Abstract

The Little Elk and the Bear Mountain terranes on the margin of the Precambrian core of the Black Hills provide the only known exposures of Archean rocks between the Archean Wyoming and Superior provinces. The Little Elk terrane consists of a supracrustal sequence dominated by biotite-feldspar gneiss (meta-arkose or metatuff) that is intruded by the slightly metaluminous to peraluminous, calc-alkaline Little Elk Granite, which has a U-Pb intercept age of 2,549 ± 11 m.y. A biotite-plagioclase schist (metagraywacke) dominates the Bear Mountain terrane and is intruded subparallel to foliation by the peraluminous granite at Bear Mountain. An isochron calculated from six severely discordant U-Pb zircon analyses yields an age of 2,393 ± 230 m.y. The Archean terranes participated in the Trans-Hudson orogenic event, suggested by a published Rb-Sr age for the Little Elk Granite (∼1,850 m.y.), 1.87-1.97 Gaigneous activity in the Black Hills, and the similarity in orientation of the Proterozoic deformational fabric with geophysical anomalies interpreted to be associated with the Trans-Hudson orogen. The metamorphic conditions estimated for the Archean rocks may be the result of a thermal event that produced the Proterozoic Harney Peak Granite and thermal high northeast of Lead, South Dakota. The unexposed Precambrian basement is probably dominated by granites similar to the Little Elk Granite and supracrustal rocks consisting of continental margin deposits, based on the REE contents of the Archean schists and on the heavy-mineral assemblages in, the uraniferous nature of, and the metasedimentary clasts in, the lower Proterozoic Box Elder Creek Formation. The ages and the relatively long-lived crustal sources for these Archean granites, and the similarity of the inferred basement characteristics to those of rocks in the Wyoming province, suggest that the Black Hills Archean rocks may be the easternmost exposures of this province. The probable Trans-Hudsonian reworking of the Black Hills Proterozoic rocks and their similarity to those in northern Saskatchewan imply that the Black Hills occupies a tectonic position comparable to that of rocks described from the Cree Lake Zone of Saskatchewan.

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