[B-CG05] 地球史解読：冥王代から現代まで

[BCG05-15] パンサラッサ海遠洋域で形成した上部三畳系層状マンガン鉱床のコノドントー放散虫化石層序による年代制約

★招待講演

*冨松 由希¹、尾上 哲治¹、Rigo Manuel² (1.九州大学 大学院理学研究院 地球惑星科学部門、2.パドヴァ大学)

キーワード：層状マンガン鉱床、チャート、コノドント、放散虫、カーニアン多雨事象、ランゲリア巨大火山岩岩石区

Numerous stratiform manganese deposits occur in the Permian to Jurassic bedded chert successions of the Jurassic accretionary complexes of the Chichibu, Mino–Tamba, and North Kitakami belts, which is considered to have accumulated in a pelagic, open-ocean setting within the Panthalassa Ocean. Although a number of previous studies investigated the depositional age and process of Permian and Jurassic manganese deposits in these accretionary complexes, very few studies have focused on the Triassic bedded chert–hosted stratiform manganese deposits. We thus investigated the occurrence and stratigraphic distribution of Triassic stratiform manganese deposits in Japan, constraining their ages by means of integrated conodont–radiolarian biostratigraphy. This study investigated the stratiform manganese deposits in the following four accretionary complexes; (i) the Takahira section, in the Chichibu Belt; (ii) the Tamaiwa section in the Tamba Belt; (iii) the Kanzaki section in the Mino Belt; and (iv) the Otaniyama section in the North Kitakami Belt. In these sections, the Triassic stratiform manganese deposits range in thickness from 30 to 150 cm and generally occur intercalated within chert sequence. The chert samples from each studied section yielded abundant conodonts and radiolarians representing the same faunal assemblages. On the basis of the conodont and radiolarian biostratigraphies, the depositional age of the Triassic manganese deposits was constrained to the latest Julian to earliest Tuvalian (mid-Carnian).This age indicates that the formation of the Triassic manganese deposits occurred contemporaneously with the last eruptive phase of the Wrangellia Large Igneous Province (LIP) volcanism and subsequent changes in the deep-sea redox conditions in the Panthalassa Ocean during the Carnian pluvial episode. Therefore, the Carnian manganese deposits may have been formed with the hydrothermal processes associated with the Wrangellia LIP volcanism and/or the changes in redox conditions on the ocean floor.