ABSTRACT The metamorphic zoning and geothermobarometry of the Ryoke metamorphic complex in the Yanai area, southwest Japan, show that its thermobaric structure was buffered by the dehydration melting of biotite. The temperatures over most of the area covered by the three high‐grade zones (8.5‐ to 19.0‐km depth) are consistent with those of the dehydration melting reaction and indicate a geothermal gradient of 9.2°C/km. The deepest area (> 19‐km depth) of the highest grade zone has a domal structure and records temperatures that are > 50°C higher than the melting reaction. In contrast, the low‐grade zones ( 100°C/km. The spacing of the isobars indicates that the crust was vertically thinned at all these depths. Thermal modelling suggests that the formation of this thermobaric structure requires thermal buffering by the dehydration melting reactions of muscovite and biotite and also advective heat transfer related to the upward migration of melt produced by these reactions. The melt migration through the thermally buffered middle crust enabled heat to advect efficiently to the shallower crust and modify the conductive geothermal gradient. The spatial–temporal scale of this thermal event in the study area is comparable to that of coeval and present‐day volcanism, suggesting a close link between deep crustal and surface processes.
Ikeda et al. (Tue,) studied this question.