Pre-Hercynian gabbro-diorites from Kuban river head (Northern Caucasus): geodynamical typification, geochemistry and minerageny

Abstract

The relevance of the work. Igneous rocks in the Greater Caucasus are present in sections from the Ordovician to the Quaternary. Dozens of different ages of magmatic complexes, confined to narrow structural and formation zones, are identified here. However, geochemical studies of rocks, based on an analysis of the distribution patterns of rare-earth and rare-metal elements, are still insufficient and this problem primarily concerns the Early Paleozoic formations. Object. In connection with this object, the bodies of pre-Hercynian, fire-bound gabbro-diorites of the “Amankol” complex, locally distributed in the upper part of Kuban river, were selected for research. Methods. The research methods, in addition to geological and petrographic descriptions, included quantitative RFA and ICP-MS analyzes of magmatites, consideration of geochemical characteristics, construction of discriminatory diagrams, their comparison with rocks (samples) of similar petrochemical types formed in various geodynamic settings. Results. As a result of a comprehensive study, it was found that gabbro-diorites are high-alumina, moderately titanic, moderately magnesian rocks of the calc-alkaline series. It is assumed that the initial melt of the basic composition was formed at a low (1-3 %) degree of melting of spinel peridotites. The low values of Mg # 0.42-0.54, and the Ni content suggest that, during the evolution of the melt, olivine and pyroxenes were fractionated. Incompatible elements in gabbro-diorite and REE, normalized to N-MORB and chondrite, form slanted line spectra located between the OIB and E-MORB profiles, Laн/Ybн ratio in the range of 6.4-8.0. The rock is characterized by increased concentrations of LILe, LREE, MREE and elements indicators of the fluid subduction component – Ba, U, Sr. On the petrogenetic diagrams, the figurative points of gabbro-diorites gravitate toward the fields of island-arc tholeiites, andesites of oceanic islands, or active continental margins. Negative anomalies of Nb, Ta, P, Ti, Hf indicate that rocks were formed under suprasubduction conditions. Comparison of gabbro-diorites with rocks of similar petrochemical types showed that they are well comparable with the rocks of mature volcanic island arcs. Geochemical specialization of rocks of the “Amankol” complex is lithophilic-chalcophilic. Based on the results obtained, it was assumed that in the Silurian time (450-400 million years ago), an island-arc geodynamic situation existed in the north of the Paleotetis in the Proto-Caucasian region.