Franciscan eclogite revisited:: Reevaluation of the P-T evolution of tectonic blocks from Tiburon Peninsula, California, USA

High-grade blocks in the Franciscan complex at Tiburon, California, record relatively low temperature eclogite-facies metamorphism and blueschist-facies overprinting. The eclogite-facies mineral assemblage contains prograde-zoned garnet + omphacite + epidote +/- hornblende (katophoritic and barroisitic Ca-Na amphibole) +/- glaucophane + phengite (similar to 3.5 Si p.f.u.) +/- paragonite + rutile + quartz. The blueschist-facies mineral assemblage contains chlorite + titanite + glaucophane + epidote +/- albite +/- phengite (similar to 3.3 Si p.f.u.). Albite is not stable in the eclogite stage. New calculations based on gamet-omphacite-phengite thennobarometry and THERMOCALC average-P-T calculations yield peak eclogite-facies P-T conditions of P = 2.2-2.5 GPa and T=550-620 degrees C; porphyroclastic omphacite with inclusions of garnet and paragonite yields an average-P-T of 1.8 +/- 0.2 GPa at 490 +/- 70 degrees C for the pre-peak stage. The inferred counterclockwise hairpin P-T trajectory suggests prograde eclogitization of a relatively cold subducting slab, and subsequent exhumation and blueschist-facies recrystallization by a decreasing geotherm. Although an epidote-garnet amphibolitic assemblage is ubiquitous in some blocks, P-T pseudosection analyses imply that the epidote-garnet amphibolitic assemblage is stable during prograde eclogite-facies metamorphism. Available geochronologic data combined with our new insight for the maximum pressure suggest an average exhumation rate of similar to 5 km/Ma, as rapid as those of some ultrahigh pressure metamorphic terranes.