TECTONIC IMPLICATIONS OF LATE ARCHEAN-EARLY PROTEROZOIC SUPRACRUSTAL ROCKS IN THE GRAVELLY RANGE, SW MONTANA

The Montana metasedimentary terrane (MMT) of the northwestern Wyoming province is characterized by the widespread occurrence of supracrustal assemblages dominated by metasedimentary lithologies (marble, pelitic schist, quartzite, BIF). These assemblages range from greenschist to granulite facies and include the low- to medium-grade rocks of the Gravelly Range, and lithologically similar, but higher grade amphibolite/granulite facies rocks in the adjacent Tobacco Root and Ruby Ranges. Studies of detrital zircons from quartzites from the Tobacco Root and Ruby Ranges (Mueller et al., 1998) show they are characterized by dominant populations of 3.2-3.5 Ga zircons, with maximum ages up to ~4.0 Ga (on average, 25% of grains are >3.4 Ga). The youngest detrital grains place minimum ages of deposition at ~3.1 Ga. The lack of 2.8 Ga grains from abundant 2.7-2.8 Ga gneisses in the northwestern Wyoming province define a lower limit for their time of deposition. In the Gravelly Range, these rocks occur at three distinct metamorphic grades; a northern band contains sillimanite, the Cherry Creek occurrence contains kyanite-staurolite, and the southern occurrence is mixed greenschist-amphibolite facies with local preservation of original sedimentary structures. A detrital zircon age-spectrum from one of these quartzites is dominated by 2.7-2.8 Ga and 3.0-3.2 Ga ages, which contrasts strongly with the >3.2 Ga dominated age-spectra from high-grade quartzites from adjacent uplifts. The 2.7-2.8 Ga grains are likely to be derived from underlying gneisses, but the source of the 3.0-3.2 Ga grains is not clear. Nd model ages of schists, including range from 2.9-3.3 Ga, and suggest the metapelitic rocks also contain substantial Archean detritus. The lack of ~1.7 Ga detritus, which would have been derived from a well-documented orogeny in this area, suggests that the Gravelly sequence is likely to be latest Archean or earliest Proterozoic. Lithologic and geochemical information suggests that the Gravelly sequence was deposited in a rift environment developed on the late Archean margin of the Wyoming craton prior to 1.7 Ga. Based on these observations, correlation of the Cherry Creek rocks of the Gravelly Range with marble-bearing, high-grade metasupracrustal rocks found throughout the MMT is unwarranted.