Structural and Petrological Studies on the Ryôké Gneiss and Granodiorite Complex of the Yanai District, Southwest Japan.

OKAMURA Yoshihiko

Journal of science of the Hiroshima University. Series C, Geology and mineralogy

The Yanai district for the most part consists of the Ryôké metamorphics and granodioritic complexes which are distributed sporadically in the southern margin of the inner zone of Southwest Japan. The metamorphic rocks of the district comprise mostly siliceous banded gneisses, small amounts of semi-pelitic and pelitic rocks, amphibolites and crystalline limestones. The granodioritic rocks are grouped into two generations, i.e., the older and the younger complexes. The former includes autochthonous and parautochthonous granodiorites of Obataké, Gokenya, Gamano and Okiura, and the latter intrusive gran-odiorites and granites of Tōwa, Kibé, Murotsu, etc. It is assumed in field relation and petrography that the banded gneisses have suffered granitization and migmatization and changed gradually into gneissic granodiorites.
Structural features of the banded gneisses and associated granodioritic rocks are presented here in detail. In particular, such features as foliation and lineation in these rocks were measured and their geometry was treated statistically. The folding of banded gneisses is uniform throughout the area, and fold axis B coincides with b-lineation. The axial planes of folds are generally parallel to each other and considered to coincides with shear plane. The older granodiorites generally exhibit distinct foliation and lineation. It is note-worthy that the trend of older granodiorites is conformable and harmonic with, and locally subconcordant to those of the banded gneisses. The younger granodiorites discordantly cut the structures of the banded gneisses and the older granodiorites.
Petrofabric analyses were made for 22 selected samples, as an important aid to the interpretation of geo-logical structure and geological history of the report area. The results of the petrofabric analysis coincided with the structural megascopic observation. Quartz fabrics of the banded gneisses proper do not show any characteristic patterns or high concentration. As the granitization of the banded gneisses advances, the quartz fabrics exhibit rather higher concentration and roughly monoclinic symmetry. The fabric diagrams of the gneissic granodiorites also show high concentration and symmetry as well as those of the granitized banded gneisess. Mica fabrics are due to the bedding schistosity and show ac-girdles. Biotite orientation seems to be related to the known s-plane which represents usually shear plane. Biotite orientation of the banded gneisses tends to disperse the concentration as the granitization progresses, and the bio-tite orientation of the gneissic granodiorites presents a more dispersed girdle, presumably due to increasing mobility of the rocks during the migmatization process.

eng

HIROSHIMA UNIVERSITY

[ISSN] 0075-4374

[NCID] AA00706718