FLUID-ABSENT MELTING BEHAVIOR OF A 2-MICA METAPELITE - EXPERIMENTAL CONSTRAINTS ON THE ORIGIN OF BLACK-HILLS GRANITE

We have studied experimentally the vapor-absent melting behavior of a Proterozoic metapelite (HP-60-1) from the Black Hills, South Dakota, t o assess whether the high-delta(18)O, tourmaline-bearing granites of t he Harney Peak suite resulted from muscovite-dominated dehydration-mel ting at 10 kbar. Samples were sealed were sealed with no added H2O in gold capsules, then run in a 0.5 inch (1.27 cm) piston-cylinder appara tus over the temperature interval 812-975 degrees C. Experiments were conducted at 10 kbar to allow comparison with other metapelite melting studies. Mass balance calculations imply the melting reaction muscovi te + biotite + plagioclase + quartz --> melt + alkali feldspar + alumi nosilicate + garnet until muscovite is exhausted (<812 degrees C), the n the reaction biotite + plagioclase + quartz + alkali feldspar +/- al uminasilicate --> melt + garnet. The inferred muscovite reaction is co mpleted below 812 degrees C and procedures 1-2 wt % melt. Melt product ion of the continuous biotite reaction increases steadily with rising temperature, to 32 wt % in our highest temperature run. nle low! melt productivity of the muscovite reaction at 10 kbar suggests that meltin g at lower pressures may be necessary to general mobile melt fractions by muscovite-dominated reactions in our starting material. In additio n, melts of HP-60-1 are considerably more mafic than the tourmaline-be aring granites, with average combined TiO2 + MgO + FeO contents of 1.8 5 wt % and 0.84 wt %, respectively. In particular, melts of HP-60-1 ar e enriched in Ti, and would probably stabilize biotite rather than tou rmaline upon cooling, arguing against HP-60-1 as a potential source ro ck for the tourmaline granites.