| The Dabie orogenic belt which formed by the subduction-collision between Yangtze Block and the North China Block is exposed massive HP-UHP metamorphic rocks,and it has become a classic area for continent deep subduction and ultrahigh pressure metamorphism research.A series of scientific achievements have been obtained in continental deep subduction,continent-continent collision,ultrahigh pressure metamorphic rocks exhumation,etc.However,research is still lacking in middle-low grade metamorphic rocks in the northern margin of the Dabie orogenic belt and its pre-Mesozoic tectonic evolution.The Beihuaiyang tectonic belt is located at the northern margin of Tongbai-Dabie orogenic belt,and is the subduction zone of the Yangtze Block to the North China Block.The middle-low grade metamorphic rocks of Luzhenguan Group and Foziling Group are widely distributed in the area and is the critical area to understand the pre-Mesozoic tectonic evolution of the Dabie orogenic belt.Due to later structural superposition and lack of paleontological evidence,it is very difficult to divide and contrast the stratum.Therefore,there is much controversy about age assignment,tectonic setting,sedimentary source and tectonic evolution of Luzhenguan Group and Foziling Group.Based on detailed field geological survey of Luzhenguan Group and Foziling Group in Beihuaiyang tectonic zone and many analytical methods,including petrology,sedimentology,heavy mineral,geochemistry,and detrital zircon geochronology etc.,this study synthetically analyzes the age assignment,sedimentary environments and paleogeography,provenance and stratigraphic mineralization of Luzhenguan Group and Foziling Group.On this basis,it could provide new constraints on the Yangtze Block and the North China Block convergence model.The Luzhenguan Group in the study area is about 2900 meters thick,and it can be divided into Xiaoxihe Formation and Xianrenchong Group.The Foziling Group is>6000 meters thick,which is divided into Xiangyunzhai Formation,Zhufoan Formation and Panjialing Formation from the bottom to the top.The Xiaoxihe Formation is exposed mainly in Xiaoxihe in Huoshan and Qilihe and Hepeng in Shucheng,and it consists of metamorphic intermediate-acidic rocks and schists,and some rocks are migmatization,and its thickness is over 2,600 m.The Xianrenchong Formation with the thickness over 376 m,comprises of dolomitic marble and quartz schist,which exposes mainly in Xiaoxihe in Huoshan and Shibapan in Luan,and the lens appears intermittently.The Xiangyunzhai Formation is mainly distributed in Xiangyunzhai in Huoshan,and consists of metamorphic quartz sandstone and quartz schist with a thickness of 545 m.The Zhufoan group with the thickness of 1736 m,comprises of quartz schist and mica schist and containing microfossils,which exposes mainly in Zhufoan in Huoshan and Youdian in Jinzhai.The Panjialing Formation is exposed mainly in the Panjialing and Siguchong in Huoshan and Youdian in Jinzhai,and it consists of muscovite-quartz schist,mica quartz schist and quartzite,and its thickness is over 3805 m.The main achievements and new understandings of this thesis can sum up as follows:(1)U-Pb geochronology of detrital zircons from a clastic rock sample in the Xianrenchong Formation of Luzhenguan Group indicate that the maximum sedimentary age of the stratum is 433±12.5 Ma,which belongs to Sheinwoodian.U-Pb geochronology of detrital zircons from three clastic rock samples from the Foziling Group shows that the maximum sedimentary age of the stratum is 439-422 Ma,which indicate that era of the stratum is Silurian.According to Rb/Sr age of Foziling Group is 378±24 Ma(Frasnian),it represents the metamorphic age of Foziling Group.So it can be limit the minimum sedimentary age of Foziling Group.Therefore,the era of the Xianrenchong Formation of Luzhenguan Group and the Foziling Group are determined accurately as Silurian-Devonian.The weighted average age of Xiaoxihe Formation of Luzhenguan Group is 714.9±8.4 Ma(MSWD=2.4),which represents the crystallizing age of Luzhenguan granitic gneiss and indicate that the era of Xiaoxihe Formation of Luzhenguan Group is Nanhuaian.(2)Based on protolith,sedimentary facies and sedimentary environment research,the Xiangyunzhai Formation's protolith is quartz sandstone,followed by siltstone and shale and the Xiangyunzhai Formation represents the coastal deposition.The Zhufoan Formation and Panjialing Formation's protolith is a composition of sandy,mud-calcareous and siliceous quartz sandstone-greywacke-lithic sandstone-siltstone-shale and the Zhufoan Formation and Panjialing Formation belong to slope to bathyal turbidite deposits.The Xianrenchong Formation of Luzhenguan Group's protolith is medium-thick layer grain limestone and the Xianrenchong Formation belong to neritic carbonate platform.(3)The heavy minerals and geochemical characteristics of Foziling Group indicates that the parent rock types are mainly ancient sedimentary rocks,intermediate-acidic magmatic rocks and metamorphic rocks,and the provenance is a composite source area of quartzite and sedimentary rock and felsic igneous rock.The source area has the dual attributes of Yangtze and North China.The chronological analysis of detrital zircons of Foziling Group shows that the Early Paleozoic age group(410?542Ma,peak 450Ma,19.92%)and early Neoproterozoic age group(900?1000Ma,peak 968Ma,11.02%)represent the material source of the North Qinling Island Arc or Beihuaiyang Island Arc;the Neoproterozoic age group(542?900Ma,peak 816Ma,21.19%)representing the material source of the Neoproterozoic basement of the Yangtze Block;the Middle Proterozoic to the Paleoproterozoic(1000?2000 Ma,peak 1804Ma,22.88%)may represent the material source of the North Qinling Island Arc or basement of North China Block;the Paleoproterozoic to the Archean age group(2000?3705Ma,peak 2532Ma,25.0%)probably from the Archean metamorphic basement of North China Block.The trace elements and rare earth elements of detrital zircons in the study area indicate that the source rocks were all from continental crust and the oceanic crust were excluded,and the Formations of Foziling group had similar provenance.(4)The age-cumulative frequency characteristics of 89 detrital zircons in Xianrenchong Formation and 240 detrital zircons in the Foziling group are similar to the curve of the foreland basin in Cordilleran and Himalayas and indicate that the prototype basin of Xianrenchong Formation and Foziling Group are foreland basin in the collisional background.(5)The enrichment coefficient(K)of the ore-forming elements indicates that the Mo is enriched in the Luzhenguan group and Foziling group(enrichment coefficients are 3.93,1.75,1.84,1.32,and 2.14,respectively),and W,Pb,Ag and Cu are also enriched in different degrees.So there are potential mineralization possibilities in Luzhenguan Group and Foziling Group.The intensity of superposition coefficients(D)and the coefficient of variation(CV)shows that Mo,Ag and Cu in study area also have different degrees of later superimposition and potential mineralization possibilities.The comprehensive evaluation for mineralization ability of Luzhenguan Group and Foziling Group indicates that the dominant ore-forming element in study area are Mo.Therefore,Luzhenguan Group and Foziling Group is the ideal source bed of molybdenum ore and may provided ore-forming material.Correlation coefficients between ore-forming elements and major elements and its characteristic index shows that Cu,W,Ag,Pb,Zn are controlled by clastic rock characteristics and redox environment. |
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