Improved Provenance Constraints for Nanaimo Group Sediments, British Columbia, Canada, Through Zircon LA‐ICP‐MS Depth‐Profiling

Detrital zircon from Late Cretaceous rocks of the Insular Superterrane have become an important constraint on Cordilleran paleogeography. These interpretations are largely based on the occurrence of a unique detrital zircon facies characterized by a broad Paleoproterozoic (mode 1.70 Ga) and narrow Mesoproterozoic mode (1.38 Ga) in Cordilleran sedimentary rocks. However, uncertainty about the provenance of this facies and the broad geographic distribution of rocks in which it is found limits its usefulness in constraining paleogeography. Detrital rims found on Mesozoic, Proterozoic, and Archean‐aged zircon cores from the Late Cretaceous Nanaimo Group of Western British Columbia, Canada provide constraints into its provenance. Depth‐profiling of a large population of grains (N = 16; n = 3,000), combined with observations regarding zircon external morphology, internal growth textures, U/Th ratios, and U concentrations, indicate high‐grade metamorphism occurred in the sediment source area between 99 Ma and 72 Ma. Rapid cooling of the metamorphic source area at rates of 35 to over 100°C/m.y. between 84 and 63 Ma was required to supply these metamorphic zircon to the basin. The geological history of the sediment source region defined by the rim populations (age of metamorphism and exhumation rate) is inconsistent with previously proposed sediment source areas for the Nanaimo Group in southern (Mojave region) or northern Laurentia (Lehmi Subbasin). We hypothesize that the sediment source area was the adjacent Coast Mountains Batholith and a metasedimentary rock, similar to the Pelona‐Orocopia‐Rand‐type schists, that was exhumed during the Late Cretaceous. Plain Language Summary Decades of geophysical, paleobotanical, and detrital zircon research has suggested parts of the westernmost North American Cordillera were located ∼2,000 km south of their current position during the Late Cretaceous. However, a lack of faults to accommodate large‐magnitude northward displacement make this interpretation contentious. Detrital zircon populations from sedimentary basins deposited atop the western Cordillera have become a crucial means to test these Cretaceous paleogeographic reconstructions. The provenance of a unique detrital zircon signature, found in Cretaceous rocks in the western Cordillera from southern California through to Alaska, has become central to this issue. Previous research has supported derivation of this unique signature from a northern sediment source area