Early Paleozoic Forearc Ophiolitic Melange in the Tam Ky-Phuoc Son Suture Zone, Central Vietnam: Insights from Serpentinized Peridotites and Plagiogranites

A long-standing debate in Southeast Asian geology concerns how the Proto-Tethys Ocean closed and what role subduction played in shaping the Indochina Block during the early Paleozoic. The Tam Ky-Phuoc Son ophiolite in central Vietnam preserves key evidence of this process, but its origin and age have been uncertain. In this study, we investigated serpentinized peridotites and associated plagiogranites from this suture zone. The serpentinized peridotites contain chromian spinels with high Cr# (0.46-0.69), Mg# (0.43-0.63), and very low TiO2 (<= 0.22 wt%), indicating a mantle source modified in a supra-subduction forearc environment. LA-ICP-MS zircon UPb dating of plagiogranites gives Cambrian ages (ca. 518-502 Ma). Geochemically, the plagiogranites are depleted in Nb, Ta, Ti, but enriched in LREEs and LILEs (Ba, Th, U), consistent with fractional crystallization of mafic magma in a forearc setting. Importantly, the older samples (similar to 518 Ma) show higher HFSE contents and more radiogenic epsilon Hf(t) than the younger ones (similar to 511-504 Ma), recording a transition from incipient subduction to a fluid-enriched mature arc environment. Integrated with regional magmatism (ca. 507-460 Ma) along the Truong Son Belt (TSB), these results indicate that northward subduction of the Proto-Tethys began in Early Cambrian and ended with Ordovician collision between the TSB and Kon Tum Massif (KTM), when the forearc ophiolite was obducted. This study provides direct evidence that the Tam Ky-Phuoc Son complex represents one of the earliest forearc ophiolites linked to Proto-Tethys subduction in Southeast Asia.