J. metamorphic Geol., 1996, 14, 763–781 Metasomatic effects related to channelled fluid streaming through deep crust: fenites and associated carbonatites (In Ouzzal Proterozoic granulites, Hoggar, Algeria) S. FOURCADE, 1 J.-R. KIENAST 2 AND K. OUZEGANE 3 1 Ge ´osciences Rennes, University of Rennes 1, Campus de Beaulieu, 35042 Rennes Cedex, France (email: Fourcade@univ-rennes1.fr) 2 Laboratoire de Pe ´trologie, University of Paris VI-VII, URA-CNRS 736, Tour 26–0, 4 Place Jussieu, 75252 Paris Cedex 05, France 3 Institut des Sciences de la Terre, U.S.T .H.B., B.P. 9, Dar el Beida, Algiers, Algeria ABSTRACT The In Ouzzal granulitic unit (IOGU ) consists predominantly of felsic orthogneisses most of which correspond to granitoids emplaced during the Archaean, plus metasediments, including olivine–spinel marbles, of late Archaean age. All units were metamorphosed at granulite facies during the Eburnean (2 Ga). The stable isotope signature of the marbles (d13C=−0.8 to −4.2‰/PDB; d18O=7.9 to 18.9‰/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. Metasomatic trans- formation of granulites into layered potassic syenitic rocks and emplacement of carbonate veins and breccias occurred during retrogressive granulite facies conditions. The chemistry of these rocks is compar- able 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 compositions of Nd (eNd (T) =−6.3 to −9.9), of Sr (87Sr/86Sr (T) =0.7093–0.7104), and the O isotopic composition of metasomatic clinopyroxene (d18O=6.9 to 8‰), all indicate that the fluid had a strong crustal imprint. On the basis of the C isotope ratios (d13C=−3.5 to −9.7‰), the fluid responsible 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 during the upward flow of the fluid may explain its chemical and isotopic signatures. The zones of metasomatic alteration in the In Ouzzal granulites may be the deep-seated equivalents of the zones of channelled circulation of carbonated fluids described at shallower levels in the crust. Key words: carbonatites; granulites; Hoggar; In Ouzzal; isotope geochemistry; metasomatism. source. Stable (O, C) isotope data suggest these features INTRODUCTION reflect contamination of the hypothetical mantle- derived magmatic precursors of carbonatites by the Several carbonatite massifs occur in western Hoggar (Algeria) within the In Ouzzal granulitic unit (IOGU). Archaean crust (Bernard-Griffiths et al., 1988). The carbonatite complexes have some problematic This unit is made up of Archaean felsic orthogneisses and metasediments metamorphosed at high P–T features: they are always associated with slightly oversaturated potassic syenitic rocks which, on differ- granulite facies during the Eburnean (#2 Ga). Some carbonatites have been the subject of mineralogical, ent scales, display a banded texture of alternating feldspathic and pyroxenitic layers. These syenitic rocks petrological and geochemical studies (Bernard-Griffiths et al., 1988; Ouzegane et al., 1988), leading to the were interpreted first as layered magmatic rocks genetically related to carbonatitic magmas, emplaced following conclusions: 1 the carbonatites were emplaced at c. 2 Ga (U–Pb, at the end of the granulite facies metamorphic event. An alternative suggestion is that they have been zircon) within the granulites at the end of the episode of granulite facies metamorphism; metasomatically transformed. The aim of this paper is to constrain the origin of 2 they had high REE abundances and high SiO 2 activity during crystallization, factors contributing to these complexes using new field observations, stable (C, O), radiogenic (Sr, Nd), major and trace element the formation of britholite (a mineral derived from apatite through a coupled substitution Ca<REE and data on carbonatites, syenitic rocks and country-rocks, especially the marbles. This approach provides a P<Si ), and wollastonite; 3 high Sr and low Nd initial ratios (87Sr/86Sr typically framework to elucidate the relationships between the carbonatitic magmatism and the granulite facies event # 0.709 and eNd (T) #−8) characterize an enriched 763