The Ryoke granites and Ryoke metamorphics at the Ina district, Nagano Prefecture

Abstract

Ina district is located at the northernmost of the Ryoke zone and mainly composed of Ryoke-granites, Ryoke-metamorphics and non-metamorphic sediments. The eastern margin of this district along the median tectonic line is composed of the Kashio Gneiss. These complex grade into non-metamorphic sediments toward west. The Ryoke zone of this area mainly comprises metamorphic rocks, granites and seine migmatites. The Ryoke metamorphic rocks are composed of schistoe hornfels, mica schists, banded gneisses, meta-diabases, quartz schists and crystalline limestones. From mode of occurrence and petrographic feature, migmatites of this area can be divided into the following three rock types. 1 Schistose fine grained biotite granite and granitic gneiss. 2 Gneissose granite. 3 Fine grained two mica migmatite. Schistose coarse and medium grained hornblende-biotite grancdioritee are widely distributed in the southern part. Medium and coarse grained biotite granites occur in the northern part with the form of stock. Kashio Gneiss is composed of porphyritic biotite gneiss and hallefiintaic gneiss. The relation between metamorphism, granitization, and granite intrusion may be explained as follows: 1. The Ryoke metamorphics have been introduced by regional metamorphism and some of them at the eastern part have been passed into the migmatite of the first type. 2. After the metamorphism and granitization, metamorphic rocks were intruded by schistose coarse grained nornblende biotite granite. As a result of tne intrusion, some of invaded rocks have been locally migmatized to the second and third type.