内蒙古北山造山带变辉绿岩和片麻状花岗岩锆石U-Pb年龄、Hf同位素组成及地质意义

北山造山带位于西伯利亚、哈萨克斯坦和塔里木三大板块交汇部位。通过对内蒙古北山造山带变辉绿岩和片麻状花岗岩的岩石学、岩石地球化学、锆石阴极发光结构、锆石SHRIMP U-Pb年龄、LA-MC-ICP MS锆石Lu-Hf同位素的多方面系统研究，并结合前人的研究成果，取得了如下主要认识：变辉绿岩具有略向右倾的稀土元素分布型式，Eu异常不明显，并亏损Ta、Nb等高场强元素；片麻状花岗岩稀土元素分布型式明显右倾，具Eu负异常与Nb、Ta、Zr、Hf、HREE亏损。变辉绿岩中的锆石主体结晶于114.8Ma±3.3Ma，为燕山期岩脉；片麻状花岗岩中锆石主体的结晶年龄为423Ma±4Ma，为加里东晚期的岩体，从而更正了过去对其年龄为古元古代或石炭纪的认识。变辉绿岩在岩浆侵位过程中同化混染了地壳组分，捕获了地壳中从中太古代到早白垩世的继承锆石。研究区从中太古代晚期至古元古代晚期，至少曾发生过5期壳幔分异事件，导致了地壳增生，并形成了该区古老的地壳基底。

The U-Pb ages and Hf isotopes of zircons in the metadiabase and gneissic granite,Beishan orogenic belt,Inner Mongolia,China and its significance

By means of integrated study, including petrology, trace element geochemistry, zircon cathodoluminescence images，zircon SHRIMP U-Pb dating and LA-MC-ICP MS zircon Hf isotopic composition of the metadiabase and gneissic granite in Beishan orogenic belt, Inner Mongolia, together with the previous results, following conclusions are drawn. The metadiabase is characterized by slightly right-inclined REE distribution pattern, without obvious Eu anomaly, but with depletion of HSFE, such as Ta, Nb in a primitive mantle-normalized spider diagrams. All the detected zircon domains are characterized by igneous origin with >0.1 values of Th/U ratio. The gneissic granite displays a right-inclined REE distribution pattern with negative Eu anomaly and depletion of Nb, Ta, Zr, Hf and HREE in a primitive mantle-normalized spider diagrams. The main zircon crystalline ages of the metadiabase are about 115Ma, which indicates its tectonic episode of Yanshanian. The main zircon crystalline ages of the gneissic granite are 423Ma, which suggests that the rock was intruded in Late Caledonian. Therefore, the two samples were not Paleo-proterozoic or Carboniferous rocks as suggested by previous researchers. The metadiabase assimilated crustal component during the upwelling/intrusion, indicated by the inheritance zircons with Mesoarchaean to Early Cretaceous ages. From Late Mesoarchaean to Late Palaeoproterozoic, there are at least five episodes of crustal accretion from the upper mantle in the region, which led to formation of the regional crustal basement.