Two types of carbonatic cumulate xenoliths occur in alkali basalts of the northern part of the Carpatho-Pannonian region, Central Europe. One is dominated by Ca-Fe-Mg carbonates with randomly distributed bisulphide globules (Fe1+xS2, x = 0–0.1), Mg-Al spinel, augite, rhönite, Ni-Co-rich chalcopyrite, and a Fe(Ni,Fe)2S4 phase. The second, carbonatic pyroxenite xenolith type, is composed of diopside, subordinate fluorapatite, interstitial Fe-Mg carbonates, and accessory K-pargasite, F-Al-rich ferroan phlogopite, Mg-Al spinel, albite and K-feldspar. All accessory minerals occur in ultrapotassic dacite-trachydacite glass in primary silicate melt inclusions in diopside, together with calcio-carbonatite and CO2-N2-CO inclusions. Textural evidence is provided for multiphase fluid-melt immiscibility in both xenolith types. The carbonatic pyroxenite type is inferred to have accumulated from differentiated, volatile-rich, ultrapotassic magma derived by a very low-degree partial melting of strongly metasomatized mantle. Mineral indicators point to a genetic link between the carbonatite xenolith with olivine-fractionated, silica-undersaturated alkalic basalt ponded at the mantle-crust boundary.