THE PETROCHEMICAL CHARACTERISTICS AND TECTONIC SETTING OF THE LATE PERMIAN AND THE LATE TRIASSIC FOR THE GRANITIC BATHOLITH IN BUYANT AREA,INNER MONGOL

Based on the new data from some recent studies,the granitic batholith in Buyant area in the western part of northern margin of North China platform is composed of 4 granitic intrusive bodies of the Late Permian and the Late Triassic Period,with the emplacement age of 261 Ma±1.3 Ma,220.9 Ma±0.3 Ma and 204.9 Ma±5.9 Ma respectively.The intrusive rocks consist of biotite granodiorite,porphyritic biotite granite and two-mica granite.Late Permian biotite granodiorite is characterized by mean Na2O/K2O ratio of 2.06,mean A /CNK contents of 1.03 and mean σ contents of 1.9,belonging to calc-alkaline series.The Late Triassic granite has an average Na2O/K2O ratio of 0.82,mean A /CNK contents of 1.03 and mean σ contents of 2.18,belonging to calc-alkaline series.Petrochemical plots for diagnosing tectonic environments of rocks indicated that the Late Permian biotite granodiorite and the Late Triassic biotite granite were a series of island arc dioritoid and granitoid formed in the process of subduction.However,the Late Triassic two-mica granite was post-orogenic continental granitoid.