Early Paleozoic Tectonic Evolution Of The Ondor Sum-Bainaimao Region In Inner Mongolia

There are two distinct first-order tectonic units in the Ondor Sum-Bainaimiao area,Inner Mongolia.It is bounded by the Bayan Obo-Chifeng Fault,the northern part belongs to the Xing'an-Mongolia Orogenic Belt and the southern part belongs to the North China Craton.Combining the petrological,geophysical and geochemical characteristics with the tectonic setting,tectonic evolution stage and relationships of superimposition and transformation,we conclude that the study area can be divided into four second-order tectonic units from north to south during the Early Paleozoic: Ondor Sum subduction-accretion complex belt,Bainaimiao island arc belt,Bainaimiao back-arc sedimentary basin and the foreland fold and thrust belt.These tectonic units are formed in the process of the paleo-Asian ocean subducted under the North China Craton and record the information of the Early Paleozoic tectonic evolution in the study area.The Ondor Sum subduction-accretion complex belt is located in the northern part of the study area.The subduction accretionary complexes which including the Ondor Sum Group,ultrabasic plutons,Deyanqimiao amphibolite and Late Ordovician trondhjemite,was formed by the subduction of paleo-Asian ocean during the Cambrian-Ordovician.The Ondor Sum Group and the ultrabasic plutons are SSZ-type ophiolite,which are the residues of the ocean crust subduction.The Deyanqimiao amphibolite belongs to island arc igneous rocks.The Ordovician trondhjemite belongs to low-k tholeiitic series and is the transitional type between the plagioclase granite and adakitic rock,which is formed in the island arc tectonic setting.Thus,the Ondor Sum subduction accretionary complexes not only include the residual oceanic crust but also contains the island arc igneous rocks.This suite of accretionary complexes had been subjected intense deformation and metamorphism.The glaucophane schist have been found in the high pressure metamorphic belt,which belongs to B-type glaucophane schist,indicating that it formed in the subduction and collision tectonic setting of the oceanic crust.Whereas,the Deyanqimiao amphibolite represents the high temperature-low pressure metamorphic belt in the island arc system.There are three stages of fold deformation and one stage of ductile shear tectonic deformation in the Ondor Sum Group.The early tight overturned-recumbent folds generally,having smaller scale,are the products of the rocks underwent plastic flow deformation.The middle East-West erect-isoclinal folds,having large scale and morphological diversity,are the products of north-south compressional stress field.The late nearly North-South folds were superimposed by the early and middle fold axes,causing the deformation of the fold axes.At this time,the principal stresses changed from North-South to East-West direction.The ductile shear deformation zone cropped out to the south of the folds belt.The rock lineations are well developed,forming a lot of L-type,L>S type and L-S type tectonites.And,they are the products of the North-South subduction,extrusion and exhumation.Bainaimao island arc Belt is mainly made up of volcanic sedimentary strata and magmatic rocks formed in volcanic arc.The sedimentary strata is called Bainaimiao Group,whose lower part is felsic volcanic rocks and upper part is dominated by basaltic volcanic rocks.These basaltic volcanic rocks,forming in Andean-type active continental margin,originate from fluid metasomatic mantle which formed in the process of the subduction and the mantle is hybridized by crustal materials when intruding upwardly.The late Cambrian garnet-muscovite granite is a highly-differentiated I-type one,a production in the early stage of oceanic crust subduction,going through heavy hydrothermal alteration,therefore REE in granite show the effect of quartering group.Late Ordovician quartz diorite,the production of magmatism in Bainaimiao area,has similar characteristics of the magmatic rocks formed in island arc of subduction belt and volcanic rocks of continental margin arc.Early Silurian granodiorite and quartz diorite are typical island arc igneous rocks produced by subduction of oceanic crust.The Xuniwusu Formation is a flysch formation,lying in the south of Bainaimiao Island Arc magmatic rock belt.This strata has a clear sedimentary rhythm and can be divided into three big sedimentary circles including thirteen sub-sedimentary circles.For each circle,the grains increasingly become fine from bottom to top,but on the whole the grains are coarse,which equals to the coarse sedimentary at the lower part of flysch formation.According to the dating results of detrital zircons from Xuniwusu Formation,the detrital zircons of 490 Ma coming from the early-paleozoic volcanic arc are common,the detrial zircons of 700-1400 Ma originated from CAOB and the other detrial zircons of 1500-3300 Ma represent provenance of the North Craton Basin.Xibiehe Formation is a molasse formation,discordantly lying over the Xuniwusu Formation and Bainanimiao Group.The sedimentary rhythm of this series is obviously,every circle begins with conglomerate and increasingly becomes finer upwardly.This molasse formation was formed in fore-arc basin under a compress tectonic setting,resulting from the collision of Bainaimiao Island Arc and the north margin of the North China Craton.This molasse formation represents the final evolve stage of trench-arc-basin system in the north margin of the North China Craton during the early-paleozoic.Foreland fold and thrust belt is composed of the Meso-neoproterozoic Baiyunebo group,nearly E-W direction,which is mainly developed in the south area of Xuniwusu,and distribution in Xinniwusi-Xianghuangqi nearby the south study area.Owing to the alteration late metamorphic structure and magmatic activity reformed,the spatial distribution of Baiyunebo group is discontinuous,and the unequal sized blocks or traps are founded in large areas of the Late Paleozoic granites.These Proterozoic formations were affected by the northern compressional orogeny in the early Paleozoic that a strongly metamorphism and deformation has occurred,and a series of closed fold,overturned fold and bedding-parallel detachment deformation has formed.In the middle segment of north margin of the North China Plate,the Paleozoic tectonic evolution of the Ondor Sum-Bainaimiao can be divided into four stages:(1)Precede Cambrian(>500Ma),the widely Paleo-Asian Ocean was formed between the North China plate and the Siberia plate with the formation and evolvement of Paleo-Asian Ocean;(2)During the Cambrian-late Ordovician(500-450Ma),the Paleo-Asian Ocean Mid-oceanic ridge expansion,there was a phenomenon of double subduction direction from the Paleo-Asian Ocean to the North China plate.Which the ophiolite association of Ondor Sum and the Island arc magmatic association of Deyanqimiao were formed,as while,the Bainaimiao active continental margin has formed.(3)During the Middle Late Ordovician(470-443Ma),with the subduction of plate has enhanced,back-arc basin was formed in Bainaimiao Magmatic Arc,and the clastic rocks and pyroclastic rocks of Xuniwusu Formation were deposited.At the same time,a large number of plutons rocks have formed.(4)The collisional Orogenic stage of Silurian(443-418Ma),during this period,the occurrence of the arc-continent collision in the northern margin of the North China plate.A large number of tonalite and quartz diorite were formed with the strongly collision orogeny.A strong deformation metamorphism was formed in front of the orogenic belt,which caused to the metamorphic deformation of Baiyunebo group and the development of a tightly homoclinal fold,and the foreland fold tectonic belt also formed at the same time.The end of Xibiehe Formation at Late Orogenic stage marks the trench-arc-basin tectonic evolution finished.