Résumé
Precise dating of very young silicic volcanic rocks can be achieved by measurements of 230Th-238U disequilibria in accessory minerals. It is illustrated by the isochron age of 12000 plus or minus 1000 years obtained on a trachyte from the Puy de Dome (French Massif Central). The crystallization age is virtually the same as the eruption age determined by thermoluminescence and 14C dating, indicating a short time interval between crystallization and eruption in such small volume silicic magmas.