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

Abstract

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.

Publisher

Springer Verlag

Publication Date

1-1-2007

Publication Title

Contributions To Mineralogy And Petrology

Department

Geology

Document Type

Article

DOI

https://dx.doi.org/10.1007/s00410-006-0161-9

Keywords

Eastern Blue Ridge, Western Alps, P-T evolution, Franciscan Complex, Glaucophane Schists, Phase relations, Solid solution, Metamorphic evolution, Ultrahigh pressures

Language

English

Format

text

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