1. zyxwvutsrq metamorphic Geol., zyxwvuts 1992, zyxwvuts 10, zyxwvutsr 17-31 P-T-X relationships in the Precambrian AI-Mg-rich granulites from In Ouzzal, Hoggar, Algeria zy P. BERTRAND,' KH. OUZEGANE2 AND J. R. KIENAST3 ' Laboratoire de Petrologie Physique, Universite Paris-7 and 1. P.G. P. , CNRS-URA 1093, 75251 Paris Cedex 05, France 21nstitut des Sciences de la Terre, U.S.T.H.B., Alger, Algeria France Laboratoire de Petrologie Mdtamorphique, Universite Paris 7, CNRS-URA 736, 75251 Paris Cedex 05, ABSTRACT zyxwvutsrq The Al-Mg-rich granulites from the In Ouzzal craton, Algeria, show a great diversity of mineral reactions which correspond to continuous equilibria as predicted by phase relationships in the Fe0-Mg0-A1,03- SiO, system. The sequence of mineral reactions can be subdivided into three distinct stages: (1) a highd stage characterized by the growth of coarse mineral assemblages involving sapphirine and the disappearance of early corundum and spinel-bearing assemblages; (2) a high-T stage characterized by the development of Sa-Qz-bearing assemblages; and (3) a later stage, in which garnet-bearing assemblages are replaced by more or less fine symplectites involving cordierite. During the course of early mineral reactions, the distribution coefficient, zyxw Kd, between the various ferromagnesian phases decreased significantly whereas A1203in pyroxene increased concomitantly. These observations, when combined with topological constraints, clearly indicate that the high-P stage 1 was accompanied by a significant rise in temperature (estimated at 150 f 50" C) under near isobaric conditions, in agreement with the reaction textures. By stage 2, pressure and temperature were extreme as evidenced by the low K, value between orthopyroxene and garnet (K, = 2.06-1.99), the high alumina content in pyroxene (up to 11.8%) and the high magnesium content in garnet [lo0 Mg/(Mg + Fe) = 60.61. Mineral thermometry based on Fe-Mg exchange between garnet and pyroxene and on Al-solubility in pyroxene gives temperatures close to 970 zyxwv f 70" zyxw C at 10 f 1.5 kbar. These results are in agreement with the development of Sa-Qz assemblages on a local scale. Late mineral reactions have been produced during a decompression stage from about 9 to 6 kbar. Except for local re-equilibration of Mg and Fe at grain boundaries, there is no evidence for further reactions below 700" C. We interpreted the whole set of mineral reactions as due to changes in pressure and temperature during a tectonic episode located at c. 2 Ga. Because of the lack of evidence for further uplift after the thermal relaxation which occurred at c. 6 kbar, it is possible however that the exhumation of this granulitic terrane occurred in a later tectonic event unrelated to its formation. Key words: granulites; Hoggar; P-T path; Precambrian; sapphirine. INTRODUCTION The recent discovery of dense mineral associations, such as hypersthene-corundum, hypersthene-sillimanite-quartz or sapphirine-quartz, in deep-seated granulite facies rocks from the In Ouzzal craton, Algeria (Kienast & Ouzegane, 1987; Ouzegane, 1987; Rahmani et al., 1991) is of great interest because these assemblages are rare and reflect extreme P-T conditions of granulite facies metamorphism (e.g. Ellis et al., 1980; Bertrand et al., 1991). The In Ouzzal craton is a N-S-trending unit about 450km long and 100km wide, which occurs within the Pharusian belt (Fig. 1; Bertrand & Lasserre, 1976; Caby et al., 1981). To the south, this unit becomes thinner and disappears towards the frontier of Mali, where it is replaced by the Iforas craton. The contacts between the In Ouzzal craton and the adjacent Pharusian belt correspond to N-S shear zones. The western and eastern Pharusian units have been deformed and metamorphosed during the Pan-African orogeny under amphibolite and greenschist facies conditions, contrasting with the higher metamorphic grade recorded in the In Ouzzal rocks. The craton comprises a wide range of supracrustal rocks, including metapelites, banded iron formations, quartzites and marbles, commonly associated with norite and lherzolite lenses. Numerous charnockitic gneisses are also found within this complex. The most original feature of the In Ouzzal craton is certainly the abundance and great variety of metapelites, often characterized by spectacular reaction coronas (Figs 2-4) (Ouzegane, 1987). Most of 17