CUESTA RIDGE OPHIOLITE: NEW FIELD & GEOCHEMICAL EVIDENCE FOR ORIGIN & EVOLUTION OF THE COAST RANGE OPHIOLITE, CALIFORNIA

The Cuesta Ridge ophiolite is one of the largest and best-preserved remnants of Mid-Jurassic Coast Range ophiolite in California. New geologic mapping and petrologic studies show that it comprises (1) harzburgite mantle tectonite, (2) dunite-rich mantle transition zone (MTZ), (3) sills of wehrlite and pyroxenite just above the MTZ, (4) isotropic gabbro which grades into (5) sheeted dike/sill complex, (6) volcanics including sheet flows, pillow lava, volcanic breccias, (7) late stage dikes and lava flows, and (8) tuffaceous radiolarian chert that caps the volcanics. Most of the volcanics lie directly above mantle harzburgite (possible depositional contact), but some volcanics overlie the sheeted complex. Other contacts (wehrlite/gabbro, gabbro/sill complex) are faulted. Shale of the Toro formation overlies the chert, and slices of shale within the chert show that it has been thickened tectonically.

The sheeted dike/sill complex is dominated by diorite and quartz diorite, with SiO2=52-75% and significant modal quartz and hornblende. The volcanic section is dominated by arc tholeiite and boninitic lavas, with SiO2=48-60%, TiO2<1%, Nb<4ppm, and Ti/V ratios=11-21. Boninites, with high MgO (11-14 wt%), Cr (up to 500 ppm), and Ni (up to 250 ppm), comprise 40% of volcanic rocks analyzed. Some late stage dikes that cross-cut quartz diorite sheeted complex, and some late lava flows below the overlying chert, have a MORB-like affinity, with SiO2=50%, TiO2=1.9-2.4%, Nb>5ppm, and Ti/V ratios=20-30.

These data suggest formation in a supra-subduction zone setting with three stages of arc-related magmatism (arc tholeiite, boninitic, calc-alkaline) before late stage MORB dikes and flows. We infer formation above the east-dipping proto-Franciscan subduction zone and suggest that ophiolite formation was terminated by a ridge subduction/collision event. Emplacement resulted from Neogene thrusting of the ophiolite and overlying Great Valley Sequence (Toro fm) over Miocene Monterey formation and its Franciscan basement. A Pliocene (?) molasse of serpentinite debris separates the ophiolite from the underlying Monterey; rip-up clasts of Monterey at the base of the molasse show that the Monterey was unlithified at the time of thrusting.