Petrogenesis And Tectonic Implications Of The Neoproterozoic Magmatic Rocks In The Western Jiangnan Orogen,South China

The Jiangnan Orogen was formed by the assemblage of the Yangtze and Cathaysia blocks during the Neoproterozoic,and its tectonic evolution has long been a matter of debate.Neoproterozoic magmatic and sedimentary rocks are widely distributed within the Guibei and Fanjingshan regions in the western Jiangnan Orogen,they are ideal objects to solve the above problem.Geoscientists have conducted numerous geochronological,petrological and geochemical studies on the Guibei and Fanjingshan magmatic rocks,but the different explanations about petrogenesis and tectonic settings lead to the regional tectonic evolution still remains unclear.In this study,we investigate the Neoproterozoic Guibei granitoids and Fanjingshan volcanic rocks and granites.We present petrographic,whole-rock elemental and Sr-Nd isotopic studies for these magmatic rocks,along with zircon Hf isotopes and tourmaline elements and in-situ B-isotopes,in order to figure out their pretrogenesis and tectonic settings.Meanwhile,combined our study with published research achievements,aim to decipher the Neoproterozoic tectonic evolution of the Jiangnan Orogen.The Guibei Neoproterozoic granitoids comprise biotitle granites and biotite granodiorites.Their formation ages range from 835 Ma to 805 Ma,and the granodiorites were formed earlier than the granites.The Guibei granitoids intruded the deformed early Neoproterozoic Sibao Group,and were unconformably overlain by the middle Neoproterozoic Danzhou Group.The granites are fine-grained and mainly consist of quartz(30 vol.%),K-feldspar(35 vol.%),plagioclase(25 vol.%)with minor muscovite(~3 vol.%),biotite(~5 vol.%)and tourmaline(~2 vol.%).The granodiorites are also finegrained and are composed of quartz(35 vol.%),K-feldspar(15 vol.%),plagioclase(40 vol.%)and biotite(10 vol.%)without tourmaline.The Guibei granitoids have high Al2O3(11.95-18.86 wt.%)and K2O+Na2O(5.34-11.17 wt.%)contents,and high K2O/Na2 O ratios(1.03-5.48)and A/CNK values(0.79-2.07).These features indicate that they are typical peraluminous S-type granitoids.The Guibei granitoids are characterized by fractionated LREE and flat HREE patterns with strong negative Eu anomalies.In the primitive mantlenormalized spider diagram,these granitoids show enrichments of Rb,Th and U and depletions of Nb,Ta,Sr and Ti.The Guibei granitoids also have high differentiation index values(DI = 79-97),highly variable(K2O+Na2O)/Ca O(2-84)and initial 87Sr/86 Sr ratios(0.500230-0.713264),and negative whole-rock ?Nd values(-9.31 to-5.45)with Paleoproterozoic two-stage Nd model ages of 1.83-2.34 Ga.These lines of evidences suggest that the Guibei granitoids were derived from partial melting of ancient crustal rocks,and underwent significant fractional crystallization of K-feldspar and plagioclase.In addition,high Rb/Sr(2.4-47.2)and Rb/Ba(0.5-22.2)ratios and low Ca O/Na2O(0.04-0.57)ratios of the Guibei granites indicate a clay-rich,plagioclase-poor politic source.Tourmalines in the Guibei granites can be subdivided into disseminated and nodular types based on field occurrences.The disseminated tourmalines are small and euhedral crystal and randomly distributed in the host granites,whereas the nodular tourmalines include two parts,a core contains tourmaline and quartz and a leucocratic rim consists of finegrained plagioclase,K-feldspar and quartz.All tourmalines have high Fe O/(Fe O+Mg O)ratios(0.82-0.99)and Y-site Al contents(0.40-0.84 apfu;atoms per formula unit)and do not show compositional zonations in the BSE images,consistent with the features of magmatic-origin tourmalines.Textural and compositional analyses suggest that the disseminated tourmalines crystallized from the B-rich granitic magma at the early magmatic stage,whereas the nodular tourmalines precipitated from immiscible B-rich fluid/melts that separated from coexisting granitic magma at the late magmatic stage.The tourmalines in this study show strong negative ?11B values(-13.4‰ to-9.5‰),similar to the ?11B value of average continental crust(-10±3‰),suggesting that the boron needed for tourmaline formation comes from the continental crust.Phyllosilicates(e.g.mica)have the highest B contents in the pelitic rocks,therefore,the boron derived from breakdown of mica during partial melting is the actual boron source.The boron isotopic variations within or among different types of tourmalines are dominantly controlled by Rayleigh fractionation.During the crystallization of magmatic tourmaline,Rayleigh fractionation can deplete the 11 B in remaining melt,resulting in the late-stage magmatic tourmaline with lower?11B values.The Fanjingshan volcanic rocks comprise basalts and felsic tuffs that underwent variable degrees of alteration.The basalts display typical pillow structures in the field.They are fine-grained and consist mainly of plagioclase and clinopyroxene.Plagioclase is partially altered into chlorite and sericite along its margins.Clinopyroxene is almost replaced by epidote,but its original morphology is preserved.Previous studies revealed that the Fanjingshan mafic rocks were formed at 856-830 Ma.The basalts are low to medium-K calc-alkaline series rocks,and plot in the basalt-andesite field in the Zr/Ti O2-Nb/Y diagram.These rocks have very low total REE contents(?REE=44-81ppm)and show right-inclined LREE and flat HREE patterns with negative Eu anomalies.They display arc-affinity trace element features,such as enrichment of Rb and Th,depletion of Nb and Ta and negative Sr and Ti anomalies in the spider diagram.The basalts have high La/Sm(3.09-5.18)and low Nb/La(0.35-0.59)ratios,arguing against significant crustal assimilation.Their high Th/Zr(0.03-0.08),Nb/Y(0.17-0.33)and Th/Nb(0.50-1.17)ratios and negative ?Nd values(-4.2 to +0.2)reveal that the parental magmas were derived from a lithospheric mantle that was modified by subduction components.Furthermore,positive correlations between Ca O and Mg O and moderate negative Eu anomalies suggest that the magmas experienced fractional crystallization of plagioclase and clinopyroxene during the emplacement.The felsic tuffs are interbedding with the sedimentary rocks of the Fanjingshan Group in the field.They show typical crystal tuff texture and consist of fine-grained quartz and plagioclase in a sericitized groundmass.The ages of the felsic tuffs are constrained at 840-830 Ma.The felsic tuffs are medium to high-K calc-alkaline series rocks and plot in the rhyodacite-dacite fields in the classification diagram,thus it is named as rhyolitic tuffs.The REE patterns of the rhyolitic tuffs are similar to the basalts,but the tuffs have higher total REE contents and significantly negative Eu anomalies.In the primitive mantle-normalized diagram,the rhyolitic tuffs are enriched in Rb,Th and Pb,and depleted in Nb and Ta with pronounced negative Sr and Ti anomalies.The trace elemental similarities between the basalts and rhyolitic tuffs suggest that the latter probably derived from extensive fractionation of the former.However,the rhyolitic tuffs are calc-alkaline in compositions,and they have lower ?Nd(-7.7 to-6.1)and more variable zircon ?Hf(-6.3 to +10.4),different from the felsic rocks produced by strong differentiation of basaltic melts.On the other hand,the rhyolitic tuffs have higher Si O2(63.43-79.6wt.%),Mg O(0.22-4.44 wt.%)and Fe2O3(3.25-12.4wt.%)than the experimental peraluminous melts,suggesting that they were not produced by solely partial melting of the continental crust.Two end-member mixing calculation reveals that mixing between the Fanjingshan mafic rocks and Neoproterozoic S-type granites in the western Jiangnan Orogen match the Nd-isotope variations of the rhyolitic tuffs,thus the rhyolitic tuffs were generated by mixing of mantle and crustal-derived melts.The Fanjingshan granites show obvious intrusive contacts with the Fanjingshan Group and have ages of 838-830 Ma.The granites are fine to medium-grained and are composed mainly of quartz(30-35 vol%),plagioclase(35-40 vol%),alkali