The Multistage Illite Evolution in Upper Proterozoic Claystones from the Srednii Peninsula, Murmansk Coast of the Barents Sea

Fine-grained clay subfractions (SFs) with a particle size of <0.1, 0.1–0.2, 0.2–0.3, 0.3–0.6, 0.6–2.0, and 2–5 μm separated from the claystone of Upper Precambrian Pumanskaya and Poropelonskaya formations on the Srednii Peninsula were studied by transmission electron microscopy, X-ray diffraction, and Rb–Sr methods. All subfractions consist of the low-temperature illite and chlorite, and the contribution of chlorite decreases with diminishing particle size. The crystallinity index and I002/I001 ratio increase from coarse- to fine-grained SFs. The leaching by ammonium acetate solution and the Rb–Sr systematics in combination with mineralogical and morphological data indicate that illite in Upper Proterozoic claystone from the Srednii Peninsula was formed during three time intervals: 810–830, 610–620, and about 570 Ma ago. The first generation of this mineral with a low Rb/Sr ratio dominates in coarse-grained SFs while the second and third generations with a high Rb/Sr ratio prevail in fine-grained SFs. All of the three generations are known in the Poropelon claystone, whereas the Puman claystone contains only illite of the first and second generations. Geological processes responsible for the multistage illite evolution in claystones are discussed.