Episodic rifting magmatism along the northern margin of the South China Block: Implications for the breakup of Rodinia

Mid- to late Neoproterozoic rifting-related magmatism along the northern margin of the South China Block provides key constraints on its paleogeographic position and geodynamic evolution within Rodinia and Gondwana reconstructions. New zircon U-Pb ages for volcanic and plutonic rocks from this region span ca. 740−640 Ma. The ca. 740 Ma Neixiang basalts exhibit intraplate-like geochemical features, with moderate positive εNd(t) (+0.9 to +3.2) and highly positive εHf(t) (+7.2 to +10.6), suggesting derivation from the melting of a depleted mantle source. Associated felsic volcanics and plutonic rocks from the Tuwushan region show A-type granite affinities, characterized by high 10,000 × Ga/Al and Fe/Mg ratios, high Zr-saturation temperatures (802−981 °C), negative εNd(t) (−2.1 to −1.9), moderately positive εHf(t) (+1.0 to +1.8), and low zircon δ18O signatures (0.1‰−6.2‰). These features suggest an origin from the remelting of high-temperature, hydrothermally altered infracrustal materials in a low-pressure, high-temperature, extensional environment. The ca. 717−709 Ma Banbanshan−Zhenggou granitoids exhibit relatively high Na2O/K2O ratios (1.02−1.56), moderate zircon saturation temperatures (760−780 °C), negative εNd(t) (−2.7 to −0.9), and slightly positive εHf(t) (+2.1 to +3.6). These characteristics are comparable to those of the Neixiang and Tuwushan A-type granitoids, suggesting derivation from a similar crustal source with a greater melting depth. The ca. 720 Ma Banbanshan and ca. 640 Ma Nianpangou mafic intrusions show enriched mid-oceanic-ridge basalt and oceanic-island basalt−like geochemical characteristics, which were likely derived from heterogeneous mantle sources in an extensional setting. The integrated geochemical and geochronological data for Neoproterozoic magmatic rocks suggests that variations in basaltic melt extraction depths coincide with the periodic changes in crustal thickness along the northern margin of the South China Block. These episodes of rifting magmatism and extension were likely induced by the periodic retreat of subduction along the north margin of the South China Block and ultimately initiated the breakup of Rodinia.