北祁连造山带嘉峪关北大草滩地区早志留世碎屑锆石LA-ICP-MS U-Pb年龄及其地质意义

以北祁连造山带嘉峪关北大草滩地区原划为中—下奥陶统的阴沟群碎屑岩为研究对象,进行碎屑锆石LA-ICP-MS UPb测年,探讨其形成时代、物源组成和构造背景。结果表明,碎屑岩沉积时代早于432.5Ma,为早志留世,该套沉积地层并不属于早—中奥陶世阴沟群。碎屑锆石U-Pb同位素年龄可明显分为4组:早古生代年龄组,434～521Ma,峰值为447Ma;新元古代年龄组,791～992Ma,峰值年龄966Ma;中古元古代年龄组,1017～1755Ma,并出现1120Ma、1278Ma、1427Ma和1648Ma多个峰值;古元古代早期—新太古代晚期年龄组,1879～2663Ma,并出现2089Ma、2428Ma和2543Ma多个峰值。综合分析显示,碎屑岩沉积物质来源于祁连造山带和阿拉善地块,祁连造山带早古生代岛弧型岩浆岩和新元古代岩浆岩,以及造山带结晶基底岩系为该套碎屑岩沉积提供了更重要的物源。

LA-ICP-MS zircon U-Pb dating of the Early Silurian clastic rocks in Dacaotan of Jiayuguan, North Qilian orogenic belt

The detrital rocks distributed in Dacaotan, northern Jiayuguan in the North Qilian orogenic belt were chosen as the study object, which was originally considered to be of the Yingou Group of Middle-Lower Ordovician. LA-ICP-MS zircon U-Pb iso-tope chronology was investigated to study the formation age, material source composition and tectonic background of the Yingou Group. The results show that the sedimentary age of the Yingou Group is earlier than 432.5Ma, and hence its age should be Early Si-lurian and it does not belong to the Yingou Group of Middle-Lower Ordovician. The zircon age spectrum can be evidently divided into four groups:Early Paleozoic age group from 434~521Ma, with the peak value being 447Ma; Neoproterozoic age group from 791~992Ma, with the peak value being 966Ma; Mesoproterozoic age group from 1017~1755Ma, with the peak value varying from 1120Ma, 1278Ma, 1427Ma to 1648Ma; Paleoproterozoic-Neoarchaean age group from 1879~2663Ma, with the peak value varying from 2089Ma, 2428Ma to 2543Ma. Comprehensive analyses show that the material source was derived form Qilian tectonic zone and Alashan block. The Early Paleozoic island-type magmatic rock in Qilian tectonic zone, Neoproterozoic magmatic rock and tectonic zone crystalline basement rock series contributed much to the sedimentation of the detrital rocks.