REACTION OVERSTEPPING IN GARNET SCHISTS AND P-T-T PATHS IN THE PROTEROZOIC BLACK HILLS OROGEN, SOUTH DAKOTA

Garnet in Proterozoic metapelites in the Black Hills, South Dakota, was analyzed to determine its pressure, temperature and hydrothermal conditions of its growth and to unravel the P-T-t paths of its host rocks during the Paleoproterozoic Black Hills orogeny. The garnet is the product of regional metamorphism beginning at c. 1755 Ma due to the collision of Wyoming and Superior cratons and subsequent contact metamorphism by intrusion of the Harney Peak Granite (HPG) at c. 1715 Ma. Garnet compositions indicate disequilibrium with rock matrix during its initial growth.

Garnet in the incipient garnet zone contains petrographically distinct inclusion-rich cores. In the upper garnet and staurolite zones, the cores were overgrown by inclusion-poor mantles. Centers of cores have distinctly elevated Y concentrations that mark the nucleation sites of garnet crystals. Mantles have more elevated Fe and Mg and lower Mn and Ca. A pseudosection, contoured with garnet end-member isopleths, was constructed for a(H₂O) = 0.5 to reflect the prevailing low H₂O activity in the graphitic metapelites during growth of the cores. Compositions of the cores indicate temperature overstepping of the garnet-in reaction by 50-100 °C and an apparent near-isothermal pressure increase from about 1.5 to 3 kbar during growth of the cores. The low end of the pressure range would imply an unreasonably high initial geothermal gradient. The reaction overstepping is attributed to slow intergranular transport of components needed for garnet nucleation in the low a(H₂O) environment while the apparent pressure range is attributed to lack of equilibrium between growing garnet and reactant plagioclase or calcite. Mantles show more elevated temperatures by ≥30 °C. Growth of the mantles occurred during heating by leucogranite magmas in the presence of aqueous fluids.

In the sillimanite zone next to the HPG, garnet is inclusion-poor and essentially unzoned. This garnet is attributed to recrystallization of earlier inclusion-rich garnet under high a(H₂O) conditions that prevailed near the granite. A pressure of >5 kbar in the HPG’s vicinity is indicated by composition of the garnet and an occurrence of kyanite. Resorbed edges on the garnet show that this part of the terrain has undergone decompression while intrusion of the HPG melt was occurring.