西秦岭造山带北缘大草滩群物源研究——LA-ICP-MS碎屑锆石U-Pb年龄证据

摘要

With clastic rocks of Late Devonian Dacaotan Group on the north margin of West Qinling orogenic belt in eastern Gansu as the study object, the authors used LA-ICP-MS zircon U-Pb isotope geochronologic method to explore the material source of Dacaotan Group. The results show that the age spectra of Dacaotan Group detrital zircon can be evidently divided into four groups: 422-518Ma, 756-887Ma, 1100-1231Ma and 2417-2736Ma? of which, the 422-486Ma age data possess the largest proportion, accounting for about 67% of the total data, having relatively concentrated ages, and showing a strong peak. The other ages possess a smaller proportion. The group of 422-518Ma indicates that its material sources were mainly from northern margin of western Qinling orogenic belt and Qilian orogenic belt in adjacent regions. The compositional ages suggest a sedimentary response to the orogenic erosion of a series of igneous rocks caused by the northward subduction and collision of mid-southern Qilian and western-Qinling micro-blocks in the Caledonian period. Detrital zircon U-Pb isotopic age group of 756-887Ma suggests that the material sources might have mainly come from the Qilian orogenic belt and northern margin of western Qinling orogenic belt. The age group of 1100-1231 Ma implies that the material sources might have come from Qilian orogenic belt, and 2417-2736Ma age group indicates that the material sources were probably derived from the basement of north Qilian orogenic belt and northern margin of western Qinling orogenic belt, with part of the material sources probably derived from the North China plate basement. A comprehensive analysis shows that the material sources of Dacaotan Group were much more complicated and characterized by obvious diversity, derived from the northern margin of western Qinling orogenic belt, Qilian orogenic belt and the basement of North China plate. The Qilian orogenic belt and the northern margin of western Qinling orogenic belt made significant source contribution to the sedimentation of Dacaotan Group, with the Qilian orogenic belt possessing the dominant role.%选取甘肃东部西秦岭造山带北缘晚泥盆世大草滩群碎屑岩为研究对象,运用LA-ICP-MS锆石U-Pb同位素年代学方法,探讨大草滩群的物质来源.结果表明,大草滩群碎屑锆石年龄谱明显分为4组:422～518Ma、756～887Ma、1100～1231Ma和2417～2736Ma.其中,422～518Ma的年龄数据约占总体的67％,所占比例最大,年龄相对集中,且呈现出最强烈的峰值,其余年龄段所占比例则较少.422～518Ma年龄组分指示其物源可能以邻近地区的西秦岭北缘构造带和北祁连造山带为主,该组分年龄是加里东期中南祁连和西秦岭微地块分别向北俯冲、碰撞产生的一系列火成岩在造山剥蚀后的沉积响应.碎屑锆石同位素年龄中756～887Ma年龄组分反映其物源可能来自祁连造山带和西秦岭北缘构造带.1100～1231Ma年龄组分反映其物源可能主要来自祁连造山带.2417～2736Ma年龄组分反映了物源来自北祁连造山带和西秦岭北缘构造带的结晶基底,部分物源也有可能来自于华北板块基底.综合分析显示,大草滩群碎屑沉积物质来源较为复杂,具有明显的多元性,存在西秦岭北缘构造带、祁连造山带和华北板块基底3个物源区,祁连造山带和西秦岭北缘构造带对大草滩群的沉积有重大的物源贡献.两者相比较,祁连造山带应为大草滩群最主要的物源区.