The Proterozoic Geological Formation And Its Tectonic Significance In The Southwestern Part Of The Ordos Block

The Proterozoic strata exposed on the southwestern margin of the North China block are mainly composed of the Great Wall system Xiong'er Group(Ch X),the Gaoshanhe Group(Ch G),and the Jixian system Guandaokou Group(Jx G).However,the rock types and ages are markedly different from corresponding typical strata exposed in the Xiong'ershan,Xiaoshan,and Xiaoqinling Mountains.What is the reason for this difference? What is the meaning of this difference? In order to solve these problems,the paper studied the petrology,geochronology,geochemistry and soft sedimentation deformation of the Proterozoic strata in the southwestern part of the Ordos Block.The paper is based on the Proterozoic Xiong'er Group,the Gaoshanhe Group and the Guandaokou Group exposed in the Miaopoli section of Qishan County and the Changqing area of Fengxiang County in the southwest of Ordos.Based on the field geological survey,the paper has carried out the detailed petrology,petrography,geochemistry,soft sedimentary deformation structure and detrital zircon U-Pb geochronology for each rock stratum unit,and try to reveal the spatial distribution and tectonic environment characteristics of Proterozoic strata in the study area,and further to discuss the Proterozoic tectonic evolution history in the southwest of North China Block.The research results will provide new data for further understanding about Proterozoic tectonic evolution of the North China Block.The main achievements of this paper are as follows.(1)According to the petrology,geochemistry and geochronology of the green schist of the former Xionger Group,of which the protolith is continental tholeiite,the protolith was formed during the late Triassic to the early Jurassic.It reflects the strong extensional tectonic environment in the southwestern part of Ordos massif during the late Triassic-early Jurassic.The U-Pb LA-ICP-MS dating of 36 zircons in the greenschist obtains 2455 ~423Ma,and the clastic zircon U-Pb LA-ICP-MS dating of the two interlayers of siltstone in the greenschist obtains 2462~206Ma and 3039 ~ 194 Ma,respectively.Combined with regional geological data and related research results of other researchers,it is suggested that it is more suitable to determine the forming time of greenschist protolith as late Triassic-early Jurassic,and the green schist should be disintegrated from the former Xionger Group in this area.The extensional structure setting of late Triassic in the southwestern margin of North China Block is consistent with the water body deepening and the sedimentary scope enlarging in the southwest margin of Ordos Basin at that time.Another possible metemorphic mechanism from basalt to green schist is the dynamic metamorphism of the Liupanshan fault zone.Although the geological chronology data of th is paper points the formation age of basalt to the Late Triassic-Early Jurassic,the metamor phic mechanism from basalt to green schist is still a problem worth further expansion.(2)The clastic zircon U-Pb LA-ICP-MS dating for the low metamorphic clastic rocks of the former Xionger Group has classified this set of low metamorphic clastic rocks into Nanhua system of Neoproterozoic,which may be the first time to reveal the stratigraphic information of Nanhua system in the southern part of the North China Block.The clastic zircon U-Pb dating of the low metamorphic clastic rocks gives ages of 2708 ~ 753 Ma,2507 ~ 854 Ma and 2735 ~ 1444 Ma,respectively,indicating that the depositional time of the low metamorphic clastic rocks should be later than 753 Ma.Considering that the Guandaokou Group is covered by the Sinian Luo-circle formation with parallel unconformity,and Luo-circle formation is covered by Cambrian-Ordovcian with conformity,it is concluded that the deposition time of the low metamorphic sedimentary rocks should be between 753 Ma and Sinian,which corresponds to the Neoproterozoic Nanhua period.The Nanhua low metamorphic rocks comtact with the Upper Triassic-Lower Jurassic greenschist with normal faults,and the greenschist is located in the hanging wall of the normal faults.The Nanhua low metamorphic clastic-carbonate assemblage should be formed in the trench environment near the continental margin.(3)Based on the U-Pb LA-ICP-MS dating results of clastic zircon in the low metamorphic clastic rocks of the former Xionger Group,the provenance characteristics of this set of low metamorphic clastic rocks are discussed.The age spectra of clastic zircons are very similar to that of the Ordos basement before cratonization(~ 1.9 ~ 1.8Ga)of the North China Block,but there is a great difference after Caratonization.It is shown that the southwestern Ordos massif is obviously influenced by the adjacent tectonic units after the Caratonization of the basement of the North China Block,which is consistent with the tectonic environment at the intersection of several tectonic units in this area.(4)The soft sedimentary deformation structures in carbonate rocks of Guandaokou Group of Jixian system in the Miaopoli Section,Qishan County are studied in detail,and the seismogenic structures are identified among them.The soft sedimentary structures in the carbonate rocks of Guandaokou Group in the Miaopuli section can be divided into two types according to their genesis:(a)soft sedimentary structures formed by diagenesis and(b)soft sedimentary structures caused by seismic activity,i.e.seismogenic structures.The soft sedimentary structures caused by diagenesis include flame structure,spheroidization of chert layer and inclusion bedding,etc.Soft sedimentary deformation structures caused by seismic activity include microfaults,self-detrital breccia and pillow structures.The seismotectonics reflect that the area was significantly influenced by earthquakes and seismicity events in the adjacent area at that time,which means that the area should be adjacent to the tectonic active zone at that time.Based on the regional geological data,it is inferred that the Guandaokou Group of Jixian system may be affected by the activities of the Kuanping Ocean in the south when Guandaokou Group is depositional.(5)The sedimentary environments of carbonate rocks of Guandaokou Group and low metamorphic clastic rocks-carbonate rocks of Nanhua system show that these strata are deposited in the relatively strong tectonic environment.Or it is affected by the tectonic activity in the adjacent area obviously,which indicates that the study area is at the edge of Ordos massif when the Guandaokou Group and the Nanhua system are depositional.In other words,the current southwest boundary of Ordos massif is the southwest boundary of Ordos massif at the time when the Guandaokou Group and Nanhua system are deposited.Although the Miaopoli section of Qishan County is not far away from the section of Guandaokou Group in the southwest of Fengjiashan Reservoir(about 41km),the sedimentary environment is obviously different.The former is formed in carbonate platform environment,while the latter is formed in continental margin environment.The seismotectonic structures of Guandaokou Group in Miaopoli section is resulted obviously from influence of tectonic activity in adjacent area,and this means that the sedimentary area is not far from the edge of the Ordos Block.The magmatic zircons of the adjacent tectonic unit in the Nanhua clastic rocks are abundant,which indicate that the study area was at the edge of the Ordos massif in the Neoproterozoic.(6)During the detailed field geological survey,the author and the members of the project team did not observe Gaoshanhe Group(Ch G)of the Great Wall system on several sections,where Gaodaokou Group should be outcropped according to geological data of others researchers.Therefore,the paper querys the existence of Gaoshanhe Group outcrops in the southwestern Ordos.According to previous data,the area of outcrop of the Gaosanghe Group in the southwest of Ordos block is not large.The authors and members of the project team have investigated four geological sections in the outcrop area of Gaoshanhe Group,.However,we have not observed the typical rock assemblages of Gaoshanhe Group.We have observed the green schist in the so-called outcrop area of Gaoshanhe Group,and the green schist should be a part of the original Xionger Group.The question of whether there are Gaoshanhe Group or not in the southwestern outcrop of Ordos Basin needs to be further studied and confirmed in the future.