Element partitioning and thermal-compositional patterns in coexisting muscovites and biotites, Black Hills, SD

Twenty-two elements in 50 coexisting muscovites and biotites from pelitic schists of the southern Black Hills have been analyzed by ICP spectrometry. Most elements exhibit highly systematic partitioning between the micas over a wide range of composition. Sillimanite-zone micas are typically enriched in K, Ti, and Li but depleted in Na relative to those from the staurolite zone. Also, sillimanite-zone biotites are relatively depleted in Mg/(Fe+Mg), but enriched in Mn, relative to those of the staurolite zone. Preliminary verification of the muscovite-biotite thermometer (calibration of Hoish (1989)) is indicated from a regional thermal pattern that is strikingly similar to that obtained by Friberg et al. (in prep.) using garnet (rim) - biotite thermometry. The lowest K/(Na+K)[sup mus], X[sub Li] (bio), mica temperatures, and highest X[sub Mg](bio) in the region are observed in staurolite-zone schists between the Grand Junction and Hill City Faults. These regional patterns correlate with: (1) mapped gravity lows (subsurface granitic plutons ), (2) mapped pegmatite density, and (3) proximity to the main mass of Harney Peak Granite. As such, pelitic micas appear to record the T-X signatures of vertically( )-flowing alkali metasomatic fluids that variably overprinted low- to medium-grade regional-metamorphic assemblages during emplacement of the Harney Peakmore » granite-pegmatite complex. The area between the Grand Junction and Hill City faults appears least affected by the metasomatic activity associated with this magmatic event.« less