Dahongshan area, located on the southern margin of the Tongbai-Dabie orogenic belt, has extensively exposed metamorphic gabbro-diabase. This dike intruded into the accretionary complex of the Dahongshan orogenic belt. In this paper, zircon U-Pb LA-ICP-MS dating method shows that the emplacement age of rock vein is 163±3 Ma. The age of this mafic dike is one of the earliest known records of Late Mesozoic magmatic activities after the collision in the Tongbai-Dabie orogenic belt, suggesting that the Late Mesozoic magmatic activities in the Tongbai-Dabie orogenic belt started in late period of Middle Jurassic, which is similar to the time of initial anatexis in northern Dabie orogenic belt, and is about 30 Ma earlier than the peak period of the Early Cretaceous magmatism (~130 Ma). The analytical results of Hf isotope from zircon show that εHf(t) is +5.8~+10.8 (with an average +9.7±0.7), and the corresponding single stage Hf model age t_DM1 is 387~595 Ma (mean value being 434±29 Ma), obviously larger than the age of diagenesis. These data show that the source of magma was derived from depleted mantle, resulting from the remelting of the Early Paleozoic partially melted depleted mantle basaltic rocks. All the data show that 163±3 Ma is one of the oldest magmatic rock records after the continent-continent collision of the Mesozoic Tongbai-Dabie orogenic belt, and this age is in agreement with that of root softening in Dabie orogenic belt. It possibly represents the conversion time of Dabie orogenic belt from compressional environment to extensional environment. It is therefore considered that the interval from root softening to large scale extensional collapse in Tongbai-Dabie orogenic belt is~30 Ma.