Mitsuo Shimazu

Tectonics and volcanism in the Sado-Pohang Belt from 20 to 14 Ma and opening of the Yamato Basin of the Japan Sea

Abstract Tertiary volcanism around the Japan Sea is closely related to the opening of the Japan Sea. Paleogene to early Middle Miocene formations in the Pohang-Yangnam area of the Korean Peninsula mainly consist of volcanic rocks that are stratigraphically and petrographically similar to similar age rocks on the Japan Sea side of the Japanese Islands, especially those of Sado Island. We propose that all these early Cenozoic rocks formed part of a single tectonic-volcanic belt called Sado-Pohang Belt. It is inferred that this belt was located on the south side of the Yamato Ridge in Paleogene time. After the Japan Basin opened, the Yamato Basin opened during Early Miocene to the early Middle Miocene time and part of the Sado-Pohang Belt migrated southward. The tectonics of the Sado-Pohang Belt is considered to support the hypothesis of the two-stage opening of the Japan Sea.

Potash felspars in some metamorphic rocks

Potash felspars in the Akka metamorphics (contact metamorphic rocks), several other metamorphics and granites are studied optically and by the x-ray powder method. Triclinicity values (Δ) in potash felspars of the Akka metamorphics show monoclinic symmetry, not only in orthoclase zone (higher part of amphibolite facies), but also in cordierite zone. Potash felspars in hornfelses close to granite mass seem to show diffuse reflection and large optic angle. In other metamorphic terrains, potash felspars show extremely low triclinicty values, while those in the Hida gneiss, as well as the Hida granite, show large triclinicity values. From these results, the relations between symmetry change of potash felspar and metamorphism are discussed. From the albite molecular percent in some alkali felspars, the temperature of potash felspar formation is determined by the Barths method.

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

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.