Isotope Systematics and the Age of Authigenic Minerals in Shales of the Inzer Formation, the Southern Urals
Authors: Gorokhov I. M. , Zaitseva T. S. , Kuznetsov A. B. , Ovchinnikova G. V. , Arakelyants M. M. , Kovach V. P. , Konstantinova G. V. , Turchenko T. L. , Vasil’eva I. M. 
Published: May 2019

The 2–5, 0.6–2, 0.3–0.6, 0.2–0.3 and 0.1–0.2 µm clay subfractions (SFs) separated from two shale samples of the Upper Riphean Inzer Formation, the southern Urals, were studied by the TEM, XRD, and U–Pb, Sm–Nd, Rb–Sr and K–Ar isotopic methods. All the SFs consist of the low-temperature 1Md illite; admixtures of quartz, chlorite and 2M1 illite occur only in the coarsest SFs. The clay particles are isometric, regardless of their size. The CIS (Crystallinity Index Standard) illite values for the all SFs are typical for the dia(kata)genetic zone. As the size of particles in the SF decreases from 2–5 to 0.1–0.2 µm, the CIS rises, the I002/I001 ratio on the XRD diagrams decreases, and the K content and the K/Rb ratio increase.

Leaching with 1N HCl and 1N ammonium acetate (NH4OAc) and subsequent U–Pb, Sm–Nd and Rb–Sr analyses of the untreated SF, acid (acetate) leachate and residue made possible to study the mixing systematics in mobile and silicate materials of the shales. The 238U/204Pb and 87Rb/86Sr ratios in the acid and acetate leachates are below, and the 147Sm/144Nd ratio is above those in the residues. Less radiogenic Pb and Sr and more radiogenic Nd are also common for the leachates compared to the residues. As the size of particles in the SFs decreases, the U, Pb, Sm, Nd and Sr contents in the residues are smoothly reduced, whereas the Rb content shows an increase. The 87Rb/86Sr and 87Sr/86Sr values in the residues for fine-grained SFs are well above, and the 238U/204Pb value is well below those for coarse-grained SFs. What is more, in the 87Rb/86Sr–87Sr/86Sr and 1/86Sr–87Sr/86Sr diagrams, data points for the residues of variable size are arranged on the mixing lines. The data points of respective acid and acetate leachates also form linear trends in the 238U/204Pb–206Pb/204Pb, 206Pb/204Pb–207Pb/204Pb, 147Sm/144Nd–143Nd/144Nd, and 87Rb/86Sr–87Sr/86Sr coordinates. The apparent Rb–Sr age values, calculated from the slopes of “inner isochrons” (“leachochrons”), along with the K–Ar ages are smoothly lowered from 835–836 and 721–773 m.y. for the 2–5 µm SF to 572–580 and 555–580 m.y. for the 0.1–0.2 µm SF. Hence the XRD and isotopic data testify that the clay as well as the mobile material of the shale represent the mixtures of at least two components, the silicate phase containing authigenic illites of different ages. The first illite generation enriched in coarse-grained 2–5 and 0.6–2 µm SFs was formed shortly after deposition of the Inzer sediments, and its age of 803–836 m.y. is in agreement with the stratigraphic age of the formation. Simple lithostatic burial or intensive lateral fluid flow induced by tectonic inversion in the eastern regions of the Urals paleobasin may be considered as the geological processes responsible for the forming of this illite. The second illite generation was formed 572–580 m.y. ago. As the starting points for its formation, alternatively, may be concerned either vertical tectonics or renewal of pore fluid compositions during deformations and metamorphism on the southern Urals area related to evolution of the Beloretsk metamorphic complex.