SETOUCHI HIGH-MG ANDESITES REVISITED - GEOCHEMICAL EVIDENCE FOR MELTING OF SUBDUCTING SEDIMENTS

In order to evaluate the mechanism of production of unusual high-Mg an desite (HMA) magmas, Pb-Nd-Sr isotopic compositions were determined fo r HMAs and basalts from the Miocene Setouchi volcanic belt in the SW J apan are. The isotopic compositions of Setouchi rocks form mixing line s between local oceanic sediments and Japan Sea backarc basin basalts, suggesting a significant contribution of the subducting sediment comp onent to the HMA magma generation. Mixing calculations using compositi ons of an inferred original mantle and local oceanic sediments suggest that a sediment-derived melt, neither an H2O-rich fluid nor an amphib olite/eclogite-derived melt, could have been produced first and served as a plausible metasomatic agent for the HMA magma source. The unusua l tectonic setting, including subduction of a newly-borne hence hot pl ate, may be responsible for melting of subducting sediments. (C) 1998 Elsevier Science B.V. All rights reserved.