Abstract: The Nansha Block (NB) was a significant component of the southeastern margin of the South China Block (SCB) during the Triassic period, which preserves critical records of geodynamic and tectonic transition processes. To reveal the essence of Paleo-Pacific westward subduction during the Triassic, Well NK-1 within this block was selected for petrographic study, and published research data from our team were compared. The latest petrographic and geochemical data reveal that the rock assemblage comprises two distinct types: Type I and Type II basaltic-basaltic andesitic (BA) series and a dacitic (DA) series, which were emplaced during the early Late Triassic (217.9 ± 3.5 to 218.6 ± 3.2 Ma). All three series exhibit typical geochemical signatures of arc magmatism and are derived from an EMII-type enriched mantle source. We propose that the flat-slab subduction of the Paleo-Pacific Plate transported significant volumes of the continental crust frontal accretionary wedge to depth where it underwent remelting. This process generated substantial melts/fluids, which infiltrated the asthenosphere or metasomatized the lithospheric mantle on the side of the continental margin, ultimately triggering mantle melting that formed type I and type II basaltic magma chambers (BA series). With continued subduction, the leading edge of the slab experienced flexural deformation, which induced upwelling of the adjacent asthenosphere on the continental side. This upwelling, in turn, led to persistent underplating, triggering partial melting of the mafic lower crust and generating the DA series dacitic magma chambers. The sustained ascent of mantle-derived magmas during this period resulted in significant lithospheric thinning beneath the study area. This thinning not only facilitated the eventual rifting of the Nansha Micro-block from the South China Block but also preconditioned the region for the subsequent development of early intracontinental rifting during the later seafloor spreading stage of the South China Sea.

Key words: Paleo-Pacific tectonic domain, tectonic evolution, rift volcanic rock, Nansha Block, South China Block