Qinghai Provincial Bureau of Nonferrous Metaland Geological Exploration, Xining 810001, China

Zircon U-Pb geochronology and geochemistry of the granodiorites in Xiarihamu Cu-Ni mining area are studied to determine the formation age, genesis and tectonic setting. The results show that the granodiorites formed in the Middle Triassic with weighted average age of 242±1 Ma. The granodiorites comprise of SiO₂ of 61.34%-62.54%, Al₂O₃ of 16.46%-16.87%, MgO of 2.37%-2.66%, (Na₂O+K₂O) of 5.39%-5.74% with the Na₂O/K₂O of 1.46-1.88, Mg^# of 47.03-48.04, and show metaluminous characteristics with A/CNK of 0.94-0.96, belonging to calc-alkaline I-type granites geochemically. The light and heavy rare earth elements in the rocks are distinctly fractionated and have "right-inclined" distribution patterns with weak negative Eu anomalies (δEu=0.84-0.91). Moreover, these samples show enrichments in LILEs (e.g., Rb, Ba, K and Sr), especially in Sr[(343-452)×10^-6], and obvious depletions in HFSEs (e.g., Nb, Ta, Ti and P), indicating geochemical characteristics similar to subduction-related magmatic rocks. According to the regional tectonic evolution, the Xiarihamu granodiorites were emplaced under an Andean-type active continental margin environment in the early Indosinian, possessing a crust-mantle mixing origin.