大别山地壳结构的Pb同位素地球化学示踪

Pb isotopic study on crustal structure of Dabie Mountains,central China

研究了大别山东部北大别变质杂岩、南大别变质杂岩和白垩纪花岗岩的全岩Pb同位素组成。结果表明大别变质杂岩与南大别变质杂岩相比,前者以相对低放射成因Pb同位素组成为特征。按照Pb同位素组成在地壳垂向剖面上的变化模型,指出在大别山地壳垂向结构上,北大别变质杂岩应位于南大别变质杂岩之下。这一认识得到大别山不同构造岩石单元中产出的白垩纪花岗岩Pb同位素对岩浆源区示踪的有力支持,因此南大别超高压变质带是发育在北大别杂岩之上的一个构造岩片。这对进一步确定扬子克拉通向华北克拉通俯冲碰撞的缝合线位置具有重要意义。

The whole-rock Pb isotopic composit ions of the North Dabie Complex(NDC),the South Dabie Complex(SDC)and the Cretaceous granites in the ea st Dabie Mountains are reported.The results show that the NDC(including orthogneiss and plagioclase amphibolite)is characterized by less radiogenic Pb isotopic compositions than those of the SDC(including ultrahigh pressure gneis s and eclogite).According to the variation model of Pb isotopic composition in crustal vertical section,it is suggested that t he NDC is beneath the SDC in the crusta l vertical structure of the East Dabie Mountains.This can be reinforced by Pb isotopic study of the Cretaceous granites exposed in different tecto nic units.The Cretaceous granites d isplay a fairly uniform low radiogen ic Pb isotopic composition,irrespectiv e of their rock type and tectonic unit.Their Pb isotopic ratios overlap th ose of the NDC and are distinguishable from those of the SDC,indicating that magma s ource of the Cretaceous granites is similar to the exposed NDC rather than the SDC.Therefore,Pb isotopic pro bing of the Cretaceous granites reveals that the SDC(ultrahigh pressure metamorphic uni t)is a tectonic slice,which overlies t he NDC.