Formation of a high-temperature metamorphic complex due to pervasive melt migration in the hot crust

摘要

Detailed reviews of metamorphic thermal structures and spatial distributions of granitic rocks in the high-T Higo metamorphic complex (HMC), central Kyushu, Japan, and Ryoke metamorphic complex in the Yanai district (RMC-Y), southwest Japan, reveal that the high-grade metamorphic rocks were associated with migmatites or syn-metamorphic granitoids. The close association of these rocks suggests that transport and solidification of granitic melts are appreciable heat sources. Partial melting of the pelitic host resists heating of the crust due to its endothermic reaction (i.e. absorption of latent heat). However, the effects of slower thermal evolution produced by partial melting are limited by the boundary conditions assumed for the HMC and RMC-Y. New thermal numerical modeling reveals that the volume fraction of the solidified melt products and the duration of melt migration required for crustal heating to a certain temperature decrease with increasing rates of melt migration. The model predicts that the melt migration rate during formation of the HMC is about three to 10 times higher than that of the RMC-Y, and the corresponding duration of melt migration is about one order of magnitude shorter. The model described here also explains the apparently contradictory observations made in different structural units of the RMC-Y: the metamorphic pressure-temperature (P-T) ratio of the southern unit is higher than those of the central and northern units, even though the volume fraction of granitic rocks in the southern unit is larger. The model suggests that the melt migration rate in the northern and central units was faster than that in the southern unit. Overall, the model implies that variations of metamorphic field P-T arrays and the spatial distributions of granitic rocks in high-T metamorphic complexes may be explained simply by variations in the rate and duration of melt migration in the hot crust.