飛騨帯の造構発達史 : 変成履歴と後期石炭紀～三畳紀の変動について

The Hida Terrane comprises the Hida Gneiss Zone and the Unazuki Zone. Gneisses of the former overlie schists of the latter as nappes and the metamorphites of either zone are intruded by the late Triassic to early Jurassic Funatsu granites. On the basis of the detailed analyses of mineral equilibria and the estimation of P-T conditions by means of several geothermo-barometers, the Hida Gneiss Zone is subdivided into two parts: the Inner and the Outer regions, since the P-T-t paths of the two regions are different from each other. Rocks of the Outer region, situated between the Inner region and the Unazuki Zone, are more intensely metamorphosed than those of the Inner one. The textural and chemical analyses of zoned garnets in metapelites from both regions have clarified the relationship between P-T changes and deformation. The core part of each zoned garnet has preserved growth structure characterized by the development of concentric rings of inclusions, but chemically homogeneous part overspreads independently of the growth structure. Therefore, the chemical homogenization of zoned garnet can be supposed to have occurred after the prograde metamorphism, during which the growth structure had been formed. Estimated P-T condition during the homogenization is lower than 600-650℃, 4-5 kb in the Inner region, while it is as high as 730℃, 7 kb in the Outer region. After the homogenization, metamorphites have regionally undergone brittle deformation and garnet grains have been fractured. During the deformation, chemical reset of each garnet grain has been performed along the fracture or the peripheries. P-T conditions during the reequilibration in the Inner and the Outer regions have been inferred to be 500-550℃, 3 kb and 600-650℃, 3.5-4.0 kb, respectively. Judging from the structural features, the Inner region tectonically overlies the Outer one. Along the boundary between the two regions, develops the thrust zone characterized by intensely mylonitized granites. The isotopic age of the granites is about 300 Ma. At that time, some subduction zone would have existed on the oceanic side of the Hida Terrane. The regional thrusting mentioned above is inferred to be closely associated with the early to middle Triassic medium P/T type metamorphism of the Unazuki Zone.