CHIME monazite dating as a tool to detect polymetamorphism in high-temperature metamorphic terrane: Example from the Aoyama area, Ryoke metamorphic belt, Southwest Japaniar_777 439..453

摘要

Chemical Th–U–total Pb isochron method (CHIME) monazite dating was carried out for pelitic–psammitic migmatites and the Ao granite (one of the Younger Ryoke granites) from the Aoyama area, Ryoke metamorphic belt, Southwest Japan. The Ao granite gives an unequivocal age of 79.8 ± 3.9 Ma. The monazite grains in migmatites yield an age of 96.5 ± 1.9 Ma with rims and patchy domains of 83.5 ± 2.4 Ma. The 83.5 ± 2.4-Ma overprinting on migmatites over the garnet–cordierite zone suggests a wide and combined effect of thermal input and fluid activity on the monazite grains caused by the contact metamorphism by the Younger Ryoke granites including the Ao granite. This contact metamorphism has not been detected from the major metamorphic mineral assemblage previously, possibly because the migmatites already possessed the high-temperature mineral assemblage before the granite intrusions and were immune from contact metamorphism in terms of major metamorphic minerals. However, monazite records contact metamorphism clearly. Therefore, the field mapping of the CHIME monazite age is a powerful tool for recognition of polymetamorphism in high-temperature metamorphic terrains where later thermal effects can not be easily detected by the growth of new major metamorphic minerals.