Archean Rocks of the Eastern Beartooth Mountains

The eastern Beartooth Mountains contain pendants of Paleo-Mesoarchean high-grade gneiss intruded by voluminous Neoarchean calc-alkaline rocks (Mogk et al., 2020). The oldest gneiss is dominantly of the TTG suite in composition and crystallized 3.5-3.1 Ga based on U-Pb zircon geochronology (Mueller et al., 2014). This rock is ion bi-modal association with amphibolite and are tectonically interleaved with metasupracrustal rocks (Henry et al., 1982). Detrital zircons in quartzite reveal crust-forming events at ~4.0, 3.7, and 3.5 Ga with a major crust forming event at 3.3-3.2 Ga; the latest deposition occurred at ~3.0-3.1 Ga (Mueller and Wooden, 2012). The metasupracrustal rocks were deposited in tectonically quiescent environments and were subsequently tectonically mixed with bodies of gneiss and metamorphosed between 3.0 and 2.8 Ga at pressure-temperature conditions of ~680-750o C and 6-8 Kbar (20-25 km depth of burial). The deep burial of the supracrustal sequence results from crustal thickening via horizontal tectonics (Henry et al., 1982). The Neoarchean magmatic cycle occurred over a restricted time interval (2.83-2.79 Ga) and produced large volumes of rocks of intermediate and tonalitic composition, along with the first major occurrence of true granite (Mueller et al., 2010). The Neoarchean rocks are interpreted as the products of subduction analogous to a modern-day continental arc setting.