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CHIME (chemical Th-U-total Pb isochron method) monazite ages were determined for gneisses and granitoids from the eastern and western parts of the Ryoke belt separated by about 500 km. The monazite ages for the gneisses are concentrated between 102 and 98 Ma, and are interpreted as the time of monazite formation under lower amphibolite facies conditions. The peak metamorphism seems to be contemporaneous with the emplacement of the geologically oldest plutons that are dated at c. 95 Ma in both the eastern and western parts. In the eastern part plutonism continued from c. 95 Ma to c. 68 Ma at intervals of 2-10 Ma, whereas in the western part it ceased at c. 85 Ma. The CHIME monazite ages agree well with the relative age of granitoids derived from intrusive relationships of granitoids in both parts. These lines of evidence are incompatible with a current view that the plutonometamorphism in the Ryoke belt becomes younger towards the east. The CHIME monazite ages, coupled with available data on the depth at which the Ryoke metamorphism took place and the emplacement of individual plutons, show that the western part was eroded more rapidly (about 1.5 mm year−1) than the eastern part (about 0.8 mm year−1) over the time span from 91 to 85 Ma. The denudation rates agree well with those in active orogenic belts like the Alps and Himalayas.Key words: CHIME geochronology; denudation history; monazite; Ryoke metamorphic belt; Th-U-Pb system. plutons, because the closure temperature for Pb in INTRODU CTION monazite of c. 700°C (Copeland et al., 1988; Parrish, 1990;Smith & Barreiro, 1990;Suzuki et al., 1994a) is Despite extensive investigations, many problems about the thermotectonic episodes of metamorphic belts close to the temperature for crystallization of granitic magmas. remain unsolved, in particular the denudation history of high T /P metamorphic belts. Because high T /P New data using the CHIME (chemical Th-U-total Pb isochron method) for dating of monazite have metamorphic belts are always closely accompanied by abundant syn-and post-tectonic granitoids, their recently been reported for metamorphic rocks and granitoids in the Ryoke metamorphic belt (Suzuki denudation must be at least temporally related to the intrusion of granitoids. Defining the precise emplaceet al., 1994a,b, 1995, Morishita & Suzuki, 1995, Nakai & Suzuki, 1996. Here we report additional ment age of granitoids is, therefore, an important step in developing a more complete understanding of CHIME ages of metamorphic rocks and granitoids in the eastern and western parts of the Ryoke belt the denudation history of the high T /P metamorphic belts.separated by about 500 km. In this study we emphasize how these data can be used to place temporal The Cretaceous Ryoke metamorphic belt is a typical high T /P metamorphic belt developed in southwest constraints on the denudation history of the high T /P Ryoke metamorphic belt. Japan as one of the circum-Pacific orogenic belts. The Ryoke belt is composed of metamorphic rocks representing the andalus...

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