HR: 1340h
AN: V13E-0592    [Abstracts]
TI: Franciscan eclogite revisited: Reevaluation of P-T evolution of tectonic blocks from Tiburon Peninsula, California
AU: * Tsujimori, T
EM: tatsukix@pangea.Stanford.EDU
AF: Stanford University, Department of Geological and Environmental Sciences, Stanford, CA 94305 United States
AU: Matsumoto, K
EM: keisaku@kueps.kyoto-u.ac.jp
AF: Kyoto University, Department of Geology and Mineralogy, Kyoto, 606-8502 Japan
AU: Wakabayashi, J
EM: johnwako@sbcglobal.net
AF: California State University, Fresno, Department of Earth and Environmental Sciences, Fresno, CA 93740 United States
AU: Liou, J
EM: liou@pangea.stanford.edu
AF: Stanford University, Department of Geological and Environmental Sciences, Stanford, CA 94305 United States
AB: High-grade blocks in the Franciscan complex at Tiburon, California, record relatively low temperature eclogite-facies metamorphism and later blueschist-facies overprinting. Eclogite-facies metamorphism is characterized by mineral assemblage prograde-zoned garnet + omphacite +/- glaucophane +/- hornblende (katophoritic Ca-Na amphibole) +/- paragonite + phengite (-3.5 Si p.f.u.) + rutile + quartz and the blueschist-facies by chlorite + titanite + glaucophane + epidote +/- albite. Albite is not stable in the eclogite stage. Although epidote-garnet amphibolitic assemblage is ubiquitous in some blocks, calculated P-T pseudosections imply that the epidote-garnet amphibolitic assemblage is stable at prograde eclogite-facies metamorphism. New calculations based on P-T pseudosections and garnet- omphacite-phengite thermobarometry yields the peak P-T conditions of eclogite-facies metamorphism at P= 2.1-2.3 GPa and T= 530-610°C and blueschist-facies overprinting at P= 0.8-1.2 GPa and T= -350 °C. The inferred P-T path is characterized as a counterclockwise hairpin P-T trajectory that suggests subduction metamorphism with ``warm" geotherm, and subsequent blueschist-facies overprinting by decreasing geotherm. Most Franciscan eclogite blocks may record similar eclogite-facies condition as they possess similar mineral parageneses and compositions. Furthermore, available geochronologic data with our new insight for the maximum pressure suggest an average exhumation rate of -5 km/Ma as rapid as those of some UHP metamorphic terranes.
UR: http://pangea.stanford.edu/research/hp-uhp/
DE: 3652 Pressure-temperature-time paths
DE: 3660 Metamorphic petrology
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005