Study On The Deformation Features And The Metamorphic-deformation Stage Of The Jitang Rock Group In The Easetern Segment Of Northern Qiangtang, Tibet

Abstract:The Jitang Rock Group, which is extending along the NW-SE portion, outcropping mainly in Baqing, Dingqing, Leiwuqi and chaya counties in eastern Tibet, and locating between Bangonghu-nujiang suture zone and Shuanghu-lancangjiang suture zone, belongs to the north continent of the"one ocean, two continents"system, that is, Northern Qiangtang Block. The deformation is well developed in the Jitang Rock Group. Some typical areas benefit to study the structural features and the deformation stage, on which we discuss the tectonic properties and the relationship between structure and mineralization. This study provides some new geological information on tectonic evolution in Qiangtang area, the northern Qinghai-Tibet Plateau. Combating the field geologic investigation and the microstructure study, we conducted detailed structural analysis on the Jitang Rock Group, and obtained the following conclusions:1. The Youxi Rock Formation of the Jitang Rock Group outcrop in Dingqing County, Tibet Autonomous Region, are mainly consist of biotite-quartz schist, two-mica quartz schist, mica quartz chlorite schist with metamorphic conglomerate, metamorphic baic volcanic rocks, and was latter intruded by diabase gabbro and gneissic biotite monzogranite. The Youxi Rock Formation generally appears layered structure and ordered in wholel, disordered in part. Its protolith association is clastic rock with amount of basic volcanic rocks, which was formed in Meso-Neoproterozoic, about (1048.2±3.3)Ma to (965±55)Ma, are more similar to the metamorphic basement rocks that do not show the properties of melanges in accretionary wedge.2. The Youxi Rock Formation of the Jitang Rock Group showed complex deformation and up-to high-grade greenschist metamorphism, and developed clearly structural replacement, penetrative foliation and lineation, which proves typical deformation in middle crust. According to the analysis on S-C fabric, porphyroclastic system and the foliation, the kinematic characteristics of the rock group has been obtained: dextral shear from NW275°～280°to SE95°～100°in the horizontal direction and thrust from north to south in the vertical direction.3. According to the structural superposition, foliation replacement, the cutting relationship between metamorphic minerals and the retrograde metamorphism, we classified the metamorphic-deformation into four stages: firstly, regionally metamorphism (the development of penetrative foliation and lineation); secondly, ductile shearing (S-C fabric, porphyroclastic system); thirdly, high temperature low pressure metamorphism (And. + Sil. + Bt. + Tur.); fourthly, retrograde metamorphism (chloritization of biotite, sericitization of sillmanite, the formation of spaced cleavage, and thrust fault).4. The results of simple analysis of the mineralization in Jitang Rock Group demonstrated that it is rich in As (19.7⁵7.6×10^-6), the elements Cu, Au and Ag have share a high anomaly, among which Au anomaly is up to 0.15×10^-6. According the location of mineralization, the ore-controlling factors, and alteration features, we preliminary view that there is a potential Au mineralization in Jitang Rock Group that close relative to ductile shearing.5. Combating the regional geology and the ages of 235.7±0.68Ma and 235.1±1.1Ma of the biotite monzogranite from Jitang Rock Group, we remain that the high-temperature metamorphism may correspond to the large tectonic-magmatic event in Indosinian (T₂-T₃) in eastern Tibet.