西南日本内帯のペルム紀酸性火砕岩類(その1)

Silicic volcanogenic sedimentary rocks (predominantly redeposited silicic tuffs and very rarely lapilli tuffs) occur within Permian (especially Middle Permian) strata in the Akiyoshi-Nishiki, Maizuruand Ultra-Tamba (UT3) terranesofthe Chugoku region. These rocks have received very little attention, however their existence has major implications for interpretation of the geotectonic history of these terranes. It is generally accepted that a Middle to Late Permian orogeny called the Akiyoshi or Honshu Orogenic Movement occurred in the inner side of Southwest Japan. We propose that the silicic rocks described above were the products of arc volcanisms preceding this disturbance. Silicic tuffs (mostly vitric or crystal vitric tuffs, and sometimes crystal tuffs) are described in detail from the Muikaichi and Nishiki areas of the Akiyoshi-Nishiki Terrane, Yanahara area of the Maizuru Terrane and Kozuki area of the Ultra-Tamba Terrane. Tuffaceous beds, several to tens of centimetres thick (sometimes attaining more than several tens of metres in thickness) are interlaminated with mudstones and argillites. The constituent silicic pyroclasts are interpreted to have been transported by turbidity currents and/or debris flows from the flanks of adjacent volcanic arcs into thick mudstone laminated sequences. The evidence suggests that sedimentary sequences of all three terranes received detritus from active arc volcanoes during Middle Permian.