| The Heihe-Hegenshan suture is the boundary between the Xing’an and Songnen blocks.However,due to the lack of relevant geological evidence(e.g.ophiolite)in the northern section of the suture,and the influence of later transform,the location of the suture extending northward is still unclear.And due to the lack of understanding of the suture and related magmatic arc,there are also disputes about the polarity of the subduction of oceanic slab(called Nenjiang Ocean)between the blocks,including westward subduction and bidirectional subduction.In addition,the Duobaoshan island arc,developed in the northern part of the suture,is one of the areas in the eastern part of the Central Asian orogenic belt that experienced the longest time span of mineralization and the most complex mineralization.Under the superimposed influence of the three tectonic domains,the largest porphyry copper system in northeast China was formed.However,the deep structure,dynamic background and deep driving mechanism of the mineralization system are still unclear.Deep structure can provide additional constraints for geological problems,and is of great significance for solving deep geological disputes.To reveal the deep position and structure of the suture,the magmatic arc and the metallogenic system,137 broadband magnetotelluric(MT)stations and 37long-period magnetotelluric stations were arranged in the northern section of the Heihe-Hegenshan suture.After processing and analyzing the original data,it was found that the main electrical strike direction was NE40 °,and the deep part had obvious three-dimensional characteristics,which required to carry out three-dimensional inversion.Considering the multi-solution of 3D inversion,this study used two 3D inversion techniques,tetrahedron and hexahedron,to establish a multi-scale electrical structure model for the study area.Firstly,the long-period MT data were inverted using hexahedron grid,and a 200 km smooth resistivity model was obtained.Based on the long-period model,we analyzed the structure of the lithosphere and asthenosphere in the study area.Secondly,all broadband MT data were inverted using tetrahedral grid,and a detailed resistivity model at a depth of 60 km was obtained.Based on this model,we studied the position and deep structure of the suture,early and late Paleozoic magmatic arc.Finally,after further densifying the tetrahedral grid,the broadband MT data covering the Duobaoshan island arc were inverted.And the inversion model revealed the deep structure of the metallogenic system.The multi-scale 3D electrical models obtained by three-dimensional inversion technology based on two grids have good coupling,indicating the stability of the 3D inversion results.In addition,sensitivity tests were carried out for each model to further ensure the reliability of inversion results.Long-period MT 3D electrical result revealed the characteristics of the electrical lithosphere-asthenosphere boundary(e LAB)and the lithosphere structure in the study area.The lithosphere is characterized by high resistivity with a thickness of 140-150 km,and the asthenosphere is characterized by low and medium resistivity.In the block collision area,the depth of e LAB is significantly shallower(～90 km),and the asthenosphere shows obvious high conductivity characteristics.In addition,there are two obvious high conductivity anomalies in the lithosphere that extend from the upper crust to the asthenosphere,connecting the high conductivity body in the asthenosphere.The electrical structure shows that the e LAB in the north section of the Xing’an and Songnen blocks is locally shallow,which may represent a weak zone in the lithosphere.Combined with the low terrestrial heat flow value in the study area,the lithospheric high conductivity anomaly that connects the asthenosphere may indicate a large-scale material exchange has occurred between the asthenosphere and the lithosphereThe 3D MT result of the broadband data revealed the detailed electrical structure of the crust and mantle.Combined with the deep seismic reflection,gravity,magnetic and surface geology,we have identified and divided the distribution and deep structure of the suture and magmatic arc.Based on the electrical results,the Heihe-Hegenshan suture,which is located on the east side of the Duobaoshan island arc,is characterized by a 50 km wide and "broken" electrical characteristics.Several conductors developed in the suture may be related to the fluid filling in the suture zone.The Duobaoshan island arc,as an early Paleozoic magmatic arc,is characterized by high conductivity in the deep part.The upper crust is characterized by near-horizontal high conductivity,and the lower crust is characterized by high conductivity extending to the mantle scale.Considering the multi-stage mineralization in the Duobaoshan island arc,the characteristics of high conductivity may be related to mineralization.The Late Paleozoic magmatic arc is located on the west side of the Duobaoshan Island arc,which also shows the high conductivity characteristics.The high conductor can extend to the mantle scale,which may be related to the hydrogen and incompatible(lithophile)elements introduced by the early plate subduction.According to the electrical results,the suture,the early and late Paleozoic magmatic arc are distributed from east to west,indicating the westward subduction closure of the Nenjiang OceanBased on the 3D electrical results of the Duobaoshan island arc area,the main electrical characteristics of the porphyry copper system can be inferred:(1)the high conductivity anomaly distributed in the lower crust and mantle indicated the trace of the enrichment of mantle-derived materials;(2)The upper crust conductors distributed under the deposits indicated the trace of the differentiation and crystallization of the metallogenic material;(3)The high-conductor channels connecting the upper and lower crust represented the migration path of metallogenic material.The residual sulfide phase may be the main mechanism of high conductivity.Combined with the regional tectonic background,the enrichment of the mantle-derived materials of Ordovician,Triassic and Jurassic-Cretaceous was controlled by the lower crust of the island arc,and the enriched metallogenic materials migrated to the upper crust along different paths for further mineralization.During this process,the subduction of the Paleo-Asian Ocean formed the early structural framework,and the reactivation of the pre-existing structure by the Mongolia-Okhotsk Ocean and the Paleo-Pacific Ocean is the deep driving mechanism of multi-stage mineralization.Based on the magnetotelluric method,the deep electrical structure of the northern section of the Heihe-Hegenshan suture was obtained.Based on the electrical structure,combined with previous studies,we revealed the deep structural characteristics of the suture,the magmatic arc and the metallogenic system,determined the westward subduction and closure mode of the Nenjiang Ocean,established a complete metallogenic system structure,and proposed the control of different activation of three tectonic domains on multi-stage mineralization events. |
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