堆積岩の希土類元素, Th, Sc組成からみた中・古生代地帯の後背地  [in Japanese] Geochemistry of REE, Th and Sc of sedimentary rocks for the provenance study of the Paleozoic and Mesozoic terranes of Japan  [in Japanese]

日本の中・古生代地帯である南部北上帯, 黒瀬川帯および舞鶴帯の堆積岩について, 放射化分析で得られた希土類元素組成とTh/Sc比を用いて, 各地帯の後背地地質や造構場の変遷を推定した.南部北上帯, 黒瀬川帯ともにシルル系では大陸縁辺の組成を示し, デボン系では大陸性島弧組成へと変化する.その後, 南部北上帯は石炭系〜ペルム系下部では海洋性島弧組成となるが, ペルム系上部に向かって次第に大陸性島弧組成に発達する.一方, 黒瀬川帯は石炭系までは大陸性島弧組成を示すが, ペルム系で海洋性島弧組成となる.両地帯は同起源であったが, ゴンドワナ大陸から分裂した後, 別の地殻発達の過程をとったと考えられる.舞鶴帯はペルム系からトリアス系まで一貫して発達した大陸性島弧〜活動的大陸縁辺の組成を示し, 他の2地帯とは大きく異なる.各地帯ともジュラ紀末には共通して活動的大陸縁辺の組成となり, 東アジア大陸縁辺へ収束したことが示唆される.

REE patterns and Th/Sc ratios were utilized as provenance signatures of the sedimentary suites in the Southern Kitakami, Kurosegawa and Maizuru terranes.Both Southern Kitakami and Kurosegawa terranes experienced evolution from continental arc to continental island arc in the Silurian and Devonian. The Southern Kitakami terrane rocks, however, shifted to a less-evolved composition (oceanic island arc) in the Carboniferous and Lower Permian. The terrane gradually became more evolved arc (continental island arc) toward the Upper Permian. The Kurosegawa terrane rocks had a continental island arc composition until the Lower Carboniferous, and in tern gradually became more primitive (oceanic island arc) in the Middle Permian. The geochemical indices clearly demonstrate the contrasts between the Southern Kitakami and Kurosegawa terranes, resulting from different evolution history after dispersion from the Gondwana margin. Steep REE patterns of Maizuru terrane rocks suggest existence of an evolved continental margin through Permian and Triassic times. All terranes studied here were terminated at the most evolved provenance (active continental margin) in Late Jurassic time, suggesting their amalgamation to the eastern margin of Southeast Asia.