Geochronology and geochemistry of basalts from the Yingchuan Formation, eastern Jiangnan Orogen: Implications for the Neoproterozoic tectonic evolution of the South China Block

作者：Chen, Mo; Wang, Lei; Li, Longming^*; Lin, Shoufa; Liu, Hang; Wang, Bo; Han, Xu

来源：Geological Journal, 2023, 58(4): 1673-1692.

DOI：10.1002/gj.4686

• back-arc basin
• basalt
• Jiangnan Orogen
• Yangtze Block
• Yingchuan Formation

The Neoproterozoic magmatism along the Jiangnan Orogen preserves significant information for understanding the tectonic evolution of the South China Block (SCB). We conducted a systematic zircon U-Pb geochronological and whole-rock geochemical study on the basalts from the Yingchuan Formation in the eastern Jiangnan Orogen, SCB. The basalts yielded a weighted mean Pb-206/U-238 age of 817 +/- 13 Ma, enriched in Rb, Th, U, LREE and HFSE (e.g., Zr and Ti). The basalts have positive epsilon(Nd)(t) (+1.9 to +2.1) and high (Sr-87/Sr-86)i (0.69853-0.70737), indicating a slightly depleted mantle source. The high La/Nb (2.14-2.35) and La/Ta (28.6-33.3) ratios indicate a lithospheric mantle source for the basalts. Their high Ba/Yb (7.85-44.3) and Th/Yb (0.59-0.74) ratios and high absolute Nb contents (7.64-11.6 ug/g) indicate that the lithospheric mantle source has been metasomatized by melts and fluids derived from a subducted slab. The basalts are characterized by (La/Yb) (N) ratios (3.0-3.5) and Sm/Nd ratios (0.24-0.26), comparable to intra-continental back arc basin basalts (BABBs). Their Ta/Yb (0.14-0.16) and Th/Yb (0.59-0.74) ratios are similar to those of Okinawa Trough basalts and contemporary mafic rocks formed in the back-arc basin in the Jiangnan Orogen. Combined with their source features, we propose that the basalts were formed in a back-arc basin setting. The identification of BABBs from the Yingchuan Formation shows that there were Neoproterozoic back-arc basins in the south-eastern Yangtze Block, indicating that the final assembly between the Yangtze and Cathaysia blocks did not occur before ca. 817 Ma.