LA-ICP-MS zircon U-Pb ages and geochemical characteristics of Sangejing gabbro in Beishan area of Inner Mongolia

Beishan area, located in the central segment of the southern margin of Central Asian Orogenic Belt, has undergone intense magma activities. In order to ascertain the tectonic evolution in Beishan area, the authors carried out the research of geochronology and geochemistry for Sangejing gabbro of Beishan area. The results show that the LA-ICP-MS zircon U-Pb age of the gabbro is (404.9 ± 1.6) Ma, and the formation age is Early Devonian. The geochemical characteristics show that the gabbro has high Al₂O₃ content(18.99% ~ 21.12%, above 17%) and belongs to high-alumina basaltic rock. There is evident fractionation between LREE and HREE, and (La/Yb)_N value is 7.16 ~ 9.07. The large ion lithophile elements (e.g., Rb, Ba, K, Sr) are enriched and the high field strength elements (e.g., Ta, Nb, P) are depleted. The above characteristics indicate that the formation of Sangejing gabbro is related with the subduction belt. Combined with the regional geological features, the authors thought that Sangejing gabbro could be the products of the early arc magmatism and formed during the main ocean basin southward subduction in Hongshishan area.