PRELIMINARY PETROGRAPHY AND GEOCHEMISTRY OF A NEWLY DISCOVERED SHEETED DIKE COMPLEX ON THE ELK RIVER, SOUTHWESTERN OREGON

Semi-parallel dikes and screens are exposed on river-polished surfaces of rare outcrops and abundant large boulders in a 6000-m² study area along the Elk River, southwestern Oregon. The rocks were previously mapped as the dioritic phase of the Pearse Peak Pluton. Approximately 73% of the boulders ³ 6 m³ contain dikes. Outcrops of dikes and pillow lavas within 1km suggest the boulders lie in a newly discovered remnant of either the Josephine Ophiolite (JO) or Coast Range Ophiolite (CRO). Blocks locally occur within a debris-flow matrix downslope of non-descript greenstone outcrops. Block size and abundance, exposure in a debris-flow, and upslope outcrop pattern suggest the dike-bearing boulders are locally derived; appreciable river transport seems unlikely.

Four distinct dikes are exposed on the river-polished face of a 6m³ block. Chilled margins indicate that dike B intruded screen A and dike C intruded dike B. Dikes B and C contain primary cpx, plagioclase, opaques, and a few equant grains of Cr-spinel(?). Uralite replaces cpx either partially or completely. The screen contains uralite and plagioclase. Decrease in grain size toward dike borders agrees with the intrusive sequence deduced in outcrop. ICP-MS and XRF analyses show that samples A, B, and C contain 58.03, 55.97, and 60.73 wt.% SiO₂ respectively. Small albite(?) veins possibly preserve hydrothermally added SiO₂. Cr varies from 162 (dike C) to 543 (dike B) ppm; Y, from 10 (A, B) to 12 (C) ppm; and TiO₂ from 0.30 (samples A, B) to 0.35 (dike C) wt.%. Ti vs V, Ce/Sr vs Cr, and Zr/Y vs Zr plot in the island arc tholeiite field. HREE are 4-5x chondrites (screen A, dike B), and 5-6x for dike C. Relative to average MORB, Nb and Ta are depleted whereas Th and La are enriched. The chemical signature is characteristic of rocks with boninitic affinity. Boninitic rocks are abundant in the CRO and compose a small percentage of rocks in the JO. The outcrop pattern is consistent with JO, but further mapping and geochemistry are warranted.