南京汤山寒武系-奥陶系界线地层地球化学特征

Geochemical characteristics across the Cambrian-Ordovician boundary at Tangshan,Nanjing

研究了南京汤山观音台组顶部、仑山组和红花园组底部的微量元素、稀土元素和碳、氧同位素,基于观音台组顶部白云岩中白云石和方解石δ^(18)O 值的相似性,认为它是在成岩作用初期由交代作用形成的准同生白云岩。依据同位素分析,所研究层段的碳酸盐岩主要形成于埋藏成岩环境,未经历大气淡化成岩阶段。依据地层多重划分概念修订了观音台组和仑山组的界线。依据碳同位素与有关地区对比探讨了寒武系与奥陶系界线,界线附近δ^(13)C 值仅见小的负偏移,反映这一时期海平面的升降幅度较小。

This paper deals with the isotopes(δ^(13)C and δ(18)O),trace elements of Late Cambrian-Early Ordovician age and the Cambrian-Ordovician boundary at Tangshan, Nanjing.Based on the similarity of δ^(18)O compositions occurring in dolomite and calcite in the dolostone at the upper part of the Kuanyintai Formation,it is possible that the dolomite is penecontemporaneous through alternation in early stage of diagensis.The isotopic analyses of dolomites and limestones from the upper part of the Kuanyintai For- mation,Lunshan Formation and the lower part of the Hunghuayuan Fomation suggest that these carbonates were mainly formed in the diagenic environment of burial stage. The weak anomalies of δ^(13)C of the Cambrian-Ordovician boundary exhibit that the change of sea level was not conspicuous during the latest Cambrian to early Ordovician.