Neoproterozoic rift-related mafic and felsic rocks from the Yangtze Block, South China: Insights into petrogenesis and tectonic evolution

Abstract

Neoproterozoic rift-related magmatism along the margins of the Yangtze Block presents a valuable opportunity to study the tectonic evolution of South China and the geodynamic processes resulting from the breakup of Rodinia. In this paper, we report Neoproterozoic rhyolites, diabases, and granites from the Kangdian and Nanhua Rift basins along the western and southeastern margins of the Yangtze Block, respectively. The Suxiong (ca. 819–824 Ma) and Yejia (ca. 800–804 Ma) rhyolites exhibit high Na₂O + K₂O (7.07–8.67 wt%) and Zr + Nb + Ce + Y (355–930 ppm) contents, zircon saturation temperatures (821–920 °C), and 10,000 × Ga/Al ratios (2.28–4.28), indicating an affinity with A-type granites. These rhyolites have depleted NdHf isotopic compositions (ε_Hf(t) = +1.8 to +5.6 and ε_Nd(t) = +3.5 to +4.9, respectively) and are characterized by low Sr/Y (0.14–1.42) and (La/Yb)_N (3.98–9.1) ratios, Mg^# (8–35) values, and high Fe/(Fe + Mg) (0.76–0.96) ratios, suggesting that they were generated primarily by the partial melting of quartzofeldspathic crustal rocks under low-pressure, high-temperature conditions. The ca. 812 Ma Dengxiangying diabases are of the calc-alkaline series, showing enrichments in LILEs and LREEs and depletions in HFSEs. Isotoptic data (ε_Hf(t) = +6.6 to +10.1, ε_Nd(t) = +3.4 to +5.8) suggest that they were derived from the partial melting of subduction-modified lithospheric mantle. The ca. 810 Ma Yifeng granites display a weakly peraluminous character, with low zircon saturation temperatures (752–806 °C), and 10,000 × Ga/Al (2.0–2.6) and Zr + Nb + Ce + Y (155–340 ppm) values, similar to those of I-type granitoids. These granites have depleted whole-rock Nd (+1.5 to +2.6) and zircon Hf (+4.0 to +7.2) isotopic compositions, indicating derivation from the remelting of Mesoproterozoic mafic crustal materials. By integrating the petrogenesis of these Neoproterozoic rift-related rocks, we consider that the Kangdian Rift was a back-arc basin produced in response to the long-term slab subduction beneath the western Yangtze Block, whereas the Nanhua Rift was a continental rift basin produced in response to the upwelling of asthenospheric mantle.