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

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.