EARLY TERTIARY VOLCANISM IN THE PACIFIC NORTHWEST; IMPLICATIONS FOR CONTINENTAL MARGIN EVOLUTION

Early Tertiary volcanism in eastern Oregon is represented by the Clarno Formation, a sequence of subaerial intermediate lavas and volcaniclastic rocks that crop out between the Cascade Range and the Elkhorn Mountains, and the overlying John Day Formation, which consists largely of silicic pyroclastic rocks interspersed with some basalt flows. Reported ages range from 54 to 33 Ma for the Clarno Formation and 39 to 20 Ma for the John Day Formation, with the rocks generally becoming younger to the east. The Clarno Formation consists chiefly of calk-alkaline andesite with lesser amounts of basalt, dacite, and rhyodacite, erupted from local vents. Nepheline-normative alkali basalts and transitional lavas are common in the upper parts of the sequence. The overlying John Day Formation consists largely of silicic air-fall tuffs and ash-flow tuffs that generally originated west of the present-day outcrops. Only sparse alkali-olivine basalts, alkaline rhyolites and sparse tuffs were erupted locally. The lower part of the Clarno Formation is a typical calc-alkaline, subduction-related assemblage reflecting subduction along the Columbia Arc, whereas the upper Clarno is a mixed assemblage of both calc-alkaline and alkaline rocks. The John Day Formation, particularly that portion erupted within the present-day outcrop area, consists of essentially a bimodal alkali basalt-rhyolite assemblage. This transition from calc-alkaline to alkaline volcanism generally coincides with westward migration of the subduction zone to its present-day position west of the Cascade Range at about 35 Ma. Westward migration of the subduction zone changed the tectonic environment of eastern Oregon from an active arc to a back-arc regime characterized by local extension and subsidence. Thus, the geochemistry the early Tertiary volcanic rocks of eastern Oregon reflect the tectonic evolution of the region.