THE METAMORPHIC FEATURE,AGE AND MECHANISM OF AMUGANG GROUP IN NORTHERN TIBET

Amugang Group is distributed mainly over Amugang, Jiangai Mountain, Mayigangri,Gemuri and Qiagela,etc. lt includes Gemuri Formation and Qiagela Formation. They had even been thought of the crystalline basement of Qiangtang terrain. However, Gemuri Formation and Qiagela Formation are not alike in the protolithes, metamorphism and deformation, rock association,etc.In fact, they are different in mechanism.Gemuri Formation is composed of high|Pressure, low|temperature glaucophane greenschist|faci metamorphic rocks. The petrolithes of the blueschists are glaciomarine conglomerates and basalts from the Southern Qiagtang area. The typical mineralogy include: glaucophane+ epidote + calcite +stilpnomelane and stilpnomelane+chlorite+sericite+ quartz+ glaucophane. P.T conditions for the metamorphism of blueschist are estimated to be 0 6～0 7GPa and 320～400℃.The 40 Ar/ 39 Ar dating of the crossite has yielded good plateau age of (222 5±3 7)Ma,which represents the formation of Gemuri Formation.Qiagela Formation comprises schist series, marbles, Gneisses and plagioamphiboles. The protolithes of them are a suit of argillaceous sandstones, arkoses, carbonates and mafic volcanic rocks. The Sm|Nd isochron age of the metamorphic mafic volcanic rocks is (268 0±5 6)Ma, which shows that The age of the protolithes is early Dias. The typical mineralogy include: Muscovite+biotite+plagioclase+quartz; garnet+kyanite+ staurolite+ biotite+ muscovite+plagioclase+quartz; amphibole+ plagioclase±garnet+ quartz. They are meos|pressure,meso|temperature low amphibole|faci metamorphic rocks. Qiagela Formation is coexistent with the late Triassic and the early Jurassic volcano|magmatic arc in space and overlapped by the triassic limestones of Xiaocaka Formation. So,it is suggested that the formation of Qiagela Formation be between the late Dias and the late Triassic period. Its genesis is relative to the thermal current provided by magmatic activity.