GEOCHEMICAL AND AGE CONSTRAINTS ON AN EOCENE-OLIGOCENE VOLCANIC FIELD IN SOUTHERN OREGON- THE PAISLEY VOLCANIC FIELD

Eocene-Oligocene volcanism in Oregon marks an important change in the regional tectonics of the Pacific Northwest. A westward sweep and reorganization of volcanism from the Challis Volcanics in Idaho to the Western Cascades in Oregon is thought to be controlled by stalled subduction and subsequent slab rollback of the Farallon Plate after accretion of the Siletzia Terrane around 55 Ma. Near Paisley, Oregon, a magmatic center first identified by Muntzert (1969) delineates a section of volcanic rocks with basal andesitic lahars/flows transitioning to predominantly welded and non-welded ignimbrites. These were later correlated to the Clarno and John Day Formations, respectively (Walker and McLeod, 1991). In this study zircon U-Pb geochronology, whole rock and single-mineral geochemistry, and Ti-in-quartz thermobarometry were employed in an attempt to understand the evolution of the magmas from which these rocks were derived and to place them into the broader tectonic context.

The volcanic center at Paisley, OR consists of a ~2000 m thick section of basaltic trachyandesite to rhyolite flows and ignimbrites, granitoid intrusions, and tuffaceous sedimentary rocks. These units are dissected into two geographic areas by Summer Lake Basin. On the east side of the basin (Coglan Buttes) at the base of the section, 600 m of lahars are exposed, with a ~350 m veneer of younger ignimbrites and tuffaceous sediments. On the west side of the basin (Paisley Hills), a 1000 m thick section exposes ignimbrites and plutonic units. Seven ignimbrites and two plutons were sampled for zircon and geochemistry. Quartz monzonite and monzonite intrusions have zircon U-Pb ages of 34.1±0.5 Ma and 35.6±0.9 Ma, respectively. Ignimbrite ages range from 25.6±0.6 Ma to 27.7±0.4 Ma. Quartz phenocrysts from two ignimbrites and two hypabyssal intrusions contain average Ti contents of 91 and 167 ppm, 133 and 184 ppm, respectively. Whole rock and single-mineral geochemistry suggests both the older plutonic units and the more evolved ignimbrites may have been derived from the same parent magma, evolved by progressive fractional crystallization. Therefore, it is envisaged the entire section of rocks in both the Coglan Buttes and Paisley Hills sections are derived from a singular volcanic center and need not be directly correlated to units in the John Day type area.