PRESERVED 2.8 GA ARC SECTION IN THE NORTHERN WYOMING PROVINCE: IMPLICATIONS FOR PRECAMBRIAN PLATE TECTONICS

Archean rocks of the northern Wyoming Province exposed in the Beartooth Mountains (MT/WY) provide insights into processes relevant to early Earth tectonics. A W-E transect across the southern Beartooth Mountains includes greenschist to amphibolite facies metasedimentary rocks of the South Snowy Block (SSB) to the west, but upper amphibolite to granulite facies metasupracrustal rocks in the east. Both areas are intruded by 2.8 Ga tonalite-trondhjemite-granodiorite (TTG) plutonic rocks. The SSB is interpreted as a zone of tectonic mixing of allochthonous units prior to 2.8 Ga with the last Archean tectonothermal event at ~2.55 Ga. Within the eastern portion of the SSB and Beartooth Plateau Block, the 2.8 Ga plutonic and metamorphic rocks represent a continuum of crustal levels (~10-25 km) representing a Mesoarchean continental arc section. 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 TTG magmatic rocks that volumetrically dominate over a distance of ~110 km. Magmatic rocks in the westernmost area are dominated by undeformed 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 of 2.9-3.6 Ga. Protolithic sediments exhibit similarities akin to those found in active continental margin settings. In contrast, in the eastern region 2.8 Ga magmatism of the Long Lake magmatic complex includes meta-dioritic rocks to metaluminous TTG suite rocks. In the east each igneous rock type is essentially coeval with magmatic units mixed in a mid-crustal ductile environment. In the easternmost area there are numerous pendants in the plutons including aluminous migmatites with peak metamorphic conditions of 750-800°C and 7-8 kbar. The protolithic sediments exhibit lithologic and compositional similarities akin to those of passive margin settings. Detrital zircon in the metaclastic rocks in the east have abundant 3.2-3.3 Ga (and older) detrital zircons, but have a hiatus of 2.9-3.1 Ga. The plutonic and metamorphic rocks are interpreted as belonging to a section of 2.8 Ga continental crust developed in response to subduction-zone processes.