抄録
A small amount of greenstone is found in the Jurassic accretionary complex of the Keisoku Massif. It occurs as lavas, dykes, volcanic breccias and tuffs in the melange zone of the chert-clastics sequences. The greenstones metamorphosed into the greenschist facies contain igneous augite, titaniferous augite and titaniferous hornblende.
Fourteen samples of the greenstones were analyzed in this study. On the basis of discriminating diagrams, most greenstones have a chemical affinity with T- or P-type MORB, AB, OIT, OIA, WPB, WPA or WPT. Three greenstones have high ratios of Nb/Y versus Nb/Zr at high Nb/Zr ratios.
The Permian Izuru Formation occurs in the Ashio Mountains, and mainly consists of greenstones. The chemical characteristics of Izuru greenstones are of the types OIT and OIA. Geological evidence also indicates that the Izuru Formation originated from a Paleozoic seamount. Greenstones of the Keisoku Massif are interbedded with olistostrome in the melange zone of the Jurassic accretionary complex, and have a chemical affinity with OIT and/or OIA. It is suggested that most greenstones of the Keisoku Massif were accreted as slices derived from the Izuru seamount.
