Geochronology and petrogenesis of the Xiongmei Cu-bearing granodiorite porphyry in North Lhasa subterrane, central Tibet: Implication for the evolution of Bangong-Nujiang metallogenic belt

Abstract

Bangong-Nujiang metallogenic belt is a recently recognized post-subduction porphyry-skarn Cu polymetallic metallogenic belt in Tibet. The Xiongmei Cu-bearing granodiorite porphyry was newly found in the middle segment of this belt. In this study, we combined zircon U-Pb ages, Hf isotopic, REEs data, whole-rock Sr-Nd isotopic data, and whole-rock geochemical data to constrain the geochronology, petrogenesis and metallogenic features of the Xiongmei Cu-bearing granodiorite porphyry. Zircon U-Pb and whole rock Rb-Sr isochronal age show Xiongmei Cu-bearing granodiorite porphyry formed during 110–105 Ma. The zircon Hf isotope together with whole rock Sr-Nd isotopic features and high Mg# value (39–59), relatively high Cr (10.9–35.5 ppm) and Ni (10–224 ppm), indicating a melt deriving from mixing between mantle material and an ancient crustal source. The possibility of crustal addition in the magma is further supported by varied Eu anomalies (0.5–1.5), Sr anomalies (0.6–1.9) and high Al2O3 contents (15.50–17.25 wt%). Geochemical composition including K2O contents (0.52–4.72 wt%) and K2O/Na2O ratios (0.2–5.5), and the linear relations between LOI, Cu, Rb and Sr contents indicated that the magma sourced from melting of an amphibolite source, and further suggesting that the juvenile crust derived from an enriched lithospheric mantle. The ratios between incompatible elements together with the absence of Ce anomaly suggested the contribution of fluids from subducted oceanic crust rather than melt. The negative Eu and Sr anomaly features together with relatively high HREEs and Y contents, low (La/Yb)N and Sr/Y ratios are interpreted as low pressure partial melting. Combination with the regional geologic background, we adopted a slab break-off model to explains the diverse contemporaneous magmatic rocks recognized in the North Lhasa subterrane including that of the Xiongmei Cu-bearing granodiorite porphyry.