Early To Middle Neoproterozoic Tectonic Evolution Of The Northwestern Margin Of The Yangtze Block

The early to middle Neoproterozoic tectonic evolution of the northwestern margin of the Yangtze Block is one of the hotly focused geological issues on Neoproterozoic geology of the Yangtze Block and even the South China Block.And further understanding of this geological issue is of great significance in revealing the location of the Yangtze Block(or South China Block)in,and the breakup driving mechanism related to the peripheral subduction for the Rodinia supercontinent.This thesis mainly focuses on the sedimentary filling sequences,depositional age,provenance,tectonic setting,and tectonic deformation characteristics of the Hengdan Group with the comprehensive researches of stratigraphy,sedimentology,structural geology,zircon U-Pb geochronology,whole-rock geochemistry,and zircon Lu-Hf isotope analysis.Furthermore,combining with previous works,we rebuild the tectonic evolution history of the northwestern margin of the Yangtze Block during early to middle Neoproterozoic,and discuss the location of the Yangtze Block(or South China Block)in the configurations of the Rodinia supercontinent as well as the peripheral subduction caused breakup driving mechanism of this supercontinent.Our study leads to the following conclusions:1.The Hengdan Group can be subdivided into the Baiyang,Yangtianba and Koutouba formations from bottom to top,and showing fining upward sedimentary filling sequences.The Baiyang Formation is a suite of celadon-colored volcanoclastic gravity flow sequences.The lower part of the formation is composed mainly of tuffaceous sandstone,silty-argillaceous slate,gravel-bearing sandstone,and conglomerate,while the upper part is dominated by tuffaceous sandstone and silty-argillaceous slate.The Yangtianba Formation is a suite of gray to dark gray terrigenous clastic gravity flow sequences.The lower part of the Yangtianba Formation is dominated by gray to dark gray sandstone,silty-argillaceous slate,gravel-bearing sandstone,and conglomerate,whereas the upper part of the formation is composed mainly of gray to dark gray sandstone and silty-argillaceous slate.The main rock types of the Koutouba Formation are fine grained sandstone,siltstone,and silty-argillaceous slate with a thickness of centimeter to millimeter,which are interbedded with small monolayer thickness but large accumulative thickness.And siliceous bands or clumps can be seen locally in the Koutouba Formation.According to the main supporting mechanism of the sediment particles during the deposition,the sedimentary facies of the Baiyang Formation deposits are composed mainly of the volcanic clastic turbidite sedimentary facie,volcanic clastic debris flow sedimentary facie,and volcanic clastic liquefaction flow sedimentary facie,while that of the Yangtianba Formation deposits consist mainly of the terrigenous clastic turbidite sedimentary facie and terrigenous clastic debris flow sedimentary facie.Besides.the sedimentary facies of the Koutouba Formation deposits are composed mainly of the terrigenous clastic turbidite sedimentary facie and abyssal facie.The vertical sedimentary sequences of the Hengdan group are diverse,mainly including the vertical sedimentary assemblage of collapse deposits and turbidite deposits,the multi-phase superimposed debris flow sedimentary assemblage deposits,the multi-layered turbidite vertical combination deposits,and the vertical combination of turbidite and deep water suspended deposits.In addition,the spatial distribution characteristics of these vertical sedimentary sequences indicate that the Hengdan Group is a suite of bathyal-abysmal slop gravity flow sequences.2.Sandstone petrography reveals that the Hengdan Group sandstones with low textural and compositional maturity,and the content of the matrix is high and mostly argillaceous-arenaceous.The counting results showed that the rock types of the Baiyang Formation sandstones are lithic arkose and feldspathic litharenite,while that of the Yangtianba Formation sandstones are mainly classified as feldspathic lithic sandstone and lithic sandstone.The average modal composition of the Baiyang and Yangtianba formations sandstones is Q₁₉F₁₈L₆₃and Q₃₂F₃₄L₃₄,respectively.In addition,it is also indicated that the Baiyang and Yangtianba formations sandstones were mainly sourced from a magmatic arc and which had experienced a denudation from an undissected arc or transitional arc to a dissected arc during the deposition of the Hengdan Group.Moreover,the crystallization ages(743±6 