Two contrasting types of zoned chromite of the Mt. Higashi- Akaishi peridotite body of the Sanbagawa metamorphic belt, central Shikoku.

Abstract

Two contrasting types of zoned chromite that were formed at different stages in the thermal history of the Mt Higashi-Akaishi peridotite body were examined by an electron microprobe analyser. The zoned chromite of retrogressive-type associated with antigorite and chlorite consists of chromite core and ferritchromite rim. The other type of zoned chromite, igneous-type, occurs in the olivine matrix of dunite, with or without garnet, from Gongen valley where the uppermost part of the body is exposed. The zoned chromite has a chromite core and a Fe3+-rich aluminous chromite rim in garnet-free dunite, and a Fe3+-rich aluminous chromite core and ferrian chromite rim in garnet-bearing dunite. Cation substitution is represented by Cr-(Al,Fe3+) in the former and by (Cr,Al)-Fe3+ in the latter. Fe2+-Mg zoning probably existed at the igneous stage, but would have been homogenized by the Fe2+-Mg substitution at the time of the Sanbagawa metamorphism. As subsolidus cation redistribution cannot explain the zoning with regard to Cr, Al and Fe3+, it is considered to be a relic of the igneous stage. -M.Ak.