黔东南新元古代下江群碎屑岩地球化学特征及其大地构造意义  

Geochemistry and Its Tectonic Significance of the Clastic Rock in the Neoproterozoic Xiajiang Group,Southeast Guizhou,South China

作　　者：戴维 徐亚军[1] 杜远生[1] 覃永军[2] 

机构地区：[1]中国地质大学地球科学学院,武汉430074 [2]贵州省地质矿产勘查开发局,贵阳550004

出　　处：《地质论评》2017年第5期1153-1168,共16页Geological Review

基　　金：国家自然科学基金资助项目(编号:41472086;41772106)的成果~~

摘　　要：黔东南下江群是一套具复理石特征的陆源碎屑岩夹火山碎屑岩建造,由下向上分别为甲路组、乌叶组、番昭组、清水江组、平略组和隆里组。本文研究了该套岩石的主量元素、微量元素和Nd同位素特征。结果显示:下江群各个组样品均显示出不同程度的Co、La、Ce、Nd、Y和Lu元素的富集,Ta和Ni元素亏损以及明显的Eu负异常,ε_(Nd)(t)表现出负异常(ε_(Nd)(t):-0.35^-3.36)。岩石地球化学特征显示源岩以遭受了中等程度化学风化作用(CIA:52~79)的酸性岩为主,并混有适量的基性岩。结合已有研究成果,判断物源区主要以盆地周缘的近源沉积为主,如分布在桂北一带的花岗质岩石和基性岩。构造背景判别表明盆地处于与岛弧活动相关的活动大陆边缘背景,如此的构造背景暗示,江南造山带西段在下江群沉积期可能仍存在消减作用,扬子板块与华夏板块此时还未完全拼合。综合对比江南造山带东、西两段消减—碰撞的时间,暗示扬子板块和华夏板块之间可能具有一个"东早西晚"的碰撞造山过程。Objectives： The Xiajiang Group in Southeast Guizhou is an assembly of siliciclastic rocks interbedded with volcanic clastic rock that show the feature of flysch. It can be divided into the Jialu, Wuye, Fanzhao, Qingshuijiang, Pinglue and Longli formations from bottom to top. This paper focused on the major, trace element and Nd isotope features of the group in order to identify the tectonic environment of western Jiangnan Orogen in Xiajiang period, and further to unravel evolution of the orogen and juxtaposition of Yangtze and Cathaysia blocks. Methods： 21 samples have been analyzed, respectively from Jialu, Wuye, Fanzhao, Qingshuijiang, Pinglue and Longli formations. Of these, point-counting was performed on 9 coarse-grained sandstone samples from the group. Between 300 to 500 points were point counted in each thin section following the Gazzi--Dickinson method . Major, trace and rare earth elements of 12 fine-grained sandstones were measured by XRF and ICP-MS, respectively, at the State Key Laboratory of Biogeology and Environment Geology of CUG. Both the accuracy of analysis are 〉 5%. And Nd isotopic analyses are measured by MC-ICP-MS at the Guangzhou Institute of Geochemistry, Chinese Academy of Sciences. The mass fi-actionation correction for Nd isotopic ratios are based on n（146Nd）/n（144Nd） =0. 7219. The measured n（143Nd）/n（144Nd） ratios of the La Jalla standard are 0. 511862 ± 10 （2if）. During the analytical process, within-run errors of precision are estimated to be better than 0. 000015 for n（ 146Nd）/n（144Nd） in the 95% confidence level. Results： The results show that all the samples rich in Co, La, Ce, Nd, Y and Lu and deplete in Ta and Ni, wth obvious negative Eu anomalies. Sm-Nd isotopic analyses show negative （εNd （t） values of -0.35 to -3.36. Geochemistry features indicate the source rock are dominated by granitic rocks with the infilling of marie rocks. And they suffered moderate chemical weathering （CIA = 52 -79 ） before the deposition. The tectonic

关 键 词：黔东南 下江群 沉积地球化学 物源 江南造山带 

分 类 号：P542[天文地球—构造地质学] P588.21[天文地球—地质学]