Petrological analysis of the pyroclastic and volcanic rocks in northwestern Mahneshan, northern Urmia-Dokhtar, NW Iran

The study area is a part of the northern Urmia-Dokhtar magmatic belt, which consists of andesite, basalt and acidic tuff. There are hyaloclastites with a clear bedding only in the NW part of Mahneshan area located in the northern Urmia-Dokhtar magmatic belt of Iran. In this region the volcanism commence during the Oligocene time with extremely strong hyaloclastite eruptions, and then after a pause, the volcanism continued by exceptionally extensive acidic tuff eruptions, and eventually ended up with the late Miocene basaltic andesite lava eruptions. Field observations, mineralogy and petrography on the major and minor minerals of the volcanic and sedimentary rocks indicate that three phases of volcanism occurred in the area. The hyaloclastites (Phase 1), are often composed of andesite and quartz-andesite to dacite. The highly extensive second phase 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 Burdigalian ages. Phase 3 includes andesite-basalt, andesite and quartz-andesite. Based on the geochemical diagrams all three phases are situated within the calc-alkaline field, indicating the subduction of an oceanic crust under the continental crust.