Break-away of South China from Gondwana: Insights from the Silurian high-Nb basalts and associated magmatic rocks in the Diancangshan-Ailaoshan fold belt (SW China)

Abstract

The Diancangshan-Ailaoshan fold belt (Yunnan, SW China) represents the remnants of the once vast Ailaoshan Ocean, a branch or back-arc basin of the Eastern Paleotethys. However, when and how this ocean/back-arc basin opened, and rifted South China away from Gondwana are still unresolved. In this study, Ordovician-Silurian high-Nb basalts (HNBs; 420 Ma), Mg-rich andesite (435 Ma), high-Mg basalt (446 Ma) and I-type granite (423 Ma) were newly identified in the Diancangshan-Ailaoshan fold belt. The HNBs are alkaline with high TiO2 (1.8–1.9 wt%) and Nb (16.9–23.1 ppm) contents, high Nb/U (36.8–39.1) and (Nb/Th)PM (1.17–1.21) ratios, positive Nb-Ta anomalies (primitive mantle-normalized) and positive εNd(t) values (+3.2). The HNBs were likely derived from an OIB/arc-mixed mantle source in the garnet stability field. The Mg-rich andesites are medium-K calc-alkaline andesite with negative εNd(t) values (−10.1) and negative Nb-Ta anomalies. The magma was likely derived from a mixture of depleted mantle peridotites and subducted sediments under low pressure and high H2O conditions. The high-Mg basalts are high-K calc-alkaline and characterized by high MgO contents (9.3–9.4 wt%; Mg# = ~70), high Cr (369–379 ppm) and low Al2O3 (13.6–13.9 wt%) contents. They were likely derived from an enriched mantle source metasomatized by marine sediment-derived fluids. The I-type granites show medium SiO2 content (63.6 wt%), low A/CNK ratio (0.9) and negative εNd(t) value (−9.8), and may have formed by the partial melting of meta-basalts. These HNB, Mg-rich andesite, high-Mg basalt and I-type granite were likely formed in an intracontinental back-arc rift setting. Integrated with our published detrital zircon UPb age and LuHf isotope evidence, we propose that South China may have commenced its break-up from Gondwana in response to the Late Ordovician-Silurian (446–420 Ma) Proto-Tethyan subduction, which opened a back-arc basin along the Ailaoshan fold belt.