Paleoproterozoic Magmatism And Sedimentary Event In The Susong Area,Anhui Province,and Its Implication For The Yangtze Cratonic Evolution

Craton and orogen are the two units constituting global continents,with craton taking up 69%of the total continental area.There are 35 known Archean cratons worldwide,recording ancient geological information within old rock archive.Cratonic rock records can be dated back to early Hadean era,representing crustal evolution history since the beginning of earth.The Yangtze Craton is one of the three Precambrian blocks of China.Due to the thick sedimentary covering since Nanhuan Period,Archean-Paleoproterozoic rocks are rarely exposed at the Kongling,Zhongxiang,Yudongzi in the north margin,and south-west margin,north Vietnam.In the mid-eastern Yangtze Craton,the sporadic Archean-Paleoproterozoic basement is only found in Dongling,Feidong area along the Tanlu fault zone.The inadequate exposure impedes the geological investigation and research of Precambrian history of the Yangtze Craton,resulting in the poor understanding of early geological evolution of the Yangtze Craton compared to the North China Craton.Our knowledge of the Precambrian tectonic setting of the Yangtze Craton is limited,especially for the Paleoproterozoic collisional orogeny related to the Nuna supercontinent,and the location of such an orogen.This thesis studies the exposure,field appearance,intersecting relationship and geochronology,petrogenesis,sedimentary source,tectonic background of the Paleoproterozoic metagranite,metagabbro dyke and sedimentary rocks in the Susong area of the mid-eastern Yangtze Craton.Based on the comparison with Paleoproterozoic rocks in other basements of the Yangtze Craton,this research explores the Paleoproterozoic geological evolution and tectonic setting in the Susong area and the northern Yangtze Craton.The achievements and understandings include:（1）The discovery of Paleoproterozoic metagranite,sedimentary rocks and metagabbro dyke in the Susong area of the mid-eastern Yangtze Craton,which have been deformed and metamorphosed.The metagranite is mainly gneissic monzogranite;the metagabbro has become amphibolite due to metamorphism;the sedimentary rocks are metaconglomerate,muscovite quartz schist,arkose quartzite and graphite-bearing muscovite quartz schist.（2）The zircon U-Pb dating confirms the age of the Susong metagranite is～2.0 Ga.The metagranite is high in silica（71.98～76.00 wt.%）,high alkali（K₂O+Na₂O=8.08～9.16 wt.%）,enriched in large ion lithophile element（LILE）,depleted in high field strong element（HFSE）,having negative correlation between P₂O₅and Si O₂,belonging to I-type granite.Whole-rockε_Nd（t）values are-10.4 to-9.4 with two-stage model age of 3.3-3.1 Ga;magmatic zirconε_Hf（t）values are-11.4 to-9.0 with two-stage model age of 3.2-3.1 Ga.It shows the metagranite is formed by remelting/anatexis of pre-existing ancient crustal rocks（mainly TTG rocks）.The comparative study with tectonic setting that generating coeval granitoid and high-pressure granulite in the northern Yangtze Craton indicates our metagranite formation is likely related to collisional orogenic event.（3）The Susong metagabbro dyke has high Ti O₂contents（2.63～3.54 wt.%）,enriched LREE（La _N/Yb _N=4.31～5.51）and LILE,positive anomaly in Nb-Ta,relative depletion in LILE（like Rb,K）,resembling trace element pattern of OIB,indicating asthenospheric mantle source.The zircon U-Pb dating shows that the time of magmatic emplacement is at～1.84 Ga,and the zirconε_Hf（t）values are from-2.4 to 3.8,indicating an enriched mantle source.The mafic dyke is probably formed in a within-plate extensional setting after collisional orogen.The metagabbro dyke shows clear intrusive boundary with country rocks like Daxingwu rock series and Luohanjian granite,constraining the lower limit of the sedimentary time of Daxingwu rock series.（4）The bottom of Daxingwu sedimentary rock series is metaconglomerate,the detrital zircon dating exhibits two age peaks at～2.5 and 2.0 Ga with the youngest zircon concordant²⁰⁷Pb/²⁰⁶Pb age at 1942±44 Ma（#22,concordance=91%）,constraining the maximum deposition age.The lower limit of sedimentary age is constrained by the emplacement of the Susong metagabbro dyke,suggesting the sedimentary time of Daxingwu rock series between 1.90～1.85 Ga.The gravel types of conglomerate are mainly gneissic granite and siliceous gravel.The conglomerate has moderate to poor compositional maturity and sorting,suggesting a proximal and quick sedimentary deposition,may be a piedmont alluvial fan formed in the subsiding intermontane basin after the collisional orogeny.The major sedimentary sources are～2.5 and～2.0 Ga intermediate-felsic magmatic rocks,such as the Luohanjian metagranite.During this time,there are no evident tectonic-magmatic events in the north Yangtze Craton.The time of sedimentary event likely represents a tectonic transition between collisional orogenic stage and post-orogenic extension.（5）Constructing the collisional orogen to post-orogenic extension tectonic model in the Susong area during 2.0～1.84 Ga:a)During 2.15～2.05 Ga,there is a north-west to south-east oceanic lithosphere subduction in the north margin of the Yangtze Craton.Evidence includes the Paleoproterozoic ophiolitic mélange belt,～2.12 Ga high-magnesium andesite,basalt in the Kongling Complex,and the～2.08 Ga island arc related granite found in the Houhe region in the north-west margin of the Yangtze Craton;they represent the oceanic crust subduction and island arc magmatism respectively.b)After the subduction,two micro-continents in the north margin of the Yangtze Craton collide and form an orogen,leading to the large-scale amphibolite to granulite facies high-grade metamorphism with clockwise P-T-t path.It also promotes the anatexis of ancient crustal materials,generating the～2.0 Ga anatectic granitic magmatism.c)The orogeny ends in～1.84 Ga,the post-orogenic extension becomes progressively stronger afterwards.The Paleoproterozoic extensional setting in the north margin of the Yangtze Craton facilitates the asthenospheric mantle upwelling and forms voluminous mafic magma.It provides the major deep-level heat source for ancient crustal remelting,forming the widespread intra-cratonic A-type granites and mafic dykes.The～2.0 Ga collisional orogeny and～1.84 Ga post-orogenic extension in the Susong area are likely associated with Nuna supercontinent evolution.（6）Combining this study and previous investigations,connecting the Susong-Huangtuling-Tongbai complexes in the Tongbai-Dabie orogenic belt that are related to2.0～1.95 Ga collisional orogeny,the Susong-Huangtuling-Tongbai complexes may stand for the 2.0～1.95 Ga orogen and its location within the Tongbai-Dabie region.Due to the strong tectonic activities in the Jinning and Indosinian period,especially the Indosinian North China and South China continental collision,the position of Tongbai-Dabie region may be shifted away over a long distance.Based on the similar tectonic-magmatic events between the Tongbai-Dabie orogenic belt and the Kongling Complex,it is possible to link the Tongbai-Dabie 2.0～1.95 Ga orogen with Kongling Paleoproterozoic orogen recorded in ophiolitic mélange belt.Together,they constitute a2.0～1.95 Ga orogen within the north margin of the Yangtze Craton.（7）Built on the comparison of Archean-Paleoproterozoic basement compositions and the distribution of Paleoproterozoic orogen in the Yangtze Craton,it is proposed that the Yangtze Craton may contain at least three independent micro-continents.The north margin of the Yangtze Craton can be divided into north-east and north-west micro-continents,jointed by a 2.0～1.95 Ga orogen studied by this thesis.The time and orogenic position for amalgamation of south-west cratonic segment and the north Yangtze Craton still desire further investigation.