Major and trace element compositions, stable H and O isotope compositions and Fe³⁺ contents of amphibole megacrysts of Pliocene-Pleistocene alkaline basalts have been investigated to obtain information on the origin of mantle fluids beneath the Carpathian-Pannonian region. The megacrysts have been regarded as igneous cumulates formed in the mantle and brought to the surface by the basaltic magma. The studied amphiboles have oxygen isotope compositions (5.4±0.2 ‰, 1 σ), supporting their primary mantle origin. Even within the small δ¹⁸O variation observed, correlations with major and trace elements are detected. The negative δ¹⁸O-MgO and the positive δ¹⁸O-La/Sm(N) correlations are interpreted to have resulted from varying degrees of partial melting. The halogen (F, Cl) contents are very low (<0.1 wt. %), however, a firm negative (F+Cl)-MgO correlation (R² = 0.84) can be related to the Mg-Cl avoidance in the amphibole structure. The relationships between water contents, H isotope compositions and Fe³⁺ contents of the amphibole megacrysts revealed degassing. Selected undegassed amphibole megacrysts show a wide δD range from –80 to –20 ‰. The low δD value is characteristic of the normal mantle, whereas the high δD values may indicate the influence of fluids released from subducted oceanic crust. The chemical and isotopic evidence collectively suggest that formation of the amphibole megacrysts is related to fluid metasomatism, whereas direct melt addition is insignificant.