江南造山带西段新元古代下江群岩石地球化学特征及沉积构造环境

Lithogeochemistry Characteristics and Sedimentary Tectonic Environment of Neoproterozoic Xiajiang Group in the Western Jiangnan Orogenic Belt

江南造山带西段广泛分布一套厚达万米的区域浅变质陆源碎屑岩夹火山碎屑岩岩系,其中位于武陵运动界面之上、雪峰运动界面之下的地层,在贵州称下江群、湖南称板溪群和广西称丹洲群,是研究江南古陆新元古代沉积盆地演化的重要对象之一。下江群广泛出露于黔东南地区,自下而上分为甲路组、乌叶组、番召组、清水江组、平略组和隆里组。本文选择下江群为研究对象,在广泛的区域地质路线调查和剖面测制基础上,选取代表性的剖面开展系统的岩石地球化学测试,结果显示:下江群的SiO_(2)含量介于61.10%75.06%,平均为67.45%;K_(2)O/Na_(2)O比值较高;TFe_(2)O_(3)+MgO含量较低,平均5.74%;Sr、Nb元素整体含量较低,显示亏损;Y元素富集;轻稀土元素相对富集,重稀土则相对亏损;稀土配分模式总体为右倾,轻稀土斜率较大,重稀土趋于平坦。其中,甲路组平均微量元素含量总体与后太古代页岩的接近,而乌叶组、番召组、清水江组、平略组和隆里组的平均微量元素含量总体与上地壳接近。通过地球化学特征参数和参数判别图解,结合下江群沉积学特征,认为下江群的沉积构造环境属于一个活动大陆边缘的弧后盆地沉积环境,沉积物可能来自盆地北侧的扬子陆块和南侧龙胜岛弧。

The southeastern Guizhou Province in the western section of Jiangnan orogenic belt is widely distributed a set of continental clastic rocks with volcanic clastic rock series up to ten thousand meters thick.The strata above the Wuling movement interface and below the Xuefeng movement interface are called Xiajiang Group,which is equivalent to Banxi Group in Hunan and Danzhou Group in Guangxi.It is one of the important objects to study the evolution history of Neoproterozoic sedimentary basins in Jiangnan oldland.Xiajiang Group is widely exposed in southeastern Guizhou in the western part of Jiangnan orogenic belt,which is a set of regional shallow metamorphic terrigenous-volcanic clastic formation.From bottom to top,Xiajiang Group is divided into Jialu Formation,Wuye Formation,Fanzhao Formation,Qingshuijiang Formation,Pinglue Formation and Longli Formation.In this study,each stratum of Xiajiang Group in the study area was taken as the research object.On the basis of extensive regional geological route survey and profile measurement,representative profiles are selected to carry out systematic rock geochemical tests.The results show that the SiO_(2) content of Xiajiang Group is between 61.10%and 75.06%,with an average of 67.45%.The K_(2)O/Na_(2)O ratio is higher,while the TFe_(2)O_(3)+MgO content is lower,with an average of 5.74%.Sr,Nb elements overall content is low,showing a loss;Y element enrichment;light rare earth elements are relatively enriched,while heavy rare earth elements are relatively depleted;the distribution pattern of rare earth is generally right-leaning,the slope of light rare earth is large,and the heavy rare earth tends to be flat.Among them,the average trace element content of Jialu Formation is generally close to that of post Archean shale,while the average trace element content of Wuye Formation,Fanzhao Formation,Qingshuijiang Formation,Pinglue Formation and Longli Formation is generally close to that of upper crust.Comprehensive geological and geochemical research results of each group,That suggests that the sedimentary period of Xiajiang Group in the study area is in a back-arc basin sedimentary environment on the active continental margin.The sediments are mainly from the Yangtze continental block on the north side of the basin and the Longsheng island arc belt on the south side of the basin.