The Neoproterozoic Magmatism And Sedimentation Of The South Qinling Belt And Its Tectonic Implications

The reconstruction of South China in the Rodinia supercontinent has long been a hot research topic.The Yangtze Block is an integrant of the South China,and thus its tectonic evolution during the Neoproterozoic is of highly debated.Neoproterozoic magmatism and sedimentation arewidely distributedalongthe margins of the Yangtze Block,and are good carries to solve the above problems.The scientistshave obtained numerous achievements in the southeastern and western margins of the Yangtze Block,whereas much less attention has been paid to the northern Yangtze Block.In this paper,we investigate the early Neoproterozoic Dahongshangabbros,thesedimentary rocks from the middle to late Neoproterozoic Suixian Group,and the late Neoproterozoic mafic-ultramafic dikes in the northern Yangtze Block.We present petrography,geochronology,major and trace elements,Sr-Nd-Pb-Hf-O isotopesfor the igneous and sedimentary rocks.For the mafic-ultramafic rocks,we aim to decipher the nature of the mantle source,magma differentiation process,and tectonic evolution of the northern Yangtze Block during the Neoproterozoic,especially the change from the subduction to the extensional environments and from rift to drift processes.The sedimentary rocks from theSuixian Group are used to address their source characteristics andpossible position relationshipsofthe South China Craton with the other continents inRodinia throughcomparing similarities and differences of the equivocal strata.The ca.815-Ma mafic plutons in the Dahongshan region are composedof fine-to medium-grained gabbro and diabase,which show variable major and trace elementcompositions with?La/Yb?_N ratios of 0.53–3.43.The mafic rocks also have large ranges of initial ⁸⁷Sr/⁸⁶Sr ratios?0.70318–0.70810?,?Nd values?-2.8 to+4.3?and Pb isotopes(²⁰⁶Pb/²⁰⁴Pb=17.57–18.46,²⁰⁷Pb/²⁰4Pb=15.43–15.56,²⁰⁸Pb/²⁰⁴Pb=37.45–38.12).These lines of evidence suggest that the Dahongshan mafic rocksunderwent significant crustal contamination during the magma emplacement.AFC calculations reveal that 10–20%crustal materialswere involved in their petrogenesis.Two primary samples from the Dahongshan intrusionsdisplay lowU/Th?0.36–0.37?,Th/Zr?0.001–0.002?,and Nb/Y?0.02–0.03?ratios,and showDMM-like?Nd and Pbisotopes,indicating that their primary melts were derived from an asthenosphere mantle which had not beenmodified by the slab-derived materials?melts and fluids?.The Suixian Group consists ofsandstone and siltstone associated withminor interlayered volcanic rocks.Detrital zircons show three U-Pb age populations with peaks at?2450 Ma,?1990 Maand?750 Ma,and the minimum age is?700 Ma.Their?Hf?t?values range from–22.5 to+11.1.Zircon U-Pbages and Hf isotopic data are similar to those of the magmatic zircons from thePrecambrian igneous rocks in the South Qinling Belt,in combination with the euhedral to subhedralshapesof the detrital zircons,suggestingtheir local derivation from the rocks within the belt.The sedimentaryrocks from the Suixian Group have high La/Sc?0.91–7.28?,Th/Sc?0.56–2.16?and low Cr/Th ratios?0.94–5.28?,implying derivation from felsic volcanic rocks.Similarly,the detrital zircons from the lateNeoproterozoic sedimentary rocks along the margins of the Yangtze Block show a dominant U-Pb age peak of770 Ma to 710Ma,suggesting significant input from an approximately syn-depositional magmatic source.TheNeoproterozoic detrital zircons show similar age distributions and?Hf?t?values to those of the Neoproterozoicigneous rocks around of the Yangtze Block,suggesting that these sedimentary rocks were sourced from amagmatic arc.The Yangtze Block and northern India share similar Neoproterozoic sedimentary successionsand detrital age populations,indicating that they were closely connected and probably located at marginal position ofRodinia.The ca.650 Ma ultramafic-mafic dike swarms are the youngestmagmatic episode in the South Qinling Belt during the Neoproterozoic,and therefore provide a window to address their mantle sources,and the continental rift to drift transition.These mafic dikes have distinct geochemical signatures,and can be classified into N-MORB-,E-MORB-,and Arc-affinity subgroups.The Shiziyan amphibole gabbros have low Rb/Y?