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 Alm52Grs30Pyp6Sps12 cores to inclusion-poor Alm62Grs25Pyp12Sps1 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.
Page, F. Z., L. S. Armstrong, E. J. Essene, and S. B. Mukasa. 2007. "Prograde and retrograde history of the Junction School eclogite, California, and an evaluation of garnet phengite-clinopyroxene thermobarometry." Contributions To Mineralogy And Petrology 153(5): 533-555.
Contributions To Mineralogy And Petrology
Eastern Blue Ridge, Western Alps, P-T evolution, Franciscan Complex, Glaucophane Schists, Phase relations, Solid solution, Metamorphic evolution, Ultrahigh pressures