Geochemical Features and Tectonic Setting of Greenstones from Kunimiyama, Northern Chichibu Belt, Central Shikoku, Japan

摘要

We report whole-rock chemical data for the greenstones from the Kunimiyama area in the Northern Chichibu Belt and their implications on the tectonic setting of these rocks. The Kunimiyama greenstones are associated with stratiform ferromanganese deposits or bedded cherts in the northern part of the study area, but are closely associated with a thick limestone block or bedded cherts in the southern part. The constituent minerals of greenstones are albitized plagioclase, clinopy-roxene, chlorite, calcite, epidote, pumpellyite, prehnite, quartz, celadonite, sericite and opaque minerals such as iron oxyhy-droxide and hematite. These mineral assemblages, epidote + pumpellyite + chlorite and chlorite + pumpellyite + prehnite, suggest that the metamorphic grade of greenstones from the Kunimiyama area is prehnite-pumpellyite facies. The whole-rock chemical compositions of greenstones associated with ferromanganese deposits are generally similar to those of normal mid-ocean ridge basalt (N-MORB). In contrast, the chemical compositions of the greenstones associated with the limestone block are comparable to those of ocean island alkaline basalt. Greenstones associated with bedded cherts are of enriched MORB and ocean island basalt, as well as N-MORB origins, suggesting they probably formed as a result of plume-related MOR volcanism in the Panthalassa Ocean in Early Permian and by tectonic mixing of ocean island basalts with oceanic ridge crustal fragments during accretion/subduction processes. These geological and geochemical lines of evidence suggest that the Kunimiyama greenstones are allochthonous blocks of accreted oceanic crust and seamounts. The ferromanganese deposits are frequently accompanied by reddish greenstones. Compared to common greenish greenstones, the reddish greenstones are characterized by high MnO and rare earth element contents and distinct negative Ce anomalies, implying a slight contribution of hydrothermal component forming the ferromanganese deposits.