Abstract
The Hida belt of Southwest Japan, dominated by plutonic rocks and orthogneisses, was situated along the continental margin of eastern Asia prior to the opening of the Japan Sea. The metamorphosed Kumanogawa–Nagatogawa gabbroic complex of the Hida Belt contains well-preserved igneous structures and textures, making it suitable for geological and petrological studies as an igneous suite. The complex contains gabbro, diorite, and granodiorite–granite. The gabbro is subdivided into coarse-grained gabbro showing igneous layers with cumulus texture, and fine-grained gabbro that occurs as syn-plutonic dykes and micro-magmatic enclaves with ophitic texture. The fine-grained gabbro has a tholeiitic composition similar to that of subduction-related basalts. In contrast, the diorite and the granodiorite–granite show various textures indicative of magma mixing and mingling (e.g., bladed biotite, acicular apatite, boxy cellular plagioclase, and rapakivi feldspar). The chemical composition of the diorite overlaps with the evolved composition of the fine-grained gabbro. The occurrence and geochemical features of the gabbroic complex indicate that the various magmas, ranging from basaltic to granitic compositions, coexisted with each other and that differentiation, magma mixing, and mingling occurred in the magma chamber to produce the complex. The magma is suggested to have formed in an active continental margin at a subduction zone.
