大兴安岭大洋山钼矿区侵入岩年代学、岩石地球化学特征及岩石成因

Geochronology, Geochemistry and Genesis of Igneous Rocks Associated with Dayangshan Mo Deposit in Great Xing’an Range

本文研究了黑龙江省大兴安岭东北部大洋山钼矿区中生代侵入岩的年代学和岩石地球化学特征,探讨了研究区中生代侵入岩的形成时代、岩石成因及构造背景。采用电感耦合-等离子体质谱仪(LA-ICP-MS)锆石U-Pb定年方法,获得细中粒二长花岗岩加权平均年龄分别为(177.3±2.6)Ma和(176.5±1.6)Ma,为早侏罗世;闪长玢岩加权平均年龄为(158.0±2.7)Ma,为晚侏罗世。岩石地球化学研究表明:细中粒二长花岗岩具高Si,富K和贫Fe、Mn、Mg等特征,大离子亲石元素Rb、K和LREE富集,高场强元素Nb、Zr、P、Th和HREE亏损,具有显著的Eu负异常(δEu=0.39),结合其低Cr、Ni、Co质量分数和低Mg#值(Mg#为27.25),指示壳源岩浆特征;闪长玢岩偏碱,富K和Na, Ca、Ti、Mg等元素质量分数中等,大离子亲石元素Rb、K、Sr相对富集,高场强元素Nb、Ta、Ti、Th和HREE相对亏损,具俯冲带弧岩浆岩的特征。综合研究表明,细中粒二长花岗岩和闪长玢岩的形成均受到北部蒙古—鄂霍茨克洋俯冲作用的影响,蒙古—鄂霍茨克洋残留部分在大兴安岭东北部俯冲闭合时间为晚侏罗世—早白垩世期间。

The geochronology and geochemistry of Mesozoic intrusions in the Dayangshan Mo deposit in the Northeastern part of the Great Xing’an Range are studied in this paper, and the petrogenesis and tectonic setting are discussed. The U-Pb dating of the medium fine-grained granite using(LA-ICP-MS) is(177.3 ±2.6) Ma and(176.5 ±1.6) Ma, namely the Early Jurassic. The dating of the diorite porphyry is(158.0 ±2.7) Ma, that is, the Late Jurassic. The study of geochemistry shows that the medium fine-grained granite is characterized by its high Si, rich K poor Fe, Mn, and Mg. It is depleted of HFSE(such as Nb, Zr, P, Th, and HREE), and rich in LILE(such as Rb, K, and LREE), with a significant negative Eu anomaly(δEu = 0.39). Combined with its low Cr, Ni, Co contents and low Mg#, the medium fine-grained granite came from a source of lower crust. The diorite porphyry is relatively rich in Na, K,and alkaline, and the contents of Ca, Ti, and Mg are medium. It is rich in LILE(such as, Rb, K, and Sr) and depleted of HFSE(such as, Nb, Ta, Ti, Th, and HREE), indicating the characteristics of arc magmatic rocks in subduction zone. Combined with previous studies, we believe that the formation of medium fine-grained granite and diorite porphyry is influenced by the subduction of the Mongolian-Okhotsk Ocean, and the closing time of the residual Mongolian-Okhotsk Ocean in the Northeast of Great Xing’an Range can be limited to the Late Jurassic-Early Cretaceous.