K-AR AGES OF THE BASALTIC ROCKS FROM FAR-EAST RUSSIA - CONSTRAINTS ONTHE TECTONO-MAGMATISM ASSOCIATED WITH THE JAPAN SEA OPENING

K-Ar ages of the Cenozoic basaltic rocks from the Far East region of R ussia (comprising Sikhote-Alin and Sakhalin) are determined to obtain constraints on the tectono-magmatic evolution of the Eurasian margin b y comparison with the Japanese Islands, Northeast China, and the forma tion of the back-are basin. In the early Tertiary stage (54-26 Ma), th e northwestward subduction of the Pacific Plate produced the active co ntinental margin volcanism of Sikhote-Alin and Sakhalin, whereas the r ift-type volcanism of Northeast China, inland part of the continent be gan to develop under a northeast-southwest-trending deep fault system. In the early Neogene (24-17 Ma), a large number of subduction-related volcanic rocks were erupted in connection with the Japan Sea opening. After an inactive interval of the volcanism similar to 20-13 Ma ago, the late Neogene (12-5 Ma) volcanism of Sikhote-Alin and Sakhalin beca me distinct from those of the preceding stages and indicated within-pl ate geochemical features similar to those of Northeast China, in contr ast to the Japan Are which produces island are volcanism. During the J apan Sea opening, the northeastern Eurasian margin detached and became a continental island are system, and an integral part of continental eastern Asia comprising Sikhote-Alin, Sakhalin and Northeast China, an d the Japan Are with a back-are basin. The convergence between the Eur asian Plate, the Pacific Plate and the Indian Plate may have contribut ed to the Cenozoic tectono-magmatism of the northeastern Eurasian cont inent.