Abstarct: Abstract The studied mafic igneous rocks crops out in Alborz structural zone, from eastern to western Alborz, consists of S-Azadshahr, Tazareh, Talu, Astaneh, Kalateh, Fulad-Mahaleh, Kamarbon, N-Qazvin and Sangrood mine areas, respectivley. Generally these mafic rocks emplaced at the Carboniferous- Permian laxyers and in the baxse of Shemchak Formation, as stocks, basaltic flows sills, and dikes. The lithological composition of these rocks in the Azadshahr and Talu regions is variable and contains olivine gabbro, gabbro, alkali-diorite, monzodiorite and monzosyenite. The igneous rocks of Tazareh and Sangrood area outcropped as microgabbroic sills, in Kalateh village as gabbroic stock, in Astaneh and N-Gazvin region as basaltic flows, in Fulad-Mahaleh as monzonitic sills and also in the Kamrbon village they are alkali gabbro and alkali basalt together with under-saturated minerals. The rock-forming minerals of these rocks mostly includes olivine, clinopyroxene, plagioclase, amphibole, rhönite, and nepheline. EPMA results indictes clinopyroxenes of the studeid rocks are augite, diopsite and salite in composition and crystallized from basaltic alkaline magmas, which derived from OIB-type mantle source in intra-plate rifting extensional environments. Clinopyroxene thermobarometry data indicate that they crystallized in temperatures between about 1150 to 1300 ˚C, low to moderate pressures and high Oxygen fugusity. Amphiboles are igneous pargasite, magnesiohastingsite and kaersutite, they have mantle or mantle-crust nature. According to amphibole thermobarometric diagrams, their crystallization temperatures and pressures are ranged from 800-950 ˚C and 300-750 megapascal, respectivly. Mineral chemistry of olivines and plagioclases in these mafic rocks determined as chrysolite and albite to anorthite and orthclase, respectively. The whole-rock geochemical data consists of major and trace elements concentrations along with Sm-Nd, Rb-Sr, and Pb isotopic results indicate alkaline nature of parental magma and it’s OIB-type mantle source. These enriched sources can be originated from the recycled oceanic crust (HIMU), recycled mantle wedge with sediments (EMI), or continental sediments (EMII). Geochemical characteristics generally show that the probable contamination with crustal components at source region and during magma ascent in lithosphere in all of the studied samples is insignificant. baxsed on the obtained U-Pb zircon dating on the rock samples from Talu (alkalidiorite), Kalateh (gabbro) and Azadshahr (alkalidiorite) area, the crystallization age of these rocks yields 250.03 ± 0.55, (Olenekian, Lower Triassic), 218.5 ± 0.5 (Norian, Upper Triassic) and 97.26 ± 0.31 Ma (Cenomanian, Early Upper Cretaceous), respectively. According to the field observations, petrography, mineral chemistry, whole-rock geochemistry, isotopic ratios and U-Pb zircon dating, several major magmatic events occurred at Permian-Triassic and Triassic-Jurassic boundary and Upper Cretaceous in Alborz belt. These magmatic events are attributed to intraplate BABB-type extensional rift basins which occurred as resulting of subduction of Paleothetys and Neothetys oceanic crusts underneath Turan, Northern and Central Iran plates.