EARLY, PROLONGED AND VARIABLE METAMORPHISM OF EXOTIC BLOCKS IN THE FRANCISCAN SUBDUCTION COMPLEX

Understanding convergent margin processes requires determination of the onset of subduction, the number and polarity of subduction zones, and the duration of subduction zone metamorphism. The Franciscan subduction complex in California is held as the archetype subduction zone, yet all of these issues and the thermal history of the subduction zone remain debated. New U-Pb zircon and Lu-Hf garnet ages of eclogite and amphibolite, combined with petrologic data, suggest that: (1) subduction was well established by no later than ~176 Ma and appears to pre-date the overlying Coast Range ophiolite; (2) subduction metamorphism was continuous until at least ~130 Ma and is consistent with a single, long-lived, east-dipping subduction zone; (3) the thermal histories of exotic blocks exposed at the surface today reflect variations in their times of formation and locations within the evolving subduction zone. Many high-grade Franciscan rocks record a counterclockwise P-T path, with amphibolite overprinted by later eclogite and blueschist facies assemblages. Early workers, however, also recognized high-grade rocks that recorded apparent clockwise paths with early eclogite overprinted by amphibolite and late blueschist. Zircon inclusions in garnet and Lu-Hf estimates of garnet core ages from such early eclogite record metamorphism at ~176 Ma. Zircon inclusions in matrix phases, Lu-Hf estimates of garnet rim ages, and ⁴⁰Ar/³⁹Ar hornblende ages record an amphibolite facies overprint of the eclogite at ~155 Ma. The new dates of early eclogite, combined with previous work, indicate that Franciscan subduction was well under way by at least ~176 Ma. The timing of eclogite metamorphism pre-dates the formation of plagiogranite within the Coast Range ophiolite. Abundant previous work shows that Franciscan metamorphism was continuous from ~176 Ma to ~130 Ma and is consistent with a single east-dipping subduction zone. The formation of eclogites at widely separated times during the evolution of the subduction zone, as well as the recognition of amphibolite overprints on eclogite and eclogite overprints on amphibolite, suggest there is no unique Franciscan P-T path and that the thermal histories of exotic blocks exposed at the surface today reflect variations in the time of formation and location within the evolving subduction zone.