内蒙古中—东部地区基底岩石、花岗岩和玄武岩的Nd同位素组成及其对该区地壳演化的指示

对出露于内蒙古地区的华北地台北缘中段及兴蒙造山带内共21件不同岩性的样品进行Nd同位素研究。这些样品的Nd模式年龄值表明:兴蒙造山带与华北地台具完全不同的特征,兴蒙造山带以年轻的亏损地幔模式年龄为特征(tDM=0.4~1.1 Ga),普遍低于华北地台西段的tDM值(1.8~3.4 Ga)。锡林浩特地块作为独立块体具与兴蒙造山带不同的特征,锡林浩特地块的亏损地幔模式年龄介于兴蒙造山带年龄与华北地台年龄之间。Nd模式年龄计算结果表明内蒙古地区华北地台北缘的地壳增生事件主要集中于中元古代之前,而兴蒙造山带地壳增生事件自新元古代开始。通过对内蒙古地区华北地台北缘εNd(t)值随时间的变化分析可知,在中元古代及海西期均存在古老地壳的再循环及新地壳的增生事件。内蒙古兴蒙造山带地壳增生速率表明该区地壳主要增生事件发生于1 000~700 M a,其后形成的岩浆岩所反映的增生过程表明有古老地壳组分的参与。

Nd Isotopic Compositions of Basement Rocks, Granitoids and Basalts from Middle-Eastern Inner Mongolia: Implication for the Regional Crustal Evolution

Twenty one samples,including basement rocks,granitoids and basalts from the west segment of the North China Craton and the Xing-Meng Orogen,Inner Mongolia were chosen to study their Nd isotopic compositions.Through the comparison of their Nd depleted mantle model ages,it is indicated that the Xing-Meng Orogen and North China Craton show overall different features.The Nd depleted mantle model ages of the rocks from the Xing-Meng Orogen are generally lower than those in the west segment of the North China Craton in Inner Mongolia.The Nd depleted mantle model ages of the Xilinhot Block are between the ones in the North China Craton and the Xing-Meng Orogen.The samples'tDM values indicate that the accretion of the North China Craton in Inner Mongolia primarily occurred before Mesoproterozoic;in contrast,the accretion of the Xing-Meng Orogen in Inner Mongolia predominantly began at Neoproterozoic.The evolution diagram of εNd values with time from the north margin of the North China Craton suggests that both reworking of old crust and crustal accretion dominated the regional crust in Mesoproterozoic and Hercynian periods.The accretion rate of the Xing-Meng Orogen shows that its crustal accretion primarily occurred at 1,000-700 Ma and the later formed igneous rocks show the involvement of the old crustal components.