Résumé
In a study of the 'La Porcherie' leucogranite which cuts major Variscan thrusts in the Saint-Germain-les-Belles, Limousin, and the Grandrieu leucogranite associated with the Margeride calc-alkaline granite, Lozere, it is demonstrated that the U-Pb system is unsuitable for dating their emplacement. In 'La Porcherie' leucogranite the zircons have xenocryst cores with euhedral overgrowths; this inherited component seems to be characteristic of leucogranites associated with major shear zones. Such a feature is absent from the 'Grandrieu' leucogranite. Monazites, however, appear to be useful for geochronology of such granites. U/Pb data yield sub-concordant ages interpreted as emplacement ages: 317 + or - 3 m.y. for 'La Porcherie' leucogranite and 305 + or - 4 m.y. for the 'Grandrieu' leucogranite.