Pressure and temperature conditions of contact metamorphism of the Suo metamorphic rocks in Asakura area, Fukuoka Prefecture, Japan: constraints on uplifting process

Abstract

The Suo metamorphic rocks of the Asakura area of Fukuoka Prefecture underwent contact metamorphism during the intrusion of granitic magmas in the Cretaceous, forming a contact aureole. This aureole is divided from south to north with ascending metamorphic grade into chlorite, chlorite–biotite, biotite, and andalusite zones, based on pelitic mineral assemblages. The andalusite isograd is defined by a muscovite + quartz = andalusite + K-feldspar + water reaction, based on the coexistence of andalusite and K-feldspar. Garnet–biotite thermometry and garnet–biotite–plagioclase–quartz barometry indicate that the andalusite zone formed at temperatures of 560 ± 50°C and at pressures of 2.3 ± 0.90 kbar. The Suo metamorphic rocks were exhumed to ca. 9 km depth by around 95 Ma, assuming that heat was supplied from the Asakura granodiorite to the north, and reached the surface and were overlain by sediments of the Nogata Group sometime before 44 Ma.