Abstract:

The age of the Jingangtai volcanic rocks, which spread over the central part of North Huaiyang volcanic rock belt, has been in controversy for a long time. New observations indicate that Jingangtai volcanic rocks unconformably overlie the early Cretaceous Shangcheng Pluton, hence they must have not  formed in the Jurassic. A SHRIMP UPb dating on individual zircons from three representative samples of Jingangtai volcanic rocks shows that the age of extrusive biotite trachyte (B09) is (1284±36) Ma, the age of subvolcanic diorite porphyrite (B21) is (1291±22) Ma and the age of trachyandesite (B26) representing the lower part of volcanic rock series is (129±2) Ma. The dating error for sample B09 is relatively large, however, the age of sample B21 is consistent with B26, which suggests that Jingangtai volcanic rocks formed at 129 Ma, and the duration of volcanic eruption was short. Jingangtai volcanic rocks are the products of largescale magmatism in Dabie Orogen in the Early Cretaceous. Combined with the foregoing research works, the regional geological features and the denudation mechanism of Shangcheng Pluton show: that (1) before the forming of Jingangtai volcanic rocks, the region had undergone intensively compressional uplift; that (2) Jingangtai volcanic rocks are the products of lithosphere delamination; that (3) after the forming of Jingangtai volcanic rocks, the regional lithosphere had been extended for a long time, thus 129 Ma is the boundary of tectonic transition for Dabie Orogen.

Key words: Dabie Mountains, Jingangtai volcanic rocks, SHRIMP zircon U-Pb dating, Early Cretaceous, tectonic transition