Precambrian Tectonic Evolution Of The Jiuling Terrane In The Central Jiangnan Orogen

The NE-SW-trending Jiangnan Orogen is located in the central part of South China Block and follows the Shaoxing-Jiangshan-Pingxiang-Qidong-Chalin-Longsheng fault zone,which separates the Yangtze Block to the northwest and the Cathaysia Block to the southeast.Early-middle Neoproterozoic rocks comprising meta-sedimentary suite with mafic-intermediate-felsic volcanic intercalations,granitic plutons and ophiolitic rocks are widely developed within this Jiangnan Orogen,together with a major regional angular unconformity.Previous studies have attempted to trace the tectonic regimes with a view to understand the paleogeography of South China Block in the Neoproterozoic.Although the formation time of this orogen is now widely accepted as early to middle Neoproterozoic,opinions are diverse on the geodynamic evolution.The Jiuling terrane is located in the central Jiangnan Orogen.Comparing with other domains in Jiangnan Orogen,the investigations on the geologic formations in Jiuling terrane are weak or not systematic.This paper focuse on several important scientific problems in jiuling terrane,used the research methods of field geology,zircon U-Pb chronology,geochemistry and zircon Hf isotope to research orogenic related geological bodies,by which we obtains a serious of new data and ideas.1.A research based on field geological survey found that there is a composite anticlinal structures incorporating recumbent fold and tight fold developed in Jiuling terrane of the central Jiangnan Orogen.The E-W-trending folds are intruded by the 820 Ma Jiuling granitoids,suggesting that some folding deformation predated the emplacement of the granitoids.The collision lead to the development of regional recumbent and ductile top-to-SE thrusting in southern Jiuling terrane and upright folds in the northern Jiuling terrane,indicating more intense deformation close to the suture zone.The mylonite have with NEE-trending foliation and stretching lineation that plunges NW 320°.Kinematic results indicate a top to the southeast thrusting with a component of dextral shear component.This is consistent with the schistosity of the greenschist-facies rocks,indicate that the time of ductile deformation coincides with that of regional greenschist-facies metamorphism,and further confirm that Yangtze Block obducted beneath ocean plate toward southeast during Neoproterozoic.Conbining with the regional angular unconformity,the intense contraction and deformation observed in obove Precambrian geological units,indicate that there was an early Neoproterozoic orogenic event occurred in the Jiuling terrane.Detrital zircon CL and U-Pb chronology data from Neoproterozoic meta-sedimentary rocks of the Jiuling terrane indicate that the lower(the unit below the unconformity)and upper(the unit above the unconformity)Shuangqiaoshan groups were deposited during ca.863-820 Ma and ca.797-780 Ma,respectively.Our data combining with those in previous studies,suggest that the regional angular unconformity developed along the Jiangnan Orogen during ca.830-800 Ma,related to the final collision between Yangtze and Cathaysia blocks.2.Three magmatic suites ranging in composition from felsic to mafic are identified in the Jiuling terrane of the central Jiangnan Orogen.Suite 1 including meta-diorites and meta-basites are dated at 849.9 ± 4.9,848.8 ± 4.5 and 846.3 ± 3.5 Ma.The meta-diorites and some of the meta-basites display enrichment of LREE,Rb and K and depletions of Nb,Ta and Ti anomalies,with distinctive geochemical features of continental arc setting.The Hf(t)values of zircons from meta-diorites range from 0.17 to 6.10.The other meta-basites have LREE depleted REE patterns,similar to N-MORB and intra-oceanic island arc settings,suggesting a fore-arc environment.Suite 2 consists of foliated granitoids dated at 816.8 ± 4.2,814.6 ± 3.6,812.5 ± 3.5 and 810.3 ± 3.5 Ma,with the A/CNK