GEOCHRONOLOGY AND GEOCHEMISTRY OF METASUPRACRUSTAL XENOLITHS FROM THE GREAT FALLS TECTONIC ZONE

The Great Falls tectonic zone (GFTZ) formed during the amalgamation of Laurentia at ~1.8 Ga and currently separates the Archean Wyoming province from the Medicine Hat block (MHB) and adjacent Hearne province. Exposure of crystalline basement within the GFTZ is very limited due to younger cover rocks. The Little Belt Mountains in central Montana provide exposures of meta-plutonic and meta-sedimentary rocks that formed in a ~ 1.86 Ga continental margin magmatic arc. U-Pb ages of detrital zircons from these meta-sedimentary rocks suggest that this magmatic arc formed above Medicine Hat/Hearne lithosphere. U-Pb ages and Hf isotopic ratios in zircons from crustal xenoliths also provide insights into the formation of the GFTZ and its incorporation into Laurentia. Metasedimentary xenoliths yield zircons with a range of ages from 1.02 Ga to 3.35 Ga for analyses <10% discordant. A major concentration of ages (42%) includes grains from 1.81 to 1.90 Ga, which correlates to the closure of the Little Belt ocean and formation of the Little Belt arc. Initial εHf of the Paleoproterozoic detrital grains reflect mixing of mantle-derived melts and old continental crust, ranging from 9.9 to -14.4 in their source rocks. Another significant concentration of ages ranges from 2.57 to 2.72 Ga (15%), which corresponds with published crystallization ages from the Medicine Hat Block (~2.65 Ga). The concentration of ages at ~2.6 - 2.7 Ga ages suggests that the xenoliths have a MHB provenance, which is characterized by Neoarchean ages, rather than to the northern Wyoming Province, which is dominantly Mesoarchean.