美濃帯ジュラ紀砕屑岩の堆積相と砂岩組成および堆積環境  [in Japanese] Sedimentary facies, mineral composition of sandstones and depositional setting of Jurassic clastic rocks of the Mino terrane, Southwest Japan  [in Japanese]

調査地である美濃帯のチャート-砕屑岩シークェンスは, トリアス紀中世からジュラ紀古世の層状チャート, ジュラ紀古世から中世の珪質泥岩, ジュラ紀中世から新世の頁岩・砂岩から構成される。本研究では, そのうちの砕屑岩層の堆積場の復元, およびその供給源の推定を行った。堆積相を検討した結果, 珪質泥岩の上に整合的に重なる頁岩は, 末端タービダイト相の性格を示し, 深海平原のような環境での堆積物に比較される。さらにその上位の砂岩は, 近源タービダイト相で, チャネル充填堆積物の複合体と考えられる。また砂岩組成より, この砂岩は石英・長石に富み, 岩片に乏しいことがわかった。比較的カリ長石に富み, かつて風成環境に置かれたことを示す円磨度のよい石英粒子が普遍的に含まれている。さらにわずかではあるが, 円磨された紫色のジルコンが含まれている。これらの特徴より, この砂岩の供給源は, 変成岩や深成岩が卓越するような大陸地域が考えられる。また, 当時の構造的背景として, Izanagi Plateがアジア人陸東縁で沈み込みを始めており, 本砕屑岩層はその時の未成熟な海溝において堆積した海溝充填堆積物であると思われる。

The stratigraphy, sedimentary facies, and mineral composition of the Jurassic terr igenous rocks of the Mino terrane were examined. The terr igenous rock succession is underlain by Triassic to Middle Jurassic siliceous rocks. The whole succession of the siliceous and terr igenous rocks records the migration of the depositional site of the siliceous rocks from an open-ocean setting toward an active continental margin setting, The terr igenous rocks are interpreted to be a taping wedge of trench-fill turbidites. The terr igenous rock succession, upto 110 m thick, displays a coarsening- and thickening-upward sequence. The succession begins with a bedded shale section with sandy laminae and thin beds of fine grained, graded, and parallel- and cross-laminated sandstones. The shale section is followed by a thick section of sandstone and shale interbeds, in which channeled beds of coarse-grained, indistinctly parallel-laminated and graded sandstones are predominant. These sandstones have accumulated by turbidity currents. Local beds of disorganized conglomerates are debris flow deposits. The sandstones are rich in quartz and alkali feldspar, and poor in lithic fragments. The mineral composition of the sandstones shows that their provenance area is most likely to be a continental block composed of gneissose and granitic rocks. Less conspicuous, but significant are rounded purple zircon and rounded quartz grains. The purple zircon grain simply a high metamorphic grade for the provenance rocks. The rounded quartz grains are best interpreted to be an eolian origin. The absence of tuffaceous rocks in the terr igenous succession and volcanic-rock fragments in the sandstones suggest these terrigenous rocks were derived from a volcanically in active area. The duration of their sedimentation possibly corresponds to the period of rearrangement of the plate systems from the Farallon Plate to the Izanagi Plate.