関東山地跡倉ナップのペルム紀ホルンフェルス(古領家帯と黒瀬川帯の構成要素と改変過程)  [in Japanese] Permian contact metamorphic rocks of the Atokura Nappe, Kanto Mountains, central Japan(Geotectonic evolution of the Paleo-Ryoke and Kurosegawa Terranes)  [in Japanese]

跡倉ナップのペルム紀石英閃緑岩に貫かれるホルンフェルス(風口層)は, 石英長石質の砂岩泥岩細互層を原岩とする.全岩化学組成は, SiO_2・Fe_2O_3・CaO・Na_2Oが高くAl_2O_3・K_2Oが低い.風口層の化学組成は苦鉄質成分に富む点で南部北上帯の古生界の特徴と似ているが, SiO_2・Al_2O_3・Na_2Oの量では異なっている.ホルンフェルスの鉱物共生には緑泥石・緑泥石-黒雲母・黒雲母の3つの鉱物組み合わせがある.カリ長石が出現する一方, 白雲母・菫青石・Al_2SiO_5鉱物は出現しない.ざくろ石にはコアがFe・Mnに富みCaに乏しい累帯構造をもつものがあり, コアは550〜570℃の変成温度を示す.接触変成後の弱変成作用による黒雲母の酸化緑泥石化・斜長石のカリ長石化が認められるが, 接触変成以前の広域変成作用を示す証拠は認められなかった.K_2Oに乏しい砂泥質変成岩で白雲母が生じない例は知られていたが, 白雲母が生じずにカリ長石が生じている例は初めてである.

Kazeguchi Formation is composed of contact-metamorphosed quartzo-felspathic interbedded sandstone and mudstone, which is intruded by the Permian quartz diorite of the Atokura nappe, Kanto Mountains, central Japan. The bulk chemical composition is characterized by high SiO_2,Fe_2O_3,CaO, and Na_2O and low Al_2O_3 and K_2O contents. Although the Permian quartz diorite was correlated to the granitoids of the Southern Kitakami Belt, chemical characters of SiO_2,Na_2O, and Al_2O_3 of the metamorphic rocks are different from that of the pre-Permian strata of the Southern Kitakami Belt. The mineral paragenesis of the metamorphic rocks is Qtz+Pl±Ab+Kfs±Grt±Hbl with Chl, Chl+Bt, or Bt, and three mineral assemblages are characterized by chlorite, chlorite-biotite, and biotite. Muscovite, cordierite, and aluminosilicates do not appear. Some of garnet grains have chemical zoning with the Fe and Mn rich, Ca poor core. The temperature of metamorphism obtained from core of zoned garnet is 550-570℃. Biotite and plagioclase are replaced with oxidized chlorite and K-feldspar respectively by the low grade metamorphism. The regional metamorphism prier to the contact metamorphism is not recognized. Though the muscovite free metamorphism of the K_2O-poor psammitic-pelitic rocks are known, this is the first report of the muscovite free and K-feldspar abundant metamorphism.