Emplacement ages, geochemical and Sr–Nd–Hf isotopic characteristics of Cenozoic granites in the Phan Si Pan uplift, Northwestern Vietnam: petrogenesis and tectonic implication for the adjacent structure of the Red River shear zone

Pham Thi Dung, Tadashi Usuki, Hoa Trong Tran, Nguyen Hoang, Masako Usuki, Pham Minh, Anh Thi Quynh Nong,
Y. Viet Nguyen & Pham Trung Hieu

Cenozoic magmatism in Northern Vietnam is suggested to have a genetic connection with the Indian–Eurasian collision and is associated with the formation and evolution of the Red River shear zone and adjacent structures. Cenozoic biotite and porphyritic granites are mainly recognized in the northwestern part of the Phan Si Pan uplift, with a smaller extent than previously thought, and crop out as small intrusions and dykes. Their geochemical features are similar to the high-K calc-alkaline magmatic series, which were emplaced between 35 and 31 Ma (zircon U–Pb ages), coincident with the timing of the early large displacement on the Red River shear zone. A crustal source of the Cenozoic granites is evidenced by high-Al and alkaline affinities, enrichment in LILE (Sr, Ba, and K) and LREEs, depletion in Nb, Ta, and Zr, high initial 87Sr/86Sr isotopic ratios (0.706570–0.719020), low εNd(t) values (−14.5 to −3.6), and variable zircon εHf(t) values (−16.2 to + 9.0) with whole-rock Nd and zircon Hf isotopic model ages mainly ranging from 0.341 to 1.597 Ga. The geochemical, Sr–Nd–Hf isotopic compositions and radiometric ages recorded by zircon U–Pb isotopic dating propose that the Cenozoic granitic magmas in the Phan Si Pan uplift were produced by partial melting of the lower-crustal materials driven by upwelling lithospheric mantle during the post-collisional stage of the India–Eurasia accretion. The Red River shear zone might have played a role in channelling large lateral magma migration and emplacement during magmatic formation.

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