Geochronology and geochemistry of the PiaOac granites: Implication for Late Cretaceous magmatism and metallogeny in NE Vietnam

Sn-W deposits have recently been found in NE Vietnam, which has been considered as southwestern extension of the South China Block. The timing and relationships between the Sn-W mineralization and associated granitic magmatism in this area have not been well understood. This study presents new zircon and cassiterite U-Pb geochronology, whole-rock geochemistry, and zircon Hf-isotope data for the PiaOac granites, greisens, and cassiterite veins in NE Vietnam. The PiaOac granites are characterized by high Na2O + K2O, and K2O/Na2O, and low Fe2O3T and Mg#. The presence of primary muscovite, and high A/CNK (1.23–1.26), Rb/Sr (20–25), and P2O5 (0.33–0.35%) suggest that they are strongly peraluminous S-type granites. Low K/Rb, Zr/Hf, Nb/Ta, and Eu/Eu* indicate that the granitic magma experienced strong fractional crystallization. The PiaOac granites have homogeneous zircon Hf-isotope compositions with average εHf(t) values of −9.6 to −8.7 and two-stage Hf model ages of 1.76–1.71 Ga. They probably were sourced from Neoproterozoic metasedimentary rocks with late Paleoproterozoic model ages in NE Vietnam. Low CaO/Na2O (<0.3) and high Rb/Sr and Rb/Ba ratios further suggest a clay-rich pelitic source. Zircons from greisen have significantly unradiogenic Hf-isotope compositions (εHf (t) = -12.2) and large variations, implying strong interaction with wall rocks in late fluid-rich stage during magmatic evolution. Extended magmatic differentiation was followed by interaction with wall rocks, which has altered the redox environment, facilitating the Sn-W mineralization.

Zircon U-Pb dating results reveal that the PiaOac granites formed in the Late Cretaceous (89–88 Ma), coincident with the ∼ 89 Ma mineralization constrained by cassiterite U-Pb dating, suggesting that the Sn-W mineralization is closely related to the granitic magmatism and greisenization. Late Cretaceous magmatism and mineralization in NE Vietnam are similar to those in SW South China in terms of formation age, rock association, deposit type, source composition, and tectonic setting, all together constituting an E-W trending Late Cretaceous metallogenic belt. The Late Cretaceous granites and associated deposits in the belt were probably formed in an intracontinental orogenic setting in response to the combined action of the subduction of Pacific and Neo-Tethys Plates.

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