Franciscan eclogite revisited: Reevaluation of the P-T evolution of tectonic blocks from Tiburon Peninsula, California, U.S.A

T. Tsujimori, K. Matsumoto, J. Wakabayashi, J. G. Liou

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    67 Citations (Scopus)

    Abstract

    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 (∼3.5-Si p.f.u.) ± paragonite + rutile + quartz. The blueschist-facies mineral assemblage contains chlorite + titanite + glaucophane + epidote ± albite ± phengite (∼3.3 Si p.f.u.). Albite is not stable in the eclogite stage. New calculations based on garnet-omphacite-phengite thermobarometry and THERMOCALC average-P-T calculations yield peak eclogite-facies P-T conditions of P = 2.2-2.5-GPa and T = 550-620-°C; porphyroclastic omphacite with inclusions of garnet and paragonite yields an average-P-T of 1.8 ± 0.2-GPa at 490 ± 70-°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 ∼5 km/Ma, as rapid as those of some ultrahigh pressure metamorphic terranes.

    Original languageEnglish
    Pages (from-to)243-267
    Number of pages25
    JournalMineralogy and Petrology
    Volume88
    Issue number1-2
    DOIs
    Publication statusPublished - Sep 1 2006

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    ASJC Scopus subject areas

    • Geophysics
    • Geochemistry and Petrology

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