The dates of eight diabase pebbles (pillow fragments) in the conglomerates of the Grès du val d'Illiez have been measured by the potassium-argon whole rock method. The samples come from "La Vacherie" and "Les lombardes" in Thônes syncline. These diabases have common isochron ages around 50 millions years but contain exces amounts of argon with initial ⁴⁰Ar/³⁶Ar ratios varying from 300 to 500. The age of 50 millions years corresponds to the time of system closure after a metamorphic event. It is proposed that the excess argon present may have diffused into these samples during a second metamorphic event at about 35 millions years.