The formation age, origin and tectonic environment of gneissic granite in Xiaohuangshan, Beishan area, Inner Mongolia

Through a 1∶50000 regional geological survey, gneissic medium−fine−grained granodiorite, gneissic fine−grained monzogranite and gneiss−like intrusive rocks were newly disintegrated in the Xiaohuangshan area of Beishan area, Inner Mongolia. Porphyry medium−fine−grained biotite monzonitic granite, among which gneissic−to−medium−grained granodiorite and gneissic fine−grained monzonitic granite are closely associated in space. The formation age, origin and tectonic environment of gneissic granite in Xiaohuangshan area was studied in this paper. The zircon LA−ICP−MS U−Pb method was used to obtain magma crystallization ages of 443.0±1.2 Ma and 441.9±1.6 Ma respectively in gneissic medium−fine−grained granodiorite and gneissic fine−grained monzogranite. The era is early Silurian. Rock geochemical analysis shows that both gneissic medium−fine−grained granodiorite and gneissic fine−grained monzogranite are calc−alkaline series rocks, with a low alkalinity rate AR of 1.82 to 2.86, and a small range of variation. The Gordini index τ is 21.22 ~ 64.24 (τ>10); the rock is enriched in light rare earth elements and large ion lithophile elements such as K, Rb, and Ba, and is depleted in high field strength elements Nb, P, and Ti, and has characteristics of island arc magmatic rocks or orogenic igneous rocks. Based on the comprehensive analysis of previous research results, the gneissic medium−fine−grained granodiorite and gneissic fine−grained monzogranite were formed in the island arc tectonic environment and are important components of the East Qiyishan−Gongpoquan island arc zone.