Detrital zircon U-Pb and Hf isotope signature of Carboniferous and older strata of the Yukon-Tanana terrane in Yukon, Canadian Cordillera: Implications for terrane correlations and the onset of Late Devonian arc magmatism

Abstract

The Yukon-Tanana terrane in Yukon, Canada, records Late Devonian (ca. 366–360 Ma) rifting and the onset of latest Devonian–Carboniferous arc and back-arc magmatism (ca. 360–325 Ma) in the Northern Cordillera. Detrital zircon U-Pb and Hf isotope analyses indicate that the metasedimentary basement of the Yukon-Tanana terrane was sourced in northwestern Laurentia. Sandstones in Late Devonian–Carboniferous successions generally have robust Late Devonian–Mississippian age peaks, and their Hf isotope signatures are characterized by strongly negative εHft values in Late Devonian zircons followed by progressively more juvenile εHft values in Carboniferous zircons. This Hf isotopic “pull-up” reflects the melting of Precambrian crust related to regional extension in the Late Devonian, followed by progressively more juvenile magmatism as the arc matured through the Carboniferous. Paleozoic rocks of the Tracy Arm terrane in southeastern Alaska, USA (formerly Yukon-Tanana south), have been compared with the Yukon-Tanana terrane in Yukon. Detrital zircons from the metasedimentary basement to the Tracy Arm terrane have distinct Precambrian populations that indicate sources along a different segment of the Laurentian margin compared to basement of the Yukon-Tanana terrane. Magmatism in the Tracy Arm terrane ranges from 440 Ma to 360 Ma and is characterized by an Hf isotopic “pull-down” in the Silurian to Early Devonian, followed by a “pull-up” in the Middle to Late Devonian and a second “pull-down” in the Late Devonian to early Mississippian. Thus, the Yukon-Tanana and Tracy Arm terranes record distinct pre-Carboniferous histories. Interactions between these two terranes are suggested by the influx of exotic early Mississippian clasts and detrital zircons on the Tracy Arm terrane that match sources in the Yukon-Tanana terrane.