Tectonometamorphic evolution of the Kluane metamorphic assemblage, SW Yukon: evidence for late Cretaceous subduction of oceanic crust underneath North America

Ph.D. thesis, Jochen E. Mezger

The Kluane metamorphic assemblage (KMA) forms a 160 km NW-SE striking belt of graphitic mica-quartz schist with interlayered ultramafic lenses.It is located east of the Denali fault at the northwestern margin of the Coast Belt of the Western North American Cordillera, between island arc rocks of the Insular Superterrane (INS) to the west and pericratonic assemblages to the east. The lithology of the KMA is distinct from any metamorphic assemblage in the Cordillera. The dominant structural fabric of the KMA is a pervasive NE-dipping schistosity with a generally east-trending mineral stretching lineation. Mylonitic schists are characterized by a distinct monoclinic fabric that defines westward hangingwall up sense of shear. The KMA has experienced Late Cretaceous syndeformational high P/low T metamorphism followed by low P/high T thermal overprinting resulting from Early Eocene emplacement of the Ruby Range Batholith (RRB). Trace element and neodymium isotope signatures of the KMA distinguish its protolith from adjacent flysch of the Dezadeash Formation (DF) which has juvenile signatures and Late Proterozoic model ages, and schists of the pericratonic Aishihik Metamorphic Suite (AMS), part of the Yukon Tanana terrane, with ancient continental crust source and Late Archean model ages.The KMA has intermediate epsilon Nd(0) values of -4 and TDM of 1.3 Ga, indicative of homogeneous mixing of ancient and juvenile material. A possible tectonic model for the evolution of the KMA involves deposition of fine grained pelitic sediments in a backarc basin proximal to a volcanic arc and distal to an ancient continent. Late Cretaceous eastward motion of the Kula plate resulted in collapse of the basin and oblique eastward subduction, with a left-lateral strike-slip component, underneath the North American continental margin (AMS). Sediments of the KMA and parts of the oceanic crust experienced ductile deformation and high P/low T metamorphism during subduction and were added to the upper plate. Change of plate motion in the latest Cretaceous resulted in Early Eocene sill-like intrusion of the RRB along the suture between the KMA and AMS. Continuous northward plate motion caused rapid late Eocene uplift of the KMA and juxtaposition with the DF and INS.