内蒙古呼伦贝尔早中生代花岗岩的年代学和岩石地球化学研究——对岩石成因和地球动力学机制的制约

Chronological and Petrogeochemical Studies on the Early Mesozoic Granites in Hulun Buir,Inner Mongolia:Constraints on the Genesis and Geodynamic Mechanism

内蒙古呼伦贝尔地处华北克拉通东北部,分布着大面积的中生代花岗岩,这些花岗岩对于研究华北克拉通岩石圈的演化、构造环境及动力学机制具有重要的意义。然而,前人对这些花岗岩的岩石成因、构造环境、侵位机制的研究较少。基于野外地质调查,对呼伦贝尔早中生代花岗岩开展了主量、微量元素成分和锆石U-Pb同位素测年等系统性的研究。锆石U-Pb同位素定年结果表明,这些花岗岩是早三叠世((251.1±1.2)Ma)和早侏罗世((187.4±1.1)Ma)岩浆活动的产物。早三叠世和早侏罗世花岗岩均为高钾钙碱性系列,富集轻稀土元素和大离子亲石元素(如Rb、Th、U、K),相对亏损Nb、Zr、Hf、P、Ti等高场强元素,与中国二长花岗岩平均分布模式相似。综合微量元素特征和地质研究成果表明,呼伦贝尔中生代花岗岩形成于壳源岩浆的结晶分异。早三叠世花岗岩侵入岩形成于板内拉张环境;早侏罗世花岗岩其形成的构造背景与陆壳间相互作用及洋壳(古亚洲洋)向陆壳(西伯利亚克拉通)的俯冲作用有关,即大洋俯冲-同碰撞环境,且俯冲作用混入了一定量的洋壳成分所致,同时高钾花岗岩的出现,也反映出古亚洲洋洋壳的俯冲碰撞作用,表明早中生代侵入岩可能形成于造山带(岛弧及活动大陆边缘)背景。本研究将为呼伦贝尔有色金属勘探提供重要的地质依据。

Hulun Buir,Inner Mongolia,is located at the northeastern part of the North China Craton,with a large area of Mesozoic granites.These granites are of great significance for studying the evolution,tectonic environment and dynamic mechanism of the lithosphere beneath the North China Craton.However,few have been studied on the genesis,tectonic setting and emplacement mechanisms of the monzonitic granites from the Hulun Buir.In this paper,based on field geological survey,systematic studies have been carried out on early Mesozoic granite from the Hulun Buir in terms of major and trace element compositions as well as zircon U-Pb isotope dating.Zircon U-Pb isotope dating shows that these granites were the product of magmatic activity in the Early Triassic((251.1±1.2)Ma)and Early Jurassic((187.4±1.1)Ma).Early Triassic and Jurassic granites belong to high-K calc-alkaline types.These granites are rich in light rare earth elements(LREEs)and large ion lithophile elements(e.g.,Rb,Th,U,K),but relatively depleted in high field strength elements(HFSEs)such as Nb,Zr,Hf,P,Ti,which are similar to the average distribution pattern of Chinese monzonitic granites.The incorporated results of the trace element characteristics and the geological research studies show that the Hulun Buir Mesozoic granites were formed in the crystallization differentiation of crust-derived magma.Early Triassic granite intrusive rocks were formed in intraplate extensional environments;the tectonic setting of early Jurassic granite formation was related to the interaction between continental crusts and the subduction of the oceanic crust(i.e.,Paleo-Asian Ocean)towards the continental crust(i.e.,Siberian plate),i.e.the oceanic subduction-collision environment,and the subduction was mixed with a certain amount of oceanic crust components.The occurrence of the high-potassium granite also suggested the subduction and collision of the oceanic crust of the Eurasian Ocean,implying that the Early Mesozoic intrusive rocks were probably formed in the orogenic belt(island arc and active continental margin).This study can provide an important geological basis for non-ferrous metal prospecting in the Hulun Buir.