Age of the basement rocks of southwest Montana

Rb-Sr analyses of a suite of quartzo-feldspathic gneisses that are interlayered with beds of marble, quartzite, and amphibolite in the Ruby and Tobacco Root Ranges and the Gallatin River canyon of southwest Montana show that the age of metamorphism of these strata occurred about 2,750 m.y. ago. The 13 samples analyzed are from rock units that have in the past been assigned stratigraphically to the Pony Group, Cherry Creek Group, and Dillon Granite Gneiss. Except for two samples of anomalous composition, the data define a linear array on an isochron diagram that has a best-fit value of 2,762 ± 113 m.y. Inclusion of other published data for the Tobacco Root Range yields a best-fit value of 2,730 ± 85 m.y. This age corresponds closely to that of the principal metamorphic-plutonic epoch of the Bear-tooth Mountains, to which the term “Bear-tooth orogeny” has been applied. It also demonstrates that the major Precambrian metasedimentary sequences of the region are of Archean age.