Identifying whether and when the southern continental margin of the South China Sea (SCS) was part of the South China Block (SCB) is important for Mesozoic–Cenozoic paleogeographic reconstructions of Southeast Asia. In this paper, we present geochronological and geochemical data for dacites from Well NK-1 in the Nansha Block (Dangerous Grounds) of the southern SCS margin to investigate their petrogenesis and tectonic affinity. Zircon UPb and ⁴⁰Ar/³⁹Ar datings show that these dacites formed during the Late Triassic (218–217 Ma). The dacites have high total alkali (K₂O + Na₂O = 7.39–8.42 wt%) and relatively low CaO contents (1.87–3.38 wt%), and show geochemical affinities with A-type granites, with high Zr saturation temperatures (806–855 °C), FeO^Tot/MgO ratios (5.22–7.48), Zr + Nb + Ce + Y contents (530–680 ppm) and 10,000 × Ga/Al values (2.40–2.94), and relatively low A/CNK values (0.86–1.04). The primitive magma for the dacites was likely generated by partial melting of ancient crustal materials. Late Triassic dacites in the Nansha Block and Triassic A-type granites in the SCB formed in an extensional setting at the continental margin of the SCB. This setting was probably related to the Paleo-Pacific plate subduction that restarted prior to the Early Triassic. Therefore, the southern SCS margin was part of the SCB continental margin during the early Mesozoic, which provides more detailed insights into the paleogeography of SE Asia since the Mesozoic.

确定南海南部大陆边缘 (SCS) 是否以及何时是华南地块 (SCB) 的一部分对于东南亚中新生代古地理重建具有重要意义。在本文中，我们展示了南海南部边缘南沙地块（危险区）NK-1 井英安岩的年代学和地球化学数据，以研究它们的岩石成因和构造亲和力。锆石 U Pb 和⁴⁰ Ar/ ³⁹ Ar 测年表明这些英安岩形成于晚三叠世（218-217 Ma）。英安岩具有高总碱（K ₂ O + Na ₂O = 7.39–8.42 wt%）和相对较低的 CaO 含量（1.87–3.38 wt%），并显示出与 A 型花岗岩的地球化学亲和力，具有高 Zr 饱和温度（806–855 °C）、FeO ^Tot/MgO 比率 (5.22–7.48)、Zr + Nb + Ce + Y 含量 (530–680 ppm) 和 10,000 × Ga/Al 值 (2.40–2.94)，以及相对较低的 A/CNK 值 (0.86–1.04)。英安岩的原始岩浆很可能是由古代地壳物质的部分熔融产生的。南沙地块晚三叠世英安岩和南沙地块三叠纪A型花岗岩形成于南沙地块大陆边缘的伸展环境中。这种环境可能与早三叠世之前重新开始的古太平洋板块俯冲有关。因此，南海南部边缘是中生代早期南海大陆边缘的一部分，这为了解中生代以来东南亚古地理提供了更详细的信息。