The Pre-Devonian Tectonic Evolution Of Southeast China

The unified South China Block comprises two Precambrian units,namely the Yangtze Block to the northwest and the Cathaysia Block to the southeast,with the Jiangshan-Shaoxing fault zone defining the boundary of these two units.The Jiangnan orogenic belt delineate subduction-collisional zone between the Yangtze and the Cathaysia blocks.The eastern part of this belt comprises the oceanic Huaiyu terrane,the northeast Jiangxi suture zone and the continental Jiuling terrane.The Southeast China,including the Jiangnan belt and Cathaysia Block,experienced long term tectonic evolution and polyphase tectonic reworking.A long research history in Southeast China has gained numerous achievements.However,some geological issues remain unsettled,particularly the Pre-Devonian tectonics.This study focus on Zhuji,Jiangshan-Chun'an,and Yiyang area in the eastern Jiangnan belt and Yuanbaoshan-Longsheng area in the western Jiangnan belt,as well as Chongyi-Lichuan,Jiangning-Jianou and Chencai area in the Cathaysia Block.Kinematic and structural studies of pre-Nanhua system from different segments in the Jiangnan belt indicate ductile southeastward thrusting,confirming northwestward subduction of oceanic crust beneath the Jiangnan belt.On the other hand,regional angular unconformity between Nanhua system and Pre-Nanhua system marks the first collisional suture between Yangtze and Cathaysia blocks.This study reports synthetic litho-stratigraphic relations,zircon U-Pb dating and Hf isotopes,along with whole rock geochemistry of rock suites from the studies areas,which determined the ages and settings of stratigraphic sequence,igneous rock suite and metamorphic rocks.Therefore,this study provides further constraints and evidence on studying the Pre-Devonian tectonic evolution of Southeast China.The Longsheng region in the western Jiangnan belt consists of a disrupted mafic-ultramafic assemblage of pillow basalt,gabbro,diabase and peridotite along with siliceous marble,ophicalcite and jasper mixed with basalt.Primary rock relations are largely overprinted by pervasive shearing resulting in disruption of the assemblage into series of discontinuous blocks within a phyllite matrix.West-dipping thrust faults mark the eastern contact of blocks and the overall succession has the appearance of a tectonic melange.U-Pb zircon age data from the gabbros and diabases yield crystallization ages of 867 ± 10 Ma,863 ± 8 Ma and 869 ± 9 Ma,with positive ?Hf(t)values.The gabbro,basalt,talc and serpentinite samples display LREE enriched patterns with obvious depletion of Nb,Ta and Ti,indicating a subduction related setting.These blocks are considered as autochthonous blocks.Similar characters are also displayed by the tuffaceous phyllite.MORB type mafic-ultramafic rocks have also been previously reported from this area,which are likely exotic blocks within ophiolitc melange,along with jasper.Overall lithostratigraphic relationships,age data and geochemical signatures suggest a fore-arc setting resulted in formation of an ophiolitic assemblage associated with trench sediments(phyllite,chert and marble)that were imbricated and disrupted within an accretionary prism environment.Mafic-ultramafic rocks exposed in Yuanbaoshan and Longsheng area of Northern Guangxi,Yiyang and Liuyang area of Northern Hunan and Zhuji area of Northern Zhejiang occur as sills or intrusions within the Pre-Nanhua basement of the Jiangnan belt.Whereas in Yiyang area of Northeast Jiangxi,andesite and basalt occur as olistostromes within the Nanhua system.Based on lithology beneath and above the Neoproterozoic unconformity,these olistostromal basalt and andesite were derived from the Pre-Nanhua System(Qigong Group).At Shijiao area within the Jiangshan-Shaoxing fault zone,hornblende schist is intruded by migmatized quartz diorite,which constitutes a shear zone delineating the welded boundary between the Neoproterozoic Shuangxiwu Group of Jiangnan belt to the north and the Chencai Complex of Cathaysia Block.The Shuangxiwu Group is composed mainly of basalt,andesite and flysch,whereas the Chencai Complex contains magmatic and sedimentary rocks that experienced amphibolite facies metamorphism.Zircons from Yuanbaoshan gabbro and Zhuji diabase yield weighted mean ages of 854.7±5.3Ma and 863±6 Ma,whereas those from Yiyang andesite yield weighted mean ages of 864.0±14Ma,868 ± 7 Ma and 871 ± 7 Ma.On the other hand,the quartz diorite,gabbro and hornblende schist from Shijiao are dated at 854 ? 