Formation Conditions of the Protoliths of Upper Riphean and Vendian Metapelites in the Bodaibo Zone, Baikal–Patom Foldbelt

The paper presents data on the lithochemistry of Upper Riphean and Vendian metapelites in the Bodaibo structural–facies zone of the Baikal–Patom foldbelt. The typical K 2 O/Na 2 O and SiO 2 /Al 2 O 3 ratios of the metapelites indicate that they have not been affected by potassium metasomatism and silicification processes. The ratios in the metapelites of the TM and FM and of the NAM and HM modules of the rocks suggest that their protoliths consisted mostly of the material of the first sedimentation cycle. Its sources were most likely igneous rocks of intermediate and mafic composition. The composition of the protoliths is comparable to kaolinite, kaolinite–illite–smectite, and chlorite–smectite–illite clays, i.e., it did not principally differ from the composition of most “ordinary” clay rocks. The average values of the chemical index of alteration (CIA correct ) exceed its threshold value that distinguishes between rocks formed in cold/arid and warm/humid climates, only for metapelites of the Kharlukhtakh, Khaiverga, Aunakit, and Vacha formations. The metapelites of the other levels of the Upper Riphean and Vendian of the Bodaibo zone are composed of thin-grained aluminosilicate clastic material relatively weakly altered by chemical weathering in the paleocatchment areas. The average values of the phosphorus enrichment coefficient in the formations give reason to believe that the paleoproductivity of the sedimentation areas of the Upper Riphean and Vendian was insignificant, despite the locally rather high content of TOC in the metapelites. The metapelites also show no evidence of the presence of “camouflaged” pyroclastics or products of underwater exhalations.