Geochronology of Pliocene volcanism in the Dzhavakheti Highland (the Lesser Caucasus). Part 2: Eastern part of the Dzhavakheti Highland. Regional geological correlation

Abstract

The results of isotopic-geochronological study of the Pliocene volcanic rocks in reference sections and volcanic edifices of eastern part of the Dzhavakheti Highland (the northwestern Lesser Caucasus) are considered. The isotopic-geochronological data obtained here are correlated with data on western part of the Dzhavakheti Highland, which have been considered in previous part of this work. Based on correlation, time spans of principal volcanic events of the Pliocene in the study region as a whole are determined, and general trends of the young magmatism evolution within the region are established. In sum, the isotopic-geochronological dates evidence that the Pliocene magmatism of the Dzhavakheti Highland developed practically without essential breaks during the period of about 2 Ma long, from 3.75 to 1.75–1.55 Ma ago. The areal basic volcanism that was most widespread at that time is divisible into five discrete phases according to the isotopic dates obtained. Comparatively short pauses, which separated these phases of magmatic activity, were a few hundreds thousand years long, not more. Chemical composition of moderately acidic to silicic volcanics, which are of a limited distribution in the Dzhavakheti Highland, and their age relations with basic lavas of the region suggest that they are most likely the differentiation products of parental basic mantle-derived magmas. The analyzed distribution of volcanic centers, which erupted basic lavas of the Dzhavakheti Highland, evidence that first two phases of basic magmatism were connected here with volcanic activity in southwestern part of the region (northern termination of the Egnakhag Ridge), whereas activity of volcanoes situated on the east, predominantly in water-shed part and on slopes of the submeridional Dzhavakheti Ridge, controlled development of the third and fourth phases. Consequently, magmatic activity of the Pliocene stage in history of the Neogene-Quaternary magmatism of the Dzhavakheti Highland laterally migrated from the west to the east, being controlled by development of regional submeridional extension zones. Volcanic ridges marking the latter are formed by volcanic edifices, which are amalgamated at their bases and have erupted lavas of close age and composition. The migration of volcanic activity can be described in terms of the “domino effect,” when cessation of volcanism in one zone led to formation of the other submeridional zone of extension and magmatic activity displaced from the west eastward in sublatitudinal direction. In general, evolution of the Pliocene magmatism of the Dzhavakheti Highland, was similar, despite the essential regional peculiarities, to the generalized trend of magmatism evolution in the continental rifts and intraplate zones of the “hot-spot” type.