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Journal of Petrology, Volume 39, Issue 10: October 1 1998.
We have studied experimentally the vapor-absent melting behavior of a Proterozoic metapelite (HP-60-1) from the Black Hills, South Dakota, to assess whether the high-[delta]18O, tourmaline-bearing granites of the Harney Peak suite resulted from muscovite-dominated dehydration-melting at 10 kbar. Samples were sealed with no added H2O in gold capsules, then run in a 0·5 inch (1·27 cm) piston-cylinder apparatus over the temperature interval 812-975°C. Experiments were conducted at 10 kbar to allow comparison with other metapelite melting studies. Mass balance calculations imply the melting reaction muscovite + biotite + plagioclase + quartz -> melt + alkali feldspar + aluminosilicate + garnet until muscovite is exhausted (<812°C), then the reaction biotite + plagioclase + quartz + alkali feldspar ± aluminosilicate -> melt + garnet. The inferred muscovite reaction is completed below 812°C and produces 1-2 wt % melt. Melt production of the continuous biotite reaction increases steadily with rising temperature, to 32 wt % in our highest temperature run. The low melt productivity of the muscovite reaction at 10 kbar suggests that melting at lower pressures may be necessary to generate mobile melt fractions by muscovite-dominated reactions in our starting material. In addition, melts of HP-60-1 are considerably more mafic than the tourmaline-bearing granites, with average combined TiO2 + MgO + FeO contents of 1·85 wt % and 0·84 wt %, respectively. In particular, melts of HP-60-1 are enriched in Ti, and would probably stabilize biotite rather than tourmaline upon cooling, arguing against HP-60-1 as a potential source rock for the tourmaline granites.
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Pages 1787-1804