-Miocene calderas of the McDermitt Volcanic Field and High Rock Caldera Complex, including formation in a basinal setting shortly after flood basalt eruptions ceased in the region, and forming on eruption of peralkaline ignimbrite. The volcanism at Rooster Comb Caldera postdates the main activity at McDermitt and High Rock, but, like it, begins ~ 300 ky after flood basalt volcanism begins in the area, and while flood basalts don't erupt through the silicic focus, are contemporaneous with the latest stages of eruptions nearby. High Rock and McDermitt rhyolites are associated with propagation of Steens Basalt dikes to the south, and LOVF rhyolites with later propagation of Grande Ronde Basalt dikes to the north and north-northwest.