震旦纪到早寒武世中国海水Nd同位素特征及变化  

Nd ISOTOPIC VARIATIONS OF CHINESE SEAWATER DURING THE SINIAN TO EARLY CAMBRIAN

作　　者：杨杰东[1] 薛耀松[2] 陶仙聪[1] 

机构地区：[1]南京大学现代分析中心 [2]中国科学院南京地质古生物研究所

出　　处：《地质论评》1995年第2期188-196,共9页Geological Review

基　　金：国家自然科学基金 (项目号:49373166和49070108)

摘　　要：系统采集了扬子区震旦系和下寒武统的沉积磷矿和沉积锰矿样品,测定其Sm-Nd同位素组成。测定结果表明,从晚元古代末到早寒武世,中国海水的ε_(Nd)(T)值在总体上呈下降趋势,从约-3.6下降到约-8.0;相反,Nd模式年龄逐渐增长。与这同一时期的古太平洋海水相比,中国海水的ε_(Nd)(T)值的变化幅度较小,并与古太平洋海水的值相分离。研究表明,在从晚元古代末到早寒武世,全球海洋地理格局曾发生重大变化。Samples of Sinian and Lower Cambrian sedimentary phosphatic rocks and manganese ore were system atically collected in the Yangtze region. China and analyzed for Sm-Nd isotopes. Analysis shows that the εNd(T) values of the Chinese seawater were -3- 6 to -3. 7 at the Nan-tuo ice age (corresponding to the Varanger ice age), falling within the εNd(T) range of Panthalassa and then decreased after the Nantuo ice age, when both began to be separated. By the Cambrian, the εNd(T) values of the Chinese seawater ranged between - 5. 7 and -8. 2 were obviously distinet from those ( - 10 to -20) of Pathalassa. It might be speculated that the Chinese seawater did not belong to Panthalassa proposed by Keto and Jacobsen (1988) but belonged to another ocean-Asiaustralia during the Neoproterozoic through the Cambrian. This presumption is coincident with the result of the investigation on the global biogeographic provinces in the Cambrian, as reported by Zhang (1989).The Nd model ages of the analyzed samples range from 1686 to 1928 Ma with a mean of about 1. 8 Ga, which is in agreement with the mean age (1. 8 Ga) of the continental crust today. The Nd model ages show a rise trend from the Nantuo ice age to Early Cambrian and corresponds to the fall trend of the εNd(T) values.The εNd(T) values of the Chinese seawater and Panthalassa both showed wide variations from the Neoproterozoic to Early Cambrian but their variation ranges were obviously different. These all indicate that there occurred a major geological change during the important transformation period from the Cryptozoic to Phanerozoic.

关 键 词：同位素 古海洋 元古宙 海水 钕同位素 

分 类 号：P734.21[天文地球—海洋化学]