Pressure–temperature structure of a mylonitized metamorphic pile, and the role of advection of the lower crust, Nagasaki Metamorphic Complex, Kyushu, Japan

Abstract

We document the pressure–temperature structure of a mylonitized metamorphic pile in the Nagasaki Metamorphic Complex (NMC), western Kyushu, Japan, and we discuss the role of advection of the lower crust in the formation of this metamorphic pile. The NMC consists of upper and lower units. The upper unit is a mylonitized metamorphic pile that consists of metamorphic rocks of amphibolite to granulite facies. The lower unit consists of metamorphic rocks of the epidote–blueschist subfacies. The upper unit was extruded onto the lower unit. Peak metamorphic temperatures of the upper unit show continuous increase from 600 to 780°C from the unit boundary towards upper structural levels, although peak metamorphic pressures within the upper unit are almost constant around 1.1GPa. The upper part of the upper unit is associated with migmatitic rocks that formed around 113.7±1.6Ma, and the pressure-temperature conditions are consistent with those of partial melting of mafic gneiss. The upper unit underwent high-T mylonitization in the lower crust, and was finally juxtaposed against the lower unit, also at depths of the lower crust. Thermal-advection models of a subduction zone show that the pressure and temperature conditions of the upper unit were reached underneath the region from the volcanic arc to the forearc; for the lower unit, the conditions were reached near the corner of corner flow of the shallow asthenosphere. The models also indicate that horizontal advection of the lower crust was activated by the corner flow, and that this process provides a satisfactory explanation for the formation of the mylonitized pile in the NMC.