大兴安岭北段新林区塔木兰沟组火山岩成因及地幔富集作用

Origin of Basalts of the Tamulangou Formation and Mantle Enrichment in Xinlin Area,Northern Greater Hinggan Mountains

本文报道了大兴安岭北段新林区塔木兰沟组玄武岩样品锆石U-Pb定年结果和岩石地球化学分析,探讨了塔木兰沟组玄武岩的成因及中生代岩石圈地幔的性质。锆石U-Pb定年结果显示,塔木兰沟组玄武岩形成于晚侏罗世(~153Ma),该玄武岩样品的SiO_2含量介于49.96%~59.06%之间,全碱含量介于5.21%~7.44%之间,Mg~#{100×n(Mg^(2+))/[(n(Mg^(2+))+n(Fe^(2+))]}值介于50~94之间。该玄武岩相对富集轻稀土元素(LREEs)、亏损重稀土元素(HREEs),La_N/Yb_N值介于10.7~20.0之间,具有轻微的Eu异常(δEu=0.80~1.09),结合区域研究成果,认为塔木兰沟组玄武岩形成于蒙古—鄂霍茨克洋闭合造山后岩石圈伸展环境,源区岩浆起源于石榴子石相二辉橄榄岩与尖晶石相二辉橄榄岩部分熔融的混合形成,含富含挥发分的金云母和角闪石等,暗示岩石圈地幔经历了交代作用的改造,交代类型以俯冲板片流体/熔体交代为主。结合区域已有研究表明,本区地幔交代富集作用来源于蒙古—鄂霍茨克洋的向南俯冲,塔木兰沟组火山岩形成于蒙古~鄂霍茨克洋闭合造山后岩石圈伸展背景。

Objectives：This paper presents zircon U-Pb ages dating results and geochemical data of the basalts of the Tamulangou Formation in the Xinlin area,north Greater Hinggan Mountains（Da Hinggan Mountains）,with the aim of constraining the origin of the basalts and the nature of the Mesozoic lithosphere mantle. Methods：Pulverization of the volcanic rock samples and the selection of zircons for analysis were undertaken at the Regional Geological Survey Research Institute based in Langfang City,Hebei Province,China.The samples were pulverized to 80～100 mesh,elutriated,and separated using the electromagnetic method.Zircons with good crystal shape,and lacking any significant inclusions or cracks,were selected using a binocular microscope.The preparation of the zircons for observation under transmitted and reflected light,cathode luminescence imaging,and U-Pb isotopic analysis,was completed in the Tianjin Institute of Geology and Mineral Resources,Tianjin,China. Results：The zircon U-Pb ages dating results indicate that basalts of the Tamulangou formed in the Late Jurassic （～153Ma）.They have SiO2 =49.96%～59.06%,Na2O＋K2O=5.21%～7.44%,and Mg^#={100 × n （Mg^2＋）/[（n （Mg^2＋）＋n （Fe^2＋）]}=50 ～94.These basalts are enriched in light rare earth elements （LREEs）,and depleted in heavy rare earth elements （HREEs）,LaN/YbN =10.7 ～20.0,and have weak Eu anomalies （δEu=0.80～1.09）. Conclusions：Combined with the studies on the regional geology,suggesting that the basalts of the Tamulangou Formation formed under the a lithospheric extension setting related to the closure of Mongol—Okhotsk Ocean,and were derived from partial melting of spinel lherzolite and garnet lherzolite.the source of this magma contain amphibole,phlogopite that rich in volatile,indicating that they had experienced mantle metasomatism.and that the metasomatizing agents are mainly subduction fluid/melt.Combined with the regional research results,we suggest that the mantle metasomatic fluid/melt could be originated from southern subduction of Mongol—Okhotsk Ocean crust,and these volcanic rocks formed under a lithospheric extension after the closure of Mongol—Okhotsk Ocean.