西华山花岗岩铷锶同位素组成及岩石成因的意义

西华山花岗岩为一多阶段复式岩体。不同阶段花岗岩的铷锶含量及同位素组成有显著差异。其中Rb和Sr~(87)趋向于在晚阶段花岗岩中富集。Sr的变化趋势与此相反。Rb、Sr的行为指示该岩体的主体是由岩浆作用形成,而第一阶段似斑状花岗岩可能和岩浆前锋流体相的交代作用有关。 岩体的Rb—Sr等时线年令为161.03Ma,Sr初始比为0.7193属典型的壳源花岗岩。

Rb and Sr Isotopic Composition of Xihuashan Granite and Its Petrogenetic Significance

Xihuashan granite is a poly-staged body. Their Rb, Sr contents and isotopic compositions are analysed. Granite of late stages is rich in Rb,Rb87 and Sr87 in comparison with the porphyroblastic granite of the first stage. Enrichment of these components is obviously related to magmatism, because of the incompatibility of Rb in magmatic process and the instability of radiogenic Sr87 in the crystal position formerly occupied by Rb87. Porphyroblastic granite which is poor in Rb and Sr87 is probably of non-magmatic genesis, this deduction is supported by other geological evidences.Rb-Sr isochron age of Xihuashan granite is 161.03 Ma, and the initial Sr87/Sr86 ratio is 0.7167.Calculation showes that the initial Sr isotope ratio of two samples is obviously lower than that of ther other samples. Geological meaning of this phenominon has been discussed in this paper.After getting rid of the two samples, initial Sr87/Sr86 ratio is recalculated as 0.7193, but the Rb-Sr isochron age almost remains the same.