Fluid-Absent Melting Behavior of a Two-Mica Metapelite: Experimental Constraints on the Origin of Black Hills Granite

Abstract

We have studied experimentally the vapor-absent melting behavior INTRODUCTION of a Proterozoic metapelite (HP-60-1) from the Black Hills, South Peraluminous leucogranites such as the Miocene granites Dakota, to assess whether the high-dO, tourmaline-bearing granites of the Himalaya and tourmaline granites of the Harney of the Harney Peak suite resulted from muscovite-dominated dePeak suite in South Dakota are widely considered to be hydration-melting at 10 kbar. Samples were sealed with no added the product of muscovite dehydration-melting; yet few H2O in gold capsules, then run in a 0·5 inch (1·27 cm) pistonexperimental studies of muscovite dehydration-melting cylinder apparatus over the temperature interval 812–975°C. reactions have been performed, because the low temExperiments were conducted at 10 kbar to allow comparison with peratures of the reactions hinder the approach to equiother metapelite melting studies. Mass balance calculations imply librium. In addition, melt fractions generated are typically the melting reaction muscovite + biotite + plagioclase + quartz low, resulting in small melt pockets which are very difficult → melt + alkali feldspar + aluminosilicate + garnet until to analyze with the electron microprobe. In this study, muscovite is exhausted (<812°C), then the reaction biotite + we assess the possible genetic relationship between the plagioclase + quartz + alkali feldspar ± aluminosilicate → tourmaline-bearing granites of the Harney Peak suite melt + garnet. The inferred muscovite reaction is completed below and the aluminous metasediments these granites intrude. 812°C and produces 1–2 wt % melt. Melt production of the Although the attempt to investigate the muscovite decontinuous biotite reaction increases steadily with rising temperature, hydration-melting reaction was hindered by the surto 32 wt % in our highest temperature run. The low melt prisingly low melt productivity, we were able to infer a productivity of the muscovite reaction at 10 kbar suggests that balanced reaction, illuminating the dominantly peritectic melting at lower pressures may be necessary to generate mobile melt nature of fluid-absent muscovite breakdown under the fractions by muscovite-dominated reactions in our starting material. conditions investigated. In addition, melts of HP-60-1 are considerably more mafic than Isotopic and trace element studies indicate a genetic the tourmaline-bearing granites, with average combined TiO2 + relationship between S-type granites and aluminous metaMgO+ FeO contents of 1·85 wt % and 0·84 wt %, respectively. sediments in the Black Hills of South Dakota, where the In particular, melts of HP-60-1 are enriched in Ti, and would Harney Peak Granite (HPG) intruded Proterozoic and probably stabilize biotite rather than tourmaline upon cooling, Archean schists and metagreywackes undergoing regional arguing against HP-60-1 as a potential source rock for the metamorphism. The timing and mode of emplacement of tourmaline granites. the HPG, and the deformation and regional metamorphic