西藏南部加查-曲松地区碰撞型混杂岩的厘定及其地质意义

Determination of the collisional mélange in Gyaca-Qusum area, southern Tibet of China and its geological significance

根据野外地质调查、大比例尺地质填图、锆石U-Pb年代学和Ar-Ar云母热年代学分析,对西藏南部东特提斯喜马拉雅加查—曲松地区的晚三叠世复理石建造中混杂岩的物质组成和构造变形特征进行精细解剖,探讨这套混杂岩的属性。结果显示加查-曲松混杂岩带主要由晚侏罗世—早白垩世变质基性岩块和晚三叠世复理石基质组成,早期顺层剪切作用和晚期纵弯叠加改造作用对应的变形时间分别为~59.5 Ma B.P.和56~55 Ma B.P.。结合区域地质背景,确定加查-曲松混杂岩为碰撞型混杂岩,认为其代表中古新世—早始新世期间印度被动大陆北缘向欧亚大陆之下俯冲过程中伴随的早期陆-陆碰撞作用的产物。始新世初期,印度被动陆缘的持续俯冲引发了印度-欧亚大陆板块发生了同碰撞作用。

Based on field geological survey, large-scale geological mapping, zircon U-Pb chronology and Ar-Ar mica thermochronology analysis, the material composition and tectonic deformation characteristics of the Late Triassic flysch formation in Gyaca-Qusum area, East Tethys Himalaya, southern Tibet are analyzed and the properties of this set of mélanges are discussed. The results show that the Gyaca-Qusum mélange is mainly composed of late Jurassic-early Cretaceous metamorphic basic block and late Triassic flysch matrix. Geochronological analysis shows that the deformation time of early layer-parallel shear and late buckling superposition is ~59.5 Ma B.P, and 56~55 Ma B.P. respectively. Based on the study in this paper and combined with the regional geological background data, the Gyaca-Qusum mélange is identified as a collisional mélange resulted from the early continent-continental collision in association with the subduction of the northern margin of the Indian passive continent under Eurasia during the middle Paleocene-early Eocene. In the early Eocene, the continuous subduction of the Indian passive continental margin caused the collision between the Indian Plate and Eurasian Plate.