ID-TIMS U-PB DETRITAL ZIRCON GEOCHRONOLOGY OF NEOPROTEROZOIC SEDIMENTARY SUCCESSIONS IN THE SOUTHWESTERN UNITED STATES

The correlative Neoproterozoic Chuar Group (Grand Canyon, AZ), Uinta Mountain Group (northeastern UT), and (middle) Pahrump Group (Death Valley, CA) indicate deposition in intracratonic basins during the rifting of Rodinia and prior to snowball Earth conditions. These units have yielded data indicating biotic change and large-scale agitations in the carbon cycle. Recent LA-ICPMS data of detrital zircons have constrained the maximum depositional age of these groups to range between 787-740 Ma. These previous findings provide the foundation for the hypothesis that the feedbacks responsible for "snowball Earth" conditions had been set in motion by ca. 750 Ma. We will improve the age constraints on these units by subjecting previously identified detrital zircon grains of this age range to ID-TIMS analysis, which provides more precise ²⁰⁶Pb/²³⁸U ages. Preliminary data suggest a significant reduction of errors on these previously identified grains from in excess of 12 million years to within a half-million years. These higher precision ages will refine the estimates for the timing of intracratonic basin formation and associated nascent rifting of Rodinia, and contribute to our understanding of how pre-snowball Earth conditions contributed to the onset of the Cryogenian.