PETROLOGY AND GEOCHEMISTRY OF DIKES CUTTING PILLOWS IN THE EASTERN ELK OUTLIER, OREGON

We discovered a previously unreported 95,200 m³ quarry exposure of a sheeted dike complex cutting pillow lava in the eastern Elk outlier (EO) of the western Klamath terrane. The quarry lies 40 km SE of Port Orford, Oregon and consists of layered pillows cut by dikes in fault contact with sheared serpentinite. Outcrops of dikes cutting pillows proved difficult to sample, so we studied a large float block containing three parallel, tabular intrusive bodies. Screen J , ~ 0.5 m in width, is intruded by dikes G and I. The only exposed margin of dike I (minimum width: 6.5 cm) is chilled against the screen. Both margins of 2.5 cm-wide dike G are chilled against screen J. Screen J is extensively altered to epidosite. Epidosite grades from 65% rock volume near screen margins to absent in the center. Rock textures range from hypidomorphic granular to intergranular. Average grain size is 0.2 cm. Primary minerals include plagioclase, augite, Cr-spinel(?) and opaque oxides. Secondary minerals include albite (after plagioclase), green amphibole (rimming augite), epidote, sphene and chlorite. XRF analyses were obtained at Washington State University. High Cr contents (dike G: 245 ppm, dike I: 373 ppm, dike J: 376 ppm) indicate the primitive nature of the lavas. On a Cr-Y diagram dikes I (15 ppm Y) and G (16 ppm Y) plot in the island arc tholeiite (IAT) field, whereas screen J (26 ppm Y) plots as a MORB. The diagram suggests that dikes I and G fractionated from a magma derived by a higher degree of partial melting than that of screen J. Dikes I and G are consistent with slightly less fractionation from the same parent as both the sheeted dikes of the western EO and the early dikes of the Josephine ophiolite (JO). The late dikes and upper extrusives of the JO can be related to screen J by fractionation. All lavas plot in the MORB field near its boundary with the IAT field on a Ti-V diagram (Ti/V 23-25). Thus, dikes I and G are transitional, likely from a back-arc basin setting. The geochemical signature of a pillow from this quarry suggests derivation from the same source as the IAT dikes but with more fractionation. Although the lavas correlate well with the JO, the intrusive sequence is reversed in the EO. If the correlation is correct this suggests the possibility that the quarry sampled the transitional phase from early (IAT) to late (MORB) lavas.