Evidence of Early Cretaceous lower arc crust delamination and its role in the opening of the South China Sea

Abstract

Understanding the formation mechanism of the South China Sea has important implications for research on plate rupture and continent-ocean transition globally. Granitoids dredged from the Xiaozhenzhu Rise provide new perspectives on lithosphere evolution processes of this region. Zircon UPb (127–122Ma) and amphibole/K-feldspar 40Ar/39Ar (123–115Ma) ages indicate high cooling rates of 55–64°C/myr and thus rapid magma emplacement and uplift in the Early Cretaceous. These calc-alkaline granitoids with intermediate Mg# (44–53) and slightly negative Eu anomalies (Eu/Eu*=0.63–1.00) have highly variable and well-correlated Cr (4.89–531ppm) and Ni (2.27–258ppm) contents, which indicate melt mixing. The low CrNi sample (19.4ppm Ni) displays much higher Sr (847ppm), Sr/Y (93.4), and overall stronger crustal signatures than the high CrNi samples (107–258ppm Ni) which have more mantle-like characteristics. Despite these differences, all studied samples show relatively similar and moderately enriched SrNd isotopic compositions ((87Sr/86Sr)i=0.7055–0.7064, εNd(t)=−0.6 to −1.7) and enriched Pb isotopic compositions that are comparable with those of marine sediments. They also show mantle-like depleted zircon O (δ18O=4.5–6.3‰) and mostly positive zircon Hf (εHf(t)=−0.4–4.1) isotopic compositions that indicate limited upper crustal contribution in the melt source. Their compositional features are best explained by magma mixing between partial melts of a delaminated lower arc crust and partial melts of a metasomatized arc mantle wedge. Combining our new results with literature studies of magmatism, metamorphism, sedimentary records and crustal structures from the region, we propose a new model of the Late Mesozoic–Early Cenozoic lithosphere deformation of the South China continental margin where lower arc crust delamination generated a tectonic weak zone that is essential for the rifting of the South China Sea.