东濮凹陷北部古近系沙三段地球化学特征及地质意义

沉积岩的微量元素和稀土元素蕴含了大量的地质信息,对研究物源区性质与沉积环境的重塑具有重要的指导意义。本文研究了渤海湾盆地东濮凹陷古近系沙河街组沙三段泥岩和盐岩的微量元素及稀土元素的地球化学特征。结果表明:微量元素Sr、Ba丰度高,其他Rb、V、Ni、Mn、Fe、Cr、Br等元素含量较低,多数微量元素含量比同类岩石克拉克值偏高;∑REE为47.1×10-6～268.02×10-6,平均值为181.55×10-6,接近于后太古宙页岩(PAAS)的平均值,高于大陆上地壳(UCC)平均值。轻稀土与重稀土总量的比值为7.96～11.46,平均值为9.71,其分配模式表现为轻稀土富集,重稀土相对亏损,整体特征为"右倾斜型",但重稀土元素为较平坦。铕(Eu)具有明显的亏损(0.48～0.70),铈(Ce)轻微亏损(0.89～0.97)。而盐岩样品中所含微量元素和稀土元素含量普遍很少。通过对样品中Sr/Cu、V/(V﹢Ni)、(La/Yb)N、Sr/Ba、δCe和Ceanom等特征参数的分析,总结出沙三段属于陆相沉积,处于还原、厌氧、咸水沉积环境;根据La/Th-Hf和Co/Th-La/Sc源岩判别图解、Eu、δEu以及REE—La/Yb等组合特点,显示源岩主要为中酸性长英质岩石(花岗岩)和混合长英质岩/基性岩,古老沉积岩对其贡献不大,主要来自内黄隆起和鲁西隆起的碎屑物质。

Geochemical characteristics of the 3rd Member of Paleogene Shahejie Formation in Dongpu Depression and their geological implications

Abundant geological information can be obtained from trace elements and REE in sedimentary rocks, which may reveal the provenance attribute and tectonic setting and help to reconstruct the paleo -sedimentary environment. Geochemical characteristics of mudstones and rock salt from the 3rd Member of Paleogene Shahejie Formation in Dongpu depression of Bohai Basin were studied in detail in this paper. The results show that the samples have relatively high Sr and Ba and low Rb, V, Ni, Mn, Fe, Cr and Br. Concentrations of most of the trace elements are higher than their Clark values in the same sorts of rocks. The total content ranges from 47.1 × 10~(-6) to 268.02×10~(-6), and the average content is 181.55×10-6,which is similar to the average value of PAAS. The ratios of LREE/HREE range from 7.96 to 11.46, and the average content is 9.71. These data show that LREE are obviously richer than HREE, and the distribution patterns of the samples are characterized by "V" -shaped curve with obvious negative Eu (from 0.48 to 0.70) anomaly and indistinct negative Ce anomaly (from 0.89 to 0.97). In addition, the content of rock salts is much lower than their content in other samples. According to an analysis of the characteristic parameters (Sr/Cu, V/(V+Ni), (La/Yb)_N, Sr/Ba, δ Ce and Ceanom), it is revealed that the study area belonged to continental sedimentation, and its paleo-environment was under the oxygen-free, anaerobic and saltwater condition. The characteristic parameters such as Eu, δ Eu, ωREE-ω(La)/ω(Yb), La/Th- Hf and Co/Th-La/Sc in the discrimination diagram can indicate that the source rocks were mainly intermediate-acidic felsic and mixed felsic/basic rocks as well as very insignificant sedimentary rocks, derived from detrital materials of Neihuang uplift and Luxi uplift.