Abstract
Gabbroic xenoliths from Ichinomegata crater, Northeast Japan arc, contain a large amount of amphiboles. Some of them are obviously of secondary origin in hornblende–pyroxene gabbros; clinopyroxene in the pyroxene–spinel symplectite, which is subsolidus reaction products of olivine and plagioclase by cooling, was replaced in part with amphibole. Minerals (amphibole, clinopyroxene and spinel) in the symplectite contain appreciable TiO2 (up to 1.6 wt%), although olivine and plagioclase are free from TiO2. The amphibole in the symplectite is significantly enriched in Na, K, Rb and Ba relative to the other symplectite minerals that contain very low amounts of these elements. The metasomatic formation of amphibole in the symplectite was accompanied with addition of, at least, Ti, Na, K, Rb and Ba. The metasomatism is characterized by enrichment of incompatible elements, especially LILE but not HFSE (Nb, Ta, Zr and Hf). This was caused by infiltration of fluid possibly released from hydrous arc magmas. Metasomatic processes observed in the lower crustal gabbros are also recorded in peridotite and websterite xenoliths of upper mantle origin from Ichinomegata. This means widespread modification of mineral chemistry and mineral assemblage from the upper mantle to the lower crust beneath the arc.
