Zur Kenntnis des Oberpaläozoikurns in Östlichen Chūgoku, Südwest-Japan

MITSUNO Chiharu

廣島大學地學研究報告

Geological report of the Hiroshima University

The Upper Palaeozoic formations develop in the eastern part of the Chūgoku District. They are divided into two facies, i.e., the one is the low grade crystalline schist belonging to the so-called "Sangun Metamorphic Zone", which represents a metamorphic facies due to the regional type of metamorphism, and the other is non-metamorphic facies. The Upper Palaeozoic formations are divided from the sedimentary facies into the Calcareous Sediments Facies and the Non-calcareous Sediments Facies. The metamorphic and the non-metamorphic formations are, in general, strati-graphically conformable, but they are in most localities bounded by the surface running obliquely to the bedding surface. In some places, the boundary surface has the character of thrusting shear zones or faults.
The Palaeozoic formations are tentatively classified into the following four groups.
1) The lowest group consists mainly of black-schist beds, with intercalated thin beds of quartz-schist and green-schist. The thickness amounts to about 1,500 m.
2) The lower group lies conformably on the lowest group and is characterized by thick beds of green-schists, with intercalated schistose acidic lava and its pyroclastic rocks, to which a number of bedded cupriferous pyrite deposits (Kieslager) are stratigraphically related. The total thickness amounts to 2,500～3,000 m.
3) The middle group lies conformably on the lower group and is characterized by the alternation beds of sandstone, quartz- and black-schist. A green-schist bed lies on these alternation beds. The total thickness amounts to 2,500～3,000 m.
4) The upper group lies on the middle group, but the relation between the middle and the upper group are not determined. The upper group is characterized by the acidic pyroclastic rocks and its lava intercalated into thick slate beds. The slate beds contain as intercalation of these acidic pyro-clastic rocks, sandstone and limestone lens. The total thickness of the upper group amounts to 1,500～2,000 m.
Among them, for the groups, 1, 2, and 3, the age of sedimentation and submarine volcanism in the geosyncline is safely be appointed as not younger than the Upper Carboniferous, while the group 4 indicates the Lower Permian-the Upper Permian from the lower to the upper part.
The limestone group, which is distributed in the eastern Chugoku District, belongs to the age from the Lower Carboniferous to the Upper Permian, the age being the same as that of the metamorphic facies. In other words, the limestone group and the metamorphic facies of the Upper Palaeozoic formations represent the contempaoraneous-heteropic relation in facies.
The geologic structure of the Palaeozoic terrain, except the Yanahara Mining region, is characterized by gentle dome- and basin-like folds. The trend of lineation of the metamorphic zone forms two currents along the northern and southern boundaries of the Atetsu liniestone plateau, covering the central part of the region. The lineation (micro-corrugation within the bedding schistosity surface) coincides generally with the fold-axis of the schist beds in trend, but in the central part near Tsuta and Ukan, the trend of lineation is intensely disturbed, showing no coincidence with the regional fold-axis. In the Yanahara region, the Permian formation suffered two folding cycles, i.e., the one is the regional folding in the phase of deformation of the Sangun Metamorphism, and the other is the local intense fold in the phase of the Early Triassic folding. As the result of these two folding cycles, the Permian beds indicate an intensely folded style with overturned structure.
There are many loder metagranite, metadiorite, metagabbro and metadiabase sheets, which run parallel to the Palaeozoic terrain and concordant or subconcordant to the country rocks. These rocks were mylonitized, showing protoclastic texture. It is conceivable that these rocks were intruded in the last stage of the Sangun Metamorphism.
Many cupriferous iron sulphide ore deposits (Kieslager) are contained within the Palaeozoic formation of the eastern Chūgoku District, having a close relation to their geological environments as strati-graphically and structurally as well. The writer classified these ore deposits to three types according to the difference in relation to the Palaeozoic formations.
The first is a type which has intimate stratigraphical relation to the basic or intermediate volcanism, being parallel to the original bedding of crystalline schist and harmonic to the folded structure of the country rocks. This type may safely be appointed to the category of the so-called "Besshi-type Kieslager" lying in the Sanbagawa Metamorphic Zone. Ores of this type of ore bed consist of compact pyrite ore with small amounts of chalcopyrite and pyrrhotite.
The second type falls into the category of the "Kawayama-type Kieslager", judging from similarities in its structural relation to the country rocks to the Kawayama ore deposits, Yamaguchi Prefecture. The ore deposit lies in a shear zone, which was formed in close relation to the deformation of the crystalline schists. Ores of this type of deposits consist mainly of pyrrhotite with small amounts of chalcopyrite and zincblende.
The third type falls into the category of the "Yanahara-type Kieslager" which is related closely to the Lower Permian acidic submarine volcanism. The ore bodies lie within the acidic pyroclastic beds and suffered the same deformation as the country rocks. Ores of this type of deposits consist mainly of massive pyrite with small amounts of chalcopyrite, but, owing to the thermal metamorphism by the Late Mesozoic plutonism, outer part of the ore bodies are replaced by pyrrhotite and magnetite.
It is inferred that the original beds of ore bodies of the Besshi- and the Yanahara-type Kieslager had already existed before the begginning of the Sangun Metamorphism, because in these ore deposits can be traced the effects of later phases of deformation and mineralization. The former is related to the basic or intermediate volcanism, and the latter to the acidic submarine volcanism. The Kawayama-type Kieslager is believed to have been formed at the last stage of the Sangun Metamorphism.

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廣島大學理學部地學教室

[ISSN] 0073-2303

[NCID] AN00213676