Accretionary complexes are the important geological structures as their study allows insights regarding the individual stages of geodynamic evolution of orogenic belts and the nature of the sedimentary material involved. The Ononsky accretionary complex of the East Transbaikalia mainly consists of metasedimentary rocks of ononskaya and tchindantskaya suits. The article considers the petrography, lithochemical and geochemical characteristics of metasedimentary rocks from the ononskaya and tchindantskaya suits.Based on petrochemical classification by Neelov A. N. we reconstructed the probable protoliths for these rocks: monomict psammitolites and polimict sandstones and siltstones for the ononskaya suite and polimict sandstones, siltstones and argillites for the tchindantskaya suite. Petrochemical modules demonstrate island arc sources that are likely associated with the presence of pelagic deposits in their protoliths. Rare earth characteristics of the studied metasedimentary rocks also confirm that two types of the sedimentary material played an important role in the protolith: terrigenous, present on continental slopes and biogenic (silica) typical of the pelagic zone in the ocean. Thus, on the basis of both petrochemical and geochemical characteristics it was found that the most probable geodynamic setting for the formation of sedimentary rocks from the studied ononskaya and tchindantskaya suites was an active continental margin with the participation of pelagic deposits. Similar geodynamic conditions are the case within the accretionary wedge formed in front of the subduction zone.

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