Characteristics and tectonic setting of Middle-Permian A-type granites in Tianshuijing area,west of Inner Mongolia

Intrusive rocks of Longjishan-Shensheling,which distributed in Tianshuijing region in west of Inner Mongolia,have an elliptic shape toward northwest.Its area is more than 100 km².They were named Longjishan Series,and included three units.The first unit is phenocryst bearing fine-grained biotite adamellite,the second is phenocryst bearing middle-fine grained biotite adamellite and the third is middle-fine grained granit.Granitoids enrich in SiO2 and Alk,contents of Alk vary between 7.23% and 8.37%,AKI are less than 1,ratios of A/CNK range from 1.01 to 1.07,and OX values vary betw een 0.47 and 0.83.The rocks enrich in LIL E and L REE,and deplete Ba,Nb,Sr,Zr and Ti relatively.REE patterns have distinct "V" type.Accessory mineral assem blages are Mt-Ap-Zi.Granit oids evolve twoard acidity and alkalinity from early unit to late unit,display the charact eristics of aluminous A-type granite(A2-type) and have the characteristic of post-orogeny in some tectonic setting distinguish diagram.U-Pbage of Zircon is 267.1 Ma.All of those indicated that the orog enic process occurred in Middle Permian,might end in late of Middle Permian and the tectonic setting might be shifted to the setting of interior plate in Late Permian.