The Ascertainment of Late-Yanshanian A-type Granite in Tongbai-Dabie Orogenic Belt
Wang Qiang, Zhao Zhenhua, Xiong Xiaolin (Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou 510640)
The study and discussion of petrology and geochemistry have revealed that there exists two kinds of A_type granites in Tongbai_Dabie orogenic belt: peralkaline and aluminous granites. The peralkaline A_type granites include alkaline feldspar granite and quartz syenites, characterized by ACNK=0.72～0.97 and NKA=1.02. The aluminous A_type granites also include alkaline feldspar granite and quartz syenites, characterized by SiO 2=67.73%～77.60%, rich alkali, Na 2O+K 2O=7.97%～9.60%, ACNK=0.72～1.03, NKA=0.84～0.97, rich REE, δEu=0.12～0.57 and rich Zr, Nb, Ga and Y. Compared with high fractionated I_type granites, the high Si(SiO 272%) aluminous A_type granites show rich Fe (FeO T1.50%), high Zr+Nb+Ce+Y and Ga/Al as well as high zircon saturation temperature (845～880℃). The A_type granites were formed in a post_orogenic environment. It is suggested that Tongbai-Dabie orogenic belt entered the phase of extension and collapse in Late Cretaceous, during which the orogenic movement ended.