SM-ND ISOTOPIC AGE OF PRECAMBRIAN-CAMBRIAN BOUNDARY IN CHINA

By the new method of Sm-Nd isotopic dating on phosphatic small skeleta l fossils and collophanite minerals, the Zhongyicun Member of the earl iest Cambrian Meishucun Stage at Meishucun in Yunnan, southern China, has been dated at 562.8 +/- 7.9 Ma and 562.1 +/- 5.7 Ma. Another Sm-Nd age, 570.3 +/- 17.1 Ma, has been obtained with samples from the Zhong yicun Member in Yunnan and its stratigraphic equivalents in Sichuan an d Xinjiang. These data tend to suggest that the best age estimate of t he Precambrian-Cambrian boundary is very Likely within the range of 56 0-570 Ma. As biophosphates and sedimentary phosphates are widely distr ibuted in sequences of the Precambrian-Cambrian transition, the Sm-Nd isotopic method is recommended as an effective approach for precise da ting of the initial Cambrian boundary.