PERSISTENCE OF PRE-METAMORPHIC C-ISOTOPE AND O-ISOTOPIC SIGNATURES INMARBLES SUBJECT TO PAN-AFRICAN GRANULITE-FACIES METAMORPHISM AND U-THMINERALIZATION (TRANOMARO, SOUTHEAST MADAGASCAR)

The status of fluid regimes during the Pan-African granulite-facies me tamorphism in Southern Madagascar was examined by means of a mineralog ical and stable isotope study of marble Lithologies. Ln the granulitic Tranomaro area, which is made up of metasedimentary rocks (metapeliti c, leptynitic (felsic) and calc-silicate metasediments) and intruded b y 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 sub divided into two compositional groups, either Mg-poor or Mg-rich. This division reflects pre-metamorphic compositional differences rather th an different P-T-XCO2 histories. On a regional scale, the marbles disp lay a large variation in delta(18)O (from +19 to +6.5 parts per thousa nd relative to SMOW) but have a more restricted delta(13)C range(from + 1.4 to -2.5 parts per thousand relative to PDB). The lowest delta(18 )O values are observed in magnesian marbles, whereas calcic ones do no t have delta(18)O values lower than + 13.5 parts per thousand. Both in itially low and high delta(18)O marbles may have locally been infiltra ted by the syn-metamorphic fluid flow which gave rise to pyroxenites a nd Th-U ores (thorianite). delta(18)O values of carbonate in Mg-rich m arbles evolve from +7 up to 8.5 parts per thousand towards the contact with a metasomatic pyroxenite in a meter-scale profile across an infi ltration zone. The mineralogy, as well as; the large and small-scale i sotopic results argue for: (i) a pre-metamorphic origin for the isotop ic variations, probably related to pre-granulitic dolomitization, (ii) a lack of pervasive fluid infiltration capable of erasing pre-metamor phic isotopic compositional differences, (iii) an absence of mantle-de rived C-bearing fluids in the infiltrated zones. For the area as a who le, the present data are consistent with the decarbonation of impure s iliceous limestones and dolostones coupled with fluid release from syn -metamorphic granitic intrusions. (C) 1998 Elsevier Science B.V. All r ights reserved.