羌塘地块南界班公湖-丁青断裂构造带火山岩地球化学及其形成构造环境

Geochemistry of the volcanic rock association from Bangong lake-Dingqing suture zone of the south boundary in Qiangtang block and its tectonic setting

利用岩石地球化学的方法,研究了羌塘地块南界班公湖-丁青断裂带中晚中生代火山岩。结果表明,研究区火山岩由拉斑系列大洋玄武岩(MORB)和钙碱性系列岛弧玄武岩(IAB)、玄武安山岩、安山岩、英安岩和流纹岩组成。拉斑系列的洋中脊玄武岩微量元素原始地幔的标准化配分型式,明显不同于N-MORB玄武岩,而具有岛弧型的MORB特征(Ta、Nb的相对亏损),说明其成因与岛弧环境有一定联系。本区钙碱性系列的岛弧玄武岩(IAB)、安山岩、英安岩和流纹岩在Hacker图解中,微量、稀土元素标准化配分型式上显示出岩浆同源性的特征。通过对研究区内分布的大洋玄武岩、岛弧玄武岩厘定,推断班公湖-丁青缝合带在晚中生代期间可能发育过一个完整的有限洋盆,这将对于进一步研究羌塘地块与其周边各地体的大陆动力学作用过程具有重要意义。

By means of the petrological and geochemical methods, the Late Mesozoic volcanic rocks collected from the Bangong lake-Dingqing suture zone of the South boundary in Qiangtang block were deal with in this paper. The research results indicate that the volcanic rocks in the studied abea are composed of two different kinds of associations: Tholeiitic series (MORB)and calc-alkaline series (basalt,andenite,dacite and rhyolite ) . After normalizd by primitive mantle,the distribution pattern of Tholeiitic basalt exhibits remarkable depleted in Ta, Nb, which is quite different both from the N-MORB and the typical island-arc volcanic rocks. Thus,the tholeiitic basalt should have generated from the marginal basin (back-arc basin) tectonic setting with both characteristics of MORB and IAB. Besides,the calc-alkaline series volcanic rocks in the Harker diagrams display the continual magmas evolved process respectively,which indicates those rocks not only originated from some parental magmas ,but resulted from the magmatic fractional crystallization. It is well known that the MORB and IAB are the two important units of ophiolite,accordingly,it can be proposed the studied area in the Late Mesozoic era had ever undergone a complete development of the limited ocean basin,which may be important significance to further study the continental dynamic evolved process of Qiangtang and its surrounding blocks.