A detrital record of Mesozoic island arc accretion and exhumation in the North American Cordillera: U-Pb geochronology of the Kahiltna basin, southern Alaska

The stratigraphic record of Mesozoic arc accretion in the North American Cordillera is preserved in a discontinuous belt of clastic strata that are exposed inboard (cratonward) of the allochthonous Wrangellia composite terrane in southern Alaska, western Canada, and Washington State. LA-ICPMS analyses of eight samples (n = 714 detrital zircon grains) collected at different stratigraphic intervals from the Jurassic-Cretaceous Kahiltna assemblage in southern Alaska reveals a bulk U-Pb age distribution of Precambrian-Mesozoic age grains (Mz 74%, Pz 11%, Pc 15%). A comparison of U-Pb ages from older to younger stratigraphic intervals within the Kahiltna assemblage reveals three stages of exhumation and basin development during arc accretion. Stages include (1) an initial Late Jurassic-Early Cretaceous pre/early collisional phase during which detritus was derived almost solely from Middle-Late Jurassic and Early Cretaceous magmatic sources of the outboard Wrangellia composite terrane (Mz 100%, Pz 0%, Pc, 0%), (2) a second Early Cretaceous syncollisional phase that reflects the introduction of Paleozoic and Precambrian detritus from the inboard Intermontane belt (Mz 84%, Pz 11%, Pc 5%) and an upsection increase in older detrital zircon grains compared to Mesozoic age grains (Mz 65%, Pz 11%, Pc 24%), and (3) a final Early-Late Cretaceous late/postcollisional phase that represents continued detrital contributions from Precambrian-Mesozoic source areas (Mz 19%, Pz 22%, Pc 59%) located inboard and outboard of the Kahiltna basin. Similar bulk trends in detrital zircon age populations have been reported from along-strike, age-equivalent strata of the Gravina belt (Mz 74%, Pz 20%, Pc 6%) in southeastern Alaska suggesting that similar provenance trends may exist in basins along this >2000 km -long collisional zone.