Zircon Geochronology And Directional Structures Of Pre-Devonian Sediments In Northern Jiangshan Of Zhejiang Province And Their Paleogeography Significance

Northern Jiangshan of Zhejiang province,where pre-Devonian strata are well exposed and studied,is located in the eastern segment of Jiangnan orogenic belt of southeastern margin of Yangtze Block and close to Jiangshan-Shaoxing fault belt.Thus northern Jiangshan is an important window to study pre-Devonian paleogeography of eastern Cathaysia Block which is close to the southeast of Jiangshan-Shaoxing fault belt.This paper describes the zircon features including 7 samples of terrigenous clastic rocks and 1 sample of volcaniclastic rocks from the Heshangzhen Group,Xiuning Formation,Leigongwu Formation,Yanghugang Formation and Yinzhubu Formation in Tangli,Shilonggang,Xintangwu and Shanghengtang sections of northern Jiangshan.858 groups of geochronological data were got through zircon LA-ICP-MS U-Pb isotope dating analysis.A total of 48 structura]elements data of sedimentary directional structures were measured and corrected.These directional structures include cross bedding,ripple mark,inclining stromatolite cylinder,syndepositional fold,syndepositional fault and slip-slump developed in different horizons where the zircon samples were collected or nearby,including the Heshangzhen Group,Xiuning Formation,Dengying Formation,Hetang Formation,Dachenling Formation,Yangliugang Formation,Huayansi Formation,Xiyangshan Formation,Ningguo Formation and Yanwashan Formation.According to these new materials,this study adopts the research approaches and methods combining geochronology of detrital zircons with directional structures to investigate the sedimentary basement topography and directions of sedimentary transportation in northern Jiangshan as well as paleogeographical characteristics of terrigenous detrital source area,mainly promoting the paleogeographic research of eastern Cathaysia Block.Main conclusions obtained in this study and their significances include the five aspects as below.(1)In northern Jiangshan,the formation age of volcaniclastic rocks in the Heshangzhen Group was 819.3±4.7 Ma,suggesting that the rocks belong to the Luojiamen Formation and there are no Hongchicun and Shangshu Formations.Depositional time about the first,second,third members of Xiuning Formation was no earlier than 754±14 Ma,707.3±9.1 Ma and 688±11 Ma,respectively.The Leigongwu Formation,whose depositional time should be later than 688.8±8.7 Ma,may belong to the latter of two Neoproterozoic glacial periods in south China.These new data deepen the cognition of temporal constraint on the paleogeographic evolution and sedimentary history of eastern segment in Jiangnan orogenic belt.(2)The age populations of detrital zircons from the upper Heshangzhen Group to the top of Yinzhubu Formation in northern Jiangshan are major 650-900 Ma and minor?2500 Ma,?1800 Ma,Grenville and pan-African ages.Detrital zircon spectra with the major peaks at 821 Ma?738 Ma are similarity to northwestern Fujian-eastern mid-Jiangxi,northeastern Jiangxi,southern part of south Anhui and eastern part of northwest Zhejiang,suggesting that northern Jiangshan has affinity to these regions in detrital sources.But the spectra are significant different with southwestern Fujian-southern Jiangxi,suggesting that both have no affinity in detrital sources.Thus,this study fills the gaps of pre-Devonian geochronology data of detrital zircons in northern Jiangshan.(3)From late-Tonian to early Paleozoic,there were sedimentary basement mainly inclined NW-NNW and direction of terrigenous sediment transport from SE-SSE in northern Jiangshan..It reveals the paleotopography and direction of sedimentary transportation and source location in northwestern Zhejiang during the pre-Silurian depositional periods,and provides new materials to research the paleogeography of eastern Cathaysia Block.(4)To detrital materials in the pre-Silurian strata of northern Jiangshan,most probable location of detrital source(eroded area)might be the central part of Zhejiang Province.It is possible that,at least 688 Ma to 478 Maj there should be some sediments.These sediments were eroded away after Caledonian or/and Indosinian orogenies so that there are no their outcrops in eentral Zhejiang nowadays.Besides,the eroded area is in eastern Cathaysia Block,suggesting that it is different from western Cathaysia Block in which the terrigenous sediments should be from outer south China nowadays as previous studies.(5)During the depositional period of the Heshangzhen Group and first member of Xiuning Formation,there might be the river system with mainly northwest flow direction in northern Jiangshan.In the period of the second member of Xiuning Formation to the Yanwashan Formation,sedimentary basement of northwestern Zhejiang mainly inclined NW-NNW and this slope of basement might extend to Niuwushan of Qimen-Beigong of Jingxian in southern Anhui.As to eroded area—central Zhejiang,the possible characteristics of paleogeograhic evolution are as fellows.During the depositional period of the Heshangzhen Group and first member of Xiuning Formation,the scope of eroded area was large.Up to the period of the second and third members of Xiuning Formation,the scope became small and the north of area developed littoral facies sedimentary.In Neoproterozoic glacial period,the scope presented the trend from enlarging to narrowing.In the Ediacaran,the scope trended to be narrowing because of the transgression and there were littoral facies sedimentary belts in the north.With the transgression and narrowing the scope during the early Palaeozoic,there were littoral facies sedimentary belts in the northern area where there also might be shallow sea facies.Furthermore,up to the period of the Ningguo Formation,the scope was very small,even submerged completely in the sea because of the long term planation.Thus,these understandings of framework provide new materials and ideas for the paleogeographic research of eastern Cathaysia block.