Mesozoic magmatism is widespread in the eastern South China Block and has a close genetic relationship with intensive polymetallic mineralization. However, proper tectonic driver remains elusive to reconcile the broad intracontinental magmatic province. This study presents integrated zircon U–Pb dating, Hf isotopes and whole-rock geochemistry of the Xiwan dioritic porphyry in the NE Jiangxi ophiolitic mélange. Zircon U–Pb dating by SIMS and LA-ICP-MS methods yielded an emplacement age of ~160 Ma for the Xiwan diorite, confirming its inclusion into the Mesozoic magmatic province in SE China, instead of a component of the Neoproterozoic ophiolitic mélange genetically. The dioritic rocks have low SiO2 (58.08wt%–59.15wt%), and high Na2O (5.00wt%–5.21wt%) and MgO (4.60wt%–5.24wt%) contents with low TFeO/MgO ratios (1.02–1.09). They show an adakitic geochemical affinity but exhibit relatively low Sr/Y ratios (24.8–31.1) and high Y contents (14.6–18.3 ppm) compared to the Dexing adakitic porphyries. In addition, the Xiwan diorites have moderately evolved zircon Hf isotopic compositions (?Hf(t)=?6.1–?0.1; TDM2=1597–1219 Ma). These elemental and isotopic signatures suggest that the Xiwan diorite formed through partial melting of a remnant arc lower crust (i.e., early Neoproterozoic mafic arc-related rocks) in response to the underplating of coeval mafic magmas. In conjunction with the temporal-spatial distribution and complex geochemical characteristics of the Mesozoic magmatism, our case study attests to the feasibility of a flat-slab subduction model in developing the broad intracontinental magmatic province in SE China. The flat-slab delamination tends to trigger an asthenospheric upwelling and thus results in extensive partial melting of the overlying lithospheric mantle and lower crustal materials in an extensional setting during the Mesozoic.