Formation And Evolution Of The Wuhe Complex In The Southeastern Margin Of The North China Craton

The Wuhe complex(WC),exposed at the southeastern margin of the North China Craton(NCC),is an important constituent of the Eastern Block of the NCC.However,there has been a dispute over its basement propertis and crustal evolution because of the widespread influence of the Paleoproterozoic metamorphism and the Yangtze Mesozoic continental subduction.The Wuhe complex contains a variety of rock types,such as garnet amphibolite,garnet granulite,marble and different types of granitic gneiss and other high-grade metamorphic rocks,accompanied by paleoproterozoic and Mesozoic granites.Among them,the Precambrian high-grade metamorphic rocks are an important research object and target area for the study of the formation and evolution of the crust in the southeastern margin of the NCC,which is of great significance for the comprehensive understanding of the formation and evolution of the crust in the early NCC and the metamorphic overprinting of the Mesozoic tectono-thermal events.In view of the research status and existing scientific problems,this paper takes different types of granitic rocks in the Wuhe complex(such as granitic gneiss,tonalite-trondhjemite-granodiorite(TTG)gneiss,alkaline ultrapotassic granitic gneiss)and related amphibite and granite as the main research objects.Based on the systematic field geological survey and the comprehensive study of petrology,mineralogy,elements-isotope geochemistry,zircon chronology and phase equilibrium modelling,the composition and petrogenesis of the Precambrian granitic rocks and the crustal growth characteristics of the late Archean period(2.5-2.8 Ga)in the North China Craton are discussed.This paper reveals the Mesozoic tectonic evolution history of the southeastern margin of the North China Craton.The primary innovative results are as follows:(1)The present study provides the first report of～2.6 Ga alkaline ultrapotassic granitic gneisses from the Wuhe complex.The identification of～2.6 Ga alkaline ultrapotassic granitic gneisses provides additional constraints on the Precambrian tectonic and magmatic evolution of this region.They are characterized by modally abundant alkali feldspar(more than 40%)and quartz(20-25 vol.%)while plagioclase is rare.Mafc minerals are sodic amphiboles such as arfvedsonite and eckermannite,aegirine and rare biotite.Minor constituents are rutile,muscovite,apatite and barite.Major elements geochemistry shows high SiO2(69.9%-74.5%)and K2O+Na2O(9.67%-12.2%)contents,high K2O/Na2O(7.41-22.5)ratios,and relatively low MgO(0.37%-0.87%)and CaO(0.10%-0.23%)contents.Trace elements geochemistry shows signifcant depletions of Nb,Y,Ce,Ga and REE(rare earth elements)relative to anorogenic granites.These features suggest that the magmatic protoliths of the studied gneisses belong to ultrapotassic silica-saturated alkaline series from an extensional background,perhaps in a subducti on-related rifting environment.Two samples yielded zircon 204Pb/206Pb ages of 2615±4 Ma and 2617±5 Ma,respectively,representing their precursor ages.The zircon igneous core domains exhibit oscillatory growth zoning with positive εHf(t)values(+2.5-+6.6).These data,coupled with chondrite-normalized and primitive-mantle-normalized element patterns,suggest that the precursors of the studied granitic gneisses were mainly derived from a subduction-modifed ultrapotassic syenitic parental magma,and may be considered as a particular group of A-type granites,involved in an important crustal growth and reworking event at～2.6 Ga.(2)Igneous rocks of different ages and types are widely exposed in the Wuhe complex at the southeastern margin of the North China Craton.Based on their field contact relationship,lithological,whole-rock geochemical and zircon U-Pb age and Lu-Hf isotopic evidence,five major rock associations have been identified.The ca.2.8 Ga granitic gneisses show relatively high SiO2(75.8-77.6 wt%),enriched LILE and LREE,depleted HFSE and HREE,low ∑REE content with large positive Eu anomalies,which are similar to the characteristics of continental crust.It is derived from the partial melting of the continental crust and the presence of plagioclase cumulation,this provides a new petrogenetic interpretation of Mesoarchean(～2.8 Ga)granitic gneiss in the region.The～2.7 Ga TTG magmatic zircon exhibit positive to negative εHf(t)(-4.6 to+1.5)values,indicating they were derived from partial melting of juvenile basaltic crust with a contribution of ancient crustal material.The geochemical data,in conjunction with similar ages of inherited zircon(～2.7 Ga)and zircon from Neoarchean TTG gneiss(～2.7 Ga),indicate that the～2.1 Ga granitic gneisses were derived from partial melting of the Neoarchean(～2.7 Ga)TTG gneiss.In addition,newly-identified Neoproterozoic(800-700 Ma)granitic gneiss within were derived from the Yangtze Craton(YC)Triassic-subducted continental crust rock,and the Jurassic granite derived from the partial melting of the Neoproterozoic continental crust rocks of the Yangtze subducted under the southeast margin of the North China Craton during the Triassic.(3)The Wuhe complex at the southeastern margin of the NCC were affected by multiple tectono-magmatic-metamorphic thermal events at ca.2.5 Ga,1.85-1.80 Ga,220 Ma and 160 Ma.These rocks experienced a granulite-facies metamorphic stage accompanied by partial melting,as testified by clinopyroxene+rutile+K-feldspar+spinel+quartz+apatite inclusions in zircon metamorphic domains which were dated at～2.5 Ga.The occurrence of this metamorphic stage is also supported by the lower ∑REE contents,negative Eu anomalies,and high Ti-in-zircon temperatures(>800 ℃)of metamorphic zircon mantles dated at～2.5 Ga.The studied rocks suffered a later～1.85 Ga metamorphic overprinting,possibly related to the Paleoproterozoic collisional orogeny recorded in the region.Metamorphic overprinting at～220 Ma,anatexis at～160 Ma,and the presence of～750 Ma and～220 Ma inherited zircons in a 156 Ma granite strongly suggest that the YC were underthrust beneath the southeastern margin of the NCC by the late Triassic during continental subduction associated with the Dabie-Sulu orogeny.(4)Through the whole rock phase equilibrium modelling of the amphibolite,the experimental results show that the melt composition of the partial melting of the mafic rocks is mainly controlled by the composition of the whole rocks,the melting degree and the water content.The melt formed by the low potassium mafic rocks is also very low potassium tonalitic melt,while the relatively sodium rich mafic rocks produce mainly trondhjemitic-granitic melts.The higher the temperature and the higher the water content,the more favorable the formation of tonalitic melt,on the contrary,the lower the temperature and the lower the water content,the more favorable the formation of trondhjemitic-granitic melt.In general,the source of the～2.5 Ga TTG from the North China Craton are considered to be inappropriate for partial melting of the amphibolite in this area.