Petrological Characteristics And Protolith Restoration Of Wuliangshan Group In Nanjian County, Yunnan Province

Wuliangshan Group located in Wuliangshan back-arc basin of South Lanping basin, southeastern margin of Tethysides, Sanjiang Area. Exposed in NWW trending as a belt of Gonglang Town- Wuliangshan, overall arc-shaped, for plunging anticline structure. Wuliangshan Group is divided into four lithologic, and showed a toughness fault contact between each lithologic. The rock types are mainly quartz mica schist quartzite, carbonaceous sericite phyllite, slate and metamorphic sandstone. The strata are strongly influenced by tectonic deformation, folding and stretching lineation is very developed. Wuliangshan Group is an shallow metamorphic rock series, have a low greenschist facies-epidote amphibolite facies.In detailed observe the attitude of stratum, rock combination, primary structure, mineral composition of Wuliangshan Group, with the help of zircon dating, geochemical method; comprehensive analysis of test data, using a variety of discrimination diagrams, analysis petrology characteristics and primary rock type of Wuliangshan Group. Led us to the following conclusions:The second- fourth lithologic section of Wuliangshan Group has rich floriniteslaevigatosporites combination, belongs to the Early Permian. Extraction of detrital zircon and make U-Pb isotopic dating of gray metamorphic sandstone in Dimo village, GongLang town, NanJian county, Yunnan province. Analysis show that zircon is mainly from hydrothermal, the zircon age mainly concentrated in 586±110Ma-1980±180Ma, the latest age is 242.3Ma(7 effective point). Show that the hydrothermal activity time is not later than Early Triassic and the rock formation should be earlier than Early Triassic. Guess the Wuliangshan Group is formed in Early Paleozoic. Comprehensive judgment, Wuliangshan Group is mainly an Early Permian deposits, its bottom may contain the Late Carboniferous strata. Therefore, we set the Wuliangshan Group in Middle Carboniferous- Lower Permian.The metamorphic degree of Wuliangshan Group decreased gradually from the core to the edge. It can be roughly divided into three metamorphic belt, chlorite belt, biotite belt, kyanite belt. The fourth lithologic section is chlorite belt, the main metamorphic mineral assemblages are quartz, sericite, chlorite, belongs to low greenschist facies. The second and third lithologic section is biotite belt, the main metamorphic mineral assemblages are quartz, muscovite, biotite, albite, belong to low greenschist facies. The first lithologic section is kyanite belt, the main metamorphic mineral assemblages are quartz, muscovite, biotite, sillimanite, albite, garnet, belong to epidote amphibolite facies.Experienced 3 tectonic metamorphism deformation generations, the first phase of tectonic metamorphism deformation happened in the Early-Middle Triassic(T1-T2), the first phase of schistosity(S1) parallel with the original bedding(S0). The second phase of tectonic metamorphism deformation happened in the end of Eogene Period(E). Wuliangshan region occurred N-NE direction’s thrust, and made it covered the Yunlong- WeiShan mesozoic red basin. The second phase of schistosity(S2) is high angle oblique with the first phase of schistosity(S1), and cutting the latter obviously. The third phase of tectonic metamorphism deformation occurred in the Late Pliocene(N2) in shallow tectonic level. The third phase of schistosity(S3) is low angle oblique with S2, and developed late brittle fracture.According to the macro geological characteristics, Wuliangshan Group output was layered, with sand and mud interbed phenomenon. Under the microscope blasto sand, silt sand structure, blasto Sand, blasto fine tuff structure can been seen; combined with the mineral assemblages and recrystallization and deformation characteristics of metamorphic rocks, and multiple metamorphic protolith recovery graphic, judged that the original rock of Wuliangshan group are mainly sandstone, mudstone and some volcanic clastic sedimentary formation.Through the analysis of main elements and rare earth elements, the Wuliangshan Group source materials experienced weak chemical weathering and simple sedimentary recycling, and were dominated by upper crust felsic rocks, acidic arc source. These rocks probably deposited on back-arc basin of continental island arc, close to passive continental margins. Wuliangshan Group is a set of weak reduction, volcano activity, strong erosion, rapid accumulation, epicontinental clastic sedimentary rock. The deposition area is not far from the supply area, in the process of diagenetic temperature was stable.According to the sedimentary structure and Ba-Sr diagram, Wuliangshan Group were formed in the volatile environment of marina neritic facies to delta facies, the first lithologic section is littoral facies, the remaining three lithology is mainly shallow shelf facies. In the upper part of the fourth lithologic section, thick sandstone significantly increased, and develop large scale cross bedding, with the character of delta facies.