ARCHEAN CRUSTAL EVOLUTION IN THE NORTHERN WYOMING PROVINCE: IMPLICATIONS FOR CRUST-MANTLE EVOLUTION

The ancient rocks of the Beartooth Mountains of the Wyoming province have inspired numerous geologists, including J. Tuzo Wilson, Arie Poldervaart, Don Bowes, and Hal James. Joe Wooden's contributions to the understanding of these ancient rocks have been critical, ranging from being the first person to apply common Pb isotopic systematics to these rocks, to his initial modeling that clearly demonstrated the antiquity of these rocks, to the naming of a unique unit of measure – the joebag. These studies focused on understanding the geodynamic environments that led to the formation of the earliest continental crust and the implications of early crustal segregation for the geochemical evolution of the crust-mantle system. In particular, these early trace elemental and isotopic studies suggested modern-style subduction may have played a critical role in the development of Archean, high-grade gneiss terranes. In the Beartooth Mountains of the northern Wyoming Province crustal growth has been documented in distinct episodes at ca. 3.5 Ga, 3.15 Ga, and 2.82 Ga and some rocks record all three events in the U-Pb systematics of their zircons. In aggregate, these rocks define a more or less equi-dimensional segment of crust, which contrasts with many Proterozoic orogenic belts and crustal age provinces have physical dimensions compatible with orogenic belts formed in response to modern plate tectonic processes. The Wyoming province and many Archean continental nuclei are more equi-dimensional and it remains an open question whether this is a primary or secondary feature. The TTG to granitic magmas emplaced in these event exhibit several geochemically distinct features, most notably an elevated common Pb isotopic compositions in ancient gneisses followed by inversion of U/Pb ratios during late Archean magmagenesis. This pattern suggests less recycling of pre-existing crust in the younger (2.8 Ga), more voluminous rocks that are apparently associated with the establishment of the Province's mantle keel. These characteristics are compatible with the proposed plate tectonic scenario and suggest that the cratonization of the Province and formation of the mantle keel may be special aspects of Archean plate tectonics.