New detrital zircon U-Pb and Hf isotope constraints for the Kluane Schist, southwestern Yukon, Canada, and their implications for Cretaceous Insular terrane accretion

The Kluane Schist, southwestern Yukon, Canada, represents one of the largest continuous exposures of metasedimentary rocks at the interface between the inboard pericratonic Intermontane terranes and outboard Insular terranes of the North American Cordillera. Consisting of a 160-km-long belt of metamorphosed siliciclastic and calc-silicate rocks interleaved with ultramafic bodies and local carbonate enclaves, the Kluane Schist preserves a record of Insular terrane accretion throughout the latest Mesozoic and into the early Cenozoic. To determine the geodynamic setting of the paleo−western margin of North America and its relationship to the Insular terranes prior to their accretion, we report U-Pb age and Hf isotope determinations of detrital zircon grains from metasedimentary rocks collected across various structural levels within the Kluane Schist. Our results highlight the Kluane Schist as a Late Cretaceous depocenter with strong provenance ties to the paleo−western margin of North America. These characteristics are compatible with its deposition as the uppermost part of a contractional forearc to an arc built upon the Yukon-Tanana terrane. In reconciling our data with similar datasets from other Jura-Cretaceous metasedimentary units within the northern Cordillera, we suggest the tectonic setting for the Kluane Schist is best reconciled with models of Insular terrane accretion that involve east-dipping subduction beneath a westward-migrating North American continent.