CHEMICAL GEODYNAMICS IN THE BACK-ARC REGION OF JAPAN BASED ON THE TRACE-ELEMENT AND SR-ND ISOTOPIC COMPOSITIONS

143Nd 144Nd, 87Sr 86Sr, trace and major element results are reported for Cenozoic alkaline basalts and associated tholeiitic basalts from southwestern Japan, and some from Korea and China. Most of the samples plot within the mantle array with ε{lunate}Nd values ranging from +5.0 to -3.2 and 87Sr 86Sr ratios ranging from 0.7037 to 0.7057. The isotopic variations observed along traverse sections of alkaline basalts from the trench to the continental sides of southwestern Japan do not suggest a direct relationship between the subduction of the Philippine Sea and Pacific plates, and the generation of alkaline basalt magmas. However, in southwestern Japan the alkaline basalts do show evidence of a subducted component and are more enriched in the mobile incompatible elements, such as Rb, Ba and K, relative to Chinese and Korean basalts. Negative correlations between ε{lunate}Nd and Sm/Nd and between ε{lunate}Nd and La/Th, as well as a positive correlation between 87Sr 86Sr and La/Th, are recognized in some of the basalts. These relationships indicate at least two-component magma mixing between (1) magma resulting from a small melting from a depleted mantle and (2) magma resulting from a relatively large amount of melting of an enriched mantle source. It is suggested that the Cenozoic alkaline basalts from southwestern Japan, Korea and China are product of interaction between a MORB-type mantle and a deep mantle plume. If the mantle plume has ascended from the upper mantle-lower mantle or core-mantle boundary, the subducted Pacific and Philippine Sea plates could not have extended to southwestern Japan. Had they done so they would have arrested, before they reached the melting zone, the ascent of the plumes that produced the alkaline volcanism in southwestern Japan. © 1990.