Taking a Fresh Look at the Stratigraphy of the Selemdzha and Tokur Terranes of the Mongol-Okhotsk Belt: The Results of U-Pb, Lu-Hf, and Sm-Nd Isotope Studies

The results from U-Pb, Lu-Hf, and Sm-Nd isotope studies of metasedimentary rocks from the Selemdzha and Tokur terranes of the Mongol-Okhotsk belt contradict the existing concepts on the stratigraphy of the region and indicate the need to revise the traditional principles of mapping within the belt. Two types of deposits that sharply differ in their Sm-Nd whole-rock characteristics and Lu-Hf isotope compositions of detrital zircons have been established within the terranes. Deposits of different types are involved in the formation of the two opposite accretionary systems: in front of the Siberian Craton margin (type I) and the Amur superterrane (type II). The type I deposits are divided into groups with different lower age limits: 553-498 Ma, Late Ediacaran-Cambrian; similar to 373 Ma, Upper Devonian; 333-327 Ma, Upper Mississippian; and similar to 304 Ma, Pennsylvanian. The deposits of the tectonic-stratigraphic units show a general trend of decreasing age from north southward. The same trend is observed from the upper to lower sheets. Such a structure is typical of accretionary wedge-shaped terranes with a rear part in the north and a frontal part in the south. The tectonic-stratigraphic units formed by type I deposits and previously ascribed to the Selemdzha and Tokur terranes likely belong to the Galam accretionary wedge terrane.