Persistence of pre-metamorphic C and O isotopic signatures in marbles subject to Pan-African granulite-facies metamorphism and U–Th mineralization (Tranomaro, Southeast Madagascar)

The status of fluid regimes during the Pan-African granulite-facies metamorphism in Southern Madagascar was examined by means of a mineralogical and stable isotope study of marble lithologies. In the granulitic Tranomaro area, which is made up of metasedimentary rocks (metapelitic, leptynitic (felsic) and calc–silicate metasediments) and intruded by syn-granulitic granitic bodies, syn-granulitic mineralization (Th–U) and metasomatic transformations (marbles→pyroxenites) demonstrate that fluid circulation has occurred. The Tranomaro marbles can be subdivided into two compositional groups, either Mg-poor or Mg-rich. This division reflects pre-metamorphic compositional differences rather than different P–T–XCO2 histories. On a regional scale, the marbles display a large variation in δ18O (from +19 to +6.5‰ relative to SMOW) but have a more restricted δ13C range (from +1.4 to −2.5‰ relative to PDB). The lowest δ18O values are observed in magnesian marbles, whereas calcic ones do not have δ18O values lower than +13.5‰. Both initially low and high δ18O marbles may have locally been infiltrated by the syn-metamorphic fluid flow which gave rise to pyroxenites and Th–U ores (thorianite). δ18O values of carbonate in Mg-rich marbles evolve from +7 up to 8.5‰ towards the contact with a metasomatic pyroxenite in a meter-scale profile across an infiltration zone. The mineralogy, as well as the large and small-scale isotopic results argue for: (i) a pre-metamorphic origin for the isotopic variations, probably related to pre-granulitic dolomitization, (ii) a lack of pervasive fluid infiltration capable of erasing pre-metamorphic isotopic compositional differences, (iii) an absence of mantle-derived C-bearing fluids in the infiltrated zones. For the area as a whole, the present data are consistent with the decarbonation of impure siliceous limestones and dolostones coupled with fluid release from syn-metamorphic granitic intrusions.