Geochemistry of Metasedimentary Rocks, Sources of Clastic Material, and Tectonic Nature of Mesozoic Basins on the Northern Framing of the Eastern Mongol–Okhotsk Orogenic Belt

Abstract

Abstract —We present results of geochemical studies of the upper Mesozoic deposits of the Strelka and Malaya Tynda depressions and U–Th–Pb (LA-ICP-MS) geochronological and Lu–Hf isotope-geochemical studies of detrital zircons from these deposits. It is shown that the Strelka and Malaya Tynda depressions, adjacent to the Mongol–Okhotsk Orogenic Belt in the north and extending along the boundary between the southern framing of the North Asian Craton and the orogenic belt, are marginal troughs. These troughs are filled with thick beds of Mesozoic marine (at the bottom) and continental (at the top) metaterrigenous rocks, with an increase in the grain size of clastic material up the section; the rocks should be regarded as molasses. The results of U–Th–Pb geochronological studies of detrital zircons from metaterrigenous rocks of the Strelka and Lesser Tynda depressions, on the one hand, and the eastern part of the Mongol–Okhotsk Orogenic Belt, on the other, show that orogenic processes in the east of the belt were completed at the Early–Middle Jurassic boundary. The depressions began to form after the complete closure of the Mongol–Okhotsk basin and the formation of an orogenic structure at its place. Then they were filled with material supplied both from the Selenga–Stanovoi and Dzhugdzhur–Stanovoi superterranes on the southern framing of the North Asian Craton and from the Mongol–Okhotsk Belt, which was a mountain-folded structure in the Middle Jurassic.