TRACING RETROGRADE METAMORPHIC FLUIDS IN A SUBDUCTION ZONE USING LI: FRANCISCAN COMPLEX, CALIFORNIA

Abstract

Centimeter-scale layers of eclogite and blueschist from Tiburon Peninsula, Franciscan Complex, CA were contrasted with a similarly layered sample from Dos Rios, CA. Eclogites from both localities have similar mineral assemblages (e.g., omphacite, glaucophane, phengite, garnet, epidote, and titanite). However, the Tiburon blueschist shows petrographic evidence for fluid-rock interaction, while the Dos Rios sample does not. Mineral phases common to both samples were contrasted via textural evidence, major and minor element concentrations, and lithium concentrations. Lithium concentrations of omphacite and chlorite decrease from the eclogite to the blueschist domains in the Tiburon sample. These lithium concentration differences are interpreted to be the result of fluid-rock interactions. These differences are not seen in the Dos Rios sample. I propose that a difference in the Dos Rios sample bulk composition produced the alternating eclogite and blueschist lithologies as a result of a process such as seafloor alteration prior to prograde metamorphism.