九州西部竜峰山変成岩の層序, 構造およびK-Ar年代

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タイトル別名
  • Stratigraphy, geologic structures, and K-Ar ages of the Ryuhozan metamorphic rocks in western Kyushu, Japan.
  • 九州西部竜峰山変成岩の層序，構造およびＫ‐Ａｒ年代
  • キュウシュウ セイブ リュウホウサン ヘンセイガン ノ ソウ ジョ コウゾウ オヨビ K-Ar ネンダイ

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The Ryuhozan metamorphic sequence in the southern Higo belt, Japan is lithologically divided into two formations ; the lower formation (total thickness, ca. 950 m) consisting of limestone and chloritoid rocks intercalated by metamorphosed basic-acidic volcanic and volcanoclastic rocks, and pelites and psammites, and the upper formation (total thickness, ca. 850 m) consisting of slates, and bedded and massive limestone. The geologic structure is characterized by an E-W trending and vertically dipping schistosity which was truncated by sinistral trunscurrent faults of NE-SW strike. The sinistral sense of shear can be inferred from more than 70% of the asymmetric microstructures (n=48) such as shear band, shape and lattice fabric of quartz and calcite (i.e. shear sense indicators). Considering also the development of isoclinal folds with E-W trending axis, a large amount of shortening in NS direction and E-W trending left-lateral shear seem to have concurrently occurred in the zone (a left-lateral transpression).Both type I and type II crossed girdle c-axis fabric patterns are developed in the elongated quartz aggregates parallel to the schistosity. The fact suggests that the deformation occurred at the temperature conditions equivalent to the greenschist to amphibolite facies, being consistent with the mineral assemblages (up to hornblende+andesine) observed in the closely associated basic rocks. Hornblende, muscovite and biotite forming schistosity give ca. 100 Ma in K-Ar age, suggesting that the Ryuhozan metamorphic rocks were formed in a Cretaceous regional tectonometamorphic event which affected the whole Higo metamorphic belt.The host lithologies for the Ryuhozan metamorphic rocks were a sedimentary sequence dominant in volcanic and volcanoclastic rocks with bimodal compositions intercalated by impure limestones and laterites, which suggests the depositional setting either at a volcanic arc or at a passive margin which occurred temporarily from Carboniferous to Permian in the Asian continent. This Ryuhozan metamorphic sequence is correlative with the South Kitakami belt and the Hitachi metamorphic sequence based on their protolithic and tectonometamorphic similarities.

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  • 地質学雑誌

    地質学雑誌 105 (3), 161-180, 1999

    一般社団法人 日本地質学会

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