Sandstone provenance and tectonics using detrital zircon metamorphic rim U-Pb dating

The Late Cretaceous paleogeographic position of the Insular Superterrane of the western North American Cordillera is disputed. The consensus view has been that the Insular Superterrane was accreted close to its present position on the southwestern Canadian margin, despite paleomagnetic evidence to the contrary. The paleomagnetic evidence, and more recently, several detrital zircon datasets, require that the Insular Superterrane was accreted at the latitude of southernmost California. Cretaceous strata of the Nanaimo forearc basin exposed on Vancouver Island, provide an important independent test of the Late Cretaceous paleogeographic position for the basin and associated terranes. Zircon rims are ubiquitous on Proterozoic detrital zircon populations in the basin and provide a record of metamorphism and magmatism in the source region. Here we use depth-profiling to measure the ages of these overgrowths and compare this to tectono-magmatic events in southwestern Laurentia. Cathodoluminescence imaging and U/Th ratios were used to classify zircon rim growth as metamorphic or magmatic. Grains with high U/Th ratio rims indicate metamorphism in the source region mostly between 100 Ma and 66 Ma with a peak at 86 Ma. Low U/Th ratio rims indicate magmatism in the source region mostly between 110 Ma and 64 Ma with a peak at 80 Ma. Core age populations (peaks at 1698 and 1388 Ma) are similar for grains with rims of metamorphic or magmatic affinity, suggesting the source region underwent metamorphism and partial melting in the Late Cretaceous. The timing of metamorphism and magmatism in the extra-regional source area are a good fit to tectonic events in the Mojave-Sonoran Region. Specifically, metamorphism and partial melting of the Pelona-Orocopia-Rand (POR) schists of southern California closely match to timing of rim growth in the source region for the Nanaimo Basin, supporting a southerly paleoposition for the Insular Superterrane. In addition to the above, metamorphic rim data from Cretaceous forearc basins like the Nanaimo Basin and exhumed schist complexes with froearc sediment protoliths in southern California and on the Insular Superterrane (e.g. Swakane Gneiss) provide evidence for rapid cycling in the Cretaceous forearc. For example, the youngest metamorphic rims in Nanaimo Basin samples are, on average, only 5 m.y. older than the depositional age of the sample indicating that exhumation of the metasedimentary source rock for the Nanaimo Basin was rapid following metamorphism and began before 84 Ma when the first grains with rims occur in the Nanaimo Basin. Exhumation of the metasedimentary source rock for the Nanaimo Basin predates deposition and underplating of parts of the POR schists and the Swakane Gneiss (a schist complex on the Insular Superterrane that is very similar to the POR schists), indicating that underplating and exhumation of metasedimentary rocks was contemporaneous in the Cordillera and probably part of a cyclical process involving, sediment subduction, metamorphism, underplating, exhumation, erosion and redeposition along the Southwestern margin of North America in the Late Cretaceous.