1�� Bureau of Geology and Mineral Exploration and Development of Inner Mongolia��Wuhai 016000�� Inner Mongolia Autonomous ��egion��China; 2�� Faculty of Earth ��esources��China University of Geosciences��Wuhan 430074��China

The shape��texture��content and ��EE 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��