<0.5?,Nb/Y?<0.1?,Nb/Zr?<0.03?,Th/Zr?<0.003?ratios,positive?Nd?+4.3 to+5.4?and moderate ²⁰⁷Pb/²⁰⁴Pb?15.48?and ²⁰⁶Pb/²⁰⁴Pb?17.57?,similar to N-MORB,suggesting that they were derived from an asthenosphere mantle.The Yindongshan pyroxenites display enrichment of LILEs and HFSEs,similar to the E-MORB characters.They also have low ⁸⁷Sr/⁸⁶Sr?i??0.7038 to 0.7041?,high?Nd?+4.2 to+5.4?,and ²⁰⁷Pb/²⁰⁴Pb?15.53-15.54?and ²⁰⁶Pb/²⁰⁴Pb?18.18-18.37?.They have high zircon?Hf?+13.1 to+15.0?,but low?¹⁸O?+4.43‰to+5.17‰?.These chemical compositions can be explained by the recycling of oceanic island basalts/seamounts in the mantle source.The Duchongshan olivine gabbros show arc-like geochemical features,with high Nb/Zr?0.07-0.16?,Nb/Yb?3.52-20.1?ratios,and high ⁸⁷Sr/⁸⁶Sr?i??0.7052-0.7066?,negative?Nd?-7.1 to-4.5?,large variable zircon?Hf?-0.5 to+4.2?and high?¹⁸O?+5.41‰to+6.41‰?.These features favor the process that they were derived from a subduction-modified lithosphere mantle followed by significant crustal contamination.The mafic dikes emplaced into the volcanic successions and are unconformably overlainby the late Ediacaran neritic to abysmal deposition.The magmatic termination and sedimentary facies transition indicate that the northern Yangtze Block broke up during the late Neoproterozoic.Considering an exterior position suggested for the Yangtze Block,the Proto-Tethys Ocean is proposed to have been opened at ca.650 Ma.Previous studies reveal that the early Neoproterozoic?>850 Ma?magmatism in the northern Yangtze Block is characterized bythe calc-alkalineigneous rocks,and were derived from thelithospheric mantle that was modified by slab/sediment melts or fluids,implying subduction of the oceanic slab.During the ca.830-815 Ma,the mafic igneous rocks dominantly show MORB-like geochemical features,and were probably derived from the asthenosphere mantle.Such a mantle source transition was more likely resulted from slab breakoff and upwelling of the asthenosphere mantle.The northern Yangtze Block transformed intothe extensional environment during this stage.At ca.820 Ma,the Douling complex underwent amphibolite facies metamorphism,and its clockwise P-T trajectories indicate the arc-continental collision.During the middle to late Neoproterozoic?ca.780–680 Ma?,widespread volcano-sedimentary sequences were developed in the northern Yangtze Block.They are characterized bybimodal volcanic rocks,low?¹⁸O magmatism and proximal deposition,indicating that they were formed in a rift basin.In the late Neoproterozoic?ca.680–650 Ma?,the mafic-ultramafic dike swarms show geochemical characteristics similar to those of the mid-ocean ridge basalts.Combined with the late Sinian?<620 Ma?carbonate cap?such as Doushantuo and Dengying Formation?,the northern Yangtze Block broke up,and developed into the passive margin.Therefore,the northern Yangtze Blockunderwent five stagesof evolution during the Neoproterozoic time:?1?>850 Ma plate subduction;?2?ca.840–820 Ma back-arc extension;?3?ca.820 Ma arc-continent collision;?4?ca.780–680Ma rift basin;and?5?opening of the Proto-Tethys oceanduring<680–635 Ma and formed a passive margin.The early Neoproterozoic magmatism,sedimentary and metamorphism in the northern Yangtze Block are similar to the normal accretionary orogenic belts,but significantly different from the Grenvillian collisional orogenic belt.The change from the Neoproterozoic rift to drift process is also different from the opening of the Proto-Pacific Ocean,which separatedfrom the western Laurentia and eastern Australia during the Cambrian time.The northern Yangtze Block share much similar to the northwestern India and the northeastern Tarim,and may have been located in the marginal position of Rodinia.