Sm-Nd systematics of fine-grained fractions of the Lower Cambrian blue clay in northern Estonia

Fine-grained clay subfractions, SFs (particle size <0.1, 0.1–0.4, 0.4–0.6, and 0.6–2.0 μm) separated from a sample of the Lower Cambrian blue clay of the Lontova Formation were studied with XRD and Sm-Nd methods. The relatively coarse-grained SFs include illite with a small admixture of chlorite, while the finegrained SF (<0.1 μm) consists of mixed-layer illite-smectite. The illite crystallinity index (I _c ) increases with decreasing particle size. The leaching of SFs with 1N HCl, analysis of Sm-Nd systematics of leachates and residues, as well as XRD data and results of chemical analysis show that the studied rocks contain at least two generations of minerals. The first (detrital) generation is related to the transformation of provenance material, whereas the second (authigenic) generation was formed at the postsedimentary stage of the evolution of the Lontova sediments. The Sm-Nd date of the first generation (790±90 Ma) is considered a minimal age of rocks in the northeastern and southwestern regions of the East European Platform that served as a source of sedimentary material of the Lower Cambrian blue clay. The date of the second generation of minerals reflects the timing of authigenic mineral formation in the course of burial and diagenetic and catagenetic reworking of clay sediments.