Structural Characteristics And Emplace Mechanism Of Tanitawen Mt. Range Metamorphic Complex In The Eastern Margin Of Tibet Plateau

Tanitawen Mt. Range Metamorphic Complex (TMC) is located in the central partof the orogenic belt between Nujiang and Shuanghu-Chawula-Jitang sutures. The TMCis composed of Jitang metamorphic complex belt in the north and Jiayujqiaometamorphic complex in the south. This thesis analyses the deformation and exposedmechanism of both belts on the basis of analysis of rock rheology, including micro-and macro-deformation, metamorphism and geochronological constraints.The exposed tectono-column of Jiayuqiao metamorphic complex is composed ofKagong rock formation (folding layer), Jiayuqiao schistose formation and Tongka rockformation (gneiss). They flank on each other but separated by ductile shear zone.Penetrative mineral lineation on schistosity has preferred stretch orientation ofSE140-NW320, and micro-and macro-kinematic sense indicates NW-ward (320)shearing. Strain measurement and plots in the Flinn diagram for the quartz grain in thePhengites-bearing quartz schist give k=∞, indicating the Cigar-type strain ellipsoidand monaxial extension, embodying the NW-ward linear flow was formed by straightshearing strain in deep layer. The differential stress is estimated to be 149.5 MPa fromthe dislocation density of quartz from the metamorphic complex, and the phengitesformation temperature is 383.27℃ by using Ranbort method (1959), thus themetamorphic grade of the complex is inferred to be mid-to igh-greenschist during themajor deformation. The author (1993) got the Ar-Ar amphibole plateau age of164.7±1.33 Ma, and the Ar-Ar muscovite plateau age of 166.27±0.99 Ma for thecomplex. Both ages are approximately similar to the cover deposit, or the extendingtime (J2) of the Bangong-Nujiang suture zone, and thus reveal a synchronizationprocess during the major deformation and exposing for Jiayuqiao metamorphiccomplex. It is interpreted that this process formed in a extension mode withdetachment in the shallow, while ductile shear deformation in the deeper.The exposed tectono-stratigraphic column of Jitang metamorphic complex iscomposed of Youxi rock formation (schist) and Endayan rock formation (gneiss).Youxi schistose formation outcrops are distributed in the north of Baqen country, Tibet,and are composed of quartz schist, albite-quartz schist, muscovite-quartz schist withlevel penetrative schistosity and mineral lineation. The schistosity of the rockformation manifests the open antiform and synform, and the mineral lineation haspreferred orientation at 100～280° when unfold the antiform. Micro-fabric analysis ofthe ductile shear deformation indicates eastward kinematics. Strain measurement andplots in the Flinn diagram of the quartz grains in the Phengites-bearing quartz schistgives k=∞, indicating the Cigar-type strain ellipsoid and monaxial extension. ElectronMicroprobe analysis for the metamorphic mineral identified a composition ofphengites, which was further determined to be formed at a temperature of 320~500℃and a high-pressure of 0.6-1.1 GPa. The temperature-pressure condition represents agrade of blueschist metamorphism. The Ar-Ar method for the Phengite yields an age of230±1Ma, corresponding to the end of middle Triassic.