Jurassic Evolution Of Andean-type Active Continental Margin In The Eastern South China

Extension in the inland areas of SE China in Jurassic have gradually become the consensus.However,there are two distinctly different views on the tectonic setting of eastern continental margin:Extension and compression.We present petrological,detrital zircon U-Pb geochronological analysis of Early-Middle Jurassic clastic rocks in the NW Zhejiang Basin in eastern South China to clarify the provenannce features and tectonic setting of the basin.Integrated with the petrological and geochemical analysis of the Maonong Formation volcanic rocks and other magmatic rocks in the Jurassic continental margin,the tempo-spatial distinction of tectonic setting between continental margin and the inland in the eastern South China are explored.The Maonong profile located in Songyang is a representative of the Jurassic volcanic rocks in Zhejiang Province.The volcanic rocks in this profile are mainly composed of dacitic crystal tuff and rhyolitic vitric tuff The SHRIMP U-Pb dating shows that the age of rhyolitic vitric tuff from Maonong is at 153 ± 2 Ma.Coupled with previous researches,the Maonong formation was probably formed at 180-148 Ma.Volcanic rocks from the Maonong formation are predominantly magnesian and calc-alkaline series and show prominent pattern of relative enrichment in LREE.Early Jurassic dacitic tuff has inconspicuous Eu negative anomalies whereas Late Jurassic rhyolitic tuff has prominent one.Tuffs in both periods are enriched in LILE(e.g.,K,Rb and Th)and depleted in HFSE(e.g.,Nb,Ta,Zr and Hf).Early Jurassic dacitic tuff was generated at conditions with a residual assemblage of plagioclase + hornblende ?garnet but Late Jurassic rhyolitic tuff was generated at conditions with residual assemblage of plagioclase + hornblende,presumably indicating their distinct source depths.The Sr-Nd-Hf isotope compositions of the Maonong volcanic rocks are akin to the Paleoproterozoic basement of the Cathaysia Block.Therefore,they presumably originated from the partial melting of the Paleoproterozoic basement with minor involvement of juvenile material.According the analyses of the tectonic setting,they were formed in Andean-type subduction environment with relatively low temperature.By calculations of zircon oxygen fugacity of rocks at different ages and regions,it was found that the oxygen fugacity of the Early Jurassic magmatic rocks from sea areas of SE China was higher than that from continental margin,and oxygen fugacity of magmatic rocks from continental margin was raising from Late Triassic to Late Jurassic.The tempo-spatial variation of oxygen fugacity may suggest the impact of subducted fluids with a tendency to expand from sea areas of SE China to the continental margin.Upper Jurassic and Triassic is generally absent in NW Zhejiang and the Lower-Middle Jurassic is angularly unconformable in contact with the underlying Pre-Mesozoic and overlying Cretaceous.In the study region,the Jurassic is mainly northeastward strips and intermittently exposed,and its tectonic deformation is characterized by thrust-fold deformation and contrasts sharply with the extensional deformation of the overlying Cretaceous that are universally controlled by normal faults.The lithology of the Jurassic in NW Zhejiang is detrital deposition without the development of volcanic rocks.It is continuous from the bottom up and is mainly divided into the Lower Jurassic Wangshaxi Formation,the Middle-Lower Jurassic Majian Formation,and the Middle Jurassic Yushanjian Formation.The field geological profile investigation and analysis of the particle composition of the detrital skeletons show that the Wangshaxi Formation is mainly composed of quartz conglomerate and sandstone with high compositional maturity,reflecting the strong barrier-free coastal sedimentation under high hydrodynamic forces.It probably represents the marginal facies in an large intracratonic basin in the Jurassic South China;The lower part of the Majian Formation is shallow-lacustrine facies deposition which is composed of siltstone and mudstone,and the upper part is braided fluvial facies which is composed of feldspar lithic sandstone and pebbly sandstone;The Yushanjian Formation is main composed of massive conglomerate,with purplish red sandstone and minor mudstone,which typically represent the alluvial fan deposit.The sedimentary characteristics of the Majian Formation and the Yushanjian Formation generally reflect that the Jurassic Basin in this region experienced a terrestrial basin evolution history,characterized by rapid sedimentation and low maturity.The detrital zircon U-Pb geochronology results show that the age of detrital zircons of the Wangshaxi Formation(the Lower Jurassic)in NW Zhejiang is dominated by 500-400 Ma and 1000-700 Ma age groups,reflecting that,during the depositional period of the Wangshaxi Formation in the Early Jurassic,the Caledonian-aged and Neoproterozoic magmatic rocks(metamorphic rocks)were the main provenance rocks.Judging from the spatial distribution,the Yangtze and Cathaysia blocks from both sides of the Jurassic Basin in the NW Zhejiang may be potential sources of sedimentary materials for the Wangshaxi Formation.On the other hand,the detrital zircon ages in the Majian Formation and the Yushanjian Formation are significantly different from those in the Wangshaxi Formation,and are dominated by ages of 1900-1800 Ma,250-210 Ma,and 200-180 Ma.,reflecting that Jurassic,Triassic magmatic rocks and Paleoproterozoic magmatic rocks became the main source rock.The distribution characteristics of these magmatic rocks indicate that the sources of the detrital zircons mainly come from the eastern continental margin.Based on the structural deformation and sedimentary filling characteristics of the Jurassic Basin,it is proposed that the Jurassic Basin in NW Zhejiang Province may shift from the shallow-marine intracratonic basin to the fast-filling retro-arc foreland basin.Noting that the emergency of the Early Jurassic age peak of 200-180 Ma may be a response to a significant arc magma in the Jurassic continental margin.Coupled with the arc feature of Jurassic magmatic rocks in the continental margin,an "Andean-type"margin arc related to the paleo-Pacific subduction was probably developed in the eastern margin of South China,which could be dated back to the Earliest Jurassic and expanded in Middle-Late Jurassic.Integrated with the evidences of both magmatic rocks and the tectonic-sedimentary regulations of NW Zhejiang basin,the continental margin of the eastern South China was under the retro-arc compression setting,which contrasts with synchro no us intra-continental extensions in the hinterland.The eclogite-facies metamorphism of subducted flat slabs draged the overriding continent downward to form a broad intraintracratonic basin in the Latest Triassic-Early Jurassic in the South China continent.The eventual foundering of the flat-slab caused the start of normal subduction and the re-initiation of continental arc magmatism.The process led to the direct effect of the palaeo-Pacific subduction to the continental margin and the development of the retro-arc compression environment.A retro-arc foreland basin was consequencely formed in NW Zhejiang.