TWO-FOLD DIVISION OF YUKON-TANANA TERRANE IN EAST-CENTRAL ALASKA INTO PARAUTOCHTHONOUS AND ALLOCHTHONOUS ELEMENTS AND THE NEED FOR CLARIFICATION OF TERMINOLOGY

We propose a two-fold tectonic division of rocks in east-central Alaska, historically referred to as the Yukon-Tanana terrane (YTT), on the basis of differing protolith age ranges and magmatic affinities. We infer a parautochthonous continental-margin setting for most Paleozoic assemblages in the western and southern Yukon-Tanana Upland (Lake George, Fairbanks-Chena River, Butte, Dan Creek, Blackshell) and the northern flank of the Alaska Range, based on quartz-rich metasedimentary rocks, Precambrian inherited or detrital zircons, radiogenic Sr and Nd isotopic compositions, and correlation of units north of Fairbanks with North American units across the Tintina fault in the Selwyn Basin. U-Pb zircon crystallization ages are Late Devonian-Early Mississippian (~378-360 Ma; a few as young as ~345 Ma). Magmatism was bimodal. Most mafic metaigneous rocks have within-plate geochemical signatures, as do peralkaline metarhyolites associated with VHMS and SEDEX prospects; other felsic metaigneous rocks were largely derived from continental crust. We interpret magmatism in these assemblages as a result of attenuation of the continental margin due to east-dipping subduction. In contrast, structurally higher rock assemblages in the northeastern Yukon-Tanana Upland (Chicken Metamorphic Complex; Fortymile River and Nasina assemblages) contain Mississippian (~360-341 Ma) metaigneous rocks with largely arc and back-arc affinities. Late Permian felsic metaigneous rocks and Triassic to Jurassic plutons occur only within these assemblages. These rocks continue into Yukon as a belt of continental-margin rocks of arc and back-arc affinity that define a west-facing Late Devonian to Early Permian arc that rifted away from the North American continental margin. This allochthonous continental fragment lay outboard of the intervening ocean basin (Seventymile/Slide Mountain) and was accreted to the continent margin, including the parautochthonous Yukon-Tanana assemblages, during Late Permian to Early Triassic west-dipping subduction and Jurassic shortening. Continued use of the term "YTT" for both the inferred parautochthonous and allochthonous packages, without additional explanation of the rock assemblages involved and their inferred Phanerozoic history, will cause major confusion in the literature.