GEOCHEMISTRY AND GEOCHRONOLOGY OF MID- TO LOWER CRUSTAL GRANITIC 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, details of its evolution remain enigmatic. Exposure of crystalline basement within the GFTZ is limited due to Mesoproterozoic cover. The Little Belt Mountains (LBM) in central Montana provide exposures of meta-plutonic rocks that formed within a ~1.86 Ga continental margin magmatic arc. This arc was built above Medicine Hat/Hearne lithosphere, as suggested by U-Pb ages of intercalated mafic gneiss, detrital zircons from meta-sedimentary rocks, Hf-isotope analyses of igneous zircon, as well as whole-rock Sm-Nd data. Xenoliths from Eocene diatremes in the Grassrange area of Montana (south of the LBM) reveal details of the crust in the central GFTZ. U-Pb ages of magmatic and inherited zircons from felsic xenoliths collected from these diatremes show two distinct groups at ~1.73 – 1.88 Ga and ~2.46 – 2.53 Ga. Zircon ages of metamorphosed leucogranite xenoliths indicate magmatic events at ~1.77 and ~2.65 Ga. Lu-Hf analyses suggest that the zircons yielding ~1.73 – 1.88 Ga and ~2.46 – 2.53 Ga were derived from similar crustal sources and the late Paleoproterozoic magmatism remobilized older (Late Archean) crust. Whole-rock Pb isotopic data from the felsic xenoliths show two distinct arrays of ~1.71 and ~2.47 Ga, which are consistent with the U-Pb data from the metagranitoids. Trace element analyses of the xenoliths show depleted heavy REEs, indicating that partial melting likely occurred in equilibrium with garnet or amphibole. Negative Eu and Sr anomalies indicate that plagioclase was removed from the melt during fractional crystallization 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 whole-rock and Lu-Hf zircon model ages support a Paleoproterozoic/Late Archean age for the source. Overall these data are consistent with magmatic and metamorphic events in the western GFTZ (1.86 and 1.73 Ga) and a previously recognized event in the Medicine Hat block (2.53 Ga), which further support that the GFTZ formed along the margin of the Medicine Hat block and were reworked during collision with the Wyoming craton after 1.8 Ga.