This paper reports the first documentation of ultramafic rocks exposed in the northern segment of Wuchuan-Sihui fault in Guangdong. These ultramafic rocks, classified as olivine pyroxenite, occur as a dike within early Late Jurassic (≈160 Ma) granites of the Dadongshan pluton. Zircon U-Pb LA-ICPMS analysis of the olivine pyroxenite yields an age of 160±2 Ma, suggesting that zircons were captured from the surrounding granite during dike intrusion. Additionally, the ⁴⁰Ar/³⁹Ar plateau age of biotite from the olivine pyroxenite is 135±1 Ma, indicating that the dike emplacement occurred at the Early Cretaceous, corresponding to a significant extensional event in South China. Whole-rock major element geochemical analysis reveals enrichment in K and Al, relatively high Ti and P contents, and moderate Mg. SiO₂ content (43.78%–44.80%) of the sample is less than 45%. These features classify the rock as an ultramafic rock, predominantly exhibiting characteristics of alkaline basalt. Whole-rock trace and rare earth element (REE) analyses show enrichment of Nb, Ta, Th, U, LILE (large ion lithophile elements), and LREE (light REE), along with pronounced fractionation between light and heavy REE. The rock displays signatures of high-Nb OIB-type mafic rocks (Ocean Island Basalts), suggesting a metasomatized asthenospheric mantle source modified by subduction components, primarily forming in an intraplate extensional tectonic setting. Combining fault activity, geochronological and geochemical characteristics, we propose that during the Early Cretaceous, the tearing and delamination of the subducted Paleo-Pacific Plate beneath South China created metasomatized asthenospheric mantle sources enriched for OIB-type mafic rocks. Subsequently, intracontinental extension caused crustal thinning, while the lithosphere-penetrating Wuchuan-Sihui Fault facilitated rapid ascent of OIB-type mafic magmas along the fracture system. These magmas ultimately crystallized to form the Jiangwan olivine pyroxenite, with intraplate geochemical affinities.