Thermobaric Structure of the Sefuri Metamorphic Rocks in the Itoshima Area, Tsukushi Complex, Northern Kyushu, Southwest Japan: A Possible Western Extension of the Ryoke Arc

Abstract

This article presents the pressure–temperature (P–T) structure of the high-T Sefuri Metamorphic Rocks in the Itoshima area, northern Kyushu, Southwest Japan, together with zircon U–Pb ages of the metamorphic and plutonic rocks in the area. These rocks were previously interpreted as contact metamorphic rocks formed by the intrusion of Cretaceous granitic bodies. However, they exhibit features commonly attributed to regional metamorphism, such as penetrative schistosity. The development of schistosity can be explained by deformation above the temperature of the brittle-ductile transition. Our numerical simulations suggest that such high-T conditions in regions consisting of numerous granitic bodies are generated by the superposition of heat from short-period pulses of granitic magma intrusions. The P–T structure of the Sefuri Metamorphic Rocks around Mt. Raizan indicates that significant ductile thinning occurred along the roof of the diapirically ascending middle and southern domains of the Itoshima Granodiorite. Schistosity-developed high-T metamorphic rocks associated with Cretaceous granitic intrusions are common in northern Kyushu. We define the Tsukushi Complex as comprising these metamorphic rocks and closely associated Cretaceous plutonic rocks. The compilation of zircon U–Pb ages from the Tsukushi Complex indicates that pulsed metamorphism occurred synchronously with long-period pulses of granitic plutonism. The synchronous metamorphism and plutonism that formed the Tsukushi Complex, together with contemporaneous volcanism, are comparable to the synchronous high-T metamorphism and plutonism that formed the Ryoke Complex and the associated Cretaceous volcanism in Southwest Japan. In both complexes, the isolated large (> 1000 km²) domains composed of the high-grade metamorphic rocks and granodiorite plutons may represent the deeper roots of coeval volcanic clusters at the surface. These observations suggest that the Tsukushi Complex may be an eroded remnant of a western extension of the Ryoke Arc, which comprises the Ryoke Complex and coeval surface volcanic clusters.