Denudation of the Ryoke metamorphic belt ; timing of ductile deformation revealed by CHIME monazite dating of granitic veins

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The Ryoke metamorphic belt is a typical high T/P metamorphic belt in southwestern Japan, and consists mainly of paragneisses and associated granitoids. The paragneisses show CHIME monazite age of c. 100Ma (Suzuki & Adachi, 1989), and are injected by numerous deformed and non-deformed granitic veins of several to 30cm in thickness. The deformed veins are largely concordant with the tectonic foliation but are locally folded and boudinaged. The boudin necks are filled with the same minerals as found in the matrix. This suggests that the host gneisses were still ductile and recrystallization continued after the injection of the deformed vein. The non-deformed veins are largely discordant with straight and sharp boundaries, and seem to have formed through permeation of granitic melt into brittle fractures. This suggests that paragneisses had changed from ductile to brittle deformation conditions before the injection of the non-deformed vein. Ages of individual granitic veins with distinct d … More eformation histories can, therefore, be used to constrain the timing of the ductile-brittle transition in the Ryoke metamorphic belt. Monazites were analyzed with a JEOL JXA-733 electron microprobe equipped with 4 wavelength-dispersive spectrometers (radius of Rowland circle=140mm) through spot by spot mode. The instrument was operated at 15kV acc. voltage, 0.2μA probe current and defocused 4-6μm probe diameter. The ThMα, UMβ, PbMα and YLα lines were measured simultaneously with PET crystals and sealed Xe X-ray detectors. CaKα, SKα, PKα and NbLβ1 lines were also measured. The spectral interferences of YLγ, ThMζ and NbLβ on PbMα and ThMγ and KKα on UMβ were corrected. An U-poor young monazite was used for evaluation of correction factors of Th interferences. The detection limits at a 2σ confidence level are 0.009, 0.011 and 0.006wt.% for ThO_2, UO_2 and PbO, respectively. Relative errors are about 10% for 0.03wt.% PbO, 2% for 0.6wt.% UO_2 and 0.5% for 7.0wt.% ThO_2. All analytical data for spots containing detectable amounts of S, K and Nb were discarded for CHIME age calculation.The deformed veins show CHIME monazite ages of 93.6±6.2-89.7±5.3Ma, and the non-deformed veins show ages of 86±6-75±5Ma. The ductile deformation in the Ryoke metamorphic belt continued beyond 90Ma and ended before c.85Ma. Less