ORIGIN AND CRYSTALLIZATION OF SOME PERALUMINOUS (S-TYPE) GRANITIC MAGMAS

Our experiments (700-900 degrees C, 1-7 kbar, a(H2O) = 0.1-1.0, f(O-2) <= OFM, seeded glass starting materials) were carried out on four compositions from central Victorian S-type volcanic-plutonic suites. Biotite saturation curves have steep dP/dT slopes at 800-850 degrees C for melt water-contents of 2-10 wt.%. Early crystallization of F-poor biotite with plagioclase (up to 3 kbar) and quartz (at higher pressures) requires water contents > 2.5 wt. % in the melt. Orthopyroxene has a wide crytallization field; its absence from most granitic rocks indicates relatively low initial magmatic temperatures or, more likely, reaction to biotite with falling T. Cordierite crystallizes near the liquidus at P = 1-2 kbar but does not develop above the solidus at P > 5 kbar. Almandine-rich garnet becomes a near-liquidus phase at P >= 4-5 kbar. Integration of geothermometry and barometry calculations and crystallization sequences with the experimental data indicates melt water-contents of 3-5 wt. %, with early-crystallizing phases (garnet, biotite, orthopyroxene, plagioclase) present at T <= 850 degrees C, P >= 4 kbar. These relatively high-temperature, H2O-undersaturated S-type magmas originated by partial fusion of mildly peraluminous materials under granulite-facies conditions. Calculated dehydration and melting equilibria model the partial fusion of biotite- and muscovite-bearing assemblages for a(H2O) < 1.0. Together with 5-kbar crystallization experiments, these data are used to estimate anatectic conditions and the chemical features of melts derived from aluminous high-grade metamorphic rocks. Low-temperature, H2O-rich melts generated by muscovite breakdown have compositions near the "granite minimum". Higher-temperature melts, formed by partial fusion of biotite-bearing assemblages, are strongly water-undersaturated and depart markedly from "minimum" compositions. Source-related thermal and chemical characteristics of peraluminous magmas are reflected in the emplacement mechanisms, chemistry and mineralogy of S-type granitoid rocks.