No abstract

No abstract

A B S TR A CT Quartz Al-Mg granulites exposed at In Hihaou, In Ouzzal (NW Hoggar), preserve an unusual highgrade mineral association stable at temperatures up to 1050°C, involving the parageneses orthopyroxenesillimanite-garnet-quartz, sapphirine-quartz and spinel-quartz. The phase relationships within the FMAS system show that a continuum exists between the earlier prograde reaction textures and those of the later decompressive event. The following mineral reactions involving sillimanite are deduced: (1) Grt+Qtz Opx+Sil, (2) Opx+Sil Grt+Spr+Qtz, (3) Grt+Sil+Qtz Crd, (4) Grt+Sil Crd+Spr, (5) Grt+Sil+Spr Crd+Spl, (6) Grt+Sil Crd+Spl, (7) Grt+Crd+Sil Spl+Qtz and (8) Grt+Sil Spl+Qtz. Minerals in quartz Al-Mg granulites display compositional variations consistent with the observed reactions. The Mg/(Mg+Fe2+) range of the main minerals is as follows: cordierite (0.81-0.97), sapphirine (0.77-0.88), orthopyroxene (0.65-0.81), garnet (0.33-0.64) and spinel (0.23-0.56).The reaction textures and the evolution of the mineral assemblages in the quartz Al-Mg granulites indicate a clockwise P-T trajectory characterized by peak conditions of at least 10 kbar and 1050°C, followed by decompression from 10 to 6 kbar at a temperature of at least 900°C.

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, K d , between the various ferromagnesian phases decreased significantly whereas A1203 in 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 f 70" 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.

A B S TR A CT Granulite facies quartzites from the Ihouhaouene region, in the northern part of In Ouzzal, contain the assemblage corundum+quartz+magnetite together with hercynitic spinel+quartz+magnetite, sillimanite+quartz+magnetite and almandine-rich garnet+quartz+magnetite. Two types of corundum have been recognized: the first is primary and is found with quartz and magnetite only; the second type is found together with magnetite and chlorite rimming spinel as a fine-grained corona. The textures show that spinel-rich magnetite probably exsolved primary corundum, sillimanite, spinel and garnet during the cooling history. The secondary corundum formed later from the spinel already exsolved from magnetite. The secondary corundum is certainly metastable with respect to quartz. This may also apply for the primary corundum. However, given the high-temperature setting of this rock, it cannot be excluded that the stable contacts observed between primary corundum and quartz indicate equilibrium between the two phases. Taking into account the uncertainties in the thermodynamic data, the stability of this assemblage would imply that this part of In Ouzzal has recorded very high P-T conditions, above 1100°C at 12 kbar.