|Budget Amount *help
¥1,300,000 (Direct Cost: ¥1,300,000)
Fiscal Year 1989: ¥400,000 (Direct Cost: ¥400,000)
Fiscal Year 1988: ¥900,000 (Direct Cost: ¥900,000)
In recent years, it has been suggested that the Chichibu Belt is a composite terrane consisting of three terranes. Those are, from north to south, Northern Chichibu, Kurosegawa and Southern Chichibu terranes. Among them, the Kurosegawa terrane, which contains a pre-Silurian basement assemblage, is considered to be the key terrane in elucidating the tectonics of Southwest Japan. This is because the faunal content and paleomagnetic data of the Kurosegawa terrane provide compelling support for the postulate that it may once have constituted part of Gondwana.
The purpose of this project is to provide basic data to understand whether or not the Kurosegawa terrane originated as a stratigraphic terrane characterized by the presence of crystalline basement with covering strata of coherent sequences. For this purpose, following approaches were planned; (1) Reappraisal of the age of the "Usuginutype conglomerate", which characteristically contains granitic boulders, by radiolarian fossils, (2) Pe
trological study of granitic rocks of the Kurosegawa Tectonic Zone and granitic boulders collected from the "Usuginu-type conglomerate" to determine the provenancial linkage between them, and (3) Dating of granitic rocks of the tectonic zone and granitic boulders collected from the conglomerate by U-Pb on zircons.
As a result of these studies the following points were made clear: (1) The "Usuginu-type conglomerate" in the Kurosegawa terrane, which is part of the covering strata to a continental fragment, ranges in age from mid Permian to Middle Jurassic, (2) Whole rock analysis of both granitic rocks of the Kurosegawa Tectonic Zone and granitic boulders collected from the conglomerate shows typical calc-alkaline rock series affinities, but indicates that both may not have been closely related in origin, and (3) A preliminary result of dating indicates that U-Pb ages of zircons separated from granitic boulders of the conglomerate are 250Ma surprisingly younger than expected. The last result is particularly important and might be a direct evidence for a subduction around that time along the Kurosegawa terrane. Less