Cordilleran Section - 97th Annual Meeting, and Pacific Section, American Association of Petroleum Geologists (April 9-11, 2001)

Paper No. 0
Presentation Time: 10:10 AM


GIARAMITA, Mario J., Department of Physics and Geology, California State Univ, Stanislaus, Turlock, CA 95382 and HARPER, Gregory D., Dept. of Earth and Atmospheric Sciences, S.U.N.Y, Albany, NY 12222,

Sheeted dikes and pillow lavas were discovered approximately 10 km east southeast of Port Orford, southern Oregon coast, within a long recognized outlier of the Western Klamath terrane. These rocks are part of a greenstone unit previously mapped as a diorite phase of the ~148 Ma Pearse Peak quartz diorite, which intrudes and locally contact metamorphoses slates and metagraywackes of the Galice Formation. Sheeted dikes having multiple subparallel chilled margins and screens of diabase and gabbro are evident in stream-polished exposures on the Elk River and Bald Mt. Creek. A 300 m stretch of pillow lava is continuously exposed on Bald Mt. Creek. A gabbro unit has not yet been identified during preliminary mapping; however, ultramafic cumulates were found in float, and sheared serpentinite crops out along the western margin of the greenstones. Greenstones exhibit intergranular to hypidoiomorphic-granular textures, minor relict igneous cpx, plagioclase, possibly Cr-spinel(?), and abundant secondary actinolite. Chemical analyses of two cross-cutting dikes and a diabase screen from the same exposure yielded high Cr, low Ti, and low Y concentrations. These and other chemical indicators reveal that the dikes and diabase screen are arc lavas with boninitic affinities. Such lavas and/or dikes are present in both the Josephine Ophiolite (JO) and the California Coast Range Ophiolite. The association of the sheeted dikes and pillow lavas with the Galice Fm., however, and their uncomformable relationship with the overlying Lower Cretaceous Myrtle Group(=Great Valley Group) suggest they represent an outlier of the JO, which in its type area is depositionally overlain by the Galice Formation and intruded by plutons similar in age to the Pearse Peak pluton. If the Elk river dikes, pillows, screens, and ultramafic rocks, located about 60 km northwest of the northern terminus of the main body of the JO, truly represent a western outlier, it would greatly expand the geographic range of the JO.