扬子地块与华夏地块的西段界线: 来自桂北和桂东新元古界—寒武系沉积岩的证据
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本文为国家自然科学基金项目(编号41772053,41330208)资助的成果。


Western boundary between the Yangtze and Cathaysia blocks constrained by Neoproterozoic to Cambrian sedimentary rocks in the northern and eastern Guangxi Province, South China
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    摘要:

    华夏块体与扬子块体拼合带在广西境内的界线一直没有统一的认识。本文对桂北和桂东两个地区的新元古代—寒武纪浅变质沉积岩开展系统的地球化学、碎屑锆石年代学和Hf同位素学研究,以期为此提供岩石学证据。研究显示桂东大瑶山藤县地区存在南华系—震旦系沉积岩,沉积时代晚于741~622 Ma。这些南华系—震旦系沉积岩的碎屑锆石年龄谱以明显的~1. 0 Ga年龄峰和~2. 5 Ga弱峰为特征,相似于桂东广泛分布的寒武系沉积岩以及西华夏南岭- 云开地体基底变质岩的锆石年龄谱。桂北龙胜地区新元古代沉积岩具有不同的碎屑锆石年龄谱,以一个~0. 80 Ga主峰和两个(~2. 0 Ga、~2. 5 Ga)弱峰为特征,具有扬子南缘新元古代沉积岩的典型年龄谱。桂北新元古代沉积岩具有低的K 2O/Na 2O、Al 2O 3/(Na 2O+CaO)和CIA指数,指示源区风化程度和成分成熟度较低,而桂东地区南华系—震旦系和寒武系沉积岩与之相反,说明源区的风化程度以及成分成熟度较高。主量元素和微量元素特征都表明桂北地区新元古代沉积岩的源区中有更多的镁铁质组分,相似于江南造山带其他地区新元古代沉积岩;而桂东地区南华系—震旦系和寒武系沉积岩的源区有较多长英质组分,相似于华夏南岭—云开地区新元古代沉积岩。结合桂东震旦系沉积岩具有与南岭- 云开地体基底岩石相似的碎屑锆石Hf同位素组成,可以确定桂东大瑶山地区属于西华夏地块,而桂北龙胜地区属于扬子地块南缘江南造山带。因此,扬子地块与华夏地块西段的界线最有可能从桂北龙胜地区与桂东大瑶山地区之间通过。

    Abstract:

    The boundary between the Cathaysia and the Yangtze blocks in the Guangxi Province has not been well constrained due to lack of exposed Precambrian basement rocks. This study presents whole rock geochemical and detrital zircon U- Pb- Hf isotopic compositions for Neoproterozoic to Cambrian meta- sedimentary rocks in the northern and eastern Guangxi Province, South China, in order to provide new petrologic evidence to constrain this boundary. The detrital zircon U- Pb dating results confirm that there are Nanhua- Sinian sedimentary rocks in the Tengxian area of eastern Guangxi Province, which were deposited later than 741~622 Ma. The Nanhua- Sinian to Cambrian sedimentary rocks in the Dayaoshao have similar detrital zircon U- Pb age spectra, characterized by a significant ~1. 0 Ga peak and a weak ~2. 5 Ga peak, similar to the basement rocks in the Nanling- Yunkai terrane, western Cathaysia Block. Neoproterozoic sedimentary rocks in the Longsheng area, northern Guangxi Province, contain abundant Neoproterozoic detritus with a main peak of ~0. 80 Ga and two weak peaks at ~2. 0 Ga and ~2. 5 Ga, similar to the Neoproterozoic sedimentary rocks in the Jiangnan Orogen, southern Yangtze Block. Neoproterozoic sedimentary rocks in the Longsheng area generally have low K 2O/Na 2O, Al 2O 3/(Na 2O+CaO) and CIA values, suggesting low maturity and weathering extent in the provenance, whereas Nanhua- Sinian to Cambrian sedimentary rocks in the Dayaoshan area have contrasting geochemical features, indicating higher maturity and weathering extent. All the major and trace element characteristics suggest that the provenance of Neoproterozoic sedimentary rocks in the Longsheng have more mafic components, similar to other Neoproterozoic sedimentary rocks in the Jiangnan Orogen, whereas the provenance of Nanhua- Sinian to Cambrian sedimentary rocks in the Dayaoshan consist mainly of felsic components, similar to the basement rocks in the Nanling- Yunkai terrane. Based on multiple lines of evidence i.e., the similarity of Hf- isotope compositions of detrital zircons in a Sinian sedimentary rock in the Dayaoshan with the basement rocks in the Nanling- Yunkai terrane and other available observations, it is suggested that the Dayaoshan area of the eastern Guangxi belongs to the Cathaysia Block, and the Longsheng area of the northern Guangxi is a part of the Jiangnan Orogen. Consequently, the western extension of the boundary between the Yangtze and Cathaysia blocks most likely passes between the Longsheng and Dayaoshan areas.

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王雅迪,于津海,李晓玲,蒋威,许华,李敏业.2021.扬子地块与华夏地块的西段界线: 来自桂北和桂东新元古界—寒武系沉积岩的证据[J].地质学报,95(6):1712-1726.
Wang Yadi, Yu Jinhai, Li Xiaoling, Jiang Wei, Xu Hua, Li Minye.2021. Western boundary between the Yangtze and Cathaysia blocks constrained by Neoproterozoic to Cambrian sedimentary rocks in the northern and eastern Guangxi Province, South China[J]. Acta Geologica Sinica,95(6):1712-1726.

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历史
  • 收稿日期:2020-12-13
  • 最后修改日期:2021-03-29
  • 录用日期:2021-04-01
  • 在线发布日期: 2021-04-12