Abstarct: Despite of rarity of the Ordovician rocks in Iran, there are considerable outcrops of these rocks in north and southwest of Shahrood, which are refrred as Abesto, Abarsej and Ghelli Formations. Abarsej Formation (with Upper Ordovician age) is an unformal formation; its lithology consists mainly of sandstone, gray and green shale and silty shale. Analysis of all field evidences such as sedimentary structures indicate that the Abrsj Formation has been deposited in a deep turbidity sedimentary environment as a submarine fan. Studied igneous rocks include lava and microgabroic dyke and sill, with basalt, trachy basalt and basaltic trachyandesite composition occur in the Upper Ordovician deposits (Abarsej and Ghelli formations). Plagioclase and clinopyroxene (augite type) are the main rock forming minerals in the basaltic rocks. The most important accessory minerals in these rocks are olivine and opaque minerals (magnetite, pyrite and chalcopyrite). Extent of alteration of the Olivine is too high that the initial mineral has lost its mineralogical specifications and has been generally replaced by chlorite, calcite and iron oxide. Most important txtures in basaltic rocks include porphyry, glomeroporphyry, amygdaloidal, trachytic, subophitic and intergranular textures. Basaltic rocks partly altered due to the influence of fluids, then plagioclase have been converted to epidote, calcite and chlorite due to sosoritization. The main minerals of microgabroic rocks include plagioclase and clinopyroxene (augite). Apatite and opaque minerals such as magnetite and pyrite have been observed in polished thin sections of these rocks and are the most important accessory minerals. Secondary minerals such as chlorite and calcite have been formed by alteration of plagioclase and clinopyroxene in these rocks. Main txtures of these rocks include intergranular, subophitic and ophitic txtures. Geochemical investigations of the studied igneous rocks show that they are alkaline in nature and baxsed on the tectonic discrimination diagrams are grouped in alkaline basalts field of the intra-continental rift setting. This tectonic setting corresponds to paleogeography of Alborz in lower Paleozoic (occurrence of Turan intracontinental Paleotethys rift) in Ordovician- Sillurian times. Primitive mantle and chondrite normalized diagrams indicate high LREE/HREE ratios and LREE enrichments without any depletions in HFSEs. Parallel trends on spider diagrams, indicate a common parental magma for these rocks and role of the fractional crystallization as the main process in their magmatic evolution. Geochemical investigations indicate that magma forming of the basaltic rocks has been derived from an OIB-like enriched deep asthenospheric mantle source. Magma forming of the basaltic rocks has been generated from ١٤-١٦% partial melting of an enriched garnet peridotitic astenospheric mantle source in ١٠٠-١١٠ km depths. Geochemical evidence clearly show no or minimal role of crustal contamination in the evolution of magma forming of the basaltic rocks during its ascending to the earth surface.