Monazite Behaviour and Time-scale of Metamorphic Processes along a Low-pressure/High-temperature Field Gradient (Ryoke Belt, SW Japan)

Low-pressure/high-temperature metamorphic rocks exposed in the western part of the Ryoke belt (lwakuni-Yanai area, SW Japan) include a section with increasing temperature conditions from similar to 425 to 880 degrees C. We use this setting to explore the evolution of monazite grain size, texture and composition, and variations in the whole-rock composition of 11 metapelite, metapsammite or metachert samples collected along the metamorphic field gradient. Monazite grain size increases with rising metamorphic grade, regardless of the whole-rock composition. From low- to high-grade conditions we infer: (1) the initial nucleation of monazite aggregates after allanite similar to 425 degrees C); (2) monazite coarsening and coalescence driven by incipient monazite recycling; that is, dissolution of small grains to grow larger ones by Ostwald ripening (500-600 degrees C); (3) a first major recycling stage enhanced by fluid liberation owing to muscovite breakdown (600-630 degrees C); (4) a second recycling stage assisted by an increase in the proportion of anatectic melt owing to biotite breakdown (> 850 degrees C). A succession of four compositional domains is recognized in monazite. We emphasize the usefulness of comparing their Ce/Th-Mnz, Ce/Y-Mnz, and Th/U-Mnz, molar ratios with those derived from whole-rock analyses to constrain the origin of each domain. Domain I, with variable ratios, reflects the progressive transfer of Th +/- U from allanite to monazite at low-grade conditions. Domain II, with Ce/Th-Mnz, matching the whole-rock values, indicates growth under rock(decimetre)scale equilibrium conditions. Domains II and III, with Th/U-Mnz, and Ce/Y-Mnz, departing from the whole-rock values, record the competition with zircon (for U) and garnet (for V) during growth at peak P-T conditions. Domain IV points to Y supply by garnet resorption during retrograde chloritization (< 550 degrees C). In the highest-grade sample, zircon grains included in garnet or cordierite show metamorphic rims with sillimanite and Si-rich inclusions. These rims formed at suprasolidus conditions (650-880 degrees C) and yield Pb-206/U-238 ages of 103-97 Ma (+/- 5 Ma), which bracket the timing of high-temperature metamorphism. Monazite dating by electron microprobe and laser ablation inductively coupled plasma mass spectrometry reveals two age groups. For domains I-III, some relatively old (206)pb/U-238 ages (99-95 +/- 3-5 Ma) represent minimum estimates for the timing of prograde to peak metamorphism, whereas the similar oldest (206)pb/U-238 age for domain IV (93 +/- 7 Ma) points to rapid cooling after the temperature peak. A more dominant population of younger Th-U-Pb and Pb-206/U-238 dates is ascribed to age resetting by heterogeneous annealing of the monazite crystal lattice. The youngest concordant (206)pb/U-238 age of 86 +/- 4 Ma is correlated with the end of intermediate-temperature conditions (< 200-300 degrees C) below which monazite, in metamorphic rocks as well as in the surrounding granitoids, eventually behaved as a closed isotopic system. This contribution is dedicated to the memory of Professor Kazuhiro Suzuki, for his contribution to geochronology and Japanese geology.