内蒙古不同时代花岗岩类Nd、Pb同位素特征及其地质意义

Nd-Pb Isotopic Composition Characteristics and Geological Significances of Granitoids with Different Ages in Inner Mongolia

内蒙古地区连接兴蒙造山带和华北板块两个具不同演化历史、不同性质的大地构造单元。通过分析内蒙古地区华北板块和兴蒙造山带不同位置出露的从太古宙至中生代晚期连续分布的30件花岗岩类样品的Sm-Nd同位素及Pb同位素,对比了华北板块与兴蒙造山带不同时代花岗岩类的Nd、Pb同位素性质的差异,揭示了两个构造单元的演化过程。华北板块花岗岩的Nd模式年龄分布于中元古代前,从不同时代花岗岩的Nd同位素特征可见华北板块具有太古宙、晚古生代和早中生代(三叠纪)3期地壳增生历史;兴蒙造山带花岗岩的Nd模式年龄出现于古元古代之后,集中于新元古代和早古生代,其花岗岩的Nd同位素特征表明从新元古代至中生代,该区均具有较显著的地幔物质的注入。兴蒙造山带花岗岩类初始Pb同位素含量总体高于华北板块。晚古生代华北板块花岗岩类可能接受兴蒙造山带物质的直接或间接的贡献。至中生代,华北板块和兴蒙造山带作为一个统一的整体演化。

Inner Mongolia connects two different tectonic units with distinct evolution histories,i.e.the Xing-Meng Orogenic Belt and the North China Block together.Through studies of the Sm-Nd and Pb isotopes of 30 granitoids with different ages from various locations,the Nd and Pb isotopic differences of the Xing-Meng Orogen and the North China Block are discussed to reveal their evolution processes.The Nd model ages of granitoids in the North China Block are pre-Mesoproterozoic.The Nd isotopic characters show three crustal accretion stages of the North China Block during the Archean,Late Paleozoic,and Early Mesozoic（Triassic）.The Nd model ages of granitoids in the Xing-Meng Orogen are after Paleoproterozoic,which focus in Neoproterozoic and Early Paleozoic.The Nd isotopic features indicate a continuous injection of mantle material from Neoproterozoic to Mesozoic.The initial Pb isotope ratios of granitoids in the Xing-Meng Orogen are generally higher than that of the North China Block.In the North China Block in Inner Mongolia,the granitoids that crystalled in Late Paleozoic could contain material from the Xing-Meng Orogenic Belt directly or indirectly,while in Mesozoic,the North China Block and the Xing-Meng Orogen were jointed together and evolved as a whole.