METASOMATIC EFFECTS RELATED TO CHANNELED FLUID STREAMING THROUGH DEEPCRUST - FENITES AND ASSOCIATED CARBONATITES (IN OUZZAL-PROTEROZOIC GRANULITES, HOGGAR, ALGERIA)

The In Ouzzal granulitic unit (IOGU) consists predominantly of felsic orthogneisses most of which correspond to granitoids emplaced during t he Archaean, plus metasediments, including olivine-spinel marbles, of late Archaean age. All units were metamorphosed at granulite facies du ring the Eburnean (2 Ga). The stable isotope signature of the marbles (delta(13)C= -0.8 to -4.2 parts per thousand/PDB; delta(18)O = 7.9 to 18.9 parts per thousand/SMOW) does not record a massive streaming of C -bearing fluids during metamorphism. Most of the isotopic variation in the marbles is explained in terms of pregranulitic features. Metasoma tic transformation of granulites into layered potassic syenitic rocks and emplacement of carbonate veins and breccias occurred during retrog ressive granulite facies conditions. The chemistry of these rocks is c omparable with that of fenites and carbonatites with high contents of (L)REEs, Th, U, F, C, Ba and Sr but, with respect to these elements, a relative depletion in Nb, Ta, Hf, Zr and Ti. The isotopic composition s of Nd (epsilon Nd-(T) = -6.3 to -9.9), of Sr (Sr-87/Sr-86((T)) = 0.7 093-0.7104), and the O isotopic composition of metasomatic clinopyroxe ne (delta(18)O = 6.9 to 8 parts per thousand), all indicate that the f luid had a strong crustal imprint. On the basis of the C isotope ratio s (delta(13)C = -3.5 to -9.7 parts per thousand), the fluid responsibl e for the crystallization of carbonates and metasomatic alteration is thought to be derived from the mantle, presumably through degassing of mantle-derived magmas at depth. Intense interaction with the crust du ring the upward Row of the fluid may explain its chemical and isotopic signatures. The zones of metasomatic alteration in the In Ouzzal gran ulites may be the deep-seated equivalents of the zones of channelled c irculation of carbonated fluids described at shallower levels in the c rust.