摘要
红柳河-洗肠井蛇绿混杂岩带位于内蒙古北山造山带中部,呈北西西向展布,向东延伸至白云山-洗肠井蛇绿混杂岩带,向西延伸至牛圈子-红柳园蛇绿混杂岩带。在白云山地区,蛇绿混杂岩由不同类型的岩块与基质组成,基质具有分带性,南侧以砂板岩基质为主,北侧以蛇纹岩基质及绿泥钠长片岩基质为主,反映了俯冲增生杂岩的特征。其中基性熔岩大面积发育,岩性以绿泥钠长片岩及变质玄武岩为主,TAS及AFM图解显示基性熔岩具有拉斑玄武岩的特征,微量元素蛛网图中具有弱Nb-Ta负异常及弱Sr正异常,稀土元素配分曲线显示了轻重稀土基本未分馏的曲线型式,微量元素系列判别图解显示了类似于MORB-like玄武岩的地球化学特征。由此可见,白云山地区基性熔岩为MORB-like玄武岩,反映了蛇绿混杂岩形成于弧前的构造背景。此外,本次工作获得侵入基性熔岩中的斜长花岗岩锆石206Pb/238U加权平均年龄为519.8±2.1Ma,εHf(t)-t图解显示了斜长花岗岩为地幔分异的产物。微量元素蛛网图中Nb-Ta亏损、右倾的稀土配分曲线及Th/Yb-Nb/Yb图解反映了斜长花岗岩具有岛弧岩浆岩的特征。对比研究区晚寒武世岛弧钙碱性辉长岩,我们认为白云山地区早寒武世发育MORB-like玄武岩,中-晚寒武世为岛弧钙碱性辉长岩及斜长花岗岩,反映了初始俯冲-正常俯冲的岩浆作用。
The NWW striking Hongliuhe-Xichangjing ophiolitic mélange belt(HXOMB)is exposed in the central part of Beishan orogenic belt,which extended from Hongliuhe-Niujuanzi in the west to Baiyunshan-Xichangjing in the east.In the Baiyunshan area,the ophiolitic mélanges are composed of different types of blocks and matrixes which are mainly composed of the sand-slate rocks in the south and serpentine matrix/chlorite albite schists in the north,reflecting the characteristics of the subduction accretion complex.A large area of basic lavas are exposed in the ophiolitic mélange belt,which are mainly composed of chlorite-albite schists and metabasalts.The TAS and AFM diagrams show that the basic lavas have the characteristics of tholeiite.In the trace element spider diagrams,they are characteristized by a weak Nb-Ta negative anomaly and Sr positive anomaly with very weak REE fractionation in their REE compositions.A series of discrimination diagrams based on the trace elements show that the characteristics of the basic lavas are similar to those of MORB-like basalts.Thus the basic lavas in Baiyunshan area belong to MORB-like basalts,showing the ophiolitic mélanges formed in the forearc tectonic setting.The weighted average zircon206 Pb/238 U age of the plagiogranites intruding into the basic lavas is 519.8±2.1 Ma,indicating that the ophiolite was formed in Middle Cambrian.TheεHf(t)-t diagram shows that the plagiogranites are the product of mantle differentiation.The island arc magmatic rocks are characterized by a right dipping REE distribution with a Nb-Ta deficit,and larger values of Th/Yb.In contrast with the island arc calc-alkaline gabbros of the Late Cambrian in the study area,we believe that the MORB-like basalts developed in the Early Cambrian in the Baiyunshan area,and the island arc calc-alkaline gabbros and plagiogranites developed in the Middle-Late Cambrian,reflecting the changes of the magmatism from initial subduction to normal subduction.
作者
田健
滕学建
辛后田
段霄龙
程先钰
张永
任邦方
TIAN Jian;TENG XueJian;XIN HouTian;DUAN XiaoLong;CHENG XianYu;ZHANG Yong;REN BangFang(Tianjin Center of China Geological Survey,Tianjin 300170)
出处
《岩石学报》
SCIE
EI
CAS
CSCD
北大核心
2020年第12期3741-3756,共16页
Acta Petrologica Sinica
基金
中国地质调查局项目(DD20160039、DD20190371)资助。
