Limestones of the Uk Formation (Upper Riphean, the Southern Urals): Effect of contamination and diagenetic fluids on the distribution of rare-earth elements and yttrium

Abstract

Research subject. A study was conducted into the geochemical features of various lithotypes of carbonate rocks (granular and clastic limestones, limestones with planar, planar-columnar and columnar stromatolites) from the upper subformation of the Uk Formation. This formation crowns the Upper Riphean Karatau Group on the Western slope of the Southern Urals (Bashkir meganticlinorium). Material and methods. The research material was two collections of carbonate rocks (11 and 32 samples), the general geochemical features of which were previously considered in publications in 2019. The main research method was the analysis of the correlation relationships of the main rock-forming components, trace elements and their indicator relationships. Research results. It was established that all lithotypes are characterized by a significant content of both the main rock-forming components and the trace elements that make up the insoluble residue/fine-grained terrigenous admixture. This suggests that the area of accumulation of carbonate sediments was located near the continent, which served as a source of a significant amount of fine-grained/clay suspension. Conclusions. A strong positive correlation was observed between K2O and REE in the carbonate rocks of the Upper Uk Subformation, and a strong negative correlation was observed between REE and CaO. This suggests that the REE content in carbonate rocks is controlled mainly by terrigenous/clay admixture. A positive correlation is also observed between REE and Th, Zr, and Al2O3. All this shows that REE in carbonate rocks are mainly present in clay admixtures. The correlation coefficient between Ce/Ce* and (Dy/Sm)N is 0.20, while this coefficient for Eu/Eu* and (Pr/Sm)N equals 0.12. Thus, a definite conclusion about the effect of diagenetic fluids on the distribution of REE cannot be made. The data obtained show that the distribution of REE and Y in our collection of carbonate rocks of the Upper subformation of the Uk Formation of the Southern Urals is controlled mainly by clay admixtures.