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Voluminous moderate- to high-magnesian [Mg# = molar Mg/(Mg + Fe2+) = 44–64] andesitic and dacitic rocks with high silica (mostly 61–66 wt%) adakitic affinity (Y = 13–22, Yb = 1.3–2.1, Sr/Y = 18–44, La/Yb = 10–25) and common mafic magmatic enclaves (MMEs) are first reported in the Keçiboyduran stratovolcano (KSV) from the Cappadocia volcanic province (CVP), Central Anatolia, Turkey. We present comprehensive whole-rock geochemistry and Sr–Nd–Pb isotope data, mineral chemical compositions and 40Ar–39Ar ages for KSV samples. Based on the volcanostratigraphy and 40Ar–39Ar dating results, two successive eruption ages of 2.2–1.6 Ma (stage I: amphibole-rich) and 1.6–1.2 Ma (stage II: pyroxene-rich) were established for the KSV, corresponding to the Gelasian and Calabrian stages of Early Pleistocene, respectively. Textural and geochemical evidence indicates that the KSV magnesian andesites–dacites are products of a hybrid magma formed by mixing between mantle-derived mafic and crust-derived felsic magmas with further fractionation and minor contamination during magma storage and ascent. Our new data, combined with previous geological and geophysical results suggest that parental magnesian mafic melts of the KSV rocks originated from a heterogenous mantle source generated through the metasomatism of mantle wedge material by subducted sediment-derived melts, and then partially melted through asthenospheric upwelling in response to slab break-off. The mafic magma underplated the overlying lower crust, resulting in its partial melting to generate crustal felsic magma. Both magmas mixed at lower crustal levels creating MME-rich hybrid magmas. Subsequently, the hybrid magmas were emplaced at different depths of the crust (c. 4–11 and 11–15 km for the stage I and II, respectively), where they crystallized at moderate temperatures (c. 1180–840 °C) and under relatively high oxygen fugacity (LogƒO2 = − 11.4 to − 9.2), water-rich (H2Omelt = 5.6–3.6 wt%) and polybaric (~ 1.2 to 5.1 kbars) conditions, and underwent fractionation of primarily amphibole ± pyroxene causing adakitic affinity. We propose a new petrogenetic model for the early Quaternary magnesian/adakitic andesites/dacites of the CVP in a post-subduction tectonic setting. Our results provide robust evidence for slab break-off of the eastern Cyprus oceanic lithosphere and put further constraints on the tectonic evolution of the eastern Mediterranean collision zone during the Early Quaternary. [ABSTRACT FROM AUTHOR]