Paleozoic Accretionary Orogeny And Crustal Evolution Of The Western Segment Of Chinese Central Tianshan

Sandwiched between the Junggar and Tarim basins,the Tianshan belt carries vital information on the evolution of the southwestern CAOB and northern Tarim.It is now believed that the Tianshan belt was formed via subduction and accretion happened in the Paleo-Tianshan Ocean?South Tianshan Ocean?and related oceanic realms,composed of micro blocks with different origins and overprinted by multi-stage structural events.Previous studies not only built up the preliminary sketch of the tectonic evolution of the Tianshan belt,but also left numerous puzzles and controversies.Thus,comprehensive investigations are needed to reveal the tectonic history of the Tianshan belt.The Eo-Tianshan was formed by accretion and collision during the Paleozoic.However,the Paleozoic tectonic history of the Tianshan belt,especially the transitional process between early and late Paleozoic tectonics,remains dubious.In this study,we have chosen the Mighigou ophiolitic melange at the northern margin of the Central Tianshan block as a probe into the evolution process of the Paleo Tianshan Ocean.On the basis of a thorough discussion on the paleo suture of the Paleo Tianshan Ocean,we have chosen three critical target regions for detailed field geological,structural and instrumental studies i.e.,the southern Yili block to the north of the suture,along with the southern Baluntai block and southwestern Chinese Tianshan to the south of the suture.Combining the newly obtained and previously published data,we have discussed and proposed a modified model about the early and late Paleozoic evolution of the Chinese Tianshan.Several ophiolitic melanges outcropped along the northeastern margin of the Central Tianshan Baluntai block,and thought to represent the suture of the Paleo Tianshan Ocean to the north of the Central Tianshan terrane.The investigated Mishigou melange could be subdivided into the northern and southern segments according to their lithological features.The southern segment comprises deformed Silurian sandstone and siltstone that deposited in fore-arc environment.The Silurian strata is in unconformity with the overlying Carboniferous Maanqiao formation.Detrital zircon dating of the Silurian strata have revealed age peaks at 2490 Ma,1751 Ma,1474-1352 Ma,1011-916 Ma and 443 Ma,and the main provenance is derived the nearby Central Tianshan continental arc.The basal conglomerate above the unconformity share the same main provenance with the Silurian strata,except for the youngest age peak at 337 Ma.The northern segment is mainly featured by melanges that made up by cherts,jasperite,andesite,dacite,rhyolite and serpentinized ultramafic blocks and tuffaceous and siliceous matrix.The melange were reworked into several structural blocks by WNW-ESE strike-slip faults,and intruded by a number of syn-tectonic and post orogenic granite dykes.Two main structural stages have been recognized in the field:the first stage?D1?is characterized by asymmetric tight folds?hinges dipping to SSW?in the southern segment and the south dipping foliations and cleavages in the northern segment.Both the asymmetry of the folds and the macroscopic shearing indicators suggest a top-to-the-north sense of shearing.The Zircon U-Pb dating results of the crosscutting granite dykes and overlying Carboniferous conglomerate suggest that D1 took place before 337 Ma;the second stage deformation?D2?are strictly related to the Main Tianshan Shear Zone and associated minor shearing faults.On the near vertical WNW-ESE striking foliations,dextral shearing is demonstrated by microscopic shearing indicators.Accroding to the U-Pb dating results of the syn-tectonic granite dykes and previously published thermochronological results,D2 reached its peak at early-mid Permian?290-260 Ma?.According to their zircon U-Pb geochronology,zircon Lu-Hf isotopic and whole-rock geochemical data,the igneous blocks within the melange could be subdivided into three groups:the first group contains volcanic blocks with MORB-like geochemical data and mixed with early Paleozoic siliceous matrix,probably represent the remnant of the oceanic block;the second group comprises typical calc-alkaline blocks that erupted during 447.7-428.2 Ma.Their evolved Hf