Neoproterozoic Tectonic Evolution in the Middle Segment of the Jiangnan Orogenic Belt:Implications from detrital zircon U-Pb and Lu-Hf isotopes
[Objective]Determining the depositional age,sedimentary sources and tectonic background of Neopro-terozoic sedimentary strata in Hunan province is one of the keys to understanding tectonic evolution in the middle seg-ment of the Jiangnan orogenic belt.[Methods]Six clastic rock samples were collected from the Lengjiaxi Group and Banxi Group in the middle segment of the Jiangnan orogenic belt.Provenance and tectonic setting of the sedimentary basins were determined by studying the morphology,trace elements,and U-Pb/Lu-Hf isotopic composition of detrital zircons,and collating published data for detrital zircons and zircons from source areas.[Results]The results indicat-ed that the Lengjiaxi Group was formed at about 852-825 Ma,and the Banxi Group was formed at about 820-720 Ma.The most important detrital zircon age peaks in the Lengjiaxi Group and Banxi Group occur at 920-790 Ma;age peaks also appear at 1 750-1 620 Ma and 2 500-2 450 Ma in the Lengjiaxi Group,and at 1 950-1 790 Ma and 2 420-2 330 Ma in the Banxi Group.The Hf isotopic characteristics show that the Lengjiaxi Group mainly received sedi-ments from the Yangtze Block,and the Banxi Group received detritus from both the Yangtze and Cathaysia Blocks.[Conclusions]The U-Pb and Lu-Hf isotopic compositions of detrital zircons indicate that deposition in the Lengjiaxi Group and Banxi Group occurred before and after the amalgamation of the Yangtze and Cathaysia Blocks,respectively.The Lengjiaxi Group was deposited in the back-arc basin of a trench-arc-basin system which was closed by 825 Ma accompanied by strong folding in the sedimentary strata and a large number of S-type granite intrusions.The Banxi Group was deposited in an intraplate rift setting related to post-collision extension.Rifting-related magma erupted around 780-760 Ma,and gradually weakened along the sedimentation of the Banxi Group.
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