Abstarct: A lot of gabbro-dioritic intrusions bodies as dykes, sills and small stocks out cropped in the lower red formation in north and north-west of Garmsar of Semnan province. baxsed on the filed petrography and geochemical studies these rocks have a gabbroic composition and represent a differential series from gabbro to diorite. Gabbroic samples have granular, ophitic and subophitic textures. In samples, which were collected from dyke margins, rock matrix are mainly composed of microcrysts with porphyry, amygdaloidal and glomeroporphyritic textures due to rapid cooling. Plagioclase and Clinopyroxene (augite) are the main minerals in study rocks. Apatite, opaque, olivine and phologopite are minor and accessory minerals and chlorite, prehnite, calcite, epidote and sericite are secondary minerals (alteration) in the rocks. Dioritic rocks have less abundance than the gabbroic rocks in the region and show ophitic, subophitic, granular and intergranular textures in thin-sections. Plagioclase and to a lesser extend clinopyroxene (augite) are the main minerals of dioritic rocks. Opaque, hornblende biotite, quartz and apatite are minor and accessory minerals and uralite, prehnite and chlorite are secondary minerals of the study rocks. The studied rocks are alkaline in nature and Harker diagrams of major oxides and trace elements versus Mg# and D.I indicate a same magmatic source for them and their evolution via differentiation crystallization process. Normalization and Spider diagrams show an intensive enrichment in light rare earth elements (LREEs) and relative depletion in heavy rare earth elements (HREEs). Parallel pattern of the samples in these diagrams indicate a common source for the rocks and the role of differential crystallization in their genesis. The lack of Eu anomaly, existence of positive anomaly in Cs, K, Sr and Ba, and intensive depletion from Nb and Zr are distinct characteristics of these rocks. Mafic magma formed the studied rocks was affected by contamination with crustal materials. Depletion of gabbroic rocks in HFSEs and their enrichment in LILEs could be an baxsed on the tectonic setting discrimination diagrams, the studied rocks plot in the field of intra continental plate tectonic setting (back-arc basin). On the basis of geochemical studies, magma formed the study rocks was originated from about 12-15% partial melting of an enriched garnet lherzolite mantle source at 90 - 100 km depths in an initial extensional continental back-arc basin. These back-arc basin setting proposed for the mentioned rocks is completly compatible with the tectonic setting of this part of Central Iran during the Oligocene-Miocene times.