Petrogenesis and tectonic context of the Harney Peak Granite, Black Hills, South Dakota

Abstract

Intrusion and crystallization of the Harney Peak Granite and associated plutons and pegmatites were the culminating events of the Trans-Hudson orogeny as expressed in the Black Hills. The granite was emplaced as thousands of sills and dikes at 1715 Ma, following an approximately 45 m.y. period of regional metamorphism and deformation of now exposed Proterozoic metasedimentary rocks. Isotopic ratios of neodymium and lead indicate that parts of the granite were derived from source rocks with Archean model TDM extraction-ages, whereas other parts were derived from sources with only 100–300 m.y. crustal residence times. However, oxygen isotope ratios and trace element concentrations suggest that both sources were metapelites or metagraywackes analogous to the country rocks. Boron and TiO2 concentrations suggest that the granites were generated either by muscovite or muscovite + biotite dehydration-melting reactions. Published thermobarometric and argon cooling-ages show that the country rocks cooled and decompressed from conditions at which garnet and staurolite grew at ∼7 kbar to less than 500°C and pressures of 3.5–4 kbar at the time of granite emplacement. We conducted a numerical simulation of metamorphism and generation of the Harney Peak Granite that is constrained by available data. The model involves shear-heating along a thrust to produce temperatures sufficiently high for melting of thrusted sedimentary rocks at relatively shallow levels in a thickened crust that is undergoing unroofing. The model successfully reproduces the metamorphic and magmatic events that occurred in the Black Hills segment of the Trans-Hudson orogen.