6 Ma,857 ± 5 Ma and 860 ± 5 Ma.These zircon ages are determined as the crystallization ages of magmatic suite,which are further confirmed by positive zircon ?Hf(t)values.These studied magmatic rock suites all display LREE enriched patterns,with minor negative Eu anomaly,and negative Nb,Ta,Cs and Ti anomalies,typical of arc magmatism.All samples are plotted in the field of volcanic arc,suggesting an arc setting.Relationships within the Jiangshan-Shaoxing fault zone at Shijiao suggest the ca.860-850 Ma rock suites were generated in a convergent plate margin and are part of the Jiangnan belt,and not the Cathaysia Block,thus constraining the location of suture between the two lithotectonic units in NE Zhejiang area during Neoproterozoic.Overall field geology,tectonic relationships,zircon ages as well as geochemistry data suggest the investigated magmatic rocks suites were generated in an 880-850Ma convergent plate margin of the Jiangnan belt.The pre-Nanhua basement(Qigong Group)in Yiyang of eastern Jiangnan belt mainly comprises of volcanic sedimentary rocks and has arc affinity,which is covered by continental derived Nanhua system(Dengshan Group)in an angular unconformity.This unconformity relates to the collisional events between Yangtze and Cathaysia blocks.The detrital zircons from the phyllite yield only one age population of 900-820 Ma(peak at 840 Ma).However,the conglomerate above regional unconformity yield a major age population of 863-810 Ma(peak of 833 Ma),with two minor ones at 1120-940 Ma(peak at 960 Ma)and 1780-1560 Ma(1670 Ma).The greywacke from middle Dengshan group also preserves zircons from multiple sources with age populations of 890-750 Ma,2070-1670 Ma and 2550-2385 Ma.Moreover,Hf isotopic data further reveal that most detrital zircons from the basement sequences yield positive eHf(t)values and late Mesoproterozoic model ages,whereas those of the cover sequence mostly show negative eHf(t)values.The Hf isotopic data therefore suggest that the basement sequences are soured from a Neoproterozoic arc produced by reworking of subducted late Mesoproterozoic materials.The orogeny related unconformity is constrained at ca.810-800 Ma.Our results show that the amalgamation of the Yangtze and Cathaysia blocks occurred at ca.840-810 Ma.The Yuanbaoshan granitic pluton in the western Jiangnan belt intrude the pre-Nanhua basement(Sibao Group)and mafic-ultramafic of the region,whereas the Daolinshan granitic pluton in the eastern Jiangnan belt intrude both the pre-Nanhua system(Shuangxiwu Group)and Nanhua system(Heshangzheng Group)of Daolinshan area.Zircons from the Yuanbaoshan granite show ages of 822.6±5.2,830.7±5.0,823.8±5.3 and 833.0±15.9 Ma,whereas those from the Daolingshan pluton are dated at 790±5Ma.The granitoids in northern Guangxi are typical S-type granites with high ACNK values(1.15-1.40),and are classified as post-collisional granites in tectonic discrimination diagrams,with the granitic magma generated by crustal melting(?Hf(t)=-18.99--0.84).The granitoids from Daolingshan granite are typical anorogenic K-feldspar granites,with ASK=0.91-0.94 and significant introduction of mantle composition(?Hf(t)=7.08-15.08).The Daolingshan granite displays within plate geochemical features and can be correlated with coeval within plate mafic rocks.We correlate the granitoids in Yuanbaoshan area of western Jiangnan belt with the post-collisional setting at ca.835-820 Ma,and suggest the Daolinshan granite was generated at an extensional setting at ca.790 Ma,shortly after collision.The Cathaysia Block of the Southeast China Craton includes a Proterozoic basement that experienced a prolonged Precambrian crustal evolution but to date lacks evidence of Proterozoic metamorphic ages.At Lichuan and Jianning,in the eastern Cathaysia Block,Proterozoic rock units include migmatized paragneiss of the Wanyuan Group and minor amphibolite of Tianjingping Formation,which are enveloped by schist of Mayuan Group.Detrital zircon grains from the Wanyuan paragneiss display metamorphic rims that yield concordant weighted average 206Pb/238U ages of 860 ± 6 Ma and 435 ± 5 Ma,along with variably disconcordant ages with lower intercept ages of 442 ± 41 Ma.The zircon core ages range from 3015 Ma to 851 Ma,with