Chemical characteristics of detrital chrominan spinel in the Sangun Zone southwest Japan

The Sangun zone belongs to "Inner zone of southwest Japan", and is basically composed of lower crystalline schists of high pressure intermediate type and upper unmetamorphosed part of Paleozoic formations (e.g. Miyashiro, 1973). Paleozoic formations are accompanied by many ultramafic complexes, central Chugoku district, Southwest Japan (Igi and Abe, 1969; Research Group of Peridotite Intrusion, 1967, Arai, 1980, Matsumoto et al, 1995). These complexes are consist of mainly harzburgite and dunite, and sometimes accompanied by chromitite-pods. Chemical composition of chromian spinel is good indicator for origin and their petirogenetic characteristics of its host rocks (e.g., Irvine, 1965, 1967, Arai and Okada, 1991, Arai, 1992). In this study, we separate detrital chromian spinel from river sediment in and around the ultramafic complexes, Sungun zone, SW Japan. Cr# (Cr/(Cr+Al)) of chromian spinels from relativery dunite rich ultramafic complex varies from 0.55 to 0.65. Contrastly, dunite free or dunite poor ultramafic complex, Cr# of chromian spinels varies from 0.40 to 0.60. And complex with relativery large chromitite show high in Cr# and Mg# (Mg/(Mg+Fe(2+))) of chromian spinel. The results of above chemical characteristics of chromian spinels from the Sangun zone clearly show that detail estimation of distribution of dunite and harzburgite and existence of chromitite pods.