Genesis and geochemical evolution of the Mio-Pliocene volcanic rocks in the SW of Bostanabad, NW Iran: A comparison with the classic Adakite

The Sahand volcano (Kuh-e-Sahand) is located in NW Iran, about 60 km E of Lake Urumieh and 40 Km SSE of Tabriz. The volcano is a stratovolcano and is dominated by pyroclastic materials and lava flows in the Miocene-Quaternary. The Mio-Pliocene volcanic rocks are exposed in the Southwest of Bostanabad (Arvanehkuh, Biukdagh, and Ghapandagh masses) East-Azerbaijan province. These rocks formed a part of the Urumieh-Dokhtar Magmatic Belt (UDMB) and consist of andesites and dacites. The rocks display a porphyritic texture and contain phenocrysts of plagioclase, sanidine, amphibole, biotite, and quartz. Based on the geochemical data and multi-elemental pattern, these rocks are medium to high-K calc-alkaline suite and show Large-Ion Lithophile Elements (LILE) and Light Rare-Earth Elements (LREE) enriched normalized multi-elemental patterns, and Nb and Ti depleted. Chondrite-normalized REE patterns of the volcanic rocks display a decrease from LREE to Heavy Rare-Earth Elements (HREE) without any Eu anomaly, indicating their formation in a subduction zone in an active continental margin. They have higher SiO2, Sr content, Sr/Y, and La/Yb ratios and lower MgO, Y, and Yb contents compared to those of normal calc-alkaline volcanic rocks and show high SiO2 adakites (HSA). Based on geochemical data, the origin of these rocks is garnet-amphibolite with the residual phase of garnet and amphibole with a titanium phase. HREE and Y depleted patterns suggest the existence of garnet and amphibole as a residue in the source. The source of these rocks was probably garnet-amphibolite possibly generated during subduction of the Neo-Tethyan oceanic slab beneath the Central Iran zone and in fact, can be termed as classic adakite. Therefore, considering the importance of adakites in inferring continental geodynamic processes, it is recommended to pay attention to their discovery and identification with the help of various petrological and geochemical methods.