A Garnet–Zircon Oxygen Isotope Record of Subduction and Exhumation Fluids from the Franciscan Complex, California

Abstract

Tectonic blocks of high-grade metamorphic rock hosted by the Franciscan me¤ lange commonly preserve textural and geochemical evidence 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 homogeneous prograde oxygen isotope history with both high (dO 11o) and low ( 4o) values in different samples probably recording altered ocean-crust protolith compositions. Multiple episodes of garnet resorption and regrowth record a dramatically changing bulk oxygen isotope ratio resulting in 6^7o neoformed garnet rims with sharp core^rim boundaries in both samples. Matrix omphacite and hornblende are in profound oxygen isotope disequilibrium with garnet cores but are in equilibrium with garnet rims despite the appearance of overall textural equilibrium in thin section.This cryptic metasomatism found in samples from different locations within the Franciscan Complex brings into question existing thermobarometry on non-inclusion phases for this complex, as well as interpretations based on whole-rock geochemical data. Zircon ages and oxygen isotope equilibrium with late-forming garnet are consistent with postmetasomatic formation of zircon.The ability of garnet and zircon to record multiple fluid events and tie them to P^T^t^fluid history is a powerful tool in gaining an improved understanding of the complex fluid environments within subduction zones.