Interaction of Biotite–Amphibole Gneiss with H2O–CO2–(K, Na)Cl Fluids at 550 MPa and 750 and 800°С: Experimental Study and Applications to Dehydration and Partial Melting in the Middle Crust

Abstract

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)1⁄4 0 5. Heating of solid cylinders of gneiss at both temperatures in the absence of a free fluid phase produced no changes in the gneiss phase assemblage.The equimolar H2O^CO2 fluid at 7508Q also did not significantly influence the phase assemblage. Addition of KCl to the fluid at 7508Q resulted in formation of the clinopyroxeneþK-feldspar (þ ilmenite/titanite) assemblage after biotite, hornblende and plagioclase. Orthopyroxene accompanied by amphibole appeared only at 8008C as a result of biotite breakdown in the presence of H2O^CO2 and low-salinity H2O^CO2^KCl fluids. Increase in the KCl content in the fluid at 8008Q resulted in the production of a clinopyroxenebearing assemblage. Increase of the NaCl content stabilized amphibole in an assemblage with either orthopyroxene (at low NaCl concentrations) or clinopyroxene. Nevertheless, clinopyroxene (þ albite) is stable only at high salt concentrations. Comparison of the experimental results with the results of thermodynamic modeling using the Gibbs free energy minimization method (PERPLE_X software) showed that mineral reactions and assemblages in the run products were governed by the activities of alkali components imposed by KCl and NaCl in the H2O^CO2 fluids, and decrease of the water activity served as an additional factor stabilizing anhydrous assemblages. No melts formed at 7508C in the presence of the H2O^CO2^KCl fluids. These fluids provoked melting only at 8008C with formation of rhyolitic melts.With increasing KCl content of the fluid, the melt composition changed to potassic rhyolitic with Al2O3513 5 wt %, CaO52 wt %, K2OþNa2O47 wt %, FeO/(FeOþMgO)40 8, K2O/Na2O41, and moderate enrichment in Cl (0 2^0 6 wt %). Increasing NaCl content caused melting at 7508Q and shifted the melt composition towards trachytic and trachyandesitic compositions at both 750 and 8008Q. The experiments support a model for the formation of ferroan A-type granite^syenite complexes via crustal melting in the presence of H2O^CO2^salt fluids in extensional tectonic settings.They demonstrate a possible link between A-type granitoids and mid-crustal dehydration zones in amphiboliteto granulite-facies terrains and allow a new interpretation of mineral assemblages within these zones in terms of variations in fluid salinity.