群馬県水上地域における新第三系基底の大倉高マグネシア安山岩(予報)  [in Japanese] Preliminary report on high magnesian andesites of Neogene basal Okura Formation in the Minakami district, Gunma Prefecture, central Japan  [in Japanese]

北部フォッサマグナ地域の下部中新統・大倉層 (約18 Ma) は, フォッサマグナ東縁を画する柏崎-銚子構造線に接する地域に分布する。この地層を構成する変質玄武岩のうち比較的新鮮な岩石について, 岩石記載および化学組成の検討を行った結果, これらの火山岩は, FeO^*/MgO比が1.1程度, MgOが5.9-6.6%, SiO_2 が54.1-56.0%の高マグネシア安山岩組成のものであることが明らかとなった。全岩化学組成, REE規格化パターンなどの特徴から, 本高マグネシア安山岩は瀬戸内区に出現するサヌカイト(12-14 Ma)や北部フォッサマグナ地域の内村層中の高マグネシア安山岩(約15 Ma)に岩石学的性質が類似している。この高マグネシア安山岩の地質学的位置は, 瀬戸内区〜内村地域の北方への延長上にある。

Lower Miocene Okurameta basaltic rocks are basal rocks of the Neogene series in the Minakami district of Gunma Prefecture, central Japan. The stratigraphy and petrological features of the Okurameta basaltic rocks are investigated. Three relatively fresh samples collected from a massive lava flow are characterized by high SiO_2 (54.10-55.96%) and MgO (5.93-6.58%) contents, and low FeO^*/MgO ratios (1.10-1.18), suggesting that these rocks are defined as high magnesian andesite. The Okura high magnesian andesites are petrologically similar to those of Miocene Uchimura Formation in the Central Shinshu district and sanukite of the Setouchi area. These three high magnesian andesites are arranged along the Median Tectonic Line and its northward extention. However, the Okura high magnesian andesites are probably older (about 18 Ma) than the Uchimura high magnesian andesites (15 Ma) and main period of Setouchi magmatism (12-14 Ma). Basic to intermediate volcanic rocks including high magnesian andesites produced during Early to Middle Miocene are distributed along the present Central Uplift zone in Northern Fossa Magna. Petrological study of these volcanic rocks may give important in formation for geotectonics of formation of failed rift basin of Northern Fossa Magna.