PARTIAL MELTING OF METAGREYWACKES .2. COMPOSITIONS OF MINERALS AND MELTS

A series of experiments on the fluid-absent melting of a quartz-rich a luminous metagreywacke has been carried out. In this paper, we report the chemical. composition of the phases present in the experimental ch arges as determined by electron microprobe. This analytical work inclu des biotite, plagioclase, orthopyroxene. garnet, cordierite, hercynite , staurolite, gedrite, oxide, and glass, over the range 100-1000 MPa, 780-1025 degrees C. Biotites are Na- and Mg-rich, with Ti contents inc reasing with temperature. The compositions of plagioclase range from A n(17) to An(35), with a significant orthoclase component, and are alwa ys different from the starting minerals. At high temperature, plagiocl ase crystals correspond to ternary feldspars with Or contents in the r ange 11-20 mol%. Garnets are almandine pyrope grossular spessartine so lid solutions, with a regular and significant increase of the grossula r content with pressure. All glasses are silicic (SiO2 = 67.6-74.4 wt% ), peraluminous, and leucocratic (FeO + MgO = 0.9-2.9 wt%), with a bul k composition close to that of peraluminous leucogranites, even for de grees of melting as high as 60 vol.%. With increasing pressure, SiO2 c ontents decrease while K2O increases. At any pressure, the melt compos itions are more potassic than the water-saturated granitic minima. The H2O contents estimated by mass balance are in the range 2.5-5.6 wt%. These values are higher than those predicted by thermodynamic models. Modal compositions were estimated by mass balance calculations and by image processing of the SEM photographs. The positions of the 20 to 70 % isotects (curves of equal proportion of melt) have been located in t he pressure-temperature space between 100 MPa and 1000 MPa. With incre asing pressure, the isotects shift toward lower temperature between 10 0 and 200 MPa, then bend back toward higher temperature. The melting i nterval increases with pressure; the difference in temperature between the 20% and the 70% isotects is 40 degrees C at 100 MPa, and 150 degr ees C at 800 MPa. The position of the isotects is interpreted in terms of both the solubility of water in the melt and the nature of the rea ctions involved in the melting process. A comparison with other partia l melting experiments suggests that pelites are the most fertile sourc e rocks above 800 MPa. The difference in fertility between pelites and greywackes decreases with decreasing pressure. A review of the glass compositions obtained in experimental studies demonstrates that partia l melting of fertile rock types in the crust (greywackes, pelites, or orthogneisses) produces only peraluminous leucogranites. More mafic gr anitic compositions such as the various types of calk-alkaline rocks, or mafic S-type rooks, have never been obtained during partial melting experiments. Thus only peraluminous leucogranites may correspond to l iquids directly formed by partial melting of metasediments. Other type s elf granites involve other components or processes, such as restite unmixing from the source region, and/or interaction with mafic mantle- derived materials.