Ma and 762±4 Ma)and petrographic characteristics of two granite gravels in the Yangtianba Formation conglomerates indicate that the Neoproterozoic magmatic rocks in the Micangshan-Hannan massifs are the important provenance of the Hengdan Group.The Baiyang,Yangtianba and Koutouba formations sandstones share similar geochemical characteristics.Comparing with the international standards(e.g.,the PAAS,NASC and UCC),the Baiyang,Yangtianba and Koutouba formations sandstones are characterized by higher contents of Si O₂ and Na₂O,and lower contents of Mg O,K₂O and P₂O₅.Besides,all the sandstone samples show subparallel rare earth element(REE)patterns,and are moderately enriched in light REEs(LREEs)and relatively flat in heavy REEs(HREEs).Besides,most of these sandstones show distinctly positive Eu anomalies and Ce anomalies.Sandstone geochemical characteristics indicate that the Hengdan Group sandstones experienced low degree of chemical weathering and sediment recycling,and their provenance is composed mainly of the arc-related acidic to intermediate igneous rocks.And it is also favored that they were deposited in an active continental margin siting.Detrital zircons from the Hengdan Group sandstones yield similar unimodal age distributions at ca.950 to 740 Ma,and which are comparable to those deposited in convergence tectonic settings.Besides,these aged(ca.950 to 740 Ma)detrital zircon grains are dominantly angular or subangular with clear oscillatory or broad zoning,indicating a proximal derivation from the nearby early to middle Neoproterozoic igneous rocks.In addition,the youngest age clusters(n?3)indicate that the Baiyang and Yangtianba formations deposited after ca.740 Ma while the Koutouba Formation is later than ca.722 Ma.3.Combining with previous work,it is indicated that the Hengdan Group was most likely deposited during ca.743 to 717 Ma in a forearc basin in the northwestern margin of the Yangtze Block with its provenance probably from the nearby arc-related acidic to intermediate igneous rocks in the northwestern margin of the Yangtze Block,and the Micangshan and Hannan massifs are its main source area.4.According to the present tectonic deformation characteristics and statistical analysis result of attitudes,the Hengdan Group could have experienced four tectonic deformation sequence.The first phase of deformation is characterized by the tight fold formed by the flaser shear and accompanying with the ductile thrust shear in the middle to deep tectonic level.And this phase deformation could be caused by the continental collision or arc-continent collision in the northwestern margin of the Yangtze Block in late Neoproterozoic(ca.717 to 700 Ma).The second phase deformation is characterized by the middle tectonic level oblique thrust nappe structure in the boundary of,or stress-concentrated belt in the Hengdan Group.This phase deformation could be caused by the Indo-Chinese epoch intra-continent orogeny in the northwestern margin of the Yangtze Block.The third phase deformation is characterized by the middle to upper tectonic level brittle to ductile left-slipping ductile shear deformation,and related to the Yenshanian epoch westward extrusion and escape of the Bikou Terrane.The fourth phase deformation is characterized by brittle dextral strike slipping ductile shear deformation in upper tectonic level of crust,closely related to the Himalayan epoch eastward wedge of the Bikou Terrane.5.The northwestern margin of the Yangtze Block was most likely in an active continental margin setting between ca.835 and 720 Ma.Combining with previous work,the late Mesoproterozoic to Neoproterozoic tectonic evolution of the northwestern margin of the Yangtze Block can be subdivide into three main stages,including:(1)late Mesoproterozoic(ca.1200-1000 Ma)passive continental margin tectonic setting stage;(2)early to middle Neoproterozoic(ca.950 to 720 Ma)long-lived oceanic subduction stage,and developing accretionary orogeny;and(3)middle to late Neoproterozoic continental collision or arc-continent collision stage(ca.720-700 Ma)and extension or rifting stage(ca.700 to 541Ma).In addition,the second stage can be further subdivided into advancing type orogeny stage(ca.950 to 820 Ma),tectonic regime transformation stage(ca.820 to 800 Ma)and retreating type orogeny stage(ca.800 to 720 Ma).Hence,it is favored that the Yangtze Block(or South China Block)was most likely located in the northwestern margin of the Rodinia supercontinent and the Top-down model could be the dominant breakup driving force in the supercontinental margin(s).