We present LA-ICP-MS zircon U-Pb dating, Hf isotopic and geochemical data for quartz diorite porphyries from Dexing porphyry Cu deposit to constrain their petrogenesis and ore-forming contribution of mantle-derived mafic magma. Zircon U-Pb dating yield age of 169 Ma, in accordance with that of ore-forming granodiorite porphyries, indicating that they were emplaced during the middle Jurassic. The quartz diorite porphyries are characterized by low SiO2 (58.41%~63.12%), K2O (1.68%~2.94%) contents and A/CNK (0.85~1.04), belonging to the calc-alkaline to high-potassium calc-alkaline rocks. They are enriched in large ion lithophile elements and light rare earth elements but depleted in high field strength elements and heavy rare earth elements. They yield variable zircon εHf(t) values (from 2.20 to 7.93), showing that they were crystallized from lithospheric mantle-derived magmas. Zircon grains from two quartz diorite porphyries show significant positive Ce anomaly. The calculated magmatic oxygen fugacity (logfO2) varies from ?20.05 to ?6.66, reaching the hematite to magnetite oxygen fugacity buffer, indicating that they were crystallized from magmas with high oxygen fugacity. Geochemical results show that quartz diorite porphyries, granodiorite porphyries (and the enclosed enclaves) are in accordance with magma mixing between mantle-derived mafic magma and crustal felsic magma, and quartz diorite porphyries represent the mafic endmember. These results suggest that quartz diorite porphyries in the Dexing deposit were generated by melting of subduction-modified lithospheric mantle, which was triggered by upwelling of the asthenospheric mantle related to the extensional setting during the middle Jurassic in South China. Underplating of the mantle-derived mafic magmas could have provided heat for melting of the lower crust and subsequently mixed with the crust-derived felsic magmas, which generated the parent magmas of ore-forming granodiorite porphyries. In addition, the magmatic system of quartz diorite porphyry was highly oxidized, which was favorable for preventing the early sulfide saturation and contributing volatiles and metals to the Dexing ore-forming systems.