Hydration of orthopyroxene-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 crystallization of orthoamphibole and garnet, and at a later stage of chlorite, from the original paragenesis. Calculated phase diagrams show that this retrogression occurred at about 3 kbar with the simplest model involving hydration at 650-700°C and at around 500°C, with the rocks experiencing a_H2O less than 1, except possibly in the last stages of chlorite crystallization. As the other rock types occurring in the same area as the orthopyroxene-cordierite rocks display similar features, it is concluded that regional hydration occurred, presumably related to the release of fluids during the crystallization of the Pan-African granitic and mafic magmas that are widespread in the Laouni area.