VOLCANIC HISTORY AND TECTONIC ASSOCIATION OF LAVAS OF THE BLACK HILLS AND RIVERBED BUTTE, SOUTH-CENTRAL OREGON: NEW WHOLE ROCK MAJOR AND TRACE ELEMENT GEOCHEMISTRY AND GEOLOGIC MAPPING

Located within the NW Basin and Range, the adjacent volcanic centers of the Black Hills and Riverbed Butte represent a continuous exposure of volcanic rocks ranging from basalt to rhyolite. We present new geologic mapping, petrographic analysis, and major and trace element geochemical data for 11 samples spanning the full compositional range of mapped lithologic units.

Geologic mapping focused on the SW Black Hills and elucidated an eruptive history that includes three distinct events: 1) Early explosive eruptions produced a palagonitic basaltic tuff that 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. Reconnaissance of the nearby Riverbed Butte area revealed exposure of pyroxene-olivine basaltic trachyandesite lava flows overlain by hornblende-pyroxene trachydacite lava domes and flows.

The Black Hills and Riverbed Butte expose a bimodal suite of lavas ranging from 49-75 wt.% SiO₂ with a notable gap in composition from 53-64 wt.% SiO₂. These units range from 0.1-7.0 wt.% MgO and from 0.5-4.2 wt.% K₂O, and all samples are peraluminous and calc alkaline. Trace element data demonstrates that all samples are enriched in LILEs and LREEs and relatively depleted in HFSEs. This combined major and trace element data suggest that these NW Basin and Range lavas are related to Cascadian arc magmatism.