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

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 eld exposures and represent deep peridotite in ltrated by H2O-CO2 uid mixtures that triggered antigoritization and local carbonation. Fluid in ltration continued after antigoritization forming a vein network as a potential response to hydrofracturing that precipitated tremolitite in the veins and triggered uid- 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 uid-rock interaction characterized by advection of in ltrating uid towards the blackwall and, possibly, by diffusion out of the blackwall towards the uid- lled vein. Tentative thermodynamic modeling of the blackwall domain Atg + Chl + Tr points vein network formation at 400-500 oC and 5-10 kbar during exhumation in the subduction channel, suggesting the in ltration of deep-seated pressurized uid that triggered hydrofracturing. The chemical compositions of antigoritites, veins and blackwalls indicate a LILE- and LREE- enriched uid evolved from the subducting plate, while Sr-Nd isotope systematics are compatible with an external uid composed of a mixture of uids evolved from sediments and, probably to a lesser extent, altered oceanic crust.