| The Plate tectonic paradigm is an important approach to understand geodynamics in the early Earth,as it presently controls the material circulation between the Earth’s surface and interior through magmatic activity and plate subduction,influencing the evolution of the atmosphere,hydrosphere and biosphere,and is a key geological process in the formation and evolution of a habitable Earth.The operation timing and style of plate tectonic have always been the frontier and hot topic in geosciences.However,due to different understandings of the defining indicators of plate tectonic paradigm,and the few geological records of the Earth’s early history,the proposed plate tectonic operation time is very different.Based on previous research,this thesis proposed that the modern paradigm of plate tectonics（modern plate tectonics）has the following three core features:1.the existence of rigid lithospheric plates;2.the ability to continuously disperse and converge between rigid lithospheric plates to form a new oceanic crust or make the oceanic crust disappear,i.e.,the existence of a life-death cycle of the oceanic lithosphere;3.the existence of large-scale connected plate boundaries.In fact,there were a large number of cratons or super-cratons at the end of the Archean,showing that the lithosphere was rigid enough at that time.Therefore,as long as we can find the above evidence for existence,and a lack of evidence for non-existence in the rock records of the Archean,there will be enough confidence to support that modern plate tectonics operated in the Archean.Ophiolite is a characteristic component of convergent margins,and it represents an ideal sequence of oceanic lithosphere.The complete ophiolite suites consist of temporally-and petrogenetically-related rock associations,namely,pelagic sediments,pillow basalts,sheeted dikes,gabbro complexes,mafic-ultramafic cumulates and mantle tectonic rocks（harzburgite or lherzolite）.However,during the closure of the paleo-ocean basin,the oceanic lithosphere is typically tectonically dismembered and mixed with other rocks（such as terrigenous materials）to form the ophiolitic mélange of accretionary or collisional orogens.Ophiolitic mélange thus is one of the important hallmarks to identify the boundaries of ancient convergent plates or accretionary terranes,recording the important geological information of the process of plate convergence.The North China Craton（NCC）is the largest craton in China,and has experienced a protracted evolution since the Eoarchaean.The Eastern Hebei complex is located in the north-central part of the Central Orogenic Belt（COB）of the NCC.We carried out detailed field mapping and structural analysis on the newly identified Zunhua and Shangying ophiolitic mélanges in the Eastern Hebei Complex,and also completed detailed mineralogical,geochronological,isotopic geochemistry and metamorphic petrology on the ultramafic-mafic blocks within the mélanges.The results aid determination and evaluation of the emplacement processes of these Neoarchean ophiolitic mélanges in Eastern Hebei,and place constraints on other processes,such as the duration of subduction-collision,the depth of subduction,and the emplacement mechanism of the ophiolitic mélanges.We next elaborate on the significance of these results for the tectonic evolution of the Eastern Hebei Complex and the NCC during the late Neoarchean,and finally discuss the relevance to the onset time of the modern plate tectonics and early geodynamic process.The main results obtained through this study are summarized as follows:1.Blocks within the Zunhua ophiolitic mélange are mainly metamorphosed harzburgite/lherzolite,dunite containing podiform chromite,pyroxenite,amphibolite,metabasite and a small amount of intermediate volcanic rocks,chert and lenticular BIFs,which are scattered in the matrix of strongly sheared biotite plagioclase gneiss.Based on the characteristics of field structures,petrography,zircon morphology and geochronology,the protolith of biotite plagioclase gneiss in Zunhua area is considered to be greywacke,with minor interbedded volcanic rocks and metapelite.Detailed field mapping and structural analysis show that the rock association has a typical"block-in-matrix"fabric,similar to Phanerozoic mélanges worldwide.Zircon U-Pb dating of metadiabase block,gneiss matrix and mafic dike cutting through mélange shows that Zunhua ophiolitic mélange was formed at 2.55–2.52 Ga.The weighted average 207Pb/206Pb age of the youngest