| The Cuesta Ridge ophiolite is one of the best-preserved remnants of mid-Jurassic ophiolite in California. Geologic mapping and petrologic studies show that it comprises (1) harzburgite mantle tectonite, (2) dunite-rich mantle transition zone (MTZ), (3) wehrlite and pyroxenite, (4) isotropic gabbro, (5) sheeted dike/sill complex, (6) volcanic rocks (7) late-stage dikes and flows, and (8) tuffaceous radiolarian chert.; The sheeted dike/sill complex is dominated by quartz diorite, with significant modal quartz and hornblende. The volcanic section is dominated by arc tholeiite and boninitic lavas. Boninites, with high MgO, Cr, and Ni comprise 40% of the volcanic rocks. Late stage dikes and lava flows below the overlying chert, have a MORB-like affinity.; These data suggest formation in a supra-subduction zone setting with three stages of arc-related magmatism before late-stage MORB magmatism. I infer formation above the east-dipping proto-Franciscan subduction zone and suggest that ophiolite formation was terminated by a ridge subduction/collision event. |
