New data on the age of young volcanic formations in the Chegem River basin (North Caucasus, Russia)

As a result of the isotope-geochronological studies of a number of young volcanic activity objects on the northern slope of the Greater Caucasus in the upper Chegem River basin (Kabardino-Balkaria, Russia), which were attributed by the different researchers to both the Jurassic period and the late Cenozoic. Their age and position in the regional Neogene-Quaternary magmatism geochronological scale of the Elbrus neovolcanic region were determined. The stratigraphic relationships of the studied geological objects with the products of catastrophic explosive volcanism, as a result of which the large Chegem caldera was formed in the region at the end of the Neogene, were substantiated. The assignment of volcanic apparatuses active in the late Pliocene to the pre-caldera, syncaldera and post-caldera stages of magmatism was clarified. The obtained results of K–Ar dating show that at the pre-caldera stage (about 3.1 Ma) endogenous activity in the studied region was areal. During this period, a large number of small volcanic apparatuses erupted, which now have been completely destroyed and marked by dikes and stocks of rhyolites, less often trachyandesites. At the end of the Pliocene, in the period 2.9–2.8 Ma, as a result of catastrophic explosive eruptions, the Chegem caldera was formed, filled by a thick layer of ignimbrites of rhyolite-dacite composition. According to the obtained data, synchronously with the caldera formation, a series of acid extrusions and dikes composed of vitrophyres were intruded along its western, southern and eastern periphery. At the postcaldera stage (about 2.8 Ma), the Kum-Tyube and Kuygen-Kaya stratovolcanoes were active in the western part of the caldera, have been erupted lavas of dacitic and subsequently andesitic composition. It has been established that in the Quaternary, volcanic activity was absent in almost the entire territory of the upper Chegem River basin. The exception is the Aktoprak pass vicinity in the extreme northwestern part of the region, where local magmatism manifestations with increased alkalinity were recorded in the Early Pleistocene (about 1 Ma). A number of geological objects (Bashil spindle and others), the formation of which was previously attributed by some researchers to the end of the Pleistocene – Holocene, are in fact either Pliocene syncaldera extrusions or were formed as a result of exogenous processes not associated with volcanic activity.