The Kosedag syenite crops out as a shallow-seated pluton within the Eocene volcano-sedimentary rocks and is unconformably overlain by Lower Miocene limestones in the Susehri-NE Sivas region, east-central Ponticles. It consists of syenites and quartz syenites with a phaneritic, porphyritic texture characterized by K-feldspar megacrysts set in a coarse- to medium-grained groundmass comprising K-feldspar, plagioclase, clinopyroxene, amphibole, biotite and quartz. Major element geochemistry data reveal a high-K, alkaline, metaluminous to slightly peraluminous character, with a low Aluminum Saturation Index (ASI) value. The main solidification process, which modified the composition of magma during crystallization, was fractional crystallization, in which syenites solidified first, and quartz syenites later. Trace element geochemistry data reveal that the magma source of the Kosedag pluton was a metasomatized mantle affected by subduction-derived fluids. This magma source could have been derived from partial melting of a metasomatized mantle layer which was accreted into the collision zone between the Eurasian plate and the Tauride-Anatolide platform along the Izmir-Ankara-Erzincan suture zone. A post-collisional extensional regime, induced by slab break-off following the continent-continent collision, may have melted these metasomatized mantle slices to produce the high-K, alkaline, metaluminous magma source of the Kosedag syenite.