Isotopic evidence for the sources of Cretaceous and Tertiary granitic rocks, east-central Alaska: implications for the tectonic evolution of the Yukon-Tanana Terrane
Jn. Aleinikoff et al., Isotopic evidence for the sources of Cretaceous and Tertiary granitic rocks, east-central Alaska: implications for the tectonic evolution of the Yukon-Tanana Terrane, CAN J EARTH, 37(6), 2000, pp. 945-956
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.