Geochronological and isotopic constraints on both the timing of formation and deformation of rocks in the Cheyenne belt support the idea that this region represents a major structural boundary between Archean and Proterozoic terrains. Initial investigations narrow the timing of formation of basement lithologies both north and south of the Cheyenne belt. A compilation of this data, as shown in table 1, led Chamberlain to propose the term “Medicine Bow Orogeny” to represent the ca. 1.78-1.74 Ga arc-continent collision that formed the Cheyenne belt. Geochronological constraints on the evolution of this region include: U-Pb zircon ages of gabbro dikes that put rifting at 2.1-2.0 Ga, early deformation of the Cheyenne belt at 1.78 Ga based on metamorphic zircon from the Richeau Hills, U-Pb apatite ages and K-Ar hornblende ages that suggest uplift during Cheyenne belt deformation, and reset Rb-Sr and K-Ar biotite ages from Archean rocks that record the extent of tectonic burial of the craton during orogeny.Pb isotopic compositions from the Horse Creek anorthosite represent a mixing line between the mantle and Archean rocks. Nd isotopic data from the Medicine Bow mountains have high Epsilon-Nd values nearest the belt and older values farther from the belt. This requires that older crustal material was incorporated in the igneous rocks and that the proportion of older material decreases with distance from the Cheyenne belt.
