云南晋宁梅树村剖面前寒武系—寒武系界线化石Sm-Nd同位素年龄测定

本文首次采用化石Sm-Nd等时线方法测定了前寒武系—寒武系界线的年龄。样品采自云南晋宁梅树村剖面的中谊村段磷块岩。根据4个软舌螺化石样品得出的年龄值为距今562.8±7.9Ma;由这4个样品和2个胶磷矿样品获得的年龄值为562.1±5.7Ma,这些数据表明,略低于中谊村段的前寒武系-寒武系界线参考点A的年龄距今约为570Ma,而位于中谊村段近顶部的界线参考点B的年龄距今约为560Ma。该结果为准确标定寒武系底界在国际地质年表中的位置提供了重要依据。     

中国前寒武纪—寒武纪过渡期海水Sm-Nd同位素特征及其意义

对我国3个前寒武系—寒武系界线剖面(云南晋宁梅树村剖面、四川峨嵋麦地坪剖面和新疆柯坪乌什剖面)地层中小壳化石和胶磷矿样品的Sm-Nd同位素测定表明,在前寒武纪—寒武纪过渡时期,这3个相距遥远的地区的海水是共洋的,海水的ε_(Nd)(T)值为-6.6±0.5.在距今610—500Ma期间,中国海水的ε_(Nd)(T)值在-5.7到-6.6之间,变化很小。研究结果证明,当时除古太平洋和古大西洋之外,还存在一个“古亚澳洋”,而中国海水属于“古亚澳洋”的一部分。样品测定的Sm-Nd模式年龄的为1.8Ga,它反映中国海水周围陆源区的平均年龄。     

Fossil Sm-Nd Isotopic Dating of the Precambrian-Cambrian Boundary in the Meishucun Section, Jinning, Yunnan, China

It was for the first time that the fossil Sm-Nd isochron method was used to date the Precambrian-Cambrian boundary. The samples were collected from the Zhongyicun Member in the Meishucun section of Jinning County,Yunnan Province, southern China. The date of 562.8±7.9Ma B.P. was obtained from four hyolithid fossil samples; another date of 562.1±5.7Ma B.P. was provided by four hyolithid fossil samples and two collophanite samples. These figures suggest that the Precambrian-Cambrian boundary reference point A, which is slightly below the Zhongyicun Member, might be about 570Ma old, and the reference point B, which is near the top of the Zhongyicun Member, could be approximately 560Ma in age. This result contributed valuable reference to precise calibration of the base of the Cambrian System in the global geological time scale.