PERMIAN TERRIGENOUS ROCKS OF THE NORA-SUKHOTINO TERRANE: SOURCES AND TECTONIC CONDITIONS OF ACCUMULATION

The publication presents the first results of comprehensive geochemical and Sm–Nd isotope-geochemical studies of terrigenous rocks from the Permian Bereya Formation of the Nora-Sukhotino terrane in the South Mongolia-Khingan orogenic belt, as well as the results of U–Th–Pb and Lu–Hf isotope studies of detrital zircons from them. The chemical composition of sandstones and siltstones indicates that the source area was dominated by acidic rocks, with possible minor occurrences of intermediate and/or basic rocks. The bulk composition of the investigated clastic rocks is characterized by weakly negative values ɛNd(t) = -0.6...-0.5 with a Mesoproterozoic Nd-model age (tNd(DM) = tNd(DM-2) = 1.1 Ga). Paleozoic zircons dominate in the terrigenous rocks of the Bereya Formation, with a certain amount of Neoproterozoic grains. In solitary instances, results of Lu–Hf studies on detrital zircons indicate that most of their grains have Early Paleozoic and Neoproterozoic Hf–model ages. The youngest zircon group has a concordant age of 261 ± 4 Ma, which defines the lower age limit as Guadalupian, which is Middle Permian on the international stratigraphic scale. The age of the Bereya Formation and the existing ideas about the formation of the South Mongolia-Khingan orogenic belt suggest the accumulation of Permian deposits in post-collision conditions after the closure of the sea basin that separated the Mamyn and Bureya terrranes. Apparently, the geological complexes of the Mamyn terrane or the Bureya terrane are the main sources of clastic material.