Dehydration-melting of Biotite Gneiss and Quartz Amphibolite from 3 to 15 kbar

Abstract

We performed vapor-absent melting and crystallization experiments on two bulk compositions that model metamorphic rocks containing a single hydrous phase: a biotite gneiss [37% bio (mg-number 55), 34% qtz, 27% plg (An38), 2% ilm] and a quartz amphibolite [54% hbl (mg-number 60), 24% qtz, 20% plg (An38), 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-absent solidi to (at least) the temperature at which the hydrous mineral disappeared. Dehydration-melting begins at similar temperatures in both bulk compositions, ranging from T∼850°C at P = 3 kbar T∼930°C at P = 15 kbar. The hydrous mineral disappears ∼50°C above the solidus in both systems, except in IHPV experiments at f(O2) above Ni–NiO, in which biotite stability extends up to atleast 80°C above the solidus. At the T at which the hydrous minerals disappear the biotite gneiss produces 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 controlled by three competing factors: (1) the positive d P/dT slopes of the solidi, (2) decreasing H2O activity with increasing P at constant H2O content, and (3) Na2O activity, which increases with P concomitantly with breakdown of plagioclase. Melt productivities at T = 920–950°C are maximized at intermediate pressures (∼7 kbar). The biotite gneiss produces strongly peraluminous granitic melts (SiO2>70 wt%) and residual assemblages of quartz norite (P>12·5 kbar) or garnet pyroxenite (P>12·5 kbar). The quartz amphibolite produces strongly peraluminous granodioritic melts (SiO2>70 wt%) that coexist with clinopyroxene + orthopyroxene + plagioclase + quartz ± at P>10 kbar)garnet. The results of coupled melting and crystallization experiments on the quartz amphibolite suggest that strongly peraluminous granitoid rocks (e.g. cordierite-bearing and two-mica granites) can be derived from melting of Al-poor protoliths.