Subduction of the paleo-Pacific plate recorded by arc volcanism in the South China Sea margin

Abstract

Accretionary prisms along the East Asia continental margin have been interpreted as having formed in response to subduction of the paleo-Pacific plate in the Paleozoic–Mesozoic. However, early arc magmatic records in the East Asia margin have been partly destroyed owing to episodic rifting. Widespread arc magmatism and fold and thrust belts in the South China block have recorded Jurassic and Cretaceous paleo-Pacific plate subduction. In addition, the exterior of the South China continental margin was rifted during the Cenozoic to form the South China Sea, and it is presumed that a relic arc is preserved in the northern and southern South China Sea. In this study, we obtained samples of volcanic rocks through drilling >2000 m in the southern South China Sea. The drill core consisted of interbedded basalts, trachytes, and minor trachy-andesites, with arc-type trace-element patterns. Zircon U–Pb dating of trachyte samples from the drill core yielded clustered ages with an overall weighted mean age of 211.2 ± 1.6 Ma, providing robust evidence for the existence of a volcanic arc in the East Asia margin during the Late Triassic. Primitive basalts with MgO > 8.5 wt% have the highest values of εNd(i) and εHf(i) and indicate derivation from Indian-type sub-arc mantle wedge. The increasing Th/Nb with decreasing Nd−Hf isotope values of the basalt samples indicates the incorporation of sediment into the melts. The trachyte samples have enriched K2O and isotopic compositions and are consistent with a sediment-dominated source. On the basis of a comparison with regional magmatic rocks related to plate subduction during Triassic–Jurassic time, the arc volcanic rocks in the southern South China Sea are interpreted to mark a temporal compositional transition in the mantle source, representing steepening subduction of the paleo-Pacific plate during the Late Triassic.