Generation and redox state of two episodes of Early Cretaceous granitoid in the Xiaoqinling area, North China Craton: Implications for lithosphere thinning and metallogenic potential

Abstract

The late Mesozoic granitoids in the Xiaoqinling area are of great significance for their associated gold deposits and for understanding the geodynamics of craton destruction. The Niangniangshan pluton in the Xiaoqinling area records three phases of magmatic activity, with biotite monzogranites formed in Phases1 and Phases2 and monzogranites in Phase3. Field investigations and LA–ICP–MS zircon UPb dating indicate that the Niangniangshan pluton was formed during two episodes of magmatism at ~134 Ma (Phase1 and Phases2) and ~ 123 Ma (Phase3). The ~134 Ma granitoids exhibit higher Sr/Y ratios (57–164) and higher contents of Sr (563–1114 ppm) and Ba (1455–1958) than the ~123 Ma granitoids (Sr = 88–124 ppm, Ba = 60–117 ppm, Sr/Y = 16–52). In addition, the ~134 Ma granitoids do not display significant Eu anomalies (0.75–1.06), while the ~123 Ma granitoids show negative Eu anomalies (0.73–0.84). However, all of the granitoids have low contents of MgO, Cr, Co and Ni, relatively high initial 87Sr/86Sr ratios (0.7072 to 0.7108), and low values of εNd(t) (−16.97 to −14.49) and εHf(t) (−29.62 to −14.41). These geochemical characteristics indicate that the ~134 Ma granitoids were generated by partial melting of the ancient metamorphic basement at high pressures, whereas the ~123 Ma granitoids were formed by partial melting of the ancient metamorphic basement at relatively low pressures. The differences in the formation of the two episodes of granitoid provide further evidence that there was a transformation from lithospheric thickening of the North China Craton in the Xiaoqinling area to a stage of lithospheric thinning, this probably taking place between ~134 and ~ 123 Ma. Zircon trace-element geochemistry indicates that the ~123 Ma granitoids had higher oxygen fugacities and zircon crystallisation temperatures than the ~134 Ma granitoids. We also found that ΔFMQ values show a positive correlation with temperature for the ~134 Ma granitoids, but a negative correlation for the ~123 Ma granitoids. This indicates that the ~134 Ma magma became more reduced with evolution whereas the ~123 Ma magma became more oxidised. These features, combined with the redox state of typical magmatic hydrothermal gold deposits globally, suggest that the ~123 Ma granitoids had a greater potential to form magmatic hydrothermal gold deposits than the ~134 Ma granitoids.