Sedimentary Facies,Provenance and Geochronology of the Heshangzhen Group: Implications for the Tectonic Evolution of the Eastern Jiangnan Orogen,South China |
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| Authors: | ZHOU Xiaohu HU Xiumian JIANG Ren GAO Tianshan MA Xue XING Guangfu SUN Gaoyuan SHU Xujie ZHAO Xilin |
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| Institution: | 1Nanjing Center, China Geological Survey, Nanjing 210016, China;2 State Key Laboratory for Mineral Deposits Research, Department of Earth Sciences and Engineering, Nanjing University, Nanjing 210024, China;3 Wuhan Center, China Geological Survey, Wuhan 430205, China;4 College of Oceanography, Hohai University, Nanjing 210098, China |
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| Abstract: | The Neoproterozoic Jiangnan orogen plays an important role in the study of the Precambrian tectonic evolution of South China. The tectonic nature of the Neoproterozoic sedimentary basins is still controversial, due to poor understanding of the sedimentary sequences and the lack of geochronological data. Here, we present sedimentological, provenance and geochronological data from the Heshangzhen Group in the eastern Jiangnan orogen. Sedimentological analysis shows that the Luojiamen Formation was deposited in a submarine fan, and the overlying Hongchicun Formation was deposited in front of a fan delta. The youngest detrital zircons constrain the lower Luojiamen and Hongchicun formations with ages of 827.3 ± 8.4 Ma and 825 ± 12 Ma, respectively. The sandstones of the Luojiamen Formation are characterized by a large number of intermediate to felsic volcanic grains, suggesting a volcanic arc source. In contrast, quartz and sedimentary lithic grains increase in the Hongchicun Formation, showing a new input from a collisional orogenic source. Detrital zircon from six sandstone samples in the Luojiamen and Hongchicun formations yield similar age spectra of 930–820 Ma with a peak at ca. 845–860 Ma, with one main cluster at 930–820 Ma. Detrital zircons of 930–845 Ma show a positive value of ?Hf (t) (+2.4 to +11, mean +7.6), which is similar to the volcanic arc of the nearby Shuangxiwu Group. There are a minor group of zircons with U‐Pb ages ranging from 820 Ma to 845 Ma from the middle part of the Luojiamen Formation and Hongchicun Formation, with ?Hf (t) values between –20 to +2.4, which are consistent with the characteristics of the Shuangqiaoshan Group. within light of the bidirectional paleocurrents in the Luojiamen Formation, it is speculated that the zircons of 820–845 Ma were recycled from the Shuangqiaoshan Group, which is derived from a continental arc to the northwest. Our data suggests that the Luojiamen Formation was formed in an inter‐arc basin, while the Hongchicun Formation was formed in an accretionary wedge‐top basin. When juxtaposed with the conglomeratic characteristics at the bottom of the Luojiamen Formation, it is believed that the unconformity represented by the ‘Shen Gong Movement’ reflects the rapid erosion and accumulation process of island arc volcanic material. The disconformity between the Luojiamen and Hongchicun formations is the imprint of transition from inter‐arc basin to accretionary wedge‐top basin, which represents the collision between the Shuangxiwu arc and the Yangtze Plate. |
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| Keywords: | provenance detrital zircon chronology Heshangzhen Group Neoproterozoic Jiangnan orogen South China |
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