LANTHANIDE TETRAD EFFECT IN LIMESTONE: A TOOL TO ENVIRONMENT ANALYSIS OF THE RUTEH FORMATION, NW IRAN

Exhibition of lanthanide tetrad effect in PAAS-normalized REE patterns, and non-CHARAC (non-CHArge-RAdius Control) behavior of Y/Ho and Zr/Hf in limestone of the Ruteh Formation, Kanigorgeh district (NW Iran), were studied in order to understand the reasons of occurrence of lanthanides tetrad effects. The computed values show that the third and fourth tetrads can be used as a good and powerful geochemical tool for investigation of physico-chemical conditions of the depositional environment of the limestones. Here, a new mathematical-based method using polar coordination system for tetrad effect values (T-p ) was used to evaluate under studying limestone. The correlation between T-p and some geochemical parameters revealed that the limestone was likely deposited under two different conditions. The obtained results indicated that paleo-redox conditions, adsorption and scavenging by kaolinite and metallic oxides, degree of detrital input, diagenesis intensity, and complexation by polycarbonate ligands are likely the main mechanisms for occurrence of tetrad effect phenomenon in REE distribution patterns in the limestone. This means that tetrad effect phenomenon in REE distribution patterns of limestone can be applied as a good geochemical indicator to evaluate the deposition conditions in limestones.