New finds of middle and late jurassic radiolarians in the siliceous-clayey and clastic rocks of the Nadan’khada-Bikin terrane (Jurassic accretionary wedge, western Sikhote Alin) and their geological significance

The lithostratigraphic analysis of the paleo-oceanic sediments in the Bikin segment of the Nadan’khada-Bikin terrane of the Jurassic accretionary wedge revealed late Bajocian and early to middle Bathonian radiolarians in the siliceous-clayey rocks constituting its southern part in the Perepelinaya-Cheremshanka and Kedrovka areas. Siltstones from the central part of the Bikin segment in the upper reaches of the Vasil’eka River contain radiolarians of the terminal late Tithonian age. Combined with the known dates available for siliceous-clayey rocks from the western part of the Nadan’khada Alin Range and the Ulitka-Zolotoi Klyuch area, these data made it possible to define three stages in the formation of the Bikin segment of the Nadan’khada-Bikin terrane of the accretionary wedge: the terminal Middle Jurassic, the late Tithonian, and the Berriasian. The formation of this segment lasted approximately 25 Ma.     

First Find of Trace Metal Minerals at the Albaza Gold Deposit

Doklady Earth Sciences - The results of mineralogical–geochemical study of the Albaza gold deposit (Khabarovsk Krai) are presented here. The ore mineral assemblage, in addition to already...     

Siliceous-volcanogenic complexes of western Sikhote Alin: Their stratigraphy and origin

New age and structural data are reported for the siliceous-volcanogenic complexes developed in the lower reaches of the Ussuri River. These complexes, which were previously treated as one stratigraphic unit, are subdivided into the Snarsky tectonostratigraphic complex (end of the Middle Jurassic-Middle Aptian) and the basaltic sequence (supposedly, Campanian-Maastrichtian). The Snarsky Complex is made up of basic volcanics, cherts, siliceous-clayey rocks, as well as subordinate limestones, sandstones, and conglomerates. Its distinctive features are the large amounts of genetically diverse basalts, the abundance of volcanomictic and pyroclastic material in siliceous-clayey rocks, the absence of fragmental rocks typical of the continental convergent zone, and the facies heterogeneity of the deposits. The complex is considered to be the southwestern continuation of the Kiselevka-Manoma terrane. Its origin is presumably related to the tectonic piling of genetically heterogeneous assemblages. The basaltic sequence includes basalts, basaltic andesites, their tuffs, and tuff conglomerates. The tuff conglomerates contain numerous fragments of granites and garnet-bearing felsic volcanics. The sequence was formed on the crystalline paleocontinental basement in the Late Cretaceous.     

Geochemical evidence for consecutive accretion of oceanic fragments: The example of the Samarka Terrane,Sikhote-Alin

The data of geochemical study of Late Triassic cherts from tectonic–sedimentary complexes from different structural levels of the Samarka Terrane are reported. It is shown that the concentration and character of distribution of the major petrogenic oxides and minor and rare-earth elements in cherts of the upper and lower structural levels differ significantly, which results from differences in the facies environments of chert deposition. All the geochemical characteristics of cherts show that their deposition proceeded in the pelagic area of sedimentation, but in different parts. The Katen Complex composing the lower structural level is the most distant from the continental margin. The closest is the Amba-Matay Complex composing the upper structural level. Based on the geochemical and biostratigraphic data and the age of accretion of paleooceanic fragments, the length of the subducted oceanic plate (>6000 km) is calculated.     

Geochemistry of the Samarka terrane cherts (Sikhote-Alin) and the size of the accreted paleo-oceanic plate

We have studied the geochemistry of Late Triassic cherts from tectonic sedimentary complexes of different structural levels of the Samarka terrane. The results show that the contents and distribution patterns of major rock-forming oxides, trace elements, and REE in the cherts of the upper and lower structural levels are somewhat different, which is due to different facies environments of chert accumulation. In particular, the contents of Al₂O₃, TiO₂, and K₂O gradually decrease and the contents of Fe₂O₃ and MnO increase downsection. The contents of Zr, Rb, Hf, Th, and Cr, which were supplied into the bottom sediments with a terrigenous suspension of heavy-mineral fragments, are twice to ten-fold lower than their average contents in the upper crust. In contrast, the contents of Pb, Cu, and Ni, which got into the sediments mostly with metalliferous hydrothermal solutions, are rather high and even exceed their average contents in the middle and upper crust. That is, the contents of the first-group trace elements gradually decrease and the contents of the second-group trace elements increase from upper to lower structural level. The calculated negative Ce anomaly gradually decreases from lower (0.75) to upper (0.88) structural level. The geochemical parameters altogether indicate that the cherts accumulated in the same pelagic depositional environment but in its different parts. The Katen complex composing the lower structural level is the most remote from the continental margin, and the Amba-Matay complex forming the upper structural level is the most proximal. Based on the geochemical and biostratigraphic data and on the time of accretion of paleo-oceanic fragments, we have established the location of each complex within the not yet subducted oceanic plate and estimated the extensionof this plate. Throughout the Jurassic, about 6000 km of the oceanic lithosphere was subducted beneath the eastern margin of the Paleo-Asian continent and partly accreted to it.