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 granu litic lower crust of Madagascar, part of a Precambrian crustal-scale networ k of vertical ductile shear zones. Based on three independent data sets - f ield and satellite mapping, C-, O- and H-isotope geochemistry and gravimetr y - this crust is divided into three zones: outside of shear zones, minor s hear zones (<140 km long and 7 km wide), and major shear zones (>350 km lon g and 20-35 km wide). The major shear zones are rooted in and are controlle d by the mantle. They tapped mantle-derived CO2 with carbon fluxes of the s ame order of magnitude as oceanic ridge degassing. One major shear zone sho ws abundant phlogopite-diopside-apatite-calcite mineralizations (a well kno wn paragenesis in mantle metasomatism) due to mantle-fluid infiltration and their interaction with the crust. Carbonatitic magmas possibly collected i n 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 crustal ly-derived H2O-rich fluids. Pervasive fluid circulation was restricted to t he vicinity (< 100 m) of synmetamorphic plutons. Fluid absent conditions do minate everywhere else. Mantle-CO2 flushing is not required for granulite g enesis but is a consequence of the high associated heat flux. Fluid transfe r at the mantle/crust interface is controlled by the tectonic setting and t he associated geothermal gradient. The C- and O- isotope systematics of met amorphosed carbonates sampled on a regional scale within a known petrologic al and structural framework are shown to be of great help to identify the d istribution of major fluid-rock interaction processes associated with plate tectonics.