isotopic values and Paleoproterozoic two staged model ages imply that they were formed under continental arc settings;The third group of blocks were formed during early-mid Carboniferous?345.2 Ma?.Considering lines of geochemical and geological evidences,they were likely formed at supra-subduction extensional environment.This magmatic event is most likely related to contemporaneous transition from advancing to retreating accretionary orogeny in the region.As per the dating results of the basal conglomerate and post-orogenic granite dyke,the emplacement of the melange took at 345-337 Ma,and formed the top-to-the-north shearing structure.The section was thereafter reworked by dextral shearing which reached its peak at early-mid Permian?290-260 Ma?.The Chinese Central Tianshan constitutes the centric and vital component of the Tianshan orogen.To better constrain the Paleozoic evolution of the Chinese Central Tianshan,we present new field observations,fabric analyses,zircon U-Pb ages,zircon Lu-Hf isotopic and whole rock geochemical data of mafic,intermediate and felsic intrusive rocks in the Baluntai area,the heartland of the Chinese Central Tianshan.Field-scale and microscopic structural patterns of the intrusive rocks reveal that the early Paleozoic gabbro-diorite intrusions underwent multistage structural reworking,and generally exhibit gently south dipping foliations?S1?and microscopic kinematic fabrics?L1?indicating top-to-the-north shearing.These ductilely deformed intrusive rocks were intruded by granitic sills or dikes that are generally sub-parallel or with low angle to the main foliations of the gabbros and diorites.Low angle foliations and top-to-the-north kinematics can be rarely noticed within the granitic intrusions.At the sites closer to the Baluntai fault,locally intense mylonitization is observed in felsic intrusions as well as in the deformed mafic and intermediate intrusions,where earlier flat foliations?S1?were completely or partially transformed to steep ones?S2?bearing sub-horinzontal stretching lineations?L2?,and asymmetric kinematics indicating dextral ductile shearing.Zircon LA-ICP-MS U-Pb datings constrain the intrusion ages of the gabbroic and dioritic intrusive rocks at 452-420 Ma.One sample of mylonitic K-granite yield a age of intrusion at 417 Ma.Two granitic sill samples contain plenty of zircons inherited from the host gabbroic and dioritic rocks,and dating on the dark rim of a few zircons roughly constrains the crystallization ages of the felsic sills at ca.360 Ma.The ?Hf?t?values of a gabbro?420 Ma?,two diorite samples?425 and 445 Ma?and a mylonitic granite?417 Ma?vary from-15.45 to-2.06,corresponding to Hf two stage crustal model ages T_DM² ranging from?1.5 Ga to?2.4 Ga,suggesting variable degrees of involvement of the Proterozoic crustal materials and the juvenile mantle components during the early Paleozoic magmatic generation.The geochemical data of the plutons exhibit typical continental arc features.Considering the previously published data,the early Paleozoic arc-type magmatic rocks within the Chinese Central Tianshan likely generated in an active continental margin with Precambrian basement.The Chinese Central Tianshan was thereafter affected by an intense magmatism during 370-310 Ma.Their high Sr/Y and?La/Yb?cN ratios,enrichment in LILE and depletion of Nb-Ta-Ti features and prompt involvement of mantle materials?indicated by the Hf isotopic results?.Considering the chronological constraints on the closure time of the northern and southern boundaries of the Baluntai block,and the magmatic data of the granulite in the Yushugou region,we tend to believe that the Central Tianshan was affected by a supra-subduction extensional event.The Chinese Central Tianshan was affected by the early Permian dextral strike-slip faulting due to regional large-scale transcurrent tectonics and overprinted by Permian mafic magmatism.We conducted field investigations,whole-rock geochemical,Sr-Nd and zircon U-Pb-Lu-Hf isotopic analyses on a suite of intrusive complex in the southern Nalati Range,SW Chinese Tianshan in order to better understand the Paleozoic tectonic and magmatic evolution of the belt.The intrusive complex comprises weakly foliated diorite,low-grade altered diabase,and deformed monzogranite;these plutonic rocks were in turn crosscut by undeformed coarse-grained diorite,granodiorite as well as granite stock.Foliated Late Silurian diorites?421 ± 4 Ma?show arc-type geochemical features,slightly negative whole-rock ?Nd?t?value?