Sedimentary Characteristics,Provenance And Basin Prototype Of Lower Paleozoic In Northwest Zhejiang Region

The South China Block forms part of the south-eastern segment of the Eurasian continent,and the Jiangnan Orogen separates the Yangtze Block from the Cathaysia Block to the southeast.As a tectonic-magmatic-metallogenic belt,the Jiangnan Orogen is the key to understand the tectonic evolution,magmatic process and mineralization effect of the South China.Several competing models have been proposed for the evolution of both blocks and the early Paleozoic sedimentary basin in the SE margin of Yangtze Block,which mainly focus on aspects of igneous and metamorphic rocks,limited research has been conducted on the sedimentary rocks in this region.In this paper we take the lower Paleozoic in NW Zhejiang as the research object to explain the sedimentary facies in detail,analyse the temporal and spatial evolution of the depositional environment,reconstruct the paleogeographic pattern of the SE margin of the Yangtze Block.Provenance of the late Ordovician sedimentary rocks in the northeastern Jiangnan Orogen has been constrained based on systemic analyses of petrology,heavy mineral assemblages,geochemistry,detrital zircon U-Pb ages and Hf isotope,combined with the properties of the multi-layered bentonite developed in the basin,were used for joint discussion the direction,distance,property and tectonic background of the provenance of the sediments.The above studies combined with previous regional geological research were used to explore the prototype of the basin and provide vital clues for reconstructing the evolution of South China Block during the early Paleozoic.Sedimentary characteristics show that in the Cambrian-middle Ordovician of the NW Zhejiang,normal faults are widely developed in the basin,the formation is characterized by carbonaceous siliceous rocks,carbonate turbidite and slump accumulation,indicating the slope environment of deep water at this time.The Huangnigang and Yanwashan formations of the upper Ordovician developed a large number of conglomerates and nodular limestones of detrital flow origin,and the Changwu Formation developed huge submarine fan deposits representing the deep-sea environment.These characteristics represent strong tectonic activity and continuous basin extension of the basin.Tidal flat and shallow marine facies were developed in Wenchang Formation of the upper Ordovician-lower Silurian,and delta sedimentary facies were developed in the middle-upper Silurian.The temporal and spatial distribution of sedimentary facies and paleo-current direction indicate that the basin is characterized by high SE and low NW palaeogeomorphology,and the provenance is from SE direction.The characteristics of petrology indicate that the sandstone contains lithic graywackes and feldspar-lithic graywackes,with low compositional and textural maturities,which is the result of rapid accumulation from near sources.The lithic fragments are angular,including mainly rhyolites,andesites,granites,quartz-schist,slates,phyllites,siltstones,and chert,which also occur in conglomerate form.Minor rutile,tourmaline,anatase,sphene,pyroxene,chromite,magnetite,and garnet grains occur in the sandstone.Geochemical analyses show that the composition of the samples has not been affected by sedimentary sorting and recycling,the trace elements of the clastic rocks are mainly controlled by the property of the provenance and tectonic background,and the provenance in the study area is mainly medium-felsic with some mafic rocks in the island arc.The composition characteristics of the clastic rocks above indicate that the provenance is mainly intermediate-acidic volcanic rocks with a small amount of mafic rocks,metamorphic rocks and sedimentary rocks,and the tectonic background of the provenance was an arc setting.U-Pb ages of detrital zircons demonstrate a mixed source from ca.818 Ma and ca.438 Ma,the central Zhejiang in the southeast direction is the main provenance area,and the Chencai,Heshangzhen,Shuangxiwu and Badu groups provide the main detrital materials to the study area.Combined with regional geological data,the peak age of438 Ma comes from Shuangxiwu island arc,Chencai accretionary complex with the mafic intrusions.The palaeogeographic characteristics and provenance analysis results show that the NW Zhejiang region was a back-arc basin in the early Paleozoic,which first experienced continuous extension,and then gradually shranked after being filled with clastic materials such as huge submarine fans.The basin was matched with the Shuangxiwu arc,the Chencai accretion complex and arc-related mafic intrusions,which demonstrate that early Paleozoic sedimentary deposition occurred in an active continental margin along the southeastern Yangtze Block.This understanding provide new data and ideas for the tectonic evolution of South China Block in the early Paleozoic.