羌塘盆地角木日地区二叠系碳酸盐岩元素地球化学特征及其对古沉积环境的指示

Geochemical Characteristics and Its Application to Depositional Environment Analysis of Permian Carbonates in Jiaomuri Area,Qiangtang Basin

对采自羌塘盆地角木日地区的二叠系碳酸盐岩,包括下二叠统展金组白云岩和中二叠统龙格组灰岩进行了元素地球化学分析,并讨论了其对古沉积环境的指示意义。结果表明,样品Y/Ho比值平均为45.14,Gd表现为轻微Gd正异常,符合海相灰岩特征;Si O2和Al2O3的平均含量较低,反映研究区陆源物质的侵入较为有限。碳酸盐岩中∑REE含量较低,稀土元素分异程度整体较弱,轻稀土元素相对重稀土元素呈轻微富集特征,Ce,Eu均具有明显负异常,结合V/Cr,V/(V+Ni),Sr/Ba及Sr/Ca等微量元素含量及比值特征认为,研究区早—中二叠世沉积期海水总体呈氧化环境。纵向上龙格组顶部表现出氧化性增强、盐度增大的趋势,推测可能与当时水体变浅有关,与早二叠世晚期研究区构造应力由拉张向挤压的变化有密切关系。

In order to study the Permian paleoenvironment of the Jiaomuri area （Qiangtang Basin）, geochemical analysis were taken from the dolomite samples of Lower Permian Zhanjin Formation and limestone samples of Middle Permian Longge Formation. Our data show that the average of the Y/Ho ratio is 45.14 while element Gd shows light positive anomaly, suggesting normal marine carbonate characteristics. The contents of SiO2 and A1203 are low indicating relatively limited terrestrial input. The low total REE contents （EREE） and the different LREE and HREE values of the samples indicate that REE were removed and LREE were fractionated more easily than HREE. The samples have obviously negative Ce and Eu anomalies showing an oxidizing paleoenvironment, which further supported by the analysis of V/Cr, V/（V ＋ Ni）, Sr/Ba and Sr/Ca ratios. Up-section, the Permian samples showed higher salinity and oxidbillity, which was supposed to be related to water depth shallowing related to a local compression tectonic trend.