Characterization and origin of the Taishanmiao aluminous A-type granites: implications for Early Cretaceous lithospheric thinning at the southern margin of the North China Craton

Late Mesozoic magmatic rocks from the Taishanmiao Batholith were collected for LA–ICP–MS dating, Sr–Nd–Hf isotope systematics, and whole-rock major and trace element geochemistry to help understand the nature of collisional and extensional events along the southern margin of the North China Craton. The batholith consists of three texturally distinguishable phases of a 125 ± 1 Ma medium- to coarse-grained syenogranite, a 121 ± 1 Ma fine- to medium-grained syenogranite, and a 113 ± 1 Ma porphyritic monzogranite. Most of the units in the batholith are syenogranitic in composition with high levels of silica (70–78 wt% SiO2), alkalis (8.0–8.6 wt% Na2O + K2O), Fe* (FeOT/(FeOT + MgO) = 0.76–0.90), and depletion in CaO (0.34–1.37 wt%), MgO (0.12–0.52 wt%), TiO2 (0.09–0.40 wt%), and A/CNK (Al2O3/(Na2O + K2O + CaO)) molar ratios of 1.00–1.11. All samples have high proportions of Ga, Nb, Zr, Ga/Al, and REE, and depletions in Ba, Sr, Eu, and compatible elements, indicating that the batholith consists of A-type granites. The zircon saturation temperature for these units yields a mean value of 890 °C, and zircons with Early Cretaceous magmatic ages have εNd(t) values of −14.0 to −12.0, εHf(t) values ranging from −18.7 to −2.1, and corresponding Hf model ages of 2339–1282 Ma. These geochemical and isotopic characteristics allowed us to conclude that the primary magma for the Taishanmiao Batholith originated from partial melting of Precambrian crustal rocks in the medium-lower crust. However, the high Nb and Ta contents and low normalized Nb/Ta values for the Taishanmiao granites are due to fractionation in Nb- and Ta-rich amphibole (or biotite). It is further proposed that these aluminous A-type granites were generated in an extensional tectonic setting during the Early Cretaceous, which was induced by lithospheric thinning and asthenospheric upwelling beneath eastern China toward the Paleo-Pacific Plate.