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

柳屋变质带是日本西南部典型的高温高压变质带，主要由副片麻岩和伴生花岗岩类组成。副片麻岩的CHIME独居石年龄为c。100 Ma（Suzuki &阿达奇，1989），并被大量的变形和未变形的花岗岩脉注入，厚度为几至30 cm。变形脉体与构造面理基本一致，但局部褶皱、石香肠状。石香肠颈部充满了与基质中发现的相同的矿物。这表明寄主片麻岩在变形脉注入后仍具有韧性，重结晶仍在继续。未变形的脉体与直线和尖锐的边界基本上不协调，似乎是通过花岗岩熔体渗透到脆性断裂中形成的。这表明，在未变形的脉体注入之前，副片麻岩已经从韧性变形状态转变为脆性变形状态。具有明显特征的单个花岗质脉的年龄 ...更多信息 因此，变形历史可以用来限制Ryoke变质带韧-脆转变的时间。独居石用JEOL JXA-733电子探针通过逐点模式分析，该电子探针配备有4个波长色散光谱仪（罗兰圆半径= 140 mm）。仪器工作电压15 kV，探针电流0.2μA，离焦探针直径4-6μm。用PET晶体和密封的X射线探测器同时测量了ThMα、UMβ、PbMα和YLα谱线。同时测定了CaKα、SKα、PKα和NbLβ1细胞系。校正了YLγ、ThM γ和NbLβ对PbMα和ThMγ、KKα对UMβ的光谱干扰。使用贫铀年轻独居石来评估Th干扰的校正因子。在2σ置信水平下的检出限分别为0.009，0.011和0.006wt.%对ThO_2，UO_2和PbO的反应。对于0.03wt.%的样品，相对误差约为10 PbO，2%，0.6wt.% UO_2和0.5%（7.0wt.%）ThO_2。在CHIME年龄计算中，剔除了所有含S、K、Nb的测点，变形脉的CHIME年龄为93.6±6.2-89.7±5.3Ma，未变形脉的CHIME年龄为86±6-75± 5 Ma。辽辽变质带的韧性变形持续到90 Ma以后，在约85 Ma以前结束。少

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