detrital zircon age group（～2.52 Ga）and the age of mafic dike（2.50 Ga）constrain that the Zunhua ophiolitic mélange emplaced at2.52–2.50 Ga.The geochemistry of ultramafic-mafic blocks in the Zunhua ophiolitic mélange shows that there are close geochemical,petrogenetic and spatio-temporal links between the depleted mantle tectonites and the mafic rocks produced by partial melting and metasomatism of the depleted mantle,indicating that they together represent a part of the same Neoarchean forearc（SSZ）ophiolite.Metamorphosed intermediate-mafic volcanic blocks have a systematic and continuous geochemical sequence,from N-MORB to arc geochemical features,similar to the well-studied forearc basalts formed at the initial stage of subduction（Izu–Bonin–Mariana forearc basalts）.In addition,detailed EMP analysis of chromites enclosed in dunite of harzburgite blocks shows that these chromites have high Cr#value（80.82–86.29）and low Ti O2content（0.09–0.23wt.%）,which is similar to the typical SSZ chromites.Moreover,the composition of the core of chromite indicates their affinity with the boninitic magma,suggesting that the formation of Zunhua chromite is related to the rock-melt reaction of the depleted SSZ-type peridotite and the boninitic magma,which occurred in the forearc region during subduction initiation.Based on the above evidence,blocks in the Zunhua ophiolitic mélange are considered to record a Neoarchean subduction initiation event,and their disruption to form a mélange is interpreted to be a later event related to subduction/accretion and emplacement on the proto-continental margin of the recently accreted Eastern Block of the NCC.In addition,there are a large number of arc-related calc-alkaline volcanic rocks and geological records of subduction-polarity reversal in Eastern Hebei,which suggest that the Eastern Hebei Complex records a continuous geodynamic process from subduction initiation（birth）to the arc magmatism（maturity）,then to the arc-continent collision（death）and the subduction polarity reversal（re-birth）,i.e.,the life cycle of an Archean subduction zone.2.Blocks within the Shangying ophiolitic mélange include metagabbro,amphibolite,pyroxenite,hornblende,garnet clinopyroxenite and pyroxene/hornblende magnetite quartzite,which are dispersed in the matrix dominated by strongly deformed pyroxene/hornblende plagioclase gneiss.The protolith of the gneiss matrix is mainly argillaceous rock with minor metavolcanic rock and metagreywacke.Through field regional mapping and high-precision mapping of typical outcrops in Shangying,it is found that the metagabbro blocks in this area are of different sizes and have two kinds of occurrences,layered and isotropic,similar to the typical oceanic gabbro complex.The major and trace element analysis of the metagabbro blocks and its enclosed garnet clinopyroxenite shows that they have N-MORB（normal mid-ocean ridge basalt）geochemical features.Hf isotopes of zircon in metagabbro shows the source characteristics of depleted mantle,and the major and trace elements such as Mg O,Si O2,Al2O3and Ni in garnet clinopyroxenite are also similar to those of metamorphosed oceanic gabbro.In addition,zircon U-Pb dating of metagabbro blocks and granitic veins cutting through metagabbro foliation shows that the Shangying mélange was formed between 2.53 and 2.52 Ga and underwent metamorphism at the end of Neoarchean.The crystallization zircon age of the granitic vein is 2468±23 Ma,which constrains that this stage of metamorphism occurred before 2.47 Ga,and the mélange was emplaced between 2.52–2.47 Ga.In summary,the blocks in the Shangying ophiolitic mélange are the relicts of Archean oceanic crust,which formed at the mid-ocean ridge（MOR）and then got subducted,and mixed with metasedimentary rocks and exhumed in the subduction channel,forming the Shangying MOR ophiolitic mélange.Petrographic observations reveal that garnet clinopyroxenite is mainly composed of garnet（35–40 vol.%）and clinopyroxene（50–55 vol.