Age and evolution of the Precambrian crust of the Tobacco Root Mountains, Montana

U-Pb analyses of zircons from gneisses, anatectic leucosome, metasedimentary rocks, and a younger (metamorphosed) mafic dike from the Tobacco Root Mountains of southwestern Montana document a Precambrian history that extends from at least 3.90–1.77 Ga. The oldest U-Pb age reported here (3.8 Ga) is from a detrital zircon from a quartzite within the Spuhler Peak Metamorphic Suite, although younger ages of clearly detrital grains suggest the protolith was deposited subsequent to 3.2 Ga. Alternatively, a Pb-Pb age of ca. 2.45 Ga from a single subhedral zircon from this quartzite suggests the quartzite, and perhaps other Spuhler Peak Metamorphic Suite lithologies, may have formed in the Proterozoic. An Archean age, however, seems most compatible with the Archean Sm-Nd model ages of mafic and metasedimentary components of the Spuhler Peak Metamorphic Suite and the age distribution of zircons from the quartzite, which is very similar to the age distribution present in Archean quartzites in the region.

The Spuhler Peak Metamorphic Suite lies in tectonic contact with volumetrically dominant, Archean, quartzofeldspathic gneisses and intercalated metasedimentary rocks. The protoliths of these gneisses were apparently emplaced 3.2–3.4 Ga, and are interpreted to be the basement upon which the intercalated (meta)sedimentary rocks were deposited. U-Pb analyses of zircons from anatectic leucosome near the boundary between the gneisses and the Spuhler Peak Metamorphic Suite, however, yield a significant population of 1.77 Ga grains, which are interpreted to have crystallized from the leucosome. All other grains are Archean (to 3.48 Ga) and interpreted to derive from the metasedimentary source of the leucosome. In addition, U-Pb analyses of zircons extracted from a granulite facies mafic dike that cuts across Archean gneissic banding indicate the dike was intruded at 2.06 Ga, but reached granulite facies at 1.76 Ga. Structural, petrologic, and geochronologic data suggest all lithologies experienced granulite facies metamorphism at ca. 1.77 Ga and that the Spuhler Peak Metamorphic Suite was tectonically emplaced after 2.06 Ga, but before 1.77 Ga. This Paleoproterozoic tectonic activity is most likely a result of burial during terrane collision (e.g., the juxtaposition of the Wyoming and Hearne provinces) and/or to postcollisional mafic underplating.