EXPERIMENTALLY DETERMINED SOLIDI IN THE CA-BEARING GRANITE SYSTEM NAALSI3O8-CAAL2SI2O8-KALSI3O8-SIO2-H2O-CO2

We have determined the phase relationships of melting of synthetic gra nite (two ternary feldspars + quartz) in the presence of an H2O-CO2 fl uid. Synthesis and reversed experiments were conducted in a piston-cyl inder apparatus over the range 650-900 degrees C and 6-15 kbar. At X(H 2O) = 0.75, melting occurred between 670 and 680 degrees C (15 kbar), 700 and 710 degrees C (10 kbar), and 710 and 720 degrees C (7.4 kbar). At X(H2O) = 0.5, melting occurred between 760 and 770 degrees C (15 k bar), 780 and 790 degrees C (10 kbar), and 800 and 820 degrees C (6-7. 4 kbar). At X(H2O) = 0.25, melting occurred between 830 and 840 degree s C (15 kbar), 830 and 840 degrees C (10 kbar), and 860 and 870 degree s C (6-7.4 kbar). These results provide important constraints on the m aximum temperatures of regional metamorphism attainable in vapor-satur ated metapelitic and quartzofeldspathic rocks that escaped widespread melting. At pressures below 10 kbar, a fluid phase of X(H2O) = 0.75, 0 .5, and 0.25 limits temperatures to below similar to 700-725, similar to 800-825, and similar to 850-875 degrees C, respectively. As a conse quence, the formation of granulite does not require CO2 concentrations in a coexisting fluid to exceed an X(co2) of 0.25-0.5, a range that m ay include dilution of the H2O component of the fluid through internal buffering by devolatilization reactions. Therefore, the formation of granulites by the influx of CO2 may be a less common phenomenon than p reviously thought.