扬子北缘神农架地区郑家垭组碎屑锆石年代学及其构造意义

Detrial Zircon U-Pb Dating of Zhengjiaya Formation from the Shengnongjia Area in the Northern Yangtze Block and Its Tectonic Implications

神农架地区位于扬子陆块北缘,东与黄陵地区前寒武纪变质基底相邻,对该区东部郑家垭组碎屑锆石开展U-Pb年龄谱研究可为中新元古代神农架地区与黄陵地区的相互关系和构造演化研究提供新的依据。本文运用LA-ICP-MS锆石U-Pb方法,对郑家垭组2件砂岩碎屑锆石进行U-Pb年龄测定,共获得了103组U-Pb有效年龄。碎屑锆石的谐和年龄主要集中分布在0.98-1.2Ga、1.3-1.6Ga、1.8-2.3Ga、2.45-2.9Ga 4个区间,其中最年轻的一组锆石谐和年龄为980-995Ma,表明郑家垭组的沉积作用持续到了新元古代早期。与周缘地质体中碎屑锆石年龄谱系进行比较,认为所研究样品的碎屑锆石物源一部分来自于神农架微陆块,一部分来自于庙湾蛇绿岩,具有双向物源特征。结合现有的资料,认为郑家垭组可能为神农架弧与庙湾SSZ型蛇绿岩之间的弧前盆地沉积,指示扬子克拉通内存在格林威尔运动早期的俯冲记录。

Shennongjia area is located in the northern margin of the Yangtze block,adjacent to the Huangling Precambrian metamorphic basement.It is significant to study U-Pb geochronology of the detrital zircons from Zhengjiaya Formation in the eastern part of Shennongjia area for inquiring into the Precambrian crustal evolution of South China,especially for the Mesoproterozoic-Neoproterozoic tectonic attribution between the Shennongjia area and Huangling region.Detailed LA-ICP-MS U-Pb dating analyses have been carried out on detrital zircons from 2samples of Zhengjiaya Formation,and yielded 103 valid data.The result shows that the age of the detrital zircons can be mainly divided into four groups：0.98-1.2Ga、1.3-1.6Ga、1.8-2.3Ga、2.45-2.9Ga,of which,the young zircon populations at 980-995Ma indicates that the Zhengjiaya Formation deposited continued to early Neoproterozoic.Comparing these with neighboring tectonic units,we found that the sedimentary sources of the studied samples has bidirectional provenance characteristics,some of them were drained from Shennongjia micro-continental blocks,while others may come from the Miaowan ophiolite.Combined with the published data,it is considered that the Zhengjiaya Formation may formed in a forearc basin between Shennongjia arc and Miaowan SSZ type ophiolite,which indicates that there is a subduction amalgamation record of early Neoproterozoic at the interior of Yangtze Craton.