Study of Geochemistry and Quaternary Volcanic Rocks source, East of Ahar (NW Iran)

Azerbaijan Plateau is a part of the Alpine-Himalayan fold and orogeny belt, which is located in a compressional regime and between Talesh Mountains, south of Lesser Caucasus Mountain, east of Anatolia, north of Zagros Mountain. The studied area has located in the geographical longitudes of 47° to 48° and latitudes of 38° to 39° in East Azerbaijan province. There are wide ranges of Plio-Quaternary basalts and andesite basaltic in the sheet of 1.250000 Ahar. These rocks are gray to black in color. They also are formed of lava flows with scattered sometimes prismatic structures on the Paleocene andesitic lavas or conglomerate, siltstone and red marl Pliocene.

After the field investigations, we were taken 30 fresh and unaltered samples. Then, they were prepared as a thin section, then go undrer petrographic studies with a microscope in the laboratory.  Finally, we selected 10 samples were sent to the Amdel laboratory in the Australia for chemical analyses by ICP-MS, and OES.  Consequently, the results of whole rock chemical analysis data, were processed by GCD-KIT software. Then, all data were interpreted by geochemical conditions of all analyzed samples and also with their tectonic environment in the area.

The studied area is a part of the lesser Caucasus mountain range with high deformation and seismicity, which has located in a crustal convergence in the northwest of Iran. This area has a faulted system and with active strike-slip faults (Kocyigit et al., 2001). These systems often coincided with previous crustal discontinuities and have been widely effective in generating Quaternary magmatic activities locally through pull-part zones (Shabanian et al., 2012; Avagayan et al., 2010). Almost all the Quaternary magmatism in the region has occurred due to the operation of these fault systems and in the tensile position of the continental crust. Therefore, active fracture and fault systems and even old hidden faults have played a major role in the occurrence of the Quaternary magmatism.Volcanic rocks of the studied area are hyalo-aphanitic to hyalo-microlithic porphyry texture with large crystals of olivine, plagioclase and pyroxene. Also, porphyry, trachyte, intersertal and intergranular texture canbe seen in these samples. These samples have the nature of basaltic, thrachy andesite, mojearite and dacite, which are located in a calc-alkaline range. These rocks are rich in LREE and depleted in HREE.They also have enriched of Cs, Th, Pb and depletion of Ba, Nb, Eu, La. The presence of garnet in the source of magma can indicated the involvement of processes and crystallization of olivine, pyroxene, and plagioclase. In addition, metasomatism or contamination with enriched crustal materials (Menzies & Wass, 1983) leads to the abundance of LREE elements. Therefore, the metasomatized mantle can be a suitable origin for the magma of the studied volcanic rocks. Enrichment of elements Cs, Rb, Pb, Ba, LILE and depletion of elements HFSE (Nb, Zr) indicated the magma of subduction arc zones (Wilson, 1989). The negative anomaly of Nb, P, Ti, Zr and the positive anomaly of Pb are the characteristics of crust contamination of quaternary basalts (Wilson, 1989; Hofman, 1997). Negative Nb anomaly is characteristic of continental rocks and crustal involvement in magmatic processes (Hofman, 1997). Also, the negative anomaly of Ti and Nb in the normalized pattern with primary mantle composition can indicate magmas related to subduction (Pearce, 1983; Wilson, 1989). All the samples are in the range of intraplate basalts (Figure a10) (Mesched, 1986), and show the orogenic situation (Pearce et al., 1977). The studied volcanic samples are volcanic arc (Muller et al., 1992) belong to the continental arc after the collision (Groves, 1997).  Also, the studied samples show the origin of lherzolite garnet mantle and lherzolite spinel with a rate of 2 to 5% of partial melting.The tensile positions of the continental margins are active, where partial melting has occurred due to pressure reduction and rupture of the subcontinental lithosphere. Its compressive force resulting from crustal collision and crustal shortening, thickening and uplift leads to disturbance of subcontinental lithosphere thermal levels in these regions. Therefore, it seems that since the Eocene, the post-collision regime (Omrani et al., 2008) and since the Miocene (Hasanpour, 2008) has been ruling in the Arsbaran.

The Quaternary volcanic rocks in the eastern part of Ahar area, include a set of basaltic, andesite basaltic and andesite with calc-alkaline nature. The special depletion and enrichment of rare earth elements, these rocks are very similar to enriched mantle magmas and OIB, which were formed in the volcanic arc environment after impact on the active continental margin. For these samples, the mantle origin is lherzolite garnet to lherzolite spinel with 1 to 10% partial melting. Also there were happened metasomatisms of subducting oceanic lithosphere fluids. The geochemical evidences show some degrees of crustal contaminations during magma ascent. This magmatism has occured due to the subduction and the system of faults and fractures of the region and the tension tectonic situation. This tension system in the active continental margin after the collision has led to a decrease in pressure and finally the pouring out of basaltic magma.