High-Mg andesites (HMAs) typically originate in subduction-related tectonic settings, including active continental margins and island arcs, and studies of HMAs can constrain the tectonic evolution of the South China Block and Paleo-Tethyan Ocean. This paper presents new chronological and geochemical data for high-Mg gabbroic diorites in the Funing area, southern South China. The gabbroic diorites yield ages of 254–249 Ma, and have moderate SiO2 contents (51.3%–57.8%), high MgO contents (4.6%–10.2%) and Mg# values (52–75), enrichment in large-ion lithophile elements and light rare earth elements, and depletion in high-field-strength elements—collectively displaying geochemical signatures typical of sanukites. The gabbroic diorites have radiogenic initial Sr and Pb isotopic compositions, and negative εNd(t) and εHf(t) values. These characteristics distinguish the Funing gabbroic diorites from subduction-related igneous rocks in the Paleo-Pacific tectonic domain. Instead, they are isotopically similar to HMAs and basalts along the Paleo-Tethys suture. We propose the high-Mg gabbroic diorites and basaltic andesites in Funing area share a common mantle source, linked to subduction and rollback of the Paleo-Tethyan plate beneath the South China Block, which triggered back-arc rifting and partial melting of the metasomatized lithospheric mantle during the Late Permian to Early Triassic.