New Evidence for the Age of Young Volcanic Formations in the Chegem River Basin, North Caucasus, Russia

Abstract

Isotope geochronology applied to several occurrences of young volcanic activity on the northern slope of the Greater Caucasus in the Chegem River basin, Kabardino-Balkaria, Russia, which were classified by different researchers as belonging to either the Jurassic Period or to the Late Cenozoic, aided toward determining their age and place in the regional geochronological scale of the Neogene–Quaternary magmatism occurring in the Elbrus neovolcanic area. We have established stratigraphic interrelationships between the geological objects studied here and the products of catastrophic explosive volcanism, which resulted in the formation of the large Chegem Caldera in the study region at the end of the Neogene. We determined more accurately whether the volcanic vents that were active in the Late Pliocene could be classified as belonging to the pre-caldera, syn-caldera, or post-caldera stages of magmatism. The results of K–Ar dating obtained here show that the endogenous activity in the region of study was of areal character during the pre-caldera stage (about 3.1 Ma). During that period volcanic occurrences were taking place at many small volcanic vents that are at present completely demolished and are marked by dikes and stocks of rhyolites, less frequently of trachyandesites. At the end of the Pliocene, during the period 2.9–2.8 Ma, catastrophic explosive eruptions occurred giving rise to the Chegem Caldera filled with a thick sequence of rhyolite–dacite ignimbrites. According to the data acquired by us, a series of acid extrusions and dikes composed of vitrophyres were emplaced along its western, southern, and eastern periphery simultaneously with the caldera formation. The post-caldera stage (about 2.8 Ma) saw activity exhibited by Kum-Tyube and Kyugenkaya stratovolcanoes in the western part of the caldera which discharged lavas of dacitic composition, and later on also of andesitic composition. It was found that no volcanic activity has been occurring during Quaternary time throughout almost all of the basin of the upper reaches of the Chegem River. One exception is furnished by the environs of the Aktoprak Pass in the northwestern part of the area of study where local manifestations of moderately-alkaline magmatism were recorded to occur in the Early Pleistocene (about 1 Ma). Several geological objects (Bashil Dam and others) which were previously dated by some researchers to the period between the end of the Pleistocene and the Holocene are in fact either Pliocene syn-caldera extrusions or were formed by exogenous processes unrelated to volcanic activity.