Isotopic and Geochemical Zonality of the Paleocene Magmatism of the Asia–Pacific Transition Zone

The diversity of the tholeiitic, alkali or calc-alkali, high-silica, and adakitic rocks (I-, FG- and А-types) in the northeastern margin of Asia often leads to controversial conclusions about the character of magmatism and the active margin evolution in the West Pacific during the Late Mesozoic–Cenozoic. In addition, paleogeodynamic reconstructions of the Asia-Pacific region can only be made based on the synthesis of patterns identified in compositions of magma sources, evolution and character of magmatism, and strain and kinematic analysis of fault tectonics of all parts of the eastern margin of the paleo-Asian continent. The paper presents new isotopic, geochemical, and structural data on one of the largest early Paleogene structures located at the continental margin of the southern Sikhote-Alin. Based on the new and previously published data we have concluded that the tectonic and magmatic settings of the region were shaped by oblique interaction of continental and oceanic plates during the Late Cretaceous-early Paleogene. The igneous products of the frontal (FG-type), in respect to the trench, and intracontinental (А-type) parts of the area are characterized by isotopic-geochemical variations caused by thermal changes in upwelling asthenosphere and by interaction with lithospheric components. These processes accounted for the formation of initial magmas similar to the IAB- or OIB-type mantle source, respectively.