1. College of Earth Sciences, Jilin University, Changchun 130061, China;
2. School of Resource Environment and Earth Science, Yunnan University, Kunming 650500, China;
3. The First Geological Brigade, Jiangsu Bureau of Geology and Mineral Exploration, Nanjing 210041, China

Ningwu Basin is one of the Mesozoic continental volcanic basins in the middle and lower reaches of the Yangtze River. The volcanic rocks of the Longwangshan, dawangshan, Gushan and Niangniangshan formations, as well as the homologous subvolcanic rocks or small intrusions, are developed from old to new in the Ningwu Basin. Zircon U-Pb dating results show the latialite phonolite of Niangniangshan Formation was erupted at 128±1 Ma (i.e., Early Cretaceous). The latialite phonolite contains moderate SiO₂ contents (57.28%-60.96%) with high Na₂O + K₂O contents, belonging to shoshonite series. The samples have high REE contents, and display right-inclined REE distribution pattern. They are characterized by enrichment in some large ion lithophile elements (e.g., LILEs, Rb, K), and depletion in some high field strength elements (e.g., HFSEs, Nb, Ta, Ti). All volcanic samples have relatively depleted Nd isotopic compositions (I_{Sr =}0.707 197—-0.707 878; ε_Nd(t)=-0.5—-0.9), indicating no genetic relationship with the lower crust of Yangtze plate, but a drift trend towards the EMII. The geochemical data suggest that the Early Cretaceous latialite phonolite was derived from the partial melting of an enriched lithospheric mantle metasomatized by subduction-related fluids in an arc-related setting. Based on the temporal and spatial distribution and geochemical variation characteristics of the regional volcanic rocks, it is suggested that the tectonic system within the study area changed from a subduction-related compression to an extensional environment in the early Early Cretaceous, which was caused by the ridge subduction of the Paleo-Pacific Ocean.