Fluid-Rock Interaction in the Granulites of Madagascar and Lithospheric-Scale Transfer of Fluids

The nature of synmetamorphic fluids and their flow is examined in the granulitic lower crust of Madagascar, part of a Precambrian crustal-scale network of vertical ductile shear zones. Based on three independent data sets - field and satellite mapping, C-, O- and H-isotope geochemistry and gravimetry - this crust is divided into three zones: outside of shear zones, minor shear zones (<140 km long and 7 km wide), and major shear zones (>350 km long and 20–35 km wide). The major shear zones are rooted in and are controlled by the mantle. They tapped mantle-derived CO2 with carbon fluxes of the same order of magnitude as oceanic ridge degassing. One major shear zone shows abundant phlogopite-diopside-apatite-calcite mineralizations (a well known paragenesis in mantle metasomatism) due to mantle-fluid infiltration and their interaction with the crust. Carbonatitic magmas possibly collected in the major shear zones at the base of the crust and may be the source for CO2 upwellings as well as other metasomatic agents. Small-scale minor shear zones were controlled by crustal deformation processes and focused crustally-derived H2O-rich fluids. Pervasive fluid circulation was restricted to the vicinity (< 100 m) of synmetamorphic plutons. Fluid absent conditions dominate everywhere else. Mantle-CO2 flushing is not required for granulite genesis but is a consequence of the high associated heat flux. Fluid transfer at the mantle/crust interface is controlled by the tectonic setting and the associated geothermal gradient. The C- and O- isotope systematics of metamorphosed carbonates sampled on a regional scale within a known petrological and structural framework are shown to be of great help to identify the distribution of major fluid-rock interaction processes associated with plate tectonics.