扬子陆块三峡地区莲沱组砂岩中碎屑锆石U-Pb年龄、Hf同位素组成及其地质意义

对扬子陆块三峡地区黄牛岩剖面莲沱组顶部砂岩中的120颗碎屑锆石进行了U-Pb定年和Lu-Hf同位素分析.结果显示,黄牛岩剖面莲沱组中碎屑锆石的年龄谱具有～880～800 Ma、～2 000 Ma、～2 500 Ma及～2 700 Ma的峰值,其中最年轻的碎屑锆石年龄为724±8 Ma.结合前人对该地区莲沱组顶部凝灰岩开展的年代学工作,将莲沱组顶部砂岩沉积时代限定为724～714 Ma.莲沱组砂岩沉积时间与其中最年轻的碎屑锆石U-Pb年龄接近,反映了其源区地壳物质的快速再循环.碎屑锆石Hf同位素两阶段模式年龄(TDM2)集中在～3.7～3.1 Ga、～2.5～2.0 Ga和～1.3～1.0 Ga,反映其物源区存在古-中太古代、古元古代以及中元古代末期的初生地壳生长.对比近年来三峡地区不同剖面莲沱组砂岩中已报道的碎屑锆石年龄和Hf同位素数据,黄牛岩剖面的莲沱组碎屑锆石年龄和Hf同位素组成与之以北的王丰岗剖面均存在明显差异,说明莲沱组沉积期两者的陆源物质供给区有较大差别.

Detrital Zircon Compositions of U-Pb Ages and Hf Isotope for Sandstone of Liantuo Formation from Three Gorges Area,Yangtze Block and Its Geological Significance

In this study, 120 detrital zircons from the sandstone at the top of the Liantuo Formation in the Huangniuyan Section of the Three Gorges area were measured for U-Pb dating and Lu-Hf isotopic compositions. The U-Pb ages of the zircons from the Huangniuyan Section cluster at~800-880 Ma, ~2 000 Ma, ~2 500 Ma, ~2 700 Ma, among which the youngest zircon age is 724±8 Ma. According to the previous geochronological study of tuff on the top of the Liantuo Formation, the sedimentary age of the sandstone in the upper most part of the Liantuo Formation is limited to~724-714 Ma. The sedimentary time of the Liantuo sandstone is close to the U-Pb age of the youngest detrital zircon, which may reflect a rapid recycling of crustal materials. The Hf isotopic two-stage model ages (T_DM2) of zircon are concentrated at~3.7-3.1 Ga, ~2.5-2.0 Ga and~1.3-1.0 Ga, which indicates that the provenance of the sandstone has juvenile crustal growth in the Paleo-Mesoarchean, Paleoproterozoic and Late Mesoproterozoic. By comparing the detrital zircon U-Pb ages and Hf isotopic data from different sections of the Liantuo Formation in the nucleus of the Yangtze craton reported in recent years, it is suggested that the Huangniuyan Section has distinct detrital zircon U-Pb ages and Hf isotopic compositions compared with the Wangfenggang Section, revealing different provenance between them.