砂岩のモード組成と化学組成からみた超丹波帯の古環境  [in Japanese] Paleoenvironment of the Ultra-Tamba Zone by means of modal and chemical compositions of sandstone  [in Japanese]

超丹波帯の古環境, 特に後背地について砂岩のモード組成のほかに化学組成, 岩片と重鉱物の種類および基質の構成鉱物から検討した。二畳紀中世から新世の大飯層相当層は舞鶴帯よりさらに沖合いに堆積し, 主な後背地は隆起してきた中国帯からなる島弧と考えられる。二畳紀新世から三畳紀古世前期と推定される氷上層相当層の後背地は, モード組成からは開析された島弧が, 化学組成からは発達した島弧が推定された。このちがいは塩基性岩片, 重鉱物および基質に含まれる緑泥石やパンペリー石に基因すると考えられ, 供給源は花こう岩からなる飛騨帯と考えられる地帯と塩基性岩からなる地帯の両者であると推定される。また, 氷上層相当層砂岩のモード組成が北東のものほど石英と長石に富み岩片が乏しいことは, 北東ほど飛騨帯からの供給が多かったためと考えられる。

The Ultra-Tamba Zone is composed of Upper Carboniferous to Upper Permian successions (the equivalent of Kozuki Formation) and the Upper Permian to Lowermost Triassic successions (the equivalent of Hikami Formation). The sandstone samples were collected from the equivalent of the Oi Formation (upper part of the Kozuki Formation) and the equivalent of the Hikami Formation. Sandstones equivalent to the Oi Formation are the lithic wackes, and their chemical and modal compositions reflect their derivation from an undissected arc. Sandstones equivalent to the Hikami Formation are mainly quartz rich feldspathic wackes while some are the lithic wackes. There is a progressive increase in quartz and feldspar and a decrease in lithic fragments in sandstones equivalent to the Hikami Formation from southwest to northeast. The modal composition of sandstones equivalent to the Hikami Formation reflects their derivation from a dissected arc, but the chemical composition of the formation reflects the derivation from an evolved island arc. It is considered that most of the quartz and feldspar grains are derived from the Hida Zone and the mafic lithic fragments, mafic heavy minerals and mafic components of the matrix are derived from an area which was composed of mafic rocks.