桂西地区乐平统合山组底部海绵骨针硅质岩的发现及古环境意义  

The discovery of Sponge chert on the bottom of the Lopingian Heshan Formation in western Guangxi and its palaeoenvironmental significance

作　　者：欧莉华[1] 伊海生[1] 王刚[2] 钱利军[1] 胡旭[1] 

机构地区：[1]成都理工大学沉积地质研究院,四川成都610059 [2]成都理工大学地球科学学院,四川成都610059

出　　处：《中国地质》2012年第5期1280-1289,共10页Geology in China

基　　金：中国地质调查局项目"扬子东南缘中生代盆地充填序列与构造成矿事件研究"(1212011085415);"十二五"国家科技支撑计划项目"广西田林-大新地区锰矿沉积地质特征研究"(2011BAB04B10)共同资助

摘　　要：桂西地区乐平统合山组底部广泛发育层状硅质岩沉积,硅质岩成层性好,侧向延伸稳定,层面平整。通过平果县太平镇山营村北、新圩村北及东兰县3条野外剖面的实测、硅质岩样品室内镜下鉴定及稀土元素分析,发现:硅质岩中含丰富的海绵骨针、有孔虫、藻类等化石,化石保存完整;稀土元素特征显示硅质岩稀土总量低,与海水相似,轻稀土略有亏损,但Ce负异常不明显。根据稀土元素及硅质岩生物组合的特征,判断合山组底部的硅质岩形成于滨浅海环境,属相对封闭的大陆边缘环境,同时受到了淡水和海水的影响,主要通过混合水带硅化机理形成,基本没有火山和热液的影响。硅质岩的出现表明由于海侵作用桂西地区水体深度增大,尤其是新圩村剖面位置,广泛台地被海水淹没,海水较洁净,适合海绵、有孔虫、腕足、腹足等生物繁殖,因而发育了硅质岩及灰岩。Chert is widely distributed on the bottom of the Upper Permian Heshan Formation in western Guangxi.The chert layers extend stably on the lateral side,and the bed surfaces are smooth.The authors measured three sections： the north Shanying Village section and the north Xinxu Village section in Taiping Town,Pinguo County,and the Donglan section in Donglan County.In the laboratory the authors conducted the microscopic examination and rare earth element analysis of the chert.The results show that all the chert is rich in organisms such as spongy spicules,foraminifers,and algae,which are well preserved.The low content of rare earth elements is similar to that of sea water,with the slight loss of light rare earth elements and insignificant Ce negative anomaly.According to the characteristics of the rare earth elements and bioassemblages of the chert,the authors hold that the chert on the bottom of Heshan Formation was formed in shallow water and belonged to a relatively closed environment of continental margin;the silicon of the chert occurred in mixed meteoric-marine coastal systems,without the addition of volcanic components and hydrothermal solution.the occurrence of chert indicates that the rapid transgression in late Permian resulted in the increase of the depth of water in western Guangxi,especially along the Xinxu Village section,where the platform was largely submerged,and the water was clean,which was suitable for the breeding of such living things as sponges,foraminiferas,brachiopodas,and gastropods and was hence favorable for the development of chert and limestone.

关 键 词：桂西地区 上二叠统 硅质岩 海绵骨针 稀土元素 

分 类 号：P588.248[天文地球—岩石学] P532[天文地球—地质学]