CONFIGURATION OF THE 2.0 - 1.6 GA ACCRETIONARY MARGIN NW OF THE WYOMING PROVINCE: IMPLICATIONS FOR PROTEROZOIC CONTINENTAL RECONSTRUCTIONS

Along the Neoproterozoic rifted margin W and NW of the Archean Wyoming province and S of the Medicine Hat-Hearne province, there is a temporal and spatial gap in our current understanding of the Laurentian basement. In particular, the area encompassing western Montana and central Idaho, and extending south to the Mojave province is one of the most poorly understood segments of Laurentia in terms of the age and configuration of Proterozoic crust. Geochronological and isotopic analyses of small areas of exposed basement along with the isotopic (Nd, Pb, & Sr) signatures and premagmatic zircon populations of Cretaceous-Eocene granitoids, and possible pre-Belt Supergroup metasediments (Syringa metamorphics) indicate a rich 2.0 to 1.6 Ga tectonic history, with a primitive component aged ~1.7-1.8 Ga. These data suggest that the basement is composed of accreted juvenile Paleoproterozoic lithosphere, with arc-like enrichment in incompatible elements. This crust was modified in the Mesoproterozoic by extension and mafic intrusion during Belt rifting. The accreted lithosphere was much more fertile for later partial melting than areas underlain by the Archean Wyoming and Medicine Hat-Hearne provinces, and gave rise anomalous volumes of felsic magma during Cretaceous-Eocene time, well inboard of the contemporaneous subduction zone. The geometry the of ~1.75 Ga province could explain the shape/distribution of the northern Idaho batholith, the margin-orthogonal Cretaceous SW Montana granitic province, and Eocene Challis igneous province. Direct clues about the age and structure of the former conjugate continental margin to western Laurentia in Rodina are contained in this largely hidden basement. Our present knowledge of the isotopic compositions and crystallization ages suggest similarities with the Broken Hill and Olary blocks of the South Australian craton, where sedimentary rocks of the Pandurra Formation are the same age as the Belt Supergroup. The Georgetown inlier and Halls Creek orogen of the North Australian craton also show similar Paleo- and Mesoprotoerozoic histories. Accreted post-Archean crust in this area may have also led to the development of the Idaho re-entrant on the Neoproterozoic rifted margin because it was easier to reactivate the accreted lithosphere than the adjacent Archean tectospheres.