Abstract

A question regarding the 1900–1600 Ma assembly of Laurentia is whether the Wyoming province (west-central United States) was part of the Hearne province (west-central Canada) prior to the Early Proterozoic Trans-Hudson orogeny, or a separate entity welded later to a Hearne-Superior continent (central Canada). New 40Ar/39Ar mica dates help to address this question by extending a Middle Proterozoic geochronologic front, long established along the southern Wyoming province, into the Black Hills of South Dakota. This suggests that previously unexplained, north-directed fold nappes (F1) in the Black Hills resulted from island-arc accretion to the south ca. 1780 Ma. North-northwest–trending upright F2 folds, which formed during east-west collision of the Wyoming and Superior provinces, must therefore be younger. New 40Ar/39Ar hornblende dates, recently published age data, and crustal heat-flow considerations further suggest that this collision began at or before ca. 1770 Ma and culminated with posttectonic magmatism beginning ca. 1715 Ma (the Harney Peak granite). This tectonic-magmatic interval is ∼50–60 m.y. younger than that reported for the Hearne-Superior collision (Trans-Hudson orogeny in Canada). Comparably young metamorphic dates (1810–1710 Ma) also typify the eastern and northern Wyoming province periphery (western Dakotas and southwestern Montana).

Collectively, these data suggest that the Hearne and Wyoming provinces were once separate continents that were ultimately welded to the Superior province (and to each other) during distinct Early Proterozoic orogenies. Regional relationships further suggest that final docking of the eastern Wyoming province with Laurentia began during the ca. 1780–1740 Ma interval of island-arc accretion along the southern margin of the growing craton.

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