Yangshan A-Type Granites in the Lower Yangtze River Belt Formed by Ridge Subduction: Radiogenic Ca and Nd Isotopic Constraints

The Early Cretaceous aluminous A-type granites in the Lower Yangtze River belt (LYRB) can provide important insights into the Mesozoic magmatism in eastern China, but their origin remains highly controversial. In this study, radiogenic Ca-Nd isotopic analysis was performed for syenite porphyry and alkali-feldspar granite porphyry of the Yangshan pluton, a typical aluminous A-type granitic intrusion in the LYRB, to constrain its source and geodynamic setting. The results show that εCa(126 Ma), εNd(126 Ma) and K/Casource of the syenite porphyry range from −0.24 to +0.96, −7.2 to −6.0, and 0.31 to 1.26, respectively. The corresponding values for the alkali-feldspar granite porphyry range from 0.26 to 0.84, −8.0 to −6.1, and 0.79 to 1.08, respectively. Binary mixing modeling indicates that they were originated from the same sources with different proportion, namely, a mixing of 50% to 75% Neoproterozoic crust and 50% to 25% asthenospheric mantle. Together with previous works, we propose that the Early Cretaceous subduction of the ridge between the Pacific and Izanagi plates was responsible for the formation of the aluminous A-type granites in the LYRB.