Southwestern Boundary Between The Yangtze And Cathaysia Blocks

The South China Plate is an important part of the Chinese mainland.It has experienced a complex and multi-stage tectonic evolution history.It was formed by the combination of the Yangtze Block and the Cathaysia Block in the Neoproterozoic.A lot of domestic and foreign scholars have conducted a large number research on the evolutionary history of the South China Plate and obtained numerous results.The northeastern boundary of the Yangtze Block and the Cathaysia Block is clear.However,the southwestern boundary of the two blocks is still controversial.In this study,LA-ICPMS detrital zircon U-Pb dating and in-situ Hf isotope analyses of the sandstone samples of the Early Paleozoic Liantan Formation in the Qinzhou-Fangchenggang areas are carried out in order to reveal the southwestern boundary of the Yangtze and Cathaysia blocks.The results of zircon U-Pb dating show that the age spectrum of 8 representative sedimentary rock samples shows three obvious age peaks of?2.5 Ga,?1.0 Ga,and?0.47 Ga,suggesting the Archean global crustal growth events(?2.5 Ga),Greenwellian orogeny(?1.0 Ga)and Early Paleozoic tectonic events(?0.47 Ga)were recorded in South China.In-situ zircon Hf isotope analytical results show that the ratios of ¹⁷⁶Hf/¹⁷⁷Hf of the detrital zircons range from0.280514 to 0.282573.The?_Hf(t)values of the age group 400-600 Ma are-21.4?-5.05,indicating that their sources are mainly from ancient continental crust materials.The?_Hf(t)values of the age group 900-1000 Ma range from-23.6 to+7.76,implying that they sourced from both ancient crustal material and new crustal material.The detrital zircons are characterizied by enrichment of heavy rare earth element and obvious Ce positive anomalies and Eu negative anomalies on the chondrite-normalized rare earth element diagrams.Combining the characteristics of the cathodoluminescence image of the detrital zircons and related regional geological data,it is suggested that the age groups of?2.5 Ga,?1.0 Ga and?0.47 Ga are mainly derived from the Yunkai terrane in the northeastern of the Qinfang region and the detrital zircons were likely sourced from a collisional orogenic belt.Comparing the results of stratigraphy and magmatic activities of the Damingshan,Dayaoshan and Western Yunnan terrains,we propose that the southwestern boundary between the Yangtze and Cathaysia blocks is located on the northwest of the Qinfang area,probably is the Guilin-Pingxiang-Nanning areas.