华南两类不同成因花岗岩的铅同位素组成特征

本文论述了华南两类不同成因花岗岩的铅同位素组成特征。不同类型的花岗岩,具有明显不同的铅同位素组成。改造型花岗岩相对富放射成因铅,其铅同位素比值较高,Pb/~(204)Pb>72;且相对富铀铅而贫钍铅,~(208)Pb/~(206)Pb<2.1。同熔型花岗岩则反之。利用有关的铅同位素图解和参数,可对花岗岩的成因类型进行划分。岩石的铅同位素组成与其成岩物质来源的关系,可从铀(钍)铅同位素的演化得到解释。研究结果表明,花岗岩的铅同位素组成特征,能够提供有关成岩物质来源的信息,从而作为划分花岗岩成因类型的又一个稳定同位素标志。

CHARACTERISTICS OF LEAD ISOTOPE COMPOSITION OF TWO-TYPES OF GRANITES IN SOUTH CHINA

This paper discusses the characteristics of lead isotope composition of transformation-type and syntexis-type granites proposed by Xu Keqin or S-type and I-type granites proposed by Chappell and White.In the paper, eleven granitoids in South China, whose genetic types are known, are studied and twenty-four lead isotopic data are obtained. Based on the study of lead isotope compositions of eleven granitoids, it can be seen that these two types of granites possess different lead isotope compositions.The transformation-type (or S-type) granites is relatively richer in radiogenic lead, with higher lead isotope ratios, Pb~*/~(204)Pb>72 (Pb~*=~(206)Pb+~(207)Pb+~(208)Pb). Moreover, it is richer in uraniumgenic lead but poorer in thoriumgenic lead, ~(208)Pb/~(206)Pb<2.1. The syntexis-type (or I-type) granites is poorer in radiogenic lead, with lower lead ratios Pb~*/~(204)Pb<72. In addition, it is poorer in uraniumgenic lead but richer in thoriumgenic lead, ~(208)Pb/~(206)Pb>2.1.Some diagrams of lead isotopes are used in the paper. By these diagrams, genetic types of granites can be discriminated effectively.Through above-listed studies it is stated that the char-acteristics of lead isotope composition of granites could give the information about resources of their rock-foirming material and therefore, could be used to divde different types of granites.