济阳拗陷第三纪玄武岩的Nd-Sr同位素研究

Study on Nd-Sr isotopic compositions of Tertiary basalts from Jiyang depressional basin.

本文报道了济阳拗陷29个第三纪玄武岩的Nd,Sr同位素组成。结果表明,该区早、晚第三纪玄武岩的Nd-Sr同位素组成变化较明显且具有一定的区别:早第三纪玄武岩的εNd值为-2.4～3.3,87Sr/86Sr比值为0.70481～0.70930;晚第三纪玄武岩的εNd值为0.1～2.3,87Sr/86Sr比值为0.70421～0.70530。鉴于εNd与1/Nd及87Sr/86Sr与1/Sr之间不存在相关特征,Nb正异常以及SiO2与MgO,Fe2O3+FeO,P2O5呈负相关,与Al2O3呈正相关,但与K2O不存在相关特征,因此,地壳混染作用并不是第三纪玄武岩同位素组成变化的主要原因。玄武岩87Sr/86Sr比值的升高是由热液蚀变造成的,而εNd值的变化则归因于源区混合。如果热液蚀变作用没有发生,这些玄武岩的所有数据点在Nd-Sr相关图上将可能位于地幔系列内部。这表明第三纪玄武岩主要是由DMM和EMI两个端员组分不同程度混合形成,EMII的贡献是次要的。

This paper reports Nd-Sr isotopic compositions of twenty nine samples of Tertiary basalts from Jiyang depressional basin. It is indicated that the early- and late-Tertiary basalts have different Nd-Sr isotopic compositions with evident variation. epsilon(Nd) and Sr-87/Sr-86 values of the early Tertiary basalts vary from -2.4 to 3.3 and from 0.70481 to 0.70930 respectively, while epsilon(Nd) and Sr-87/Sr-86 values of the late Tertiary basalts range from 0.1 to 2.3 and from 0.70421 to 0.70530. There is no correlation between epsilon(Nd) and 1/Nd and between Sr-87/Sr-86 and 1/Sr of the basalts. The SiO2 contents of the basalts are negatively correlated with contents of Mgo, Fe2O3 + FeO and P2O5, and positively correlated with Al2O3, but not correlated with K2O. These basalts also have positive Nb anomaly on the spider diagram. Therefore, variations of isotopic composition of the basalts may have not been caused by crustal contamination. Increases of Sr-87/Sr-86 of the basalts were caused by hydrothermal alteration, but variations of epsilon(Nd) values may have been attributed to source mixing. On the Nd-Sr correlogram, all data points are should plot inside the mantle array, if hydrothermal alteration of basalts did not occur. This suggests that the Tertiary basalt was formed by mixing process between DMM and EMI to different degrees; with minor contribution from EMII.