Tectonic affinity of the Yingyangguan Group in northeastern Guangxi: geochronological constraints from banded iron formation (BIF) of the Xialong Formation

The Yingyangguan group in northeastern Guangxi, serving as a crucial area for reconstructing the early tectonic evolution of the South China Block, has long been subject to academic debate regarding its chronological classification and tectonic affinity. To address this, we conducted detrital zircon U-Pb dating and Hf isotope analysis on banded iron formation (BIFs) from the Xialong Formation of the Yingyangguan Group, which exhibit continuous and traceable stratigraphic sequences. Three BIFs samples (YY21063, YY21074, and YY2302) yielded similar detrital zircon U-Pb age spectra, with dominant age peaks at 0.72～0.90 Ga, and subordinate populations at 0.90～1.30 Ga, 1.62～1.99 Ga, 2.12～2.36 Ga, and 2.43～2.64 Ga. Two samples (YY21063 and YY2302) displayed comparable zircon Hf isotopic compositions, with ε_Hf(t) values ranging from -21.92 to 11.68, and -12.80 to 10.73, respectively. Their two-stage Hf model ages (T_DM2) varied between 0.92～3.85 Ga and 1.09～3.31 Ga, indicating that the detrital materials were predominantly derived from the reworking of Mes-Paleoproterozoic and Archean crustal sources. The youngest detrital zircon ages from all three samples consistently constrained the maximum depositional age of the Xialong Formation at ～720 Ma, thereby confirming the Neoproterozoic age of the Yingyangguan Group. The dominant 0.72～0.90 Ga zircon age population suggests that the primary provenance was likely associated with Neoproterozoic magmatic events along the southeastern margin of the Yangtze block. The continuous and traceable banded iron formations in the Xialong Formation indicate that the Yingyangguan Group represents a coherent sedimentary sequence rather than a tectonic mélange. Combined with the provenance signatures, we propose that the Yingyangguan Group formed in a rift-related tectonic setting, further elucidating the Neoproterozoic tectonic evolution of the South China block.