Zircon U-Pb age of the Caledonian volcanic rocks in Nanjing basin of southern Jiangxi Province and its geological significance

Nanjing basin is located in the central and west part of the Caledonian fold belt in South China, and the volcanic rocks in the basin are often characterized by strong weathering and poor exposure, therefore long considered to be part of the Mesozoic rocks. The volcanic rocks exhibit conformable contact with the underlying Late Ordovician strata, and unconformable contact with the overlying Middle Devonian strata. These rocks are supposed to be Early Paleozoic volcanic rocks, but without precise chronologic and rock lithogeochemical constraints. This paper reports zircon U-Pb chronology, Sr-Nd-O isotopes and geochemistry of volcanic rocks in Nanjing basin. The zircon SHRIPM U-Pb ages of andesite and dacite are 442.1±3.9Ma and 439.9±3.7Ma respectively, indicating a late Late Ordovician-Early Silurian magmatic event. Trace element analyses show enrichment of Rb, K, LREE, and depletion of Nb, Ta, Zr, Hf, Ti, low ratios of Nb/U, Nb/Th, Ti/Yb and strong fractionated REE(LREE/HREE=7~9) and negative Eu anomalies (0.6~0.76), with the characteristics of depletion of Ti(0.67%~0.79%), richness of Al(14.3%~16.1%). It is thus concluded that the crustal thickening, lithospheric mantle delamination and upwelling of the asthenosphere resulted from the partial melting of the EMI-type mantle and the lower crust with the closure of the South China Ocean and the collision of the Yangtze block with the Huaxia block during the Early Paleozoic.