Alkaline and ultrapotassic volcanic rocks in the Afyon volcanic complex are regarded as the first products with asthenospheric signature after post-orogenic magmatism in western Turkey. The Yagcikoy volcanic complex (YVC), a key locality in the western off-axis of the Afyon volcanic complex, comprises three units consisting of domes, dykes, lava flows and associated volcaniclastic rocks intercalated with Middle Miocene fluvial/lacustrine deposits. Emplacement of the domes and dykes appears to have been controlled by NE-trending faults in a rift environment. The YVC is alkaline and shoshonitic in character, and all the volcanic units in the complex have similar geochemical patterns (enriched in LILE and LREE and depleted in Nb, Ta and Ti). The Sr-87/Sr-86 and Nd-143/Nd-144 ratios range from 0.704989 to 0.706662 and 0.512492 to 0.512595, respectively. The geochemical and isotopic data suggest that fractionation of parental basaltic trachyandesite magmas generated a magma chamber with chemically normal zonation. The geochemical data and modeling results suggest that the YVC originated from a spinel peridotite-bearing lithospheric mantle source that had been metasomatized by subduction components. The rifting environment that hosts the volcanic products is thought to have resulted from slab tear that caused the rise of fertile asthenospheric material to relatively shallow depths. The upwelling of hot asthenospheric melts through a tear subsequently melted the spinel peridotite-bearing lithospheric mantle source.