Reaction microstructures in corundum- and kyanite-bearing mafic mylonites from the Takahama metamorphic rocks, western Kyushu,Southwest Japan

摘要

High-grade mylonites occur in the Takahama metamorphic rocks, a member of the high-pressure low-temperature type Nagasaki Metamorphic Rocks, western Kyushu, Japan. Mafic layers within the mylonites retain reaction microstructures consisting of margarite aggregates armoring both corundum and kyanite. The following retrograde reaction well accounts for the microstructures in the CaO–Al_2O_3–SiO_2–H_2O system: 3Al_2O_3 + 2Al_2SiO_5 + 2Ca_2Al_3Si_3O_(12)(OH) + 3H_2O = 2Ca_2Al_8Si_4O_(20)(OH)_4 (corundum + kyanite + clinozoisite + fluid = margarite). Mass balance analyses and chemical potential modeling reveal that the chemical potential gradients present between kyanite and corundum have likely driven the transport of the CaO and SiO_2 components. The mylonitization is considered to take place chronologically after peak metamorphism and before the above reaction, based on the following features: approximately constant thickness of the margarite aggregates, random orientation of margarite, and local modification of garnet composition at a boudin neck that formed during mylonitization. The estimated peak temperature of 640°C and the pressure–temperature conditions of the above reaction indicate that the mylonitization took place at temperature between 530 and 640°C at pressures higher than 1.2 GPa, approximately equivalent to the depth of the lower crust of island arcs.