Geochemical study of mafic dikes and sills of the Riphean (Ulakhanbam) and Ordovician (Suordakh) ages in the area of the Sette-Daban ridge allowed to establish differences between these complexes. For the Ulakhanbam magmatic rocks, the mantle source was basically similar in characteristics to E-MORB. Despite the possible sediment contamination, wide range of ϵ_Nd(T) values is most likely due to the specific composition of the mantle source, which likely had an admixture of OIB material. The Ordovician reservoir had OIB characteristics, whereas the MORB material had a subordinate significance.