Abstract

The Late Archean to Early Proterozoic terrain exposed in the Black Hills, 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 part of the terrain, to sillimanite + K-feldspar grade in the southern part. 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 metamorphism occurred 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 and isotherms indicate that the Harney Peak granite was at the locus of a regional thermal high and that doming during granite emplacement was partly responsible for exposure of the low-pressure, high-temperature metamorphic terrain.

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