EVOLUTION OF YUKON-TANANA TERRANE IN THE STEWART RIVER AREA, WESTERN YUKON

A widespread tract of Paleozoic Yukon-Tanana Terrane (YTT) in western Yukon represents a succession of arcs built upon a siliciclastic substrate. The entire sequence is affected by high-strain structural transposition, and one or more amphibolite-facies metamorphisms.

The lowest structural level comprises widespread, continentally(?) derived psammite and quartzite, with lesser pelite. The youngest detrital zircon grains from rare conglomerate and a quartzite yield U-Pb ages of Devonian and early Mississippian age respectively. Intermediate to mafic amphibolite derived from volcanic or volcaniclastic protoliths, interdigitates with and lies stratigraphically(?) above the siliciclastic rocks. Course crystalline marble, inferred to have been fringing reefs or local carbonate buildups, occurs as members within both successions. Orthogneiss of diorite, tonalite, granodiorite, monzogranite and K-spar megacrystic granite protolith intrudes both the siliciclastic and amphibolitic assemblages. Preliminary zircon ages demonstrate they are latest Devonian to early Mississippian (363-345 Ma) in age, in part coeval with siliciclastic sedimentation. The metavolcanic and metaplutonic rocks have the lithological and geochemical affinities of an arc succession, which rests above the siliciclastic substrate and which is coeval with back-arc basin rocks represented in the Finlayson Lake VMS district of southeastern Yukon. A poorly dated unit of graphitic quartz-rich metasedimentary rocks (Nasina assemblage), distinguished by abundant carbonaceous phyllite, and lack of amphibolite, rests structurally(?) above the non-cabonaceous siliciclastic rocks typical of the area.

A suite of mid-Permian (263-255 Ma) augen granites and their probable volcanic equivalents represent a younger arc succession built upon the mid-Paleozoic one. Widespread amphibolite facies metamorphism and high-strain structural transposition were synchronous with the Permian magmatism.

The YTT rocks are intruded by post-tectonic Jurassic, Cretaceous and Eocene plutons, and overlapped by middle to late Cretaceous conglomerate and volcanic rocks. YTT was emplaced in the Jurassic-Cretaceous when it was delaminated from its basement and thrust as a thin(?) sheet above the ancestral North American margin.