The Formation And Evolution Of The Jiangnan Orogen

As a crucial part of the Rodinia supercontinent,the South China Block has attracted global attention on its Neoproterozoic tectonic evolution,mineralization,phosphorous,“snowball Earth”and Ediacaran multicellularity life evolution.However,the amalgamation timing and mechanism between the Yangtze and Cathaysia blocks(i.e.,the formation of Jiangnan Orogen belt)are debated,thus resulting in external versus internal paleogeography of the South China Block in Rodinia.It is a consensus that the Jiangnan Orogen belt was formed at Neoproterozoic,whereas the precise timing is confusing(880Ma vs.830 Ma).As a result,there is a dispute about whether the 870?800 Ma magmatic and sedimentary rocks in the Jiangnan Orogen belt were related to mantle plume or subduction-collision.Therefore,an accurate cognition of the Neoproterozoic tectonic evolution and dynamic mechanism of South China and its paleogeographic position in the Rodinia supercontinent is of great significance to decipher the above major global geological events.In this study,the 850 Ma Dongling A-type granite,850?825 Ma Fuchuan ophiolite and 805 Ma Chengbu S-type granite in the Jiangnan Orogen belt are chosen as the research objects.Detailed field geological exploration,petrography,zircon U-Pb geochronology,elemental and isotopic geochemistry are used to explore the timing,petrogenesis,material sources and tectonic implication for these 850?800 Ma igneous rocks in the Jiangnan orogenic belt.Then,combining with previous datasets and results,the Neoproterozoic tectonic evolution in the southeastern margin of the Yangtze block was reappraised,the link between the Jiangnan orogen and the global Greenville orogen was discussed,and the paleogeographic position of the South China plate on the Rodinia supercontinent was re-defined.The major conclusions are listed as follow:(1)LA-ICP-MS zircon U-Pb dating suggests that the Dongling granites were formed at?850 Ma.The Dongling granites have high K2O+Na2O(7.44-9.09 wt.%)and low Mg O(0.07-0.54 wt.%)and Ca O(0.16-1.21 wt.%)contents.Their high Fe O^T/(Fe O^T+Mg O)(0.82-0.94)and Ga/Al(?2.6)values and HFSE abundance(Zr+Nb+Ce+Y=443-965 ppm)group them as A-type granite.In the chondrite normalized diagrams,these granites display right-inclined REE patterns and strong to moderate negative Eu anomalies(Eu*=0.32-0.70).The Dongling granites have variable and negative whole-rock?Nd(t)(-14.6 to-8.6)and zircon?Hf(t)(-12.8 to-4.4)values with T_DM2(Nd)of 2.20-2.69 Ga and TDM2(Hf)of 2.01-2.41 Ga.Zircon saturation parameters(TZr)suggest>850? temperature and other geochemical characteristics reveal low melting pressure(>350 to<700 MPa)and 1.9-2.6 wt.%H2O content.Geochemical and isotopic characteristics indicate derivation of the Dongling A-type granites through partial melting of an ancient granulitic metasedimentary source,after a previous melt extraction.Emplacement of the Dongling A-type granites is inferred in an extensional setting following the arc-continent collision along the southeastern Yangtze Block margin.(2)LA-ICP-MS zircon U-Pb dating suggests that the Fuchuan Ophiolite were emplaced at 850-825 Ma.Mineral geochemistry of olivine,orthopyroxene and chrome spinel from the harzburgite and two types of clinopyroxene from the cumulate gabbro within the Fuchuan Ophiolite suggests formation in a Neoproterozoic subduction system similar to the IBM.High temperature(1000-1200?),low pressure(<30 km depth),high degree of melts extraction(>18%)have been recorded in the harzburgite.This evidence,in conjunction with magma evolution from fore-arc basalt to boninite to hydrous calc-alkaline magmatism,recorded in the crustal section of the Fuchuan Ophiolite indicates a transition from initial to mature subduction in a fore-arc setting.Additional evidence for Neoproterozoic ophiolites from India,Siberia and Arabian-Nubian Shield,which are also similar to the IBM fore-arc ophiolite,suggests that cold and deep subduction and modern-style plate tectonics may have developed in the Neoproterozoic(ca.850 Ma)around the periphery of the Rodinia supercontinent.This subduction system may have acted as the geodynamic trigger for the break-up of Rodinia.(3)LA-ICP-MS zircon U-Pb dating suggests that the Yexijiang and Baomuping granites were formed at ca.805 Ma.They have low and variable?REE concentrations(47-115 ppm)and show moderate to highly fractionated REE patterns([La/Sm]N=2.83-3.78)with low[Gd/Yb]N(1.34-2.31)and negative Eu anomalies(Eu*=0.34-0.57).Geochemical features and presence of peraluminous minerals classify them as S-type granites and P-T parameters indicate low to moderate temperature(730-791?)and pressure(middle crustal level)conditions.These granites show consistently negative whole-rock?Nd(t)(-5.7 to-6.6)and magmatic zircon?Hf(t)values(-0.3 to-3.8)while inherited zircon grains show variable?Hf(t)values(2.1 to-13.0)and two-stage Hf model ages(1.80 to 2.88 Ga).In particular,one Neoproterozoic(899 Ma)inherited zircon grain displays depleted Hf[?Hf(t)=+5.4 and TDM2=1.34 Ga]isotopic characteristic.These lines of evidence indicate a sedimentary source for Yexijiang and Baomuping granites,possibly the Archean to Neoproterozoic crustal rocks,and emplacement in a post-collision tectonic environment triggered by mafic magma underplating during the amalgamation of the Yangtze and Cathaysia blocks.(4)Based on previous studies,there was an evolutionary trend from the initial subduction to mature arc subduction in the southeastern margin of the Yangtze block during the Neoproterozoic 850?820 Ma,which was similar to the modern IBM island arc to back-arc subduction system.After that,the Yangtze and Cathaysia blocks were amalgamated from 820 Ma,and ended at?805 Ma,i.e.,the formation of the Jiangnan Orogen belt.Then the Nanhua rifting basin developed in South China.The formation time(820?805 Ma)of the Jiangnan Orogen belt is later than the Global Greenville Orogeny.Combined with the geological fact that the South China Block is different from the western Laurentian Block and the Australian Adelaide geosyncline but similar to the Indian Block on Cryogenian sediment detrital zircon KDE age profile and the long-term plate subduction around the Yangtze in the Neoproterozoic,we suggest that the South China Block should be located at the edge of the Rodinia supercontinent.