北部フォッサマグナ頚城地域の中新-鮮新統タービダイト砂岩の源岩  [in Japanese] Source rocks of Mio-Pliocene turbidite sandstones in the Kubiki area, northern Fossa Magna, Japan  [in Japanese]

北部フォッサマグナ最北部に分布する中新統から鮮新統にかけての田麦川層と川詰層の砂岩について, モード組成, 重鉱物組成, ならびにガーネットおよび角閃石砕屑粒子のEPMA分析などの堆積岩石学的検討を行った。両層の砂岩は不透明鉱物や黒雲母の量比を異にするが, ともにジルコン・ガーネットによって特徴づけられる砂岩と角閃石によって特徴づけられる砂岩に区別される。田麦川層の砂岩の前者をTA型, 後者をTB型, 川詰層の砂岩の前者をKA型, 後者をKB型とする。TA型砂岩は飛騨主帯・宇奈月帯・飛騨外縁帯の変成岩類および美濃帯の砂岩と同質の砂岩を含む古期堆積岩類, および新生代火山岩類を, TB型砂岩は宇奈月帯・飛騨外縁帯の変成岩類と同質の岩体および新生代火山岩類を源岩とする。一方, 川詰層のKA, KB型砂岩はともに飛騨主帯・宇奈月帯・飛騨外縁帯の変成岩類と同質の岩体および新生代火山岩類を源岩とする。

The source rocks of the Late Miocene to Pliocene Tamugigawa and Kawazume sandstones distributed in the Kubiki area of the northern Fossa Magna were clarified by modal and heavy mineral compositions and EPMA analysis of detrital garnets and amphiboles. The amount of opaque minerals and biotite are different between the two sandstones, and there exist two types of non-opaque heavy mineral assemblages in each sandstone, namely zircon and garnet dominant type and amphibole dominant type, with each sandstone. Therefore, the sandstones are classified into TA- and TB-types in the Tamugigawa sandstones, and KA- and KB-types in the Kawazume sandstones. The TA- and KA-types are generally abundant in monocrystalline quartz, although apart of the TA-type sandstones are rich in lithic fragments and poor in quartz. On the other hand, the TB- and KB-types are abundant in lithic fragments. The kind of source rocks, deduced from the EPMA analysis on the detrital garnets and amphiboles, are different more or less each other. The garnets and amphiboles of the sandstones are generally similar in composition to those of metamorphic rocks in the Hida Major, Unaduki, and Hida Marginal Belts, and those of the Neogene volcanic rocks. The sedimentary rocks in the Mino Belt might have added detritus to the TA-type sandstones. The volcanic rocks and granitic rocks supplied the characteristic detrital grains to the TB- and KB-type sandstones, respectively.