Precambrian geologic history of the Gravelly Range, southwest Montana

The meta-sedimentary rocks of the Gravelly Range are of anomalously low metamorphic grade compared to nearby Archean rocks in the Tobacco Root Mountains. This has led some previous workers to assign a Proterozoic age to the rocks of the Gravelly Range. This study examines the petrology, structure, and geochemistry of the metasupracrustal rocks of the Gravelly Range in order to better compare these rocks to those found in the Tobacco Root Mountains to the north. Metamorphic monazite from a phyllite in the Gravelly Range was dated to ca. 2.5 Ga, all but precluding a Proterozoic depositional age (Gerwin 2006). Likewise, petrologic, geochemical, and structural comparisons between the Precambrian rocks of the Gravelly and Tobacco Root Mountains show no major incompatibilities. Based on these comparisons, the simplest geologic history of the Gravelly Range is very similar to that defined for the Tobacco Root Mountains by Brady et al. (2004a). However, the structural mechanism by which the Gravelly Range escaped the high-grade metamorphism experienced in surrounding areas remains unknown