PETROLOGICALLY DIVERSE BASALTS FROM A FOSSIL OCEANIC FORE-ARC IN CALIFORNIA - THE LLANADA AND BLACK MOUNTAIN REMNANTS OF THE COAST RANGE OPHIOLITE

Basalts from two geographically separated fragments of the California Coast Range ophiolite are very different petrographically, mineralogic ally, and geochemically, Pillow lavas from the Llanada Coast Range oph iolite remnant are mostly Cr-rich and TiO2-poor basalts in which Ti do es not increase with increasing Fe/(Fe + Mg); some Cr-poor siliceous r ocks are also present. Most Llanada rocks are depleted in Light rare e arth elements (REE), although a few show slight light REE enrichment, Ti/V ratios are are-like. The Llanada basalts appear to be island-are tholeiites that have limited boninitic affinities-i.e., products of su prasubduction zone magmatism, Basalts from the Black Mountain Coast Ra nge ophiolite remnant near St. Helena are TiO2-rich relative to those from most other Coast Range ophiolite fragments, and TiO2 increases wi th increasing Fe/(Fe + Mg). Black Mountain basalts are light REE deple ted, REE enriched overall relative to Llanada lavas, and have mid-ocea nic-ridge basalt (MORB)-like Ti/V ratios. These characteristics are si milar to those of MORE, but elevated Th/Ta ratios suggest a suprasubdu ction-zone origin for the Black Mountain lavas. These results show tha t the Coast Range ophiolite is highly diverse, containing island-arc t holeiites and possibly suprasubduction zone-related MORB-like componen ts. In the Coast Range ophiolite as a whole, the apparent preponderanc e of island-are tholeiite lavas such as those at Llanada over MORB-lik e lavas such as those at Black Mountain favors a forearc over a backar c setting for the origin of the ophiolite, Other data suggest that the Coast Range ophiolite locally contains ocean-island basalts (OIBs), M oreover, serpentinite-rich ophiolitic detritus overlying Coast Range o phiolite sergentinite suggests that, in some areas, the basaltic secti on of the oceanic crust was missing completely, and serpentinized harz burgite was exposed at the sea floor. These relationships are similar to those in the modern Mariana forearc, which exposes a complex amalga m of boninitic and arc-tholeiitic suprasubduction zone-related crust, MORB and OIB crust, and serpentinized harzburgite.