-1.7;TDM¹=1.52 Ga?and variably positive zircon ?Hf?t?values(2.34 to 7.27;T_DM²:0.95?1.26 Ga).Deformed Early Devonian porphyritic monzogranites?411 ± 4 Ma?show geochemical features similar to A-type granite,and their zircon ?Hf?t?values range from-6.63 to 1.02,with T_DM² ages of 1.82 to 1.33 Ga.Metamorphosed Early Devonian diabases?ca.410 Ma?have OIB-like REE patterns,?Nd?t?values of-2.0?-0.8 and TDM¹ ages of 1.37?1.25 Ga.The undeformed Early Carboniferous diorite and granodiorite?353?344 Ma?exhibit arc?type geochemical features,positive SHf?t?values of 6.11?7.91 with T_DM² ages of 0.97?0.86 Ga,and positive ?Nd?t?value of 1.9 with TDM¹ age of 1.04 Ga.The Early Permian granite stock?292±5 Ma?has highly differentiated REE pattern,slightly negative ?Nd?t?value?-4.4?and variable zircon ?Hf?t?values of-9.73-6.36.Combining with available data,Early Paleozoic?500?410 Ma?arc-related magmatic rocks occur on both sides of the suture zone along the southern Nalati Range,likely resulted from a bi-directional subduction of the Paleo-Tianshan Ocean beneath the Yili Block to the north and the Central Tianshan to the south.Occurrences of A-type granites and OIB-like diabases?ca.410 Ma?along the Nalati Range likely indicate a hot extensional regime probably induced by the break off of the northward subducting slab of the Paleo-Tianshan Ocean.The closure of the Paleo-Tianshan Ocean and subsequent amalgamation during Early Carboniferous resulted in the regional deformation and metamorphism of the Early Paleozoic arc-related magmatic rocks.From Early to Late Carboniferous,a magmatic arc that corresponded to the well-developed Late Paleozoic Balkhash-Yili active continental margin,superimposed upon the southern Yili Block,most likely resulted from the southward subduction of the Junggar-North Tianshan Ocean.After the closure of the North Tianshan Ocean in Late Carboniferous,the study area was dominated by post-orogenic magmatism.The South Tianshan comprises the Hark Shan,Erbin Shan,Huola Shan and Keziletage regions to the south of the Nalati-Sangshuyuanzi fault zone.Despite numerous studies,the tectonic affinity and evolution of the South Tianshan remain controversial.Therefore,we conducted field geological investigations and structural analyses of the Early Paleozoic volcanic and sedimentary sequences from the Kuikewusu and Kaidu sections in the Bayinbuluk basin,which are overlain by Devonian to Carboniferous carbonates.We have identified at least three episodes of structural overprints along these two cross-sections.The first stage of deformation?D1?is represented by the wide-spread north/south dipping foliations?S1?and down-dipping lineations?L1?.Assymetric intrafolial folds and sigmoid porphyryblasts on the XZ plane suggest top-to-the-north shearing.The second deformation stage?D2?,which was only obaserved at the northern segment of the Kuikewusu section,is characterized by NEE-SWW striking steep foliations?S2?and sub-horizontal lineations?L2?.The sigma-type porphyryblasts suggest a general dextral shearing,with occasionally sinistral kinematics.Zircon U-Pb dating results of the mylonitic sandstone?301-300 Ma and 257 Ma?provide constraints on the D2 strike-slip shearing.The E-W striking,brittle high angle faults reprenting the third stage deformation?D3?crosscut both the Paleozoic unit and Cenozoic molasses,might be related to the Cenozoic tectonic reactivation.Moreover,we determined whole-rock elemental and Sr-Nd isotopic compositions and zircon U-Pb-Lu-Hf isotopic ratios of representative volcanic rocks that are ductilely deformed and metamorphosed to greenschist facies.These rocks yielded isotopic ages between Middle Ordovician and Early Devonian?460-410 Ma?,and contain zircon xenocrysts whose ages range from Neoarchean??2.75 Ga?,Paleoproterozoic??2.46 Ga?,Mesoproterozoic??1.60 Ga?to Neoproterozoic?997-963 Ma and 827 Ma?.The meta-volcanic rocks show enrichment in LREE,K,Rb,Ba,Th,and depletion in Nb,Ta and Ti.Their evolved whole-rock Sr-Nd isotopic?sNd?t?=-5 to+4?and heterogeneous zircon Lu-Hf isotopic values??Hf?t?