The Dexing porphyry copper deposit, Jiangxi Province, is the largest porphyry copper deposit in SE China. Considerable studies have been done for the ore–bearing granodiorite porphyries, ore–forming source and ore–forming fluids and etc. However, there are few researches for the quartz diorite porphyrites occurred as veins in this copper deposit. In this study, we have carried out the SHRIMP zircon U–Pb geochronologic analyses for the diorite porphyrite veins from the Tongchang mine of the deposit. Our new SHRIMP U–Pb dating yields a good concordant 206Pb/238U weighted mean age of 153.5 ± 2.4 Ma (2σ, MSWD = 2.5), suggesting that the magmatism occurred in the late Jurassic, emplaced in a back–arc or intra–arc extension environment triggered by the subduction of palaeo–Pacific plate. The high εNd(t) (～0.5) and εHf(t) values (～4.73) possibly indicating that these intrusions have a crust–mantle mixing source. Thus, the Dexing area had not only gone though a continental arc setting in the early period (170～171 Ma), but also gone though an back arc extension setting in the late period (～153.5 Ma), forming the ore–bearing granitic intrusions and ore–barren dioritic intrusions, respectively.