Petrological Characteristics And Metamorphism Of Wuliangshan Group In Nanjian,Yunnan

Wuliangshan Group is located in Wuliangshan back-arc basin in the central part of Lanping-Simao Basin,which is an important area for Tethys tectonic evolution.Influenced by the evolution of Tethys,the whole rock group appears as NW-trending upright near-horizontal anticline,with SW-trending or NE-trending thrust nappe structures and tight synclinal folds.The two-wing thrust nappe overthrusts the Triassic red beds and belongs to the older strata exposed in this area.It shows obvious three-stage tectonic deformation characteristics,which completely preserves the influence of Tethys evolution on this area.In this paper,Wuliangshan Group exposed in Gonglang-Bashicun area is selected as the research object.According to the exposed lithology and rock assemblage characteristics,and combined with previous data,Wuliangshan Group is divided into four sections.Fault contacts are mostly used between each section,and conformable contacts between two and three sections can be seen in local area.The main rock types are mica schist,quartz schist,phyllite,slate,metamorphic sandstone,quartzite and a small amount of volcanic clastic material.Based on the detailed study of its Petrological and petrographic characteristics,its metamorphism is further discussed,which provides a reference for the study of metamorphic deformation behavior and Tethys evolution of rocks in this area.According to this study,the following conclusions have been obtained:the lithology of each member of Wuliangshan Group is relatively stable.However,the number of sandstone interbeds metamorphosed from N to S increases gradually.Geochemical characteristics also show that Wuliangshan Group in the south is relatively concentrated in CaO,K₂O,Rb,Ba and Ni than in the north,but less in Zn,Cu and Sr.The Wuliangshan Group in the south is mainly deposited in freshwater oxidation environment,while the Wuliangshan Group in the north is mainly deposited in saltwater-freshwater and mild reductive-reductive environment.The sedimentary environment gradually transits from N to S,from marine neritic facies to delta facies.The degree of rock metamorphism generally belongs to greenschist facies,and the degree of local metamorphism increases to amphibolite facies.The degree of metamorphism is not related to the New-old stratigraphic relationship,but to the strength of rock deformation and fault structure.According to the mineral symbiotic assemblage and the distribution of metamorphic minerals,six metamorphic belt are divided:chlorite belt,biotite belt,garnet belt,cruciform belt,kyanite belt and sillimanite belt.