Petrogenetic links among granites and pegmatites in the Harney Peak rare-element granite-pegmatite system, Black Hills, South Dakota

Linkages between the fertile granite and associated pegmatites, linkages among pegmatites, and processes instrumental in defining their textural, chemical, and mineralogical uniqueness are obscured by their coarse grain-size and the transitional nature between magmatic and hydrothermal regimes. Studies of the Harney Peak granite-pegmatite system indicate that these systems are a culmination of distinctive processes of partial melting and fractional crystallization. Both the Harney Peak Granite and the associated pegmatite field are mineralogically and chemically zoned. Superimposed on the general zoning in the pegmatite field are swarms of pegmatites that appear to define distinct compositional-textural arrays. These may be related to distinct fractional crystallization trajectories. In addition to fractional crystallization, partial melting appears to be important in controlling the potential content and composition of the volatile component, and the incompatible element character of the distinct magma-types. -from Authors