Evolution of the Sanbagawa and Shimanto high-pressure belts in SW Japan: Insights from K-Ar (Ar-Ar) geochronology


The Sanbagawa belt sensu lato in Japan is one of the world’s classical high-pressure (HP) metamorphic belts and has long been considered to be a coherent HP belt of the Cretaceous age. However, recent progress in detrital zircon geochronology together with the accumulation of abundant phengites K–Ar (Ar–Ar) ages revealed that the belt is comprised of two petrotectonic belts—Sanbagawa sensu stricto HP belt and Shimanto HP belt. These two belts have distinctly different ages of peak metamorphism and different P–T conditions of metamorphism. The Sanbagawa schists sensu stricto were metamorphosed in the conditions of the pumpellyite–actinolite facies through the epidote-blueschist to epidote–amphibolite facies and up to the eclogite facies during a prograde stage in a subduction zone at 120–115 Ma. Phengite K–Ar geochronology revealed that the subsequent exhumation after the peak metamorphism took placed in a manner that the higher-grade rocks exhumed more rapidly in comparison with the lower-grade rocks; this is supported by a positive correlation between age and apparent metamorphic gradient that have formed during the post-metamorphic peak hydration/recrystallization. Moreover, the schists have experienced intense ductile deformation and long-term exhumation; it took longer than 31 m.y. to reset the phengite K–Ar system. In contrast, the Shimanto HP schists were metamorphosed in the epidote-blueschist/greenschist transitional facies to the epidote–amphibolite facies and have experienced a short-term deformation for less than 13 m.y. This short deformation formed a negative correlation between K–Ar age and apparent metamorphic gradient. These two contrasting age-temperature relationships suggest different exhumation processes between the two HP belts, most likely due to a change in subducting oceanic plates in the Cretaceous along the paleo-Japanese convergent margin.

Journal of Asian Earth Sciences, v. 42, no. 6, p. 1075-1090, doi:10.1016/j.jseaes.2011.06.012