Genesis and Metallogenic Potential of the 80 Ma High Sr/Y Anglonggangri Granodiorite in the Northwestern Lhasa Block, China

Abstract

Multiple instances of the Late Cretaceous granodiorites within the Anglonggangri region of the northwestern Lhasa Block were identified, their petrogenesis were explored and mineralization potential were assessed. The zircon U-Pb dating of the Anglonggangri granodiorites revealed ages of 82.8 and 80.8 Ma. Granodiorite samples have SiO₂ contents of 64.36–68.33 wt%, with high Sr/Y (55–95) and A/CNK ratios (0.99–1.06). Zircon ε_Hf(t) values range from –0.3 to +16.2. Two granodiorite samples have (⁸⁷Sr/⁸⁶Sr)_i values of 0.7034 and 0.7043 and positive ε_Nd(t) values of 3.51 and 3.83. These geochemical properties indicate that they are adakitic rocks formed by partial melting of the juvenile thickened lower crust, slightly contaminated with material from the mantle due to the small-scale delamination of the lower crust. The zircons in the granodiorites have moderate Ce/Nd (2.5–43), logfO₂ (–20.0 to –9.6), and ΔFMQ (–1.28 to +4.00) values; low (Ce/Nd)/Y (0.001–0.049) ratios; and high Dy/Yb (0.17–1.16) ratios, which indicate that these granodiorites exhibit moderate oxygen fugacity and lower magma water content than the Miocene Gangdese porphyry copper deposits associated with high-Sr/Y granites. Their ability to create porphyry-type copper deposits could have been hampered by their low magma water content and moderate oxygen fugacity.