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Reaction textures and fluid inclusions in the w2.0 Ga pyroxene-bearing dehydration zones within the Sand River biotite-hornblende orthogneisses (Central Zone of the Limpopo Complex) suggest that the formation of these zones is a result of close interplay between dehydration process along ductile shear zones triggered by H 2 O-CO 2 -salt fluids at 750e800 C and 5.5e6.2 kbar, partial melting, and later exsolution of residual brine and H 2 O-CO 2 fluids during melt crystallization at 650e700 C. These processes caused local variations of water and alkali activity in the fluids, resulting in various mineral assemblages within the dehydration zone. The petrological observations are substantiated by experiments on the interaction of the Sand River gneiss with the H 2 O-CO 2 -(K, Na)Cl fluids at 750 and 800 C and Production and hosting by Elsevier available at www.sciencedirect.com China University of Geosciences (Beijing) GEOSCIENCE FRONTIERS journal homepage: www.elsevier.com/locate/gsf GEOSCIENCE FRONTIERS 3(6) (2012) 829e8415.5 kbar. It follows that the interaction of biotite-amphibole gneiss with H 2 O-CO 2 -(K, Na)Cl fluids is accompanied by partial melting at 750e800 C. Orthopyroxene-bearing assemblages are characteristic for temperature 800 C and are stable in equilibrium with fluids with low salt concentrations, while salt-rich fluids produce clinopyroxene-bearing assemblages. These observations are in good agreement with the petrological data on the dehydration zones within the Sand River orthogneisses. ª 2012, China University of Geosciences (Beijing) and Peking University. Production and hosting by Elsevier B.V. All rights reserved.

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