铅同位素动力学模型及其在示踪花岗岩成因中的应用

LEAD ISOTOPE DYNAMIC MODELS AND THEIR APPLICATION TO GRANITIC GENESIS TRACING

介绍了壳幔铅混合模式和铅连续生长模式 ,并首次将这两个模式推广到花岗岩成因研究中。通过对世界上一些花岗岩体中长石铅同位素数据的壳幔混合模式计算 ,发现在约 3 1Ga时曾发生过一次全球性的大规模U ,Pb分异事件。将铅连续生长模式计算结果与其它一些一致性信息对比发现 ,该模式对示踪花岗岩的源区时代非常敏感。对以地壳铅为主的花岗岩 ,其铅连续生长模式年龄与其成岩年龄相当 ;而对造山带花岗岩 ,两者之间的关系具有不确定性。对中国南、北秦岭和大别—苏鲁造山带花岗岩中长石铅同位素数据的计算结果对比表明 ,即使属于同一个造山带的岩体 ,其铅的来源和演化历史也存在较大的差别。此外 ,青岛崂山碱性花岗岩的各种铅同位素性质与中国东部其它碱性花岗岩体存在差别 ,但与苏鲁地体花岗岩表现的铅同位素行为一致 ,表明崂山碱性花岗岩的成因与苏鲁花岗岩体的成因具有更强的相关性。

Crust mantle Pb mixing model and continuous Pb evolution model are presented for the Pb isotope composition of ore lead previously.This paper introduces the two models and their application to the study of feldspar Pb in granites.The calculation by using the crust mantle Pb mixing model shows that a worldwide scale U,Pb differentiation event had taken place at about 3.1Ga.The continuous Pb evolution model is more sensitive for the recovering of granite source age as compared with the other concordant results.For the granites predominated by crust Pb,the continuous Pb evolution ages are consistent with their formation ages For granites in orogen,however,the relationship between the two ages is not definite For example,the calculation results and the comparison between the granites from South and North Qinling as well as Dabie and Sulu show that there are significant differences in Pb origin and evolutional history between them,even though they belong to the same orogen In addition,the Pb isotopic evolution for alkalic granite from Laoshan in Qingdao shows significant difference from other alkalic granites in the continental margin of East China,but it is consistent with that of the granites within the Sulu orogen This may imply that the alkalic granite at Laoshan has more closer genetic connection with the Sulu granites than the other alkalic granites in East China