渤海湾盆地南堡凹陷新近系泥岩稀土元素地球化学特征及其地质意义

详细研究渤海湾盆地南堡凹陷新近系馆陶组和明化镇组泥岩的稀土元素地球化学特征。结果表明:样品稀土总量为146.2×10-6~317.1×10-6,平均值为223.3×10-6;w(LREE)/w(HREE)为8.1~14.5,平均值为11.5,相对富集LREE;Eu为明显负异常;δCe基本正常。δCe和Ceanom值的变化反映馆陶组和明化镇组为陆相沉积,水介质条件为富氧环境。稀土元素分布模式、组合特征及其与燕山褶皱带地区不同时代(期)花岗岩和中酸性火山岩稀土元素的对比结果表明:馆陶组和明化镇组地层的物源基本相同,主要来自凹陷北部燕山褶皱带地区于燕山期发育的中酸性火山岩和花岗岩体,少部分来自沉积岩。与不同构造环境杂砂岩的稀土元素特征的对比结果及δCe的特征表明南堡凹陷新近系源区属性为活动大陆边缘背景。

REE Geochemical Characteristics and Geological Significance of Mudstones from Neogene, Nanpu Sag, Bohai Basin

This study presents the REE geochemical characteristics of mudstones from Guantao Formation(Ng)and Minghuazhen Formation(Nm)of Neogene in Nanpu sag,Bohai Basin.The results show that the REE contents range from 146.2×10~(-6) to 317.1×10~(-6),and the average content is 223.3×10~(-6).The ratios of LREE/HREE range from 8.1 to 14.5,and the average value is 11.5.These characteristics show that LREE are(obviously) richer than HREE.The REE distribution patterns of samples are characterized by"V-shape"curve with no obvious Ce anomaly and obviously negative Eu anomaly(from 0.61 to 0.78).According to the characteristics of δCe and Ce_(anom)values,it can be shown that the palaeoenvironment is mainly continental oxidized water conditions.The similar REE distribution patterns of mudstone samples from Ng and Nm strata reveal that they have the similar provenance.The REE characteristics and the geo-analysis show that source rocks are mixing of(intermediate)-acidic magmatite and sedimentary rocks,and the former are the main materials.Compared with intermediate-acidic magmatite of different time from Yanshan fold and thrust belt to the north of Nanpu sag where have extensive intermediate-acidic volcanic rocks and granites in different time,it is indicated that source rocks mainly formed during Yanshan Period.In contrast to REE characteristics of graywacks of different tectonic settings and δCe values,the tectonic setting of source rocks belongs to the active continental margin.