Mesozoic Sedimentary Records In The Northern Margin Of The Middle Yangtze: The Successive Response To The Collision–uplift Process Of The Qinling Orogenic Belt

The assembly of the South China Block(SCB)and the North China Block(NCB)initiated at the Late Paleozoic and continued to the Mesozoic.The consequent continental collision,the orogenesis of the Qinling Orogenic Belt(QOB)and the massive exhumation of the QOB profoundly influenced the geological evolution of the East Asia,which has unique research significance.It has been controversial of defining the SCB-NCB collision timing and reconstructing the uplift process of the QOB.Owing to the intense overthrusting,metamorphism and surface erosion of the QOB after the Jurassic,few Early Mesozoic orogeny record was preserved in the present Qinling Mountain.In this case,sedimentary successions preserved peripheral foreland basins could provide the basis for reconstructing orogenic evolution.Zigui Basin and Dangyang Basin are Mesozoic sedimentary units located in the northernmost South SCB.These two units were emplaced on the Paleozoic passive margin of the SCB and shifted to peripheral foreland basins affected by Qinling collisional orogenesis during the Triassic.The infilled Triassic-Jurassic clastic sequence represented successively sedimentary response to the collisional orogenesis of the QOB.This thesis focused on above material and took tectonic sedimentology as guidelines.Specific methods systematically involved sedimentological observation,tectonic subsidence calculation,sandstone microscopic petrography,whole-rock geochemistry,detrital zircon geochronology and detrital zircon trace elements.In virtue of these methods,the clastic response to different phases of orogeny could be classified.On this basis,this thesis ultimately probed into constraining the timing of the SCB-NCB initial collision and reconstructing the uplift as well as the exhumation process of the QOB during the Early Mesozoic.The research contents and progress are summarized as follows:(1).The chronostratigraphic revision of the Badong Formation.The Badong Formation was a set of amaranth thick-bedded mudstone associated with grey marlstone and micrite strata.It overlies the Lower Triassic Jialingjiang Formation and pervasively distributed along the northern SCB.The Badong Formation was distinguished as the youngest marine sedimentation.It remains unclear concerning its geochronological definition.Whether the upper Badong Formation extend to the Upper Triassic should be further determined.This thesis reported newly discovered two tuff layers intercalated in the Badong Formation.Zircon U–Pb age dating by LA-ICP-MS was conducted and two samples from the middle and the top of the 2nd Member,and yielded 241.7±1.7 Ma and 237.5±2 Ma respectively.These results equally indicated the depositional age of their stratigraphic position.Combined with the biostratigraphic record,the deposition age of the Badong Formation could be revised to the Middle Triassic to the early Late Triassic.Furthermore,zircon grains from the tuffs possessed the orogenic type of geochemical features,which was indicative of the Middle Triassic volcanic influx from the QOB.(2).The Early Mesozoic sedimentary evolution of the Middle Yangtze.The Lower Triassic Daye Formation was a combination of carbonate and mudstone rocks,which collectively represented the upward shallowing slope environment.The Jialingjiang Formation primarily contained two types of subfacies including the medium layered micrite for the open platform and massive dolomite for the restricted platform.There was a set of karst breccia at the top of the Jialingjiang Formation,indicating evaporitic platform facies.Hence,the Jialingjiang Formation was interpreted as another shallowing sequence upwards.As for the Mid-Late Triassic Badong Formation,it generally demonstrated the mixed sedimentation of clastic and carbonate rocks.The Badong Formation could be divided into three Members.The 1st and the 3rd belonged to sandy-muddy tidal flat,and the 2nd Member was deposited on the carbonate ramp.The Upper Triassic Jiuligang Formation consisted of prodelta and delta front subfacies with unconformity on its top.The Lower Jurassic Tongzhuyuan Formation was a southward prograded fluvial-lacustrine facies deposition with S and SSE directed paleocurrent structures.With respect to the Middle Jurassic,both the Qianfoya Formation and the Xiashaximiao Formation were dominated by the lacustrine deposition which consisted of lakeside plain,shore,shallow lake subfacies.This lacustrine association in the Zigui Basin represented the depocenter of Jurassic deposition in the northern Middle Yangtze.(3).Microscopic petrographical studies were applied on Triassic-Jurassic clastic successions on the northern Middle Yangtze.To retrospect parent rock types of lithic fragments and lay solid foundations for succeeding provenance studies,this thesis discussed the taxonomy of different sandstone lithic fragments.For instance,the classification scheme for the igneous rock grains was established according to the acidity,while the metamorphic ones were sorted by their metamorphism ranks.Under this setting,Gazzi-Dickinson point counting criteria was applied to Triassic-Jurassic sandstones.Grains were recognized and semi-quantified,and their provenance significance was also summarized.Results showed that the detrital model of the Badong Formation was characterized as low maturity and the recycled orogen background in the Dickinson ternary diagram.Plentiful limestone grains in the sandstone certified the existence of proximal source regions such as the Upper Paleozoic in the Northern SCB and the Precambrian Shennongjia Terrane.A certain number of volcanic grains should be attributed to the adjacent volcanic activities along the Mianlue Suture Zone during the Triassic.The Upper Triassic Jiuligang Formation was dominated by detrital quartz with rounded shape and the enlarged edge.This formation lacked unstable grains such as feldspars and corresponded to the recycled orogen in the Dickinson diagram.The potential