Abstract
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 gabbro-diorite 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 50 Ma and 18 Ma, respectively.
The Niobetsu complex includes a large amount of xenoliths derived from the host S-type tonalite. These xenoliths show recrystallized-granoblastic 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 900ºC and 500 MPa, 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. Whole-rock 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.
