CENTRAL METAMORPHIC TERRANE, NORTHERN CALIFORNIA: GEOCHEMICAL EVIDENCE OF A MID-OCEAN RIDGE ORIGIN

Interpretation of the Central Metamorphic terrane (CMT) metabasites as the lower plate of a Devonian intraoceanic subduction zone is a basic tenet of paleotectonic models for the eastern Klamath province. Specifically, such models hold that (1) the east-dipping CMT represents a subducted piece of oceanic lithosphere accreted to the overlying Trinity subterrane; (2) the Trinity and Eastern Klamath subterranes represent peridotitic mantle and magmatic arc crust of the overriding lithospheric plate; and (3) the east-dipping Trinity thrust, separating the Central Metamorphic terrane and Trinity subterrane, represents the paleo-plate boundary. Recent work confirms the supra-subduction zone nature of Trinity and Eastern Klamath magmatic rocks (Metcalf et al. 2000). The subduction zone model predicts that MORB-type oceanic basalts were the protolith of CMT metabasites. This hypothesis has not been tested using geochemical data. We present new major (XRF) and trace (ICP-MS) element data for 20 CMT metabasites to test whether CMT composition is consistent with a MORB protolith. Major element data are consistent with a MORB-type tholeiite basalt. Trace element data were evaluated using a variety of established empirical geochemical discrimination diagrams including the Th-Hf-Ta plot, the Cr-Y plot, the Th/Yb vs. Nb/Yb plot, the V-Ti plot and MORB normalized spider diagrams (Wood, 1980; Shervais, 1982, Pearce, 1982; Pearce et al. 1984). On plots that focus on the least mobile elements (e.g. Th, Hf, Ta, Cr, Y, Yb, Ti, V) 18 of 20 samples plot in the MORB fields. Fluid mobile elements (e.g. Cs, Rb, Pb, Sr) show varying degrees of enrichment relative to MORB. Such enrichments suggest post-accretion metasomatism by subduction-derived fluids, consistent with the interpretation of Peacock (1987) based on stable isotope data. These results argue in favor of the interpretation of the CMT as oceanic lithosphere accreted to the upper plate of a intraoceanic subduction zone.