Discovery of the Siling aluminous A-type granite in Hangzhou and its tectonic significance

Abstract：The Siling rock mass, which was formerly regarded as typical S-type granites or reworked anatectic granites, should be aluminous A-type granites. This rock mass is rich in silica, alkalis and potassium and distinctively poor in calcium and magnesium; it has high K2O+Na2O/ CaO values (average 16.79) and a high AKI index (average 0.92)；high FeO?鄢/MgO ratios (average 13.95), higher than that of M-type, S-type and I-type granites but close to the average value (13.4) of A-type granites worldwide；the rock is weakly alkaline, metaluminous-weakly peraluminous quality, rich in REE and HFSE , high in Rb, F and Nb, which are also the characteristics of A-type granites. The δ18O values of the Siling rock mass range from +8‰ to +9‰. All these suggest that the magma of the granites was mainly derived from the lower crust. The rock mass was emplaced into an extensional environment, which was an important tectono-magmato-thermal event in the early Late Cretaceous. It also indicates that the regional Xiaofeng-Sanmenwan fault and Xuechuan-Huzhou fault were in their extended stage in the early Late Cretaceous.