Sedimentary Characteristics And Provenance Analysis Of The Mugagangri Group In Nima Area?Tibet

The Nima area is located in the central part of the Tibet Plateau,and it has exposed a large number of the Bangong-Nujiang suture zone's landmark strata,Mugagangri group.The Mugagangri group in the area can be divided into disordered center uplifted tectonic melange belts and coherent western clastic rock belts.Previous researchers have extensive research on ophiolitic rocks and magmatic rocks mixed in the Mugagang group,but very little work has been done on coherent strata.In this paper,we choose the coherent strata of the Mugagangri group in the Nima area of the central Tibet Plateau as the object of study.We selected the Dongezha section and the Emongle section with good outcrop,continuous deposition,and applied the sedimentary facies analysis and the detrital composition analysis,and provenance analysis to study the sedimentary characteristics and material sources of coherent strata in the Mugagangri group,and obtained the following understanding:In the coherent strata of the Mugagangri group of the Nima area,clastic rock is dominant,with signs of shallow metamorphism.Lithology includes gray-green conglomerate,gray-green coarse sandstone gravel,silty sandstone,gray mudstone,gray-green-yellow-green metamorphic quartz sandstone,gray-red breccia,gray-green slate-phyllite.Thirteen types of lithofacies were identified in the section of the Mugagangri group in the area,and a total of six types of lithofacies sequence were identified.According to the characteristics of lithofacies assemblages and provenance analysis,submarine fan facies was identified in Dongezha section.Including the inner fan subfacies,the middle fan subfacies,the inner fan main channel microfacies,the natural levee microfacies,the middle fan branch channel microfacies,and the distributary channels microfacies.The outer fan subfacies of submarine fan were identified in the Emengle section.Concretely,it can be subdivided into the proximal end of the outer fan,the outer end of the outer fan,and the microfacies of the deep-cut water channel.According to the sandstone thin section measurement method,the sandstone of the Dongxuzha section is basically feldspathic lithic sandstone and lithic sandstone.The rock debris is dominated by sedimentary rock debris,followed by magmatic rock debris,and contains a small amount of metamorphic rock debris.The sedimentary debris is dominated by oolitic limestone debris and sandstone debris.The magmatic rock debris mainly consists of basic basalt and andesite debris.The cathodoluminescence test showed that the quartz particles are dominated by non-luminescence and blue-violet light,indicating that most of the quartz particles come from sedimentary rocks and magmatic rocks.The Qiangtang block on the northern side of the Nyima area mainly reveals intermediate-basic magmatic rocks,carbonate rocks,and clastic rock strata.The south Lhasa land mass mainly reveals carbonate rocks.And clastic rock formations.According to the Dickinson triangle plot,the results show that the source area tectonic background is dominated by the recycled orogenic belt and magmatic arc source regions,and they have the characteristics of the source area of the mixture.The results of previous studies indicate that at this time,the Qiantang block on the northern side of the study area is in the inland evolution stage after the closure of the Paleo-Tethys Ocean,where the internal magmatic-tectonic uplift activity was intense,and eventually think the inferred source originated mainly from the northern Qiangtang body.