Isotopic evidence for the sources of Cretaceous and Tertiary granitic rocks, east-central Alaska: implications for the tectonic evolution of the Yukon-Tanana Terrane

Magnetotelluric traverses across the southern Yukon-Tanana terrane (YTT) re veal the presence of a thick conductive layer (or layers) beneath Paleozoic crystalline rocks. These rocks have been interpreted to be flysch of proba ble Mesozoic age, on the basis of the occurrence of Jurassic-Cretaceous fly sch in the Kahiltna assemblage and Gravina-Nutzotin belt flanking the YTT t o the southwest and southeast, respectively. The Pb, Nd, Sr, and O isotopes in Cretaceous and Tertiary granitic rocks that crop out throughout the YTT were measured to determine if these rocks do in fact contain a component o f flysch. Previous limited analyses indicated that the Pb isotopes of the g ranitic rocks could be a mixture of radiogenic Pb derived from Paleozoic cr ystalline rocks of the YTT with an increasing component of relatively nonra diogenic Pb with decreasing age. Our Nd, Sr, and O data, along with additio nal Pb isotope data, eliminate flysch as a likely source and strongly sugge st that the nonradiogenic end-member was derived from mafic rocks, either d irectly from mantle magma or by melting of mafic crust. The lack of a sedim entary component in the granitic plutons suggests either that the plutons d id not incorporate significant amounts of flysch during intrusion or that t he conductive layer beneath the YTT crystalline rocks is not flysch.