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 ε_Nd(i) and ε_Hf(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.

东亚大陆边缘的增生棱柱被解释为是在古生代-中生代古太平洋板块俯冲时形成的。然而，东亚边缘早期的弧形岩浆记录因幕式裂谷而被部分破坏。华南地块广泛的弧形岩浆作用和褶皱冲断带记录了侏罗纪和白垩纪古太平洋板块俯冲。此外，华南大陆边缘外部在新生代时发生裂陷形成南海，推测南海北部和南部保存有遗迹弧。在这项研究中，我们通过在南海南部钻探> 2000 m获得了火山岩样品。钻芯由互层玄武岩、粗面岩和少量粗面安山岩组成，具有弧形微量元素模式。来自钻芯的粗面岩样品的锆石 U-Pb 测年得出了总体加权平均年龄为 211.2 ± 1.6 Ma 的聚类年龄，为晚三叠世东亚边缘存在火山弧提供了有力的证据。MgO > 8.5 wt% 的原始玄武岩具有最高的 ε 值_Nd(i)和 ε _Hf(i)表示源自印度型亚弧地幔楔。随着玄武岩样品的 Nd-Hf 同位素值的降低，Th/Nb 的增加表明沉积物并入熔体中。粗面岩样品富含 K ₂ O 和同位素组成，与沉积物为主的来源一致。在与三叠纪—侏罗纪板块俯冲相关的区域岩浆岩对比的基础上，南海南部弧形火山岩被解释为标志着地幔源的时间成分转变，代表着古-侏罗纪俯冲变陡。三叠纪晚期的太平洋板块。