AN OVERVIEW OF PALEOGENE SILICIC VOLCANISM IN OREGON

Onset of Cascade volcanism was marked by eruption of large silicic ash-flows in the southern and central Oregon Cascades, at about the same time that caldera-forming ash-flows were erupting in central and eastern Oregon. Early Cascade volcanism is coincident with the poorly defined transition between Clarno volcanism (largely calc-alkaline and presumed to be subduction-related) and John Day volcanism (largely per-alkaline rhyolites with strongly tholeiitic mafic lavas) in central Oregon. Some early Oligocene Cascade silicic volcanic centers are spatially associated with magnesium-rich pyroxene basalt complexes. Largest Paleogene caldera mapped thus far, the Oligocene Crooked River Caldera, is similar in size to the McDermitt Caldera, at the west end of the Yellowstone Hot Spot Track, Like McDermitt, the Crooked River Caldera is made up of peralkaline rhyolites and tholeiitic basaltic andesite and andesite flows that compositionally resemble middle Miocene Grande Ronde Basalt flows. Similar late Oligocene and early Miocene tholeiitic lavas overlie silicic ash-flow tuffs in the central Cascades. Smaller late Oligocene and early Miocene silicic volcanic centers in both the Cascades and Blue Mountains of central and eastern Oregon are associated with porphyritic andesite and dacite flows and domes.