New U—Pb age for Precambrian —Cambrian boundary in the Meishucun section, China

Category Other
Group GSI.IR
Location International Geological Congress,oslo 2008
Author Wang, Yinxi۱; Zhang, Kaijun۲; Chen, Junyuan۳; Song, biao۴; Wang, Yuanyuan۵; Yang, Jiedong۶; Li, Huiming۶
Holding Date 07 October 2008

The Meishucun section is one of the most important and ideal sections, which can tell us the early evolutional history of animal on the Earth, the "Cambrian Explosive" in the world and the determining age of the international Precambrian-Cambrian boundary. By using a new technique of combing with the Cathodoluminescence (CL) graphs and Raman Spectroscope Analysis to choose the suitable samples[1] and based on the U—Pb zircon determination by SHRIMP, this cluster consists of 13 spots and the new more credible zircon 206Pb/238U weighted mean age 541.3±1.3(δ)Ma have been obtained in layer M5 of the betonies from the Zhongyicun Member of the Meishucun section, Yunnan, China. The layer M5 is just between two international Precambrian—Cambrian boundary stratotypes point A and B. The volcanogenic event occur between the first emergence of monophyletical evolution’s small shelly fossil assemblages of a phylum (i.e. the Anabarites-Protohertzina Zone) and the second emergence of polyphyletic evolution small shelly fossil and the vestige of limb animal assemblages of the polyphyletic phylum (i.e. the Anabarites-Protohertzina Zone, Siphogonuchites-Paragloborilus Zone, Phycodes Pedum, Sellanlichnus meishucunensis, Rusophycus and Chondrites). From the stratigraphy and the paleontology angle, the Oman section( J. E. Amthor,et al. 2003) is not the most ideal section(Sun Weiguo,et al.,1999). Because the most of the layer of M5 80% zircons exsit the metamictization[1] in the different degrees, now still there is not announced on the authentic U—Pb age data of M5 layer zircons ID—TIMS. Because the announced M5 layer SHRIMP U—Pb age data (W. Compston,et al. 1992; .R.J.F. Jenkins,. et al. 2002) lacks the studies of the mineralogy and the Raman spectrumit peaks, it is very indispensable that these U— Pb data quantity and reliability can be test again. So, we proposed that the zircons 206Pb/238U weighted mean age (541.3.0 ± 1.3Ma) of the bentonites within the layer M5 of the Meishucun section, dated by SHRIMP, may be the now best choice for the global criteria age of Precambrian—Cambrian boundary. It is very necessary that the date of 541.3Ma (the base of the Cambrian )will be received by GEOLOGIC TIME SCALE 2008.
[1]Wang Yinxi,et al.2007. Geological Review,Vol.53,No 1,22-30