Step-leach Pb-Pb dating of inclusion-bearing garnet and staurolite, with implications for early Proterozoic tectonism in the Black Hills collisional orogen, South Dakota, United States

Dahl, P.S.; Frei, R.

Geology 26(2): 111-114


ISSN/ISBN: 0091-7613
DOI: 10.1130/0091-7613(1998)026<0111:slppdo>;2
Accession: 018422064

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We report (super 207) Pb/ (super 206) Pb stepwise-leaching (PbSL) ages of 1762+ or -15 Ma, 1759+ or -8 Ma, and 1760+ or -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 (super 208) Pb/ (super 206) Pb (Th/U) trends reveal that staurolite contains inclusions of both cogenetic monazite (ca. 1760+ or -7 Ma) and older detrital zircon (> or =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 approximately 400 degrees C and approximately 550 degrees C (i.e., well below Pb closure temperatures). We interpret the 1760+ or -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.