Lithium isotopes as a tracer of fluids in a subduction zone mélange: Franciscan Complex, CA

Abstract

Abstract High-grade blocks of eclogite, blueschist, and garnet amphibolite found in melange terrains in the Franciscan Complex, CA display petrologic and geochemical evidence for a complicated history of interactions with aqueous fluids. These blocks have features (actinolitic rinds, a blueschist layer) that have petrologic and geochemical indicators of interaction with retrograde aqueous fluids. Lithium concentrations and isotopic compositions of different portions of three high-grade blocks are different from those of MORB (their presumed protolith) and show systematic variations associated with fluid-related features. Block cores and an eclogite layer have high whole-rock Li concentrations relative to MORB (17 to 39 ppm) and low to moderate δ7Li (0.4 to 2.1‰). This range of values is inconsistent with dehydration of MORB and is interpreted as the product of infiltration of aqueous fluids with an elevated Li concentration and low δ7Li. Rinds and a blueschist layer exhibit lower Li concentrations (7 to 24 ppm) and higher δ7Li (2.7 to 4.4‰). The differences between cores and rinds, and the eclogite and blueschist layers are interpreted as the product of retrograde infiltration of an aqueous fluid with low Li concentrations and high δ7Li. Results of a small-scale traverse across the boundary between the blueschist and eclogite layers show that the boundary is gradational over ∼ 0.5 cm, suggesting small-scale cross-layer diffusion/dispersion associated with the fluid infiltration. The high Li concentrations and low δ7Li of block cores suggests that the source of early fluids was likely subducted sedimentary rocks. The lower Li concentrations and higher δ7Li values of the rinds and blueschist layers do not fully constrain the source of retrograde fluids, but indicate that the fluid source likely had a lower Li concentration and higher δ7Li.