Garnet clinopyroxene amphibolites occur in the mylonite unit structurally at the top of the Takahama metamorphic rocks a member of the Nagasaki Metamorphic Rocks, western Kyushu, SW Japan. Geothermobarometry applied to an equigranular garnet clinopyroxene amphibolite, which escaped severe mylonitization, yields the metamorphic condition of 770 ± 70°C and 1.1 ± 0.2 GPa. This condition is comparable with the high pressure granulite facies, and is much higher grade than those of the underlying crystalline schists of the Takahama metamorphic rocks that belong to the epidote glaucophane schist subfacies. Despite the presence of such a gap in metamorphic condition between the crystalline schists and the mylonite unit, nearly identical muscovite K Ar ages have been obtained from these units, which indicate that both units were juxtaposed at depth before they cooled below the closure temperature of the muscovite K Ar system.
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 2 O 3 -SiO 2 -H 2 O system: 3Al 2 O 3 + 2Al 2 SiO 5 + 2Ca 2 Al 3 Si 3 O 12 (OH) + 3H 2 O = 2Ca 2 Al 8 Si 4 O 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.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.