Abstarct: In the south of Biarjomand (one of the town of Shahrood in SE) has cropped out a thickness sequence of Eocene volcanic and volcanosedimentary rocks. The main composition of volcanic rocks is trachybasalte. The major forming minerals of these rocks are olivine, titan-augite and calcic plagioclase. Apatite and magnetite are accessory minerals. Calcite, natrolie, analcime and chlorite and Fe oxids and hydroxides are secondary mineral. This sequence overlaid a sequence of sandstone - congolomerate with Jurassic age (most probably) and covered by fossiliferous limestons and marls of upper Eocene. A few trachybasaltic dikes with porphyry and microlitic textures cutting the Jurrasic congolomrates and sandstones and they terminated to the lower parts of the Pahnavaz volcanosedimentary sequence. Indeed these dikes are magma feeding channels that theirs magma erupted into the sedimentary basin. At first, due to very high differences of temperature and pressure between thracybasaltic lavas and water entered thracybasaltic lavas exploded and intensively fragmented, then deposited in the form of agglomerate accumulations. Therefore agglimeratic facieses and epiclastic depositions are very common with high frequency. With continuing of extension and subsidence and more deeping the sedimentary basin, explosive activities has been lower and lavas deposited in the form of laxyers and horizons. Therefore in the end stages of volcanic activities, the erupted lavas show pillow structure and associated copper mineralization (mainly in the form of malachite and chalcocite). These volcanic activities have been occurred in the middle Eocene. Then volcanosedimentary sequence exited from water and in the late Eocene fossiliferous carbonate sedimentary of upper Eocene deposited on them. Geochemically Pahnavaz volcanic rocks mostly have trachybasaltic composition and have alkaline nature. Typical evidences of magma fractionation and crust contamination have been seen in them. Enrichment of light rare earth elements (LREE) and large ion lithophile elements such as Rb, Sr, K, Th, and depletion of Nb, Ta and also Ti, indicate that magma forming of this volcanic sequence produced in a volcanic arc setting. Meanwhile the magma forming trachybasaltic lavas originated from 10 – 20 % (in avrage 15%) partial melting of a garnet lehrzolitic source. From tectonic setting point of view, these extensional environments were related to continental margin magmatic arc or back arc.