Rare-earth-element compositions of rocks from Quaternary volcanoes in Northeast Japan arc and their magmagenetic implications.

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Contents of large-ion-lithophile (LIL), rare earth (RE), and high-field-strength (HFS) elements are determined for representative rock samples from volcanoes at Moriyoshi and Chokai zones, situated in back-arc side of Northeast Japan arc. Based on these results together with the preceeding research made by myself ( Grant-in-aid No. 63540643 ), across-arc inhomogeneity in magma source region with respect to incompatible elements is investigated.Across-arc variation in respect of lighter REE / heavier REE ratios is clearly recognized among basaltic members of the magma suites; the ratios increase systematically from trench side toward back-arc side zone. This tendency becomes distinct more and more as the relevant magmas enrich in silica. Since different degrees of partial melting of common magma source can not account for the systematic variations, it seems plausible that across-arc variation with regard to REE exists within source mantle region. Further, role of magma mixing is more important in magmatic evolution process for back-arc side volcanoes, with acidic end-member magma suffering fractionation ( or remnant ) of some HREE-bearing minerals such as amphibole, clinopyroxene, and garnet.Detailed examination of the LIL-RE-HFS systematics for concurring magma suites at Kampu, Adatara, and Funagata volcanoes revealed source heterogeneity in respect of incompatible element geochemistry. The inferred chemical characteristics for metasomatically added agent(s) are well-reconciled with the two-stage stepwise wedge dehydration model postulating two pressuresensitive dehydration reactions ( proposed by Tatsumi; 1989 ), suggesting strongly that differential infiltration of slabderived fluid into the mantle source is a crucial process in producing a variety of magmas.