AGarnet^Zircon Oxygen Isotope Record of Subduction and Exhumation Fluids from the Franciscan Complex, California

Tectonic blocks of high-grade metamorphic rock hosted by the Franciscan me ¤ lange commonly preserve textural and geochemical evi-dence for metasomatism and blueschist-facies overprinting within a subduction zone. These exhumation events can obscure information on the nature of subduction fluids at depth that might otherwise be preserved by these rocks in this tectonic setting. Cation and oxygen isotope zoning in garnets from eclogite and garnet^hornblende rocks with minimal textural evidence of retrogression reveals separate prograde and retrograde fluid histories. Garnet cores preserve a homo-geneous prograde oxygen isotope history with both high (d18O 11ø) and low (4ø) values in different samples probably re-cording altered ocean-crust protolith compositions. Multiple episodes of garnet resorption and regrowth record a dramatically changing bulk oxygen isotope ratio resulting in 6^7ø neoformed garnet rims with sharp core^rim boundaries in both samples. Matrix omphacit