Experimental and petrological constraints on local-scale interaction of biotite-amphibole gneiss with H₂O-CO₂-(K, Na)Cl fluids at middle-crustal conditions: Example from the Limpopo Complex, South Africa

Reaction textures and fluid inclusions in the ∼2.0 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 H₂O-CO₂-salt fluids at 750-800°C and 5.5-6.2 kbar, partial melting, and later exsolution of residual brine and H₂O-CO₂ fluids during melt crystallization at 650-700°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 H₂O-CO ₂-(K, Na)Cl fluids at 750 and 800°C and 5.5 kbar. It follows that the interaction of biotite-amphibole gneiss with H₂O-CO ₂-(K, Na)Cl fluids is accompanied by partial melting at 750-800°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.