The Vrondou pluton is an Oligocene high-K calc-alkaline composite granitoid body emplaced in the Lower Tectonic Unit of the Western Rhodope Massif, adjacent to its western margin, and borders the Strymon and Serres basins to the west and south, respectively. Hornblende-biotite quartz-monzonite and biotite-hornblende granite s.s. crop out in the central-western and southern areas, whereas biotite-hornblende granodiorite crops out in the southeastern areas. Clinopyroxene-hornblende monzonite and porphyritic monzonite occur in the central-eastern and northeastern areas. Minor rock types include gabbro, microgranular enclaves and amphibole-bearing lamprophyre dykes. Textural evidence, especially the abundant presence of plagioclase with disequilibrium zoning features (patchy zoning, calcic spikes) in the granitoids and their enclaves, suggests that hybridisation was responsible for their genesis. Enclaves are interpreted as magmatic, probably with a multi-stage crystallisation history, the final stage of crystallisation taking place in chemical and thermal equilibrium with their hosts, as is also indicated by the close chemical affinities between enclave and host ferromagnesian minerals. Major and trace element modelling suggest that the quartz-monzonites, granites and granodiorites are inter-connected by the fractionation of parental quartz-monzonite, and possibly that the monzonites and porphyritic monzonites are inter-connected by the fractionation of a parental monzonitic magma. Rare earth element and trace element patterns of the granitoids suggest that they have a common source and have features similar to magmas generated in active continental margins or post-collisional settings. The basic rock-types show subduction-related trace element enrichments and a garnet-absent source, and are inferred to have been generated from subcontinental mantle lithosphere, enriched during subduction. The Sr and Nd isotopic compositions of the granitoids and the gabbro rule out a single stage basic-to-acid fractionation process. Instead, it is proposed that basic magmas underwent fractional crystallisation concurrent with assimilation in a transient deep magma chamber to produce an initial quartz-monzonite batch, parental to the evolved rocks.