Chemical geodynamics in a back arc region around the Sea of Japan: Implications for the genesis of alkaline basalts in Japan, Korea, and China

Abstract

Major and trace element have been analyzed from alkaline basalts from southwestern Japan, Korea, and northeastern China. No significant differences were found in the immobile incompatible element ratios, such as Zr, Y, Hf, Th, and Ti. These ratios, as well as normalized incompatible element patterns, resemble those of continental and oceanic island alkaline basalts. However, southwestern Japanese alkaline basalts show evidence of K, Ba, and Rb enrichment and a slight depletion in Ta relative to La, implying a weak island arc signature. Korean and Chinese alkaline basalts do not have such a signature. Rare earth elements (REE) show near‐constant La/Sm ratios and a crossover at the high REE end of patterns for each areas studied. The parallelism in light REE can be derived if the magmas are mixtures formed by (1) relatively large degrees of partial melting of an enriched mantle plume from deeper in the mantle and (2) a small degree of partial melting of a depleted mid‐ocean ridge basalt (MORB)‐type source. These observations when combined with seismic results suggest that the upper mantle beneath southwestern Japan has been weakly affected by metasomatism caused by dehydration and/or partial melting of subducted Pacific plate (not Philippine Sea plate). The mantle plume may have reacted with weakly metasomatized MORB‐type depleted mantle to produce alkaline basalt magmas retaining mild island arc characteristics in southwestern Japan. However, the metasomatism by the subduction of the Pacific plate has not affected the mantle beneath Korea and northeastern China. Here the interaction between plume and MORB‐type mantle produced alkaline basalt magma similar to normal continental and oceanic alkaline basalts.