CONTEMPORANEOUS MIDDLE MIOCENE TO QUATERNARY ULTRAPOTASSIC AND CALK ALKALINE VOLCANISM IN NORTHWEST IRAN

Study of subduction-related calc-alkaline and collision volcanism in northwestern Iran and eastern Anatolia can give invaluable insights into the geodynamic evolution of a young mountain belt. This study concerns contemporaneous Late Cenozoic calc-alkaline and mafic ultrapotassic magmatism in the Azarbaijan Province, NW Iran. Sahand volcano (3695m above sea level) is a huge dacitic volcanic massif (with basal diameter of ~100 km. It consists of extensive tuffs and pumice deposits which date from ~13 to ~7 Ma (Middle to Late Miocene). The pyroclastic complex is cut and covered by numerous andesite, dacite and rhyolite flows and domes, and they date from ~12 to 0.14 Ma. Sahand rocks are mostly normal K, with K2O/Na2O ratios <1. Based on mineralogy and style of eruptions, the Sahand magmas were silica saturated and volatile-rich. The early period of eruptions at Sahand corresponds well with the Middle Miocene subduction-related magmatism in Urumieh-Dokhtar zone of central Iran, and especially with its SE sector in Kerman Province. However, the terminal phase of volcanism in Sahand is clearly post-collision, although the overall rock compositions are not different from the earlier subduction-related phase. A very K-rich mafic volcanic center occurs very close to the west of the Sahand system on the eastern shore of the Urumieh Lake, in the Island of Shahi. With ages of 8-7 Ma, the Shahi Island volcanic center is contemporaneous with the Sahand. The unusual rocks in Shahi volcano include basanites and leucitites. Unlike the Sahand system, Shahi volcanics are poor in OH-bearing minerals. K2O/Na2O ratios in Shahi basanites and leucitites are normally >2 and reach values as high as 12. The only other large example of alkaline mafic volcanic center of the same age -although very sodic (K2O/Na2O=0.2-0.8)- is reported from Van Lake area in eastern Turkey. The occurrence of alkaline mafic volcanoes in Shahi Island and Van Lake areas can be attributed to divergent motion of the Iranian and Anatolian continental blocks due to indentation tectonics at the collision site of Arabian and Iran-Anatolia continental plates. The tectonic escape must have been associated with local transpressional systems along strike-slip faults. One corollary of this study is that whether the Urumieh Lake with its leucitite volcano can be viewed as a pull-apart basin.