Zircon SHRIMP dating and geochemical characteristics of Neoproterozoic granites in southeastern Hunan

Abstract：There occur Neoproterozoic granitic bodies in Yueyang-Pinjiang area, northeast Hunan. Zircon SHRIMP U-Pb dating of the granite from Zhangbangyuan pluton was performed, yielding an age of 816±4.6 Ma. The rocks are lightly basic with an average SiO2 content of 70.25%, have low potassium and high calcium with an average CaO content of 2.38 %, and contain poor alkali with an average Na2O+K2O content of 5.97 % and the feature Na2O＞K2O. The ASI values are 0.97～1.29 with an average of 1.16. Thus the granites are of the magnesian peraluminous calc-alkaline series. Rb and Th are rich, while Ba and Nb are poor. The rocks have low ΣREE values with an average of 130 μg/g, (La/Yb)N values of 6.76～8.50, δEu values of 0.54～0.78, εNd (t) values of -1.5～1.6, T2DM ages of 1.35～1.60 Ga, and δ18OV-SMOW‰ values of 8.1～11.1. According to the above data, the granites should belong to I-type or ACG formed by crust-mantle magmatic mixing, and the main source was derived from the depleted mantle with the participation of a small amount of crustal matter. Geochemical characteristics suggest that the granites are of the island arc volcanic system, and derived their materials from magmatism in the convergent margin of the plate instead of from mantle plume magmatism. Hence they cannot serve as the indicator of Rodinia Breakup.