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. (c) 2006 Elsevier
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