Abstract

A pressure-temperature-time path determined from zoned garnet and associated minerals in a metapelite from a dome in the western part of the Black Hills, South Dakota, Precambrian core shows two clockwise loops that link the previously postulated thermal and tectonic events affecting Early Proterozoic age rocks between ca. 1.850 and 1.715 Ga. The shape of the loops indicates that two distinctly different styles of tectonic processes occurred during this time period. The first loop established from the garnet core is correlated with crustal thickening by thrusting and nappe development followed by a period of erosional uplift. The second loop, from the garnet rim, resulted from homogeneous thickening represented by north-northwest-trending isoclinal folds. The peak conditions of this event are interpreted to be associated with anatexis, doming, and intrusion of Harney Peak S-type granite (1.715 Ga).

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