New Early to Middle Triassic U-Pb ages from South China: calibration with ammonoid biochronozones and implications for the timing of the Triassic biotic recovery.
Résumé
New zircon U–Pb ages are proposed for late Early and Middle Triassic volcanic ash layers from the Luolou and Baifeng formations (northwestern Guangxi, South China). These ages are based on analyses of single, thermally annealed and chemically abraded zircons. Calibration with ammonoid ages indicate a 250.6±0.5 Ma age for the early Spathian Tirolites/Columbites beds, a 248.1±0.4 Ma age for the late Spathian Neopopanoceras haugi Zone, a 246.9±0.4 Ma age for the early middle Anisian Acrochordiceras hyatti Zone, and a 244.6±0.5 Ma age for the late middle Anisian Balatonites shoshonensis Zone. The new dates and previously published U–Pb ages indicate a duration of ca. 3 my for the Spathian, and minimal durations of 4.5±0.6 my for the Early Triassic and of 6.6+0.7/−0.9 my for the Anisian. The new Spathian dates are in a better agreement with a 252.6±0.2 Ma age than with a 251.4±0.3 Ma age for the Permian–Triassic boundary. These dates also highlight the extremely uneven duration of the four Early Triassic substages (Griesbachian, Dienerian, Smithian, and Spathian), of which the Spathian exceeds half of the duration of the entire Early Triassic. The simplistic assumption of equal duration of the four Early Triassic subdivisions is no longer tenable for the reconstruction of recovery patterns following the end Permian mass extinction.