Notes on petrography and rock-forming mineralogy (11). Green biotite in Sanbagawa basic schists in the Kanto Mountains, Japan.

In the northeastern part of the Kanto Mountains, central Japan, in basic schists, green biotite widely occurs in a lower-grade part where garnet is common in pelitic and psammitic schists, while brown biotite is present in a higher-grade part where brown biotite commonly occurs in them. Comparing with brown biotite from higher-grade part of the Sanbagawa metamorphic terrain in this area and central Shioku, green biotite in lower-grade part of this area contains lower Al2O3 and higher Fe2O3, and slightly lower TiO2. On the basis of the paragenetic relation of the green biotite-bearing assemblages, the occurrence of green biotite is explained by high Fe2O3, (FeO+MgO), and low Al2O3 contents of the host rocks. Brown biotite in basic schists in a higher-grade part is formed by the following reaction; green biotite+ (Fe, Mg-rich) chlorite+epidote+albite+quartz=brown bitite+hornblende+(Al-rich) chlorite+magnetite+H2O. This reaction enlarges the stability field of biotite, and in a higher-grade part, brown biotite-bearing assemblages widely occurs in the common basic schists.