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The magmatic rocks of the Ben Giang-Que Son complex exposed in the southern part of the Truong Son belt have petrographic compositions including gabbro, gabbrodiorite, granodiorite, and granite. Geochemically, the magmatic rocks are of subalkaline affinity and belong to the high-K calc-alkaline series. All analyzed samples contain hornblende and biotite as mafic minerals and are defined as metaluminous with a aluminum saturation index of <1.1. Chondrite-normalized rare earth element diagrams are characterized by fractionation between light and heavy rare earth elements and show small to moderate negative Eu anomalies (Eu/Eu* = 0.81–0.44). Primitive mantle-normalized trace element patterns exhibit enrichment in LILEs such as Rb, K, U, and especially Pb and depletion of HFSEs (Nb, Ta, and Ti), indicating arc-magma. The BG-QS gabbro and gabbrodiorite have low Cr, Co, and Ni and display enrichment in ⁸⁷Sr/⁸⁶Sr (0.7084 to 0.7147), Ꜫ_Nd(t) (−0.5 to −1.9), and positive Ꜫ_Hf(t) (+2.4 to 4.5), suggesting generation from the enriched mantle source. The BG-QS diorite and granodiorite contain small mafic enclaves, have a wide range of SiO₂ contents and enrichment in Sr-Nd isotopes (⁸⁷Sr/⁸⁶Sr = 0.7109–0.7178; Ꜫ_Nd(t) = −3.3 to −3.7), and display high Mg# (43–51). All these features indicate that they were formed by the mixing of magmas, which originated from an enriched mantle source and the pre-existing juvenile mafic lower crust. The whole-rock Nd and zircon Hf model ages are of 1160–760 Ma. The Ben Giang-Que Son complex yields LA-ICP-MS zircon U-Pb ages of 285 ± 3.1 and 278 ± 3.5 Ma that corresponds to the Cisuralian epoch (early Permian), which is linked to the subduction and amalgamation of the Indochina and South China blocks due to the closing a branch of the Paleotethys along the Song Ma suture.