希土類元素組成からみた東北日本中新世アイスランダイトの成因  [in Japanese] The REE composition of Miocene icelandite in Northeast Japan, and implication for the origin of icelandite magma  [in Japanese]

18〜12 Maの東北日本に出現するアイスランダイトは, 高K_2O量・Ce/Yb比タイプと低K_2O量・Ce/Ybの2種類に区分できる。前者は関東地方北部(茂木・大子地域)や北部阿武隈地域(毛無山)に産し, 後者は東北地方中部脊梁地域(古川〜新庄間)に分布する。周辺の苦鉄質火山岩類の化学組成との関係から, 前者はある種の下部地殻物質の部分融解によって, また後者はソレアイト質苦鉄質マグマの結晶分化作用によって生成されたものと推定される。K_2O/TiO_2比は前者で高く後者で低い。これは, 両者における初源マグマの化学組成上の違いを反映しているものと考えられる。前期中新世に生じた, マントル・プリュームの上昇による日本海の急速な拡大と日本列島の移動は, 高温でH_2Oに乏しく還元的な地殻環境と伸長応力場の発達を促し, アイスランダイト質マグマの生成に寄与したものと思われる。

There are two types of Miocene icelandites in Northeast Japan. One is characterized by high K_2O and Ce/Yb, and the other formed low K_2O and Ce/Yb. High K_2O-Ce/Yb type icelanditeis distributed in the northeast Kanto (Motegi and Daigo) and northern Abukuma (Kenashiyama) districts, whereas low K_2O-Ce/Yb type icelandite occurs in the central region of the Tohoku back-bone range (between Furukawa and Shinjo). The fromer was probably generated by melting of the lower crust, and the latter produced by simple crystallization differentiation from mafic tholeiitic magma. The estimated chemical compositions of the primitive magma for the two types of icelandite are different. The K_2O/TiO_2 ratios of the high K_2O-Ce/Yb type icelandite are higher than those of the low K_2O-Ce/Yb type. Chemical composition and mineralogy of these icelandites suggest that hot, dry, andreducing conditions existed in the crust where the icelandites evolved. The mode of occurence of these icelandites and their geological relation indicate that they erupted in an extensional regime. These conditions were probably related to the opening of the Sea of Japan which was caused by upwelling of a mantle-plume during a short time span in the early to middle Miocene.