Neoarchean Accretionary-to-collisional Orogenesis In The Southern North China Craton And Its Geodynamic Implications

The formation and growth processes of continental and oceanic crusts,and the tectonic/geodynamic mechanisms on the early Earth are hotly debated.The North China Craton(NCC)contains a few scattered remnants of Eoarchean crust,but the craton formed largely through Neoarchean crustal growth,terrane assembly,and Paleoproterozoic rifting and continent–continent collision leading to a prolonged period of relative stability until the Paleozoic.The NCC thus provides a natural laboratory for exploring continental crustal growth and secular changes of geodynamic regimes on the early Earth.However,due to the limited outcrops of Archean-Paleoproterozoic rocks and the emphasis of different methods,diverse viewpoints still exist regarding the detailed evolutionary process of the NCC during the Archean-Paleoproterozoic,with intense debates,in particular,on the nature and geodynamic backgrounds of the late Archean(?2.5 Ga)tectonic events.The southern margin of the NCC contains outcrops of typical late Archean lowmiddle grade granite–greenstone belts,which provide a critical window to decipher the tectonic evolution of the NCC and the Neoarchean geodynamic regimes in general.This dissertation focuses on the Neoarchean granite–greenstone belts(Dengfeng and Angou complexes)in the southern part of the Central Orogenic Belt(COB)of the NCC,and uses multidisciplinary methods of structural geology,sedimentology,petrology,geochronology,geochemistry,and phase equilibrium modelling,to determine the age,petrogenesis,metamorphism,deformation characteristics,and tectonic settings of different litho-tectonic units of these complexes,and then discuss the geodynamic backgrounds and implications of the ?2.5 Ga tectonic events.Based on the recognition of intra-oceanic arc–forearc–accretionary complexes and associated paired metamorphism,a model that includes the Neoarchean intra-oceanic subduction,forearc accretion and subsequent arc–continent collisional processes was proposed to account for the tectonic evolution of the southern NCC,which also provides key evidence for the operation of plate tectonics at the end of Archean.Therefore,this study is of great importance for deepening current understanding of major scientific issues including the Neoarchean tectonic evolution and crustal accretion history of the southern NCC,and late Archean geodynamic processes in general.The main achievements were summarized as follows:(1)An integrated study including field mapping,structural analysis,geochronology,and geochemistry was carried out on the Dengfeng greenstone belt(metavolcanosedimentary assemblage)to determine its formation age,structural deformation,and tectonic settings;and structurally juxtaposed intra-oceanic arc–forearc–accretionary complexes were recognized.The Dengfeng greenstone belt in the southern section of the COB of the NCC consists of structurally juxtaposed slices of meta-ultramafic,metamafic,and felsic igneous rocks,metasedimentary rocks,including minor banded iron formation(BIF).The complex was metamorphosed to greenschist to amphibolite facies and intruded by ca.2.50–2.42 Ga mafic and felsic plutons/dikes.Detailed field mapping and structural analyses show that the different lithostructural units,including 1)a metamafic-dominant unit;and 2)a metasedimentary-dominant unit,are in tectonic contact,with complex thrust imbrication and multiple brittle and ductile deformation.The metamafic-dominant unit in the west is composed of metagabbro,metabasalt,high-Mg O diorite,granitic sill/dikes and minor sedimentary rocks.The metabasalts in this unit possess geochemical characteristics of mid-oceanic ridge basalt(MORB)and island arc tholeiite(IAT);coupled with coeval sanukitoid-like metadiorite,and adakitic sills/dikes,this unit was interpreted as the structurally dismembered Neoarchean forearc complex.The metasedimentary-dominant unit in the east consists of coherent schistmetabasalt sequences,metaturbidites,and chaotic mélanges.The metasedimentarydominant unit closely resembles Phanerozoic accretionary complexes in terms of lithostratigraphy and structural patterns.Accordingly,we interpret the Dengfeng greenstone belt as Neoarchean forearc and accretionary complexes consisting of a dismembered forearc complex in the west and accreted oceanic plate stratigraphy in the east that were structurally imbricated at a convergent plate margin.In addition,geochronological data indicate that the western Dengfeng Complex consists of 2.54–2.51 Ga TTG rocks,which is interpreted as the nucleus of an oceanic arc in the COB;whereas the eastern Dengfeng Complex is relatively old(2.66–2.55 Ga)and is interpreted to be a part of the Eastern Block of the NCC.The kinematic indicators and the spatial configurations of different tectonic units suggest a near-southwest-dipping intra-oceanic subduction zone beneath the arc in the COB,which later evolved into an arc–continent collision with the Eastern Block.The metamorphic age and widely distributed mafic-felsic intrusions/dykes indicate that the collision occurred at about 2.50 Ga.The accreted arc and accretionary prism are unconformably overlain by a clastic sedimentary wedge,the lower part of which has a maximum depositional age of ca.2.45 Ga and is interpreted as a foreland basin sequence related to this collision.Documentation of the Neoarchean Dengfeng forearc and accretionary complexes demonstrates that ca.2.5 Ga intra-oceanic subduction,oceanic plate stratigraphy accretion,and arc–continent collisional events occurred in the southern section of the COB of the NCC.The accretion of the 2.54–2.51 Ga arc to the continental margin of the Eastern Block marks an early episode of mountain building in the COB.