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

摘要

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 metamor-phism 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.