GEOCHEMISTRY OF MID- TO LOWER CRUSTAL XENOLITHS FROM THE GREAT FALLS TECTONIC ZONE

The Great Falls Tectonic Zone (GFTZ) separates the Archean Wyoming province from the Hearne/Medicine Hat craton. The GFTZ formed during the amalgamation of Laurentia at ~1.8 Ga; however, the evolution of the GFTZ remains enigmatic. Exposure of crystalline basement within the zone is very limited due to Mesoproterozoic and younger cover rocks. The Little Belt Mountains in central Montana provide exposures of meta-plutonic rocks that formed within a ~ 1.86 Ga continental margin magmatic arc. U-Pb ages of detrital zircons from meta-sedimentary rocks, as well as whole-rock Sm-Nd data, suggest that this magmatic arc formed above Medicine Hat/Hearne lithosphere. U-Pb ages from magmatic and inherited zircon from xenoliths from the Grassrange area of the GFTZ show two distinct ages at ~ 1.77 Ga and ~2.65 Ga. These ages correlate to a magmatic/thermal event in the western GFTZ (1.77 Ga) and the age of the older Medicine Hat block (2.65 Ga). Paleoproterozoic metamorphism and cooling in the GFTZ contrasts with the Archean tectonothermal history of rocks of the northern Wyoming province. The xenoliths included in this study include meta-granitoids, quartzofeldspathic orthogneisses, quartzites, and schists. Ages for the meta-granitoids and orthogneisses define 2 distinct groupings at ~1.77 and ~2.65 Ga. The quartzite and schist samples show a range of detrital zircon ages, but prominent peaks at ~1.9 and ~1.1 Ga. Lu-Hf analyses reveal that the ages clustered together at ~1.77 and ~2.65 derive from the same magmatic source and the 1.77 Ga ages are likely metamorphic and/or represent anatexis of older crust. Secondary isotopic systematics of common Pb for the xenoliths show two distinct arrays of ~ 1.79 and ~ 2.64 Ga, clearly correlating with the two U-Pb age clusters from the metagranitoids. Trace element analyses of the xenoliths show depleted heavy REEs, indicating that the samples were in equilibrium with garnet at one point. Negative Eu and Sr anomalies indicate that plagioclase was removed from the sample during formation or remained as a residue from partial melting. Lastly, a negative Nb anomaly in the samples is an indicator of formation in a subduction modified environment, however, Sm-Nd model ages suggest that the source originally formed in the Paleoproterozoic.