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

Ryoke变质带是日本西南部典型的高T/P变质带，主要由副片麻岩及其伴生的花岗岩类组成。副片麻岩显示约100 Ma的嵌套独居石年龄(Suzuki&Adachi，1989)，并由许多变形和未变形的花岗岩脉注入，厚度几到30厘米。变形矿脉与构造面理基本一致，但局部褶皱并呈束状分布。布丁的颈部充满了与基质中发现的相同的矿物质。这表明寄主片麻岩在注入变形矿脉后仍具有韧性，重结晶作用仍在继续。未变形的矿脉与直线和尖锐的边界基本不一致，似乎是通过花岗岩熔体渗入脆性裂缝而形成的。这表明在未变形的矿脉注入之前，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d…的单个花岗岩脉的年龄因此，更多的地层历史可以用来限制Ryoke变质带韧性-脆性转变的时间。独居石的分析采用JEOL JXA-733电子探针，配有4个波长色散光谱仪(罗兰圆半径=140 mm)，通过逐点模式进行分析。该仪器工作在15千伏交流电压下。电压，0.2μA探头电流，离焦4-6μm探头直径。用PET晶体和密封Xe X射线探测器同时测量了THMα、UMβ、PbMα和YLα谱线。测定了CAK-α、SK-α、PK-α和NBL-β-1细胞系。校正了YLγ、THMζ和NbLβ对PBMα的光谱干扰，以及THMγ和KKα对UMβ的光谱干扰。用贫铀的年轻独居石来评价Th干扰的校正因子。在2σ置信度下，THO_2、UO_2和PbO的检出限分别为0.009、0.011和0.006wt.%。0.03wt.%PbO、0.6wt.%UO_2和7.0wt.%ThO_2的相对误差分别约为10%、2%和0.5%。所有含有可测得的S、K和Nb的斑点的分析数据都被丢弃用于计算脉钟年龄，变形脉脉的独居石年龄为93.6±6.2~89.7±5.3 Ma，未变形脉脉的年龄为86±6~75±5 Ma。Ryoke变质带的韧性变形持续至90 Ma以上，并在约85 Ma之前结束。较少

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