About the marbles of the Dizi series (Greater Caucasus): geochemical features, the nature of the protolith, and paleoecological conditions of their formation

The Dizi series formed on the southern passive margin of a small oceanic basin on the Southern Slope of the Greater Caucasus and is exposed in the structural zone of the same name. The series is composed of metaterrigenous phyllite-like and volcanogenic rocks of the Devonian-Triassic age; marbles also are quite widespread in the series. During the Variscan orogeny, all rocks of the series underwent low-temperature regional metamorphism; later, in the Middle Jurassic time, intensive contact metamorphism took place. The object of presented research is marble, found in the rocks of the series both in the form of lenses and blocks. Interest in the study of marbles spread in the Dizi series is caused by several factors, namely, there is still no consensus on the position of marbles in the series, their detailed geochemical study has not been carried out, and the conditions of their formation have not been determined. There is an opinion that the Dizi series represents part of the Triassic Upper Karakaya complex, which was probably deposited in the forearc of the southern margin of Laurasia. The marbles are included in the series as exotic blocks of shallow marine Carboniferous and Permian limestone, most likely formed in Gondwana. During the study of the Dizi series marbles petrographic and geochemical research methods were applied. Petrographic characteristics of the marbles from all outcrops show a similar mineral composition, and structure and texture as well. The authors analyzed the main components, RE, and REE of the Dizi series marbles. The ratios Ca VS Mg, Mg/Ca VS SiO₂, and Sr VS Mg/Ca show that the protolith of the marbles was pure limestone. The reconstruction   of the deposition conditions of the marble protolith, based on the concentrations of Sr, Rb, and Ba, indicates the formation of limestones in the conditions of the continental margin. The Ce-negative anomaly observed in the marble samples is possibly due to the existence of an oxygenated marine environment, probably of shallow water conditions. These conditions fully correspond to the tectonic interpretation of geochemical data from other metasedimentary rocks of the series. The results of the presented studies confirm that the marbles under consideration were formed in situ, together with the other rocks of the Dizi series in the conditions of the continental slope and its foot on the southern passive margin of a small oceanic basin of the Southern Slope of the Greater Caucasus.