THE STRATIGRAPHIC AND ISOTOPIC RECORD OF NEOPROTEROZOIC FLOOD BASALT MAGMATISM, SNOWBALL GLACIATION, AND CONTINENTAL BREAK-UP IN NORTHWESTERN LAURENTIA

Northwestern Canada preserves an exceptional stratigraphic record that spans from the time of the purported assembly of Rodinia ca. 1.0 Ga to Laurentia’s definitive break-out from the supercontinent sometime close to the Precambrian–Cambrian boundary. It has long been appreciated that western Laurentia’s Neoproterozoic history involved more than a simple Wilson cycle, but details of its tectonostratigraphic evolution have remained obscure. The Neoproterozoic succession in Yukon reveals a remarkably complex history of basin development, punctuated by Cryogenian snowball glaciation. Multiple phases of continental extension occurred in the early Neoproterozoic, prior to regional uplift and tilting associated with emplacement of the c. 779 Gunbarrel large igneous province (LIP). Subsequent extrusion of the ca. 720 Ma Franklin LIP in northern Laurentia was temporally associated with mafic volcanism in western Yukon, and transtension, which generated thick pull-apart basins filled by Rapitan glacial deposits. In the eastern Yukon, northwest-vergent folds in the Basinal Assemblage of the Little Dal Group are sealed by the post-Rapitan Keele Formation. Snowball glaciation subsequently stripped northwestern Canada of its abundant basalt carapace. Final opening of a passive margin in northwestern Laurentia did not occur until the late Ediacaran–early Cambrian. This convoluted history is reflected in the Nd isotope record preserved in fine-grained clastic sediments. This isotopic record, which reveals a prominent peak in εNd prior to the Rapitan glaciation, can also be applied to testing paleogeographic models of Laurentia’s place within a Rodinian supercontinent and its possible role in the initiation of snowball glaciation. The global seawater Sr isotope record also reflects the interplay of continental break-up, extensive early–mid Neoproterozoic flood basalt magmatism, and the profound influence of snowball glaciation on continental weathering.