Metamorphism of early Proterozoic schists of the southern Black Hills, South Dakota

The Late Archean to Early Proterozoic terrain exposed in the Black Hills of South Dakota, records a history of deformation, metamorphism, and igneous activity extending from ~2.5 to 1.7 Ga. Metamorphism began during crustal thickening; however, maximum temperatures were attained after crustal thickening during a regional low-pressure, high-temperature event coinciding with the emplacement of the Harney Peak granite at 1.7 Ga. Metamorphic grade increased over a distance of 10 to 15 km from garnet grade in the northern portion of the terrain, to sillimanite + K-feldspar grade in the southern portion. Isograds mapped in the terrain include the first appearance of staurolite + biotite, andalusite + biotite, sillimanite, sillimanite + biotite + garnet, and sillimanite + K-feldspar. In the extreme western exposure of the terrain, kyanite + biotite occurs instead of andalusite + biotite. Geobarometry indicates that metamorphic pressures were between 2.0 and 4.4 kbar. Geothermometry indicates that temperature increased from 469-500 °C in the garnet zone to 528-555 °C in the sillimanite zone on the northern flank of the Harney Peak granite. Higher temperatures are recorded along the southern flank of the granite in the sillimanite + biotite + garnet zone (585 °C) and in the sillimanite + K-feldspar zone (≤ 662 °C). The distributions of isograds, isotherms, and major fold axes indicate that the Harney Peak granite was at the locus of a regional thermal high and that doming during granite emplacement locally deformed preexisting isograds and fold axes. Pressure-temperature paths based on compositionally zoned garnets from garnet-grade and staurolite-grade rocks indicate that prograde metamorphism from 400 °C to 500 °C occurred under almost isobaric conditions (± 0.5 bars). PT paths based on compositionally zoned garnets also have a clockwise orientation. Because of volume diffusion and retrograde decomposition, prograde PT paths could not be determined from sillimanite and sillimanite + K-feldspar zone garnets; however, reaction textures in these rocks strongly suggest clockwise decompression and cooling paths. Homogenization of garnets at a temperature of ~565 °C indicates heating and cooling occurred at a minimum of -4x10⁷ years. Low-pressure, high-temperature metamorphism in the Black Hills followed crustal thickening associated with continental collision and the suturing of the Archean aged Superior and Wyoming provinces. One- and two-dimensional numerical models were constructed to examine the thermal consequences of rapid homogeneous crustal thickening and slow uplift. The uplift rate (0.014 mm/year) was estimated from the amount of uplift determined from geobarometry (14 to 16 km) and the time interval between peak metamorphism at 1.7 Ga and the age of the Cambrian (~0.6 Ga) lithologies unconformably overlying the Proterozoic crystalline complex. This value requires that uplift was constant and continuous. At an uplift rate of 0.014 mm/year, low-pressure, high-temperature metamorphism occurs with a moderate mantle heat flux (33 mW/m²) and a measured crustal radiogenic heat production rate of 3.3 μW/m³. The mantle heat flux is consistent with present-day reduced heat flux measurements and the rate of radiogenic heat production is estimated from the concentration of U, Th, and K in major lithologic units from the Black Hills. Estimated PTt paths are consistent with nearly isobaric heating, clockwise decompression, and with a thermal perturbation that lasts on the order of 10⁷ years.