GETTING YOUNGER WITH AGE: NEW ID-TIMS U-PB DETRITAL ZIRCON AGE CONSTRAINTS ON NEOPROTEROZOIC SEDIMENTARY SUCCESSIONS, SOUTHWESTERN U.S

The correlative Neoproterozoic Chuar Group (Grand Canyon, AZ), Uinta Mountain Group (northeastern UT), and (middle) Pahrump Group (Death Valley, CA), together referred to as ChUMP, 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 fluctuations in the carbon cycle. Knowing when these units were deposited provides insight into how Earth conditions contributed to the onset of the Cryogenian at 720 Ma. Recent laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) U-Pb geochronology of detrital zircons obtained from the bases of these units have constrained the maximum depositional age of these groups to ca. 787 Ma.

We have improved the maximum age constraints on these units by subjecting detrital zircon grains previously identified by LA-ICP-MS to high-precision isotope dilution thermal ionization mass spectrometry (CA-ID-TIMS) analysis, which provided more precise ²⁰⁶Pb/²³⁸U ages. These new data significantly reduce the errors on previously identified grains from in excess of 11 million years to within a million years, and reduce the maximum depositional ages of the ChUMP units by millions of years. Significantly, the same ca. 775 Ma zircon population is identified in multiple mineral separates from samples of both the Horse Thief Springs Formation of the Pahrump Group, and the Nankoweap Formation of the Chuar Group. Using these higher precision ages we have refined the estimates for the timing of intracratonic basin formation, associated nascent rifting of Rodinia, biotic diversification, and carbon cycling prior to the onset of global glaciation.