ZIRCON LA-ICPMS AND CA-TIMS U-PB DATES OF LATE PALEOZOIC VOLCANIC TUFFS IN THE MIDLAND BASIN, WEST TEXAS

Radiometric ages of the upper Paleozoic strata in southwestern Laurentia are poorly constrained mainly because of the paucity of datable materials. Consequently, the correlation between the North America regional time scale and the International Geological Time Scale remains problematic. Recent discovery of volcanic tuffs in the Lower Pennsylvanian Atoka and lower Permian Wolfcamp B, Wolfcamp A and lower Spraberry units in the Midland Basin, USA, provides a great opportunity to fill this knowledge gap. Our zircon LA-ICPMS U-Pb dates show that the Maximum Depositional Ages (MDAs) of the Atoka, Wolfcamp B, Wolfcamp A and lower Spraberry units are 315.6±2.5 Ma, 285.5±3.7 Ma, 283.9±1.7 Ma, and 272.7±1.5 Ma, respectively, and the Peak Ages (PAs) are 327.4+3.1/-1.7 Ma, 286.7+8.8/-4.5 Ma, 283.5+4.8/-2.6 Ma and 276.2+3.4/-3.0 Ma, respectively. The MDA of each tuff unit was calculated using the weighted mean date of four or more of the youngest grains overlapping at 2σ uncertainty and the PA of each tuff unit was calculated as TuffZirc date of six or more grains within >95% confidence interval from the youngest peak. The MDAs may significantly underestimate the depositional ages of the tuffs because of Pb loss in the youngest zircon grains. Here we date the youngest zircon grains (N=3-5) in each tuff unit using CA-TIMS in order to test the potential influence of Pb loss on the MDAs and improve the precision of zircon U-Pb date for each unit. This study is the first to constrain the absolute ages of the upper Paleozoic strata in the Permian Basin, USA. The results can shed light on the correlation between the late Paleozoic North America regional time scale and the International Geological Time Scale. Comparing zircon CA-TIMS and LA-ICPMS dates has implications for how to better determine zircon LA-ICPMS MDAs.