Document Type: Research Paper
Department of Geology, Science and Research Branch, Islamic Azad University, Tehran, Iran
Department of Geology, Faculty of Earth Sciences, Shahid Beheshti University, Evin, Tehran, Iran
Department of Earth Science, Science and Research Branch, Islamic Azad University, Tehran, Iran
The study area is part of the northern Urmia-Dokhtar Magmatic Belt. The northern part of this belt consists of andesite, basalt and acidic tuff. There are hyaloclastites with a distinctive bedding only in the northwestern Mahneshan area and northern Urmia-Dokhtar magmatic belt. In this region volcanism began during the Oligocene time with extremely strong hyaloclastite eruptions, and then with a stop, the volcanism continued with exceptionally extensive acidic tuff eruptions, and eventually ended up with basaltic andesite lava eruptions in the late Miocene. Field, mineralogical and petrographic studies on the major and minor minerals of the volcanic and sedimentary rocks indicate that three phases of volcanism occurred in this area. These three phases are: hyaloclastites, acidic tuffs and volcanic Lavas. hyaloclastites (Phase 1), are often composed of andesite and quartz-andesite to dacite. The highly extensive phase 2 consists of acidic tuffs interbedded with limestone. According to the fossil content and the 87Sr/86Sr age determination, this phase formed during the Chattian to Aquitanian ages. Phase 3 includes andesite-basalt, andesite and sometimes quartz-andesite. Based on the geochemical diagrams all three phases are located within the calc-alkaline field, indicating the subduction of an oceanic crust under the continental crust.