Interaction of Biotite-Amphibole Gneiss with H₂O-CO₂-(K, Na)Cl Fluids at 550MPa and 750 and 800^° C: 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/(H₂O+CO₂) ratios with molar CO₂/(CO₂+O)=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 clinopyroxene+K-feldspar (+ 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 (+ 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 Al₂O₃<13.5wt %, CaO<2wt %, K₂O+Na₂O47wt %, FeO/(FeO+MgO)40.8, K₂O/Na₂O>1, 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 800^°C. 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 amphibolite-to granulite-facies terrains and allow a new interpretation of mineral assemblages within these zones in terms of variations in fluid salinity.