Study And Numerical Simulation Of Archean Tectonic Patterns In The Eastern Part Of The North China Craton

The tectonic pattern of the Archean crust has been one of the hot topics in the field of geology.The Anshan area of Liaoning Province is located in the eastern margin of the North China Craton,where the Archean TTG series,greenschist facies metamorphic basic-intermediate-acid volcanic/volcanic sedimentary rocks and Fe-Si sedimentary formations are widely distributed,which have formed a typical Archean granite-greenstone belt.In addition,the rich records of tectonic deformation in the granite-greenstone belt provide important clues for the exploration of the early crustal tectonic evolution.Based on the investigation of Archean rock compositions and macro-microstructure styles in Anshan area and the detailed anatomy of the shear deformation zones in Qiashashan,Hujiamiaozi and Baijiafen,this paper tries to discuss the evolution mode of the early crustal tectonic system with the numerical simulation method.The BIF iron mine of Qidashan and Hujiamiaozi is composed of iron-rich BIF and the iron-poor BIF beside.The two are in fault contact.The surrounding rocks,including Yingtaoyuan Formation of Anshan group and Qidashan granite,all develop strong ductile shear deformation,forming the Qidashan-Hujiamiaozi ductile shear zone(Qidashan shear zone).In Qidashan ductile shear zone,the granitic rocks are characterized by the plastic elongation of feldspathic minerals,development of banded structures,north-dipped foliations,the asymmetric fabric characteristics and the occurrence of steep normal sliding shear action toward NWW.The macroscopic structural pattern shows that the closer the deformed rock is to the rich iron ore body,the more developed the schistose,gneissic and lineation,and the stronger the deformation degree.The results of microfabric analysis show that the characteristics of quartz wave extinction,deformation zone,bulge-type and subparticle rotational recrystallization.Feldspar plastic elongation,fine granulation and a series of meso-low temperature microdeformation fabric characteristics.The structure analysis of quartz c-axis electron backscattering diffraction(EBSD)reveals that quartz is dominated by the slip system of reshaped surface <a> and bottom surface <a>,and the rock has undergone medium and low temperature non-coaxial deformation.According to the deformation behavior of mineral and quartz crystal of preferred orientation of ductile shear zone in the rock deformation temperature of about 400~500?,the rock deformation characteristics mainly dislocation creep.The finite strain analysis results show that,close to the direction of iron ore belt,the tectonite type changes from L=S tectonite to LS tectonite,and the strain strength of the rock shows an obvious strengthening trend.The Baijiafen ductile shear zone,located in the west of Qidashan and Hujiamiaozi BIF iron minedevelop in the contact position of the greenstone belt and the Archean granitic rocks,Anshan,NWW oriented and NEE dipped.In the Baijiafen shear zone,there are mainly two rock units,namely,Chentaigou porphyritic granitic gneiss and Baijiafen gneiss,and a small amount of strongly deformed chlorite schist and phyllite in Anshan Group.The S-C fabric and the mineral rotating debrisboth indicate the characteristics of the steep sliding shear in the direction of SEE in the shear zone.The microdeformation characteristics of rocks show that the quartz grains are characterized by wave extinction,deformation grains and lamella,and recrystalization consists mainly of expansion recrystallization and subparticle rotation recrystallization.Plagioclase develops of mechanical twins and deformation zone.The plagioclase remnant spot is usually in the form of rotating eyeball structure,indicating the dextral shear action and the rock deformation temperature condition is relatively low.Finite strain analysis shows that the porphyry granitic gneiss in Chentaigou has planar-tensile strain characteristics and belongs to L=S type tectonites.Baijiafen Ordovician granitic gneiss is LS tectonite,which shows the deformation characteristics of general tensile strain.Quartz C axial fabric analysis showed that the quartz mainly experienced the sliding on the underside <a> and rhomb<a> with the deformation temperature of ~450?.Also,the closer the deformation rock is to BIF iron ore,the stronger the deformation is.From themacro-micro structural analysis above,we found that Qidashan and Baijiafen ductile shear zones are situated along the Qidashan-Xidabei BIF iron ore belt on both sides linearly and symmetrically,while they are on the different wings of the synclines,respectively.These two ductile shear zones are situated on the contact of the syncline structure and granitic rocks.In the Baijiafen ductile zone,the foliations are characterized by the NEE dipping and moderate dipping angles,which are consistent with the southwestern wing of the syncline,and the mineral stretching lineation are SEE dipped with a middle-large angle,indicating a SEE-directed steeply dipping shearing action along the southwestern wing of the syncline;In Qidashan ductile shear zone,there develop NWW-directed steeply dipping foliations(~ 90°),which is consistent with the ore body in the northeastern wing of the syncline.And the mineral stretching lineation reflects the nearly-west steeply dipping shearing action along the northeastern wing of the syncline.The kinematic chracteristics of steeply dipping sliding on both sides is consistent with the geometry characteristics of the greenstone belt.Thus,we speculated that the strong shear deformation beside the green belts is related to the formation of the vertical dome and keel structure.The kinematic characteristics of rock deformation is consistent with the diapirism and sinking involved in the vertical tectonic mode,which reflects vertical tectonism in synform greenstone belt.The ductile shearing action,occurring in the formation of greenstone vertical dome and keel structure,is characterized by a certain degree of gravitational disequilibium between the lighter granitic rocks and heavier ferrous greenstone formation.Driven by the regional geological movement and magmatism,the upward diapirism of granitic rocks and the downward sinking of the ferrous formation occurred.In order to further verify this tectonic model and its occurrence process,we carried out a numerical simulation on the above tectonic model.Numerical simulation is based on previous geological data,combined with macro and micro geological researches and supplemented by geophysical data and chronology data,etc.,to build a comprehensive model of boundary conditions and geological evolution model.2D finite difference(I2VIS)and mark-in-cell(MIC)technology were used to perform the numerical simulation on a large-scale computing unit in the Marine dynamics laboratory of the school of earth sciences,Ocean University of China.The simulation results show that the vertical tectonic genesis of the granite-greenstone belt in Anshan area should be controlled by the upper crust rock activation,lower viscosity and lower density of TTG caused by large-scale magmatic activities in the late Neoarchean.The simulation results also indicate that the Archean vertical tectonic system may be an important mechanism of crust-mantle material exchange cycle in the early earth evolution.The overall characteristics and structural changing law of granitic-green belt,and the morphology,and distribution of BIF in Anshan can compare to other granite-greenstone belts in the world.The ore-body enrichment characteristics associated with the downward subsidence of the iron-bearing green rocks under the vertical tectonic mechanism and the formation and evolution process of the greenstone belt syncline structure provides certain theoretical basis for the study of the ore-controlling model of the sedimentary metamorphic iron ore deposit and the strategic arrangement of deep ore prospecting in Anshan area.