Abstarct: In the baxse of Shemshak Formation in Alborz zone, basic igneous rocks have been reported as lavas. However, precise investigations of geological outcrops in different parts of eastern Alborz show that these rocks in Gheshlagh (Khosh Yeilagh area), Tazare, Talo and Kalate Rodbar (in north of Damghan) regions are intrusive bodies as same as sill, dike and small stocks and in Cheshmeh Ali region are lavas. baxsed on, field and petrography observation, Gheshlagh basic intrusion as a fractionated sill (with about 100 meters thickness) intruded into the baxse of Shemshak Formation and has a complete fractional sequence of olivine gabbro, gabbro, diorite to monzonite from up to the bottom. Talo basic pluton, intruded in the lower part of the Shemshak Formation as same as small bodies and continued as sill or dikes parallel or cutting the shale and sandstone laxyers respectively. The existence of a fractional sequence of olivine gabbro to diorite has been proved in field and petrography observation on the igneous rocks. Petrography characteristics of Gheshlagh and Talo region are same. Plagioclase, clinopyroxene and hornblende are the most abundant minerals in these rocks and typical textures are accumulated, granular, microgranular and ophitic. The presence of phlogopite, apatite and primary and secondary sphene in these rocks suggest high contents of TiO2 and P2O5 and alkaline nature of parental magma. Igneous rocks in Tazare and Kalate Rodbar region, has been observed in the form of sill and diabasic dike, or gabbroic intrusion in shale and sandstone laxyers of Shemshak Formation. These rocks have intergranular and ophitic texture and contain plagioclase and clinopyroxene phenocrysts and apatite microcryst. Basic rocks in Cheshmeh Ali region are alkali olivine basalts and have been strongly altrated. These rocks are composed of plagioclase, pyroxene and olivine in microcrystal matrix and show porphyry, intersertal and gelomeroporphyry texture. The samples of Cheshmeh Ali, Tazare and Kalate Rodbar regions have no petrographic variations, therefore, variations diagrams of these regions do not show special trends in constant to Gheshlagh and Talo regions, where petrographic varieties are abundant and differential trends can be seen. Identification diagrams of magmatic series and tectonic setting discrimination diagrams have been proved an alkaline nature and intracontinental setting for these rocks. Enrichment in LREE and high incompatible and depletion in HREE are obvious in chondrite and primitive mantle normalized spider diagrams. Parallel trends of REE variations and Pbpositive anomaly associated with patterns of compatible and incompatible variation diagrams, indicating an unique source for these igneous rocks and the main role of fractional crystallization in the evolution of magma, although, minor amounts of assimilation and contamination of magma by crustal rocks occurred also. These signatures indicate that this alkaline magma has been formed by low degree partial melting (10-15 %) of an enriched sub-continental lithospheric garnet-lerhzolite mantle in an extensional intracontinental environment. This tectonic setting corresponds with the extensional regimes governed on fluvial-deltaic basins of upper Triassic-lower Jurassic passive continental margins in Alborz.