Unraveling the fluid-present metamorphism of schists from garnet compositions in the Black Hills, South Dakota

Garnets in Proterozoic metapelites in the Black Hills, South Dakota, were analyzed to determine their pressure, temperature and hydrothermal conditions of growth and to elucidate pressure-temperature-time (P-T-t) paths of their host rocks. The metapelites are the product of garnet-grade regional metamorphism beginning at ∼1755 Ma due to the collision of Wyoming and Superior cratons and subsequent contact metamorphism by intrusion of the Harney Peak Granite (HPG) at ∼1715 Ma. Garnet occurs in the garnet zone, and contact-metamorphic staurolite and sillimanite zones. X-ray element maps and compositional profiles across garnets were determined using electron-probe microanalysis (EPMA). Mineral assemblage diagrams (pseudosections) with garnet composition contours were constructed using the THERIAK-DOMINO software. The diagrams provide a foundation for determining the conditions of garnet growth and P-T-t paths for their host rocks. In the garnet zone, garnets have inclusion-rich centers and almost inclusion-free rims. The garnets display low-Ca cores (Xgrs = 0.08-0.1), followed by slightly more elevated Ca annuli (Xgrs ∼ 0.1) and low-Ca rims (Xgrs5 kbar to ∼4.2 kbar. The geochemical characteristics of the garnets and the derived P-T-t paths were used to determine the cause of zoning and the dynamo-thermal metamorphic history of the Proterozoic core of the Black Hills. Calculations show that internal chemical diffusion would have been too slow for modification of garnet chemistry after crystallization. Thus, the chemical zoning must reflect conditions during garnet growth. In the garnet and staurolite zones, garnets initially grew in a low a(H2O) environment and the temperature of initial growth may have overstepped the equilibrium temperature. In the sillimanite zone, low-grade garnet partially or fully recrystallized in a higher a(H 2O) environment and compositions suggest equilibrium growth commensurate with sillimanite-grade temperatures. Garnet compositions suggest that the early regional metamorphism occurred at relatively low pressures (∼3 kbar), whereas the region in close proximity to the HPG had a high-P history (>6 kbar) that is supported by an occurrence of kyanite in one sillimanite-zone sample