U-Pb age and Lu-Hf isotopic characteristics of S-type granite in the Bac Tra My area, Quang Nam and their regional tectonic implications

Abstract

The Kon Tum Massif (KTM) in central Vietnam is a key part of the Indochina Block, where Early Paleozoic magmatism is attributed to subduction and subsequent collision between the KTM and the Truong Son Belt (TSB). However, the timing, source and tectonic setting of S-type granites of the Chu Lai Complex, especially near Bac Tra My area, remain unclear. This study presents new zircon U-Pb geochronological and Lu-Hf isotopic data from two granitic gneiss samples of the Chu Lai Complex in the Tra My area to better constrain their formation age, magma source and tectonic implications. Zircon U-Pb dating by MC-LA-ICP-MS yields concordant crystallization ages of 438.7 ± 2.9 Ma and 441.7 ± 1.9 Ma, corresponding to the Late Ordovician. Inherited zircon core ages (1.45÷2.62 Ga) indicate that the magma was derived from partial melting of Paleoproterozoic-Mesoproterozoic metasedimentary sources. Zircon εHf(t) values from -11.1to -2.4 support derivation from ancient crustal materials. Petrographic features, including muscovite, garnet and tourmaline, further indicate an S-type peraluminous affinity. Integrating these data with previous results suggests that Chu Lai magmatism (455÷426 Ma) represents a syn- to post-collisional episode following the closure of the Proto-Tethys Ocean and the collision between the KTM and the TSB. This event, likely during the Middle to Late Ordovician, marks the termination of subduction beneath the KTM and the onset of a widespread post-collisional thermal regime across the Indochina Block. The study refines the timing of Early Paleozoic orogenesis in central Vietnam, provides new isotopic evidence for the crustal evolution of the KTM and contributes to a better understanding of the tectonic assembly of the eastern Indochina Block during the Paleozoic