Abstract This study reports the first observation and compositional data of tourmaline overgrowths in the post-Caledonian basal Gdoumont conglomerates of the Belgian part of the Rhenohercynian basin. Authigenic tourmaline occurs as monopolar hemimorphic and bipolar asymmetrical foitite overgrowths within fractures and along the margins of detrital tourmaline grains and tourmalinite pebbles. Formulae calculated from electron-microprobe analyses reveal high amounts of vacancies in the X-site (61 to 84%), placing this tourmaline amongst the most alkali-deficient tourmaline recorded to date. Besides the alkali-deficiency, Gdoumont overgrowths are characterized by high Al, low Na and Ca, and low-to-moderate Mg/(Fe + Mg) ratios. These characteristics reflect the composition of the host conglomerates and suggest local derivation of the chemical components. Slightly differing zoning patterns (sample scale) and differences between localities (regional scale) indicate heterogeneity of the low-grade host rock. Host tourmaline grains as well as tourmaline occurring in quartz-veins crosscutting the Gdoumont metasediments have distinctly different compositions. The absence of discontinuous zoning suggests growth of the authigenic tourmaline in just one stage. It is most likely that tourmaline formed from boron released by metamorphic breakdown of detrital minerals, e.g. illite and/or K-feldspar. Based on their crystal chemistry and the observed optical properties, the overgrowths must have developed during diagenetic to lower-greenschist facies conditions. Scarce concurrency of bipolar overgrowths, isolated neoblasts and cryptocrystalline nests of tourmaline indicates limited growth until middle-greenschists facies conditions.