Abstract
The Tam Ky–Phuoc Son Ocean in Central Vietnam, a southern branch of the Prototethyan Ocean, holds critical evidence of Early Paleozoic subduction processes in Southeast Asia. However, the polarity and tectonic evolution of its subduction system remain controversial. Here we present new petrographic observations, zircon U-Pb geochronology, in situ zircon Hf isotopes and whole-rock geochemistry for mafic–intermediate rocks from the Kham Duc Complex along the northern margin of the Kontum Massif. These data document arc magmatism at ca. 455–451 Ma. Geochemically, the rocks display pronounced enrichment in large-ion lithophile elements (LILEs; e.g., Rb, Ba) and depletion in high-field-strength elements (HFSEs; e.g., Nb, Ta, Zr, Hf), coupled with εHf(t) values of +2.32 to +15.96 and εNd(t) values of −0.86 to +0.35, consistent with derivation from a depleted mantle wedge metasomatized by subducted slab-derived melts and sediment-derived fluids. Integrated with regional geochronological and geochemical data, these results suggest that southward subduction of the Tam Ky–Phuoc Son Ocean initiated at ca. 485 Ma beneath the Kontum Massif. Subsequent northward slab rollback drove the progressive northward migration of arc magmatism and the accretion to the southern Truong Son Belt between ca. 485 Ma and 420 Ma. This evolution was accompanied by increasing crustal involvement and progressively shallower melting toward the north, reflecting vertical thickening and lateral expansion of the accretionary wedge. These findings provide robust evidence for a southward-directed subduction system with northward slab rollback during Early Paleozoic evolution of the Protote thyan Ocean in central Vietnam.
Zhang Y.; Li S.; Gan C.; Qian X.; Yu X.; Zhang F.; Hieu P.T.; Wang Y. (2026) Lithos.