UPPER-TO-MIDDLE LEVEL EXPOSURE OF A 2.8 GA CONTINENT IN THE NORTHERN WYOMING PROVINCE, USA

The 2.8 Ga plutonic and metamorphic rocks in the northern portion of the Wyoming Province in Montana and Wyoming (USA) represent a continuum of crustal levels (~10-25 km) within a Mesoarchean continent. Although these rocks contain a diverse record of crustal evolution extending before 2.8 Ga (as early as ~4.0 Ga), it is the 2.8 Ga magmatic rocks (tonalite-trondhjemite-granodiorite, i.e., TTG suite) that volumetrically dominate the region from the South Snowy Block (partially in Yellowstone National Park) to the Main Beartooth Block (~110 km distance). Magmatic rocks in the westernmost area are dominated by undeformed bulbous, peraluminous, epizonal, quartz-monzonitic plutons that cut metasedimentary rocks with low-P assemblages, e.g., andalusite-staurolite (~580°C, ~3.5 kbar). The rocks cut by the plutons include metamorphosed turbiditic rocks with detrital zircon populations ranging from 2.9-3.6 with a significant population at 3.0-3.1 Ga. The protolithic sediments exhibit lithologic and compositional similarities akin to active continental margin settings. In contrast, in the eastern part of the area 2.8 Ga magmatism in the Long Lake magmatic complex includes a diverse series of meta-dioritic rocks to metaluminous TTG suite rocks. Individual igneous rock types do not exhibit consistent sequential field, geochemical or geochronologic relationships, i.e., they are essentially coeval, but independent, magmatic units that were mixed in a ductile environment. In the easternmost area there are numerous enclaves/pendants including aluminous migmatites with common metamorphic assemblages of qtz + pl + K-fsp + bt + sil ± grt ± crd with peak metamorphic conditions of 750-800°C and 7-8 kbar. The protolithic sediments exhibit lithologic and compositional similarities akin to passive margin settings. The detrital zircons in the metaclastic rocks in the east do not contain the younger 2.9-3.1 Ga, but have abundant 3.2-3.3 Ga (and older) detrital zircons. Between the west-east PT extremes, the transition is not continuous – there is a shear zone along the Yellowstone River that exhibits a P change of 3 kbar. These plutonic and metamorphic rocks are interpreted as belonging to a section of 2.8 Ga continental crust developed in response to subduction-zone processes and ultimately exposed at varying depths.