四川江油马角坝地区船山组核形石灰岩稀土元素分布特征及其意义  

REE Characteristics and Their Significance of Oncolite Carbonates of the Chuanshan Formatiom in Majiaoba,Jiangyou region

作　　者：徐锦龙[1,2] 洪天求[2] 贾志海[2] 王伟[2] 罗雷[2] 

机构地区：[1]安徽省地质调查院,合肥230001 [2]合肥工业大学资源与环境工程学院,合肥230009

出　　处：《高校地质学报》2012年第4期724-734,共11页Geological Journal of China Universities

基　　金：国家973前期预研项目<川东地区晚二叠世隐伏生物礁的识别及其对油气储集的控制研究>(2010CB434809)资助

摘　　要：该文采集四川江油马角坝地区沉水剖面和双龙剖面船山组核形石灰岩样品,采用电感耦合离子质谱对样品进行了系统的稀土元素测试分析。结果表明,研究区核形石灰岩稀土元素总量较低,介于6.26×10-9~25.1×10-9之间,均值为15.3×10-9。LREE相对富集,HREE相对亏损,但总体轻重稀土元素分馏程度不高,且轻稀土元素分馏程度比重稀土元素稍高,分馏程度可能与核形石的成因有密切关系。δCe介于0.43~0.74之间,均值为0.59,表明Ce亏损严重,指示了船山期的海水总体属于强氧化状态。Y/Ho值接近现代海水Y/Ho值的下限,表明当时海水可能受到冰融淡水影响。稀土元素不仅来源于海水,还与生物碎屑的含量和核形石的成因有密切关系。Oncoids are well preserved in the Chuanshan Formation in Majiaoba,Jiangyou county. ICP-MS is used to determine REE contents of oncolite carbonate rocks of the study area. The analyses of the REEs were conducted to determine their depositional environments including ∑REE, LREE, HREE, δCe, δEu and Y/Ho. The total REE contents （ ∑REE ） are between 6.26 × 10^-9 and 25.12 × 10^-9. The average ∑REE value of all samples is 15.29 × 10^-9 and below the average value of NASC （ North American Shale Composite ） （ 160.12 × 10^-9 ）. LREE are relatively rich and HREE are a little depleted. The different LREE and HREE values of the samples indicate that REE were removed, LREE were fractionated more easily than HREE, and REE values are closely related to sedimentary environment and the genesis of oncolite. NASC normalized REE patterns display low REE enrichments and relatively high REE loss. The LREE patterns show a little leftward heeling, and the HREE patterns show flat one. If the interference of the terrestrial detritus and iron manganese concretion are suppressed, the chemical property of cerium provides an important clue for inferring redox environments of ancient seawater masses. The δCe is between 0.43 and 0.74, and the average value of all samples is 0.59. They all have obviously negative Ce anomalies,indicating strong oxidation environments during the Chuanshan Stage. Ce anomalies are compatible with their sedimentary environment. Some samples have negative Eu anomalies and some have positive ones,which may indicate that REE characteristics were affected by ancient climate cycles （ zlacial-inter^lacial cycle ） and the periodical sea level changes, and should be further studied. Y/Ho ratios are closed tothe lower limit of modem seawater,which implies that the ancient seawater was affected by the ice melt water. The REE were not only derived from the seawater, but also affected by the bioclast and the genesis of oncolite.

关 键 词：稀土元素 核形石灰岩 碳酸盐岩 船山组 江油马角坝地区 

分 类 号：P539[天文地球—古生物学与地层学] P595[天文地球—地质学]