Chronology of Magmatic Activity and Petrologic–Mineralogical Characteristics of Lavas of Kazbek Quaternary Volcano, Greater Caucasus

Abstract

The paper presents detailed isotope-geochronological, geological, and petrologic–mineralogical data on lavas of one of the greatest Quaternary magmatic area in the Greater Caucasus, the Kazbek neovolcanic center, including polygenetic Kazbek stratovolcano and a number of subordinate volcanic cones in its vicinities. The research was conducted based on a representative collection of more than 150 geological samples that characterize most of the volcanic cones and lava flows of different age, some of which were known previously, and other were discovered by the authors. The high-precision K–Ar data obtained on these materials make it possible to reproduce the evolutionary history of youngest magmatism at the Kazbek center and evaluate the total duration of this evolution at ~450 ka. The magmatic activity was subdivided into four phases (at 460–380, 310–200, 130–90, and <50 ka) with long-lasting interludes in between. Because the latest eruptions occurred in the Kazbek vicinity in the Holocene, this volcano is regarded as potentially active. The volcanic rocks of the Kazbek center make up a continuous compositional succession of basaltic (trachy)andesite–(trachy)andesite–dacite and mostly belong to the calc–alkaline series. The principal petrographic characteristics of the rocks and the composition of their phenocryst minerals are determined, mineral assemblages of these minerals are distinguished in the lavas of different type, and the temperature of the magmatic melts is evaluated. A principally important role in the petrogenesis of the Kazbek youngest magmas is proved to have been played by fractional crystallization and replenishment of mafic melts in the magmatic chambers beneath the volcano, which resulted in their mixing and mingling with the residual dacite melt and the origin of high-temperature hybrid andesite lavas. The comprehensive geological studies, involving interpretation of high-resolution satellite images, allowed the authors to compile the first detailed (1: 25 000) volcanologic map of the Kazbek center and a geochronologic chart supplemented with a stratigraphic column, which illustrate the origin sequence of the volcanic vents and their lava flows, geological relations between them, as seen in reference geological sections, and variations in the composition of the magmatic products with time.