九州の点在変成岩類の特徴と構造地質学的位置

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The metamorphic rocks, scattered in North and Middle leyushu (to the north-west of the Butsuzo tectonic line), may be grouped into five divisions, the Sangun, Sanbagawa, Nagasaki, and Ryoke metamorphic rocks, and rocks of the Kurosegawa structural belt. 1) Of these divisions the most important is the Sangun, which includes metamorphic rocks of the Sangun (the derivation of the name), Seburi, Asakura, and Chikuhi mountainlands, thenorthwestern part of Fukuoka Prefecture, the southern part of the Chikuho district, and the Konoha, Kiyama, Asaji, and Manotani areas. The Sangun rocks consist of pelitic, psammitic, and basic crystalline schists, with a few limestone beds, and in a number of localities undouted metagabbros (including metadiabases) and serpentinites are found. The original sediments may be stratigraphically divided into four groups, A, B, C, and D, and may range in age from late Ordovician (?) to early and middle Permian. This metamorphic terrane is divided in to three metamorphic zones by using pumpellyite, actinolite, epidote, and hornblende as the index minerals in basic schists. The zones are 1) pumpellyite-actinolite (-alkali amphibole) zone (lowest grade), 2) epidote-actinolite zone, and 3) epidote-hornblende zone, the first zone being developed in a very limited area at Yamaga and also the last zone at Kiyama. The great part of the associated ultrabasic rocks, largely serpentinized, is dunite and the small part harzburgite. The Kiyama schists are dated by radiometric methods at 297-454 m.y. and the preliminary Rb-Sr ages of the Chikugo schists are of the order of 300 m.y. 2) The Sanbagawa belt undoubtedly extends westwards to the Saganoseki peninsula and its further extension has not been distinct. In the Kuma area of the Chichibu belt, there are the upper Paleozoic formations which have been subjected to the glaucophanic metamorphism characterized by the growth of lawsonite and pumpellyite in both pelitic and basic rocks. This area is considered by some petrologists to represent the western extension of the Sanbagawa belt. 3) The Nagasaki metamorphic rocks include crystalline schists of the Nishisonogi and the Nomo peninsulas and the Amakusa-Shimoshima island. They are characterized by the epidote-actinolite (-alkali amphibole) assemblage in basic rocks and the ultrabasic rocks intercalated to them are made up of dunite and wehrlite largely serpentinized. Muscovites from the schists have radiometric ages of 59-94 m.y. These rocks may represent the northeastern extension of the Ryukyu metamorphic belt. 4) The regional metamorphism of the andalusite-sillimanite type are shown in the Higometa-morphic area, where the lower-grade pelites contain andalusite as a characteristic mineral and the higher-grade sillimanite. Radiometric ages of biotite from the Higo biotite gneisses are 104, 106, and 108 m.y. Thus the Higo metamorphism is very similar in type and age to the Ryoke metamorphism. The original sediments are composed mainly of psammitic rocks, limestones, and basic tuffs and lavas, and are accompanied by many metamorphosed ultrabasic masses, whose original rock association is dunite-harzburgite. These features are characteristic of the Sangun belt. In addition, stratigraphically the Higo rocks pass gradually into the Manotan irocks, which belong to the Sangun belt. On the basis of these facts it may be inferred that the Higo metamorphic rocks are polymetamorphic: that is, they have been subjected to the Sangun metamorphism followed by the Ryoke metamorphism. 5) The metamorphic rocks of the Kurosegawa structural belt occur as small patches along some thrust zones in the Chichibu belt and are closely associated with granitic rocks and the Silurian formations. The metamorphic rocks are represented by mica gneisses and garnet-bearing amphibolites, which always show various cataclastic features. Radiometric age determinations on biotite and zircon from these gneisses and granitic rocks give ages of 209-600 m.y. The
日本地質学会の論文
1969-01-31

九州の点在変成岩類の特徴と構造地質学的位置

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