Zircons U-Pb Geochronology of the Mtkvari Ignimbrites Flow, Samtske-Javakheti Volcanic Highland, Lesser Caucasus

摘要

In the Samtskhe-Javakheti Volcanic highland (Lesser Caucasus), in the Mtkvari river canyon a large volcanic pyroclastic flow outcrops, which is represented by ignimbrites of andesitic-dacitic composition. Despite the fact, that one of the important monuments of Georgian cultural heritage cave-city Vardzia is hewn into this flow, the question of its geometry and geochronology is still unclear. Field works showed that the Mtkvari ignimbrites flow traced continuously from the Karzameti fortress to the Khertvisi fortress for more than 35 km. Thickness of the flow increases obviously from the magma centre (40 m on average) to the periphery (80 m on average), though its width is not still defined. The flow is inclined to the northern direction by 2-4° angle. Perfectly straight surface of the flow and its bottom conforms to the relief, which confirms its magmatic formation. The flow is cut by a regional fault the Mtkvari canyon. At the same time, together with its enclosing rocks it is divided into smaller blocks by cutting local faults. In isotopic laboratory of the National Taiwan University the zircons of this pyroclastic flow were dated by U-Pb method, using LA-ICP-MS equipment. The samples were taken from three places: at the end of the flow, near the Khertvisi fortress (at 35 km; #13Geo4), in the central part of the flow, near the Vardzia cave-city (at 15 km; #13Geo5) and at the beginning of the flow, near the Karzameti fortress (#13Geo6). The results are practically identical within the margin of error and correspond to the Upper Miocene epoch: #13Geo4=7.50±0.42 Ma; #13Geo5=7.54±0.21 Ma; #13Geo6 =7.52±0.21 Ma. These almost identical results of the U-Pb geochronology from various sections of the Mtkvari volcanic flow indicate great credibility of this dating.