ERUPTIVE HISTORY OF A BIMODAL VOLCANIC TERRANE: NEW GEOLOGIC MAPPING AND WHOLE ROCK GEOCHEMISTRY OF THE BLACK HILLS, SOUTH-CENTRAL OREGON

Located within the NW Basin and Range, the Black Hills of Central Oregon expose a bimodal suite of volcanic rock types. This landscape consists of resistant bluffs of rhyo-dacite, mesas of olivine basalt, and gentle slopes of basaltic tuff intruded by basaltic dikes.

We present new 1:10,000 scale geologic mapping of the Black Hills, petrographic analysis, and major element geochemical data of the mapped units. This study represents the first detailed mapping and petrologic analysis of this area, which has been mapped only in reconnaissance by previous authors.

The eruptive history of the mapped area includes three distinct events. 1) Early explosive eruptions produced a palagonitic basaltic tuff. Within the tuff, scoria and lithic clast size and bed thickness increase to the SW, suggesting the eruptive center is located SW of the mapped area. This unit underlies all other mapped units. 2) Later, there was an eruption of a large rhyolite dome complex, including an older, biotite rhyolite, and younger, volumetrically larger plagioclase pyroxene rhyolite. 3) Finally, olivine pyroxene basalt lava flows were erupted, which overlie the tuff and onlap the rhyolite domes. Geochemically similar dikes feeding these flows are observed in the SW Black Hills cutting older units, and are arranged radially. Their radial arrangement suggests that since-eroded vents were present just outside of the mapped area.