Prograde and retrograde history of the Junction School eclogite,California, and an evaluation of garnet–phengite–clinopyroxene thermobarometry |
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| Authors: | F Zeb Page Lora S Armstrong Eric J Essene Samuel B Mukasa |
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| Institution: | (1) Department of Geological Sciences, University of Michigan, 2534 C.C. Little Building, Ann Arbor, MI 48109-1005, USA;(2) Department of Geology and Geophysics, University of Wisconsin—Madison, 1215 W. Dayton St., Madison, WI 53706, USA;(3) Present address: Department of Earth Sciences, University of Bristol, Wills Memorial Building, Queen’s Road, Bristol, BS8 1RJ, UK |
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| 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. |
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