Tetonic Styles Of The High-Grde Metamorphic Terrain Of Hengshan

The high-grade metamorphic Hengshanterrain in the northern part of the Shanxi Provinceis located at the centre of the North ChinaCarton. Itis a typical exposed crustal cross sectionof the middle to lower crust of the early Preca-mbrian metamorphic terrain in the Wutaishan-Hengshan-Datong-Jining region.The amphibolite to granulite facies Hengshangneisses consist mainly of the TTG gneissesand supracrustal rocks by adding of mantlematerials horizontally. The supracrustal rocksare composed of the mafic volcanic rocksand sedimentary rocks. The plenty ofgranitoid magma activities in the region maybe a result of anatexes.The sub-horizontal ductile shearing was acco-mpanied by an underplating of mafic-ultramaficmagma. The mafic-ultramafic rocks are alsosubjectedto granulite facies metamorphism (IBCtype). The tectonic style (Hengshan type) of asubhorizontal ductile shearing deformationexhibits characteristics of four layers framework:the supracrustal sequence free of early strongdeformation, the supracrustal sequence undergoneearly strong deformation, the straight gneisses,and the oval-typed gneisses. Separated bydetachment zone, the upper two parts similitudea hanging plate, and the lower two parts is likea footwall plate of the metamorphic core in thetwo-layer configuration of metamorphic corecomplexes.1. Supracrustal sequence free of earlystrong deformationThe early deformation of the JingangkuFormation of the Wutai Group in the HengshanMountains formed a foliation( S1) parallel tobedding ( S0) . It is characterized by gneissosityand schistosity, and intraformational and intrafolialfolds occasionally. The early deformation of thesub-horizontal ductile shearing is developedweakly in the WutaiGroup. The orientation ofthe shear movement is NNW-SEE.2. Supracrustal sequence undergoneearly strong deformationThe lower part of the Jingangku Formation,Wutai Group, in the Hengshan Mountain under-gone an early stronger deformation has a lot offelsic laminae parallel to gneissosity and ischaracterized by augen, banded and laminarstructures, flow folds and intraformationalrecumbent folds various in size, which are mostlyA-type fold with the hinge of fold pallalal tothe stretching lineation. They reflect an obviousfeature of the horizontal structure transpositionwhich is the result of sub-horizontal shearing.They may be acted as a middle crust ductileflow layer. It is suggested that the stratigraphicsequence as a whole is only slightly influencedby these deformation .3. Detachment zonesequence of the Jingangku Formation and theHengshan straight gneisses is a detachment zone.It is different types of rocks or strata of theJingangkuformation in the Hengshan and WutaiMountains that are generally overlaid on theHengshangrey gneisses. It is a discontinuousplane between the grey gneisses and the JingangkuFormationundergone later strong deformations,especiallyas a sub-horizontal ductile shear zone.The detachment zone exhibits the characteristicsof a largearea lower angular normal fault.4. Straight gneissesUnder the Wutai Group in the Hengshan-Wutai-Fuping areas, there is generally a zone ofstraight gneisses from several kilometers tomore than ten kilometers in thickness. It wasformerlycalled the Qianzhuangwang and Dian-fangtai Formation in Hengshan, the LongquanguanGroup andYulinping Formation in Fuping. Thestraight gneisses have a straight gneissositybearing a few, fine and straight felsic streaks, withstrong stretching lineation, equigranular andfiner grain size ( 1mm±), and no or weaklypreferred crystallographic orientation whichindicateanorigin of superplastic flow mechanism.5. Thearea of oval-typed gneissesThe main parts of the Hengshan grey gneissesare the oval-typed gneiss area. The alternatedistribution of stronger strain zones and weakerstrain domains shows a feature of irregular ovaland dome-shaped style on the surface, and ananastomosing pattern on the cross-sections. Itinfers three dimensional tectonic frameworksand styles of the strong strain zones and theweaker strain domains formed by sub-horizontalductile shearing. The Hengshan oval-typedgneisses are mainly augen, layering and strippedgneisses, which are formed by the adding ofmafic intrusive rocks of mantle origin andmaterials of part-melting and auto-differentiationof rocks as the result of a dilatation structuresparallel to shear zone boundary. These dilatationveins were ultramafic-mafic intrusive sheets inthe early stage, and were suffered strongboudinages and shearing in the later deformationalstage forming , , type porphyroclastssystems, and appear as a seriesof deformationzones with different sizes generally. The featuresof the leucocratic felsic veins in the middle stageand the pink granitoid veins in the later stagereflect an evolution process ofgneisses fromgneisses from deep tectonic facies to the middletectonic facies. There is a stretching lineationdeveloped in the strong deformation gneisses andmafic rocks. The deformation behavous of horn-blendes and plagioclases of thegneisses showsthat it might be the characteristics of superplasticflow mechanisms. The boudinages appear aschocolate-tablet typewith very strong extentionon sub-horizontal plane. Aseries of movementsenses showed by the boudinages andporphyroclasts of the mafic rocks are developedin the scale from hand specimen, outcrops to morelarge size. There areflow fold layers, intrafo-rmational folds and shearth folds reflectinghorizontal transposition in the gneisses. Thevarious movement senses indicatethe hanging-wallmoving down towards SEE direction. The featuresof the sinistral and dextral shearing towards NEor SW direction have found in a few part in thenorthern part of Hengshan.Consequently, the Hengshan amphibolite-granulite facies metamorphic terrain regardingas a transition zone between the middle crust andlower crust is a giant broaden ductile shear zonewith characteristics of superplasticflow mechanism.The amphibolite facies Wutai Group is madeThe contact between the supracrustalup of mafic volcanic rocks and sedimentaryrocks. Itis deformed as a bundle of cylindricalfolds striking NEE with the axial plane dippingto S at the southern parts of Hengshan. Thetectonic framework of the Hengshan-Wutaishanterrain isa huge fan-typed anticlinorium (socalled theWutai type) with large-scale magmaactivities,metamorphism (ITD type) and anatexes.It is suggested that this is a collision orogenicbelt of Wutaishan at 2 500Ma which formed anArcheancraton in the North China. This may bethe earliest surpracontinent convergent event. Itrepresents a middle crust tectonic modal of theNorth China Craton in early Precambrian.The tectonic style of the Hutuo Group mainlyconsisting of sedimentary rocks in the WutaiMountains is a huge invert-fan synclinorium(so called the Hutuo type). It illustrates thematerial components of the upper crust and theintra-continental orogenesis, and may be acted asa model of the tectonic-magma event in theworld at 2 200~1 800Ma whichwould be a newresearching hot spot after Rodinia.It is suggested that the Wutaishan, Hengshanand Fuping areas can be divided into three(Hengshan-Fuping,Wutaishan and Hutuo)structural layers and two geotectonic units bycomprehensive studies on structures, petrology,geochemistry, metamorphism andgeochronology.Two geotectonic units are divided by the NNW-NEE structural zones, i.e. the Hengshan-Fupingblock to the NE and the Wutai block to the SW,accordinly, the North China Craton may alsodivided as the NE and SW blocks.On the basisof geological events occurred in the Hengshanand adjacent area, its crustal evolution can bedescribed as formation of continental cores in theMesoarchean, passive continental margin/volcanic arc (flych) and collision orogene inthe Neoarchean, Paleoproterozoicrift, Paleoproterozoic continental orogen and break-up of rigidicplate in the Mesoproterozoic stages.