四川喜德地区九盘营组变英安岩锆石U-Pb年龄:兼论登相营群新认识

ZIRCON SHRIMP U-Pb DATING OF META-DACITE FROM JIUPANYING FORMATION IN XIDE AREA,SICHUAN:DISCUSSION ON THE NEW UNDERSTANDING OF THE DENGXIANGYING GROUP

通过采用锆石SHRIMP U-Pb测年方法,针对四川喜德地区冕山乡以北九盘营组同沉积火山岩(岩性分别为变质英安岩和含凝灰质碎屑岩)中的变质英安岩进行锆石微区同位素定年,获得英安岩成岩年龄为824 Ma±6 Ma(MSWD=0.7,n=9),此结果表明登相营群上部九盘营组地层时代应归属于新元古界,而非前人厘定的中元古代。综合登相营群和区域上同期沉积地层沉积特征、同位素年代学资料及构造演化特征认为:首先,九盘营组应从登相营群中解体出来,划归青白口系;其次,九盘营组变英安岩年龄(824Ma±6 Ma)为继续探索扬子陆块西缘前寒武纪变质基底演化提供了新的年代学资料,同时对重新界定"晋宁运动"的时限,进一步开展前南华纪地层的区域性对比、构造演化及成矿地质背景等研究都具有重要意义。

Zircon U-Pb isotopic geochronology study on meta-dacite and tuffaceous rocks in the Jiupanying Formation of Dengxiangying Group in Mianshan area of Xide County is carried out. It shows that the zircon SHRIMP U-Th-Pb z06 pb/238U weighted age of the meta-dacite in the Jiupa- nying Formation is 824 Ma＋ 6 Ma （MSWD= 0.7, n = 9）, suggesting that the Jiupanying Forma- tion belongs to Neoproterozoic period,instead of previously proposed Mesoproterozoic period. On the basis of regional tectonic evolution, combined with the characteristics of the sedimentary stra- ta and geochronology of the Dengxiangying Group,it is considered that the Jiupanying Formation should be disintegrated from the Dengxiangying Group. The newly obtained age of meta-dacite leads to the redefinition of the time limit of the ＂Jinning movement ＂ in the western margin of the Yangtze Block. Therefore, it is of significance on the study of regional stratigraphy correlation, tectonic evolution and mineralization geological background of Pre-Nanhuan metamorphic base- ment in the western margin of the Yangtze Block.