Abstract
In the Sanbosan unit of the Southern Chichibu Belt, west to central east Shikoku, small but significant amounts of greenstones occur as various-sized blocks or lenses embedded within mudstone or siliceous mudstone. Some greenstones preserve pillowed structure with or without interpillow limestone, and related hyaloclastite and brecciated pillowed structures. They are rarely aphyric but largely phyric with phenocrysts of plagioclase, olivine and clinopyroxene. Bulk rock compositions divide the greenstones into high and low Nb/Zr types. The former has an affinity with oceanic island basalts and the latter with mid-oceanic ridge basalts or back-arc basin basalts. Phenocryst of clinopyroxene is restricted to the high Nb/Zr type. With increasing iron content, clinopyroxene compositions of phenocryst and/or groundmass indicate a Ca-enrichment in the high Nb/Zr type, but a Ca-depletion in the low Nb/Zr variety. Metamorphic mineral assemblages reveal that the greenstones have been metamorphosed under the medium- to high-grade zeolite facies in the high Nb/Zr type and the prehnite-pumpellyite (PP) to pumpellyite-actinolite (PA) facies in the low Nb/Zr type and a part of the high Nb/Zr variety. The high Nb/Zr type of the zeolite facies is widespread in the Sanbosan unit forming a narrow zone just north of the Butsuzo Tectonic Line, whereas the low Nb/Zr and high Nb/Zr types of the PP to PA facies are restricted to the Sanbosan unit called the Tosaka Window and Nakatsugawa Window (Matsuoka, 1998), respectively, in west Shikoku. In particular, the greenstones occurring in the Tosaka and Nakatsugawa Windows have some similarities in chemistry and metamorphic facies to those of the Northern Chichibu Belt. These results lead to the following two interpretations ; 1) all these greenstones occur forming several stratigraphic horizons in the Sanbosan unit of the Southern Chichibu Belt, or 2) the greenstones occurring only in the Tosaka and Nakatsugawa Windows belong to the Northern Chichibu Belt.
