The significance of the Cretaceous Kluane Schist in the Mesozoic Assembly of the North American Cordillera.

The Cordillera of western North America is an archetypal accretionary orogen. During the Mesozoic, two significant periods of terrane accretion are recognised within the Canadian-Alaska ("Northern") Cordillera: 1) the accretion of the Intermontane terranes to the western Laurentian margin from ca. 200 to ca. 180 Ma and, (2) the subsequent accretion of the Insular terranes to the Intermontane terranes, possibly as early as ca. 174 Ma and as late as ca. 70 Ma. Information regarding the precise timing and tectonic drivers of this Mesozoic accretionary history is held within a series of inverted Jura-Cretaceous basins exposed at the interface between the pericratonic Intermontane and exotic Insular terranes. Of these inverted basins, the Kluane Schist, southwest Yukon, represents one of the largest and most continuous exposures. The Kluane Schist consists of metamorphosed and deformed low-Al pelites that were deposited during the very latest stages of Insular terrane accretion. Detrital zircon geochronology reveals a Late Cretaceous maximum depositional age for the Kluane Schist and Hafnium-isotope geochemistry provides strong provenance ties to Triassic–Cretaceous zircon sources currently exposed within the southern Yukon Tanana terrane. Detailed petrography, petrological modelling, and in-situ monazite petrochronology demonstrate the metamorphism of the Kluane Schist occurred during the override of the Yukon-Tanana terrane between ca. 70–55 Ma. This metamorphic evolution, combined with its continentally-dipping, Buchan-style inverted metamorphic sequence and tops-to-the-southwest shear structures are consistent with the Kluane Schist representing the uppermost part of a contractional forearc to an arc built upon the Yukon-Tanana terrane. This geodynamic setting for the Kluane Schist is most compatible with models of Insular terrane accretion that involve east-dipping subduction below a westward migrating North American continent during the Mesozoic.