Abstract

We report 207Pb/206Pb stepwise-leaching (PbSL) ages of 1762 ± 15 Ma, 1759 ± 8 Ma, and 1760 ± 7 Ma for almandine garnet, staurolite, and garnet-staurolite (combined) in an Early Proterozoic metapelite from the Black Hills collisional orogen, South Dakota. PbSL-derived 208Pb/206Pb (Th/U) trends reveal that staurolite contains inclusions of both cogenetic monazite (ca. 1760 ± 7 Ma) and older detrital zircon (≥2040 Ma), whereas coexisting garnet contains only the zircon. These results and petrologic data indicate prograde sequential growth of garnet, monazite, and staurolite during tectonic burial at temperatures between ∼400 °C and ∼550 °C (i.e., well below Pb closure temperatures). We interpret the 1760 ± 7 Ma date as a maximum age for Wyoming-Superior continental collision in the Black Hills, which apparently postdated the 1800–1900 Ma Trans-Hudson orogeny in Canada, but was nearly synchronous with north-directed accretion of the Central Plains orogen in southeastern Wyoming.

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