The Lower Cambrian Lontova claystones contain well crystallized authigenic 1M illite mixed with detrital 2M illite. Part of the authigenic illite probably precipitated during sedimentation and subsequently experienced a temperature up to 140°C, despite a limited burial to 500-1000 m. The difference between the Rb-Sr and K-Ar dates of this authigenic illite are attributed to preferential loss of radiogenic 40Ar induced by a short-lasting thermal pulse. This interpretation is based on an Ar-diffusion model suggesting that the Lontova blue clays were heated to around 140°C for 2-5 Myr. This thermal pulse could have occurred in relation with hot-fluid migration induced by either tectonic dislocations in the basement rocks during the Late Silurian-Early Devonian, or rifting conditions during the Permian. By comparing Rb-Sr and K-Ar data, evaluation of radiogenic 40 Ar losses by small authigenic illite particles at low temperatures could be a powerful tool for reconstruction of evolutionary histories of sedimentary successions that were either shallowly buried, or percolated by migrating fluids released during tectono-thermal activities.