HIGH-GRADE METAMORPHISM, DEHYDRATIONS AND CRUSTAL MELTING - A REINVESTIGATION BASED ON NEW EXPERIMENTS IN THE SILICA-SATURATED PORTION OF THE SYSTEM KALO2-MGO-SIO2-H2O-CO2 AT P-LESS-THAN-OR-EQUAL-TO-1.5GPA

The system KAlO2-MgO-SiO2-H2O-CO2 has long been used as a model for th e processes of granulite-facies metamorphism and the development of or thopyroxene-bearing mineral assemblages through the breakdown of bioti te-bearing assemblages. There has been considerable controversy regard ing the role of carbon dioxide in metamorphism and partial melting. We performed new experiments in this system (at pressures of 342 to 1500 MPa with T between 710 and 1045 degrees C and X-H2O(Fl) between 0.05 and 1.00), accurately locating most of the dehydration and melting equ ilibria in P-T-X-H2O(Fl) space. The most important primary result is t hat the univariant reaction Phl + Qtz + Fl = En + Sa + melt must be al most coincident with the fluid-absent reaction (Phl + Qtz = En + Sa melt) in the CO2-free subsystem. In conjunction with the results of pr evious measurements of CO2 solubility in silicate melts and phase equi librium experiments, our theoretical analysis and experiments suggest that CO2 cannot act as a flux for partial melting. Crustal melting in the presence of H2O-CO2 mixed fluids will always occur at temperatures higher than with pure H2O fluid present. Magmas produced by such melt ing will be granitic (s.l.) in composition, with relatively high SiO2 and lots' MgO contents, irrespective of the H2O-CO2 ratio in any coexi sting fluid phase. We find no evidence that lamprophyric magmas could be generated by partial fusion of quartz-saturated crustal rocks. The granitic melts formed will not contain appreciable dissolved CO2. The channelled passage of hot CO2-rich fluids can cause local dehydration of the rocks through which they pass. In rock-dominated (as opposed to fluid-dominated) systems, minor partial melting call also occur in ve ins initially filled with CO2-rich fluid, as dehydration and local dis equilibrium drive the fluid towards H2O-rich compositions. However, CO 2 is unlikely to be a significant agent in promoting regional granulit e-grade metamorphism. melting, magma generation, metasomatism or long- range silicate mass transfer in Earth's crust. The most viable model f or the development of granulite-facies rocks involves the processes of fluid-absent partial melting and withdrawal of the melt phase to high er crustal levels.