孙吴地区中侏罗世白云母花岗岩的年代学与地球化学:对蒙古-鄂霍茨克洋闭合时间的限定

Geochronology and geochemistry of muscovite granite in Sunwu area,NE China: Implications for the timing of closure of the Mongol-Okhotsk Ocean

本文对孙吴地区白云母花岗岩行了锆石LA-ICP-MS U-Pb年代学和岩石地球化学研究,确定其形成时代、岩石成因,从而揭示区域构造背景。孙吴地区白云母花岗岩中的锆石均呈半自形-自形,振荡生长环带明显,暗示其岩浆成因。测年结果显示,白云母花岗岩形成于168Ma,为中侏罗世岩浆事件的产物。岩体具有高硅(Si O2=74.61%~80.16%)、富铝(Al2O3=10.59%~13.90%)、贫铁(Fe2O3=0.11%~0.3%)等特征,在化学上属于准铝质-过铝质(A/CNK=0.97~1.13)系列。孙吴地区白云母花岗相对富集轻稀土元素(LREEs)和大离子亲石元素(LILEs),亏损重稀土元素(HREEs)和高场强元素(HFSEs)。锆石的εHf(168Ma)=+7.53^+11.66,二阶段的模式年龄(tDM2)变化于595~966Ma之间。上述特征表明,孙吴地区中侏罗世白云母花岗岩的岩浆起源于新增生加厚陆壳物质的部分熔融。结合在大兴安岭与小兴安岭衔接地区变质杂岩和冀北-辽西地区区域性地层不整合面的存在,表明大兴安岭西坡-冀北-辽西地区中侏罗世经历了一次与蒙古-鄂霍茨克洋闭合有关的陆壳加厚过程。因此,研究区内中侏罗世白云母花岗岩的形成应与蒙古-鄂霍茨克洋闭合过程中的陆-陆碰撞作用有关,而与古环太平洋向欧亚大陆的俯冲作用无关,其形成时代也限定额尔古纳地块西北部蒙古-鄂霍茨克洋的闭合时间应为中侏罗世。

Zircon U-Pb dating and whole-rock geochemical analyses are undertaken for the muscovite granite in Sunwu area, NE China, with the aim of constraining its formation time, petrogenesis and the regional tectonic setting. Zircons from the muscovite granite in the study area are euhedral-subhedral in shape, and display fine-scale oscillatory growth zoning, indicating a magmatic origin. LAICP-MS zircon U-Pb dating indicates that the muscovite granite formed in the Middle Jurassic （ - 168Ma）. The muscovite granites have SiO2 = 74. 61% - 80. 16% , Al2O3 = 10. 59% - 13.90% , Fe2O3 = 0. 11% - 0. 30%, and belong chemically to metaluminous- peraluminous series （ A/CNK = 0. 97 - 1.13 ）. In addition, these muscovite granites are enriched in light rare earth elements （LREEs） and large ion lithophile elements （LILEs）, and depleted in heavy rare earth elements （HREEs） and high field strength elements （HFSEs）. The εHf（t） values and tDM2 of zircons range from ＋7.53 to ＋ 11.66 and from 595Ma to 966Ma, respectively, indicating that the primary magma was derived from partial melting of a juvenile thickened lower crust. Combined with the metamorphic complexes in the conjunction of the Great and Lesser Xing＇ an Ranges and the regional stratigraphic unconformity in western Liaoning and northern Hebei provinces, it is suggested that a significant collisional and thrusting event took place during Middle Jurassic, implying the closure of the Mongol-Okhotsk Ocean. Taken together, we conclude that the muscovite granite in the study area formed in a compressing collisional setting, which is related to the closure of the Mongol-Okhotsk Ocean instead of the subduction of the Paleo- Pacific oceanic plate beneath the Eurasian continent.