REFINING THE PROVENANCE OF THE NANAIMO BASIN, BRITISH COLUMBIA, CANADA BY LA-ICP-MS DEPTH PROFILING

The Late Cretaceous latitude of the western terranes in the North American Cordillera is contentious. Paleomagnetic and paleobotanical estimates of large Late Cretaceous to Cenozoic displacements are difficult to reconcile with geological estimates based on offsets along faults in the western Cordillera. Recent detrital zircon dating of Nanaimo Group strata are consistent with derivation from the Mojave-Sonoran region and support a southerly paleogeographic position. However, derivation from the Lemhi sub-basin of Idaho cannot be ruled out. High U/Th rims on some detrital zircons from the Nanaimo Basin are interpreted to record one or more metamorphic events that affected the sediment source area and may further constrain the provenance interpretation.

LA-ICP-MS depth-profiling was used to date <10 µm rims found on ~30-40% of zircon grains in 6 samples from Maastrichtian formations. 240 grains from each sample were depth profiled. Depth-profiling reveals that 78% of high U/Th rims (n=213) grew between 95 and 71 Ma (mode 87 Ma). The high U/Th rims become younger with decreasing depositional age and are between 7-10 Ma older than the maximum depositional age of the sample. Rims were found predominantly on cores yielding Proterozoic dates but also on Mesozoic and Archean cores.

If the high U/Th rims are indicative of metamorphism in the source area, then the timing of rim growth can be used to distinguish between potential northern and southern source areas. Our results suggest the source area underwent metamorphism between 95 and 71 Ma. The timing of metamorphism of the Lemhi sub-basin is constrained by Lu-Hf Garnet dates to be approximately between 110 and 85 Ma and thermochronology indicates the basin was cooling by 83 Ma. Metamorphic rim growth in the source area for the Nanaimo Basin sediments post-dates cooling in the Lemhi sub-basin by ~12 m.y. and derivation from this region in unlikely. The timing of rim growth overlaps with metamorphism of the POR schists of southern California (95 to 50 Ma) and is consistent with the period of flat-slab subduction in the Mojave-Sonoran region (90 to 70 Ma). Our observations support the paleomagnetic estimates for the Late Cretaceous latitude of the basin and derivation of Maastrichtian Nanaimo Group sediments from the Mojave-Sonoran Region.