USING DETRITAL ZIRCON TO CORRELATE AND RECONSTRUCT NEOPROTEROZOIC AND EARLY PALEOZOIC COMPONENTS OF PERI-LAURENTIAN TERRANES IN THE NORTHERN CORDILLERA AND ALASKA

Neoproterozoic and early Paleozoic metasedimentary successions are exposed throughout allochthonous and parautochthonous parts of the Yukon-Tanana terrane (YTT) in western Yukon and eastern Alaska. The successions are thought to be Laurentian in origin and are correlated across the Tintina fault with the Selwyn Basin of western Canada. The Wickersham unit, comprising low-grade “quartz-eye grit,” mudstone, and minor limestone of inferred late Neoproterozoic to Cambrian age, contains primarily Neoarchean and Paleoproterozoic detrital zircon that define bimodal age populations with broad peaks between 2.8 and 2.5 Ga, and 2.0 and 1.7 Ga. Rocks with this distinctive bimodal age signature are recognized in exposures spanning 700 km of central and eastern Alaska and as part of the Snowcap assemblage of the allochthonous Yukon-Tanana terrane. The Wickersham signature is similar to the Windermere Supergroup (“Sequence A”), consistent with a western Laurentian source. The Fairbanks Schist is a heterogeneous package of metasedimentary and metavolcanic rocks that is dominated by quartzite and quartz-muscovite schist and interpreted to be in thrust contact with the Wickersham. The Fairbanks Schist contains a wider range of Precambrian detrital zircon ages, with prominent populations between 1.6 and 0.9 Ga that distinguish it from the Wickersham. The detrital zircon age spectra suggest affinity with the McKenzie Mtn. Supergroup (“Sequence B”). The Chatanika assemblage includes eclogite-facies schist, amphibolite, quartzite, and small amounts of marble. It is interpreted to be in thrust contact with the underlying Fairbanks Schist, and protoliths are presumed to be Devonian or Mississippian. Chatanika detrital zircon age populations span 2.7 to 0.9 Ga and reflect a mixture of Windermere and McKenzie Mts. signatures, suggesting mixed western Laurentian sources and/or recycling of the Wickersham or Fairbanks successions. Comparison of detrital zircon U-Pb age spectra of representative units with each other and with presumed Laurentian equivalents provides key criteria for testing local and regional correlations and helps identify and interpret the peri-Laurentian components of the Cordilleran accretionary collage.