Long-lived center of youngest volcanism in the Borzhomi region of Georgia: Isotopic-geochronological evidence

Abstract

One of the well-known areas with youngest volcanism in the Lesser Caucasus is located in the vicinity of the Borzhomi City (central Georgia) famous by mineral water springs. This relatively small area hosts four volcanoes (Tsitelidabadzveli, Tsikhisdzhvari, Mukhera, and Sargavi), which yielded extended (from 2 to 13 km long) flows of andesite to, subordinate, basaltic andesite lavas (figure). In scientific works dedicated to different aspects of Neogene‐Quaternary magmatism of the Caucasus region, manifestations of basaltic andesite‐andesite volcanism in the examined part of the Lesser Caucasus mountainous system are usually united into the Bakuriani‐Borzhomi neovolcanic area [1 and others]. It is composed of Upper Cretaceous carbonate bands, Paleocene‐Lower Eocene terrigenous rocks of the Borzhomi series, and Middle Eocene‐Oligocene volcano-sedimentary sequences. Young lava flows a few tens of meters thick known in this area lie with stratigraphic and angular unconformities upon the Paleogene basement spreading over significant part of studied territory (Fig. 1). It should be noted that, according to the recent tectonic map of Georgia [2], magmatic activity in the Bakuriani‐Borhomi area was concentrated in the Adzharia‐Trialeti zone of the Lesser Caucasus fold system with volcanoes confined to the linear near-latitudinal Bakuriani abyssal fault zone at its intersection with the meridional Samsari Fault. Tsitelidabadzveli Volcano (2320 m) is located on the northern slope of the Trialeti Range 12 km south of Borzhomi. It represents an oval meridionally extended edifice up to 100 m high composed of pyroclastics (scoria, breccia, lapillis) with subordinate interbeds of dark gray and reddish lavas. Basaltic andesite lava flows erupted by this volcano fill a spacious depression 8 × 6 km in size and form the Dabadzveli Plateau, the surface of which is located at altitudes of 2100 to 1600 m and is slightly inclined in the northern direction toward the Kura River valley. Basaltic andesites of the Tsitelidabadzveli Volcano (samples YUG-156 from the volcano cone and YUG-157 from lavas of the Dabadzveli Plateau) represent massive to vesicular porphyric to subordinate aphyric rocks. Phenocrysts in porphyric varieties are dominated by plagioclase (labradorite) and clinopyroxene (augite) accompanied by subordinate orthopyroxene (hypersthene) and olivine. Tsikhisdzhvari Volcano (1818 m) is located 2.5 km