Utilisation de la méthode rubidium-strontium pour la datation de niveaux sédimentaires du Précambrien supérieur de l'Adrar mauritanien (Sahara occidental) et la mise en évidence de transformations précoces des minéraux argileux

Amineralogical-geochronological study has been made of sedimentary levels I₂ and I₅ in the upper Precambrian of the Adrar, Mauritania, showing that clay minerals can be used for geochronology. This is, however, only possible where early diagenesis has homogenised the ⁸⁷Sr/⁸⁶Sr ratio of clay minerals and interstitial waters. Rocks whose clay minerals consist of unaltered mica or illite are not suitable for this approach, as is the case for sediments rich in detrital feldspars: carbonates may however be used.Ages of 964 ± 32 M.Y. (I₂) and860± 35 M.Y. (I₅) were obtained using a⁸⁷Rb decay constant of 1.47 × 10?11 yr^?1, confirming that the beginning of the late Precambrian sedimentation in the Adrar may be placed at 1000 M .Y.