NEW GEOLOGIC AND GEOCHRONOLOGIC DATA ON THE LAKE OWYHEE VOLCANIC FIELD, OREGON: A SILICIC CENTER CONTEMPORANEOUS WITH FLOOD BASALT VOLCANISM

Silicic volcanism concomitant with eruption of mid-Miocene Steens/Columbia River flood basalts spread across ~150,000 km² from northern Nevada to the Oregon/Idaho border. One of three major caldera centers in this region, the Lake Owyhee Volcanic Field in eastern Oregon, is the least studied due to difficult access and overprinting by younger Miocene volcanism, faulting, sedimentation, and alteration (Cummings et al., 2000). Rytuba et al. (1991) identified two calderas in the central part of this field: Mahogany Mountain and Three Fingers calderas, which they attributed to collapse on eruptions of the Tuff of Leslie Gulch (TLG) and Tuff of Spring Creek (TSC), respectively. Our new geologic and geochronologic data from this area suggest that there is a single ignimbrite sourced from a single caldera. We interpret the TSC as a later-erupted, more crystal-rich (10%) phase of the crystal-poor TLG, with the appearance of two units being (as suggested by M. Ferns, pers. comm., 2012) a result of alteration. New ⁴⁰Ar/³⁹Ar ages on intracaldera TSC (15.95 ± 0.06 Ma, 2σ) and what is mapped as extracaldera TLG (15.91 ± 0.05 Ma) are consistent with these units being the same ignimbrite (and agree with the Ekren et al. (1984) K-Ar age of 15.8 ± 0.6 Ma on the TLG).

We interpret the Three Fingers caldera of Rytuba et al. (1991) to be the northern part of a larger, ~25x30 km caldera. This part of the caldera preserves thin-bedded caldera lake sediments intruded by rhyolitic and trachytic lavas, including the 15.77 ± 0.11 Ma lava of eponymous Three Fingers Rock. In the southern part of the caldera, mapped by Vander Meulen (1989) and Rytuba et al. (1991) as Mahogany Mountain caldera, a slightly deeper level is exposed in which pervasively altered intracaldera ignimbrite is cut by numerous N-S-trending silicic and mafic dikes—feeders for postcaldera lavas preserved to the north. This new interpretation makes caldera formation contemporaneous with eruption of the ~15.9 Ma Dinner Creek Tuff (Streck et al., 2011; Nash and Perkins, 2012) from the Castle Rock area to the north, and with activity at two other flood-basalt-associated caldera centers: High Rock Caldera Complex, Nevada (15.8-16.4 Ma; Coble and Mahood, 2012), and McDermitt and Whitehorse centers (15.0-16.5 Ma; Rytuba and McKee, 1984; Henry et al., 2012) in northern Nevada and southern Oregon.