source regions for the Jiuligang Formation should include the pre-existed clastic strata in the SCB and the QOB.In contrast,the Lower Jurassic Tongzhuyuan Formation was distinguished by poor sorted grains and higher contents of matrix.The Tongzhuyuan Formation had a variety of lithic fragments corresponding to the influx from the recycled orogen and the magmatic arcs.Specifically,the medium to low ranked metamorphic lithic fragments associated with volcanic grains probably sourced from the adjacent Precambrian volcaniclastic assemblages.As for the radiolarianbearing siliceous fragments,they should originate from the banded chert among the Paleozoic carbonates on the SCB or deep-water siliceous rocks developed in the Mianlue Suture Zone.The Middle Jurassic Qianfoya Formation was mainly composed of detrital quartz,and fell into recycled orogen and magmatic arc zones in the Dickinson diagram.Some unprecedented grains such as doleritic textured basic rock fragments and intermediate metamorphosed grains with zoisite-epidote inclusions added to the sandstone.This change was consistent with increased amount of microcline,which might suggest the persistent denudation of the source region.The Xiashaximiao Formation was characterized as abundant detrital feldspars especially K-feldspars.This might indicate the influence of uplifted basement-type provenance.Multiple types of lithic fragments suggested the comparatively complex parent rock associations,which linked to the further exhumation and flux of the QOB.(4).The timing of initial SCB-NCB collision was constrained at the eastern Qinling.The initial continental collision usually triggers the tectonic subsidence and provenance change on the subducting side.Tectonic subsidence curve of the northern Middle Yangtze demonstrated a rapid subsidence during the Early Triassic and the uplift initiated at the Middle Triassic.Moreover,as revealed by the detrital zircon geochronology,the Daye Formation was typical of ~1.9 Ga and ~850 Ma peaked age clusters,which reflected recycled autochthonous material within the SCB.By comparison,mudstones from the Jialingjiang Formation contained many unprecedented 270 Ma—245 Ma aged grains.It matched well with the zircon age spectra of the South Qinling Belt and represented its early-staged detrital input.The detrital zircon age population of the Mid-Late Triassic Badong Formation was between~2500 Ma,~1850 Ma,920–800 Ma,~450 Ma and 350–220 Ma.Above age distribution combined with sandstone microscopic petrography features and the whole rock geochemistry characteristics suggested a mixed provenance ranging from the North Qinling Belt,the South Qinling Belt to the northern SCB.Taking the youngest marine strata position and the foreland basins initiation into consideration,the initial SCB-NCB collision at the eastern Qinling segment should have taken place in the Early Triassic.(5).The detrital zircon U–Pb ages integrated with zircon geochemistry analysis could be more sensitive to some specific source regions.The high value of ?Eu(Eu/Eu*)in zircons has genetic relations to the crustal thickness during their crystallization.Similarly,the Th/U value could indicate the tectonic stress condition of the source regions.There were many ~215 Ma clustered detrital zircons with high Eu/Eu* and low Th/U incorporated in the Lower Jurassic.This intersected feature exclusively suggested the thickened crust meanwhile compressional background,which was consistent with voluminous post-collisional granitoids in the South Qinling Belt.Reversely,the South Qinlng Belt should have experienced massive exhumation down to the deep intrusions.In addition,the widely spread ~190—~170 Ma clusters of detrital zircons in the Middle Jurassic also possessed high Eu/Eu* and low Th/U value.As supported by W-directed paleocurrent,this age peak should be interpreted as the influx from the magmatic arc located on the east coast SCB near the Paleo-Pacific Subduction system.Above case studies suggested that trace elements of detrital zircons were good complements for provenance studies.It could precisely indicate the thickened crustal provenance in the collisional belt as well as the active continental margins.Besides,it could pinpoint the tectonic conversions between the compression and the extension,and the denudation rate of source region could also be quantified.Thus,Eu/Eu* and Th/U values are expected to be a promising analysis method for future provenance studies.(6).Sedimentary record is a comprehensive proxy to reconstruct the Early Mesozoic uplift-exhumation process of the Qinling Orogenic Belt.The Early Triassic initial collision between the SCB and the NCB marked the orogenesis beginning of the eastern Qinling Orogenic Belt.During this stage,the interaction between two collided continents was not intense and the QOB had not primarily uplifted.The northern Middle Yangtze mainly performed as rapid loading flexural subsidence and received~249 Ma clustered detrital zircons from the South Qinling Belt.By the Middle Triassic to the early Late Triassic,the compression between the SCB and the NCB turned to be stronger.Fold and thrust system and marine foreland basins formed along the northern SCB.The South Qinling Belt started growing and became as primary source regions in conjunction with the North Qinling Belt.By the Early Jurassic,the Tongzhuyuan Formation was characterized as ~215 Ma clustered detrital zircons with thickened crustal background.This feature along with abundant volcanic and meta-sedimentary lithic fragments indicated the extreme exhumation of the South Qinling Belt.As for the Middle Jurassic,the Qianfoya and the Xiashaximiao formations are the main filled body in the Zigui Basin.They were both enriched with feldspar grains,complicated protolith types and ~250—200 Ma detrital zircon age intervals.These signals were indicative of detrital influx from the South Qinling Belt,and the fold-thrust belt.The development of the frontal river along the northern SCB suggested that the South Qinling Orogenic Belt was still being compressional uplifted and severely denuded.