THE LITTLE BELT ARC OF THE GREAT FALLS OROGEN

The Late Paleoproterozoic Little Belt magmatic arc (LBA) forms a key component of the Great Falls orogeny (GFO) along the northern margin of the Wyoming Craton (WC). Outcrop observations, analyses of crustal xenoliths in Eocene volcanic rocks, and the radiogenic isotopic composition of Cretaceous plutons show that the LBA extends along most of the length of the GFO and was constructed on Neoarchean continental basement of the Medicine Hat Block (MHB). The largest exposures of the Little Belt arc occur in the Little Belt Mountains, Montana, where mapping and U-Pb zircon geochronology indicate emplacement of a complex suite of dioritic to granitic plutonic rocks between ~1870 and 1790 Ma. The Paleoproterozoic plutonic rocks intruded Archean meta-diorite (c. 2700 Ma), and Paleoproterozoic pelitic gneisses and metavolcanic rocks. The LBA igneous rocks have geochemical and isotopic characteristics typical of subduction-generated magmas, suggesting that they formed during closure of an ocean basin leading to collision between the WC and MHB at about 1770 Ma. LBA rocks were metamorphosed to upper amphibolite to granulite facies during and after the main pulse of intrusion between c. 1840-1870 Ma and prior to c. 1800-1770 Ma when they cooled below 350˚C, based on ⁴⁰Ar/³⁹Ar thermochronology. These cooling ages are consistent with the WC partially subducting beneath the southern MHB forming the Great Falls suture. Metagranitoid xenoliths from the Montana Alkali Province give crystallization ages of c. 1820-1870 Ma and record crust formation and metamorphic events similar to exposed LBA. Rocks of the LBA were also sources of magma for the Late Cretaceous (75-68 Ma) SW Montana metaluminous suite, on the basis of whole rock Sm-Nd and zircon Lu-Hf isotopic data, and xenocrystic zircons. Dioritic gneisses, c. 1870 Ma, associated with Neoarchean gneisses in the Clearwater core complex, ID, suggest that the LBA extends west along the Lewis and Clark fault zone.