CHIME monazite dating as a tool to detect polymetamorphism in high‐temperature metamorphic terrane: Example from the Aoyama area, Ryoke metamorphic belt, Southwest Japan

Abstract

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 Ϯ 3.9 Ma. The monazite grains in migmatites yield an age of 96.5 Ϯ 1.9 Ma with rims and patchy domains of 83.5 Ϯ 2.4 Ma. The 83.5 Ϯ 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.

Age gap between the intrusion of gneissose granitoids and regional high‐temperature metamorphism in the Ryoke belt (Mikawa area), central Japan

“…Indeed, rejuvenation of monazite is likely to have occurred during the intrusion of granitoids postdating the Ryoke regional metamorphism. In migmatite samples from the Aoyama area (Figure ) monazite grains record two age populations ascribed to the timing of regional metamorphism and massive granite intrusion, respectively (Kawakami & Suzuki, ). The latter event is not recorded in zircon from the same area (Kawakami et al, ), suggesting that monazite is more easily rejuvenated than zircon during later thermal events.…”

Possible polymetamorphism and brine infiltration recorded in the garnet–sillimanite gneiss, Skallevikshalsen, Lützow–Holm Complex, East Antarctica

“…This was presumably followed by the zircon growth during the melt crystallization and partial resorption of garnet. Some of the young-aged monazite may be affected by the diffusion of Pb and partial resetting of the U-Th-Pb system under high-temperature conditions (e.g., Suzuki et al, 1994;Kawakami and Suzuki, 2011).…”

“…Therefore, possibility of polymetamorphism and role of fluids including brines should be carefully considered in the future study of the LHC. Recognizing the timing of fluid release/infiltration during metamorphism and crystallization of plutonic bodies, and understanding the composition of fluids is of great importance especially because brines are able to recrystallize datable minerals such as monazite and rarely zircon (e.g., Seydoux-Guillaume et al, 2002;Kawakami and Suzuki, 2011;Higashino et al, 2013), which makes the interpretation of geochronological data more complex (e.g., Kawakami et al, 2014). More reliable interpretation of zircon and monazite dating in high-grade metamorphic rocks is required to understand the rate and duration of crustal processes.…”

Behavior of zircon in the upper-amphibolite to granulite facies schist/migmatite transition, Ryoke metamorphic belt, SW Japan: constraints from the melt inclusions in zircon

“…The former two were 68 observed at amphibolite facies and the latter two was observed at granulite facies. They ascribed 69 prismatic zoning to be due to the growth in amphibolite facies H 2 O saturated melt whereas isometric 70 zoning to be due to the growth in granulite facies H 2 O undersaturated melt (Vavra et al 1999).Recently, melt inclusions are found in migmatites and granulites (Cesare et al 2003; 2009;2011). metamorphic zone, utilizing mineral assemblages in pelitic lithology (Kawakami 2001a).…”