Petrogenesis of Early Cretaceous Duorenlieqian igneous rocks (113 ∼ 117 Ma) in the western Bangong-Nujiang metallogenic belt, Tibet, China: Implications for tectono-magmatic evolution and porphyry Cu-Au mineralization

Abstract

Porphyry copper deposits are classic products of arc magmatic activity at convergent plate boundaries. Understanding the signatures of arc magmas is essential for reconstructing the dynamic evolution of plate boundaries and predicting copper resource potential. The southern Qiangtang subterrane in the Bangong-Nujiang metallogenic belt hosts several subduction-related porphyry and epithermal copper-gold deposits. However, such deposits are quite scarce in the northern Lhasa subterrane, where the potential for subduction-related porphyry copper mineralization remains poorly understood. Here we present zircon U-Pb geochronology, trace elements, Hf isotopes, and whole-rock geochemical compositions for newly discovered mafic to felsic igneous rocks in the Duorenlieqian area, northern Lhasa subterrane. The zircon U-Pb geochronology results show that the hornblende gabbro, diorite porphyry and quartz diorite formed at 114.4 ± 0.9 Ma, 116.8 ± 1.5 Ma, and 113.4 ± 1.7 Ma, respectively. The hornblende gabbro and diorite porphyry exhibit arc magmatic geochemical affinities with low SiO₂ contents (48.4–53.0 wt% and 52.3–56.4 wt%, respectively), high Mg^# values (49–51 and 46–50, respectively), depleted whole-rock Sr-Nd isotopic compositions [(⁸⁷Sr/⁸⁶Sr)_i = 0.7040 to 0.7043, 0.7053 to 0.7055, respectively; ε_Nd(t) = +4.20 to + 5.96, +2.72 to + 5.06, respectively] and zircon Hf isotopic compositions [ε_Hf(t) = +11.8 to + 15.4, +9.1 to + 11.3, respectively]. Additionally, they have high Th/Ce and Th/Nb ratios, as well as variations in Nb/Zr ratios. These characteristics suggest that they were likely derived from mantle wedge metasomatized by hydrous sediment melts. The quartz diorite, sharing arc magmatic affinities but relatively more enriched whole-rock Nd isotopic compositions (+2.09 to + 3.48) and lower Mg^# (34–48) values compared to the gabbro and diorite porphyry, is interpreted to be derived from partial melting of juvenile lower crust with minor mantle input. Based on our new data, integrated with recent studies, we propose that during the same geological period (113 ∼ 117 Ma), the Bangong-Nujiang Ocean basin exhibited a diachronous geodynamic background from east to west, characterized respectively by lithospheric delamination, slab break-off and slab roll-back. This provides new evidence for the diachronous closure of the Bangong-Nujiang Ocean from east to west during the Cretaceous. The diorite porphyry and quartz diorite have similarly high zircon Eu_N/Eu_N* ratios (averaging 0.62 and 0.56, respectively) that are comparable to those of fertile igneous rocks formed in comparable tectonic settings, but they contain notably higher zircon Ti contents (averaging 15.5 ppm and 9.5 ppm, respectively). This suggests that although these magmas are relatively water-rich, they have not yet reached the super-wet conditions that are fundamental to porphyry copper deposit mineralization. Meanwhile, both the diorite porphyry and quartz diorite exhibit lower degrees of magma evolution and oxygen fugacity than typical subduction-related fertile magmas. These features indicate that they may represent precursor igneous rocks for the development of porphyry copper mineralization. Consequently, we suggest that the search for such deposits in the Duorenlieqian area or the western segment of northern Lhasa subterraneshould be focused on more evolved igneous units of similar age to the quartz diorite.