(2)Through comprehensive petrology,petrography,mineral chemistry,zircon and titanite U-Pb geochronology and phase equilibrium modelling,the metamorphic ages and peak metamorphic P–T conditions of variable 2.54–2.51 Ga metabasites,metapelites and TTG gneiss in the Dengfeng Complex were constrained,which suggests that the Dengfeng Complex can be viably explained as a remnant of a paired metamorphic belt.In the western part of the Dengfeng Complex,tonalitic gneiss from the intra-oceanic arc unit partially melted and preserved leucosome and leucogranitic veins.Phase equilibrium modelling shows that its peak P–T condition of partial melting is ?750–810°C/5.6–8.8 kbar,corresponding to high geothermal gradients of 875–1400°C/GPa.The garnet amphibolite from the high-strain zone in the forearc unit also preserved evidence of partial melting.It has peak metamorphic P–T conditions of 685–755°C/6.3–8.4 kbar,corresponding to geothermal gradients of 925–1190°C/GPa.The accretionary complex unit in the eastern part of the Dengfeng greenstone belt contains some garnet amphibolite blocks and garnet-bearing metapelite matrix.Two garnet amphibolite blocks and three garnet-bearing metapelites yielded peak metamorphic P–T conditions of >9.8 kbar/525–655°C,which correspond to moderate geothermal gradients of ?425–600°C/GPa.SHRIMP and LA-ICP-MS zircon and titanite U-Pb geochronological data indicate that the peak metamorphism occurred at ?2.52–2.50 Ga.Therefore,metamorphic evidence indicates that the arc–forearc complex in the west recorded metamorphism with high geothermal gradients,while the accretion complex in the east recorded moderate-T/P metamorphism,which together can be explained as a spatially and temporally linked Neoarchean paired metamorphic belt.The moderate geothermal gradients recorded by the accretion complex are consistent with the slab-top geotherms reconstructed from the Neoproterozoic–Phanerozoic “warm” subduction zones.The metamorphic P–T data indicate that the subduction-accretion complex was buried to >30 km,then exhumed to the near-surface by ca.2.45 Ga,which was unconformably covered by the Paleoproterozoic foreland basin sequences of the Songshan Group.The documentation of the Neoarchean Dengfeng arc-forearc and subduction-accretion complexes and the Neoarchean paired metamorphic belt suggest that modern-style asymmetric plate subduction has been operating at the end of the Neoarchean,which provides key geological evidence for the operation of plate tectonics.(3)Integrated field,petrology,geochemistry,zircon U–Pb geochronology and LuHf isotope,and phase equilibrium and trace element modelling studies have been carried out on the Angou Complex,which provides new constraints on the composition,formation age,petrogenesis and tectonic settings of different litho-tectonic units that could have been formed in the divergent and convergent plate boundaries.Field mapping and structural analysis show that the Angou Complex can be divided into three main lithostructural units: 1)the western Angou Complex composed mainly of TTG gneisses and minor amphibolite;2)the central Angou Complex,composed predominately of amphibolite,metavolcanic rocks,and metasedimentary rocks;3)the eastern Angou Complex,dominated by amphibolite and metasedimentary rocks.Different lithostructural units are in contact with thrust faults or shear zones,characterized by strong foliations.Zircon U-Pb dating data indicate that the Angou Complex was formed at ?2.54–2.51 Ga and metamorphosed at 2.50–2.48 Ga.The protoliths of amphibolites are basalt,which can be divided into MORB-like and island arc basalt(IAB)-like types according to geochemical compositions.MORB-type metabasalts are distributed in the eastern Angou Complex,with locally preserved pillowlike structures.They have nearly-flat rare earth element(REE)patterns,without obvious anomalies of high field strength elements(HFSEs),suggesting that they were derived from partial melting of MORB-type mantle in the spinel peridotite stability field.IABtype metabasalts in the central Angou Complex have enriched light REE(LREEs)and large-ion lithophile elements(LILE)and significant negative anomalies of HFSEs(e.g.,Nb,Ta and Ti),indicating that they were likely formed by partial melting of subduction melts/fluids-metasomatized enriched mantle wedge in the garnet-spinel peridotite stability field.TTG gneiss