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

Abstract

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 byV-shapecurve 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.