Emplacement age and tectonic implications of the Xilinhot A-type granite in Inner Mongolia,China

A new rock type of granite, approximate 45 km² in area and located about 10 km south of Xilinhot, Inner Mongolia, was found in the Sunitezuoqi (or called Su-zuoqi)-Xilinhot tectonic belt and identified as an A-type miarolitic intrusion. The pluton has miarolitic structure and is composed chiefly of perthite, quartz, euhedral albite and potassium feldspar. Various types of textures occur in the pluton, such as perthitie, graphic and myrmekite textures. Only quartz is found in miarolitic cavity. This A-type granite with seagull-shaped REE patterns and obvious negative Eu anomalyδEu = 0.24–0.28) is high in SiO₂ (76%–77%), K and Na (Na₂O + K₂O = 7.75%–8.15%) and low in Ca (CaO = 0.20%–0.22%), Fe and Mg. Both petrographical observations and chemical compositions indicate that it is an A-type granite. Zircon SHRIMP U-Pb analyses indicate that this A-type granite was emplaced at 276 ± 2 Ma and coeval with the same type of granites in the adjacent areas. Therefore, it suggests that this pluton was likely formed in a post-orogenic extensional setting and probably related to break-off of subducted slabs in Central Asian Orogenic Belt (CAOB), which indicate that the Sunitezuoqi-Xilinhot belt was tectonically evolved into post-orogenic stage since early Permian.