大兴安岭中北段老道口闪长岩锆石U-Pb年龄、地球化学特征及构造意义

SIMS Zircon U-Pb Age,Petrogeochemistry and Its Tectonic Implication of Laodaokou Diorite in the Mid-North Part of Great Xing'an Range

鄂伦春自治旗老道口闪长质岩体位于兴蒙造山带东段、大兴安岭中北段,原定义为老道口单元的老道口岩体主要由闪长岩、石英二长岩和闪长玢岩组成。锆石SIMS U-Pb定年测定闪长岩年龄为（126.09±0.95）Ma,属于早白垩世岩浆活动的产物,并非前人认为的形成于寒武纪。闪长岩的w（SiO_2）为56.13%～57.91%,w（TiO_2）为0.97%～0.99%,w（MgO）和w（TFe_2O_3）分别为2.00%～2.12%和6.73%～7.41%,Mg#值为36.00～38.00,w（Na_2O）为4.29%～4.53%,w（K2O）为1.38%～1.59%,K_2O/Na_2O为0.31～0.37,w（Al_2O_3）为17.95%～18.36%,相对富钠高铝,稀土元素球粒陨石标准化曲线显示其具有富含轻稀土元素（LREE）、贫重稀土元素（HREE）以及Eu（0.85～0.87）弱负异常的特点。老道口闪长岩强不相容元素Th、U、K显著富集和高场强元素Nb、Ta明显亏损,其岩浆源区可能为俯冲流体交代的岩石圈地幔。闪长岩富集Ba、K、Rb、Th、U等大离子亲石元素,高场强元素Nb、Ta、Ti、P等明显亏损,指示了与俯冲作用相关的地球化学特征。结合老道口岩体地球化学特点和区域构造背景,认为该岩体很可能形成于蒙古—鄂霍茨克洋闭合造山后的岩石圈伸展构造环境。

Laodaokou unit is located in Oroqen Autonomous Banner,Inner Mongolia,the east of Xing＇an Mongolian orogenic belt,northern Great Xing＇an Range.The unit is mainly composed of diorite,adamellite,and dioritic porphyrite.SIMS U-Pb dating of zircon of the diorite provides the age of（126.09±0.95）Ma,indicating that the plution was formed in Early Cretaceous instead of Cambrian as thought before.The major element data show that the diorites contains 56.13%-57.91% of SiO_2,0.97%-0.99% of TiO_2,2.00%-2.12% of MgO,6.73%-7.41% of TFe_2O_3,36.00-38.00 of Mg#,4.29%-4.53% of Na_2O,1.38%-1.59% of K_2O,17.95%-18.36% of Al_2O_3.The K_2O/Na_2O ratios range from 0.31 to 0.37.The diorites are rich in alumina and sodium relatively.The rare earth elements chondrite standardized curves show that they are rich in light REE and depleted of heavy REE with weak negativeδEu（0.85-0.87）.Laodaokou plution is rich in large ion lithophile element（LILE）（e.g.,Ba、K、Rb、Th、U）,but depleted of high field strength elements（HFSE）（e.g.,Nb、Ta、Ti、and P）.The geochemical signatures above imply a characteristic related to subduction,and this suggests that the magmatic source of Laodaokou diorites were from the lithospheric mantle,which was metasomatosed by subduction fluid.Combining with the geochemical characteristics of Laodaokou plution and its regional tectonic settings,we conclude that the plution might be formed under a lithospheric extension after the closure of Mongolia-Okhotsk Ocean.