羌塘盆地南部古油藏带布曲组碳酸盐岩稀土元素特征及意义

摘要

The REE characteristics of different carbonate rock types were studied based on petrologic analyses of carbonate rocks from the Buqu Formation in ancient oil pools in the southern Qiangtang Basin. The carbonate rocks in the study area were classified into limestones, clay?dolomicrites, ( residual) grain dolostones, crystalline dolo?stones and saddle dolomites. All kinds of carbonate rocks have a low REE content (∑REE ) , and some of the saddle dolomite samples have a higher ∑REE than limestones, indicating the mixing of outside liquids. Different carbonate rocks have different REE contents, but their distribution patterns are similar due to the standardization of surface waters of the Pacific Ocean. The LaN/YbN ratio averages 2.86, with a slight light rare earth concentration and a heavy rare earth loss. Light rare earth concentration degree decreases with the increase of dolomitization. The positive δCe anomaly (average 2.94) and positive δEu anomaly (average 1.43) showed that the Buqu carbonate rocks were deposited in an oxic environment. Dolomitization fluids mainly came from oceanic formation water in the same period. The diagenetic environment changed to reductive after hydrocarbon charging. At the end of Early Cretaceous, the Lhasa and Qiangtang blocks shear collided, helping atmospheric fresh water enter the Buqu Forma?tion. Affected by the Cenozoic tectonic thermal event again, saddle dolomites were formed. Tectonic thermal events provided temperature conditions for Eu2+ concentration.%在羌塘盆地南部古油藏带布曲组碳酸盐岩的详细岩石学分析基础上,研究不同类型碳酸盐岩的稀土元素特征,结果表明:①研究区碳酸盐岩包括灰岩、泥—微晶白云岩、(残余)颗粒白云岩、晶粒白云岩及鞍形白云岩;②各类碳酸盐岩均具有较低的稀土元素总量,能够指示沉积或成岩流体的性质,部分鞍形白云岩样品稀土元素总量高于灰岩,预示着可能有外来流体的加入;③不同类型碳酸盐岩稀土总量具有差别,但经太平洋表层海水标准化后,具有相似的稀土元素配分模式,LaN/YbN平均值为2.86,轻微的轻稀土富集和重稀土亏损,随着白云石化程度的增加,轻稀土富集程度变弱,δCe正异常(平均值2.94)和δEu正异常(平均值1.43),这些特征说明布曲组碳酸盐岩沉积于氧化性质的水体中,白云石化流体主要为同期海源地层水,但在烃类充注以后成岩环境变为还原环境,受早白垩世末期拉萨地体与羌塘地块剪刀式碰撞,研究区布曲组地层中有大气淡水加入,再次受到新生代构造热事件影响,形成鞍形白云岩,构造热事件为Eu2+富集提供温度条件.