NEW U-PB ZIRCON AND GEOCHEMICAL EVIDENCE FOR BIMODAL MID-PALEOZOIC MAGMATISM AND SYNGENETIC BASE METAL MINERALIZATION IN THE YUKON-TANANA TERRANE, ALASKA

New SHRIMP U-Pb zircon ages and trace-element geochemistry for mafic and felsic metaigneous rocks of the Yukon-Tanana terrane (YTt) help define the tectonic setting of magmatism, basinal deposition, and base-metal mineralization within the ancestral western margin of North America. We compare data from metaigneous rocks in the Bonnifield Mining District (BMD) in the Alaska Range with presumed equivalents 150 km to the NE in the Big Delta 1°x3° quadrangle (BDq) in the Yukon-Tanana Upland; both areas also contain mid-Paleozoic siliciclastic and carbonaceous metasedimentary rocks. Syngenetic Zn-Pb-Ag mineral deposits or occurrences are hosted by metarhyolites of the Mystic Creek Member of the Totatlanika Schist in the BMD and by carbonaceous metasiltites in the BDq. Metarhyolites from the Mystic Creek Member have U-Pb zircon ages of ~372±3 Ma and have high Zr, Nb, Ta, Y, and Yb concentrations indicative of a peralkaline composition and a within-plate origin. Metabasalts from the immediately underlying Chute Creek Member also have alkalic within-plate immobile element geochemistry. Felsic metaigneous rocks from the two members that underlie the Chute Creek have arc geochemistry and zircon ages of 370±6 (upper member) and 381±3 Ma (lower member). In the BDq, zircons from two areas within a 700 sq km augen gneiss body yield U-Pb ages of 388±3 and 362±3 Ma. A 1000-ft drill hole into the body encountered biotite- and augen gneiss interlayered with ~10 intervals of amphibolite and mafic gneiss. Zircons from a 125-ft-thick amphibolite have magmatic habits and give a U-Pb age of 361±3 Ma. Trace element abundances of the dated amphibolite plot in or near the calc-alkalic/volcanic arc field, as do those of most 360-390 Ma felsic metaigneous rocks from the BDq. Most other amphibolites and greenstones in the BDq, and two metarhyolites (374±7 and ~350 Ma) from within the carbonaceous metasiltite unit, have alkalic, within-plate geochemistry. Our data indicate that the extensive Devonian-Early Mississippian sequence of the northern YTt formed in a continental-margin arc that experienced periods of extension accompanied by bimodal volcanism, local anoxic basinal deposition and syngenetic hydrothermal mineralization.