PETROLOGY AND GEOCHEMISTRY OF OPHIOLITIC DIKES OF THE EASTERN ELK OUTLIER OF THE WESTERN KLAMATH TERRANE, SOUTHWESTERN OREGON

The Elk outlier (EO) of the western Klamath terrane (WKT) is about 30 km west of the WKT proper where metasediments of the Late Jurassic Galice Formation sit on the Josephine ophiolite (JO) and are intruded by Early Cretaceous arc-related intrusives. The Galice and intrusives are exposed in the EO, but the basement to the Galice is unrecognized. We examined two previously unmapped dike-bearing outcrops in the eastern EO. The first, a road cut located ~ 250 m west of the Coquille River Fault zone, exposes dikes intruding cumulate gabbro and cumulate olivine websterite ~ 8 km South of Powers, Oregon (N42°43.81’ lat. and W124°4.59 long.). Outcrop dimensions are ~ 30m x 10m x 10m. The dikes have chilled margins; average ~ 6 cm in width and 0.1 mm in grain size; and consist of primary albitized plagioclase and augite, with secondary sericite, prehnite, (in veins) and chlorite. Geochemical analyses were performed at Geoanalytical Laboratory, Washington State University. The dike contains 460 ppm Cr, 15 ppm Y, has a Ti/V ratio of 16.7, is LREE-depleted, and plots in the IAT field on Cr-Y, Ti-V and Th/Yb (0.11) vs Ta/Yb (0.03) discriminant diagrams. The second locality, ~ 6 km WNW of the first, is very steep-faced, cliff-forming outcrop (N42°45.70’ lat. W124°8.42’ long.), ~20 m tall x 30 m long, that exposes what we interpret to be sheeted dikes. Chilled margins are weathered out in outcrop. The presence of steeply dipping dikes striking ~ S64E is evidenced by repeated sequences of gradually increasing grain size from ~ 0.25 mm to 0.65 mm in one direction punctuated by an abrupt decrease in grain size, presumably as the chilled margin is crossed. A chilled margin was observed in a thin section. The dikes are hydrothermally altered and contain albitized plagioclase, uralitic amphibole, epidote, and sphene. Geochemical analysis yielded 162 ppm Cr, 22 ppm Y and a Ti/V ratio of 30.7. This sample is a slightly LREE-enriched MORB by Ti/V and an IAT by Cr vs Y, making it transitional. The Th/Yb (0.11) vs Ta/Yb (0.07) ratios show it to be more enriched and to have less of a subduction zone component than the dike cutting gabbro. The dikes are likely the lower and middle portions of a sheeted dike complex. Both samples are geochemically similar to samples from the lower pillows and early dikes of the JO suggesting the possibility that the JO is basement to Galice in the eastern Elk outlier.