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

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. 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. -from Authors