内蒙古中部构造混杂带晚古生代-早中生代变质基性岩的地球化学特征及其大地构造意义  

Geochemical characteristics and tectonic significance of Late Paleozoic-Early Mesozoic meta-basic rocks in the mélange zones,Central Inner Mongolia

作　　者：张晋瑞[1] 初航[2] 魏春景[1] 王康[1] 

机构地区：[1]北京大学地球与空间科学学院,北京100871 [2]天津地质矿产研究所,天津300170

出　　处：《岩石学报》2014年第7期1935-1947,共13页Acta Petrologica Sinica

基　　金：国家"973"项目(2013CB429801);中国地质大调查项目(121201091100)联合资助

摘　　要：内蒙古中部构造混杂带中的变质基性岩可分为南、北两带：南带位于乌兰沟-图林凯地区,被划分至温都尔庙群下部的桑达来因组,主要为一套变质玄武岩和辉长岩、辉绿岩,局部含有超基性岩和碳酸岩透镜体,其中变质基性火山岩以绿片岩相变质为主,局部保留枕状构造或发育蓝片岩,已有的锆石U-Pb年代学数据表明南带变质基性火山岩形成于晚古生代到早中生代;北带位于芒和特-二道井-红格尔一线,主要呈岩块状保存在由绢云绿泥石英片岩、硅质岩、含铁石英岩和少量的大理岩组成的基质中,岩石类型包括蓝片岩、冻蓝闪石片岩、阳起片岩、绿帘角闪片岩等。地球化学研究显示南、北两带的变质基性岩相对低Al（Al2O3=10.66%~14.97%）、低Ti（TiO2=1.27%~1.96%）、高Na（Na2O=1.02%~4.20%）、贫K（K2O=0.02%~0.71%）,具有拉斑玄武岩到碱性玄武岩系列的过渡特征,高的Na2O/K2O比值（6.89~454）暗示这些基性岩在变质作用前发生了细碧角斑岩化。不活动元素Zr与其他高场强元素（HSFE;Th、Nb、Hf、Ti）和稀土元素（REE）显示良好的线性关系,表明在变质过程中,高场强元素和稀土元素基本保持稳定,可以反映原岩的性质。根据稀土、微量元素配分型式和相关比值可以将内蒙中部构造混杂带中的变质基性岩分为两类：一类稀土含量相对较低（∑REE=46.00×10-6~78.08×10-6）、轻重稀土分异不明显（（La/Yb）N=0.50~1.04）,无明显Eu负异常,Hf/Ta=6.82~15.18,具有正常的大洋中脊玄武岩（NMORB）特征;另一类稀土含量相对较高（∑REE=58.66×10-6~151.3×10-6）、轻重稀土分异明显（（La/Yb）N=2.28~4.68）,无明显Eu负异常,Hf/Ta=2.06~4.70,与富集型洋中脊玄武岩（E-MORB）相似。部分变质基性岩样品轻微富集大离子亲石元素Rb和Ba,可能暗示原岩在就位过程中遭受了微弱的陆壳混染。以上地球化学特征表明这些变质基性岩The meta-basic rocks in the melange zones from the Central Inner Mongolia are divided into southern and northern zones. The southern zone meta-basic rocks are distributed in Wulan Valley and Tulinkai areas and have been grouped into the lower part of the Ondor Sum Group, called Sangdalaiyin Formation. The rocks mainly include meta-basalt, meta-gabbro and meta-diabase intercalated with lenses of serpentinite and carbonate. The meta-basic rocks mainly underwent greenschist facies metamorphism, with blueschist developing and pillow structure preserved locally. Available zircon U-Pb age data suggested that the southern zone meta-basic volcanics were formed in the Late Paleozoic to Early Mesozoic. The northern zone meta-basic rocks are distributed from Manghete, eastwards to Erdaojing and Honger and mainly occur as tectonic blocks in a matrix consisting of sericite-chlorite-quartz schist, siliceous rock, ferrnginous quartzite and a small amount of marble. The rocks mainly include blueschist, barroisite schist, actinolite schist, epidote amphibole schist. Geochemical study shows that the meta-basic rocks in both the southern and northern zones are characterized by relatively low Al（Al2O3 = 10. 66% - 14. 97% ）, low Ti （TiO2 = 1.27% - 1.96% ）, high Na （Na2O = 1.02% -4. 20% ） and low K （K2O =0. 02% -0. 71% ）, suggesting a transitive feature from tholeiitic series to alkali basalt series. The high ratio of Na2O/K2O （6. 89 - 454） indicates that these basic rocks might have experienced spilitization before they underwent the regional metamorphism. As immobile element Zr shows good linear relationships with other high field strength elements （HSFE; Th, Nb, Hf, Ti） and rare earth elements （ REE）, it is concluded that most HSFE and REE in the recta-basic rocks remain stable during the metamorphic process, and thus can represent the characteristics of their protoliths. According to the distribution patterns of rare earth and trace elements and their ratios, the meta-basie rocks can be subdivided into

关 键 词：变质基性火山岩 地球化学 构造混杂带 内蒙古中部 中亚造山带 

分 类 号：P588.145[天文地球—岩石学]