南部北上帯氷上花崗岩類の岩相区分, 構造および岩石化学(古領家帯と黒瀬川帯の構成要素と改変過程)  [in Japanese] Lithology, structure and petrochemistry of the Hikami Granitic Rocks in the South Kitakami Belt(Geotectonic evolution of the Paleo-Ryoke and Kurosegawa Terranes)  [in Japanese]

氷上花崗岩類は, 氷上山を中心とした氷上花崗岩体と, その北方に分布する小岩体から構成される.氷上花崗岩体は鉱物組成と組織から9つの岩相に細区分できる.大野-西山地域で大野型花崗閃緑岩はマイロナイト化していないのに対し, 隣接する片状花崗岩類は上盤北の剪断センスを示すマイロナイト化を被っていることから, 大野型花崗閃緑岩は片状花崗岩類のマイロナイト化の後に貫入したものと考えられる.氷上花崗岩類には複数の放射年代が知られている(約440Ma, 350Ma, 250Maなど)が, 少なくとも壷の沢変成岩に密接に伴う片状花崗閃緑岩は, 440Ma前後の貫入岩であると考えられる.主成分元素と副成分元素の岩石化学的特徴については, 異なる年代, 異なる岩相をもつ花崗岩類の間でも良い一致を示し, 同一の分化傾向を示す.氷上花崗岩類のK_2O-CaO-Na_2O図における領域は, 日本列島の平均的花崗岩類の化学組成と一致する.

The Hikami Granitic Rocks are distributed mainly around the Mt. Hikami (Hikami granitic mass), and small masses, such as Komatsu-Pass, Shiraishi-Pass, Yokamachi, Ezo, Hirasawa, Obata and Okuhinotsuchi masses, are exposed to the north. The Hikami granitic mass can be subdivided lithologically into nine on the basis of mineral assemblage, texture and existence of foliation; i.e., porphyritic hornblende-biotite granodiorite, biotite granodiorite (Ono-type), schistose biotite granodiorite, schistose hornblende-biotite granodiorite, schistose hornblende-bearing biotite tonalite, schistose biotite-hornblende tonalite, schistose medium-grained biotite tonalite, schistose fine-grained biotite tonalite and schistose medium-grained muscovite-biotite granodiorite. Magnetic susceptibility shows that most of the Hikami Granitic Rocks belong to ilmenite series, however, some of them belong to magnetite series.NNW-SSE trending gently- to moderately-dipping mylonite zone with about 500m thick can be identified in the Ono-Nishiyama area. The mylonite zone with top-to-the-north shear sense is transected by high-angle later stage cataclasite zone extending N-S from Ono to Kayooka Pass. The Ono-type granodiorite is not mylonitized even though the adjacent schistose granites are strongly mylonitized. Accordingly, the Ono-type intruded after the schistose granites was consolidated and mylonitized. From the correlation between the isotopic age data and lithologic classification, at least the schistose medium-grained muscovite biotite granodiorite which is closely associated with the Tsubonosawa metamorphic rocks, must have been intruded and consolidated at c. 440Ma.Although several radiometric age data have been reported on the Hikami Granitic Rocks (c. 440,350,250Ma) and younger ages (350,250Ma) also suggest thermal events or igneous activities, petrochemical characteristics of major and minor elements are fairly identical. The K_2O-CaO-Na_2O diagram indicates that the Hikami Granitic Rocks have the characteristics of average chemical composition of Japanese granitic rocks.