NEW GEOLOGIC PERSPECTIVES ON THE MID-MIOCENE LAKE OWYHEE VOLCANIC FIELD, OREGON

The Lake Owyhee Volcanic Field (LOVF) in eastern Oregon is one of three major mid-Miocene silicic centers associated with Steens and Columbia River flood basalts. Vander Meulen (1989) delineated two calderas at this center: Mahogany Mountain Caldera, formed by the eruption of the Tuff of Leslie Gulch, and Three Fingers Caldera, formed on eruption of the Tuff of Spring Creek. Our new mapping and geochemical and age data indicate that there is a single ~25x30 km caldera that formed on the eruption of single zoned ignimbrite at ~15.9Ma.

The shallowest levels of the ~1-km-thick section of intracaldera tuff are exposed in the northern part of the caldera, where diatomaceous caldera lake sediments overlie the ignimbrite. Postcaldera rhyolitic and trachytic lavas that erupted at ~15.7 Ma locally interacted with the sediments. Feeder dikes for these lavas intrude intracaldera ignimbrite in the southern half of the caldera where the deepest levels of the section are exposed in Leslie Gulch.

New XRF ­analyses on intra- and extra-caldera ignimbrite samples show that the Tuff of Spring Creek is an intrinsic part of the Tuff of Leslie Gulch, with the physical differences being the result of variable degrees of alteration that have destroyed many primary phenocryst phases and textures. The ignimbrite is compositionally zoned from high silica alkali rhyolite high silica rhyolite. Intracaldera lavas and dikes overlap the most evolved portion of the compositional range of the ignimbrite.

Large-volume rhyolite lavas around the margins of the caldera at Mahogany Mountain, Bannock Ridge, and McIntyre Ridge have a range of compositions similar to the Tuff of Leslie Gulch, with the exception of higher Ba and Sr values. In contrast, the McCain Rhyolite south of the caldera, with its higher concentrations of Sr, Ti, and Fe, and lower Y, has chemical similarities to the nearby 10.6 Ma Jump Creek Rhyolite (Cummings et al., 2000).

Eruptions at ~15.9-15.5 Ma at LOVF postdate most silicic volcanism at the other two mid-Miocene caldera centers in the region, High Rock Caldera Complex (16.4-15.7 Ma; Coble & Mahood, 2012) and McDermitt Volcanic Field (16.6–15.6 Ma; Benson et al., 2013). This progression is spatially and temporally consistent with the northward transition from Steens basalts to Columbia River flood basalts (e.g., Camp et al., 2013).