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.