U-PB ZIRCON AND WHOLE-ROCK GEOCHEMISTRY OF BASEMENT SAMPLES FROM BORE HOLES AND XENOLITHS: AGE AND TECTONIC SIGNIFICANCE OF THE GREAT FALLS TECTONIC ZONE

The Great Falls tectonic zone (GFTZ) separates the Archean Wyoming province from the Hearne/Medicine Hat craton. This zone records a series of events in the amalgamation of Laurentia; however, the development of GFTZ remains enigmatic. Studies of the GFTZ are complicated by limited exposure of crystalline basement due to Mesoproterozoic and later cover. The Little Belt Mountains, Montana, provide some exposure, and recent work indicates the presence of a 1.86 Ga continental margin magmatic arc developed from subduction of oceanic material. U-Pb ages of detrital zircons from meta-sedimentary rocks, as well as whole-rock Sm-Nd data, suggest that this arc formed above Medicine Hat/Hearne lithosphere. U-Pb ages from magmatic and xenocrystic zircon and 40Ar/39Ar cooling ages from exposed crust in the Little Rocky Mountains, drill cores, and xenoliths from the eastern GFTZ and beneath the Williston Basin, support a similar history of crustal development for the eastern and central GFTZ, and adjacent parts of the Trans-Hudson orogen. These data reveal Paleoproterozoic metamorphism and cooling as opposed to the Archean cooling history evident in rocks of the northern Wyoming province. The western GFTZ has a more complex crustal structure and metamorphic history involving Paleoproterozoic reworking of Archean crust (~1.77 Ga) as well as magmatic additions of younger (to ~1.65 Ga) and older (to 2.45 Ga) crust beyond the major metamorphic event at 1.77 Ga. This suggests that the western GFTZ records collisions between multiple terranes with distinct histories in addition to the Medicine Hat/Hearne block. This complexity contrasts with the central and eastern GFTZ, which likely record a single post-arc formation collision. The spatial coincidence of this complex collisional zone in the west and the formation of the Belt basin provide an analog to the formation of the Williston basin beneath the eastern GFTZ/Trans-Hudson region. This is consistent with interpretations indicating that the Great Falls tectonic zone and Medicine Hat block developed independent from Wyoming and were not accreted to Wyoming until ca. 1.8 Ga. This information could provide better constraints on the age and character of the GFTZ, increasing its value as a piercing point between the Cordilleran Laurentian margin and proposed conjugates.