AGE AND PROVENANCE OF THE NERUOKPUK FORMATION, NORTHEASTERN BROOKS RANGE, BASED ON AR-AR AND U-PB GEOCHRONOLOGY

The Late Neoproterozoic to Cambrian Neroukpuk Formation is a thick succession of turbiditic, locally feldspathic sandstone and siltstone deposited on the continental margin of Laurentia following the breakup of Rodinia. It is currently exposed in several outcrop belts in the northeastern Brooks Range of Arctic Alaska and in adjacent northwestern Yukon. Detrital muscovite from three localities in the Neroukpuk Formation in northern Yukon were dated by the step-heating ⁴⁰Ar/³⁹Ar method. Plateau ages of 1800 – 1900 Ma are interpreted to represent the age of metamorphism of the source terrane and are consistent with U-Pb ages from detrital zircons from Alaska and the Yukon. A sandstone sample containing fine-grained sericitic muscovite has an Ar-Ar whole rock weighted age of 544 +/-3 Ma, which we interpret as the age of diagenesis. All samples show evidence of Mesozoic Ar loss, perhaps due to Jurassic/Cretaceous burial and heating. These Cambrian and Mesozoic ages illustrate the versatility of the muscovite dating method in revealing insights into the complex geologic history of detrital minerals. Sandstone from a fourth locality yielded a population of detrital zircons that were analysed using the SHRIMP facility at the Geological Survey of Canada. Most grains are late Paleoproterozoic in age, with some Archean and some Mesoproterozoic grains. The youngest grain, at 720 Ma, clearly documents a maximum age well within Neoproterozoic time, supporting previous correlations of the Neruokpuk with Windermere strata of widespread occurrence throughout the Canadian Cordillera. These ages are consistent with a sedimentary provenance within northwestern Canada, tying the eastern part of Arctic Alaska to the North American continental margin in latest Proterozoic to Cambrian time.