阿尔金喀腊大湾地区卓阿布拉克组沉积岩系年代及其地质意义——来自碎屑锆石LA-ICP-MS U-Pb年龄的证据

Geochronology of the sedimentary series from Zhuo'abulake Formation in Kaladawan area of Altun and its geological significance: LA-ICP-MS U-Pb age evidence of detrital zircons

以阿尔金喀腊大湾地区卓阿布拉克组沉积岩系砾岩为研究对象,应用LA-ICP-MS方法对碎屑锆石进行U-Pb同位素年龄测定,探讨其形成时代、物源组成和构造背景。研究结果表明:(1)碎屑锆石年龄谱明显分为2组:522~427Ma和638~1766Ma,另有2个古老的同位素年龄数据分别为2450Ma、3394Ma;(2)最年轻的碎屑锆石年龄为427±4Ma,说明该砾岩的形成时代晚于427Ma,沉积岩系可能形成于晚志留世—早泥盆世(427~410Ma);卓阿布拉克组存在晚寒武世—早泥盆世的物质组成,应将其解体重新划分;(3)碎屑锆石峰值年龄为456Ma,表明喀腊大湾地区存在该时期的岩浆活动;522~427Ma的年龄数据约占总体的81%,构成了砾岩中最主要的碎屑锆石群体,说明在早古生代,尤其是晚寒武世—奥陶纪形成的岩浆岩是砾岩最重要的物源,该沉积岩系是敦煌地块向南俯冲、碰撞产生的一系列火成岩在造山剥蚀后的沉积响应;(4)结合目前北祁连造山带的研究进展和研究区碎屑锆石的研究结果,初步认为喀腊大湾地区卓阿布拉克组沉积岩系和北祁连天祝组、鹿角沟组具有相似的沉积年龄和物质来源,可能同为北祁连-北阿尔金局限洋盆闭合后,陆-陆或弧-陆碰撞造山的产物。

Choosing the conglomerate from the sedimentary series of Zhuo＇abulake Formation in Kaladawan area as the study object and using LA-ICP-MS zircon U-Pb isotope chronology, the authors studied the sedimentary age, material source and tectonic setting of the sedimentary series. Some results were obtained：（1） The age spectra of detrital zircon can be evidently divided into two groups：522~427 Ma and 638~1766 Ma, with other two old ages 2450 Ma and 3394 Ma.（2） The youngest age of detrital zircons is427±4 Ma, suggesting that the formation of the conglomerate was later than 427 Ma, and hence the formation age of the strata may beconstrained from Late Silurian to Early Devonian（427~410 Ma）; Zhuo＇abulake Formation should be divided into more units since it consists of materials from Late Cambrian to Early Devonian.（3） The peak value of the age spectrum is 456 Ma, indicating that there existed a magmatic event during this period. The 522~432 Ma age data possess the largest proportion and account for about 81% of the total data, showing that the magmatic rocks that were formed in early Paleozoic, especially from late Cambrian to Ordovician constituted the most important source of the conglomerate. The compositional ages suggest a sedimentary response to the orogenic erosion of a series of igneous rocks caused by the southward subduction and collision of Dunhuang massif in early Paleozoic period.（4） Combining the recent research on North Qilian orogenic belt and the study of the detrital zircons in Kaladawan area, the authors consider that sedimentary series of Zhuo＇abulake Formation in Kaladawan area and the Tianzhu Formation and Lujiaogou Formation in North Qilian have similar ages and material sources, and they may be the products of continental collision or arc-continent collision after North Qilian—North Altun limited ocean closure.