瀬戸内海地域領家帯の黒雲母片麻岩の起源に関する1つの提案 : 玖珂層群とは別の地質単元から由来した黒雲母片麻岩がある

The Kuga group is a Jurassic accretional complex that has been believed by many workers to be a unique geologic unit transformed into the Ryoke gneisses in the Seto Inland Sea region. I will now suggest the presence of a different geologic unti in addition to the Kuga group. Most of the biotite gneisses found at the Iwakuni-Yanai area are probably derived from rocks of the Kuga group, but the rest biotite gneisses that occur in the Murotu Peninsula, the Yashiro-jima, the E-no-shima, and the other several islands in the Seto Inland Sea region were derived from a un-known and a different geologic unit to the Kuga froup. Some pieces of evidence proving the distinct origin of the latter biotite gneisses are: (1) the biotite gneisses commonly occur as small to large fusiform bodies generally enclosed in the older Ryoke granite or as large masses contacting only with the older Ryoke granite; (2) tight folds with parallel axial surfaces are very common, and dupli-cated fold systems are found in many localities; (3) microtextures of some minerals, e.g. garnet and K-feldspar, often suggest a plural metamorphic history of the gneisses (Nuraki et al., 1982a; Miyashita % Komatsu,1993); and (4) the sillimanite zone (Ikeda,1991 : Okudaira et al.,1993) is only a metamorphic zone that can be found. The sillimanite zone is interpreted to be excluded from the progressive metamorphism in the Iwakauni-Yanai area (Wallits et al.,1992). Biotite gneisses of the sillimanite zone are found here and there and show no zonal arrangement of distribu-tion in the Ryoke belt. The estimated metamorphic temperature and pressures for the biotite gneisses of the silliman-ite zone are 530-710℃ and 2-4kb respectively, both of which are lower than of the peak metamorphism, estimated as 550-820℃ and 5-6kb, at the Iwakuni- Yanai area.