GEOCHEMICAL AND LITHOLOGIC EVIDENCE OF POSSIBLE RATTLESNAKE CREEK-LIKE BASEMENT ASSOCIATED WITH A COASTAL OUTLIER OF THE JOSEPHINE OPHIOLITE, SW OREGON

An ~ 2.5 km² exposure of serpentinite-matrix mélange has been mapped in the Elk Outlier of the Western Klamath Terrane (WKT), ~10 km ESE of Port Orford, southern coastal Oregon. Greenstone blocks in the mélange were previously mapped as part of the ~145 Ma Pearse Peak Pluton. The mélange lies in fault contact with the Colebrooke schist (=South Fork Mountain Schist) to the west; sheeted dikes we previously correlated with the ~162-164 Ma Josephine Ophiolite (JO) to the northeast; and the Late Jurassic Galice Formation to the north, east, and south. The western margin of the Pearse Peak quartz diorite, which intrudes the Galice Fm., is <100 m east of mélange. The Early Cretaceous Humbug Mountain Conglomerate of the Myrtle Group (=Great Valley Group) unconformably overlies the Galice Formation and the Pearse Peak Pluton. The map pattern suggests the mélange is the structurally lowermost unit and the core a domal upwarp. The mélange consists of a wide variety of blocks that vary in diameter from <1 m to ³100's of m in a sheared serpentinite matrix. The attitude of the shear foliation in the serpentinite varies considerably, even in a single outcrop. Blocks in the mélange include intermediate to mafic pillow and non-pillowed lava, dacite, volcanic breccia, ophiolite breccia, micro-diorite/gabbro, gabbro, garnet-amphibolite, hornblende-granulite, cumulate and non-cumulate peridotite, radiolarian chert, metasandstone, and, possibly, Colebrooke Schist. The relative proportion of blocks to matrix varies from >95% blocks to >95% matrix. XRF and ICP-MS analyses show that mafic volcanic, some hypabyssal, and amphibolite and granulite blocks all have MORB magmatic affinities, ranging from very depleted to enriched MORB. Other hypabyssal blocks have calc-alkaline magmatic affinities. The affinites of the mélange blocks is different from that of the adjacent sheeted dike complex, which is transitional MORB-IAT and boninitic. Lithologies and geochemistry of the mélange are very similar to the lower ophiolitic unit of the Lower Mesozoic Rattlesnake Creek terrane (RCT) that is thrust over the WKT elsewhere; RCT-like rocks, however, also occur as older basement in parts of the WKT. The geology of the Elk outlier is similar to the southernmost, Devils Elbow remnant of the JO, which has been interpreted as a rift facies of the JO.