The Compositional And Structural Characteristics Of The Subduction-Accretion Melange Belt In Hailesitai,Inner Mongolia

In the past three decades,the study of Hegenshan-Heihe suture zone has achieved rich results in petrology,structural geology,geochemistry,isotope geochemistry and chronology.However,there is no evidence for the existance of the geological records of suture zone in the middle part from Xiwuqi to Zhalantun for hundreds of kilometers.In addition,the material composition,subduction direction,closing time and structural attribution of the Hegenshan-Heihe suture zone are still controversial.Using the new theory and method of geological mapping of orogenic melange,in 1:50,000 regional geological survey of Niufentai Forest Farm area of Inner Mongolia,the original Jiageda Formation in the area was determined as Early Paleozoic Hailestai subduction accretion complex based on Geology,petrology,rock geochemistry,isotope geochemistry and geochronology.This is the first time that the distribution of subduction accretive melanges has been found in the area of Hellestai,Qianqi,right wing of Horqin.The discovery fills the gap in the middle of the Hegenshan-Heihe suture zone and is of great significance to the determination of the early Paleozoic ocean continent transition process and tectonic evolution of the Central Asian orogenic belt and the paleo Asian Ocean tectonic domain.The melange belt in Hailestai is divided into matrix and rock block.The matrix mainly includes mylonite,thousand mylonite,ultramylonite and a small amount of siltstone,fine sandstone,tuffaceous sandy conglomerate and sedimentary tuff.The protolith is mainly a set of sandy argillaceous clastic rock formation intercalated with volcanic rocks,which is formed in the shallow sea and semi deep sea in front of the arc.Two detrital zircon U-Pb ages are obtained in the matrix.The youngest detrital zircon age and peak age are comprehensively analyzed.It is considered that the formation age of the matrix is late Cambrian Middle Ordovician.The matrix of the early Paleozoic Hailestai subduction-accretionary melange has strong ductile deformation,schistosity wrinkle is widely developed,and shows the characteristics of superposition and replacement of two stages of ductile shear deformation.The strike of the ductile shear zone in the matrix is about NE50°.It is considered that the early ductile deformation tends to northwest,mainly left strike slip shear,and the late ductile deformation zone tends to Southeast,which is characterized by left strike slip and thrust shear,through the comprehensive analysis of the macro and micro deformation structures developed in the belt,such as sheath fold,asymmetric fold,“domino”,“S-C fabric”and rotating fragment system.There are oceanic island seamount blocks,Mid ocean ridge block,intra ocean arc block and micro paleocontinent block.The overall characteristics of the blocks are basically non-deformed,and the schistosomes surrounding the blocks（eyeball and strip）mylonite and ultra-mylonite revolve around the blocks.The lithology of the oceanic island seamount block is accumulative amphibolite,amphibole gabbro,basalt,olivine basalt,and so on,which is ultrabasic rock-basic rock.The LA-ICP-MS zircon U-Pb isotopic age of 478 Ma is obtained from amphibole gabbro,and its formation environment belongs to oceanic island seamount environment（OIB）.The lithology of Mid ocean ridge block is mainly composed of（amphibolite）gabbro,actinolite basalt and diabase,with the Si O₂content ranging from 40.56 to 51.35%,belong to ultrabasic rock-basic rock range.The LA-ICP-MS zircon U-Pb isotopic age of 463 Ma was obtained in gabbro.The lithology of intra ocean arc block is mainly andesite,basalt andesite,amphibolite diorite,diorite,granodiorite.The LA-ICP-MS zircon U-Pb ages of schistosized andesite and andesite are 540 and 512 Ma respectively.The lithology of the Neoarchean rift block is quartz diorite,which transited partly to quartz diorite and formed in the environment of volcanic arc environment related to thrust.The LA-ICP-MS zircon U-Pb isotopic age of 2544 Ma was obtained from fine-grained biotite amphibolite quartz diorite.LA-ICP-MS zircon U-Pb ages of 496±4 and 460±2Ma were obtained from arc basalt and basaltic andesite on Duobaoshan Island,respectively,which representatives has large-scale island arc volcanism and eruption in the Middle Ordovician wangyuanshan area.The LA-ICP-MS zircon U-Pb ages of 460±3 Ma and 450±2 Ma were obtained from hornblende gabbro and Ordovician granite intruded into Hailestai subduction accretionary complex,respectively,which were formed in continental margin arc environment.Combined with the age of matrix and rock block of subduction accretionary melange,hornblende gabbro of intrusive melange（460 Ma）,age of TTG rock association（450 Ma）,deformation degree,contact relationship,The final formation and emplacement age of Hailestai subduction accretionary complex is determined as late Ordovician.The Duobaoshan island arc belt is about 40km to the northwest of the subduction-accretion melange belt in Hailesitai,which mainly includes dark gray,grayish green,grayish white,light gray basalt,andesite,dacite crystal debris（rock debris）tuff,dacite