1． Bureau of Geology and Mineral Exploration and Development of Inner Mongolia，Wuhai 016000， Inner Mongolia Autonomous Ｒegion，China; 2． Faculty of Earth Ｒesources，China University of Geosciences，Wuhan 430074，China

The shape，texture，content and ＲEE characteristics of zircons from the O2 tnd-1，O3 tnd-2，O3 tnd-3 pyroclastic rock of Tanjianshan Group on the north margin of Qaidam Basin indicate that the O2-3 tnd is the product of volcanism during the transitional period from ocean-land interim crust to oceanic crust． The U-Pb surface ages obtained from O3 tnd-2 and O3 tnd-3 can be divided into 9 groups，every age group coincides with the period when significant tectonic-heat event took place at Oulongbuluke micro-continental base of northern Qaidam，suggesting that the base rocks have provided materials for the formation of sedimentary and volcanic rock in O2-3 tnd ． The volcanic rocks of O3 tnd-3 formed at 440 Ma，with time gap 46 Ma to those of O1 tna-1 and O3 tnd-3 may represent the minor period that Xitieshan back-arc extension have lasted，the scale of back-arc basin that formed in Xitieshan extension may approach to 1 400 km． Based on the test of ithochemistry data for major elements and analysis of Sr isotope geochemistry for the clastic sedimentary rock in O1 tna-2 and O3 tnd-2，the authors get the conclusion that the O1 tna-2 and O3 tnd-2 of Tanjianshan Group may form in back-arc basin environment，while the lithology difference between these two formations may reflect the changes of geodynamic processes as the diagenesis tectonic environment transformed from continental margin depression to adjacent sea basin．