Abstarct: Neotethyan oceanic lithospher subduction under the southern margin of central Iran microplate during the Eocene, in addition to creating the Eocene magmatic arc at the South edge of central Iran, causes development of the extensional environments in inner parts of the central Iran block. Extensional movements result in a proto back-arc basin in the Saveh, Kashan, Qom, Garmsar and Shahrood areas and create a depressed basin, filled with continental (Lower red), shallow sea (Qom) and continental (Upper red) deposits. Basaltic magmatism had occurred in this basin, as extrusives and intrusives, as a result of the extensional processes. Existence of the thick continental-marine sequences associated with igneous rocks in the different area of the central Iran, are the consequences of these processes.The basic igneous rocks occurred as lavas in the Oligocene red marl around the Ahmadabad village but there were not any comprehensive petrological study about these rocks. Ahmadabad igneous rocks involve few lava flows, in wich a continuous series from Olivine basalt to basalt are seen. In the previous reports, these rocks have been called Quaternary basalts witch overlied the Neogene red marls. Because, the same basic rocks had been reported in the Lower red formation in Garmsar area, we decided to study them and compare with each other. Garmsar igneous rocks involve a number of intrusive and subvolcanic bodies as dyke, sill and small stocks with gabbro-dioritic composition, in which a continuous series from gabbro to diorite can be seen. These rocks introduced as alkali basalt, micro alkali basalt and diabaxse in the form of submarine volcanics and dykes in the previous research reports. Investigation of stratigraphic, tectonic, geochemical and petrogenetic features of these rocks, show all of them relate to the same magmatic event in Oligocene, and were formed in an extentional proto- back arc basin. The most aboundant minerals in the basaltic rock are, olivine, augite and plagioclase and these rocks have porphyry, glomeroporphyry and microlity-trachity texture. Garmsar intrusive and sub-volcanics have the same petrographic features. Theire most aboundant minerals are plagioclase and augite and have granular, intergranular, ophitic and subophitic textures. The alkaline nature and intra-continental plate (Back-arc) environment have been proved for these rocks baxsed on both magmatic series and tectonic setting discerimination diagram. Enrichment in LREEs and depletion in HREEs are obvious in chondrite and primitive mantle normalized spider diagrams. Spider diagrams of these rocks, parallel trends of REE variations, associated with the trends of major oxides, trace, compatible-incompatible elements variation diagrams and existence of some anomalies such as Pb positive anomaly and low Nb/U, Ce/Pb and high La/Nb, can be all indication of indicated a common unique source for these igneous rocks and confirm the role of the fractional crystallization in addition to assimillation and contamination with crustal rocks in the evolution of the magma forming the rocks of the areas. Depletion in HFSE and enrichment in LILE in gabbroic rocks, also may indicate the role of subduction processes in the evolution of the source of magma. The magmas of the igneous rocks occurred in Lower red formation originate from 10-15 % partial melting of an enriched garnet-lherzolitic mantle source in the depth range of 90-100 Km dephts, in an extensional intracontinental environment. This setting has perfect concordance with the extentional regime prevailing in central Iran back arc basin during the Oligo-Miocene times.