LITHOLOGIES, FIELD RELATIONS, AND PETROTECTONIC AFFINITIES OF A PROBABLE OUTLIER OF THE JOSEPHINE OPHIOLITE, ELK RIVER AREA, SW OREGON

In a recent abstract, we reported the discovery of an ophiolite remnant associated with the Galice Fm. along the western margin of the ~148 Ma Pearse Peak quartz diorite in a long-recognized outlier of the Western Klamath Terrane near the Oregon coast, approximately 10 km E-SE of Port Orford. We tentatively correlated it with ~162 Ma Josephine Ophiolite. A sheeted-dike complex, locally with gabbro screens, is exposed along the Elk River and consists of multiple parallel dikes having distinct chilled margins. Dike cores have fine-phaneritic textures and typically contain plagioclase and amphibole pseudomorphs after clinopyroxene (cpx) and commonly relict cpx. The sheeted dike unit is in contact with the Galice Fm. along two faults: a well constrained N60°W-trending fault and a less-well constrained N20°E-trending fault. The faults appear to intersect, but their relative ages are not known. Three pillow-lava localities have been recognized in Bald Mountain Creek, a tributary of the Elk River; pillows appear to occur as isolated fault blocks. The pillows are finer-grained than the dikes and typically contain plagioclase and relict augite. Coarse-grained mafic rocks have been discovered at several localities and consist of plagioclase and amphibole, some of which is uralitic and replacing augite; these plutonic rocks are interpreted as the gabbro unit of the ophiolite. Sheared serpentinite, locally with enclosed pyroxenite, may represent the ultramafic section of the ophiolite. Whole-rock XRF and ICP-MS data from the sheeted dike complex and pillow lavas reveal a remarkable range in geochemistry. Two sheeted dike samples are boninitic (TiO₂=0.3 to 0.32 wt %, Y=10 ppm, Ti/V=9), one is an island-arc tholeiite (IAT; TiO₂=0.35 wt %, Y=12 ppm, Ti/V=12), and four are transitional IAT-MORB (TiO₂=0.71 to 0.97 wt %, Y=18 to 21 ppm, Ti/V=18 to 24). Sampled pillow lavas have MORB affinities (TiO₂=1.58 to 3.02 wt %, Y=36 to 66 ppm; Ti/V=26 to 36), three of which are unusual highly fractionated Fe-Ti basalts such as those that occur at modern propagating rifts on mid-ocean ridges and back-arc basins. These unusual geochemical characteristics are identical to the those of the type Josephine ophiolite. Consequently, the correlation of the Elk River area ophiolite with the Josephine ophiolite appears to be valid.