Petrogenetic relationships between pegmatite and granite based on geochemistry of muscovite in pegmatite wall zones, Black Hills, South Dakota, USA

Abstract

The compositions of large samples of granitic pegmatite wall zones have been determined for a suite of ten pegmatites of diverse geochemical character and degree of compositional evolution in the Keystone area of the Black Hills. Whole-rock compositions are strongly peraluminous, and they deviate substantially from the granite minimum composition in quartz-albite-orthoclase normalized components, showing considerably more scatter than Harney Peak Granite whole rocks. Wall-zone minerals are commonly coarsely segregated, leading to large modal variability among whole rocks. These features make whole-rock samples of wall zones unsuitable for the determination of initial pegmatite bulk compositions. Trace and minor element compositions of muscovite separates from the wall zones were thus determined to eliminate the effects of modal variability on trace element concentrations so that geochemical differences between pegmatites could be modeled. Estimates of initial pegmatite melt trace element concentrations range from 800–4000 ppm Rb, 100–1000 ppm Cs, 200–2000 ppm Li, and 1–50 ppm Ba. Trace element concentrations of muscovite from a given pegmatite generally cluster together, although several show considerable intra-pegmatite scatter, and there are large overlaps among different pegmatites. The geochemical characteristics of samples from the Etta pegmatite indicate mixing with and assimilation of country rocks. Exceptionally low Rb Cs ratios of muscovite from the Etta pegmatite are similar to those of muscovite from K-feldspar-rich assemblages of other pegmatites where the Rb concentration of melt may have been buffered by crystallizing assemblages that had bulk Rb distribution coefficients close to 1. The large degree of scatter of geochemical parameters among these pegmatites precludes derivation of the entire suite by either single-stage crystallization or partial melting processes, nor can the pegmatites be satisfactorily related by any fractionation trajectory from a single starting composition. Fractional crystallization is difficult to model due to an apparent geochemical decoupling that exists among several of these elements in different pegmatites, e.g., Rb from Cs, or Li from Rb/Cs, and nonsystematic variation of Ba concentrations. However, the magnitude of observed rare-element enrichments could be achieved by 75–90% fractional crystallization of melts that crystallized Harney Peak Granite in nearby exposures. The inter-pegmatite scatter and apparent geochemical decoupling that characterize the suite indicate an origin by partial melting of a heterogeneous source rock or rock sequence, followed by an intermediate episode of fractional crystallization, prior to final emplacement of the pegmatite melts. Additional compositional variations among samples from the wall zones of individual pegmatites were caused by processes during or after emplacement, including local interaction with host metamorphic rocks and fractionation during internal evolution of the pegmatites as they crystallized.