values>1.1,and are classied as post-collisional granites in the Rb/(Y+Nb)diagram.Zircon grains from the granitoids show Hf(t)values in the range of-1.99 to 8.49 and Hf model ages of 1835-1041 Ma.Suite 3 is composed of meta-diabase dated at 792.1 ± 2.8 Ma,with zircon Hf(t)values of 8.46-13.83.This suite shows E-MORB REE patterns,and plot within continental rift field.Their zircon Hf model ages mainly clustered at 1046-811 Ma,indicating magma sourced through partial melting of early Neoproterozoic lithosphere mantle.These data suggest that the three Neoproterozoic Jiuling magmatic suites in the Jiuling terrane correlate with(1)the continental arc,fore-arc,(2)collision and(3)continental rift settings in the interval from 850 Ma to 790 Ma.These tectonic settings are consistent with a geodynamic process from the oceanic subduction beneath the Yangtze Block(850 Ma),collision between Cathaysia and Yangtze blocks(820 Ma)and intracontinental rift(790 Ma).3.Provenance analysis shows that the Neoproterozoic detrital zircons of lower and upper Shuangqiaoshan groups were mainly derived from Jiangnan Orogen.Detrital zircon with>1000 Ma age of the lower Shuangqiaoshan Group were mainly derived from the western Yangtze Sub-block,whereas the detrital zircons from the upper Shuangqiaoshan Group with ages>1000 Ma and<800 Ma(with negative sHf(t)values)mainly come from the northern and northeastern Yangtze Sub-block.Four periods of magmatism are identified in the source area of the Neoproterozoic meta-sedimentary rocks at 900-780 Ma,1500-1900 Ma,1900-2200 Ma and 2300-2700 Ma.Another crustal growth during ca.1400-1000 Ma was also detected from the source area of both the lower and upper Shuangqiaoshan groups.Meanwhile,a comprehensive analysis based on available zircon U-Pb ages and paleomagnetic data reveal that there was a close affinity between Jiangnan Orogen,Tarim,Greater and Lesser Himalaya and East India-East Antarctica blocks during the assembly of Rodinia.4.Base on the data presented in our work,in conjunction with the geochronological and isotopic geological evidence on the five-phase tectonomagmatic evolution,we propose a tentative model for the tectonic evolution of Neoproterozoic Jiangnan Orogen.As follows:(1)Ca.1000-880 Ma intra-oceanic northwestward subduction and island arc magmatism resulting in the formation of Huaiyu arc in the eastern Jiangnan Orogen.The overriding plate retreat caused upper plate rifting and the back-arc extension during the initial stage.The metasedimentary rocks and volcanic rocks from Shuangxiwu Group were formed in the ocean island arc and the NJO were formed in the intra-oceanic back-arc basin.(2)Combining with the 880 Ma leucogranitic lenses,866 Ma blueschist facie metamorphic rocks within the NJO and the 863 Ma basal conglomerate from Henshangzhen Group,we infer that the Huaiyu arc amalgamated and collided with the Yangtze Block along the NJO during 880-860 Ma.(3)The continuous northwestward subduction and slab rollback of the paleo-South China Ocean might have triggered the formation of an extensive back-arc,fore-arc extension above the continental crust,and continental arc magmatism along the whole Jiangnan Orogen during ca.860-830 Ma.The Neoproterozoic Jiuling continental arc and Xiushui back-arc basin are formed during this stage.(4)Ca.830-800 Ma,with the gradually closure of paleo-South China Ocean,the Yangtze Block collided with the Cathaysia Block.The collision lead to the emplacement of syn-collision and post-collision granitic magmas.A series of molasse formations were deposited in the intermontane basin,accompanied by the formation of regional angular unconformity in the Jiuling terrane.(5)During ca.800-750 Ma,influenced by the breakup of Rodinia supercontinent,widespread bimodal magmatism and rifting occurred throughout SCB,including the emplacement of ca.792 Ma Anyi meta-diabases in the eastern Jiuling terrane and the deposition of upper Shuangqiaoshan Group in Xiushui area.