拉萨地块中段查孜地区典中组火山岩锆石U-Pb年龄及地质意义
Zircon U-Pb Ages and Implications of the Dianzhong Formation in Chazi Area, Middle Lhasa Block, Tibet

林子宗群火山岩是在青藏高原南部广泛发育的新生代火山岩，形成于新特斯洋闭合向印度-亚洲大陆碰撞过渡的背景下，其底部典中组火山岩的年龄对限定印度-亚洲大陆的碰撞时限具有重要意义.然而林子宗群火山岩的研究主要集中在拉萨地块东部林周盆地及其附近，其中部和西部的火山岩研究较少，系统的年代学研究则更少.对拉萨地块中段查孜地区的一条林子宗群典中组火山岩剖面进行了系统的锆石U-Pb年代学研究，获得了火山岩锆石U-Pb年龄分别为70.7±1.4 Ma、69.9±1.5 Ma、68.3±1.2 Ma.结合前人资料，对拉萨地块林子宗群火山岩年代学进行了区域对比，结果显示其底部火山岩的年龄沿东西走向存在一定的差别，中段年龄略早于东段和西段，表明印度-亚洲大陆碰撞中段略早于东部和西部.
The Cenozoic volcanic rocks in Linzizong Group are widely exposed in southern Tibet Plateau and it is considered to have been formed in the transitional process from the end of subduction of Neo-Tethyan oceanic plate to India-Asia continent collision.The age of Dianzhong Formation is significant to constrain the time of collision between India and Asia.However,research on the Linzizong Group volcanic rocks is mainly concentrated on the Linzhou basin and its vicinity in the eastern part of the Lhasa block.There are few studies on volcanic rocks in the central and western parts of the Lhasa block,and very few systematic chronology studies.After the stratigraphic and petrological study,we conducted systematic U-Pb zircon dating on 3 selected samples taken from the geological section of Dianzhong Formation in Chazi area.The results show the ages of Dianzhong Formation are 70.7±1.4 Ma,69.9±1.5 Ma and 68.3±1.2 Ma,respectively.Combining with previously reported chronological data,it is revealed that there were certain differences of the ages from bottom layer of the Linzizong volcanic rocks in Lhasa block.The initial collision between India and Asia continent took place earlier in middle Lhasa block but later in eastern and western parts