Petrology of Sirdan volcanic rocks in Alborz Mountains, northern Iran

The study area is located around Sirdan town in Sofla-Tarum Mountains in Alborz, south west of Manjil City and west of Loshan, northern Iran. Talesh-Western Alborz volcanic belt was formed by the subduction of Neotethys ocean toward north. However, some researchers believe that the continental rift is responsible for its formation. Firstly, geological field trip was carried out and lithogeochemical samples were collected and prepared for chemical and mineralogical studies. Genetic pattern of the samples was also introduced based on the geochemical analysis and microscopic studies. Sirdan volcanic rocks (northern Iran) include pyroclastic and lava units. Lavas are basalt, andesite, trachyandesite, dacite and riolite. Pyroclastic rocks also consist of different kind of tuffs (lattice crystal tuff, halo lattice tuff, sandy tuff, clay tuff and tuffit), ignimbrite and breccia. Age of the formations belongs to tertiary (mostly Eocene to Oligocene). Basic lavas include olivine, clinopyroxene, plagioclase phenocrysts as well as opaque minerals and plagioclase and clinopyroxene microlite. The dominated texture of the samples is porphyritic microlite. Medium-silica lavas have plagioclase-pyroxene, alkali feldspar, and biotite and amphibole phenocrysts. They also have silica and feldspar microlite and the dominated texture is halo porphyritic. Acidic lavas consist of plagioclase, quartz and alkali feldspar phenocrysts with xenocryst of quartz, feldspar and silica with a dominated texture of porphyry with micro-granular and hyaline. Based on the geochemical analysis, the studied rocks are sub alkaline and tholeiitic. Pattern of rare earth elements shows a high contamination of magma with crustal surface. Finally, the tectonic situation of the studied samples have been introduced from mantle with a source of MORB toward the active marginal continental rift