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The Cuesta Ridge ophiolite is a well-preserved remnant of the Middle Jurassic Coast Range ophiolite tectonically overlying rocks of the Franciscan complex. It is a nearly complete ophiolite section, consisting of over 1 km of serpentinized harz-burgite and dunite, sills of wehrlite, pyroxenite, and lherzolite, isotropic gabbro, a sheeted complex of quartz-hornblende diorite, an ∼1200-m-thick volcanic section, late-stage mafic dikes, and 5–10 m of tuffaceous radiolarian chert. The volcanic section at Cuesta Ridge has two chemically distinct volcanic groups. The lower volcanic section is characterized by low Ti/V ratios (11–21), enriched large ion lithophile element (LILE) concentrations, and depleted high field strength elements (HFSEs). Boninitic lavas with high MgO, Cr, and Ni abundances are present in this suite, along with arc tholeiites (basaltic andesites to dacites). Basalts of the upper volcanic section, which conformably overlie the lower volcanic section, and late-stage basaltic dikes that crosscut the hornblende–quartz diorite plutonic section are characterized by higher Ti/V ratios (20–27) and HFSE abundances and lower LILE abundances than the underlying section. These late-stage volcanic rocks have mid-ocean-ridge basalt–like chemistry.

The field and geochemical data indicate formation in a suprasubduction-zone setting above an east-dipping proto-Franciscan subduction zone due to the onset of subduction and subsequent slab rollback. Multiple stages of magmatism ensued, until the emplacement of the late-stage dikes and uppermost flows. These late-stage dikes, which are present in several Coast Range ophiolite remnants, signify the end of ophio-lite formation and are interpreted to represent a Late Jurassic ridge collision.

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