MAGMATIC ORIGIN AND TECTONIC SETTING OF THE TERTIARY MAGMATIC ROCKS FROM NORTHWEST OF AHAR- NW IRAN (ORUMIEH-DOKHTAR BELT)

Ahar area occurs in the Tertiary magmatic Orumieh-Dokhtar belt. The Cenozoic volcanic, volcaniclastic and plutonic rocks produced after the closure of Neotethyan Ocean between Arabia and Central Iran in upper Cretaceous. The Eocene extrusive rocks intruded in Cretaceous sedimentary rocks. Their composition is andesite, trachyandesite, dacite, trachydacite, rhyolite, porphyritic diorite and porphyritic monzodiorite. Upper- Eocene volcaniclastic rocks are breccia and agglomerate. The intrusive rocks intruded volcanic and volcaniclastic rocks in Oligocene. These rocks are consist of monzogranite, granodiorite, monzonite, syenogranite, diorite. The rocks with such composition with spatial and temporal similarities have spreaded in the Orumieh- Dokhtar belt. All types of rock's composition are similar and value of SiO₂ is different from 50% to 70% which shows diffraction processes in initial magma. According to the geochemical diagrams, this area's tectonic environment in Eocene-Oligocene is determined as continental arc setting, thus this area’s magmatism is interlocked to Neotethyan post-collision activities. According to mineralogy and geochemistry studies, all of area’s magmatic rocks have been derived from unit mantle-derived magma that probably has formed in more than 100 kilometers depths due to partial melting of upper mantle and geochemical diagrams certifies it. The formed magma as the time of rising experience fractional crystallization, magmatic mixing and contamination in crust. Petrography evidences, field observations and geochemistry diagrams affirm magma mixing and contamination processes. Plutonic rocks show sub-solvus, hybrid, cordileriate I type granitoids properties and volcanic rocks have high- K orogenic andesites properties. The enrichment of Th in spider diagrams and high value of K can be related to mantle metasomatism source and increasing the ratio of Y/Ti shows diffraction and mixing processes between initial magma and high Y acidic magma. Depletion in compatible elements is related to magma contamination with crustal rocks. Magmatic series of different types of rocks in the Ahar area is determined alkaline but they seldom shows high-K calc-alkaline and shoshonit properties too that is related to mixing and contamination processes.