The Lancang Group are widely distributed within the southern segment of the Sanjiang area,western Yunnan Province.It consists dominantly of low-grade metamorphosed mudstones and basic volcanic rocks,which recorded the procedure of closure of the Paleo-Tethys Ocean and subsequent subduction between ocean and continent.And it is unique research subject for understanding subduction and collision of Paleo-Tethys.However,the age and mineral source of the Lancang Group are still under controversial,as too is tectonic signification.In this paper,we present LA-ICP-MS detrital zircon U-Pb dating results of three quartzite and a sericite-bearing metamorphosed quartz sandstone samples.The detrital zircons selected from four metamorphosed phyllite samples have high Th/U ratios and show strongly or weakly oscillatory zoning,suggesting their magmatic origin.Those detrital zircons yielded two major age peaks at 530Ma and 930Ma,570Ma and 915Ma,540Ma and 960Ma,570Ma and 910Ma in age histogram,respectively.And all detrital zircons clustered around 570 ～ 530Ma and 960 ～910Ma,with the youngest age peak of ～ 530Ma.The dating results strongly support the previous views that the depositional time of the Lancang Group has been constrained during the middle Ordovician (462 ～ 454Ma).Age distributions of detrital zircons provide evidence for their magmatic origin during the Pan-African event and assembly-breakup of Rodinia supercontinent.The detrital zircons with remarkable rounded shape suggested that the protoliths of the Lancang Group originated from distant external sources,which were similar to those of metasedimentary or stratigraphic rocks in Qiangtang,Tethyan Himalaya and Lhasa terrane.%澜沧群出露于滇西“三江”地区的南段,其主要岩石由遭受低级变质作用改造的泥质岩和基性火山岩组成.这些岩石普遍经历了古特提斯洋的闭合以及随后的洋陆俯冲过程,是研究古特提斯洋俯冲-碰撞过程的重要窗口.但是有关澜沧群的形成时代、物质来源以及形成的构造背景等一系列问题长期以来存在着多种争议.本文对澜沧群中3件石英岩和1件绢云母变质石英砂岩样品中分选出的碎屑锆石进行了阴极发光图像分析和LA-ICP-MS U-Pb年代学研究.澜沧群浅变质岩系碎屑锆石具有明显或弱的振荡环带和较高的Th/U比值,表明岩浆成因.年代学分析结果表明,4件浅变质岩石样品均得到了两组主要年龄峰值,分别为530Ma和930Ma、570Ma和915Ma、540Ma和960Ma、570Ma和910Ma.本次研究中碎屑锆石U-Pb年龄主要分布在570～530Ma和960～910Ma.其中,最年轻的碎屑锆石年龄峰值～530Ma,支持了前人认为澜沧群沉积时代为中奥陶纪(462 ～ 454 Ma)的认识.本研究中澜沧群浅变质岩系碎屑锆石年龄分布特征表明源区可能主要为泛非期和罗迪尼亚超大陆聚合-裂解过程中形成的岩浆岩.碎屑锆石磨圆较好指示其经历了较长距离的搬运.澜沧群浅变质岩系碎屑锆石与羌塘、特提斯喜马拉雅和拉萨地体变沉积岩或地层中碎屑锆石具有相似的年龄分布特征,表明它们可能具有相似的源区.
展开▼
