УЛЬБАНСКИЙ ТЕРРЕЙН (ЗОНА) КАК ЧАСТЬ ЮРСКОГО АККРЕЦИОННОГО КОМПЛЕКСА СИХОТЭ-АЛИНСКОГО ОРОГЕННОГО ПОЯСА

Abstract

New data are reported on the age of Triassic-Jurassic cherts replaced up the section by siliceous mudstones, and then by mudstones, which together with basalts form thin (up to hundreds of meters) plates and lenses that pinch out along the strike at different levels of the multi-kilometer terrigenous section of the Ulban terrane (zone). In the Nimelen subzone (the Albazino gold deposit area), their time interval of formation is Norian-Early Toarcian and that of host terrigenous deposits is Pliensbachian-Bathonian. In the Mewachan subzone, cherts and siliceous–clayey rocks are Middle Triassic-Callovian in age and terrigenous rocks are Middle-Late Jurassic (including Tithonian). A comparison of deposits of the Ulban terrane with deposits of the Samarka terrane which are well-studied and similar in age and composition allows us to consider the Ulban terrane (zone) as part of the Jurassic accretionary complex of the Sikhote-Alin orogenic belt. There are also sufficient indications to believe that its structural features are consistent with the subduction model. The basis for this is that the Triassic-Jurassic cherts and siliceous–clayey rocks accumulated for about 45 million years at a rate of about 1.8 m/million years characteristic for the oceanic plate sedimentary cover. At the same time, a gradual transition from cherts to mudstones is observed evidencing the movement of the oceanic plate into the subduction zone. The inclusion of its fragments in the terrigenous section is the result of their being detached in the trench and under the overhanging slab. Tectono-stratigraphic columns of the subzones of the Ulban terrane and a scheme of zoning of the Jurassic accretionary prism have been compiled with four structural-age levels along its entire length.