The composition and sources of Kalba-Narym terrain sediments (Eastern Kazakhstan)

The Kalba-Narym terrain is part of the Western sector of the Central-Asian Orogenic Belt (CAOB). In Late Paleozoic - Early Mesozoic, structural complexes of this terrain evolved as part of a single geological megastructure - Altai accretionary-collision system. In spite of the duration and detail of investigation of the Kalba-Narym terrain, its nature remains a debating point. The aim of this work is a detailed study of the Devonian-Carboniferous sedimentary strata composition that can give new information about the nature of the Kalba-Narym terrain. The paper presents the results of petrographic and petrogeochemical studies of sedimentary sections of the Kalba-Narym terrain. Sedimentary strata, outcropping on the territory of the Kalba-Narym terrain, are divided into follow suites: Kystav-Kurchum (D2gv), Ablaketka (D ₃-Cit), Burabay (Civ), Dalankara (Qs), Taubinka (C ₂b) (Fig. 1). In this work sediments from the lower part of the sedimentary section of the Kalba-Narym block (D ₂-Ci) associated with the early stages of terrain development were studied. Research indicates slight variations of major and trace element compositions of the studied sedimentary rocks from different formations of the Kalba-Narym terrain (Fig. 2). Data on the content of major components of sediments based on 100 % dry material are given in Table 1. Rare and rare-earth elements in representative samples of the Devonian-Carboniferous black shale rocks of Kalba-Narym are shown in Table 2. In addition to the sedimentary strata in the northern part of the Kalba-Narym terrain, plates of metabasalts have been studied. On the level of trace elements composition, these rocks are closest to the ocean-island basalts described as part of the West Kalba area. It can be assumed that these greenstone rocks of basaltic composition are fragments of oceanic lithosphere, included in the accretionary complex. The results of these studies indicate no significant differences in major and trace element composition of Kalba-Narym terrain sedimentary rocks from the Givetian stage of the Middle Devonian to the Visean stage of the Lower Carboniferous. On this basis, we can assume that the sources for sedimentary rocks of the Kalba-Narym block have not changed in the interval from the end of the Middle Devonian to the Early Carboniferous. Based on the obtained data on the composition of the black shale rocks, it is assumed that the formation of these strata occurred in the destruction of the substrate, characterized by relatively low maturity; most likely this could be a source of the immature volcanic complex of the Altai active continental margin. The presence within the terrain units of metabasic composition also shows that the Kalba-Narym terrain formed as a basin with oceanic crust in the basement and with sediments corresponding to the base of the continental slope facies.

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