Geologic History of the Precambrian of the Central Laramie Range, Wyoming

Abstract The oldest rock type exposed in the central Laramie Range is an intermediate gneiss, probably equivalent to gneiss dated elsewhere at 2700 m.y. and older. This gneiss was intruded at deep crustal levels by granite at probably 2550 m.y. ago. At 2490 m.y. Rb-Sr systems in both the granite and gneiss were reset (Hills and Armstrong, 1974). The Slate Creek metamorphic terrane, a sequence of amphibolite and metapelite with thin quartzite layers, overlies the Archean granite-gneiss basement. The contact between the two is everywhere tectonic. During metamorphism of the Slate Creek terrane (which reached 5.5 kb and 640 to 660°C), basement material rose diapirically upward through the overlying metamorphic rocks to produce gneiss domes and the current distribution of folds in the area. Near 1510 m.y. ago, both basement and the Slate Creek terrane were intruded by syenite. Metamorphism of the Slate Creek terrane occurred after 2550 m.y., but on the basis of available data, it cannot be firmly established whether precursors to the metasediments were deposited before or after granite emplacement. However, based on field relations and compositional dissimilarites to basement geniss, it is suggested that precursors to the metasediments were deposited unconformably on the basement, and hence are younger than 2550 m.y.