深成岩体中の不均質構造:日高変成帯主帯, トッタベツ複合深成岩体の例  [in Japanese] Heterogeneous structures in a plutonic complex : Inferences from the Tottabetsu plutonic complex, the Main zone of the Hidaka metamorphic belt, Hokkaido  [in Japanese]

北海道の日高変成帯主帯には大陸性地殻-マントル断面が露出し, プレート収束境界における火成プロセスを検討するうえで重要なフィールドである。変成帯主帯北部に位置するトッタベツ複合深成岩体はかつての日高地殻中部から上部にかけて貫入・定置した塩基性から酸性にいたる様々な深成岩類から構成される複雑な内部構造をもつ岩体である。そこではプライマリーな火成活動と, マグマと固結相の反応からなる2次的な火成活動を区別することができる。プライマリーな火成活動はソレアイト質系列の塩基性火成活動と, カルタアルカリ質系列の酸性火成活動からなり, 塩基性火成活動の方が時期的に早い。2次的な火成活動には塩基性マグマと壁岩や堆積岩起源の変成岩ゼノリスとの反応によるノーライト質岩の形成, 酸性マグマが塩基性岩類を同化することによる閃緑岩類の形成がある。閃緑岩類はカルタアルカリ質系列に属する混成岩であり, 地殻下部物質の部分溶融で形成されたカルタアルカリ質なアナテクサイトとは成因を異にする。

The Tottabetsu plutonic complex, situated in the Main zone of the Hidaka metamorphic belt, Hokkaido, has heterogeneous and complicated structures. These features are described in order to elucidate magmatic processes in the middle to upper horizons of the juvenile Hidaka continental crust. The first stage of the magmatism in the complex is recorded in olivine gabbro and ferrogabbro, which were accumulated from evolving mafic magma. Wide compositional ranges of mafic constituent minerals (e.g., olivine, Fo74 to Fo48 in olivine gabbro and Fo33 to Fo6 in ferrogabbro) and presence of inverted pigeonite in ferrogabbro suggest that the mafic magma followed a tholeiitic differentiation trend. These cumulates are includedas blocks (several tens of cm to 1 km in dimension) with rather fine-grained pyroxene hornblende gabbros, which occupy the bulk of the complex (Zone I: 8×8 km). These modes of occurence indicate that the sequence of cumulates was collapsed and disrupted by injection of pyroxene hornblende gabbroic magma during the second stage of magmatism. In Zone I, chilled margins are observed along the boundaries of some irregular sheet-like injections (ca. 20 cm to 50 min thickness) of the pyroxene hornblende gabbros, indicating the multiple injections of mafic magma batehes during the secondary stage. Although compositional range of the pyroxene hornblende gabbros in rather limited, the iron-enriched, tholeiitic differentiation trende is depicted. The third stage of magmatism is characterized by biotite-bearing granitic and granophyric rocks (Zone III: 2×10 km), which are exposed in the uppermost part of the complex. Also observed are numerous granitic dikes in the pyroxene hornblende gabbros and some cumulates of Zone I. As similation by the granitic magma of pyroxene hornblende gabbros and some cumulates resulted in the formation of heterogeneous calc-alkaline dioritic rocks (Zone II:2×8.5 km) between Zones I and III. Another type of assimilation is observed from surroundings of xenolithic meta sedimentary rocks (several cm to 30 min thicknss). The meta-sedimentary xenoliths enveloped in norites are observed at many locations in the complex, especially in Zone I. The mode of occurrence and petrology of the norites suggest that they crystallized from the assimilated gabbroic magma. These observations in the Totabetsu pluton clearly visualize the complicated magmafic processes operated in the magma chamber in the middle to upper levels of the juvenile Hidaka continental crust.