Magma oxygen fugacities of granitoids in the Xiaoqinling area, central China: implications for regional tectonic setting

Abstract

The trace element compositions of zircons from four granitoids/metagranitoids in the Xiaoqinling area in the TransNorth China Orogen of the North China Craton were determined by in-situ laser ablation (LA) ICP-MS. The results were used to compute the zircon Ce anomaly, Ti-in-zircon temperature, and the magma oxygen fugacity. Generally, the Ce anomalies determined with La-Pr interpolation are prone to be underestimated due to the influence of tiny inclusions or analytical errors. This underestimation can markedly influence the data processing and generate unrealistically low oxygen fugacity values. In contrast, the lattice strain model yields more reliable results, and offers a robust tool in determining the magma oxidation state. The calculated magma oxygen fugacities are in the range of FMQ-1.41 to +3.21, FMQ-3.25 to +3.74, FMQ +3.66 to +5.27, and FMQ +2.51 to +4.67, respectively. The high average oxygen fugacities of FMQ +4.51 and FMQ +3.46 for a 1881 Ma old pegmatitic granite and 1866 Ma old granitic veins suggest these rocks were probably formed in arc setting, which further supports the ~1.8 Ga collision between the Western and Eastern Blocks of the North China Craton.