Geochemistry and Evolutionary Characteristics of Rare Earth Elements in Ordovician Carbonate-Evaporite Rocks of the Central-Eastern Ordos Basin, Central China

This study aims to reveal the geochemical behavior characteristics of rare earth elements (REEs) in the process of seawater evaporation and concentration by analyzing the REEs and their variation characteristics in different stages of rocks from the rock system of carbonate-gypsum-salt (C-G-S system) of the Ordovician in the central-eastern Ordos Basin, China. The results indicate: despite the low overall content of REEs in the endogenous deposition process of the C-G-S system in this area, a relatively consistent REEs distribution pattern is still exhibited in different types of rocks formed in different stages of seawater evaporation and concentration, showing a relative enrichment of light rare earth elements (LREEs) and obvious negative europium (Eu) anomalies; and its REEs content is not directly related to the content of continental source detrital minerals. All this means that REEs do not enter the crystal structure of the main rock-forming minerals mainly in the form of isomorphous substitution during the mineral crystallization process but are more likely to aggregate in the form of extremely fine particles, exist in the lattice defects and mineral surfaces of the main rock-forming minerals, or be enclosed by subsequent growth of mineral crystals. Also, during the dolomitization transformation from limestone to dolomite, REEs undergo relatively significant outward migration.