The isotopic compositions of basaltic lavas from the Quaternary and Plio-Pleistocene Italian volcanics (Tuscan Magmatic Province, Roman Magmatic Province, Vesuvius, Aeolian Islands, Etna, and Iblean Basin) define binary hyperbolic relationships in the 87Sr/86Sr--206Pb/204Pb--ϵNd--ϵHf space. The isotopic compositions of the two end-members of these mixing arrays are assessed by least-squares regression. The mantle-derived component (206Pb/204Pb = 19.8, 87Sr/86Sr = 0.7025, ϵNd = +8, ϵHf = +9) is a rather homogeneous mixture of the standard high-¿ (HIMU) and depleted mantle (DM) components. The crust-derived component (206Pb/204Pb = 18.5, 87Sr/86Sr > 0.715, ϵNd = -12, ϵHf = -11) accounts for the enrichment of K and other large-ion-lithophile elements in the Italian volcanics. As shown by the relationship in ϵHf--ϵNd space and the lower-than-chondritic Hf/Sm ratio, this crustal component is dominated by pelagic sediments rather than terrigenous material. The overall scarcity of calc-alkaline compositions in the Italian volcanics and the presence of a HIMU component, which is the hallmark of hot spot basalts, raise the question of how plume mantle source contributes to volcanism in a subduction environment. At about 13 Ma, the Apennine collision terminated the westward subduction of the Adria plate under the European margin and rotated the direction of convergence to the northwest. The cumulative differential of subduction between the fossil plate under Tuscany and the active plate under Sicily since the opening of the Tyrrhenian Sea amounts to at least 300 km and is large enough to rift the dipping plate and open a plate window beneath the southern part of the peninsula. This model is consistent with recent high-resolution seismic tomography. We propose that the counterflow of mixed upper and lower mantle passing the trailing edge of the rifted plate is the source of Italian mafic volcanism. Alternatively, material from a so-far unidentified plume may be channeled through the plate window. The crustal signature is probably acquired by interaction of the mantle advected through the window with the upper part of the subducted plate.