内蒙古北山造山带白云山地区上泥盆统墩墩山组火山岩的厘定及其构造意义  

The determination of volcanic rocks in Upper Devonian Dundunshan Formation in the Baiyunshan area of Beishan orogenic belt,Inner Mongolia

作　　者：田健[1] 辛后田[1] 滕学建[1] 段霄龙 程先钰 张永[1] 任邦方[1] TIAN Jian;XIN HouTian;TENG XueJian;DUAN XiaoLong;CHENG XianYu;ZHANG Yong;REN BangFang(Tianjin Center of China Geological Survey,Tianjin 300170,China)

机构地区：[1]中国地质调查局天津地质调查中心

出　　处：《岩石学报》2020年第2期509-525,共17页Acta Petrologica Sinica

基　　金：中国地质调查局地质矿产调查项目(DD20160039)资助

摘　　要：北山造山带中部发育北西西向展布的洗肠井-白云山-牛圈子-红柳河蛇绿混杂岩带,白云山地区上泥盆统墩墩山组呈近东西向分布于蛇绿岩带两侧,该地层的岩石组合为底砾岩、岩屑砂岩、安山岩、英安岩、英安质角砾熔岩及英安质凝灰岩。锆石U-Pb年代学特征表明,英安质角砾熔岩及英安质凝灰岩的206 Pb/238 U加权平均年龄分别为368.1±1.8Ma、363.5±2.5Ma。白云山地区墩墩山组岩石组合及时代与牛圈子地区墩墩山群十分相似,具有可对比性。墩墩山组火山岩微量元素蛛网图中Nb-Ta负异常及Th/Yb-Nb/Yb构造判别图解显示了大陆边缘弧的地球化学特征,结合区域蛇绿岩及晚泥盆世岩浆岩的时空展布,我们认为,北山造山带中部晚泥盆世陆缘弧的岩石组合主要为花岗闪长岩(英安岩)和二长花岗岩(流纹岩),其形成可能与柳园洋的向北俯冲有关。墩墩山组酸性火山岩正的εHf(t)值(+1.4^+16.4)反映其源区具有幔源物质的贡献;Sr/Y-Y图解、MgO-SiO2图解及变化的Mg^#(17.6~62.5)进一步指示俯冲洋壳+地幔楔及弧增生物(变质玄武岩为主)为墩墩山组火山岩的主要源区,即洋壳及弧增生物的部分熔融是北山造山带中部晚泥盆世陆壳增生的方式之一。The NWW-trending Xichangjing-Baiyunshan-Niuquanzi-Hongliuhe ophiolite belt is an important component of the central Beishan orogenic belt.The Upper Devonian Dundunshan Formationis distributed on both sides of the E-W trending Baiyunshan ophiolite belt.The rock assemblages of Dundunshan Formation are composed of basal conglomerate,lithic sandstone,andesite,dacite,dacitic breccia lava and dacitic tuff.Zircon U-Pb chronological characteristics of the volcanic rocks shows that the weighted average age of 206Pb/238U is 368.1±1.8Ma^363.5±2.5Ma.The rock assemblages and ages of Dundunshan Formation in Baiyunshan area are very similar and comparable to those of the Dundunshan Group in Niuquanzi area.Nb-Ta negative anomaly in the spider diagram of trace element and Th/Yb-Nb/Yb tectonic discriminant diagram of the volcanic rocks show that they have the geochemical characteristics of a continental margin arc.By combining the spatial and temporal distribution of regional ophiolites and magmatic rocks of the Late Devonian,the continental arc rock assemblages were mainly composed of granodiorite(dacite)and monzogranite(rhyolite),and the formation of the Late Devonian continental margin arc in the central part of the Beishan orogenic belt may be related to the northward subduction of the Liuyuan ocean.The positive zirconεHf(t)values(+1.4^+16.4)of the acidic volcanic rocks of Dundunshan Formation indicate the contribution of mantle materials to its sources.As shown in the Sr/Y vs.Y and MgO vs.SiO2 diagrams,the source properties for the acidic volcanic rocks are composed of subduction oceanic crust+mantle wedge and accretionary arc(metabasalts),so partial melting of oceanic crust and accretionary arc are a few ways of continental crust accretion in Late Devonian in the central Beishan orogenic belt.

关 键 词：内蒙古 北山造山带 晚泥盆世 墩墩山组火山岩 陆壳增生 大陆边缘弧 

分 类 号：P588.14[天文地球—岩石学] P597.3[天文地球—地质学]