Abstarct: Bande Hezar Chah mextamorphic – igneous complex is located in 25Km from Southwest of Biarjomand (southeast Shahrood - in Semnan province) and in the northern margin of Central Iran structural zone. The complex crossed cut by Late Neoproterozoic quartz dioritic dykes and Middle Jurassic gabbroic dyke swarms with east – west strike. The gabbroic dykes intruded in the granitoid pluton of Bande-Hezar chah with Late Neoproterozoic- Early Cambrian age and the detrital sedimentary sequence (conglomerate, sandstone, shale and siltstone, equivalent to Shemshak Formation in Alborz) with Late Triassic – Early Jurassic age; but they have not been observed in the limestone- sandy limestone sequence of upper Jurassic (equivalent to Delichay Formation). The study gabbroic dykes generally consist of plagioclase and augitic clinopyroxene as main minerals. The minor minerals includ apatite and magnetite and the secondary minerals mainly consist of sericite, chlorite, calcite and amphibole (due to uralitization of clinopyroxene), and represent intergranular, ophitic and subophitic textures. The rocks have sub-alkaline affinity and plot in Back-arc field in discrimination tectonic diagrams. The samples display LREEs enrichment in compare with HREEs on chondrite-normalized spider diagram. This enrichment can be duo to the effects of forming materials of subduction zone and/ or contamination by crustal materials. Positive anomalies of K, Rb and negative anomalies of Nb and Ta in gabbroic samples can be related to subduction environment or magma contamination with crustal material during ascending and emplacement. The parent magma(s) of gabbroic dikes was/were generated via 6-10% partial melting of an enriched spinel lherzolitic mantle source in depths less than 80 Km and consequently arose through spaces and fractures of normal faults to shallow depths. It was undergone contamination and assimilation during ascending and finally emplaced in a sedimentary sequence equivalent to Shemshak Formation as dyke small intrusive plutons and even lava flows in Neotethyan middle Jurassic back-arc basin of central Iran.