Abstract

Basalts from two geographically separated fragments of the California Coast Range ophiolite are very different petrographically, mineralogically, and geochemically. Pillow lavas from the Llanada Coast Range ophiolite remnant are mostly Cr-rich and TiO2-poor basalts in which Ti does not increase with increasing Fe/(Fe + Mg); some Cr-poor siliceous rocks are also present. Most Llanada rocks are depleted in light rare earth elements (REE), although a few show slight light REE enrichment. Ti/V ratios are arc-like. The Llanada basalts appear to be island-arc tholeiites that have limited boninitic affinities—i.e., products of suprasubduction zone magmatism.

Basalts from the Black Mountain Coast Range ophiolite remnant near St. Helena are TiO2-rich relative to those from most other Coast Range ophiolite fragments, and TiO2 increases with increasing Fe/(Fe + Mg). Black Mountain basalts are light REE depleted, REE enriched overall relative to Llanada lavas, and have mid-oceanic-ridge basalt (MORB)-like Ti/V ratios. These characteristics are similar to those of MORB, but elevated Th/Ta ratios suggest a suprasubduction-zone origin for the Black Mountain lavas.

These results show that the Coast Range ophiolite is highly diverse, containing island-arc tholeiites and possibly suprasubduction zone–related MORB-like components. In the Coast Range ophiolite as a whole, the apparent preponderance of island-arc tholeiite lavas such as those at Llanada over MORB-like lavas such as those at Black Mountain favors a forearc over a backarc setting for the origin of the ophiolite. Other data suggest that the Coast Range ophiolite locally contains ocean-island basalts (OIBs). Moreover, serpentinite-rich ophiolitic detritus overlying Coast Range ophiolite serpentinite suggests that, in some areas, the basaltic section of the oceanic crust was missing completely, and serpentinized harzburgite was exposed at the sea floor. These relationships are similar to those in the modern Mariana forearc, which exposes a complex amalgam of boninitic and arc-tholeiitic suprasubduction zone–related crust, MORB and OIB crust, and serpentinized harzburgite.

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