Geodynamic Model for the Formation of Neoproterozoic A-type granites in the Western Yangtze Block

Traditionally called the Jingning-Chengjiang period S-typegranitic belt in the western margin of Yangtze Block may haveformed by different tectonic evolutional processes. The newlyidentified A-type granite in this granitic belt is a good diag-nostic indicator of tectonic setting and provides insight intounderstanding these processes. We obtained zircon U-Pb age,elemental, and isotopic geochemistry data of granites fromShimian pluton, revealing that these traditionally thought S-type granites have A-type affinity. Furthermore, we concludethat Neoproterozoic Shimian A-type granite originated fromthe partial melting of a Mesoproterozoic juvenile felsic uppercrust heating by the upwelling of asthenosphere through theslab window. The formation of Neoproterozoic A-type granitesin the region is probably linked to an ancient ridge subduction.