Geochronological constraint on the Cambrian Chengjiang biota, South China

摘要

The Cambrian Chengjiang biota of South China provides compelling fossil evidence for the rapid appearance of metazoan phyla in Earth history (the 'Cambrian explosion'). However, the timing of the Chengjiang biota is poorly constrained owing to lack of datable rock materials within the Maotianshan Shale that yields the fossils. Here we integrate secondary ion mass spectrometry (SIMS) and chemical ablation isotope dilution thermal ionization mass spectrometry (CA-ID-TIMS) U-Pb analyses of detrital zircons fromthe Maotianshan Shale to provide high-precision geochronological constraint on the Chengjiang biota. The youngest group of SIMS U-Pb detrital zircon dates yields an age peak at 520 Ma. Six zircons from this group are further dated by the CA-ID-TIMS U-Pb technique, but suggesting that they were not formed from a single zircon growth event. Thereby neither the age peak nor the weighted mean age defined by the youngest SIMS U-Pb dates could represent the maximum depositional age of the Maotianshan Shale. Instead, the youngest CA-ID-TIMS U-Pb date, 518.03 +/- 0.69/0.71 Ma (2s, analytical uncertainty/incorporates U-Pb tracer calibration uncertainty), provides the first robust maximum age of the Chengjiang biota. This new geochronological constraint on the Chengjiang biota indicates that the Cambrian explosion reached its major phase around 518.03 +/- 0.69/0.71 Ma, demonstrating a protracted process ( 22 myr) for the Cambrian explosion.