Metamorphic fluids transport heat and matter. In the Boucheville Basin (North Pyrenean Zone), the circulation of metamorphic fluids is attested by abundant synmetamorphic quartz – calcite veins. The Boucheville Basin formed during the Albian extensional regime and was filled by the so-called “Albian flysch”. The basin underwent a thermal overprint (the North Pyrenean Metamorphism) related to the exhumation of nearby mantle rocks that advected large amounts of heat to the upper crustal levels. The oxygen and carbon isotope compositions of quartz – calcite veins and their host rocks show strong buffering of the composition of the fluids by the Albian metamorphic host rocks. Some host rocks are depleted in calcite near vein contacts showing that some of the elements implicated in crystal growth in veins were derived from the local host rock. The Albian rocks display a range of oxygen and carbon isotope compositions potentially related to closed-system processes of decarbonation–dehydration. We argue then that the fluids that circulated throughout the Boucheville Basin were generated within the basin itself. Their upward migration led to heat advection in the metamorphic pile, a consequence of which was some homogenization of the recrystallization temperatures in the basin.