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

Tamanrasset (Central Hoggar, Algeria) is part of the 2000 Ma Eburnean terrane in which spectacular examples of granulite-grade metapelites and metabasites occur. Reactional textures in metabasites, supplemented by geothermobarometry, fluid inclusions, and mineral equilibria indicate a complex metamorphic history, characterized by a strong decompression during postpeak uplift. Peak metamorphic pressure-temperature (PT) conditions of 800 degreesC and 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 pyroxenites). This isothermal decompression path was followed by nearly isobaric cooling. Throughout this history, water activity remained consistently low (a(H2O) = 0.1-0.5), comparable to values in neighboring migmatitic metapelites. 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 evolution path is consistent with crustal thickening during collisional tectonics.