Nature of polygenetic to monogenetic transition of volcanism of Gegham volcanic ridge (Armenia)

In this contribution we discuss the geological structure, temporal and spatial relationships of Gegham upland between polygenetic and monogenetic volcanic activity as well as transitions from one to another as well as geochemical features of magma generation processes.

Armenia is situated in the NE part of the Anatolian-Armenian-Iranian plateau, an intensely deformed segment of the Alpine-Himalayan belt. The complex geological structure of the region is represented by a mosaic of tectonic blocks comprising fragments of volcanic arcs, continental crust and exhumed oceanic crust of the Mesozoic Tethys ocean basin (Meliksetian, 2013). The Gegham volcanic upland is located in the center part of the Neogene-Quaternary volcanic belt formed within the territory of the Armenian Highland. The duration of volcanism within the Gegham ridge spans from the Late Miocene to the Holocene (Karakhanyan et al. 2003, Karakhanyan et al. 2002). Temporal and spatial relationships between polygenetic and monogenetic volcanic activity as well as transitions from one to another are among fundamental problems in volcanology. Geological evidence such as presence of thick (abouth 500m) Vokhchaberd volcanoclastic suite at foothills of Gegham volcanic ridge suggests presence of stratovolcano (caldera-?) activity in Late Miocene-Pliocene (K-Ar dating data 3.4-6.7Ma; Bagdasaryan and Ghukasyan 1985) in Gegham, that was switched later to monogenetic activity and crater (or caldera) and slopes of former stratovolcano covered by monogenetic vents and their lava flows. After the polygenic volcanism the volcanism of Gegham upland is accompanied by fissure (plateau basalt) and monogenic volcanism.

Plateau basalts of Gegham upland distributed within town Gavar and Kotayk plateau, gorg of Hrazdan river up to village Parakar and age of these are 40Ar/39Ar 2.37±0.03 Ma (Neill et al., 2015). According to K. Karapetyan (1962, 1973) the youngest, Upper Pleistocene-Holocene volcanism of the upland is confined to the watershed part of the upland and the Eratumber plateau. According to Meliksetian (2017), there are data from extended flows from the Gegam upland - Argavand (221.1±5.0 Ka), Gutansar (314.1±16.2 Ka), Garni columnar flow of basaltic trachyandesites (127.7± 2.6 Ka) and lavas overlapping the Garni flow (49.9±9.2 Ka), which show the chronological and stratigraphic position volcanic activity of Gegham upland.

Taking into account the available and new reliable data, it is obvious that the volcanism of the Gegham upland continued from the Late Miocene-Early Pliocene time and up to the Upper Pleistocene and Holocene, and at the turn of the Pliocene-Quaternary period, due to changes in volcano-tectonic conditions, a change occurred in polygenic explosive-effusive volcanism to predominantly effusive areal.

Geochemical typification of the volcanic series of the Gegham upland indicates the predominance of "subduction" related fingerprints in them, however, some transitional to "intraplate" geochemical features are also found. The geochemical features and the petrogenetic model of the evolution of the volcanic series of the Gegham upland suggest a single magma-generating source and similar conditions for the evolution of melts within the entire Gegham upland.