The Paleoproterozoic To Middle Neoproterozoic Tectonic Evolution Of The Suizhou-Yingshan Terrain And Its Surrounding Area In The Northern Yangtze Block

The Paleoproterozoic to Middle Neoproterozoic tectonic evolution of the Yangtze Block was involved in the assembly and breakup of the Columbia and Rodinia supercontinents.However,the Paleoproterozoic to Mesoproterozoic geological records are mainly reported in the Kongling terrain and its surrounding areas,the Houhe Complex in the northwestern Yangtze Block,the Huangtuling area in the Dabie Orogen,the Paleoproterozoic sequences in the southwestern Yangtze Block and northern Vietnam area in Vietnam.Whereas in the other areas there have been no such geological records.Some questions about(1)the records of Paleoproterozoic-Mesoproterozoic magmatic,sedimentary and metamorphic events in different regions in the Yangtze Block,(2)tectonic affinity of these events,(3)Paleoproterozoic-Mesoproterozoic tectonic evolution of the Yangtze Block,(4)the location of the Yangtze Block in the Columbia supercontinent are still unclear.In addition,the location,geological feature and magma-dynamic mechanism of the Yangtze Block in the Rodinia supercontinent are still controversial.Some debates are as follows:(1)When is the termination of subduction of the Neoproterozoic oceanic crust in the Yangtze Block?(2)What is the dynamic mechanism of 860-700 Ma magmatism,mantle plume,plate subduction,post collisional extension or others?(3)Is the Southern China in the center or the margin of the Rodinia supercontinent? These questions are essential for understanding the crustal evolution of the Yangtze Block and the reconstruction of the Rodinia supercontinent.The Mesoproterozoic-Neoproterozoic strata were widespread outcropped in the Suizhou-Yingshan area and surrounding area,northern Yangtze Block.The Dagushi Group in the Dahongshan area and the Shennongjia Group in the Shennongjia area are the only two Mesoproterozoic stratigraphic units in the northern margin of the Yangtze Block.The Huashan Group and tectonic melange in the Dahongshan area were formed in the Neoproterozoic.It has always been the research focus of the Lithostratigraphic assemblage,formation age,tectonic setting and provenance of these Mesoproterozoic to Neoproterozoic strata(such as Dagushi Group,Shennongjia Group,Huashan Group and tectonic melange).Researches have shown that the tectonic evolution of the Yangtze Block has a close affinity to the Columbia and Rodinia supercontinents.Therefore,the study of the Mesoproterozoic to Neoproterozoic strata,tectonic melange and volcanic rocks in the northern Yangtze Block would provide information about the evolution history of these two supercontinents and the tectonic evolution of the Yangtze Block.This thesis carried out a study on the Neoproterozoic Wudang group,Huashan Group,Mesoproterozoic Dagushi Group and Dahongshan tectonic melange.Through detailed field work,section analysis,deformation analysis,lithostratigraphic analysis and regional stratigraphic correlation,this thesis figured out the petrotectonic assemblages of these lithostratigraphic units.Integrated study of detrital zircon geochronology and Hf isotopic composition as well as zircon trace element characteristics of the Wudang Group,Huashan Group and Dagushi Group provide insights into the deposition age,provenance and tectonic affinity of these groups.Petrology,geochronology and geochemistry studies of the bimodal magmatic rocks in the tectonic melange belt were performed to constrain the crystallization age,petrogenesis and its tectonic affinity.These studies also gave constraints on the closing time of the Neoproterozoic oceanic basin in the northern Yangtze Block.Furthermore,through comparison of the magmatic,sedimentary and metamorphic events in different regions in the Yangtze Block,this thesis further discussed the tectonic affinity of the Paleoproterozoic to Middle Neoproterozoic tectonothermal events in the northern margin of the Yangtze Block and reconstructed the tectonic framework of the Dahongshan melange belt.This thesis also discussed the Paleoproterozoic to Middle Neoproterozoic tectonic evolution of the Yangtze Block.In addition,by comparison of the stratigraphy,magmatic and metamorphic events between different plates,a reconstruction model of the Yangtze Block in the Columbia supercontinent was put forward,and the position of the Yangtze Block in the Rodinia supercontinent was also suggested.The main conclusions and progresses are as follows:1.Some events in the northern Yangtze Block related to the assemblage and break-up of the Columbia supercontinent obtained in this thesis are as follows:(1)Crustal growth in the Paleoproterozoic.The upper part of the Wudang Group shows a major detrital zircon age group of ~750 Ma.Whereas the lower part of the Wudang Group shows one age group of ~2.03 Ga.The ~2.03 Ga detrital zircons have high positive ?Hf(t)values,close to the one stage Hf model ages.However,the ~750 Ma detrital zircons have high negative ?Hf(t)values.The two stage Hf model ages(1837-2230 Ma)of ~750 Ma detrital zircons are compared well with one stage Hf model ages of ~2.03 Ga detrital zircons.These lines of evidence indicate crustal growth in the Paleoproterozoic.