U/PB GEOCHRONOLOGY OF DETRITAL ZIRCONS IN QUARTZITES FROM THE SAWTOOTH METAMORPHIC COMPLEX, IDAHO

The Sawtooth metamorphic complex in central Idaho is an isolated exposure of high-grade metamorphic rocks surrounded by Cretaceous and Eocene plutonic rocks of the Idaho and Sawtooth batholiths. U/Pb geochronological data on detrital zircons from quartzites in the Sawtooth metamorphic complex have been utilized to constrain the depositional age and provenance of the paragneiss sequence. More than half of the U/Pb data from the detrital zircons are discordant due to regional metamorphism and, therefore, not used in defining age distributions. Zircons that give concordant U/Pb ages (less than 10% discordant) from biotite-quartzite samples show age distributions that are consistent with North American sources. The majority of the detrital zircons are Mesoproterozoic to Paleoproterozoic with peaks at ~1100 Ma and ~1800 Ma on KDE plots. Smaller age populations occur at ~2700 Ma, ~1750 Ma, and ~1430 Ma. The youngest concordant detrital zircon age from samples in this study is ~900 Ma and the oldest is ~3000 Ma. These data are consistant with and expand upon other recent detrital zircon studies from the SMC that indicate similar North America provenance and a maximum depositional age of about 510 Ma for the quartzites. The results suggest that at least some of the SMC paragneisses are lower Paleozoic in age and likely correlate with rift and shelf facies sequences in the northern Great Basin.