ISLAND IN THE STREAM: PRE-LAURENTIAN WANDERING OF THE WYOMING CRATON

The amalgamation of Laurentia in the Paleoproterozoic largely occurred as Archean cratonic units coalesced across Paleoproterozoic mobile belts (1.8-1.9 Ga). Many of these Paleoproterozoic belts began as accretionary orogens and culminated as collisional orogens. Classic examples include the Trans-Hudson, Penokean, and Great Falls orogens; other Paleoproterozoic, craton-bounding structures developed primarily as transpressional systems with minor subduction-driven magmatism (e.g., Cheyenne Belt). Between the end of the Archean and their late Paleoproterozoic juxtapositions, the history of these Archean cratonic units is not well known. Several proposals for pre-Laurentian supercontinents (s.l.) comprised of Archean cratonic units have been made (e.g., Kenorland, Superia, Itsaqia, Ur, etc.). If one or more of these supercontinents existed, then each modern (surviving) Archean craton should contain some evidence of pre-1.9 Ga rifting and/or accretionary orogenesis. In the case of the Wyoming craton (WC), recent studies have shown the presence of 2.45-2.55 Ga tectonic activity, including anatexis, on all sides of the craton as currently defined. These include: 1) 2.45 Ga metamorphism and crustal melting along the southwestern margin (e.g., Uinta and Wasatch Ranges of Utah); 2) 2.4-2.5 Ga high-T metamorphism and anatexis of older gneisses in the Montana metasedimentary terrane of southwestern Montana (e.g., Ruby, Tendoy, and Gallatin ranges); 3) 2.4-2.5 Ga crustal xenoliths recovered from Eocene ultramafic rocks of the Montana alkali province, which formed within the 1.7-1.9 Ga Great Falls tectonic zone (e.g., Highwood and Little Rocky Mountains); and 4) mafic magmatism in the Black Hills (e.g., Blue Draw gabbro). If these occurrences mark the breakout of the WC from a larger mass of continental crust, then the distinctive U-Pb systematics of the Archean crust of the WC should provide a clue as to which, if any, modern cratonic units comprised an earlier “supercontinent” that contained the WC. At present, the mostly likely partner for the WC in Laurentia is the Slave Province based on its enriched U-Pb systematics, evidence for old crust (3.5-4.0 Ga), and extensive, Mesoarchean, arc-related magmatism. Further afield, gneisses of the Limpopo belt also share many of these characteristics.