Petrogenesis and tectonic setting of the granites in Honggeertu area, central Inner Mongolia: Constraints from LA-ICP-MS zircon U-Pb chronology and geochemistry

Honggeertu granites, located in central Inner Mongolia and belonging to southern Solon-Linxi fault, is mainly composed of middle-fine to middle-coarse grained syenite granites and monzonitic granites. These granites have high SiO₂ (70.44%~78.80%) and are enriched in alkali (Na₂O+K₂O=7.46%~10.74%) but depleted in Mg, Fe and Ti. The A/CNK ratios are in the range of 0.95~1.41, alkali aluminum indexes (AKI) are 0.68~0.97, and alkalinities (AR) are 3.30~6.68, suggesting that the granites belong to weakly peraluminous-peraluminous alkaline series. The granite intrusion has widely varying total REE values (ΣREE=25.96×10^-6~654.50×10^-6), and is enriched in LREE and depleted in HREE with negative Eu anomaly (δEu=0.03~0.89). It is relatively enriched in high field strength elements (Th, U, Hf, Ta and Y) and depleted in large ion lithophile elements (Ba, Eu and Sr). The values of high field strength elements (Zr+Nb+Ce+Y) are higher than 350×10^-6. The rocks belong to highly fractionated I-type granites formed in a post orogenic tectonic environment. Zircon LA-ICP-MS dating yielded ²⁰⁶Pb/²³⁸U weighted average ages, i.e., 267.2Ma±1.4Ma (MSWD=1.5), 269.2±1.6Ma (MSWD=1.7) and 272.1±1.2Ma (MSWD=0.38), indicating that the rock was formed in the Middle Permian. The new data suggest that the collision between NCC and Siberian Craton was earlier than the formation of the rock, and should be at least as early as 267.2~272.1Ma.