松辽盆地北部中二叠统碳酸盐岩元素和稳定同位素地球化学特征与古环境

松辽盆地中二叠统哲斯组发育一套暗色泥岩、灰色砂砾岩与灰岩沉积组合,灰岩中产海相腕足类和双壳类化石。由于受钻井进尺和取心资料限制,前人对盆地覆盖区晚古生代地层及其沉积环境研究较少。对采自松辽盆地杜101井、杜103井剖面哲斯组的白云质灰岩进行了系统的稳定同位素和微量元素、稀土元素分析,首次获取了盆地覆盖区中二叠统灰岩元素地球化学资料。测试结果显示碳同位素δ¹³C_PDB值一般>0(均值为1.68‰),氧同位素δ¹⁸O_PDB值全部<-15‰(均值为-19.98‰),碳、氧稳定同位素关系图解显示二者呈正相关关系,表明白云岩化作用对灰岩同位素改造作用明显。稀土元素(REE+Y)总量为56.88×10^-6~143.72×10^-6,均值为108.92×10^-6,PAAS标准化后显示具有轻稀土相对于中稀土和重稀土亏损、δEu(均值0.94,最大值1.57)负异常、δCe(均值0.87,最大值1.06)普遍负异常和相对高的Y/Ho均值(均值34.59)等特点,与正常海相具有大体相同的稀土元素配分模式;而微量元素Ba(均值281.55×10^-6)、Zn(均值71.91×10^-6)、Y(均值17.44×10^-6)、Zr(均值117.75×10^-6)和Rb(均值66.84×10^-6)等元素含量整体较高,Sr/Ba(均值2.91)、Th/U(均值2.75)、V/Cr(均值1.45)和V/(V+Ni)(均值0.66)等比值适中,指示研究区哲斯组沉积时期主体为一种频繁受陆源碎屑混染作用影响的碳酸盐台地或滨海岸环境。

Element and Isotope Geochemical Characteristics of Middle Permian Carbonates and Paleoenvironment in the Northern Songliao Basin

The Middle Permian Zhesi Formation comprises a suite of dark mudstone, gray sandstone, and carbonate in the Songliao Basin, of which the limestone contains marine brachiopod and bivalve fossils. Limited by drilling footage and core data, few previous studies were conducted on the upper Paleozoic strata and sedimentary environment of the basin. Compositions of trace elements (incl. rare earth elements (REEs)) and carbon-oxygen isotopes were measured on the Zhesi Formation dolomitic limestone in the well D101 and D103, and we first presented the element geochemical data of these Middle Permian limestone in the basin. The results show that the δ¹³C_PDB values are generally >0 (mean 1.68‰) and δ¹⁸O_PDB values < -15‰ (mean -19.98‰), the carbon-oxygen isotope plot indicates positive correlation between the two isotopes, which suggests that dolomitization has obvious effect on the isotopic transformation of limestone. Total $sum$REE+Y contents of the dolomitic limestone range from 56.88×10^-6 to 143.72×10^-6 (mean 108.92×10^-6). Compared with MREE and HREE, PAAS-normalized LREE depletion was observed with no or negative δEu anomalies (mean 0.94, max 1.57). Negative Ce anomalies are common (mean δCe 0.87, max 1.06). The samples have relatively high Y/Ho ratios (mean 34.59), and the same REE distribution pattern as normal marine facies. The mean trace element contents are relatively high in general, such as Ba (281.55×10^-6), Zn (71.91×10^-6), Y (17.44×10^-6), Zr (117.75×10^-6) and Rb (66.84×10^-6), and the samples have medium mean ratios of Sr/Ba (2.91), Th/U (2.75), V/Cr (1.45), V/(V+Ni) (0.66). We concluded that the region was a carbonate platform or coastal environment affected by terrigenous debris in the Middle Permian.