crystal debris fusion tuff mixed with（fine）sandstone,siltstone,silty mudstone and mudstone.LA-ICP-MS zircon U-Pb isotopic ages obtained from basalt and basaltic andesite in Duobaoshan island arc are 496±4 MA（n=13,mswd=0.13）and 460±3 MA（n=20,mswd=0.14）,respectively,which show that they are middle Ordovician.It is proposed that the Duobaoshan island arc volcanic sedimentary rock assemblage in the study area was formed in the island arc（continental arc）environment under the background of Ocean plate subduction,and was the product of the island arc volcanism of the subduction and accretive orogeny of the hellestai ocean（ancient Asian Ocean branch）to the Xing’an block.And A NE trending Ordovician island arc magmatic belt is developed on the south side of the Hailesitai subduction-accretion melange belt,which includes hornblende gabbro,hornblende diorite,diorite and TTG rock series（granodiorite,tonalite and Ordovician granite）.The rock mass intrudes into the hailestai subduction accretionary melange,locally develops mylonite xenoliths,and no obvious ductile deformation occurs,indicating that the set of rocks is not involved in the subduction accretionary melange belt,and its diagenetic age restricts the upper limit of the formation of the set of melanges.LA-ICP-MS zircon U-Pb isotopic ages of 460±3Ma（n=23,mswd=0.24）,461±2 Ma（n=19,mswd=0.37）,461±4 Ma（n=13,mswd=2.0）,450±2 Ma（n=16,mswd=0.35）are obtained from hornblende gabbro,augite dioritediorite and Ordovician granite,which show that they are middle Late Ordovician.The characteristics of rock geochemistry and Hf isotope show that this set of rocks is the product of dehydration of oceanic subducted basalt plates and partial melting of metasomatic wedge-shaped mantle.It was formed in the island arc or continental margin arc environment under the background of subduction/collision orogeny.Combined with the matrix of Hailestai subduction accretionary melange belt,the diagenetic ages of various rock blocks and Duobaoshan island arc volcanic rocks,and the formation age of magmatic arc at the end of subduction/collision orogeny,the initial subduction age of Hailestai subduction accretionary melange is determined as early Cambrian,and the end of subduction/collision is constrained to late Ordovician.In the study area,the initial subduction occurred in the oceanic crust of Hailestai ocean before the Middle Cambrian,forming intra oceanic arc volcanic rocks,further evolved into volcanic island arcs,and began to form a series of tectonic melanges formed by early subduction and accretion.From the late Cambrian to the early Ordovician,the Hailestai ocean continued to subduct in both directions to cratons and micro paleolandmasses on both sides,forming a series of island arc volcanic rock belts and island arc magmatic rock belts,accompanied by ocean island and mid ocean ridge volcanism.The subduction accretionary melange is formed near the trench on both sides,the subduction accretionary complex belt formed in the early stage is further uplifted,and the craton has sustained lateral accretion.In the late Ordovician,due to the continuous convergence and compression of the region,the ocean basin of Hailestai ocean gradually disappeared,and gradually changed from ocean arc subduction to arc continent collision.The subducted accretive melanges on both sides underwent continuous compression and collage.The ductile deformation belt with southeast and northwest tendency was formed in the collapse,fragmentation and scraping of the subduction accretionary complex belt formed in the early subduction stage,which was stacked and mixed with the new rock blocks formed in the collision stage again,which Superimposed or reconstructed early ductile deformation zone.The two stages of ductile deformation developed in the matrix correspond to the high angle subduction orogeny from southeast to northwest in the Middle Cambrian and the two-way subduction collision orogeny under the southeast distal compressive stress in the late Cambrian Middle Ordovician.At the end of the ocean land conversion process in the late Ordovician,TTG rock association and medium basic magma invaded the melange belt at the end of orogeny.The analysis shows that the Hellestai ocean,a branch of the paleo Asian Ocean,developed on the southern edge of the Siberian plate and the northern side of the paleo Asian Ocean,and experienced the process of ocean land conversion in the early Paleozoic Cambrian Ordovician.According to the tectonic position and closing process of the branched ocean,it is considered to be a branched ocean basin between the Siberian Craton and the micro paleolandmass on its edge.It belongs to the accretionary orogeny of Early Paleozoic micro Paleozoic landmass and assembled to craton.Its material composition is complex,with residual records of trench arc basin system,seamount and mid ocean ridge.It is proposed that in the middle of the Hegenshan-Heihe suture belt in the eastern part of the Central Asian orogenic belt,there is an ocean land conversion process of the paleo Asian Ocean tectonic domain in the early Paleozoic,which is in the form of accretionary orogeny of micro continental blocks and branch ocean basins on the margin of Craton.