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. 

首次报道广东吴川–四会断裂带北段出露的超基性岩脉。该超基性岩位于粤北韶关市江湾镇, 岩性为橄榄辉石岩, 呈脉状侵入大东山晚侏罗世花岗岩体(约 160 Ma)。橄榄辉石岩锆石LA-ICPMS U-Pb年龄为160±2 Ma, 可以解释为岩脉侵入过程捕获的围岩花岗岩的锆石结晶年龄。岩石黑云母⁴⁰Ar/³⁹Ar坪年龄为135±1 Ma, 表明岩脉侵位时代为早白垩世, 对应华南地区一期显著的伸展事件。岩石富集K和Al, 具较高的Ti和P含量以及中等的Mg含量, SiO₂含量(43.78%~44.80%)小于45%, 属于超基性岩, 具有碱性玄武岩的特征。全岩微量元素和稀土元素分析结果显示, 岩石富集Nb, Ta, Th, U, LILE和LREE, 轻重稀土分异显著, 具有高Nb含量的OIB型基性岩特征, 表明其源区可能为被俯冲组分交代的软流圈地幔, 形成于板块内部拉张环境。结合区域断裂活动、岩石年代学和岩石地球化学特征可以认为, 华南地区早白垩世因俯冲古太平洋板块的撕裂而发生拆沉, 俯冲组分交代软流圈地幔形成OIB型基性岩的富集源区, 随后陆内伸展导致地壳减薄, 切割至岩石圈地幔的吴川–四会断裂带, 为岩浆运移提供通道, 诱发OIB型基性岩浆沿断裂快速上涌, 形成具有板内属性的江湾橄榄辉石岩。