Chemical Composition of Basic Schists from the Sangun Metamorphic Terrain in the Nishiki-chô District, Yamaguchi Pref., Japan

In the Sangun metamorphic terrain of the Nishiki-chô district, characterized by glauco-phanitic metamorphism, basic schists such as greenschists and glaucophane schists occur abundantly as lenticular or amoeba-shaped bodies within some definite stratigraphic horizons. The origin and the chemistry of the basic schists have been investigated on the field as well as on the petrochemical point of view hitherto applied mainly to igneous rocks. Original rocks of the basic schists are regarded as sills intruded into water-saturated pelites in the geosynclinal phase. The basic sill is divided petrographically and petrochemically into two parts. One is the most peripheral zone of the sill, which is composed of pale-green, schistose, fine-grained rocks and shows fairly abnormal bulk composition between gabbroic and pelitic rocks (type II). The other is the main part of the sill, which consists of coarse-grained green rocks, showing normal gabbroic composition (type I). The bulk chemistry in the main part varies systematically from the marginal layer to the central part wihin a sill. The regularity of features of the intruded basic body is described in detail and genetically considered. Petrochemical characters of the basic schists along with the mineralogy of relic clinopyroxenes demonstrate that the basic schists of the Nishiki-chô district have been derived from rocks of the alkalic rock series.