INTERMONTANE AND INSULAR SUPERTERRANES: SEPARATED BY AN OCEAN BASIN FROM LATEST TRIASSIC TO LATE CRETACEOUS TIME

The Kluane Schist is a meta-flysch that crops out in the core of a major structural culmination in SW Yukon. The meta-flysch is thrust west over Late Jurassic to mid-Cretaceous flysch of the Gravina-Nutzotin assemblage, part of the Insular Superterrane (ST). Geochemical and isotopic data indicate that the meta-flysch is correlative with and probably originated as a distal portion of the more westerly Gravina-Nutzotin flysch. The syn-kinematic 58 Ma Ruby Range batholith, the northern continuation of the Great Tonalite Sill, overlies the Kluane Schist and constitutes a west-verging thrust fault. Pericratonic strata of the Yukon-Tanana terrane, part of the Intemontane ST, forms the hangingwall to the thrust. Late Cretaceous to Tertiary west-verging thrust faults imbricate the Kluane Schist and merge up-section to the west into the Ruby Range batholith forming a crustal-scale duplex. These faults carry slices of metamorphosed ultramafic rocks. Zircons separated from a sample of the ultramafic rocks yielded a U-Pb age of 202 Ma that we interpret as the age of primary crystallization. This latest Triassic age is older than the age of deposition of the sedimentary protolith of the Kluane Schist and suggests that the ultramafic rocks originated as mantle or crustal cumulates within the oceanic lithosphere on which Kluane Schist meta-flysch was deposited. This relationship requires the presence of an ocean basin separating the Intermontane and Insular STs from the latest Triassic until at least 105 Ma (the age of the youngest known detrital zircons in the Gravina-Nutzotin assemblage) and is inconsistent with interpretations of the superterranes as having accreted prior to the Late Jurassic. Models that explain mid-Cretaceous to Tertiary Coast Belt deformation as the result of intra-arc processes are also called into question. Separation of the Insular and Intermontane STs into the Cretaceous provides an explanation for Gravina-Nutzotin deposition being restricted to the Insular ST, and implies that Cretaceous – Tertiary Coast Range magmatism and deformation is attributable, in part, to closure of an ocean basin by east-dipping subduction beneath the Intermontane ST of oceanic lithosphere that was continuous with the Insular ST.