Discussion On Anatexis Of Garnet Granite In Daqingshan-Wulashan Area,Inner Mongolia

Anatexis has now become a focus in the research of the high-grade metamorphic rocks.It has great significance to the origin of magma and deep crustal activities in orogenic belts.The Daqingshan-Wulashan area in Inner Mongolia is located in the middle section of the northern margin of the North China Craton.In the area,Early Precambrian basement rocks were widely exposed and partially melted generally,forming large areas of anatectic granite.So,this area becomes a natural laboratory for studying anatexis.This paper takes the garnet-biotite gneiss,garnet granite,and garnet-bearing granite which were colleted from the Deersutai and Xiaomiaozigou of Daqingshan-Wulashan area in Inner Mongolia as the research object.On the basis of summarizing and absorbing a large number of previous research results and field geological surveys,this paper studied the anatexis mechanism of garnet granite in the study area by petrography,EPMA,whole-rock geochemistry and SHRIMP zircon U-Pb dating and other technical.The mainly achievements are as follows:(1)Through SHRIMP zircon U-Pb dating,the results show the garnet granite and garnet-bearing granite which were colleted from the Deersutai iron ore deposit and Xiaomiaozigou area has the metamorphic age of 2.30?2.46 Ga and?1.97 Ga.Combined with previous research data,it is confirmed that the study area has experienced at least two metamorphic events in the late Neoarchean and late Paleoproterozoic.Among them,the samples from the Deersutai area has not the metamorphic age of the late Paleoproterozoic,and the samples from the Xiaomiaozigou area has not only the metamorphic age of the late Paleoproterozoic,but also the metamorphic age of the late Neoarchean.Based on the chronological research results obtained by the predecessors in the research area,this paper thinks that the metamorphic events in the late Paleoproterozoic belong to the local tectonic thermal events.(2)The garnet-biotite gneiss and the garnet granite are closely symbiosis,gradual transition,and have inheritance in mineral composition and chemical composition.The garnet in the garnet granite has a sieve-like metamorphic structure developed at the core but not developed at the edge,the core-packed mineral has a small grain size and a large grain size close to the edge,at the same time it packs the front of the metamorphic peak(fine-grained quartz feldspar)and the metamorphic peak period.(Sillimanite)Two generations of products and other characteristics.And after the electron probe test,The garnet in the garnet granite and the garnet biotite gneiss have similar chemical composition characteristics.Based on these,it is determined that the garnet in the garnet granite is peritectic,which reflects the close relationship between the garnet granite and the garnet biotite gneiss.It is comprehensively indicated that the garnet granite is formed by partially melted of garnet biotite gneiss.(3)Garnet granite closely symbiosis with garnet-biotite gneiss in space.A large number of peritectic garnet and various types of enclaves are retained in the rock,and not evenly distributed.The rock type has a large range(garnet quartz monzonite?garnet granodiorite?garnet monzonite).And the mineral composition and main trace elements are extremely uneven in garnet granite.These indicate that the garnet granite in the study area has not undergone long-distance transportation to make the whole uniform,and it is in-situ or semi-in-situ cause.(4)Through the systematic study of field geological characteristics,petrographic characteristics and geochemical characteristics,it is found that the garnet-bearing granite is more uniform than the garnet granite.In the garnet-bearing granite,the content of garnets and inclusions in the granite is less,and the type of rock,mineral composition and chemical composition is more uniform.It has similarity in chronological characteristics,and the garnet in the garnet-bearing granite shows the characteristics of magmatic garnet which is different from the peritectic garnet in garnet granit.These indicate that the garnet-bearing granite is the product of further evolution of garnet granite.(5)The metamorphism-anatexis evolution of garnet biotite gneiss?garnet granite in the study area is divided into four stages about prograde metamorphic stage,peak metamorphic stage,post-peak metamorphic stage,and cooling and pressure reduction stage.The prograde metamorphic stage is characterized by garnet cores and the fine-grained minerals Quartz+Biotite+Plagioclase+Sillimanite±Spinel wrapped in it.The peak metamorphic stage is characterized by garnet and Sillimanite+Quartz+Biotite+K-feldspar+Plagioclase in the matrix,and there is a brief process of decompression and heating during the peak period.This paper thinks that the deep melting first occurs in this process,starting with the dehydration reaction of biotite,and the metamorphic reactions is as follows:Bt+O₂?Melt(K₂O+Al₂O₃+Si O₂+H₂O)+Ilm+Mt.The post-peak metamorphic stage is characterized by the development of cordierite at the edge of the garnet,and the metamorphic reactions is as follows:Gt+Sil+Qz?Cord,Gt+Melt?Cord+Bi+Pl,Gt+Melt?Cord+Qz±Pl.And this paper thinks that a large-scale deep melting occurred at this stage.The cooling and pressure reduction stage is characterized by the development of reverse reaction biotite at the edge of the garnet and plagioclase cumulus crystal,and the metamorphic reactions is as follows:Gt+Melt?Bt+Qz.In addition,the reverse reaction biotite can continue to react with garnet to form fine-grained biotite and felsic minerals,and the temperature and pressure condition is T=527.9?610?,P=0.48GPa.(6)Through the comparative analysis of the chemical composition of garnet and biotite,some regularities and differences are obtained,which reflect the migration of elements in metamorphism-anatexis evolution.According to the current research,the Fe-Mg ion exchange reaction between garnet and biotite reflected in the garnet diffusion zone is closely related to the isomorphism.