Fluid flow in the subduction channel: Tremolite veins and associated blackwalls in antigoritite (Villa Clara serpentinite mélange, Cuba)

Abstract

Exotic blocks of massive antigorite-serpentinite (antigoritite) document a deep-seated subduction channel in the Villa Clara serpentinite-matrix m ́elange, central Cuba. The petrological and geochemical characteristics of antigoritite allow distinguishing two types of rock: i) antigoritite and ii) dolomite-bearing antigoritite. Both types are intimately related in field exposures and represent deep peridotite infiltrated by H2O-CO2 fluid mixtures that triggered antigoritization and local carbonation. Fluid infiltration continued after antigoritization forming a vein network as a potential response to hydrofracturing that precipitated tremolitite in the veins and triggered fluid- antigoritite reaction forming blackwalls. The mineralogical and chemical zoning in the blackwalls (Atg + Chl + Tr adjacent to antigoritite and Chl + Tr adjacent to the tremolitite vein) attest for multi-step metasomatic processes during fluid-rock interaction characterized by advection of infiltrating fluid towards the blackwall and, possibly, by diffusion out of the blackwall towards the fluid-filled vein. Tentative thermodynamic modeling of the blackwall domain Atg + Chl + Tr points vein network formation at 400–500 ◦C and 5–10 kbar during exhumation in the subduction channel, suggesting the infiltration of deep-seated pressurized fluid that triggered hydrofracturing. The chemical compositions of antigoritites, veins and blackwalls indicate a LILE- and LREE- enriched fluid evolved from the subducting plate, while Sr–Nd isotope systematics are compatible with an external fluid composed of a mixture of fluids evolved from sediments and, probably to a lesser extent, altered oceanic crust.