Petrological Evidence for Subduction of the Early Paleozoic Oceanic Crust in Bart - Obo, Inner Mongolia

In the Xing'an-Mongolia fold belt in the north of Bayan Obo, there is a sequence of arc-type intrusion rocks distributing in Bart-Obo and Chagenhosao, Inner Mongolia, which have an identical chemical composition with a typical adakite: SiO2≥56 %,Al2O315 %,MgO3 %(Less6 %),Yb(0.726 ～ 2.789)×10~(-6),La/Yb10 ～ 20,Y(6.842 ～ 28.734)×10~(-6),Sr400×10~(-6),Sr/Y≥20 ～ 40, Mg~#=0.51 ～ 0.58(most 0.40), rich LREE and depleted HREE, positive or little negative Eu-anomaly, and obvious Sr apex and Nd vale and Ti vale. Nine isotopic ages of U - Pb (~(206)Pb - ~(238)U) arrange between 345.06 ～ 552.671 Ma, indicating the subduction time of the subducted ocean crust. A series of diagrams for tectonic setting suggest that these intrusion rocks formed in an island arc environment, representing the subductive and partial-melting production of the ancient ocean crust under oblique subduction of the southern Mongolia oceanic plate towards the Buyant micro-block during the early Paleozoic.