藏南白垩系泥、页岩微量、稀土元素特征及氧化—还原环境分析  

The Characteristics of Trace Element and REE Geochemistry of the Cretaceous Mudrocks and Shales from Southern Tibet and Its Analysis of Redox Condition

作　　者：熊国庆[1] 江新胜[1] 蔡习尧[2] 伍皓[1] 

机构地区：[1]成都地质矿产研究所,四川成都610082 [2]中石化石油勘探开发研究院西北分院,新疆乌鲁木齐830011

出　　处：《地球科学进展》2010年第7期730-745,共16页Advances in Earth Science

基　　金：国家自然科学基金项目"青藏高原隆起前东亚白垩纪大气环流样式研究"(编号:40372064);中国地调局青藏高原专项"青藏高原构造-岩相古地理编图与研究"(编号:1212010610101)资助

摘　　要：为了解藏南定日—岗巴盆地和江孜盆地白垩纪沉积时底层水体的氧化—还原环境,采用ICP-MS方法进行微量元素及稀土元素测试分析。结果表明:研究区微量元素在时空上均发生了变化,这与各元素对沉积水体的氧化—还原敏感程度有关。元素Al标准化后变化趋势也不尽相同,其垂向上的变化反映了沉积物底层水体氧化—还原状况、生物产率等沉积环境,与海平面升降、陆源碎屑输入、成岩作用再活化及沉积后氧化作用等多因素有关。V/(V+Ni)、U/Th、V/Cr、Ni/Co、Re/Mo、Uau及δU比值表明早白垩世及晚白垩世中、晚期藏南地区盆地内底层水体环境总体上为中等分层的含氧环境,但定日—岗巴盆地内晚白垩世早期Cenomanian-Turonian界线附近微量元素发生突变,显示为强烈的还原环境。稀土元素球粒陨石标准化后均表现为轻稀土富集的右倾型,LREE/HREE介于7.42～14.80之间,LaN/YbN变化范围为7.93～18.54,δEu负异常,δCe异常不明显,表明为含氧环境。因此微量、稀土元素微观分析与野外宏观露头观察相结合,不但可以作为地层划分对比的直接依据,且能更准确、有效地判断地层沉积时底层水体的环境状况。To understand the redox conditions of bottom water column of the Cretaceous mudrocks and shales during their deposition from two main marine basins in southern Tibet,the tests and analysises of trace elements and REEs from the samples were done by the ICP-MS method. The results reveal that the trace elemens in study area have changed both spatially and temporally,and the changes are related to redox-sensive extent of those elements to the water-column. The variable trends of all Al-normalized elements are almost different and their vertical variations have reflected redox condition of bottom water column of sediments and biological production,related to multiple facters：sea-level osccilation,the flux of terrigenous debris,remobilization of diagensis and post-deposition oxygen. The ratios of V/（ V ＋ Ni） 、U/Th、V/Cr、Ni/Co、Re/Mo、Uau and δU indicate that condition of bottom water column in southern Tibet basins is generally moderate-stratified oxic condition during the early cretaceous and middle and late cretaceous,but those elements have suddenly changed between the Cenomanian-Turonian boundary during the early cretaceous in Gamba-Tingri basin,and revealed anoxic condition. Chondritic-normalized REEs both exhibit LREE enriched,right-tilted,LREE/HREE between 7. 42 and 14. 80,LaN /YbN range from 7. 93 to 18. 54,δEu negative abnormal,δCe unclearly abnormal,showing oxic condition. The analysis of trace elements and their REEs combined with field observation of outcrop can be direct foundation for divided and contrasted strata,and determine the condition of bottom water column more accurately and effectively during the deposition.

关 键 词：微量、稀土元素 底层水体 氧化—还原环境 含氧环境 Cenomanian-Turonian界线 球粒陨石标准化 

分 类 号：P595[天文地球—地球化学]