EXPERIMENTAL GENERATION OF HYBRID SILICIC MELTS BY REACTION OF HIGH-AL BASALT WITH METAMORPHIC ROCKS

Melting and crystallization experiments (T = 1000 degrees C; P = 0.5, 0.7, 1.0, 1.2, and 1.5 GPa) were performed on two hybrid bulk compositions consisting of 50% anhydrous high-Al olivine tholeiite glass (HAOT) and 50% metamorphic rock (alumino-silicate-bearing metapelite and aluminosilicate-free biotite gneiss). The objective of the experiments was to determine phase equilibrium constraints attending bulk assimilation of crustal rocks by basaltic melts. Experiments generated 32-38 wt % of H2O-undersaturated granitic melt (SiO2 > 70 wt %). Thermal and phase equilibrium arguments show that production of this amount of granitic melt at the chosen experimental conditions is a plausible model for bulk assimilation in nature. The compositions of the melts and of the coexisting crystalline assemblages are affected to a comparable extent by pressure and by composition of the crustal source. Reaction of HAOT with biotite gneiss at P less than or equal to 1.0 GPa produces calc-alkaline melts in equilibrium with gabbronoritic cumulates (plag + opx + cpx). Reaction of HAOT with metapelite at P less than or equal to 0.7 GPa produces strongly peraluminous melts that resemble S-type granites from the Lachlan Fold Belt, in equilibrium with noritic cumulates (plag + opx +/- spi +/- gar). At P > 1.2 GPa, both source compositions produce strongly peraluminous leucocratic melts (< 2 wt % FeO+MgO+TiO2) in equilibrium with garnet-rich and plagioclase-poor residues (opx + cpx + gar + plag from the biotite gneiss, gar + plag from the metapelite). The experiments show that a wide spectrum of high-SiO2 melts can be hybrids formed directly by reaction of basaltic melts with amphibolite-facies metamorphic rocks. Accumulation of the complementary mafic crystalline assemblages in the deep crust will generate granulites which are neither restitic nor the products of subsolidus dehydration. Mafic granulites and granitic melts formed by reaction of basaltic melts with metamorphic rocks will share isotopic and trace element signatures reflecting inheritance from both crustal and mantle sources.