Two ultramafic rock types in the Hongseong area, South Korea: Tectonic significance for northeast Asia

Several isolated lensoidal mafic and serpentinized ultramafic bodies occur in the Hongseong area, South Korea, which is the Korean extension of the Dabie-Sulu collisional belt of China and the northern margin of the South China Block. The mafic and ultramafic bodies are important in any interpretation of the Neoproterozoic tectonics of northeast Asia related to the amalgamation and break-up of the Rodinia supercontinent. The Singok, Gwangcheon, and Hongseong ultramafic rocks are significant bodies of ultramafic rock in the Hongseong area. The orthopyroxenes in the Singok harzburgite plot mostly in the forearc peridotite fields of major element classification diagrams. The Singok olivines also classify as forearc peridotite on the basis of their Fo and NiO contents. The spinels of the Singok harzburgite plot within the mantle array on an Al₂O₃ versus Cr₂O₃ diagram, and they fall in the ophiolite and supra-subduction zone peridotite (SSZ) fields on several tectonic discrimination diagrams. These data indicate that the Singok harzburgite is a segment of residual mantle that formed in a SSZ tectonic setting before the amalgamation of the Rodinia supercontinent as the Bibong and Baekdong ultramafic rocks in the Hongseong area. The spinels from the Gwangcheon harzburgite and Hongseong dunite have similar geochemical characteristics with those from ultramafic rocks formed in the Lesvos incipient rift. They plot in the mantle array on the Al₂O₃ versus Cr₂O₃ diagram and have higher Mg/(Mg+Fe²⁺) than abyssal peridotites and lower Cr/(Cr+Al) than SSZ and forearc peridotites. These data together with field evidence indicate that the Gwangcheon and Hongseong ultramafic rocks formed in a continental rift tectonic setting during the break-up of the Rodinia supercontinent. These two types of ultramafic rock from the Hongseong area, which formed in different tectonic settings, are correlated well with the two periods of Neoproterozoic magmatic activity along the northern margin of the Yangtze Block which experienced a subduction and continental rifting stages before and after the amalgamation of the Rodinia supercontinent, respectively.