SEQUENTIAL GROWTH OF A PROTO-CONTINENT BY ARC MAGMATISM: THE PALEO- TO MESOARCHEAN RECORD OF CRUSTAL EVOLUTION IN THE NORTHERN WYOMING PROVINCE, USA

Felsic (meta)plutonic rocks, developed in a 2.8 Ga Mesoarchean arc, are found in the Beartooth-Bighorn magmatic zone of the northern Wyoming Province. In the eastern Beartooth Mountains of MT-WY, these rocks contain fragments of crust predating the Mesoarchean arc plutonism that provide insights into the nature of earlier continental crust. Fragments of older crust can be km-sized enclaves, comprised of continuous-to-tectonically superimposed lithologic associations, to cm-sized xenoliths. Preserved lithologies include tonalitic-trondhjemitic-granodioritic gneiss (protoliths as old as 3.5 Ga), quartzite (detrital zircons 3.2-4.0 Ga), aluminous gneiss and migmatite, amphibolite, mafic granulite, meta-ultramafic and rare meta-ironstone. Many lithologies contain relict mineral assemblages indicative of an early (M1) granulite-facies metamorphism (750-800C; 5-7 kbar) with a later amphibolite-facies overprint (M2) and a subsequent local granulite-facies development associated with dehydration proximal to cross-cutting 2.8 Ga trondhjemitic veins. Tectonic superposition developed at high temperature is indicated by thoroughly recrystallized blastomylonites and high-temperature thermometry (>750C) recorded in reaction-zones between superimposed lithologies. U-Pb zircon geochronology shows that most of the pre-2.8 Ga gneisses are 3.1-3.3 Ga, similar in age to the maximal concentration of ages in detrital zircon suites. Sm-Nd systematics of the gneisses confirm the oldest ages indicated by the detrital zircons (e.g., single stage Tdm of 4.0-4.1 Ga). The interval from 3.1-3.3 Ga was a major crust-forming interval throughout the northern Wyoming Province and is common worldwide based on detrital zircon studies. In the northern Wyoming Province whole-rock geochemistry demonstrates that the magmatic rocks have varied origins, including both a depleted mantle source and ancient lower crust. A majority of the gneisses were derived through melting of hydrous sources in presence of garnet (Lu(n) ~1), have calc-alkaline affinities, and have elemental patterns (HFSE depleted) consistent with formation in a continental volcanic arc. Growth and evolution of the northern Wyoming Province appears to have occurred through arc-related crust-forming events at 3.3-3.1 Ga and again at 2.8 Ga.