Major and trace elements and Sr–Nd isotopic data are presented for the Quaternary alkaline volcanism NW of Ahar (NW Iran). The exposed rocks mainly consist of alkali basalts, trachybasalts, basaltic trachyandesites and trachyandesites. Alkali basalts and trachybasalts display microlithic porphyritic texture with phenocrysts of olivine, clinopyroxene, and plagioclase in microlithic groundmass. In the more evolved rocks (basaltic trachyandesites and trachyandesites), amphibole and biotite have appeared. Major and trace element abundances vary along continuous trends of decreasing MgO, TiO₂, Fe₂O₃*, CaO, Co, Cr, V and Zn, and increasing K₂O, Al₂O₃, Ba and Th with increasing SiO₂. The Sr and Nd isotopic ratios vary from 0.704463 to 0.704921 and from 0.512649 to 0.512774, respectively. Alkali basalts with high ¹⁴³Nd/¹⁴⁴Nd ratio, low ⁸⁷Sr/⁸⁶Sr ratio and high MgO, Ni and Cr contents indicate that they were generated from relatively primitive magmas. Ba, Cr and La/Sm ratios versus Rb suggest that fractional crystallization of alkali basalts could have played a significant role in the formation of evolved rocks. Assimilation and fractional crystallization modelling, as well as Rb/Zr, Th/Yb and Ta/Yb ratios clearly indicate that crustal contamination accompanied by the fractional crystallization played an important role in petrogenesis of the trachyandesites. The small compositional differences between magma types, isotopic composition, mineralogy and nonlinear trends on Harker diagrams also indicate that magma mixing was not an essential process in the evolution of the Ahar magmas. Petrogenetic modelling has been used to constrain sources. Trace element ratio plots and REE modelling indicate that the alkali basalts were generated from a spinel-peridotite source via small degrees (~2.5%) of fractional melting.