feldspar(15-25 vol%)and minor muscovite(<8 vol%)and tourmaline(~2 vol%).The granites are also high-K calc-alkaline series rocks and have A/CNK values of 1.06-1.34,typical of peraluminous S-type granites.These granites have different REE patterns compared to the volcanic rocks,which are characterized by flat LREE and fractionated HREE patterns with strong negative Eu anomalies.They display enrichments of Rb,Th and U,and depletions of Nb,Ta and Sr in the trace element spider diagram.The granites have highly variable Fe O/Mg O(1.3-113)and(K2O+Na2O)/Ca O ratios(7-109),and high Rb/Sr(24-124)and Ca/Sr ratios(58-1552).These geochemical features suggest that the granites underwent strong fractional crystallization.In addition,their high Ta contents(3.39-25.5 ppm)and low Nb/Ta ratios(1.6-3.8)reveal the existences of different degrees of hydrothermal alteration.The granites have negative ?Nd(-13.0 to-7.30)and ?Hf(-2.7 to +0.7)values with two-stage Nd model ages of 2.10-2.56 Ga and Hf model ages of 1.67-1.87 Ga,indicating that they were formed by partial melting of ancient crustal rocks.The strong Neoproterzoic magmatisms in the Jiangnan Orogen produced variety of magmatic rocks,from ultrmafic to felsic in compositions.Granites are the representative of felsic magmatic rocks in the Jiangnan Orogen.They are widely distributed in the orogen and have formation ages range from 970 Ma to 770 Ma.Although there is a large time span,the Neoproterozoic granites in the Jiangnan Orogen can be divided into three groups according to their ages: >880Ma,840-810 Ma and <800Ma.Granites of the first group include the plagiogranites and S-type leucogranites within the ophiolite suite and I-type granites in the Shuangxiwu Group.The S-type leucogranites were derived from the partial melting of metesedimentary rocks,whereas the plagiogranites and I-type granites were formed by partial melting of subducted oceanic crust and subductionmodified mantle wedge,respectively.Granites of the second group are mainly composed of peraluminous S-type granites,their Hf-Nd-O isotopic variations show some regularities.For example,S-type granites in the eastern Jiangnan Orogen have higher ?Hf and ?Nd,and lower ?18O values than those from the western Jiangnan Orogen,some granites in the eastern Jiangnan Orogen display intermediate geochemical features between S-type and I-type granites.Granites of the third group are A-type granites,their formation is closely related to the intracontinental rifting during this period.It is noteworthy that the three groups of granites corresponding to different evolution stages of the Jiangnan Orogen.Combined with published research achievements,the Jiangnan Orogen underwent four stages of evolution during the Neoproterozoic:(1)Early subduction stage(ca.1000-880Ma).Northwestward subduction of Paleo-South China Ocean lead to the formation of the Shuangxiwu arc igneous rocks and Northeast Jiangxi ophiolite suites in the eastern Jiangnan Orogen.(2)Late subduction stage(ca.880-830Ma).The continued subduction of Paleo-South China Ocean might have triggered the arc magmatism along the whole Jiangnan Orogen,resulting in the formation of numerous arc igneous rocks(e.g.Fanjingshn volcanic rocks)and early Neoproterzoic sedimentary strata(e.g.Fanjingshan Group and its equivalents).At the same time,the Shuangxiwu arc amalgamated with the Yangtze block along the Northeast Jiangxi Fault.(3)Collision stage(ca.830-800Ma).With the closure of Paleo-South China Ocean,the Jiangnan Orogen was formed by the collision of the Yangtze and Cathaysia blocks.The early Neoproterzoic sedimentary strata underwent strong deformation and peraluminous granites(e.g.Guibei and Fanjingshan granites)were derived from partial melting of crustal rocks resulted from orogenic collapse.(4)Intracontinental rifting stage(ca.800-750Ma).Influenced by the breakup of Rodinia supercontinent,intracontinental rifting occurred throughout the whole Jiangnan Orogen,generating the middle Neoproterozoic sedimentary strata and bimodal magmatic rocks.