PROVENANCE OF THE BELT BASIN: A SNAPSHOT OF PRE-RODINIA WESTERN NORTH AMERICAN PALEOGEOGRAPHY

The Mesoproterozoic (1.47-1.37 Ga) Belt Basin in northwest United States and southern Canada is a thick (~16 km) assemblage of predominantly fine-grained siliciclastic strata with a much-debated tectonic setting. We have dated over 400 detrital zircon grains (U-Pb SHRIMP) from formations that cover the entire stratigraphic thickness of the Belt as well as from geographically diverse sections along the east, southern and western sides of the preserved basin. The new data offer an enhanced view of the mysterious western craton, the source area for a substantial portion of the Belt that has been removed by post-Belt tectonic processes. The results indicate that three general sources have contributed detrital zircons to the Belt Basin. An eastern source (proximal Canadian Shield) supplied clastic material found in the sandstones in the Grinnell, Siyeh, Creston formations. In Montana, a southern source for the Lahood, Newland and Aldridge-Prichard formations is compatible with derivation from the Wyoming Province but also contains a substantial proportion of young Proterozoic grains (1760-1900 Ma)possibly derived from the Great Falls Tectonic Zone. A western source appears in the upper Prichard quartzite facies and persists into Missoula Group deposition. The western source is inferred from detrital zircons that range in age from 1510-1610 Ma, distinctly non-North American ages. Significantly, the Prichard quartzite also contains zircons with ages that nearly overlap with those for intraBelt magmatic rocks (1467-1435 Ma), suggesting that some Belt sedimentation may have been contemporaneous with magmatism occurring in the western craton. The new data confirm that the western craton was similar to present day eastern Australia, supporting a pre-Rodinia juxtaposition of eastern Australia and western North America. Based on rapid rates of sedimentation implied by geochronology of intraBelt magmatic rocks, the apparent presence of syndepositional magmatism in the western source, and persistent reactivation of the western craton, we envision that a oblique extensional setting with a magmatically and tectonically active source area, broadly speaking a back-arc basin, may be a plausible tectonic setting for the Belt basin.