Petrogenesis of the Middle Jurassic andesitic dikes in the giant Dexing porphyry copper ore field, South China: Implications for mineralization

摘要

The giant Dexing porphyry copper ore field, containing 9.7 Mt copper, is the largest porphyry copper system in South China. The petrogenesis of andesitic dikes in the Dexing ore field and their genetic link with porphyry mineralization remain unclear. This paper presents LA-ICP-MS zircon U-Pb dating, elemental and Sr-Nd-Hf isotopic data of the andesitic dikes in the Dexing porphyry copper ore field. The Dexing andesitic dikes were emplaced during the Middle Jurassic at ca. 166 Ma. The Dexing andesitic dikes have relative high MgO contents and Mg# ratios but low TiO2 contents. They also have obvious Nb, Ta and Ti depletion in the primitive mantle normalized spider diagrams. The geochemical affinity of arc magma and low Ti contents indicate that the magma source is probably lithosphere mantle modified by subduction process. A non-arc setting during the Middle Jurassic is inferred due to the absence of typical arc rocks and the presence of rifting-related magmatic rocks in the interior of South China. The non-arc setting implies that the Neoproterozoic Jiangnan Orogen was likely responsible for the arc geochemical features. The concentration of copper in the andesitic dikes is not high, supporting that remelting of the metal-rich Neoproterozoic juvenile lower continental crust instead of the mantle is the critical factor for the giant porphyry style mineralization. The mantle derived mafic magma may play a key role for providing significant heat for remelting of the metal-rich lower continental crust via underplating into the crust-mantle boundary.