H2O-saturated experiments with synthetic metapelite compositions (muscovite-quartz-albite and muscovite-quartz-albite-biotite ± aluminum silicate ± cordierite) performed over the temperature interval of 600–750 °C at 200 MPa (H2O) reveal that partial fusion commences at 625 °C along the metastable extension of the reaction muscovite + quartz + albite + H2O = melt + aluminum silicate. Biotite is stable over the entire temperature interval, although it reacts progressively to hercynite + melt at the high end of the temperature range. Muscovite that survives initial melting breaks down to corundum + orthoclase between 700 and 725 °C. Minor corundum and aluminum silicate are present at 650 °C, whereas corundum with a large sapphire (Fe + Ti) component is present at and above 700 °C. Finally, corundum and hercynite exist with orthoclase-rich feldspar and remaining biotite at 750 °C. The normative composition of melt at the minimum is approximately Ab30Or15Ms20QtZ35 (in weight percent) without other minor components (e.g., cryolite). Both muscovite and its equivalent orthoclase + corundum assemblage contribute substantial excess Al to melt, bringing the value of the Al saturation index (ASI) of melt to 1.4. Li, Rb, Cs, and F are strongly enriched in melts because of the continuous reaction and reequilibration of biotite and muscovite over the temperature interval. Concentrations of the femic components (TiO2 + FeO + MgO + MnO) are low (<1 wt%) but rise with temperature primarily because of increasing solubilities of FeO in melt. Calculated partition coefficients, D(M)Bt/gl, between biotite (Bt) and glass (gl) for the element M show that D(Li)Bt/gl (1.7−1.0), D(Ba)Bt/gl (~14−6), and D(F)Bt/gl (2.5–1.5) all decrease with increasing temperature, whereas partition coefficients for Sr (≈0.04), Rb (≈2.0), and Cs (≈0.4) remain constant with temperature over a large range of concentrations. Partition coefficients for muscovite (Ms) were also determined at 650 °C; D(M)Ms/gl ≈ 0.8 (Li), 3−6 (Ba), 1.6 (Rb), 0.05 (Sr), 0.3 (Cs), and 1.8 (F). These results, together with other data for feldspar, suggest that Rb, Cs, and Ba become strongly fractionated from one another during anatexis of aluminous metasediments and the ensuing crystallization of melts. Finally, the low partition coefficients for F between micas (biotite or muscovite) and melt indicate that F-rich melts can be generated by the incipient hydrous anatexis of aluminous metasediments; models that invoke remelting of a dehydrated protolith to generate such F-rich melts may not be necessary.

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