飛騨外縁帯ペルム系森部層の砂岩組成とその後背地  [in Japanese] Sandstone composition and provenance of the Permian Moribu Formation in the Hida Marginal Belt, central Japan  [in Japanese]

飛騨外縁帯のペルム系森部層の層序と砕屑岩組成について検討した.森部層は砂岩・礫岩が卓越する基底層, 石灰岩を挟み, 泥岩や砂岩からなる下部層, 厚い塊状砂岩が卓越する中部層, 砂岩泥岩互層や泥質岩が卓越する上部層の4部層に区分される.森部層基底層は安山岩片を含む石質砂岩や酸性火山岩片を含む石質砂岩, 下部層は火山岩片の卓越する石質砂岩, 中部層はやや石英・長石類に富む長石質〜石質砂岩により特長づけられる.砂岩のモード組成および基底層の礫組成から, 基底層から下部層の後背地として未成熟な活動的火成弧の活動が推定される.中部層では酸性火成岩の卓越する火成弧が主要な後背地であったと考えられる.これらの後背地は, 活動を終えた先ペルム紀やペルム紀の火成弧であったと推定される.森部層における後背地の時代的変化は, 南部北上帯ペルム系のそれと同様の傾向を示すので, 両者の後背地は類似した環境にあったと考えられる.

The permian Moribu Formation in the Hida Marginal Belt is composed mainly of non-carbonate clastic rocks. The formation can be subdivided lithologically into four members : The basal (conglomerate and sandstone), lower (mudstone and sandstone with minor amount of limestone), middle (massive sandstone) and upper (mudstone) members. Andesitic lithic sandstones in the basal member indicate a primitive arc volcanics provenance. Quartz-rich lithic sandstones and feldspathic sandstones in the lower and middle members were derived from an active volcanic arc and pre-Permian strata which consist of acidic volcanic and volcaniclastic rocks.Sandstone compositions of the Moribu Formation and the Permian of South Kitakami Belt exhibit two common traits. First, both sequences start with coarse terrigenous clastics indicative of andesitic volcanic provenance. Second, stratigraphic transition of the clastic composition in both Permian sequences shows a similar mode from volcanic to volcano-plutonic provenance. These lithologic and petrographic features suggest that the Moribu Formation is the equivalent of the Permian part of the South Kitakami Belt.