惠民凹陷渐新世玄武岩地球化学特征及大地构造意义

采用主量、微量元素和Sr-Nd同位素地球化学研究手段和大地构造分析方法,旨在阐明惠民凹陷渐新世玄武岩地球化学特征和揭示渤海湾盆地形成的地球动力学背景。研究认为:渐新世沙河街组三段玄武岩为碱性橄榄玄武岩;呈轻稀土富集型分布模式,无Eu异常,稀土总量较低(∑REE=109.4～146.8μg/g);大离子亲石元素富集,蛛网图显示负斜率较大的右倾曲线,并有不同程度的Nb-Ta和Sr元素富集、以及Ti和Y元素的轻度亏损,含量较低的不相容元素基本上呈平滑分布;Sr-Nd同位素组成较为均一,玄武岩岩浆主要来自亏损软流圈地幔,同时存在轻微的岩石圈地幔混染作用。综合分析区域地质和地球物理研究成果认为,包括惠民凹陷在内的渤海湾盆地是地幔上隆而造成岩石圈伸展减薄的产物,是太平洋板块向欧亚板块之下俯冲形成的陆内扩张盆地。

GEOCHEMISTRY OF BASALTS DURING OLIGOCENE IN HUIMIN DEPRESSION AND ITS GEOTECTONIC SIGNIFICANCE

In order to show the geochemical characteristics of the Oligocene basalts in Huimin depression and to discover the original geodynamic background of Bohai Bay Basin, this paper studies the major elements, trace-REE elements and Sr-Nd isotopes in the bulk rock samples by using a variety of analytical methods. The results indicate that the basaltic rock samples collected from the three members of the Shahejie Formation (E_3 s 3) during Oligocene belong to alkaline olivine basalt and all samples show LREE enrichment distribution pattern and no Eu anomaly, ∑REE is relatively low (109.4～146.8 μg/g), while large iron lithophile elements (LILE) in the basalts are generally enriched, the spider diagram shows a negative slope curve, the sample areas being enriched in Nb-Ta and Sr to different degrees but lightly depleted in Ti and Y relative to other elements. In addition, the incompatible elements basically show smooth patterns. The comparatively homogeneous Sr-Nd isotopic components suggest that the basalts were probably derived from asthenosphere-depleted mantle and underwent slightly lithosphere-mantle contamination. By combining field geologic research with geophysical investigation, a conclusion was drawn in the paper that Bohai Bay Basin, including Huimin depression, was derived from lithosphere extensional attenuation due to mantle upwelling and it is the inner-continental extensional basin that is formed from the Pacific tectonic plate being subducted into the Eurasian tectonic plate.