LATEST PERMIAN TO MIDDLE TRIASSIC ACCRETIONS OF THE YUKON-TANANA, STIKINE, AND QUESNEL TERRANES TO NORTH AMERICA: NEW DETRITAL ZIRCON AGE DATA FROM TRIASSIC ROCKS IN YUKON

Provenance data from conodont-constrained Triassic sedimentary rocks of eastern Yukon support latest Permian accretion of the Yukon-Tanana terrane (YTT) and Middle Triassic accretion of the composite Stikinia-Quesnellia arc to the western edge of Ancestral North America. We present a new tectonic model for the early Mesozoic in the northern Cordillera, which includes: (1) latest Permian convergence between the YTT (upper plate) and North America (lower plate) leading to the development of an Early to Middle Triassic peripheral foreland basin superimposed on the distal Cordilleran miogeocline with sediment sourced mainly from the west; and (2) a Middle Triassic collisional event between the YTT and Stikinia (Semenof block) which shed sediment to the east onto the YTT and further east into the peripheral foreland basin of North America. The YTT has been interpreted to be the Paleozoic basement to Quesnellia in southern Yukon; therefore we conclude that the composite Late Triassic to Early Jurassic Stikine-Quesnel arc that subsequently developed in this part of the northern Cordillera was built on an amalgamated North American Plate.

North American Early Triassic (Smithian) sedimentary rocks of easternmost Yukon contain Mississippian (343-348 Ma) detrital zircons, which are ages diagnostic of the YTT and absent from western Ancestral North America, demonstrating erosion of an uplifted hinterland to the west in Early Triassic time. Whole-rock provenance data from Early Triassic rocks also indicate a partial source from Slide Mountain terrane, the intervening marginal ocean basin which closed by subduction under the YTT.

North American Middle Triassic (late Ladinian) rocks of the Selwyn Basin in southeastern Yukon and Middle Triassic conglomerate of the YTT in central Yukon contain Triassic (mainly 238-252 Ma) and Pennsylvanian to Early Permian (295-317 Ma) detrital zircon whose source is interpreted to be the Semenof block of Stikinia.

Late Triassic (Carnian-Norian) to Middle Jurassic sedimentary rocks of the YTT and Slide Mountain terrane, as well as the parautochthonous Cassiar terrane and North American miogeocline, record sedimentary systems sampling the amalgamated YTT-Stikinia-Quesnellia complex.