GEOCHRONOLOGY AND LITHOGEOCHEMISTRY OF VOLCANIC ROCKS OF THE AMBLER DISTRICT, SOUTHERN BROOKS RANGE, ALASKA

New U-Pb zircon ages and whole-rock lithogeochemical analyses of felsic and mafic metavolcanic units from the Arctic volcanic-hosted massive sulfide (VMS) deposit of the Ambler district, southern Books Range, Alaska further support previous correlations between these deposits and VMS deposits found within the Yukon-Tanana Terrane (YTT). U-Pb zircon analyses of four drill core samples from three aerially extensive porphyritic metarhyolite units were performed using the SHRIMP-RG at the USGS-Stanford analytical laboratory. These samples yielded 207Pb-corrected weighted mean 206Pb/238U ages of 376 ± 3, 376 ± 3, 376 ± 2, and 379 ± 3 Ma (2-sigma errors). X-ray fluorescence analyses of 23 whole-rock samples yielded two geochemically distinct populations. Felsic samples (n=21) have subalkaline rhyolitic-andesitic affinities and elevated high field-strength element (HFSE) and rare-earth element (REE) concentrations relative to the primitive mantle consistent with formation from a source that includes melt derived from continental crust. Mafic samples (n=2) have subalkaline basaltic affinities and low HFSE and REE concentrations relative to the primitive mantle consistent with formation from melt derived from undepleted mantle. The geochemical trends of the Arctic deposit metavolcanic rocks closely resemble those of volcanic units attributed to high temperature melting of continental crust and upwelling of enriched asthenospheric or lithospheric melts in an extensional back-arc basin environment. Metavolcanic units associated with VMS deposits in the Pelley Mountains district, the Finlayson lake district, the Bonnifield district, and the Nasina assemblage of the YTT also have Late Devonian-Mississippian U-Pb zircon ages, albeit somewhat younger, and lithogeochemical trends consistent with melting of both crustal and undepleted mantle sources. Observations from both the Arctic Alaska terrane and the YTT are consistent with an extensional back-arc basin setting along the western margin of North America from the late Devonian until at least the early Mississippian.