九州西部竜峰山変成岩の層序, 構造およびK-Ar年代

Stratigraphy, geologic structures, and K-Ar ages of the Ryuhozan metamorphic rocks in western Kyushu, Japan

坂島 俊彦

竹下 徹

板谷 徹丸

早坂 康隆

地質学雑誌

The Journal of the Geological Society of Japan

熊本県, 肥後帯南縁部に位置する竜峰山変成岩の原岩堆積場は, 暗灰色(泥質)石灰岩や砕屑岩を挟在する火山岩および火山砕屑岩優勢な原岩構成に基づき, 受動的大陸縁辺部もしくは島孤縁辺部であった可能性が推察される.竜峰山変成岩中で東西走向・鉛直傾斜のペネトレーティブな片理形成で示される主要な変成・変形作用は, K-Ar鉱物年代測定から白亜紀前期末頃(100 Ma前後)に生じたことが推測された.また, その運動像が北盤水平西ずれのトランスプレッションであったことが大構造および非対称変形微細構造の解析から示唆された.竜峰山変成岩と地体構造的に対比可能な地質体としては, 石炭紀からペルム紀頃の陸棚古生層を原岩とし, 白亜紀に変成作用および左ずれ変形作用を被った南部北上帯や日立変性岩が推察される.

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.

Ryuhozan metamorphic rocks

Paleozoic shelf deposit

K-Ar age

deformation microstructure

Cretaceous left-lateral transpression

日本語

日本地質学会

© The Geological Society of Japan 1999

[ISSN] 0016-7630

[ISSN] 1349-9963

[NCID] AN00141768

[DOI] https://doi.org/10.5575/geosoc.105.161

[DOI] 10.5575/geosoc.105.161