three major age populations at 930-865 Ma,1850-1200 Ma and 2650-2400 Ma.Detrital zircon grains from Mayuan schist samples at Jianning generally lack core-rim structures and yield three main age populations at 860-750 Ma,1835-1775 Ma and 2720-2500 Ma.Metamorphic ages of ca.860 Ma and ca.440 Ma for the paragneiss along with the youngest detrital zircon constrain the depositional age of the protolith to ca.865-860 Ma,whereas the Mayuan Group is younger and probably accumulated between 600-540 Ma.Characteristics of detrital zircon age populations along with regional geological data suggest accumulation of the Wanyuan Group in a convergent and/or collisional setting.Metamorphism and subduction-collision process within the Cathaysia Block at around 860 Ma suggest it is not a unified block in early Neoproterozoic.The Mayuan Group may be a local correlative of the Sinian Group succession.The sparse evidence for early Neoproterozoic metamorphism likely reflects the widespread overprint by the Paleozoic tectonothermal event at around 440 Ma.The protoliths of the metamorphic suite of the Chencai Group in the northeastern margin of the Cathaysia Block are composed of Neoproterozoic argillaceous-arenaceous rocks of turbidite facies,marine carbonates,gabbro and basalt.Zircons from the migmatite and gabbro yielded weighted mean 206Pb/238U ages of 438 ± 3 Ma and 453.2 ± 3.5 Ma,respectively.Hf isotopic analysis reveals ?Hf(t)values of +0.02 to-2.55 and +1.80 to +5.25 for zircons from the migmatite and gabbros,respectively.The gabbroic enclaves in the migmatites are characterized by subduction related geochemical characteristics,which differentiate them from the migmatites.Based on regional geotectonics and sedimentary environments,the zircon ages of gabbro enclaves are suggested to represent age of resetting of protolith ages during regional migmatization.The gabbro enclaves do not support an early Paleozoic arc setting in Cathaysia.The migmatite and gabbro record a Paleozoic tectono-thermal event,which are consistent with metamorphic ages of ca.440.Ma traced by metamorphic zircon rims fromw estern Wuyi block of Cathaysia.We present field geology,detrital zircon U-Pb age data and Hf isotopes from Chongyi,Jianou area in Cathaysia and Jiangshan,Chun'an area in the eastern Jiangnan belt,where abundant Precambrian units are exposed.Detrital zircon from Devonian strata in Jiangshan define four age populations:470-410Ma,860-740Ma,1200-900Ma,2000-1550Ma,2700-2300M;while Ordovician strata from Chun'an yield four age populations:850-740Ma,1000-900Ma,2000-1550Ma,2700-2550Ma.Those from Ordovician strata in Chongyi area determine four age population at 2560-2380 Ma,1930-1520 Ma,1300-900,850-730 Ma and 670-530 Ma.However,detrital zircons from Triassic strata in Jian'ou area yield only three age populations at 2630-2300 Ma,2030-1770 Ma and 280-200 Ma.We also report zircon U-Pb concordia age of 3.96 Ga,which is the oldest age so far obtained from Cathaysia.The age peaks correlate with similar ages reported for Neoarchean global continental growth,Late Paleoproterozoic to Mesoproterozoic amalgamation and disruption of the Columbia supercontinent,early Neoproterozoic assembly of Rodinia,breakup period of Rodinia and late Neoproterozoic assembly of Gondwana.Age data suggest different segment of the Cathaysia Block response to major tectono-thermal events in different ways,confirming that the Cathaysia Block can be divided into sub-blocks.Numerous euhedral Archean zircons,as well as the presence of 3.96Ga zircons indicate the Cathaysia Block likely has an Archean basement.It is noteworthy that the detrital zircons from Cathaysia traced the imprints of a major Late Neoproterozoic-Cambrian orogeny related to assembly of Gondwana.However,direct geological evidence for this event has not yet been found within the studied area.This clear age peak suggests Southeast China Block has close affinity with Gondwana,which contribute to the study of Pan-African orogeny in Southeast China.Our results suggest distinct crustal evolution histories for the Yangtze and Cathaysia Blocks albeit with close affinities.Obtained age spetra confirm early Paleozoic western expansion of the Cathaysia Block,followed by intracontinental tectono-thermal and metamorphic event.During the interval of Sinian to Early Paleozoic,the Southeast China was in a stable depositional environment.