Plutonic rocks ranging in composition from gabbro to granite are widespread in the province of Thrace in northern Greece, intruding the Rhodope Massif and the Circum Rhodope Belt. Magmatism in this area is considered to be of Eocene to Miocene age. The Maronia pluton is situated south of the Komotini town, in southwest Thrace intruding the Circum Rhodope Belt. The Circum Rhodope Belt is divided into two units: the Makri unit (also known as the phyllite series) and the overlying Drimos-Melia unit. Late studies reject the concept of the Circum Rhodope Belt being a Mesozoic cover of the Rhodope Massif and accept the idea that its rocks belong to two distinct greenschists belts. To the west the Maronia pluton intrudes marbles and calc-phyllites of the metasedimentary series of the Makri unit while to the east it intrudes a metamorphic sequence, comprised mostly of phyllites, greenschists and gneisses of the overlying metavolcanosedimentary series of the same unit. The intrusion caused thermal metamorphism in the country rocks. The pluton was emplaced during the Oligocene and is considered to be the youngest of the Tertiary plutons that intrude western Thrace. Based on the Q’/ANOR classification diagram three main rock groups can be distinguished: a) the basic, b) the intermediate and c) the acid group. a) The basic group is composed of gabbro, a dark-coloured, medium grained rock, having clinopyroxene, red-brown biotite, plagioclase and Fe-Ti oxides. Olivine and orthopyroxene are rare. Interstitial K-feldspar is also present. b) The intermediate group is composed of monzonite, quartz monzonite, monzogabbro, quartz monzogabbro and mafic microgranular enclaves. Monzonite shifts to quartz monzonite with an increase of interstitial quartz up to 6 vol%. These rocks are fine- to medium-grained with monzonitic texture. They consist of plagioclase, clinopyroxene, orthopyroxene, hornblende, red-brown biotite, orthoclase and Fe-Ti oxides and apatite as accessory minerals. Non-perthitic orthoclase encloses poikilitically plagioclase, biotite, orthopyroxene, clinopyroxene and Fe-Ti oxides. Patches of a second generation of K-feldspar is often recognised in the poikilitic K-feldspar. Biotite often encloses grains of accessory minerals as well as orthopyroxene and clinopyroxene. The latter is the most abundant pyroxene. Pinkish orthopyroxene is often altered to uralite. It is partially enclosed by either biotite or clinopyroxene and it seems to be the first mineral to crystallize. Hornblende, wherever present, occurs both as primary crystals and as an alteration product of pyroxenes.