Sr-Nd-Pb-B isotope and incompatible trace element investigations were carried out on volcanics of the Phlegrean Volcanic District, which includes Campi Flegrei and the islands of Ischia and Procida (Campanian region, southern Italy). The results of these investigations allowed us to define several components in the genesis of magmas, including transitional mid-ocean-ridge basalt (MORB)–type asthenospheric mantle, aqueous fluids, oceanic sediment melts derived from the subducted Ionian slab, and continental crust. A source contamination process that introduces variable proportions of slab-derived components into the mantle can explain the observed isotopic variations among the products of Ischia and Procida, and of Campi Flegrei older than 39 ka. Furthermore, comparison between Phlegrean Volcanic District and Aeolian arc volcanic rocks shows a different role of aqueous fluids and sediment melts from southeast to northwest. Energy-constrained assimilation and fractional crystallization modeling suggests that younger than 39 ka Campi Flegrei potassic trachybasalt magmas could have assimilated up to 2% of crustal melt, while evolving to trachyte during stagnation at ∼8 km depth. Crustal assimilation and mixing/mingling processes among isotopically distinct magma batches at shallower depth account for geochemical and isotopic variations of younger than 39 ka Campi Flegrei volcanic rocks.