PETROLOGY AND GEOCHEMISTRY OF PILLOW BASALTS, EASTERN ELK OUTLIER OF THE WESTERN KLAMATH TERRANE, SOUTHWESTERN OREGON

A previously unmapped 150 x 96.5 x 30 m quarry exposure of pillow basalts averaging 1 m in diameter was discovered in the eastern Elk outlier (EO) of the western Klamath terrane (WKT), SW Oregon. The EO lies ~ 50 km west of the WKT and consists primarily of metasediments of the Late Jurassic Galice Formation cut by Early Cretaceous diorite intrusives. Sheeted dikes and basaltic pillows in a serpentinite-matrix mélange are present in the western EO. Pillows were analyzed petrographically and geochemically to characterize tectonic affinity and test correlation with the rocks of the western EO and the Josephine ophiolite (JO). The pillows lie 8 km south of Powers, OR (lat. N45°48.64'; long. W124°4.65'), 386 m west of the Coquille River fault where Tertiary sediments crop out to the east. Pillows appear mildly flattened and some reveal keels indicating they are overturned. Hyaloclastite breccia commonly rims larger pillows with small faults and veins throughout. Grain size grades from ~ 1.5 mm in cores to altered-glass chill zones at the rinds. Cores reveal plagioclase microphenocrysts up to 13 mm in length in a matrix of plagioclase, augite, and secondary sphene having intergranular texture. Additional secondary minerals include albite(?), calcite, quartz, chlorite, pyrite, pumpellyite(?) and prehnite(?). Pillow basalt 21B (50.8. wt% SiO₂) and 21D (50.6 wt% SiO₂), collected near the base and top of the exposed pillows, were chosen for XRF and ICP-MS analysis performed at the Geoanalytical Lab, WSU. Samples 21B (331 ppm Cr, 28.89 ppm Y) and 21D (315 ppm Cr, 31.32 ppm Y) plot in the MORB field on a Cr-Y diagram. Both have MORB-like Ti-V ratios of ~27. Th/Yb vs Ta/Yb for 21B (0.07, 0.05) and 21D (0.07, 0.06) and La/Sm vs TiO₂ for 21B (0.98, 1.349) and 21D (0.98, 1.449) are MORB like. Both samples are LREE-depleted and slightly HREE-depleted at ~ 20 x chondrite, and are MORB-like on a MORB normalized trace element plot except for a possible slight Th-Ta anomaly suggesting a possible minor slab component in the lavas. The pillows were erupted on the sea floor at a spreading center, possibly a back-arc basin. Geochemically, they appear similar to MORB like pillows of the western Elk outlier except for the minor slab signal. They are similar in some respects to the upper pillows of the JO and might represent basement to the Galice in the Elk outlier of the WKT.