U-Pb geochronology of detrital zircons from the para-metamorphic rocks of the Zhoutan Group, central Jiangxi Province
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摘要:
对赣中周潭群副变质岩中的碎屑锆石开展了LA-ICP-MS U-Pb定年工作，两个石榴黑云母片岩和一个黑云母片岩三个样品中碎屑锆石给出的最年轻的年龄峰值分别为834±11Ma、830±11Ma和809±15Ma。这些碎屑锆石具有与江南造山带内冷家溪群及其相当地层类似的年龄谱型式和相近的最年轻的年龄峰值，表明周潭群应与它们有相似的形成历史和源区，产生于相似的构造环境。因此，周潭群在新元古代扬子地块与华夏地块拼接的过程中，应与冷家溪群、双桥山群等地层一样靠近扬子一侧，而不应属华夏地块。从黑云母片岩部分碎屑锆石的变质边获取的年龄分别为444±11Ma和438±11Ma，表明周潭群地层的变质作用发生在加里东期，与华夏地块其他地区的角闪岩相变质岩的变质年龄一致。相反，与周潭群同期的双桥山群、冷家溪群等地层均未达到角闪岩相变质。这表明，在扬子和华夏地块新元古代的拼接之后，二块体在(现在的)赣中地区又裂开，从而导致了周潭群和相邻的冷家溪群及双桥山群在加里东时期有不同的变质响应。
Abstract:LA-ICP-MS zircon U-Pb dating work was carried out for the detrital zircons from the Zhoutan Group, central Jiangxi Province. Detrital zircons from three samples, including two garnet biotite schist and one biotite schist, show the youngest age peaks of 834±11Ma, 830±11Ma and 809±15Ma, respectively. The age spectrum and maximum depositional ages of detrital zircons from the para-metamorphic rocks of the Zhoutan Group is similar to those for the Lengjiaxi Group and its equivalent sequences in the Jiangnan orogen, suggesting that the Zhoutan Group should have similar formation history, sources and tectonic settings with those sequences. Therefore, the Zhoutan Group, similar to the Shuangqiaoshan and Lengjiaxi groups, should have affinity with the Yangtze Block, rather than the Cathaysia Block, during the amalgamation between the two blocks. The metamorphic rims of the detrital zircons from the biotite schists yielded mean age of 444±11Ma and 438±11Ma, respectively. The metamorphic ages suggest that the metamorphism of the Zhoutan Group happened at Caledonian age, similar to other amphibolite-facies metamorphic rocks in the Cathaysia Block, but rather different from the coeval Shuangqiaoshan and Lengjiaxi groups along the Jiangnan orogen. It probably suggest that the rifting might have taken place at the central Jiangxi Province after the Neoproterozoic assembling between the Yangtze and Cathaysia blocks, which finally led to the different metamorphic process between the Zhoutan Group and the above two adjacent sequences (the Shuangqiaoshan and Lengjiaxi groups).
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Key words:
- Zircon U-Pb dating /
- Para-metamorphic rocks /
- Zhoutan Group /
- Neoproterozoic /
- Jiangnan orogen
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图 1
赣中周潭群地层分布地质简图(据胡恭任等，2004修改)
Figure 1.
Simplified geological map for the distribution of the Zhoutan Group in central Jiangxi Province (modified after Hu et al., 2004)
图 2
周潭群定年样品代表性锆石CL图像
Figure 2.
Representative CL images of dated zircons from samples of the Zhoutan Group
图 3
周潭群副变质岩碎屑锆石U-Pb谐和图
Figure 3.
U-Pb concordia for zircons from the para-metamorphic rocks of the Zhoutan Group
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