Sources of Terrigenous Rocks of the Lower-Middle Devonian(?) Siltstone–Sandstone Sequence of the Zeya–Selemdzha Fragment of the Nora-Sukhotino Terrane: Geochemistry and U–Th–Pb (LA–ICP–MS) Zircon Dating

The article presents the first results of geochemical and U–Th–Pb (LA–ICP–MS) isotope studies on terrigenous deposits of the inferred Lower–Middle Devonian siltstone–sandstone sequence of the Kamenushka River basin of the Zeya–Selemdzha fragment of the Nora-Sukhotino terrane. It has been found that the sandstones we studied correspond in their geochemical characteristics to sediments that formed in a setting of an active continental margin or an island arc, with felsic and intermediate igneous rocks and minor recycled (second cycle) material predominant in the source area. According to the concordant age of the three youngest zircon grains, the lower age limit for the accumulation of sandstones of the siltstone–sandstone sequence is 459 ± 9 Ma. Large variations in the U–Th–Pb age of detrital zircon grains from a sandstone sample indicate heterochronous material in the source area. The obtained data along with the existing paleogeodynamic models for the formation of the South Mongolia–Khingan orogenic belt suggest that the main sources of clastic material were Paleozoic and Neoproterozoic igneous rocks of the Mamyn terrane, as well as Early Paleozoic island-arc rocks of the Nora-Sukhotino terrane.