PETROGRAPHY AND GEOCHEMISTRY OF PILLOW BASALT AND IGNEOUS BRECCIA FROM THE EASTERN ELK OUTLIER OF THE WESTERN KLAMATH TERRANE, SOUTHWESTERN OREGON

The Elk outlier (EO) of the western Klamath terrane (WKT), in the southern Oregon Coast Ranges, contains metasediments of the Late Jurassic Galice Formation cut by Early Cretaceous intrusives. 30 km west, the Galice sits on the Josephine ophiolite (JO) in the WKT proper. A 95 m x 72 m area in and around a rock quarry (42°36.54' N lat.,124°6.10' W long.) in the eastern EO contains steep exposures of igneous breccia to the west separated from pillow basalt to the east by 10 - 20 m of soil and/or talus. The breccia contains angular clasts ranging from 0.9 mm to 8.8 mm in diameter including: basalt containing primary augite; fine, medium, and coarse intermediate to mafic igneous rocks with aligned plagioclase; and both medium and coarse grained altered gabbro/diorite with rare primary augite. ~85% of the mafic minerals in the clasts are altered to uralite or chlorite. The matrix contains subangular igneous rock fragments and grains of plagioclase and augite < 0.4 mm in diameter. Pillows average ~33 cm in diameter and have weathered glassy rinds. Microphenocrysts of sub- to euhedral augite (~0.2 mm) and plagioclase (~0.9 mm), uncommonly in clusters, are set in a matrix of plagioclase (0.1 mm) and augite (0.05 mm) grains having intergranular texture. Secondary minerals include sphene, calcite, prehnite, and quartz. XRF and ICP-MS analysis of a pillow (Geoanalytical Lab, Washington State University) yielded 49.58 wt.% SiO₂, 2.6 wt.% TiO₂, and 13.3 wt.% FeO total, making the rock a Fe-Ti basalt. The basalt plots in the MORB field on a Cr (96 ppm) vs Y (55.4 ppm) discriminant diagram and has a MORB-like Ti-V ratio of ~33. Th/Yb (0.05) vs. Ta/Yb (0.05) plots in the MORB range with no trace of a subduction-zone component. The sample is depleted in LREE at ~25 x chondrite, with a negative Eu anomaly, suggesting fractionation of plagioclase or its presence in the parent . The pillow was erupted on the sea floor at an oceanic spreading center, possibly in a back-arc basin, and like modern Fe-Ti basalts, may have erupted along a propagating rift tip or transform fault. The breccia was derived from uplifted oceanic or arc crust, possibly along a fault. The basalt sample is geochemically simililar to pillows from a mélange unit in the western EO, and differs from the Fe-Ti basalts of the upper pillows of the JO in that the EO sample is LREE-depleted whereas those of the JO have flat REE patterns.