下扬子二叠系泥页岩稀土元素地球化学特征及地质意义

Geochemical Characteristics of Rare Earth Elements and Their Geological Significance of Permian Shales in Lower Yangtze Area

为探讨下扬子地区二叠系泥页岩的稀土元素特征、物质来源及其地质意义,对研究区昌桥剖面大隆组、龙潭组和孤峰组的泥页岩样品进行了微量和稀土元素地球化学测试.测试结果表明该地区稀土元素(∑REE)含量较高,∑REE平均值为259.15μg/g,轻稀土元素富集,重稀土元素亏损.稀土元素的配分曲线整体表现为"右倾",Eu呈明显的负异常,Ce有轻微的负异常.样品的Ceanom值(平均值为-0.07)和δCe值(平均值为0.90)反映了沉积时水体为缺氧的还原环境,有利于形成优质烃源岩,(La/Yb)N值的变化反映研究区在垂向上的沉积速率为:大隆组<孤峰组<龙潭组.稀土元素数据还表明研究区物源来自上地壳,母岩为长英质岩石,以花岗岩为主.利用La-Th-Sc、Th-Sc-Zr/10、Th-Co-Zr/10三角图版和La/Th-Hf图解,判断得到研究区的构造背景为被动大陆边缘.

In order to find out the characteristics,provenance and geological significance of rare earth elements(REEs)in Permian shale in Lower Yangtze area,16 samples collected from Dalong Formation,Longtan Formation and Gufeng Formation were selected for the trace and REE analyses.The results show that the REE distribution in research area is similar with upper crust,which is characterized by obvious"right-leaning"distribution pattern,with enrichment in LREE and depletion in HREE.In addition,Eu shows negative anomaly in a distinct"V"shape and Ce generally shows a slight negative anomaly based on REE curve.The characteristics of Ceanom(-0.07)andδCe(0.90)indicate that the mudstone is deposited in an anoxic environment which is conductive to the formation of excellent source rock.The value of(La/Yb)Nreflects the vertical sedimentation rate in the study area as follows:Longtan Formation<Gufeng Formation<Dalong Formation.In the diagrams of La/Th-Hf and La/Yb-∑REE,it can be found that the parent rock is feldspathic-quartz,mainly intermediate to granite.By the comparison of REE characteristic parameters from graywackes in different settings,combined with La-Th-Sc,Th-Sc-Zr/10,Th-Co-Zr/10 triangle diagram,a conclusion can be drawn that the tectonic setting of the study area is determined to be a passive continental margin.