In order to understand the role of mica-rich rocks as a source of granite magmas, a series of melting experiments was performed on two different starting materials. The first composition is a model biotite gneiss consisting of 30 wt % biotite, 30 wt % plagioclase, and 40 wt % quartz. The second composition is a model two-mica pelites consisting of 15 wt % biotite, 15 wt % muscovite, 30 wt % plagioclase, and 40 wt % quartz. Experiments were performed under vapor-absent conditions at 1.0 GPa and between 750 ø and 950øC. With only biotite in the starting material the volume of melt is always less than 15 vol % below 900øC and reaches 25 vol % at 950øC. In experiments that involve both biotite and muscovite in the starting material, the melt proportion increases up to 28 vol % at 825øC and reaches 60 vol % at 950øC. For the biotite-plagioclase-quartz (BPQ) assemblage, the solidus is located at 800øC at 1.0 GPa. The melting reaction produces a metaluminous granitic liquid and leaves a residuum consisting of garnet + biotite + orthopyroxene + plagioclase + quartz. In addition, the experiments show that at 1.0 GPa biotite can be stable above 950øC. With both micas in the starting material (BPQM), the solidus at 1.0 GPa is located at 750øC. The melting reactions produce a peraluminous granitic liquid and leave a residuum of garnet + sillimanite + biotite + quartz + plagioclase + Kfeldspar in experiments below 900øC. At 950øC the residuum consists of garnet + orthopyroxene + biotite + plagioclase. The melt fraction is determined by the proportions of the hydrous phases and of the amount of feldspar relative to quartz. Mineral modes of the source rocks, particularly the amount of quartz, are at least as important as the amount of available H20 in controlling the melt fraction generated during crustal anatexis. Paper number 95JB00916. 0148-0227/95/95JB-00916505.00 asthenospheric impingement under continental crust following delamination of eclogitic thickened lower crust. Biotite-bearing gneisses are abundant in exposed lower crustal terrain. Their protoliths may have been graywackes and/or compositionally intermediate metavolcanic and plutonic rocks. Vapor-absent melting of biotite in metapelites begins at higher temperature than that of muscovite [Vielzeuf and Holloway, 1988; LeBreton and Thompson, 1988; Patiho-Douce and Johnston, 1991] but at a lower temperature than that of hornblende in amphibolite [Rushruer, 1991], metavolcanics [Conrad et al., 1988], and granodiorite to tonalite [Naney, 1983; Schmidt, 1993]. Fluid-absent melting of muscovite occurs abruptly by virtually univariant reaction because of the restricted variation in composition of this mineral. By contrast, fluid absent-melting of biotite and hornblende takes place over a relatively broad temperature interval because of the large variation in composition of these phases (due to solid solutions). Muscovite-bearing metapelites are extremely fertile because they are sensitive to vapor-absent melting in collision thickened lower crust. For example, trace elem...

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