=-13.5 to+11.6?indicate a mixing between continental-crust derived magma and juvenile materials probably derived from depleted mantle.These geochemical data resemble those of the Andean-type continental arc volcanic rocks.The coexisting poorly-sorted tuffaceous sedimentary rocks probably indicate a depositional environment of coeval shallow marine intro-arc basin.These new data suggest that the South Tianshan was located in a continental arc setting during Early-Mid Paleozoic,comparable to the synchronous arc magmatism along the northern margin of Tarim.Combined with the regional stratigraphic and tectonic data,we propose that the continental arc-type rocks in the South Tianshan,Central Tianshan and northern Tarim likely formed in a wide active continental margin related to the Early Paleozoic southward subduction of the Paleo-Tianshan Ocean beneath the Tarim plate.The active margin was broken up by rifting of several continental fragments from the Tarim plate by the opening of the South Tianshan back-arc basins during Mid-Late Paleozoic.This process marked an important transition from advancing to retreating accretionary orogenesis in the southwest CAOB in Mid-Paleozoic time.Subsequently,the South Tianshan was subjected to regional deformation during the successive closure of the major oceanic basin and the back-arc basins,and was locally reworked by the ductile strike-slip faulting.Considering the data mentioned above,we have proposed a multi-stage evolutionary model for the Chinese western Tianshan:?1?The western Chinese Tianshan was affected by the divergent double subduction of the Paleo-Tianshan Ocean during Paleozoic.The suture of the Paleo-Tianshan Ocean extended from the south Nalati fault to the Bingdaban-Mishigou-Gangou belt.The northward subducting branch of the oceanic basin induced the active continental margin at the southern Yili?500-410 Ma?.The southward subduction formed the Central Tianshan-northern Tarim continental arc?490-410 Ma?,which covered the Baluntai block,the northeastern Tarim margin and the southwestern Chinese Tianshan;?2?The northerly subducting slab of the Paleo-Tianshan Ocean broke away at early Devonian?410-400 Ma?,triggered the opening of a slab window and upwelling of anthenosphere at the southern Yili.This resulted in the extesional magmatism in the Nalati range,and was followed by a magmatic tranquisent period?400-370 Ma?.During late Devonian to late Carboniferous,the Yili block was affected by the southward subduction of the North Tianshan-Junngar Ocean and accompanied calc-alkaline magmatism;?3?The stress status of the southern subducting branch of the Paleo-Tianshan Ocean evolved from compressional to extensional.Concequently,back-arc basins opened in the Heiyingshan-Kule lake and Keziletage region in the South Tianshan,separated the Central Tianshan Baluntai block and the Kaidu continental arc from the northern Tarim.These back-arc basin?s?closed diachronously during late Devonian to early Carboniferous,formed top-to-the-north kinematics.The drifted Central Tianshan block was under supra-subduction extensional settings,affected by corresponding magmatism;?4?The Mishigou ophiolitic melange at the norhtenr border of the Central Tianshan emplaced at 345-337 Ma,formed the top-to-the-north ductile shearing structures.Considering the top-to-the-north shearing structures and chronological constraints?330-315 Ma?of the southern Nalati metamorphic belt,the Paleo Tianshan Ocean closed and demised at mid-late Carboniferous;?5?After the closure of the Paleo Tianshan Ocean,the Central Tianshan block amalgamated with the Yili block.This assemblage was affected by the southerly subduction of North Tianshan-Junggar Ocean,which resulted in intensive calc-alkaline magmatism.The subduction induced the opening of the Guluogou-Wuwamen back-arc basin?332 Ma?at the southern boder of the Baluntai block,which eventually closed at end Carboniferous?ca.300 Ma?;?6?The western Tianshan was thereafter dominated by post-orogenic tectonics,and reworked by the regional dextral transcurrent shearing.The shearing deformation is clearly controlled by the main shearing zones and their affiliated faults,and reached its intensity peak at early-mid Permian?290-260 Ma?.The pre-Permian units were unconformably overlain by Permian terrestrial clastic deposits,or crosscut by Permian alkaline intrusions.