New Perspectives on Prolific and Voluminous Rhyolite Volcanism of the Mahogany Mountain--Three Fingers Rhyolite Field, Eastern Oregon

Voluminous silicic volcanism of the Mahogany Mountain--Three Fingers rhyolite field (MM--TFrf) is spatially and temporally associated with mid-Miocene flood basalts of the Columbia River Basalt province. Early studies of the area advocated for a two-caldera model consisting of the Mahogany Mountain and the slightly younger Three Fingers caldera with pre- and post-caldera effusive rhyolite eruptions. Although close in time, the calderas were thought to be spatially offset producing the tuff of Leslie Gulch and the tuff of Spring Creek. Finding that the tuff of Spring Creek, that is exposed in Leslie Gulch, is an altered product of the tuff of Leslie Gulch, Benson & Mahood (2016) suggested only one large caldera with pre- to post-caldera lavas. With the new data of my study, building on results by Marcy (2013), we can address key outstanding questions regarding the stratigraphic and geochemical evolution of mid-Miocene rhyolite volcanism at the MM--TFrf. Abundant and compositionally variable effusive rhyolites largely postdate the tuff of Leslie Gulch, including the Mahogany Mountain as well as the McIntyre Ridge rhyolite that were considered pre-caldera before. New ages of the Mahogany Mountain rhyolite suite (15.82-15.71 Ma) and stratigraphic, mineral & compositional data, and age relationships along Succor Creek indicate both rhyolites are post-caldera. The only rhyolite underlying the tuff of Leslie Gulch was identified in the Leslie Gulch locality itself, yielding an age of 16.02 Ma. Stratigraphic data reveal that the tuff of Leslie Gulch is a complex, multi-phase deposit with eruptive breaks in between. There are additional discrete explosive events with deposits along Succor Creek and north of Leslie Gulch that are distinguished by age or composition. Similarly, geochronological and compositional data can be used to identify distinct post-caldera rhyolite magmas. In summary, the MM--TFrf represents a prolific rhyolite center that was active from 16.02 to 15.71 Ma thus shifting initiation of rhyolite activity back, to the end of Grande Ronde Basalt volcanism. Rhyolite eruptions recommenced in the SW of the field with the eruption of the 14.94 Ma Birch Creek rhyolite, and 14.42 Ma McCain Creek rhyolite.