日高変成帯野塚岳地域,ニオベツ岩体の貫入による中新世の複変成作用,部分溶融および花崗岩マグマの形成(<特集>日高衝突帯研究の最近の進歩(1)-その深部過程と上昇過程)  [in Japanese] Poly-metamorphism, anatexis and formation of granitic magma due to intrusion of the Niobetsu complex during Miocene, the Nozuka-dake area, Hidaka metamorphic belt, northern Japan(<SPECIAL ISSUE>Recent Advances in Research of the Hidaka Collision Zone, Hokkaido, Northern Japan: Deep-Seated Processes and Tectonic Uplift (Part 1))  [in Japanese]

日高変成帯の中部に位置する野塚岳地域には,S・I-タイプトーナル岩とそれらに伴って産するはんれい岩〜閃緑岩,ニオベツ岩体,野塚岳花崗岩および変成岩類が産する.S・I-タイプトーナル岩と変成岩類は日高変成帯の上昇に伴うマイロナイト化を被っている.一方,ニオベツ岩体と野塚岳花崗岩はマイロナイト化の後に貫入した.ニオベツ岩体はS-タイプトーナル岩起源の捕獲岩を含む.ニオベツ岩体に取り込まれたトーナル岩捕獲岩は部分溶融を起こした.Sr・Nd同位体比組成を含めた火成岩類の化学組成は,野塚岳花崗岩がトーナル岩捕獲岩の部分溶融によって生じたメルトとニオベツ岩体マグマの混合によって形成されたことを示す.日高変成帯からこれまで報告されている火成岩や変成岩の年代測定結果を考慮すると,ニオベツ岩体と野塚岳花崗岩は中新世における千島弧と東北日本弧の衝突に関連した火成活動の産物であると推察される.

The Hidaka metamorphic belt is considered to be a tilted island-arc crustal section, as resulted by the arc-arc collision. In the lower sequence of the belt, granulite-facies metamorphic rocks and S- and I-type tonalites underwent mylonitic deformation. Hornblede gabbrodiorite complex (Niobetsu complex) and biotite granite (Nozuka-dake granite) were emplaced after mylonitc deformation. The magma activity of the S- and I-type tonalites and the Niobetsu complex occurred at ca. 55 to 50Ma and 18Ma, respectively. The Niobetsu complex includes a large amount of xenoliths derived from the host S-type tonalite. These xenoliths show recrystallized-gra-noblastic texture with locally remaining mylonitic foliation, and consist of plagioclase-orthopyroxene-cordierite-quartz-biotite-K-feldspar-spinel±garnet-apatite-opaque minerals. The metamorphic P-T conditions of the xenoliths are up to 900℃ and 500MPa, which are fairly lower pressure conditions compared with the highest-grade metamorphic rocks of the Hidaka metamorphic belt. The mineral assemblages and textures of the tonalite xenoliths are produced through reactions such as biotite+quartz=orthopyroxene+melt, and/or biotite+garnet+quartz=orthopyroxene+cordierite (or+spinel)+melt. Wholerock geochemical studies combined with metamorphic analyses indicate that the tonalite xenoliths are regarded as restites after releasing granitic melt from the S-type tonalite. Geochemical analyses including Sr and Nd isotope compositions reveal that the Nozuka-dake granite are derived through the process of mixture with the Niobetsu complex and the granitic melt released from the tonalite xenoliths. This magma process occurred probably at the syn-collisional events in the Hidaka Collision Zone.