Geochemistry and genesis of the Nadun Nb-enriched arc basalt in the Duolong mineral district, western Tibet: Indication of ridge subduction

The Duolong mineral district in western Tibet is one of the largest porphyry Cu–Au deposit fields with significant metallogenic potential in China. Its tectonic environment relevant to Early Cretaceous Cu–Au mineralization remains controversial. Here we report new whole-rock major and trace element, and Sr-Nd-Hf-Pb isotopic data for the newly discovered basalt in the Nadun area, Duolong mineral district, to decipher their genesis and further constrain the tectonic environment. A contemporaneous rhyolite sample interbedded with the basalt in the lower part of the volcanic section in the Nadun area yields an LA-ICP-MS zircon U–Pb age of 122.5 ± 1.2 Ma. The basalt samples exhibit high-K calc-alkaline/shoshonite properties and are enriched in high field strength elements, e.g., high TiO₂ (1.43–1.79 wt.%) and Nb (14.6–19.5 ppm) contents, with high Nb/La ratios (0.4–0.6), which are compositionally comparable to those of Nb-enriched arc basalts (NEABs). The (⁸⁷Sr/⁸⁶Sr)_i ratios of 0.7052 to 0.7056, negative ε_Nd(t) (− 0.7 to − 0.2) and ε_Hf(t) values (+ 6.0 to + 6.5), and high (²⁰⁶Pb/²⁰⁴Pb)_i, (²⁰⁷Pb/²⁰⁴Pb)_i, (²⁰⁸Pb/²⁰⁴Pb)_i and ratios (18.522 to 18.561, 15.641 to 15.645 and 38.679 to 38.730, respectively) suggest that the Nadun NEABs are more enriched than those of the island arc basalts (IABs) in the area. The slightly enriched radiogenic isotopes for the Nadun NEABs indicate that the subducting sediments play an important role in the source. Furthermore, their high Nb, Ti, and Cu contents indicate that the source mantle wedge was metasomatized by slab melts. The Nadun NEAB and other coeval magmatic rocks in the Duolong mineral district, including adakite, OIB-like basalt, MORB-type basalt, A-type rhyolite, and common IAB, are typical rock assemblages of ridge subduction. We infer that the Duolong mineral district were formed by ridge subduction in the Early Cretaceous.