Newly Identified A-type Granite in Western Yangtze Block and Its Geochemistry Characteristics

Abstract

Numerous Neoproterozoic granitic intrusions occur over a north-south distance of more than 700 km in western Yangtze Block. In this paper, we newly identified Shimian granitic pluton as A-type granite which was previously considered as S-type granite in the region, with presenting its zircon U-Pb age, Lu-Hf isotopic and whole-rock geochemical data. The selected fresh samples for this study are mainly porphyritic biotite monzogranites. They are weak peraluminous, A2-subtype granites, displaying high contents of SiO2 and K2O, low Cao and Na2O with the characteristics of high silica, rich potassium, and low sodium. They are enriched in incompatible trace elements, but low in trace elements compatible in mafic silicates and feldspars, and chemically characterized by having high REE and Ga abundances and low Al, Ni, Ba, Sr, and Eu abundances. The major and trace element behavior suggests that the granites are formed by advanced fractional crystallization. Zircon U-Pb age shows the Shimian granitic pluton was emplaced at 728 ± 20 Ma. The studied granites yield an average 176Hf/177Hf value of 0.282363 (εHf = 0.8) with a 94% confidence interval of 0.282334 to 0.282384 (εHf = -0.1 to 1.7), indicating that they are derived from remelting of juvenile continental crust. We suggest that this newly identified A-type granite at Shimian, southwestern China was more plausibly generated in an extensional setting as a result of slab window caused by a ridge subduction, which can account not only for the formation of voluminous granitoids, but also for the basalts, boninitic pillow lavas, and SSZ-type ophiolites in western Yangtze Block.