The Qingbaikouan-Cambrian Volcanic And Sedimentary Sequences In The Western Jiangnan Orogen And Its Implication For Tectonic Evolution Of South China

Extensive change on the earth,such as transition from Rodinia to Gondwana accompanied by global tectonic events,metallogenic process,paleoclimate change and life evolution,occurred during the transition from Neoproterozoic to early Paleozoic.Therefore,reconstruction of plates/terranes in the configuration of the supercontinental cycles are the fundament and of great importance in understanding the above changes happed at that time.One of the controversial issues is the tectonic evolution and reconstruction of South China during the Rodinia-Gondwana transition.The South China Block was amalgamated by the Yangtze and Cathaysia blocks along the Jiangnan orogenic belt.However,the amalgamation timing is controversial with ages proposed ranging from Neoproterozoic to early Paleozoic,while the suture zone is also poorly constrained in the southwest segment in contrast to the well-known Jiangshan-Shaoxing Fault zone as the northeastern boundary.In view of the above-mentioned scientific issues,the Qingbaikouan-Cambrian volcanic-sedimentary sequences,located at the junction of Hunan,Guangxi and Guangdong provinces,are investigated in this thesis.Based on detailed field investigation and mapping on representative sections,samples were selected for petrography,sedimentology,structural analysis,geochronology and geochemical study.According to the data obtained in field and laboratory,the rock assemblages,formation age(s)and tectonic setting of the Yingyangguan Group are constrained in details.Moreover,the provenance and tectonic implications of the Qingbaikouan-Cambrian clastic rocks are investigated to offer useful insight into the process and timing of amalgamation between the Yangtze and Cathaysia blocks and into the boundary between the two blocks.The main conclusions are as follows:(1)Essentially,the Yingyangguan Group is a tectonic mélange showing typical block-matrix structure,with metamorphic tuffaceous lutites as the matrixes and metamorphic volcanic(-clastic)rocks,microlitic quartzite and marbles as the blocks.Andesitic and rhyolitic rocks collected from the blocks were formed at 822-816 Ma and?765 Ma,respectively.The youngest zircon age of metasandstone in the matrix is 764Ma.Combined with the youngest detrital zircon ages of 661 Ma and 700 Ma from marble in the block and metasandstone in the matrix,as well as the intrusion of Ordovician(?450 Ma)gabbro and Silurian(440-417 Ma)granite,the Yingyangguan Group was restricted to be Qingbaikouan-Cryogenian.(2)Field investigation and structural analysis indicate five phases of deformation for the Yingyangguan Group.First stage schistosity(S₁)paralleling to S₀ is indicated by residual bedding.Tight-homoclinal fold(S₂)and felsic vein rootless folds(S₂)were formed during second stage of deformation.Rolling fold,separation cleavage and left strike slip movement are superimposed on S₂.The above three stages represent structural deformation(D₁-D₃)occurred at Jinning Period-Caledonian.Subsequently,the early deformations(D₁-D₃)were superimposed by the W-NWW compression during the Indosinian and early Yanshanian(D₄),and the NNE strike faults were under intense dextral strike-slip shearing.The extensional regime of the late Yanshanian is mainly represented by normal fault activity(D₅).In the region,there is an angular unconformity between Devonian and underlying sequences,and the brittle deformation is obviously different from the D₁ and D₂ ductile deformation of the Yingyangguan Group.Therefore,the Yingyangguan Group is the result of the initial tectonic mélange during the Neoproterozoic along Yangtze-Cathaysia blocks amalgamation and was superimposed by Caledonian intracontinental orogeny.(3)The 822-816 Ma andesitic rocks of the Yingyangguan Group are high-Mg in nature,originating from partial melting of the lithospheric mantle metasomatized by sediment melts under subduction setting.The?765 Ma rhyolite rocks have the characteristics of S-type granite and was formed in an intraplate rift environment,resulting from partial melting of ancient sediments caused by the upwelling of the asthenosphere.These rocks record the transition from subduction(822-816 Ma)to intraplate rift settings(?765 Ma),similar to what happened synchronously along the southeastern margin of the Yangtze Block.Together with detrital zircon data,geophysical and geochemical evidence,the study indicates the Yingyangguan area is a part of the southeastern margin of the Yangtze Block.(4)The Cryogenian-Cambrian greywackes contain low to medium quartz,and variable lithic fragments and feldspar grains,which are poorly sorted and angular to subrounded.The average ICV values of Cryogenian,Ediacaran,and Cambrian greywackes increase gradually from 0.90 through 1.02 to 1.03,while mudstones have ICV values of 0.70,0.72,and 0.79,respectively.All samples have medium-high CIA values,indicating that the source area has accumulatively experienced moderate to intense chemical weathering.Besides,all rock show similar chemical characteristics with light rare earth enrichment,heavy rare earth depletion and negative Eu anomalies,suggesting These geochemical features suggest felsic rock and/or low-grade metamorphic rock as the principle sedimentary sources.Moreover,both recycling and first-cycle materials contributed to the Cryogenian-Cambrian sedimentary basin with contribution of the latter increases from base upwards(5)During the late Qingbaikouan(820-720 Ma),the study area mainly received sediments supplying from the Jiangnan Orogen and interior of the Yangtze Block.Sedimentary transportation changed during deposition of the Tianzidi Formation in the Cryogenian with sediments from the Cathaysia direction reached the study area.Afterwards,the Cathaysia derived detritus migrated northwestwards,and reach the Yongfu area in the Ediacaran,the Longsheng area in the Cambrian,which is also confirmed by the W-NW paleocurrent direction.Take all the evidence mentioned above,it is believed that the late Qingbaikouan-Cambrian sedimentary basin is formed in rift-passive setting and there is no wide ocean separation between the Yangtze and Cathaysia blocks after early Qingbaikouan.(6)In combination with the existence of ophiolite mélange,arc magmatic rocks,back-arc basin deposits,post-collision granites,and regional unconformities in the Jiangnan orogenic belt,it is believed that the amalgamation of the Yangtze-Cathaysia Block occurred at about 820-805 Ma,and the southwestern segment of the convergence boundary is located to the east of the Yingyangguan Group.Collectively,the evolution of South China during Qingbaikouan-Cambrian can be summarized as:(1)Ocean-ocean subduction(970-880 Ma);(2)Arc-continent collision(880-860 Ma);(3)Ocean-continent subduction(860-825 Ma)(4)Collision assembly(825-805 Ma);(5)Continental rift(805-750 Ma);(6)Tectonic uplift(750-720 Ma);(7)Continental rift deposition(<720 Ma).