بررسي پترولوژي و محيط تكتونوماگمايي سنگ هاي آتشفشاني آشتيان

Petrology and Tectonomagmatic Investigation of Volcanic Rocks of Ashtian

The area under study is located in central Iran and 235 kilometers of south west of Tehran is a part of Qom quadrangle. Volcanic sequences, which constitute the most major parts of the outcrops in the region mainly, have seen in north eastern and east of Ashian. These volcanic rocks are included of andesite basalt, andesite, trachy andesite, dacite, rhyodacite and rhyolite that are emplaced during Eocene (middle Lutetian to late Lutetian) along the Oromieh-Dokhtar volcanic belt that divided to six members from Ei to E6 in Tafresh and Ashtian area. Plutonic rocks related to Oligo-Miocene. There are not any rocks older than Triassic in area under study. Volcanic rocks both pyroclastics and laves have covered the most of area and could be seen in the continental and marine so we can see ignimbrite and tuffs. Acid eruptions are more than basic in this area. Mineralogically, these rocks have plagioclase, alkali feldespar, quartz, clinopyroxene, amphibole, biotite and a lesser amount olivine. Plagioclases have zoning structure and amphibole crystals have opacite rimmed. This phenomenon shows that magma responsible for rocks had a high temperature. Geochemically, petrographically and petrologically studies based on major and trace elements and differentiation index versus Si02 indicate that changes in the volcanic processes at the area are the result of magmatic crystallization and differentiation magmatic rocks magma of area under study is calc-alkaline and they belong to subduction. Using of petrological and tectonic setting diagrams indicate that these Eocene magmatic rocks are compared to calc-alkaline series of subduction orogenic andesitic belts. The rare and trace element patterns of thes rocks have investigated and the examples of anomalies are related to Sr and Nb. Also the study of trend show that the calc- alkaline magma has formed in the upper mantle. Similarities between volcanic and plutonic rocks in Ashtian area show that these rocks may be having the same origin.