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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。
The Eduochuan Autonomous Banner Laodaokou dioritic rock mass is located in the eastern segment of the Xingmeng orogenic belt and the central-northern section of the Greater Hinggan Mountains. The old-pass rock mass formerly defined as an old-crossing unit consists mainly of diorite, quartz monzonite and diorite porphyrite . The dating of zircon by SIMS U-Pb dating is (126.09 ± 0.95) Ma, belongs to the product of the Early Cretaceous magmatism, which was not thought to have been formed in the Cambrian. The content of w (SiO 2) is 56.13% -57.91%, w (TiO 2) is 0.97% -0.99%, w (MgO) and w (TFe 2 O 3) are 2.00% -2.12% and 6.73% -7.41% # Value is 36.00-38.00, w (Na2O) is 4.29% ~ 4.53%, w (K2O) is 1.38% ~ 1.59%, K2O / Na2O is 0.31 ~ 0.37 and w (Al2O3) is 17.95% ~ 18.36% The normalized curves of sodium aluminate and rare earth chondrule meteorites show that they have the characteristics of LREE, HREE and Eu (0.85-0.87) weak negative anomalies. The strong enrichment of Th, U, K and strong depletion of high field strength elements Nb and Ta in the Laodokou diorite are obviously depleted. The magma source area may be lithospheric mantle subducted by subduction fluid. Diorite is enriched in large ion lithophile elements such as Ba, K, Rb, Th, U and high depletion of high-field elements such as Nb, Ta, Ti and P, indicating the geochemical characteristics related to subduction. Combined with the geochemical characteristics and regional tectonic setting of the rock in Raojiaokou, it is believed that the rock mass is likely to form in the lithospheric extensional tectonic setting after the closure of the Mongolian-Okhotsk Ocean.