岡山県西部，井原市付近の弱変成古生層

The Weakly Metamorphosed Paleozoic Formations near Ibara City, Western Okayama Prefecture

HASE Akira

MONDO Masayuki

HIDE Kei

廣島大學地學研究報告

Geological report of the Hiroshima University

The weakly metamorphosed Paleozoic rocks distributed in the vicinity of Ibara City, Okayama Prefecture, Southwest Japan, are classified lithostratigraphically as follows.
Komachi formation
Main part (Km, 1000～4500 m. in approximate thickness): Composed mainly of pelitic rock and basic volcanic and pyroclastic rocks, with subordinate acid pyroclastics. Thin layers of psammitic rock and siliceous rock are intercalated and small lenses of limestone are also included.
Upper part (Ku, 400～500 m. thick): Composed almost exclusively of pelitic and psammitic rocks. They are often alternated rhythmically, showing a graded bedding.
A conglomerate occurs locally.
Sabara formation
Main part (Sm, 2000～3000 m. in approximate thickness): Similar in lithofacies to the main part of the Komachi formation, but there is no lens of limestone.
Upper part (Su, 700～800 m. thick): Similar to the upper part of the Komachi formation, but the psammitic rock is less developed.
From the occurrence of Yabeina shiraiwensis OZAWA the upper part of the Sabara formation can be correlated to the Upper Permian. The main part may possibly range from the Middle to the Lower Permian, though no fossil has yet been found. It is questionable whether the Komachi formation is contemporaneous with or older than the Sabara.
Metagabbro-metadiabase and mylonitic granite-felsite, which have been grouped under the name of the Yakuno complex, occur closely associated with basic volcanic and acid volcanic rocks, respectively. These volcano-plutonic associations are considered to be a product of the geosynclinal igneous activity preceding the deposition of flysch facies, and are traceable along the boundary between the Central non-metamorphic zone and the Sangun metamorphic zone.
Based on the distribution of metamorphic minerals in basic rocks, the surveyed area is divided into three zones, I, II and III. The zone I is characterized by the assemblage of prehnite-pumpellyite, and is represented by rocks of the Sabara formation. Schistosity is scarcely observed and recrystallization is weak. The zone II is characterized by the assemblage of pumpellyite-actinolite, and is represented by rocks of the Komachi formation. Schistosity and lineation are in general well developed and recrystallization is more advanced. These two zones are probably correlative with the weakly metamorphosed upper part of the Sangun complex. As for the zone III, the assemblage of actinolite-epidote is common, and furthermore, hornblende appears in some cases. This zone occupies the limited areas adjacent to the Cretaceous granite and granodiorite, and therefore is undoubtedly related to the later contact metamorphism.
Both the Komachi and Sabara formations take a general strike of E-W direction and dip usually to the north with low to moderate angle. The structure is, however, by no means simple. The Komachi formation exposed in the northern part of the surveyed area is, as a whole, overturned, and is in contact by a tectonic slide with the gently folded Sabara formation on the south.

jpn

廣島大學理學部地學教室

[ISSN] 0073-2303

[NCID] AN00213676