Late Vendian unfolded Molasse in the northeastern, eastern, and southwestern peripheries of the east European platform: Distinctive lithogeochemical features

The paper reports data on the lithogeochemistry of sandstones and silty mudstones from Upper Vendian sedimentary sequences in the northeastern, eastern, and southwestern peripheries of the East European Platform belonging to the so-called unfolded molasse. The sequences are dominated by wackes, arkoses, subarkoses, litharenites, and sublitharenites, i.e., chemically immature and moderately mature psammites, and can be classed with rocks produced by clastic material brought from orogens surrounding the platform. The higher TiO₂, Al₂O₃, FeO_tot, MgO, Na₂O, and K₂O concentrations of the psammites than those in the average cratonic Phanerozoic sandstone (APhSa) testify that the chemical maturing of the rocks was not completed. The silty mudstones accompanying the sandstones have a composition closer to those of the average cratonic Phanerozoic shale (APhSh), but this is likely explained by the fact that the rocks were produced of material brought from erosion territories of much greater area. The lithogeochemical data generally indicate that these territories were dominated by acid and intermediate magmatic rocks with variable fraction of sedimentary rocks when the Late Vendian sedimentary associations in question were produced. The distribution of certain indicator trace elements in the sandstones and silty mudstones show that the average composition of the eroded complexes was close to the composition of the post-Archean upper continental crust, but the erosion areas occasionally (in the Vychegorskii trough and the Shkapovsko-Shikhanskaya depression) contained relatively primitive source rocks. The data points of the great majority of the sandstones and silty mudstones plot in the SiO₂-K₂O/Na₂O and F1–F2 diagrams in the fields of sediments typical of the environments of active continental margins, which is consistent with the arrangement of the data points of these rocks in the La-Th-Sc, Th-Sc-Zr/10, and Th-Co-Zr/10 diagrams. All of these features confirm that the sedimentary rocks in question affiliate with rock associations produced at an active tectonic regime.