U-Pb Zircon Age,Geochemical and Sr-Nd-Hf Isotopic Compositions of Neoproterozoic Granitoids in Northwestern Margin of Yangtze Block （South China）:Implications for Neoproterozoic Tectonic Evolution

摘要

The widespread Neoproterozoic magmatism along the Yangtze block carries critical in-formation for understanding the Neoproterozoic evolution of the Yangtze block. In the northwestern margin of the Yangtze block,the Hannan （汉南） intrusive complex includes the Wudumen （五堵门）,Erliba （二里坝） and Zushidian （祖师殿） granitoids. Using LA-ICP-MS U-Pb zircon dating method,the Wudumen and Erliba granitoids yielded magma crystallization ages of 785±4 and 778±3 Ma,respec-tively. Samples from these three granitoids show variable SiO2 contents ranging from 58.8% to 72.6%. They are characterized by enrichment of Al2O3 （14.97%-17.87%）,Na2O （3.80%-5.33%） and Sr （504 ppm-741 ppm）,and depletion of Y （＜19 ppm） and HREE （e.g.,Yb＜1.6 ppm）,resulting in high Sr/Y （29-161） and （La/Yb）N （7.3-27.8） ratios. The geochemical features of the granitoids are comparable with those of adakite. The granitoids have zircon εHf（t） values of +3.65 to +10.05,whole-rock εNd（t） val-ues of -0.09 to +2.98 and whole-rock initial 87Sr/86Sr ratios of 0.703 4-0.703 9,indicating that their magma was derived from a juvenile crustal source. Together with geochemical and Hf-Sr-Nd isotopic compositions,it is suggested that the granitoids formed in island-arc setting and originated from partial melting of a subducted oceanic slab. The results support a model that the Yangtze block was sur-rounded by ocean and arc magmatism in its northern and northwestern margins in Neoproterozoic.