Ages and SR isotope systematics of Archean basementrocks from the south^central Beartooth Mountains

The Precambrian basement rocks of the Beartooth Mountains north of Cooke City, Montana are dominated by granitic gneisses. Consistent intrusive relationships indicate that the pink gneiss, of granitic-to-adamellitic composition, is the younger. It yields a well-defined Rb-Sr whole-rock ages of 2,745 plus/minus 37 Ma with primitive initial {sup 87}Sr/{sup 86}Sr = .7005 plus/minus .0006 (all errors 2 standard deviations). The low initial Sr ratio is consistent with either a mantle source or a granulitic lower-crustal source. The older, tonalitic-to-granitic grey gneiss cannot be dated precisely by Rb-Sr methods, but does contain systematically more radiogenic Sr for given Rb/Sr ratio than does the pink gneiss. These data, combined with previously published zircon data suggest two alternative interpretations for the age of the grey gneiss: either it is up to several hundred million years older than the pink gneiss; or the two are nearly contemporaneous, but the grey gneiss has inherited some radiogenic Sr from assimilated pre-existing crustal material. No evidence has been found for earlier Archean rocks in this region, in contrast to portions of the eastern Beartooth Plateau block, where 3.4-Ga granulites and possible 3.9-Ga detrital zircons have been identified.