日高帯の緑色岩の意義 : 海嶺の多重衝突  [in Japanese] Significance of greenstones in the Hidaka belt, Hokkaido : Evidence for polycollision of ocean ridges  [in Japanese]

北海道中央部を構成する日高帯には, 周囲の砂泥質堆積岩と同時期に形成された現地性緑色岩が多数分布している。これらは, 日高帯西縁のイドンナップ帯, 日高帯西部, 日高帯東部の3帯の緑色岩に区分される。年代は, イドンナップ帯のものが白亜紀中頃, 日高帯西部は後期白亜紀後半, 日高帯東部は古第三紀暁新世-始新世と推定される。緑色岩の全岩組成はいずれもN-MORBの特徴を示す。これらは, イドンナップ帯においてはユーラシアプレートとイザナギ-クラプレートとの洩れ型トランスフォーム境界, もしくはイザナギ-クラ海嶺, 日高帯西部については沈み込み境界に対して高角な海洋プレート内の洩れ型トランスフォーム断層, 日高帯東部についてはクラ-太平洋海嶺に由来すると推定される。日高帯は, 後期白亜紀後半から古第三紀にかけて海嶺の多重衝突を経験した特異な付加体で, 日高火成・変成作用の発生によりこの付加体は大陸性地殻へと転化した。これは, 海嶺の相次ぐ衝突による付加体深部の異常な温度上昇, 大量の陸源砕屑物の供給による付加体の急激な成長, 東側から古千島弧が接近してきたなどの複合によっていると考えられる。

Many greenstone bodies which were extruded upon or intruded into clastic sediments cropout ubiquitously in the Hidaka belt, which is accretionary complexes produced during Cretaceous to Paleogene and exposed in cental part of Hokkaido. The in situ greenstones are divided into three zones from west to east; those of the Idon'nappu belt (western margin), the western and the eastern greenstones. The ages of the greenstones are presumed to be middle Cretaceous for the Idon'nappu belt, the latest Cretaceous for the westernones (e.g., Shimokawa, Aibetsu and Poroshiri), and Paleocene to Eocene for the eastern ones (e.g., Kamiokoppe, Uenkotan, Rurochi, Tomuraushi and Nipesotsu), respectively. They have N-MORB signatures in respect to major and minor element compositions. Thus the Hidaka belt records the contemporaneous occurrence of N-MORB and terrigenous sediments with three different ages. The first one might be produced at Izanagi-Kula ridge or leaky transform fault between Eurasian and Izanagi-Kula plates. The second one may be formed at another leaky transform fault within the Izanagi-Kula plate and the last one at the Kula-Pacific ridge. Several tectonic events, such as unconfomity or abrupt shallowing of a forearc basin (Yezo Supergroup), olistostrom due to collapse of inner trench-slope, and exhumation of high pressure metamorphic rocks, occurred at about middle Cretaceous and latest Cretaceous to Plaeogene, respectively. The Hidaka metamorphic belt, an upthrust crustal section of continetal type, would becaused by overlapping of several factors. Repeated subduction of spreading ridges, vast supply of terrigenous clastics which resulted a rapid growth of accretionary complexes, and approaching of paleo-Kuril arc from east, may be responsible for the transformation from the accretionary complexes into the Hidaka continental crust.