PETROGRAPHY AND GEOCHEMISTRY OF NEW PILLOW LAVA AND DIKE LOCALITIES, ELK RIVER AREA, SOUTHWESTERN OREGON

New exposures of basaltic pillow lava and possible dikes cutting serpentinite have been discovered in rocks previously mapped as the diorite phase of the ~148-Ma Pearse Peak Pluton, ~10 km ESE of Port Orford, SW Oregon. The pillow outcrops are located 1.2 km north of previously discovered sheeted dikes on the Elk River. The dikes suggest the presence of an ophiolite which recently was proposed to be an outlier of the Josephine ophiolite (JO). The pillows are exposed along a 30 x 15 m stream cut on Bald Mt. Creek. Larger pillows average 25 x 40 cm, and clusters of 7.5 cm-radius pillows are common. Primary igneous minerals include plagioclase and augite. The rock has a very fine-grained matrix and displays dendritic to feathery textures suggesting undercooling, consistent with submarine eruption. Grain size decreases significantly from cores to rims further corroborating pillow structure. Chemical analyses of two pillow lavas yielded SiO₂ contents of 51.8 and 47.9 wt% indicating basaltic composition. The samples plot in the MORB field on Ti-vs-V (Ti/V=36 and 32) and Cr-vs-Y diagrams. The pillows have unusually high TiO₂ contents of 3.0 and 2.9 wt % and FeO contents of 11.2 and 15.5 wt%, similar to lavas in the upper pillow lava unit of the JO. Exposed in road cuts 700 m SW of the pillow lavas are at least four 5-m-wide exposures of mafic rock alternating with 20-m-wide exposures of sheared serpentinite. The mafic rocks are fine-phaneritic, contain zoned hornblende (brown core, green rim) and plagioclase, and are interpreted as dikes. One mafic dike(?) was analyzed: it has relatively high TiO₂ (1.7 wt%) and FeO (12.2 wt%) and plots in the MORB field on Ti-vs-V (Ti/V=24) and Cr-vs-Y diagrams. This rock has geochemical similarities to the pillow lavas, but the presence of magmatic hornblende suggests it may be a post-ophiolite dike similar to bodies that cut the type JO. A dacite dike (65 wt % SiO₂) also cuts sheared serpentinite approximately 300 m NW of the mafic dikes(?). It is composed of plagioclase and a few quartz microphenocrysts in a fine-grained matrix and is interpreted to be related to the Pearse Peak quartz diorite. The discovery of the new pillow locale supports the earlier interpretation that an ophiolite is present. The petrography and geochemistry of the pillows and dike(?) are consistent with the hypothesis that the ophiolite is an outlier of the JO.