Mineralogical and chemical evolution of a rare-element granite-pegmatite system: Harney Peak Granite, Black Hills, South Dakota

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The Harney Peak Granite (1.7 b.y.) in the Black Hills, South Dakota, is a well-exposed granite complex surrounded by a rare-element pegmatite field (barren to Nb-, Ta-, Be, Li-enriched pegmatites). It consists of a multitude of large and small sills and dikes, which exhibit great variation in texture, mineralogy and geochemistry. This granite is moderately to strongly peraluminous with the following mineralogy: plagioclase (An 0–An 21) + potassium feldspar (Or 70–96) + quartz + muscovite ± apatite ± biotite ± garnet ± tourmaline. The granitic intrusions in the interior of the complex have similar K Rb ratios (> 190), whereas this ratio decreases and is more variable for intrusions which are structurally higher or along the perimeter of the complex. Substitutions of (Fe, Mn)Mg −1 in the ferromagnesian minerals, NaCa −1 in plagioclase and RbK −1 in muscovite and potassium feldspar increase in the perimeter granites and vary systematically with K Rb . These more evolved intrusions are commonly enriched in incompatible elements such as Nb, Li, Cs, Be, and B and depleted in Ba, Ca, and Sr relative to the interior, primitive granites. Biotite-bearing assemblages are common in the interior granites but are replaced by tourmaline-bearing granites in the more evolved intrusions. A series of discontinuous reactions may explain this assemblage transition. Observations and trace element modeling suggest that: (1) within individual units volatile transfer mechanisms have resulted in mineral and chemical segregation; (2) 75–80% fractional crystallization of a primitive biotite-muscovite granite was the dominant mechanism in producing the more evolved tourmaline-bearing granites; and (3) extreme fractional crystallization aided by high volatile activity produced the associated rare-element pegmatites.