A NEWLY DISCOVERED SHEETED DIKE COMPLEX ON THE WESTERN MARGIN OF THE IRON MOUNTAIN PERIDOTITE, WITHIN EASTERN ELK OUTLIER OF THE WESTERN KLAMATH TERRANE, SOUTHWESTERN OREGON

The Western Klamath terrane (WKT) consists of the ca. 162 Ma Josephine Ophiolite (JO) conformably overlain by the Upper Jurassic Galice Formation. Both are intruded by 150 to145 Ma calc-alkaline intrusives and are unconformably overlain by Lower Cretaceous strata. The Elk outlier (EO) of the WKT, in the southern Oregon Coast Range, has geology similar to the WKT except for the absence of a complete ophiolite. Sheeted dikes in the western EO tentatively correlate with the JO. This study concerns what we interpret to be foliated sheeted dikes, and dikes cutting both gabbro and cumulate ultramafic rocks exposed in a 4.5-km long, north-trending belt east of the Iron Mountain peridotite and west of the Galice Formation in the eastern EO.

A 300 m east-to-west traverse reveals vertically faulted, variably serpentinized cumulate ultramafics, diabase dikes cutting gabbro, and mafic schist. The cumulate ultramafics contain primary augite, opaques, serpentine pseudomorphs, and secondary uralite, tremolite, chlorite and prehnite. The gabbro contains augite, brown hornblende, plagioclase altered to epidote(?) opaques, secondary actinolite, tremolite, and rare serpentine. Diabase dikes cut gabbro, are roughly parallel (ca. N25°E, 75°E) and contain plagioclase altered to albite, and secondary uralite, epidote, and sphene. A steep foliation in the mafic rocks increases in intensity to the west where gabbro is absent. Here, the rocks are chlorite-actinolite schists consisting of aligned albitized plagioclase, uralite, and chlorite, with minor epidote, sphene and opaques. The presence of a chilled margin in the schist and its similarity in mineralogy and texture to a deformed dike cutting gabbro suggest that the schists are deformed sheeted dikes.

Geochemical analysis of two samples reveals that dike I, which cuts gabbro, is LREE-depleted, plots in the island arc tholeiite (IAT) field on Ti vs V, Cr vs Y, and Th/Yb vs Ta/Yb diagrams. The dike has IAT affinity. Schist P is LREE-enriched, has higher REE concentrations, plots in the MORB field on a Ti vs V diagram, within the IAT field on a Th/Yb vs Ta/Yb diagram, and is a transitional IAT-MORB derived from more enriched mantle than dike I. The rocks are part of a suprasubduction zone ophiolite and are geochemically and petrographically very similar to the two types of intrusives from the JO.