GEOLOGIC EVOLUTION OF THE MIDDLE EOCENE WILDCAT MOUNTAIN CALDERA, CENTRAL OREGON, USA

The Wildcat Mountain caldera, located along the western edge of the Blue Mountains in central Oregon, is a 16 × 11 km, northeast-trending vent complex that collapsed and filled with more than 90 km³of rhyolitic tuff during the middle Eocene. It is one of only a few caldera sources for Paleogene ash-flow tuffs identified within the outcrop belt of the Clarno and John Day formations and is the first recognized to be Eocene in age. Rocks of the Wildcat Mountain caldera record the onset of widespread Paleogene ash-flow tuff eruptions in central Oregon and overlap the regional transition from dominantly calc-alkaline magmatism that characterizes the Clarno Formation to the bimodal basalt-rhyolite suite typically associated with the John Day Formation.

Systematic geologic mapping, supported by geochemistry, geochronology, and petrography, has distinguished three major volcanic phases associated with evolution of the Wildcat Mountain caldera. The earliest phase is associated with the development of the pre-caldera Ochoco volcanic field, a series of calc-alkaline, porphyritic to aphyric andesite and dacite lavas, domes, and shallow intrusions emplaced between 43.86 ± 0.89 and 41.50 ± 0.48 Ma. Pre-caldera magmatism was strongly associated with northwest-oriented (N50°W) intrusions and dikes. Initial intermediate volcanism in the field was followed by eruption of the Tuff of Steins Pillar at 40.21 ± 0.39 Ma and synvolcanic subsidence of the Wildcat Mountain caldera. The deeply eroded, rhyolitic lithic-pumice tuff is thickly ponded (>425 m thick) within the southern confines of the caldera, but no correlative outflow sheets have yet been recognized. Rhyolite and dacite lavas, domes, and intrusions were emplaced along the ring fracture and in central vent areas around 39.35 ± 0.30 Ma, following the main subsidence phase. Repeated injection of silicic magma within central areas of the caldera during this phase formed a prominent central resurgent dome that was accompanied by the emplacement of linear breccia pipes, hydrothermal alteration, and localized mercury mineralization. Post-mineralization extrusion of basaltic andesite and andesite flows and plugs occurred along caldera margins up until ca. 38 to 36 Ma, when major volcanic activity ceased.