Recambrian evolution of the Beartooth Mountains, Montana-Wyoming, U.S.A.

Abstract

The Beartooth Mountains ofMontana and Wyoming contam more than 5,000 km2 of exposed Archean rocks. The southeastern portion of the range contains the only well documented segments of early Archean crust. The oldest rocks identified are a supracrustal assemblage contaning basaltic, ultramafic and felsic metavolcanics, metagraywackes, quartzites, meta-ironstones, and metapelites. These rocks yield a Rb-Sr whole-rock age of 3,400 Ma which is interpreted as the time of a granulite fades metamorphic event (800°C and 6 kb). This granulite facies event was followed at about 3,000 Ma age by the development of extensive volcanic and sedimentary packages with some minor silicic plutonism. The last major episode of crustal development began with an amphibolite fades rnetamorphic deformational event (about 2,850 Ma) and culminated with the intrusion of large volumes of granitoids (about 2,800 Ma). These granitoids were subsequently intruded by mafic dikes at least four separate times between 2,800 and 700 Ma age. Many of these events, including mafic dike emplacement, are correlative with geologic events reported from other ranges in the Wyoming Province; however, the Beartooth region appears to contain the longest and most complete record.