【摘要】 蒙古–鄂霍茨克缝合带在东亚大陆形成演化历史上占有极为重要的位置,但对其南向俯冲的演化历史还存在较多争议。本文对蒙古–鄂霍茨克缝合带南缘的额尔古纳地块乌尔根地区花岗斑岩进行了锆石U-Pb年代学、Lu-Hf同位素和全岩主量、微量元素以及Sr-Nd同位素的分析研究,在此基础上限定了蒙古–鄂霍茨克洋在额尔古纳地块的闭合时间。锆石U-Pb定年结果显示,花岗斑岩侵位年龄为174.9±1.1 Ma,是早侏罗世晚期岩浆事件的产物。花岗斑岩属于过铝质高钾钙碱性岩石,其SiO₂、Al₂O₃和Sr含量高,而Y和Yb含量较低,类似于典型埃达克岩的地球化学特征。该岩石富集轻稀土和大离子亲石元素,强烈亏损重稀土和高场强元素,且无明显Eu负异常。岩石具有相对亏损的Sr-Nd同位素组成,其(⁸⁷Sr/⁸⁶Sr)_i=0.706665～0.707989ε_Nd（t）=1.22～4.18。锆石ε_Hf（t）=5.3～12.0,二阶段模式年龄(t_DM2)为451～879 Ma。这些特征揭示乌尔根地区花岗斑岩岩浆是由年轻的加厚下地壳部分熔融形成,而非俯冲板片熔融。结合前人研究成果以及区域地质演化规律,认为乌尔根地区花岗斑岩形成于蒙古–鄂霍茨克洋闭合后的碰撞向造山期后发展的转折时期。更多还原

【Abstract】 The Mongolia-Okhotsk suture zone plays an extremely important role in the formation and evolution of the East Asian continent.However,the study on the evolution of its southward subduction is still very weak and exits many controversies.In this paper,the LA-ICP-MS zircon U-Pb age and Lu-Hf isotopes,whole rock major and trace elements and Sr-Nd isotopic compositions are analyzed for the granite porphyries in the Ulgen area of the Erguna Block on the southern margin of the Mongolian-Okhotsk suture zone,and the closure time of the Mongolia-Okhotsk Ocean is constrained.The results indicate that the granite porphyry in the Ulgen area was emplaced in the EarlyMiddle Jurassic （174.9±1.1 Ma）.The granite porphyry belongs to the peraluminous and high-K calc-alkaline series with high SiO₂,Al₂O₃,and Sr contents,and low Y and Yb contents,similar to typical adakitic rocks.The granite porphyry samples are enriched in LREE and LILE （such as Rb,Th,U）,depleted in HREE and HFSE （such as Ta,Nb）,with significant differentiation between LREE and HREE and weak Eu negative anomalies （δEu=0.78–0.88）,and also characterized by low (⁸⁷Sr/⁸⁶Sr)_i （0.706665–0.707989） and a positive ε_Nd（t）（1.22–4.18）.The zircons haveε_Hf（t） values ranging from 5.3 to 12.0 and two-stage model ages (t_DM2) of 451–879 Ma.These characteristics reveal that the adakitic granite porphyry was formed by partial melting of the young thickened lower crust rather than subducted slab melting.Combined with previous research results and the regional geological history,we propose that the granite porphyry in the Ulgen area formed in the transition period from the collision to the post-orogenic stage after the closure of the Mongolia-Okhotsk Ocean.更多还原