滇西澜沧岩群碎屑锆石U-Pb定年及其地质意义

摘要

Although numerous previous studies,no direct evidences of geological ages are recorded for the Lancang Group Complex in western Yunnan. The U-Pb age data of the detrital zircons in sericite quartz schist from the Lancang Group Complex in the Changputang-Datianshan zone have provided convincing evidence for the formation age of the Lancang Group Complex. The cathodoluminescence (CL ) images show that these zircon crystals generally display the well-defined oscillatory zonation and suggest a magmatic origin of the zircons. The age data for the detrital zircons in the sericite quartz schist sample PM08-B1 range between 452 and 3016 Ma,with the youngest age group of 452 ± 26 Ma,implying that the maximum depositional age of the Lancang Group Complex is less than 452 ± 26 Ma. One zircon grain with a U-Pb age of 255 ± 3Ma exhibits a slight cathodoluminescence image and has no internal structures,which is characteristic of a type of metamorphic recrystallization zircons and in general aggrement with the previous data. In general,the Lancang Group Complex is dated at a depositional age of less than 452 ± 26 Ma,The U-Pb age (255 ± 3 Ma)of the metamorphic crystalline zircons reveals that the metamorphic events once took place during the Late Permian.%虽然前人对澜沧岩群做了大量的研究,但缺乏同位素年代学方面的研究.此次研究针对澜沧江南段菖蒲塘-大田山地区澜沧岩群绢白云母石英片岩进行碎屑锆石U-Pb定年,为澜沧岩群的形成时代增添年代学证据.极大多数(129粒)锆石阴极发光(CL)图像显示其有明显的结晶振荡环带,指示其为岩浆成因的碎屑锆石,129粒碎屑锆石具有多组峰值年龄,最年轻一组年龄加权平均值为452 ± 26Ma,表明澜沧岩群最早沉积时限不早于452 ± 26Ma;极少数(1粒)锆石具弱阴极发光,缺乏内部结构特点,认为其为变质重结晶锆石,变质结晶锆石U-Pb年龄为255 ± 3Ma,与前人研究所得澜沧岩群变质时代基本吻合.综合分析,澜沧岩群的沉积时限不早于452 ± 26Ma,在二叠纪末期可能发生变质作用.