Zircon Lu-Hf isotopic constraints on the suture boundary of the eastern South China Block

Abstract

The Jianshan-Shaoxing (JSFZ) Fault Zone in Zhejiang Province has been proposed to represent the suture between the Yangtze and Cathaysia blocks in South China. Limited Neoproterozoic outcrops and similar Sr and Nd isotopic data across the fault zone do not show distinctive characteristics for the two separate blocks. Thirteen Cretaceous granitic bodies within the fault zone, 6 and 7 from the NW and SE sides of the fault zone have been collected for this study. These samples have compositions of diorites to granites (SiO2 = 56.2 to 76.6 wt.%). In-situ zircon U-Pb and Hf isotopic compositions have been analyzed. These granitoids yielded U-Pb ages from 87 to 134 Ma, and show systematic differences in Hf isotopic compositions (εHf(t) = 6.67 to -7.01 in the NW side vs 1.91 to - 12.63 in the SE side, and TDM C = 0.52 to 1.48 Ga in the NW side vs 0.58 to 2.85 Ga in the SE side). The TDM C data from NW side shows two peaks at 876 and 1170 Ma respectively, whereas those for the SE side show a single peak at 1135 Ma. The Hf isotopic disparity for the two sides may indicate the different basement compositions for the Yangtze and Cathaysia blocks. The available geological data indicate that the Neoproterozoic magmatic activities are important for the Yangtze Block, possibly due to the break-up of the Rodinia supercontinent, but such activities are weak for the Cathaysia Block. This study shows that the Neoproterozoic juvenile materials are important component of the eastern Yangtze basement, but the Cathaysia basement is mainly made up of Mesoproterozoic materials. Our data may also imply that the two continental blocks were away from each other in the Neoproterozoic. Relatively wide range distribution of TDMC (820 to 900 Ma) from NW side and lack of 800 to 900 Ma records from SE side also suggests that the Cathaysia block is underneath the JSFZ, implying a northward subduction of the Cathaysia Block.