PROGRADE AND RETROGRADE HISTORY OF THE HEALDSBURG ECLOGITE, CALIFORNIA

The Healdsburg eclogite is a tectonic block in the Franciscan ~12 miles SW of Healdsburg CA near the site of the old Junction School.  The matrix assemblage is omphacite-garnet with minor rutile-epidote and trace quartz. The eclogite block has been variably overprinted by chlorite, phengite and minor blue amphibole.  Matrix rutile is sheathed by sphene. Garnets are ~1 mm in diameter and contain a diverse inclusion assemblage: quartz-omphacite-epidote-sphene-rutile-phengite-apatite-zircon-chalcopyrite. No early amphibole is entrapped in garnet. Matrix omphacite ranges in diameter from 0.5-1.0 mm and contains small (1-10 µm) inclusions of sphene-garnet-zircon.  The garnets retain a prograde zoning pattern from Alm₅₅Pyp₆Grs₂₆Sps₁₃ cores to maximum Mg Alm₅₆Pyp₁₆Grs₂₆Sps₂ ~30 µm from the rim. The outer rims are Alm₅₈Pyp₁₁Grs₂₈Sps₃. Matrix pyroxenes are zoned from Jd₅₀Di₃₉Hd₁₁ to Jd₃₈Di₃₈Hd₁₃Ac₁₁.

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.