PROGRADE AND RETROGRADE HISTORY OF THE HEALDSBURG ECLOGITE, CALIFORNIA
Inclusions in the garnets are concentrated in Mn-rich cores but are also found sporadically in the rims. Phengite (Si=6.7-7 apfu)-omphacite (Jd35Di32Hd10Ac23)-garnet thermobarometry in garnet cores give prograde conditions of 11-13 kbar and 400°-475°C using Waters' 1996 formulation, 17-19 kbar less than that calculated by Massonne (1991) on the same rocks. The inclusion assemblage in the intermediate zone is buffered on the reaction clinozoisite + sphene=grossular + rutile + quartz + H2O and gives pressures of 12-16 kbar between 450°-700°C bounded at high and low temperature by the stability field of clinozoisite (Cz80Ps20). Garnet - clinopyroxene thermometry yields ~500°C. Omphacite inclusions (Jd50Di30Hd16Ac4) constrain prograde conditions in the intermediate garnet zone to greater than 14 kbar in the absence of albite. Garnet - clinopyroxene thermometry between core and intermediate garnet zones and matrix clinopyroxene cores yields temperatures of 500°-600°C and pressure minima of 13-14 kbar in the absence of albite. Matrix phengite combined with rim garnet and clinopyroxene gives retrograde conditions of 15-17 kbar at 475°-500°C. The combined thermobarometry suggests that the Healdsburg eclogite followed a counter-clockwise P-T path from the blueschist to eclogite facies and back again, similar to that inferred for other eclogites in the Franciscan.