Abstarct: Tuyeh- Darvar granitoid pluton is situated in 50 Km SW of Damghan in Semnan province in the eastern Alborz zone. This pluton has intruded into the early Paleozoic rock units (Barut and Lalun Formations). baxsed on the field observation and petrographic studies, the pluton composed of monzonite, quartz monzonite and monzodiorite. Mineralogically, the granitoid consists mainly of plagioclase and orthoclase, accompanied by minor amounts of quartz, hornblende and biotite. Accessory minerals include ilmenite, magnetite, zircon, apatite, titanite, pyrite, and chalcopyrite. Sericite, epidote, calcite, and chlorite are also often present, as secondary phases. This granitoid shows dominantly granular subhedral and porphyritic textures, but intergrowth textures, such as graphic and myrmekitic, are also observed. The composition of the plagioclase is albite-oligoclase and feldspar alkaline 98-75% or). The iron-rich biotite are the most significant mafic minerals which laying in the alkaline and anorogenic biotite fields. The amphiboles of the Touyeh- Darvar rocks are low pressure calcic type with ferro-hornblende composition and igneous origin. Application of the geothermobarometry methods yielded temperature ranges of 650–730°C and pressures less than 2 Kb for stopping of exchange and final equilibrium of amphibole in the pluton that corresponds to less than 5 km depths for emplacement depth of the pluton. Existence of fine grained hyper-solvous to sub-solvous reaction textures (perthitic, myrmekitic, graphic and granophyric) in the rocks of pluton and the lack of apparent contact mextamorphism around it indicate diapiric ascend and emplacement of the pluton in a relatively cool and solid state in low depths of the crust. The veins of mineral deposits such as Fe-Mn oxide-hydroxides, fluorite, barite, lead and zinc in the pluton host rocks are the obvious evidence for these states. The granitoid is mostly mextaluminous, ferroan and alkalic monzonite to monzodiorite. These rocks have high ratios of FeOT/MgO, Ga/Al, high concentrations of K2O+Na2O, low abundances of MgO and transitional elements, strong enrichments of LREE relative to HREE and LILES (such as Rb) and HFSES and depletions in Sr and Ba relative to primitive mantle. They contain Fe-rich hydrous mafic minerals and magnetite. These features are typical of A-type granites. Zircon U-Pb age show Carboniferous age (327 Ma) for the formation of this granitoid. Sr-Nd isotopic geochemistry, with initial ɛNd values from -1.1 to -1.5 and initial 87Sr/86Sr ratios between 0.70562 and 0.70678, are consistent with magmatic differentiation from mafic melts produced from an enriched mantle source. However, other models such as melting of mafic crust or mixing of components from depleted mantle and continental crust cannot be discarded. On the basis of the U-Pb zircon age (327 Ma) and the known magmatic tectonic regime in Iran during the Paleozoic, it is suggested that the pluton, formed in a rift environment related to extension of the Paleo-Tethys in Early Carboniferous time.