FIELD, PETROGRAPHIC, AND GEOCHEMICAL VARIATION IN ROCKS PREVIOUSLY MAPPED AS THE PEARSE PEAK DIORITE, ELK RIVER, SOUTHERN COASTAL OREGON

In recent abstracts, Giaramita and others reported the presence of an outlier of the Josephine Ophiolite (JO) associated with the Galice Formation in southern coastal Oregon. Sheeted dikes and pillows occur on the Elk River and Bald Mt. Creek, ~10 km ESE of Port Orford, in what was previously mapped as the diorite phase of the Pearse Peak (PP) pluton: a predominantly quartz diorite body ~148-Ma in age. Here we present the results of new mapping, petrography and geochemistry of greenstones formerly mapped as PP diorite. Sample 4, near the summit of Bald Mt., is a mafic dike cutting Galice metasediments just south of the PP pluton. It is phaneritic and contains augite (rimmed by green hornblende), brown hornblende, plagioclase, biotite, and Fe-Ti oxides. The rock contains 1.4 wt% TiO₂, has relatively high Y (27 ppm), plots on the edge of the MORB field on a Cr-Y diagram, and has Ti-V ratio of 22:1. A similar dike cuts Galice on the eastern margin of the pluton. The remaining 3 samples studied differ from sample 4 although they too were previously mapped as PP Diorite. Samples 9 and 11, about 1 km apart along the Elk River, lie near the northern and eastern boundaries of the sheeted dikes in fault contact with the Galice. Sample 11 shows distinct chilled margins. These greenstones contain plagioclase and hydrothermal green amphibole, are cut by prehnite veins, and are similar to previously analyzed sheeted dikes in the area in their low Y contents (14 and 16 ppm), Ti-V ratios (16-20:1), and elevated Th and depleted Ta relative to average MORB. Sample 16 occurs in a block(?) in sheared serpentinite which also contains blocks of cumulate peridotite. This fine-phaneritic rock bears hornblende and plagioclase phenocrysts (unlike the sheeted dikes), and contains secondary prehnite, calcite, and quartz. Its Ti-V ratio (19:1) and low Y (20 ppm) are, however, not unlike the sheeted dikes. The results demonstrate field, petrographic, and geochemical differences among a wide variety of rocks that have been grouped as the PP diorite. Samples 9 and 11 demonstrate a 1 km minimum aerial extent of sheeted dikes. Both are distinctly different from sample 4 (transitional arc/MORB?) which cuts the Galice and clearly postdates formation of the JO. Sample 16, is petrographically different than, but geochemically similar to the sheeted dikes and interpreted to be an arc-related intrusive that post-dates the ophiolite.