Revisiting The Neoproterozoic Orogenic Process At The Eastern Segment Of Jiangnan Orogen

The Jiangnan Orogen is one of the important Precambrian geological units in the South China Block(SCB),which divides the SCB into the Yangtze Block to the northwest and the Cathaysia Block to the southeast.The tectono-magmatic evolution history of the Jiangnan Orogen has remained controversial.What kind of tectonic setting were the 860-820 Ma sedimentary sequences and interlayered mafic rocks associated with?When did Neoproterozoic post-orogenic rifting in South China Block take place?What are the tectonic features of the Cathaysia Block?This paper deals with the synthetic studies on the petrology,mineralogy,geochemistry,geochronology and isotopic geology of the Precambrian sedimentary sequences and igneous rocks from Eastern Jiangnan Orogen of northern Zhejiang Province in the(part samples of mafic rocks were from Jiangxi Province).It reveals the tectono-magmatic evolution history of eastern segment of the Jiangnan orogenic belt during the Neoproterozoic period.It is first to report that there are Mesoproterozoic A-type granites in the northern Cathaysia Block which supports a connection between the Cathaysia Block and the Laurentia Block at late Mesoproterozoic.These findings have important significance for the study of the assembly and breakup process of the Rodinia supercontinent,and clarify the paleogeography of the SCB.The tectonic setting of the 860-820 Ma sedimentary sequences and interlayered mafic rocks have been controversial.The Heshangzhen Group in northern Zhejiang Province unconformably overlies the early Neoproterozoic Shuangxiwu arc volcanic rocks.Here,we present new detrital zircon U-Pb and Hf isotopic data of the lower Heshangzhen Group(includes Luojianmen Formation and part of Hongchicun Formation).According to the zircon U-Pb dating,the Luojiamen Formation formed during 850-820 Ma,serving as a part of the Jiangnan orogen basement metasedimentary sequence.The sedimentary sequence is generally horizontally bedded and shows a fining-upward grain-size trend,suggesting that these sedimentary sequences formed in a setting of increasing water depth(deposition in a littoral to abyssal environment).The Luojiamen Formation contains detritus derived from the shuangxiwu arc terranes.The Hf isotopic compositions of zircons reveal a rhythmic change in source coincident with a progressive increase in the amount of j uvenile and old crustal detritus within these sediments.This result,combined with the petrography of these sediments,provides evidence of bidirectional sources that are consistent with a backarc setting.The juvenile crustal material within these sediments was sourced from adjacent arc terranes to the east,whereas the old crustal detritus was derived from the Yangtze block to the west.In addition,ca.860-840 Ma mafic rocks within equivalent sedimentary sequences have arc-or mid-ocean-ridge basalt-like geochemical features,also suggesting the presence of a backarc basin system at ca.860-820 Ma along the southeastern margin of the Yangtze block.This in turn indicates that Rodinia assembly was not completed until ca.820 Ma,with the South China Block possibly acting as a connection between a Neoproterozoic Andean-type active continental margin and Grenville belts on the paleo-western margin of the Rodinia supercontinent.The mafic dykes that intruded the Shuangxiwu arc volcanic rocks and the Heshangzhen Group in northwestern Zhejiang Province provide new evidence of postorogenic extension or rifting for the Jiangnan Orogen.The U-Pb dating results indicate that the mafic dykes formed at ca.760 Ma,rather than ca.860-850 Ma as previously suggested.They formed in a post-orogenic extensional setting.All the rocks can be divided into three subgroups based on their geochemistry,and two end-members for the formations of the three types of mafic dykes:depleted asthenospheric mantle and metasomatized lithospheric mantle.The upwelling of asthenosphere at ca.760 Ma led to partial melting of the pre-existed metasomatized lithospheric mantle to generate the Group-1 rocks with arc-like geochemical features.Continued extension resulted in lowdegree partial melting of asthenospheric mantle and subsequent interaction with Group1 magmas form the Group-2 rocks.The Group-3 rocks were resulted from AFC processes of the Group-2 magmas.These mafic dykes represent the transition of the tectonic regime from otogenic to to post-orogenic withinplate process for a typical orogenic belt.Based on our new data and the available U-Pb ages of Neoproterozoic magmatic rocks,the rifting of South China from Rodinia should have not happened before ca.820 Ma.We first report the Mesoproterozoic granites in the Chencai Complex,the north Cathaysia Block,which supports a Cathaysia-west Laurentia connection at late Mesoproterozoic.The results of SIMS zircon U-Pb dating suggest the crystallization age of the protolith is ca.1320±9 Ma,then metamorphosed into grantic gneiss in 430 Ma.The whole-rock geochemistry of the ca,1320 Ma granitic gneiss indicates A-type granites of the protoliths,with low LILE/HSFE ratio,high high-field-strength elements(HFSE such as Th,Nb,Ta,Zr,Hf,Y)and rare earth elements(REE)concentrations.The low ?18O values(5.14-6.49‰)of magmatic zircon domains are close to that of typical mantle zircons,suggesting less incorporation of supracrustal materials in magma source.In addition,zircon ?Hf(t)values of the protoliths are-2.78?+3.27 and Hf model ages(TDM2)is in the range of 1.9?2.3 Ga.Combined with zircon ?18O values,the Hf isotopes indicate the rocks were originally derived from partial melting of preexisting 1.9 to 2.3 Ga intracrustal materials.According to the similarities in age and Hf isotope compositions,the ca.1320 Ma granitic gneisses in Chencai Complex have an affinity with the Mesoproterozoic A-type granites in Mojave,western Laurentia.Therefore,we propose a Cathaysia-west Laurentia connection at Mesoproterozoic(?1320 Ma),and the connection was destroyed during late Mesoproterozoic.After separation from the west Laurentia,the Cathaysia Block was welded together with the Yangtze Block during middle Neoproterozoic.