Development of migmatites and the role of viscous segregation in high- T metamorphic complexes: Example from the Ryoke Metamorphic Complex, Mikawa Plateau, Central Japan

Abstract

The Ryoke Metamorphic Complex of the Mikawa Plateau, Central Japan, contains well-preserved geological and thermal structures formed during Cretaceous high-temperature (high- T) metamorphism. These structures can be explained by a pervasive melt migration model. However, the estimated flux of melt is much higher than that expected for compaction flow of felsic melt. An additional tectonic force and/or melt segregation to network of veins or layers is therefore required. This paper presents the results of numerical modeling of the structural evolution of immiscible viscous fluids under simple-shear deformation, and discusses the role of viscous segregation in enabling pervasive melt flow in hot crust. The results of numerical modeling of simple-shear deformation of immiscible viscous fluids reveal that low-viscosity fluids segregate into lenses and layers. This segregation is a type of viscous segregation that takes place to minimize the dissipation of energy in the system. The rate of energy dissipation is proportional to viscosity. Therefore, this process is expected to occur universally in immiscible viscous fluids with large viscosity contrasts, and is an important process in focusing melt in partially melted rocks. The structures produced by the numerical simulation are similar to those of migmatites. The simulation results suggest that melts distributed randomly in partially melted rock will segregate into lenses or layers with progressive simple-shear deformation. During deformation-enhanced segregation, a melt network will form dynamically. If this process occurs at small to large scales in the hot crust, melts will be effectively transported upward due to buoyancy forces. It is possible that pervasive migmatites at low structural levels within high- T metamorphic complexes may represent “fossil” melt pathways formed by crustal-scale viscous segregation.