The Late Cretaceous mafic to intermediate igneous rocks of northern Ajabshir include gabbro, basalt, basaltic andesite, and andesite. They crop out with a NW–SE trend in the northwestern Urumieh–Dokhtar magmatic belt. Petrographic, whole-rock geochemical, and Rb–Sr and Sm–Nd isotopic studies were conducted to determine the magma source and evolutionary processes. The rocks mainly consist of plagioclase (andesine to bytownite), clinopyroxene (augite), quartz, titanite, apatite, and magnetite. Geochemical data show enrichment in large-ion lithophile elements, including Ba, Rb, Th, and K, together with negative Nb and P anomalies. These features indicate the influence of slab-derived fluids and partial melting of a metasomatized spinel lherzolitic mantle source. The enrichment of Sr and Rb further supports the contribution of fluids released from the subducting slab to the enrichment of fluid-mobile incompatible elements. Sr–Nd isotopic compositions confirm a predominantly mantle-derived origin for the parental magmas. Nevertheless, isotopic variations indicate that crustal assimilation and fractional crystallization also affected magma evolution within an open magmatic system. Comparison between the Ajabshir and Salmas rocks reveals similar geochemical and isotopic characteristics; however, the Salmas data are used only for regional comparison and do not necessarily demonstrate an identical genetic origin. Fractional crystallization and assimilation–fractional crystallization processes operated during magma differentiation and emplacement. Overall, the studied magmas were generated by partial melting of a metasomatized mantle wedge above the subducting Neo-Tethyan slab and subsequently evolved in a subduction-related magmatic arc setting.