Abstarct: The volcanic rocks of the study area are located in the north and north-east of Semnan and are parts of the structural zone of Alborz and Central Iran. The main type rocks can be divided into two groups including igneous and sedimentary ranged in age between Jurassic and Quaternary. The Middle- Upper Eocene volcanic rocks are mainly basalt, andesite, trachyandesite, rhyolite and dacite. The intrusive rocks have gabbro composition and granodiorite compositions, which penetrated into the Eocene volcanic rocks. The pyroclastics are composed of tuff, tufite and agglomerate, which form parts of the Eocene volcanic belt of the South Alborz-North Central Iran. The sedimentary rocks are shale, sandstone, marl, lime and conglomerate, which are related to Shemshak, Dilchai, Lar, Fogen, Karaj, Qom and Hezardarreh formations. The volcanic rocks contain hyalophorphyry, porphyry and glomerophorphyry textures. Plagioclase and clinopyroxene are the primary minerals and sericite, kaolinite, calcite and chlorite are secondary minerals of these rocks. The composition of the clinopyroxene is iron-magnesium-calcium type and plot in diopside field. The investigating of the chemical compositions of medium-sized diopside shows that the middle zone between the core and rim is depleted in Fe, Al and Ti elements, while it enriched in Mg, Ca, and Si that is the reason for the oscillatory zonation of this diopsides. The magmatic series and tectonic discrimination diagrams baxsed on mineral chemistry of clinopyroxene indicate their crystallization from an alkaline to subalkaline magma with a water contianing of 2-5% and high oxygen fugacity. Moreover, the thermobarometry studies show that diopside in the basalts has been crystallized in pressures about 1 kb and temperature of 1110 to 1210° C. An-Ab-Or triangular diagram shows that the study feldspars in volcanic rocks are albite and sanidine with lack of zonations.