(2)1750-1500 Ma rifting process.A significant detrital zircon age group of ~1.6 Ga was recorded in the Dahongshan area,northern Yangtze Block.Age groups of 1750-1350 Ma and 1200-1100 Ma are obtained in the paragneiss from the lüwang-Gaoqiao ophiolitic melange belt in the Hong'an HP eclogite belt,with peak ages of ~1557 Ma and ~1163 Ma.The 1750-1500 Ma detrital zircons show mostly positive ?Hf(t)values and have ?18O values similar to the normal mantle.Together with the detrital zircon trace element feature and stratigraphic research,it is suggested that the 1750-1500 Ma magmatic rocks were formed in rifting setting in the northern Yangtze Block.2.Based on detailed field geological survey and section analysis,the Dahongshan melange belt was divided into four tectonic units from northeast to southwest.(1)Island arc magmatic belt,which is composed of island arc volcanic rocks(basalt,dacite,rhyolite and tuff)and intrusive rocks(gabbro,diabase and granite)as well as a small amount of mudstone and siltstone.(2)Northern back-arc complex belt,which contains mainly siliceous rocks,argillaceous rock and mafic rock mass(basalt,diabase and gabbro).(3)Southern back-arc complex belt,which contains terrestrial and lacustrine clastic rock(Huashan group),fragment of the Dagushi Group,a large number of intruded gabbro dykes and veins,the deep-sea siliceous rocks and pillow lava;(4)Sedimentary cover sequences and passive continental margin Dagushi Group.The sedimentary cover sequences include the Liantuo Formation,Nanhuan,Sinian and Paleozoic strata.3.Some events in the northern Yangtze Block related to the assemblage and breakup of the Columbia supercontinent obtained in this thesis are as follows:(1)The detrital zircons show periodical ?Hf(t)values.The detrital zircon ?Hf(t)values from the Wudang Group in the Suizhou-Yingshan area and Wudang area have a significant change at 840 Ma,as marked by an increase in negative ?Hf(t)values of the <840 Ma detrital zircons relative to those at 950–840 Ma,which probably reflects an arc–continent collision at 840 Ma.Another change occurred at 790 Ma,as characterized by a lack of high positive ?Hf(t)values of the <790 Ma detrital zircons relative to those at 840–790 Ma,which may indicate that the extensional collapse phase of the orogen occurred after 790 Ma.(2)~820 Ma back-arc bimodal magmatic rock assemblage.The ~820 Ma bimodal rocks were outcropped in the Dahongshan back-arc complex belt.The mafic rocks show left-leaning to right-leaning rare earth elements(REE)characteristics with(La/Yb)N = 0.76-2.21,negative Nb,Ta and positive Pb values.They sourced from an old lithospheric mantle and were modified by subducted slab fluids.It is suggested that these mafic rocks were formed in a back-arc setting.The rhyolite chemically belongs to the A2 type granitoid.Zircon ?Hf(t)values of the rhyolite are consistent with those of the basalt.The rhyolite is suggested to be produced by both crystallization differentiation of mantle derived magmas and crustal contamination.(3)The oceanic basin closed at ~794 Ma.In the northern back-arc complex belt,siliceous and argillaceous rocks and mafic rocks are covered by the thin dolomite layer with carbonate breccia lenticular,which is further covered by the thick conglomerate and sandstone,indicating the closure of the ocean.LA-ICP-MS zircon U-Pb dating of tuffaceous calcium slate demonstrates that the depositional age is 794 ± 7Ma,which should represent the timing of the collisional event.(4)Back-arc rifting sediments.The detrital zircon age spectrum of the Huashan group shows two major age groups(~1950 Ma and ~2650 Ma),and three secondary age groups(~820 Ma,2400-2500 Ma and 2800-3300 Ma).The sedimentary rocks are mainly composed of mature continental detritus(dolomite,chert and quartz).Combined with the zircon Hf isotopic composition,these results indicate the Huashan group sourced mainly from the basement of the Yangtze Craton rather than Dahongshan island arc belt.4.Comprehensive studies suggests that the northern margin of the Yangtze Block has experienced two major tectonic evolution processes and ten tectonic stages during the Paleoproterozoic to Middle Neoproterozoic.The two tectonic evolution processes were corresponded to the Columbia supercontinent and Rodinia supercontinent,respectively.One-five and six-ten stages were corresponded to assemblage and break-up of the Columbia supercontinent and Rodinia supercontinent,respectively:(1)~2.2-2.0 Ga,western and northern margin subduction of the Yangtze Block;(2)~2.0-1.94 Ga,collision between west and east Yangtze Block;(3)~1.85 Ga,collision between west Yangtze Block and another continent(Siberia/Laurentia?)as well as post-collisional extension in the east Yangtze Block;(4)1.8-1.5Ga,rifting of the Yangtze Block;(5)1.5-1.1 Ga,evolution of the passive continental margin carbonate platform in the northern Yangtze Block;(6)~940-840 Ma,Dahongshan ocean island arc evolution stage;(7)~840-820 Ma,Dahongshan continental arc evolution stage;(8)~820-800 Ma;Dahongshan back-arc basin evolution stage;(9)~800-780 Ma,Dahongshan arc-continent or continent-continent collision stage;(10)<780 Ma,Dahongshan post-collisional extension stage.