Will Fergus Mckenzie, Brendan Dyck, H. Daniel Gibson, Matthew Steele-MacInnis

A wealth of information regarding the Mesozoic evolution of the Northern Canadian and Alaskan Cordillera is held within a series of variably metamorphosed and deformed Jura-Cretaceous basins. Located at the interface between the pericratonic Intermontane and exotic Insular terranes, these basins may prove key to understanding the timing and tectonic style of Insular terrane accretion, a topic of longstanding debate. This study unravels the structural and metamorphic evolution of one of these basins, the Kluane Basin, within southwest Yukon Territory.

The Kluane Schist is the primary assemblage of the Kluane Basin. It consists of metamorphosed and deformed low-Al pelites that were intruded by granodioritic plutons of the Paleocene-Eocene Ruby Range Batholith. The Kluane Schist preserves a complex metamorphic history that includes both regional and contact events. Previous workers have suggested the variable metamorphic character of the Kluane Schist represents a large thermal aureole related to Ruby Range Batholith emplacement. Our work, however, indicates the Kluane Schist can be divided into seven distinct petrologic zones, highlighted by a unique combination of mineral assemblage and structure, that together record a period of regional metamorphism coeval with protracted deformation. Further, we propose the Kluane Schist represents a single lithological package which experienced two distinct phases of deformation, 1) an early greenschist-facies phase that resulted in the development of a bedding-parallel chlorite–muscovite–titanite fabric, preserved by its lowest grade units, and 2) a later amphibolite-facies phase that manifests across higher grade units as the progressive transposition of the earlier chlorite–muscovite fabric into a penetrative biotite-rich schistosity that transitions upgrade into a segregated gneissic fabric comprised of biotite–cordierite and plagioclase–quartz (+/- sillimanite–K-feldspar–melt).

By integrating the results of detailed petrography and petrological modelling, we demonstrate that the Kluane Schist preserves metamorphic conditions that align with other Buchan-Style terranes worldwide. Our data defines a field gradient across the Kluane Schist ranging from 3.0–3.5 kbar at 375–400 °C to 4–4.5 kbar at 700–750 °C. This record of a coupled Buchan-style metamorphic-deformational evolution and tops-to-the WNW to W non-coaxial shear structures are consistent with the override of the thermally mature Yukon-Tanana terrane as the principal driver of Kluane Schist metamorphism, rather than intrusion of the Ruby Range Batholith.