Geochronology and geochemistry of Mesozoic dykes in the Qingchengzi ore field, Liaoning Province, China: Magmatic evolution and implications for ore genesis

The Qingchengzi Pb-Zn-Au-Ag-Mo polymetallic ore field located in the Liaodong Peninsula, Liaoning Province, China, is spatially associated with multiple magmatism, which provide a classic example for studying the multiple tectonic and metallogenetic events. In this paper, we report some new geochemical and geochronological data of igneous rocks in the Qingchengzi ore field, including diabase, lamprophyre, microdiorite, granite porphyry, and quartz porphyry. Zircon U-Pb dating yielded the formation ages of these rocks as Triassic, Jurassic, and Early Cretaceous. Low-Mg adakitic granite porphyries formed at 233 Ma have zircon epsilon(Hf)(t) values of -17.7 to -14.5, suggesting that they were mainly derived from partial melting of thickened lower crust. Mafic dykes formed during 226-203 Ma have zircon epsilon(Hf)(t) values of -28.6 to -12.8, suggesting that they were mainly derived from partial melting of ancient lithospheric mantle. Zircons from the 168Ma Jurassic diabase have high Nb/Ta and low Nb/Th ratios, with zircon epsilon(Hf)(t) values of -25.2 to -18.5. This suggests that ancient lithospheric mantle was influenced by melts/fluids from the subducted Palaeo-Pacific Plate. The Early Cretaceous microdiorites (126 Ma) are high-Mg adakite rocks, with zircon epsilon(Hf)(t) values of -27.3 to -19.6. Melting of delaminated eclogitic lower crust might produce these high-Mg adakitic microdiorites. The upwelling asthenosphere could heat the residual lower crust to form the quartz porphyries. The Early Cretaceous diabase (100 Ma) shows low Nb/Ta and high Nb/Th ratios, with zircon epsilon(Hf)(t) values of -0.7 to 5.8, implying a depleted mantle source. The characteristics of Triassic-Cretaceous intermediate-mafic intrusions show that the lithospheric mantle evolved from enriched ancient mantle to depleted juvenile mantle. This transformation of the lithospheric mantle might have resulted from the delamination of the lower crust that was caused by subduction of the Palaeo-Pacific Plate. Based on the field observation and geochemical data, Au, Pb-Zn-Ag, Mo mineralization in the Qingchengzi ore field show closely spatial and genetic relationship with intrusive rocks, indicating that the Mo, Pb-Zn-Ag mineralization might be related to the Jurassic magmatic activity, whereas Au mineralization related to the Early Cretaceous magma.