The Analysis Of Characteristics,Metamorphism And Deformation Of Rocks In Devonian Wenquan Formation,Fengqing,Western Yunnan

The outcrop from Devonian"Wenquan Formation"studied in this paper is located at the Changning-Menglian Belt,experienced complex tectonic evolution and developed multi–stage metamorphism deformation during the geohistory period,between the Baoshan Block and the Licang Block in the western Yunnan.Combined with detailed field profile measurementand geological survey,based on the deep research of petrographic and geochemical characteristics,it is believed that the main rock types of Wenquan Formation are biotite quartz schists,mica schists and shallow Metamorphic sandstones with a small amount of phyllonites,granulites,metamorphic siliceous rocks,cataclastic rocks which containing structuralbrecciasor carbonaceous slates.There are two stages of metamorphic mineral symbiosis assemblages in the rocks which as follows:1st mineral assemblage?M₁?:Biotite?Bit?+Chlorite?Chl?+Oligoclase?Olgc?+Quartz?Q?;Biotite?Bit?+Oligoclase?Olgc?+Quartz?Q?;Biotite?Bit?+Quartz?Q?;2nd mineral assemblage?M₂?:Muscovite?Mus?+Oligoclase?Olgc?+Sericite?Ser?+Quartz?Q?;Biotite?Bit?+Sericite?Ser?+Quartz?Q?;Muscovite?Mus?+Oligoclase?Olgc?+Quartz?Q?.According to the geochemical analysis,and the study of the palimpsest sedimentary structure and associations,the original rocks are identified as a set of paramorphic rock assemblages,mainly arkose sandstones and argillaceous rocks.Metamorphic siliceous rocks in the formation are of normal biosedimentary origin,and its original sedimentary environment has the characteristics of the changes from"coastal"to"tidal flat",then to"shallow sea shelf"and then"bay",then"semi-deep-shallow sea shelf"and finally to"shallow sea",which hasproved that they formed in the characteristics of initial ocean basin deposition under passive continental margin environment.It can be concluded that the original rock assemblage,sedimentary sequence and sedimentary environment are consistent with the profiles of“Wenquan Formation”.Based on the electron microprobe analysisof biotites andmuscovites in different period time,combined with the regional geological background and microscopic characteristics,It is believed that the metamorphism of Wenquan Formation mainly undergoes regional cryogenic dynamic metamorphism,the metamorphism mainly includes recrystallization,metasomatism,and metamorphic crystallization,meanwhile,deformation,fragmentation and metamorphic crystallization were also involved.The pressure metamorphism of two processes were0.2 Gpa±,and 0.31 Gpa⁰.67 Gpa,and the metamorphic temperature<600?,which shows the progressive metamorphism in the orogenic metamorphism.Based on the above analysis,it is concluded that the formation of the Devonian Wenquan formation in Fengqing,western yunnan,along with the changning–menglian Ocean evolution,underwent the first stage of synmetamorphism and deformation in the late Permian continuous subduction stage,resulting in the formation of the new mineral biotite and the regional extrusion plane S₁.In the late Permian to early Triassic main collision period,the second stage of synmetamorphism and deformation occurred,which was characterized by early biotite metasomatism and the appearance of axial plane plane S₂.During the period of intracontinentalorogenic stage from late Triassic to lateCretaceous,the third process of deformation was occurred,forming thecompression shear foliation Cand the mineral of sericiteaccompanied by a weak metamorphism.Finally,due to the uplift of thrust and the formation of plateauin the Himalayan period,the deformation was strengthened,a series of wide and gentle folds and overlying brittle faults are formed.The Changning–Menglian Belt ended its evolution and formed the present pattern,which indicates that Wenquan Formation in this area has quasi-autochthonous characteristics.