梵净山群火山岩锆石SHRIMP年龄及其年代地层学意义

SHRIMP age of the volcanic rock from the Fanjingshan Group and its chronostratigraphic significances

梵净山群为一套出露于黔东北地区、厚达万米、变形强烈的浅变质火山岩、陆源碎屑岩系,其下未见底,上被下江群所覆盖。梵净山群的层型剖面位于贵州西部的梵净山地区,自下而上划分成上、下2个亚群。样品T0731采自下亚群上部的回香坪组,岩性为灰绿色熔结火山集块岩,首次分选出100余粒岩浆型锆石,完成SHRIMP U-Pb定年测点15个,获得加权平均年龄(840±11)Ma。这表明梵净山群主体属新元古界,进而分析、讨论了江南造山带主要地层单元的地层对比关系。

The Fanjingshan Group, outcropped in the northeastern part of Guizhou Province, is over 10000 m thick succession of strongly-deformed, low-grade metamorphic volcanic rocks and terrigenous clastic rock, overlaid unconformably by the Xiajiang Group. It is divided into 2 subgroups： Baiyansi sub-Group and Heta0ping sub-Group in ascending order. Sample T0731 is welded volcanic agglomerate, collected from the Huixiangping Formation, the uppermost formation of the Baiyansi sub-Group, and over 100 grains of zircon were sorted out from it. Fifteen of the zircons were analyzed with SHRIMP, and a weighed-mean U-Pb age of 840±11 Ma was obtained indicating that the volcanic rock of the Huixiangping Formation belongs to Neo- Proterozoie. Based on this new zircon SHRIMP U-Pb age combined with the other high-quality ages obtained recently in the Jiangnan Orogen, the stratigraphic correlation between Fanjingshan Group with the other main strata, such as Lengjiaxi Group, Shuangqiaoshan Group, and Shuangxiwu Group, has been discussed.