HYDRATION OF ORTHO-PYROXENE-CORDIERITE-BEARING ASSEMBLAGES AT LAOUNI,CENTRAL HOGGAR, ALGERIA

In the Laouni region (Central Hoggar, Algeria), retrogression of high- grade orthopyroxene-cordierite-bearing rocks led to the crystallizatio n of orthoamphibole and garnet, and at a later stage of chlorite, from the original paragenesis. Calculated phase diagrams show that this re trogression occurred at about 3 kbar with the simplest model involving hydration at 650-700 degrees C and at around 500 degrees C, with the rocks experiencing a(H2O) less than 1, except possibly in the last sta ges of chlorite crystallization. As the other rock types occurring in the same area as the orthopyroxenecordierite rocks display similar fea tures, it is concluded that regional hydration occurred, presumably re lated to the release of fluids during the crystallization of the Pan-A frican granitic and mafic magmas that are widespread in the Laouni are a.