Pressure-temperature-fluid evolution in Eburnean metabasites and metapelites from Tamanrasset (Hoggar, Algeria)

Tamanrasset (Central Hoggar, Algeria) is part of the 2000 Ma Eburnean terra ne in which spectacular examples of granulite-grade metapelites and metabas ites occur. Reactional textures in metabasites, supplemented by geothermoba rometry, fluid inclusions, and mineral equilibria indicate a complex metamo rphic history, characterized by a strong decompression during postpeak upli ft. Peak metamorphic pressure-temperature (PT) conditions of 800 degreesC a nd 10 kbar, respectively, were followed by rapidly decreasing pressure (to 700 degreesC and 6 kbar) at nearly constant temperature, during which time the rocks acquired their present mineral assemblages and textures (garnet p yroxenites). This isothermal decompression path was followed by nearly isob aric cooling. Throughout this history, water activity remained consistently low (a(H2O) = 0.1-0.5), comparable to values in neighboring migmatitic met apelites. Fluid inclusion studies suggest that the low water activities are due to CO2-rich fluids, identified as a synmetamorphic fluid (CO2 density: 1.05 g/cm(3)) generated by reactions during decompression. This fluid may have been generated during melting of the metapelite. The clockwise PT evol ution path is consistent with crustal thickening during collisional tectoni cs.