Isothermal decompression history in the Western Granulite terrain, central Tanzania: Evidence from reaction textures and trapped fluids in metapelite

The Mozambique Belt (MB) of the East Africa Orogen contains large areas of gramilite-facies migmatitic gneisses with Archaean and Palaeoproterozoic protolith ages and that were recycled during the Neoproterozoic Pan-African orogeny. The study area is situated along the Great Ruaha River and within the Mikumi National Park in central Tanzania where migmatitic gneisses and mafic to intermediate granulites are interlayered with Neoproterozoic gramilite-facies migmatitic metapelites. Mineral textures suggest isothermal decompression, with the peak mineral assemblage comprising Grt-Bt-Ky-Kfs-Pl-Qtz +/- Phn +/- Ti-Oxide +/- melt and amphibolite-facies retrograde assemblage Grt-Bt-Sil-Ms-Kfs-Pl-Qtz +/- Fe-Ti-Oxide. The near isothermal retrograde overprint is seen in well-developed formation of pseudomorphs after garnet. The HP gramilite-facies assemblages record P-T conditions of 13-14 kbar at 760-800 degrees C. Retrogression and the release of fluids from crystallizing melts occurred at similar to 7 kbar and 650-700 degrees C. A fluid inclusion study shows three types of fluid inclusion consisting of nearly pure CO2, as well as H2O NaCl and H2O-CO2 mixtures. We suggest that a immiscible CO2-bearing brine represents the fluid composition during high-grade peak metamorphism, and that the fluid inclusions containing H2O-NaCl or nearly pure CO2 represent trapped fluids from in situ crystallised melt. The results suggest strong isothermal decompression, which is probably related to a fast exhumation after crustal thickening in the central part of the Mozambique Belt in Tanzania. (c) 2008 Elsevier Ltd. All rights reserved.