PERMO-TRIASSIC ACCRETION OF YUKON-TANANA AND RELATED TERRANES: NEW CONSTRAINTS ON THE EVOLUTION OF THE NORTHERN CORDILLERAN MARGIN FROM DETRITAL MINERAL GEOCHRONOLOGY

New detrital zircon and muscovite ages provide constraints on the source, paleotectonic setting, and paleogeography of mid- to late Paleozoic, and especially Triassic, North American strata in the northern Cordillera. Late Devonian to Mississippian Ellesmerian clastic wedge and clastic shelf deposits in Yukon and Northwest Territories record south-directed dispersal of sediment sourced from the Innuitian orogenic belt (and other fragments of Baltican-Siberian affinity?) along the eastern margin of the Slide Mountain Ocean. Innuitian-derived detrital zircons were consistently recycled in post-Late Devonian time and their occurrence in Cordilleran margin strata defines a northwestern Laurentian provenance.

Triassic sedimentation in Yukon was largely related to collision of the Yukon-Tanana terrane (YTT) with western North America. Permo-Triassic closure of the Slide Mountain Ocean, whose remnants comprise the Slide Mountain terrane (SMT), juxtaposed YTT and related terranes against the Ancestral North American margin. Triassic units in southeastern Yukon underlain by the SMT and Cordilleran margin strata contain mid- to late Paleozoic detrital zircon and muscovite derived from outboard pericratonic terranes, such as YTT. Triassic units also contain ubiquitous early Paleozoic and Precambrian detrital zircons that are characteristic of northwestern Laurentia; some grains of this age may be recycled through YTT rocks but we interpret the majority of them to be of Cordilleran margin affinity.

Early to Middle Triassic strata underlain by North American units in southeastern Yukon, which have detrital mineral ages that correlate with U-Pb zircon reference frames of YTT and the Cordilleran margin, delineate the extent of a collision-related basin along western Laurentia. Subsequently, broadly similar Late Triassic units formed an overlap assemblage that blanketed the YTT, SMT, and former North American margin. Late Triassic strata contain both northwestern Laurentian and Cordilleran terrane detrital mineral signatures. This sedimentary overlap provides a “hard pin” or geodynamic linkage that ties northern pericratonic terranes to the North American plate by the early Mesozoic.