MESOARCHEAN ROCKS OF THE EASTERN BEARTOOTH MOUNTAINS, MT AND WY

Meso- to Paleoarchean rocks occur as km-scale pendants in the Eastern Beartooth Mountains in 4 locations: Hellroaring Plateau, Quad Creek, Line Creek-Wyoming Creek, and Christmas Lake areas. Originally mapped by Poldervaart and his students in the 1950’s and ‘60’s, in aggregate these rocks provide the earliest insights into crustal genesis and evolution in the Wyoming Province. The dominant lithologies are migmatitic quartzofeldspathic gneisses ranging in composition from tonalite to granite. Significantly, there are also extensive suites of: 1) high-grade metasupracrustal rocks that include peraluminous gneisses (gar-sill-cord-kfs), 2) quartzites, 3) banded iron formations (gar-opx-cpx-qtz-mt), 4) metabasites (hbl-plag+/-cpx+/-opx), and 5) minor ultramafic rocks (hbl-opx). These rocks are now tectonically mixed, and occur as meter-scale macro-lithons separated by high-grade ductile shear zones. The metamorphic history, determined from numerous independent mineral geothermobarometers and mineral assemblages, reveals initial peak metamorphic conditions in the granulite facies (M1) of 0.6-0.8 GPa and 750-800^oC; followed by decompression and cooling into the amphibolite facies (M2); followed by a second late heating of the system to 750-800^oC, interpreted to be coeval with the emplacement of the 2.8 Ga Long Lake Magmatic Complex (LLMC); and final retrogression and cooling through the greenschist facies by about 2.75 Ga. U-Pb zircon geochronology of the quartzofeldpathic gneisses yields ages that range from 3.5 to 3.1 Ga with a dominant crust-forming event recorded at 3.3-3.2 Ga. Whole rock and trace element data demonstrate that these rocks have geochemical characteristics similar to modern day continental arc magmas (e.g., LREE enrichment, HFSE depletion). These rocks are significant because they document the occurrence of thick, evolved continental crust in the northern Wyoming Province in the time interval of 3.5-3.2 Ga. This is a major crust-forming event that is also evident in detrital zircons analyzed from quartzites from across the northern Wyoming Province. This suite of rocks represent part of the crustal infrastructure to the Mesoarchean (2.8-2.9 Ga) arc that created the LLMC.