Abstract

The Teslin suture zone (TSZ) comprises a portion of the Yukon–Tanana terrane (YT), in the Yukon, formed by steeply dipping layering and L–S tectonite foliation. The TSZ forms the fundamental tectonic boundary between rocks deposited along the ancient margin of North America and allochthonous terranes to the west. TSZ tectonites evolved during initial penetrative dip-slip deformation (Dds) and later dextral strike-slip shear (Dss) along steep, 1–3 km wide shear zones. Several workers have speculated that the TSZ and related YT heterogeneous ductile deformation and associated metamorphism are Devonian to Mississippian in age and related to the intrusion of a similar-age orthogneiss throughout the Yukon and Alaska. However, recent structural and metamorphic studies of the TSZ provide evidence contradicting this view. New isotopic evidence, presented herein, indicates that TSZ dynamothermal metamorphism was cooled by Early Jurassic time, that it cannot be related to Devonian–Mississippian and Permian granitic intrusion, and that it predates Cretaceous plutonism.U–Pb zircon dating of peraluminous orthogneiss constrains primary peraluminous granite crystallization at 355 ± 25 Ma. Three Rb–Sr whole rock + muscovite and three K–Ar muscovite cooling dates of rocks containing Dds and Dss fabrics place a younger age limit of 182–213 Ma (latest Triassic to Early Jurassic) on Dds/Dss deformation. In addition, three Rb–Sr whole-rock + muscovite isochrons and one K–Ar date on biotite indicate peraluminous orthogneisses in the eastern portion of the study area were affected by a mid-Cretaceous thermal event. These data, togemer with structural and metamorphic relationships reported elsewhere, are summarized in pressure–temperature–time–displacement diagrams illustrating the evolution of TSZ and adjacent rocks from Devonian to Late Cretaceous time. Tectonites within the TSZ can be differentiated from peraluminous orthogneiss east of the d'Abbadie fault on the basis of their respective cooling histories.

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