北海道北部, 下川地域の中新世火山活動と未分化玄武岩 : 初生マグマの生成条件からみた中新世テクトニクスへの制約  [in Japanese] Miocene volcanism and primitive basalt from the Shimokawa district, north Hokkaido, Japan : constraints on Miocene tectonics from petrogenesis of primary magma  [in Japanese]

下川地域は北海道北部に位置し, 中期〜後期中新世にかけて広域的隆起運動をともなう陸上火山活動が激しく生じた地域である。本地域の下川層群は, 下位の二股火山噴出物と上位のフーレベツ火山噴出物よりなる。二股火山噴出物は, 玄武岩〜安山岩質溶岩流および火山砕屑性堆積岩を主体とし, 安山岩〜デイサイト質溶結凝灰岩を介在する。本層のK-Ar年代は10.5±0.5 Maと12.1±0.7 Maを示す。フーレベツ火山噴出物は, 安山岩〜デイサイト質溶岩流からなり, 安山岩のK-Ar年代は11.2±0.3 Maを示す。これらの年代値は, 本地域の火山岩類が, 中期中新世後半〜後期中新世前半のほぼ同時期に形成されたことを示唆している。二股火山噴出物中には, MgO, Cr, Niに著しく富む未分化な玄武岩が含まれる。本地域の初生マグマは, マントルカンラン岩から部分溶融程度の高い条件で生じ, 上部マントルの最上部 (〜30 km) から分離上昇し, 枯渇したハルツバージャイトが残存マントルであったと推定される。北海道北部において12 Ma頃に開始された火山活動は, 日高山脈の形成を含む北海道北部の日高帯の隆起運動をもたらしたと推定されるマントル・ダイアピルの上昇と関連して起こった可能性があり, 下川地域の未分化玄武岩はこのマントル・ダイアピルに由来する玄武岩の一つと考えられる。

The Shimokawa district is located in the north Hokkaido, where intense terrestrial volcanism associated with regional uplifting took place during the Middle to Late Miocene. The Shimokawa Group in this districtis divided into the Futamata volcanics and Furebetsu volcanics in ascending order. The Futamata volcanics is composed mainly of basaltic to andesitic lava flows and volcaniclastic rocks with intercalation of andesitic to dacitic welded tuff in the lower part of this formation. K-Ar ages of volcanic rocks from the Futamata Volcanics are as 10.5±0.5 Ma and 12.1±0.7 Ma. The Furebetsu Volcanics consists of andesitic and dacitic lava flows, which have been dated at 11.2±0.3 Ma (K-Ar). Thus, volcanism in this district occurred simultaneously during the late Middle Miocene to early Late Miocene. Basaltic rocks in the Futamata volcanics comprise unusually high MgO rocks. These basalts are characterized by Primitive compositional feature (such as high Cr and Ni), suggesting that they are largely unfractionated, and in equilibrium with mantle peridotite. In comparison with NE Japan arc magmas and experimental studies, primary magmas in this district have segregated from shallow parts of the upper mantle (〜30 km). Geochemical and mineral ogical features are consistent with a high-degree of partial melting leaving a depleted harzburgite residual. Middle to Late Miocene volcanism after about 12 Ma in the north Hokkaido may have a close association with an upwelling mantle diapir, which may have resulted in the uplift of the Hidaka Belt, followed by the formatioin of the Hidaka Mountains. Petrogenetic features of primitive basalt from the Shimokawa distrid suggests that the Shimokawa basalts were derived from this upwelling mantle diapir.