Interaction of Biotite^Amphibole Gneiss with H2O^CO2^(K, Na)Cl Fluids at 550MPa and 750 and 8008Q: Experimental Study and Applications to Dehydration and Partial Melting in the Middle Crust

To constrain effects of chloride-bearing H2O^CO2 fluids on complex natural assemblages during high-grade metamorphism and anatexis, we report the results of experiments on the interaction of biotite^ hornblende tonalitic gneiss from the Sand River Formation (Limpopo Complex, South Africa) with H2O^CO2, H2O^CO2^ KCl, H2O^CO2^NaCl, and H2O^CO2^(K, Na)Cl fluids at 550MPa, 750 and 8008C, and varying chloride/(H2OþCO2) ratios with molar CO2/(CO2þH2O) 05. Heating of solid cy-linders of gneiss at both temperatures in the absence of a free fluid phase produced no changes in the gneiss phase assemblage.The equi-molar H2O^CO2 fluid at 7508Q also did not significantly influ-ence the phase assemblage. Addition of KCl to the fluid at 7508Q resulted in formation of the clinopyroxeneþK-feldspar (þ ilmen-ite/titanite) assemblage after biotite, hornblende and plagioclase. Orthopyroxene accompanied by amphibole appeared only at 8008