九州,甲佐地域の間の谷-肥後変成岩の地質構造(予報)  [in Japanese] A preliminary report on geological structure of the Manotani-Higo metamorphic rocks of the Kosa district, Kyushu  [in Japanese]

The Manotani metamorphic rocks are situated just on the north of the Higo metamorphic rocks in the Kosa district, Kumamoto Prefecture, Kyushu. YAMAMOTO (1962) explained that the former is in fault contact with the latter at the west side of the Miyauchi fault (Fig. 1), the fault being associated with intrusion of serpentinite. Both metamorphic rocks show an andalusite-sillimanite type metamorphism, though the Higo metamorphic rocks are of higher grade. In the Manotani metamorphic rocks have been found relicts of minerals such as alkali amphiboles, pumpellyite and lawsonite, suggesting that they were initially metamorphosed under high P/T condition (SATO & INOUE, 1968; KARAKIDA et al., 1984). Through our detailed geological survey, it has been clarified that the Manotani metamorphic rocks are not in fault contact with the Higo metamorphic rocks, and that the former lithologically gradually changes to the latter, though in the former basic metamorphic rocks of probably ophiolitic rock origin are predominant and in the latter siliceous metamorphic rocks of chert origin are predominant. In the transitional horizon between the Manotani and the Higo metamorphic rocks, serpentinite bodies as small lenses are found with various rock types such as pelitic, basic and siliceous metamorphites and the former stratigraphically con formably cover the latter. The Higo metamorphic rocks must have undergone high P/T type metamorphism together with the Manotani metamorphic rocks before the andalusite-sillimanite type metamorphism occurred. The geological structure of the Manotani-Higo metamorphic rocks is characterized by the development of gentle upright folds and of a nappe (Figs. 1, 2). The nappe (Kamakurayama nappe) is developed in the southern part of this district. The Manotani-Higo metamorphic rocks of the nappe is lithologically characterized by the predominance of calcareous metamorphic rocks derived from giant limestone masses, associating gneissose granitic rocks. The Manotani-Higo metamorphic rocks are, as a whole, mylonitic. The mylonitization is distinctly intense around the basal thrust (Kamakurayama thrust) of the Kamakurayama nappe, showing that it is related to the formation of the Kamakurayama nappe. In the highly mylonitized metamorphic rocks are found such de formation textures as the development of ribbon quartz and mica fish (Plate I). Ribbon quartz has some time aspect ratio of larger than 17. The Manotani-Higo metamorphic rocks around the Kamakurayama thrust are initially of the stability field of sillimanite associating disappearance of muscovite. However, they now generally show such textures and mineral characteristics of retrograde metamorphism as defined by the appearance of muscovite, chlorite and actinolite. The mylonitization related to the formation of the Kamakurayama nappe is followed by the cataclasis associating the formation of prehnite veins.