長野県飯田市三穂地域の塩基性岩類I

野外の産状と岩石学的性質

抄録

The basic metamorphic rocks of the Miho area are divided into meta-diabase and gabbroic rocks. The meta-diabase, finer-grained basic rocks characterized by granoblastic texture, is widely distributed as small masses sporadically scattered in this area, and the gabbroic rocks are exposed in limited zone around Mts. Suisho-zan and Jo-yama. The gabbroic rocks can be classified petrographically into ten rock-types, cortlandite, allivalite, anorthositic gabbro, gabbroic anothosite, olivine bearing hornblende eucrite, hornblende eucrite, hornblende gabbro, quartz gabbro, olivine bearing pyroxene dolerite, and pyroxene dolerite. The last two rock-types are chilled facies of the gabbroic rocks. In the gabbroic rocks layered structure is recognized in the field. From the field and petrographical observations and the chemical characters of the constituent minerals, it is concluded that these gabbroic rocks have been formed through crystallization differentiation of basic magma and emplaced as a layered intrusion.
Petrological characteristics of the cortlandites scattered throughout the Ryoke metamorphic belt were reviewed. There is some local variations and a difference between the cortlandites from the Chubu (especially in the north) and ones from the Kinki-Setouchi districts. The cortlandites from the Chubu district contain clinopyroxene and chromium-bearing spinel, but the cortlandites from the Kinki-Setouchi districts do not contain these minerals. Mg/(Fe+Mg) ratio of the constituent minerals and the An content in plagioclase of the cortlandites from the Chubu district are higher than those of Kinki-Setouchi districts.