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Geoscience ›› 2019, Vol. 33 ›› Issue (05): 937-956.DOI: 10.19657/j.geoscience.1000-8527.2019.05.02

• Structural Geology and Stratigraphy • Previous Articles     Next Articles

Neoproterozoic Post-orogenic Tectonic Transformation in Eastern Margin of the Yangtze Block:Petrologic and Geochronologic Constraints of the Weng’an Dome

SONG Zhidong1(), YAN Danping1,2()   

  1. 1. School of Earth Sciences and Resources, China University of Geosciences, Beijing 100083, China
    2. State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences, Beijing 100083, China
  • Received:2019-02-28 Revised:2019-07-26 Online:2019-10-26 Published:2019-10-28
  • Contact: YAN Danping

Abstract:

The timing and processes of the transformation from the Rodinian post-orogenic tectonics to extension in the Yangtze block have been an important question in the Neoproterozoic Yangtze tectonic evolution. The Weng’an Dome in the southeastern Yangtze block has preserved the complete stratigraphic sequence, and the unconformity between the Neoproterozoic Banxi Group and the Nantuo Formation in the Nanhua period, and is thus an ideal target to investigate the Neoproterozoic post-orogenic tectonic transformation. We study the sedimentary sequence of the low-grade metamorphosed sedimentary rocks of the Neoproterozoic Banxi Group and Nantuo Formation (Nanhua period), via detrital zircon U-Pb dating and whole-rock geochemical analyses. Field geology shows that the Nantuo Formation (Nanhua period) lies unconformably on the Banxi Group. Age dating results indicate the maximum depositional ages for the Banxi Group and Nantuo Formation are 772 Ma and 691 Ma, respectively. Neoproterozoic rocks in the Weng’an Dome (Banxi Group) have intermediate SiO2 contents, with the average values of SiO2/Al2O3=5.53, K2O/Na2O=7.14 and (TFeO+MgO)=3.47%. The Neoproterozoic Banxi Group sedimentary rocks may have sourced from the feldspar-quartz-dominated upper crust. The Weng’an Dome may have formed in an active continental margin, and the clastic material may have derived from the western margin of the Neoproterozoic Yangtze block. The Nantuo Formation sedimentary rocks have intermediate SiO2 contents, with average values of SiO2/Al2O3(4.69), K2O/Na2O (20.41) and (TFeO+MgO) (6.64 %). The REE chondrite-normalized patterns of the clastic rocks resemble those of the upper continental crust, as characterized by LREE-enrichment, flat HREE patterns and negative Eu anomalies. The Nantuo Formation sediments may have come from the feldspar-quartz-dominated (with minor intermediate rocks) upper crust. The Weng’an Dome was likely formed in a rifting-related tectonic setting. In summary, we suggest that the stratigraphic transition from the Banxi Group to the Nantuo Formation represents the tectonic transformation from syn-collisional orogeny (till ~772 Ma) to post-orogenic rifting (after ~691 Ma). Therefore, the syn-to post-orogenic tectonic transition occurred at about 772-691 Ma.

Key words: Yangtze Block, Weng’an Dome, Neoproterozoic, sedimentary sequence, detrital zircon U-Pb age, post-orogenic extension

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