%）,and preserves the information of the mineral paragenesis assemblage,revealing a complete sequence of prograde metamorphism,a peak assemblage,and overprinting by retrograde metamorphism.Through mineralogical and metamorphic petrological analysis,the metamorphic P-T conditions of the peak mineral assemblage（garnet core+clinopyroxene core）are estimated as 792–890°C/1.98–2.45 GPa,reaching eclogite-facies P-T conditions.In addition,the petrographic microstructure of garnet clinopyroxenites also supports the UHP-HP peak metamorphic conditions,with textures such as exsolution of needle-like rutile on the surface of garnet,exsolution of banded garnet on the clinopyroxene,and the separation of Al-rich titanite in the middle of idiomorphic ilmenite,all characteristics of retrogression from HP-UHP metamorphism.Based on the above evidence,this study proposes that metagabbros and garnet clinopyroxenites in the Shangying MOR ophiolitic mélange represent subducted Archean oceanic crust,which has been subducted to at least 65–70 km depth and was later exhumed.Our result is the first report of the eclogitic Archean oceanic crust,which extends the known age of subduction-related eclogite-facies metamorphism back by 400Myr.Combined with a series of reported geological records of the Neoarchean subduction in the eastern margin of the COB of the NCC,including the Jianping-Zunhua ophiolitic mélange in the north,the Zanhuang tectonic mélange and large-scale nappe-sheath fold structures in the center,and the Dengfeng subduction-accretionary wedge and paired metamorphic belt in the south,regional relationships indicate that there is large-scale（up to 1600 km）asymmetric subduction at the end of the Neoarchean,and locally deep subduction of oceanic crust.3.Two different types of ophiolitic mélanges are preserved in the Zunhua-Shangying structural belt of the Eastern Hebei Complex.The Zunhua ophiolitic mélange in the west mainly contains a set of genetically related forearc SSZ-type ultramafic-mafic blocks（peridotite,podiform chromite,pyroxenite,metagabbro,metabasalt,etc.）,which records the petrological evidence of the forearc oceanic lithosphere extension during the Neoarchean subduction initiation;whereas the Shangying ophiolitic mélange in the east mainly contains metagabbro and garnet clinopyroxenite blocks with N-MORB geochemical characteristics,representing the oceanic lithosphere formed in the MOR,recording the geological information of subduction and exhumation of rocks in the subduction channel.This co-existence of the contemporaneous SSZ-type and MOR-type ophiolitic mélanges in a paleo-suture is similar to the Phanerozoic Cyprus-Hatay ophiolite belt,the North Qilian orogenic belt and the Central Asian orogenic belt.The Zunhua SSZ ophiolitic mélange was emplaced in the forearc region of the Archean subduction zone through accretionary thrusting,while the Shangying MOR ophiolitic mélange in the east was emplaced in the front of the accretionary wedge near the trench in the subduction channel.The spatial configuration of the Zunhua and Shangying ophiolitic mélanges reveals a west-northwest dipping Archean subduction zone,which is consistent with the analysis of kinematic fabrics recorded in the ophiolitic mélanges.The paleo-tectonic facies in the Zunhua-Shangying tectonic belt indicates that it is a Neoarchean forearc and subduction channel complex,and also suggests that the Eastern Hebei Complex and the NCC were in a tectonic setting of convergent continental margin（subduction zone）at the end of the Neoarchean era.4.Detailed geochronological studies of the blocks and other rock units in the Zunhua and Shangying ophiolitic mélanges show that the geodynamic processes of the Archean subduction zone,including sea-floor extension,subduction initiation,arc-continent collision,forearc thrusting,and subduction channel exhumation,all occurred within the timespan of<80 Myr（2.55–2.47 Ga）,which is comparable to the duration of the same processes recorded in Phanerozoic subduction-collision zones.For example,the onset time of subduction recorded by the Mesozoic Oman SSZ ophiolite is105–95 Ma,and the timespan to ophiolite emplacement and arc-continent collision（75–70 Ma）is～35 Myr,which can be compared with the duration of the same geodynamic process（2.55–2.50 Ga）recorded by the Zunhua SSZ ophiolitic mélange.The subduction initiation of the Piemonte ocean in the western Alps is 95–85 Ma,while the time of ocean basin closure and eclogite exhumation along the subduction channel is generally considered to be 33 Ma.This duration（62–52 Myr）is also comparable to the same geological process（2.55–2.47 Ma）revealed by the Zunhua and Shangying MOR ophiolitic mélanges in the Eastern Hebei Complex.All the above indicators reveal that the plate tectonic pattern and products at the end-Neoarchean were not significantly different from those of modern times,and the modern-style plate tectonics operated in the late Neoarchean. |
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