内蒙古林西地区中上二叠统界线碳酸盐岩微量元素和稀土元素地球化学特征及其地质意义

Geochemical characteristics and geological significance of trace elements and rare earth elements in carbonate rocks at Middle--Upper Permian boundary in Linxi area,Inner Mongolia

内蒙古林西地区二叠系哲斯组和林西组分布广泛,发育一套暗色泥板岩、砂岩和碳酸盐岩沉积组合,是区域上重要的烃源岩。利用ICP-MS电感耦合等离子体质谱仪分析方法,对林西县官地镇哲斯组与林西组界线附近的碳酸盐岩进行了稀土元素和微量元素分析。结果显示,林西组/哲斯组界线上、下的微量元素和稀土元素构成差异明显。哲斯组稀土元素(∑REE+Y)总量均值为40.51×10^-6,明显小于林西组的均值82.12×10^-6;PAAS标准化后,二者均具有轻稀土相对于中稀土和重稀土亏损的特征,且哲斯组碳酸盐岩相对于林西组显示更明显的δEu正异常和δCe正异常,与正常海相和湖相碳酸盐岩具有不同的稀土元素配分模式。哲斯组/林西组界线碳酸盐岩中的微量元素含量呈现突变特点。其中表征陆源混染作用的Cu、Zn、Rb、Zr等微量元素含量在界线之上显著增大。Sr/Ba>1、V/Cr>4.25和V/(V+Ni)>0.54等特征参数表明研究区中、晚二叠世之际沉积主体为海相、强还原环境。古亚洲洋板块俯冲和岩浆作用及其产生的极端强还原环境和热液影响,是导致研究区哲斯组/林西组界线碳酸盐岩δEu显著正异常和δCe普遍正异常的主要原因。

The Permian Zhesi and Linxi formations are widely distributed in Linxi area,Inner Mongolia,consisting of a set of dark mudstone,sandstone and carbonate rock sedimentary assemblages,which are important source rocks in the region.Rare earth elements and trace elements in carbonate rocks near the boundary between Zhesi and Linxi formations in Guandi,Linxi County were analyzed by ICP-MS inductively coupled plasma mass spectrometry.The results show that there are obvious differences in the composition of trace elements and rare earth elements between Linxi and Zhesi formations.The total average value of rare earth elements(REE+Y)in Zhesi Formation is 40.51×10^-6,which is significantly lower than that in Linxi Formation(82.12×10^-6).After PAAS standardization,both of them are deficient in light rare earth elements relative to medium rare earth elements and heavy rare earth elements.Moreover,the Zhesi Formation carbonate rocks show larger positiveδEu andδCe anomalies than Linxi Formation,with different REE distribution patterns from normal marine and lacustrine carbonate rocks.The content of trace elements in the carbonate rocks at the boundary between Zhesi and Linxi formations shows a sudden change.Among them,the contents of trace elements such as Cu,Zn,Rb and Zr,which characterize terrestrial contamination,increase significantly above the boundary.The characteristic parameters of Sr/Ba>1,V/Cr>4.25 and V/(V+Ni)>0.54,indicating that the main sedimentary bodies were deposited marine and strong reductive environment during the Middle and Late Permian.The subduction and magmatism of the Paleo-Asian Ocean plate and their extreme strong reduction environment and hydrothermal effection are the main reasons for the significant positive anomalies ofδEu and the general positive anomalies ofδCe in carbonate rocks along the boundary between the Zhesi and Linxi formations in studied area.