FLUID-MEDIATED MINERAL CONSUMPTION AND GROWTH IN METAPELITES OF THE BLACK HILLS, SOUTH DAKOTA

Late Archean and Early Proterozoic continental margin pelites, graywackes, and quartzites were regionally metamorphosed during collision of the Archean Wyoming and Superior Provices beginning ~1760 Ma (Dahl et al., 2002). The metamorphism reached at least garnet-biotite grade conditions. This assemblage occurs in much of the middle portion of the Precambrian core of the Black Hills. It is characterized by inclusion-rich unzoned garnets that are Sps₄₄Alm₄₄Pyr₃Grs₉. Mn-ilmenite (up to 47% pyrophanite component) also occurs. The broad distribution of the grt-bt assemblage is attributed to the stabilizing effect of Mn and low aH₂O due to the presence of CH₄ and CO₂.

During late stages of the collision, the rocks were intruded by the Harney Peak leucogranite (HPG) at 1720-1705 Ma. Vigorous fluid flow, evidenced by abundant quartz veins, metasomatism, and consumption of graphite from metapelites, occurred in the granite aureole. The lowest-grade aureole assemblage includes chlorite that overgrows regional foliaton and new, clear, Sps-poor, Alm-rich rims on the garnet. Higher-grade facies include staurolite, then sillimanite, and finally second-sillimanite. Mineral compositions suggest 4-4.5 kbar pressure. Within the sillimanite zone, garnet is mostly inclusion free and ~1720 Ma old. Textures suggest that this new garnet grew after dissolution of the old garnet, for which the only remaining evidence may be remnant inclusions or quartz-biotite clots in which biotite is coarser then in the matrix. Andalusite typically occurs as euhedral crystals in quartz veins or as poikiloblasts along foliations planes, where it appears to have grown as Si-rich fluids passed through. Most andalusite probably grew during decompression of the fault-bounded HPG block.