The link between subduction-modified lithosphere and the giant Dexing porphyry copper deposit, South China: Constraints from high-Mg adakitic rocks

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The giant Dexing porphyry copper deposit is one of the largest porphyry copper deposits in South China. This paper presents new LA-ICP-MS zircon U–Pb dating, element and Sr–Nd–Hf isotopic data for the ore-related porphyries in the Dexing porphyry deposit. The Dexing ore-related porphyries were emplaced during the Middle Jurassic (ca. 170Ma), and were characterized by high-Mg adakitic geochemical features, including high MgO and Sr contents and high Mg#, Sr/Y and LaN/YbN ratios, low Y and Yb contents, and lacking a negative Eu anomaly, which suggest that they were probably derived from the delaminated thickened lithosphere. A non-arc setting in the Middle Jurassic is indicated by the absence of arc rocks and the presence of rifting-related igneous rock associations in the interior of South China. The non-arc setting suggests that the copper-rich lithosphere in the Dexing area was enriched by an ancient subduction event prior to the Late Mesozoic. The regional geological history and the Neoproterozoic two-stage Nd–Hf isotope model ages imply that the ancient subduction event occurred in the Neoproterozoic in response to the Jiangnan Orogen between the Yangtze and Cathaysia blocks. Thus, we infer that the giant Dexing porphyry copper deposit was formed in the Middle Jurassic by remelting of the copper-rich Neoproterozoic subduction-modified lithosphere. Previous studies of the low-Mg ore-related porphyries in the adjacent Yinshan copper polymetallic deposit proposed a copper-rich juvenile crust as their magma source. In this study, the high Mg contents of the Dexing ore-related porphyries stress the significant contribution of the subduction-modified lithospheric mantle or a higher melting temperature.