Petrological-geochemical characteristics of lavas, sources and evolution of magmatic melts of the Kazbek neovolcanic center (Greater Caucasus)

Abstract

The results of petrological-geochemical and isotope-geochemical studies of the Late Pleistocene-Holocene lavas of the Kazbek Neovolcanic Center, one of the largest centers of youngest magmatism in the Greater Caucasus, are presented.
 It has been established that the volcanic rocks of the Kazbek center arise a continuous compositional series basaltic (trachy-)andesites(trachy-)andesitesdacites with a predominance of calc-alkaline intermediate and moderately-acid lavas. The obtained results indicate that the processes of fractional crystallization and mixing of melts had a leading role in the petrogenesis of the rocks. The crustal assimilation was of limited importance; its influence is noticeable only in the rocks of the earliest and late pulses of magmatic activity within the Kazbek center. The common crustal lithologies participated in the assimilation were presented by metamorphosed Jurassic sediments (mainly shales and sandstones), forming the foot of the Kazbek center, and Mesozoic mafic metamorphosed volcanites very rarely. The specific features of AFC processes during the development of the studied magmatic system (including the presence of noticeable amount of water in the melt, the leading role of Amp in the cumulus and the absence of Pl fractionation) led to the appearance of dacitic lavas with geochemical signs of adakites as an evolutional end-member.
 The volcanic rocks of the Kazbek center are derived from trachybasalt magmas, the source of which was presented by the mantle reservoir of OIB-type. Recent and previously published results of studies of the Neogene-Quaternary magmatism manifested within the Greater Caucasus show that the main petrological and geochemical characteristics of this regional mantle reservoir remained constant from the end of the Miocene to the present time.

Quaternary Collision-Zone Magmatism of the Greater Caucasus

“…By contrast, to increase from least to the most radiogenic Gudauri Formation, lava would require excessive amounts of crustal assimilation, inconsistent with its bulk chemistry. It should be noted that Parfenov et al (2019) have suggested that Jurassic sediments from the Mt. Kazbek region could be a potential candidate for a crustal contaminate, but although they have elevated 87 Sr/ 86 Sr ratios (0.718621), their low Sr content (80 μg g −1 ) would again require large (Neill et al, 2015).…”

“…suggest there is a common Caucasus mantle composition that is represented by trachy-basalts similar to oceanic island basalts (OIB) erupted in central Georgia, such that intermediate composition rocks from the central range of the Greater Caucasus have undergone small amounts of crustal assimilation (e.g. Parfenov et al, 2019), whereas the silicic volcanics of the western range require significant (50%) crustal assimilation (e.g. Lebedev et al, 2010).…”