Abstract
Geochemical data are compiled for basalts (SiO2 30) from the Nasu zone (17 tholeiites) at the trench side and the Chokai zone (2 high-alumina and calc-alkalic basalts) at the back-arc side, Northeast Japan arc. The source peridotites of these basalts above the subducted Pacific plate have large LIL (large ion lithophile)/HFS (high field strength) element ratios but are differentially enriched in individual LIL elements. The Chokai basalt sources are distinctly more enriched in K and Rb relative to Sr, Ce and Zr and lower in K/Rb than the Nasu tholeiite sources. A phlogopite component appears to have been important in metasomatic fluids related to the Chokai basalts which were generated at relatively great depths (~ 140 km deep). On the other hand, fluids high in Sr and K/Rb and low in K/Sr and Rb/Sr, which are probably generated by dehydration of amphibole at a relatively shallower level (~80 km deep), enrich the Nasu tholeiite sources. These metasomatic fluids derived ultimately from the subducted Pacific crust are, before added to the magma sources, probably trapped at the base of the mantle wedge descending by induced convection.
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