PETROGRAPHY AND GEOCHEMISTRY OF OPHIOLITIC DIKES AND PILLOW LAVAS FROM BLOCKS IN SERPENTINITEZED-MATRIX MELANGE NEAR THE EASTERN MARGIN OF THE ELK OUTLIER OF THE WESTERN KLAMATH TERRANE, NEAR POWERS, OREGON

The western Klamath terrane (WKT) of the Klamath Mountains in California and Oregon (OR) exposes the ~164 Ma Josephine ophiolite, overlain by meta-sediments of the Jurassic Galice Formation, intruded by ~150 Ma calc-alkaline intrusives. The Elk outlier (EO) of the WKT anomalously exposes the Galice Formation cut by Cretaceous intrusives and unconformably overlain by strata of the Cretaceous Myrtle Group in the Oregon Coast Ranges. In addition, the EO contains abundant sheared serpentinite, gabbro, diabase dikes, and pillow lavas suggesting the presence of an ophiolitic basement as is observed in the WKT.

This study concerns ophiolitic rocks exposed as two blocks in serpentinite-matrix mélange near the eastern margin of the EO, ~ 20 km SSW of Powers, OR. The northern block contains dikes cutting gabbro and dikes cutting dikes. The gabbro contains primary plagioclase and augite, with secondary green calcic clino-amphiboles. The dikes consist of primary augite and Fe-Ti oxides, albite after plagioclase, and secondary uralite, and epidote. The southern block, ~ 3.5 km SW of the northern block, consists of multiple parallel dikes intruding dikes, in fault contact with pillow lavas. Except for its abundant zoned augite, the southern dike is petrographically similar to the northern dikes. The pillows are variolitic, amygdaloidal, and have primary augite, with secondary albite after plagioclase, sphene, quartz and pumpellyite.

Two northern-block dikes and a southern-block dike and pillow were analyzed by XRF and ICP-MS at the Geoanalytical Lab, Washington State University. Trace elements were plotted on discriminant diagrams to determine petro-tectonic affinities. All samples have negative Ta anomalies on mid-ocean ridge basalt (MORB)-normalized trace element plots. All dikes are LREE-depleted, whereas the pillow has a flat REE pattern. The northern dikes plot as island arc tholeiite (IAT) on Cr vs Y, Ti vs V, and Th/Yb vs Ta/Yb diagrams. The southern dike has calc-alkaline affinity on the Th/Yb vs Ta/Yb plot. The pillow is transitional plotting in the IAT field on the Cr vs Y and Th/Yb vs Ta/Yb but in the MORB field on the Ti vs V plot. All the rocks are arc-related. The northern dikes and the southern pillow correlate geochemically with the JO and the southern dike, with the arc-intrusives that cut the JO in the WKT.