Prograde and retrograde history of the Junction School eclogite, California, and an evaluation of garnet-phengite-clinopyroxene thermobarometry

Quantitative thermobarometry of inclusions in zoned garnet from a Franciscan eclogite block record a counter-clockwise P-T path from blueschist to eclogite and back. Garnet retains prograde zoning from inclusion-rich Alm52Grs30Pyp6 Sps12 cores to inclusion-poor Alm62Grs25 Pyp12Sps1 mantles, with overgrowths of highly variable composition. Barometry using the Waters-Martin version of the garnet-phengite-omphacite thermobarometer yields conditions of 7-15 kbar, 400-500°C (garnet cores), 18-22 kbar, ∼550°C (mantles), and 10-14 kbar, 350-450°C (overgrowths), in agreement with clinozoisite-sphene-rutile-garnet-quartz barometry. These pressures are ∼10-15 kbar less than those obtained using more recent, fully thermodynamic calibrations of the phengite-omphacite-garnet thermobarometer. Low early temperatures suggest that the block was subducted in a thermally mature subduction zone and not at the inception of subduction when prograde temperature is expected to be higher. Franciscan high-grade blocks likely represent crust subducted throughout the history of this convergent margin, rather than only at the inception of the subduction zone. © Springer-Verlag 2006.