has strongly enriched LREE,depleted heavy rare earth elements(HREE)and significant negative anomalies of HFSEs such as Nb,Ta and Ti.Phase equilibrium and trace element modelling show that they were likely derived from low-degree(<18%)partial melting of subducted MORB-type metabasalts with the main residual phases of garnet–clinopyroxene–rutile–quartz ± amphibole ± plagioclase in the source at P–T conditions of 750–900°C/1.35–2.0 GPa,which corresponding to depths of ?45–65 km and moderate-to-low geothermal gradients of ?450–600 °C/GPa.Those P–T conditions are consistent with warmer geotherms and shallower degrees of Neoarchean subduction zones.Felsic metavolcanic rocks in the central Angou Complex are dominated by metaandesite,metadacite,metarhyolite,and minor metabasaltic rocks.Felsic metavolcanic rocks are enriched in LREE and LILE,and depleted in HREE and Nb,Ta and Ti,and high Sr/Y ratios,typical of subduction-related volcanic rocks.Accordingly,the felsic metavolcanic suite could have been produced by partial melting of the subducted oceanic subducted slab.According to above geochemical and structural characteristics,it's proposed that the western Angou Complex(TTG and metabasalt)could represent the nucleus of an oceanic island arc;the central Angou Complex(IAB,felsic volcanic rocks and metasedimentary rocks)represents island arc–forearc complexes;and the eastern Angou Complex represents accreted oceanic plate stratigraphy(OPS,MORB–BIF–chert –shale–limestone)and passive continental margin sedimentary sequences.Therefore,the Angou Complex exhibits obvious asymmetric lithological–structural–geochemical zoning,including passive continental margin sequences,MORB-dominated OPS,oceanic island arc–forearc complexes from east to west.Spatial configuration and kinematic data from different tectonic units indicate the westward polarity of subduction.Structurally juxtaposed litho-tectonic units record typical magmatic and tectonic processes in the divergent and convergent plate boundaries,which thus suggest that the plate tectonic regime has been operating at the end of Archean.(4)Based on a comprehensive study on the Neoarchean Dengfeng Complex and Angou Complex in the southern NCC and a review of recent regional data,a model is proposed to explain the Neoarchean-Paleoproterozoic tectonic and geodynamic evolution of the NCC.There was a Neoarchean(ca.2.54–2.50 Ga)intra-oceanic arc terrane(WutaiDengfeng Arc)outboard of the western margin of the Eastern Block.Westward intraoceanic subduction and forearc accretion events occurred along the eastern margin of the arc.TTG rocks were produced in the nucleus of the arc through partial melting of the subducted mafic oceanic crusts with contributions of subducted sediment-derived fluids/melts.The basalt–high-Mg diorite–adakitic sills/dikes were developed in the Dengfeng forearc region,while the Angou region preserves a subduction-related basalt–andesite–dacite–rhyolite arc–forearc assemblage.The structurally repeated OPS sequences were locally accreted onto the forearc to form an accretionary complex,where a moderate T/P metamorphism consistent with slab-top geotherms of warm subduction zones was recorded.Together with high T/P metamorphism in the arc–forearc regions,a remnant of a Neoarchean paired metamorphic belt is locally preserved.In the late Neoarchean to early Paleoproterozoic(ca.2.50–2.47 Ga),an arc–continent collision event occurred,which led to the accretion of the Wutai-Dengfeng arc terrane onto the western margin of the Eastern Block of the NCC,leading to the formation of the Neoarchean accretionary-to-collisional COB.The newly-assembled COB was unconformably overlain by the Proterozoic Songshan Group that has a maximum depositional age of ca.2.45 Ga in its low section,and was intruded by potassic plutons.Other evidence of coeval accretion–collision events are recorded in the Zanhuang mélange,Zunhua mélange,and the Jianping forearc complex in the central and north sections of the COB,which together with the Dengfeng and Angou complexes in the southern section,probably represent one of best-documented Neoarchean linear arc–continent collisional suture zones in the world.The accretion of an arc terrane and arc–continent collision reveal details of an important Neoarchean accretionary to collisional orogenic event in the NCC,which played an important role in the lateral growth of the NCC.The coeval metamorphic,magmatic,and sedimentary records suggest that extensive collision and amalgamation between micro-continental blocks and arc terranes led to the formation of the proto-NCC consisting of the Eastern Block and the accreted arc(s)of the COB.The accretion of juvenile arcs to the continental nuclei was likely the main mechanism for the lateral crustal growth of the NCC.On a global scale,many cratons record similar metamorphic and magmatic events during the period of ca.2.8–2.5 Ga,which may represent the formation events of cratons/supercratons by the amalgamation of arc/microcontinents in the late Neoarchean,following the widespread operation of the plate tectonic regime.