ALBERT

Description: To constrain effects of chloride-bearing H 2 O–CO 2 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 H 2 O–CO 2 , H 2 O–CO 2 –KCl, H 2 O–CO 2 –NaCl, and H 2 O–CO 2 –(K, Na)Cl fluids at 550 MPa, 750 and 800°C, and varying chloride/(H 2 O + CO 2 ) ratios with molar CO 2 /(CO 2 + H 2 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 H 2 O–CO 2 fluid at 750°C also did not significantly influence the phase assemblage. Addition of KCl to the fluid at 750°C resulted in formation of the clinopyroxene + K-feldspar (+ ilmenite/titanite) assemblage after biotite, hornblende and plagioclase. Orthopyroxene accompanied by amphibole appeared only at 800°C as a result of biotite breakdown in the presence of H 2 O–CO 2 and low-salinity H 2 O–CO 2 –KCl fluids. Increase in the KCl content in the fluid at 800°C resulted in the production of a clinopyroxene-bearing 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 H 2 O–CO 2 fluids, and decrease of the water activity served as an additional factor stabilizing anhydrous assemblages. No melts formed at 750°C in the presence of the H 2 O–CO 2 –KCl fluids. These fluids provoked melting only at 800°C with formation of rhyolitic melts. With increasing KCl content of the fluid, the melt composition changed to potassic rhyolitic with Al 2 O 3 〈 13·5 wt %, CaO 〈 2 wt %, K 2 O + Na 2 O 〉 7 wt %, FeO/(FeO + MgO) 〉 0·8, K 2 O/Na 2 O 〉 1, and moderate enrichment in Cl (0·2–0·6 wt %). Increasing NaCl content caused melting at 750°C 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 H 2 O–CO 2 –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.