DO SUPRA-SUBDUCTION-ZONE DIKES CUTTING GABBRO IN THE EASTERN ELK OUTLIER OF THE WESTERN KLAMATH TERRANE, SOUTHWESTERN OREGON, CORRELATE WITH THE JOSEPHINE OPHIOLITE?

The Elk Outlier (EO) of the Western Klamath terrane (WKT) in SW Oregon consists of anomalous coastal exposures of the Late Jurassic Galice Formation cut by ~145 Ma calc-alkaline intrusives; both are unconformably overlain by the Cretaceous Humbug Mountain Conglomerate. The EO lies ~40 km NW of the WKT in the Klamath Mountains of California and Oregon where the Galice conformably overlies the ~ 164 Ma Josephine ophiolite (JO). This study concerns an approximately 5 x 0.3 km, N20°E-trending unit of intermediate to mafic diabasic dikes cutting cumulate diorite, gabbro, and peridotite in fault contact with the Iron Mountain Peridotite to the east and the Galice Formation to the west. We examined field relations, petrography, and geochemistry of dikes and gabbro from a 900 m² area at the northern margin of the intrusive unit to determine its origin, tectonic affinity, and relationship to ophiolites of the western EO and the WKT.

Outcrops expose 10 to 30 cm wide dikes dipping ~ 45° east with chilled margins against diorite, cumulate gabbro and peridotite. Dikes and coarser intrusives contain albitized plagioclase, actinolite, and Fe-Ti oxides cut by epidote and albite veins. Dike 42 contains chromite; 44 has prehnite veins. Diorite and gabbro contain sphene. Gabbro 44 has cumulate euhedral augite partially altered to actinolite with intercumulate albitized plagioclase. Diorite 41 has brown amphibole that possibly pseudomorphed augite.

Dikes were analyzed by XRF and ICP-MS at the Geoanalytical Lab, WSU. Silica content is 58.1 wt. % in 41, 52.8% in 44, and 51.97% in 42. Least evolved is 42 with 366 ppm Cr; 44 has 49 ppm Cr and 41, 15 ppm Cr. The Ti-V ratio of dike 42 (19.6) is in the island arc tholeiite (IAT) field whereas dikes 41 (21.6) and 44 (30.9) plot in the mid-ocean ridge basalt (MORB) field. All lie in the IAT field on Cr vs. Y plot. MORB-normalized and Th/Yb vs Ta/Yb plots indicate a subduction zone component, less pronounced in dike 44. All have slightly LREE-enriched chondrite-normalized REE plots.

Mineralogy, textures, intrusive relationships, and alteration suggest that the rocks formed at the base of a sheeted dike complex or the top of a cumulate section of an ophiolite. Chemically the dikes are transitional MORB-IATs, all clearly showing a subduction zone signature. The rocks correlate well with dikes from the western EO and the JO.