DETRITAL ZIRCON PROVENANCE STUDY OF THE YUKON-TANANA TERRANE IN YUKON, CANADA

Rocks of the main body of the Yukon-Tanana terrane (YTT) consist of a lower sequence of pre-Devonian metasedimentary rocks and minor amphibolite of the Snowcap assemblage, overlain by three unconformity-bounded sequences of arc-type metavolcanic and metasedimentary rocks of Late Devonian through Permian age. Arc and back-arc sequences in YTT have been divided into the Upper Devonian–Lower Mississippian Finlayson assemblage, Middle Mississippian-Lower Permian Klinkit assemblage, and the mid- to Late Permian Klondike assemblage. The YTT arc rocks formed outboard of coeval oceanic assemblages of the Slide Mountain terrane (SMT). Permian to Triassic clastic rocks of the Simpson Lake and Jones Lake assemblages overlap YTT, SMT and continental margin strata of western Laurentia.

U-Pb geochronologic analyses have been conducted on zircons separated from eleven detrital samples to better constrain the tectonic evolution of the YTT. One Snowcap assemblage sample is characterized by Proterozoic and Archean zircons. Finlayson assemblage samples are dominated by mono-peaks around 360 Ma with a small percentage of mid-Devonian to Precambrian grains. Two Klinkit samples yield populations of 725 Ma, 2.0-1.7 Ga, 2.8-2.5 Ga, but the proportions of each group are variable. One Klinkit sample contains a young population of ca. 330 Ma zircons. Two samples of the Permo-Triassic Simpson Lake assemblage are dominated by Proterozoic and Archean grains with minor but significant populations at 720, 430, 355, and 260 Ma.

Current interpretations of YTT illustrate a mid to late Paleozoic continental arc/back-arc system recorded by multiple episodes of magmatism that developed upon a metasedimentary basement (Snowcap) inferred to have rifted off the Laurentian margin following opening of the Slide Mountain back-arc ocean. The detrital zircon record of the Devonian-Mississippian sequences is consistent with recycling of western Laurentian detritus, and syn-magmatic deposition indicated by prominent Devonian-Mississippian mono-peaks. Upper Ordovician to Lower Devonian zircons are only significant in Permo-Triassic clastic rocks of the Simpson Lake assemblage indicating limited interactions prior to latest Permian between the main YTT in Yukon and YTT-south and the Alexander terrane exposed in SE Alaska.