Internal structures and U-Pb ages of zircons from a tuff layer in the Meishucunian formation, Yunnan Province, South China

摘要

The Precambrian/Cambrian (PC/C) boundary is one of the most important intervals for the evolution of life, represented by prominent biological evolution from the first appearance of soft-bodied animals from the late Neoproterozoic to the sudden diversification of animals with mineralized skeletons in the Cambrian. In South China several areas contain many fossils and are well exposed, suitable for the investigation of PC/C boundary. However, geochronological relationships are still poorly known because of lack of combined detailed investigations of internal structures of zircons and in-situ U-Pb dating. We focus on the internal structure of zircons from a tuff layer within Bed 5 in the Meishucun section on which we undertook cathodoluminescence (CL) imaging and in-situ U-Pb dating with LA-ICP-MS and nano-SIMS. Over 600 zircons from the tuff layer were classified into three types based on their CL images: oscillatory rims, inherited cores and dull structures. U-Pb dating of the internal structure of the zircons by LA-ICP-MS clearly shows a distinct unimodal age population dependent on the structure: 531 +/- 17 Ma for the oscillatory rims and 5 +/- 15 Ma for the dull structures. The clear oscillatory zonation, the prismatic morphology, and their occurrence indicate that the oscillatory rims were formed from felsic magmatism, and that the U-Pb nano-SIMS age of 536.5 +/- 2.5 Ma records the depositional age of the tuff. Our results indicate that the PC/C boundary is situated below Bed 5, and therefore the bottom of Zone I (Marker A) is more appropriate for the PC/C boundary than is the top of Zone I (Marker B). The age of a positive anomaly (P2) in the early Cambrian is estimated to be ca. 536 Ma.