DEHYDRATION-MELTING OF BIOTITE GNEISS AND QUARTZ AMPHIBOLITE FROM 3 TO 15 KBAR

We performed vapor-absent melting and crystallizatian experiments on t wo bulk compositions that model metamorphic rocks containing a single hydrous phase: a biotite gneiss [37% bio (mg-number 55), 34% qtz, 27% plg (An(38)), 2% ilm] and a quartz amphibolite [54% hbl (mg-number 60) , 24% qtz, 20% plg (An(38)), 2% ilm]. Experiments were performed at 3 and 5 kbar in internally heated pressure vessels (IHPV), and at 7, 10, 12.5 and 15 kbar in piston cylinder apparatus (PC), from the vapor-ab sent solidi to (at least) the temperature at which the hydrous mineral disappeared. Dehydration-melting begins at similar temperatures in bo th bulk compositions, ranging from T similar to 850 degrees C at P = 3 kbar to T similar to 930 degrees C at P = 15 kbar. The hydrous minera l disappears similar to 50 degrees C above the solidus in both systems , except in IHPV experiments at f(O-2) above Ni-NiO, in which biotite stability extends up to at least 80 degrees C above the solidus. At th e T at which the hydrous minerals disappear the biotite gneiss produce s 2-3 times more melt than the quartz amphibolite (50-60 wt% vs 20-30 wt%). In both systems, variations in melt productivity with P are cont rolled by three competing factors: (1) the positive dP/dT slopes of th e solidi, (2) decreasing H2O activity with increasing P at constant H2 O content, and (3) Na2O activity, which increases with P concomitantly with breakdown of plagioclase. Melt productivities at T = 920-950 deg rees C are maximized at intermediate pressures (similar to 7 kbar). Th e biotite gneiss produces strongly peraluminous granitic melts (SiO2 > 70 wt%) and residual assemblages of quartz norite (P less than or equ al to 12.5 kbar) or garnet pyroxenite (P > 12.5 kbar). The quartz amph ibolite produces strongly peraluminous granodioritic melts (SiO2 > 70 wt%) that coexist with clinopyroxene + orthopyroxene + plagioclase + q uartz +/- at P > 10 kbar) garnet. The results of coupled melting and c rystallization experiments on the quartz amphibolite suggest that stro ngly peraluminous granitoid rocks (e.g. cordierite-bearing and two-mic a granites) can be derived from melting of Al-poor protoliths.