Geochronology,geochemistry and geological significance of Early Cretaceous volcanic rocks from Niangniangshan Formation,Ningwu Basin,middle and lower reaches of the Yangtze River

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(ISr =0.707 197--0.707 878; εNd（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.