MELT PRODUCTION DURING GRANULITE-FACIES ANATEXIS - EXPERIMENTAL-DATA FROM PRIMITIVE METASEDIMENTARY PROTOLITHS

We conducted fluid-absent partial melting experiments, at 0.5 and 1.0 GPa in the temperature range 750 to 1000 degrees C, to investigate the influence of bulk rock Mg# [100Mg/(Mg+Fe)] and the effects of additio nal TiO2 on the granulite-grade anatectic evolution of relatively magn esian metapelites and metagreywackes. In these experiments, melting be gan between 780 and 830 degrees C by the incongruent breakdown of biot ite to produce quartz-saturated, granulite-facies residual mineral ass emblages in equilibrium with H2O-undersaturated granitic melt. The gla ss (quenched melt) compositions produced in this study vary little. Ge nerally, the glasses have compositions similar to those of many natura l strongly peraluminous leucogranites. The solidus temperatures in bot h rock types increase with increasing Mg#, but are unaffected by the p resence or absence of a TiO2 component. At 0.5 GPa the metapelites mel ted at temperatures up to 50 degrees C lower than the equivalent metag reywackes, but at 1 GPa there was no discernible difference. This stud y suggests that the fluid-absent solidus has a steep positive dP/dT sl ope in metapelites and steep negative dP/dT slope in metagreywackes. T he pattern of melt production with increasing creasing temperature is strongly controlled by the upper limit of biotite stability. In TiO2-f ree compositions this was found to increase by 15 to 20 degrees C in t he metapelites and by 30 to 40 degrees C in the metagreywackes, as a f unction of increasing Mg# from 49 to 81. The presence of a TiO2 compon ent increases the upper limit of biotite stability by similar to 50 de grees C in the metapelites and by similar to 80 degrees C in the metag reywackes, over that observed in the equivalent TiO2-free compositions . In consequence, in the TiO2-free samples large pulses of melt (up to 35 wt%) are produced over narrow temperature ranges (as little as 15 degrees C in these experiments) between 830 and 875 degrees C. In the TiO2-bearing samples the major pulse of melt production occurs more gr adually between 830 and >900 degrees C.