Early Paleozoic Accretionary Orogenesis And Crustal Evolution Of The Eastern North Qilian Orogenic Belt

Reconstructing the tectonic evolution of accretionary orogenic belts is a frontier topic in tectonics and crustal evolution.Accretionary orogenic belts are major sites for crustal growth and metallogenesis.Therefore,deciphering the tectonic evolution of accretionary orogenic belts is of great importance for understanding the geodynamic processes of subduction-accretion-collision at convergent plate boundaries and the mechanisms of crustal growth.The North Qilian orogenic belt(NQOB),located between the Alxa block and the Central Qilian-Qaidam block in northeastern Tibet,is a typical Phanerozoic accretionary-to-collisional orogenic belt that represents the termination of the northern branch of the Proto-Tethys Ocean.It contains representative geological records of ancient divergent and convergent plate boundaries including those related to the breakup of the Rodinia supercontinent,spreading of ocean basins,subduction-accretion of oceanic crusts,closure of ocean basin,and collision of micro-continents/arcs,which thereby provides a natural laboratory for studying accretionary orogenesis.The NQOB contains two subparallel ophiolitic belts,arcs,and subduction complexes;the ophiolitic rocks in the northern belt have generally been considered to have formed in a back-arc setting.However,the subduction initiation,subduction polarity,accretionary process,and timing of the closure of the back-arc ocean basin remain equivocal,which limits the understanding of the overall tectonic evolution of the Proto-Tethyan NQOB.To address these problems,this dissertation focuses on the Laohushan Complex in the eastern NQOB and conducted detailed structural,magmatic,sedimentologic,and metamorphic investigations through integrated field mapping,structural investigations,mineralogy,geochronology,geochemistry and isotopes,and phase equilibria modelling.The dissertation determined the compositions and structural characteristics of the accretionary complex,the age and origin of ophiolitic rocks,the P-T evolution of high-pressure metamorphic rocks,and the petrogenesis of collision-related magmatic rocks.Accordingly,a comprehensive tectonic model is proposed to account for the evolution of the North Qilian back-arc ocean basin.This study thus provides key evidence and new understanding for the accretionary orogenesis and crustal growth of the NQOB.The following conclusions were achieved.(1)The Laohushan Complex is herein divided into several litho-tectonic units,including an ophiolite(peridotite-gabbro-basalt-plagiogranite)-forearc basin unit from the northern upper plate and an accretionary complex unit(pillow/massive basalt-chert(OPS),melange,high-pressure metamorphic rock and coherent trench turbidite)off-scraped from the lower plate.They together constitute a complete forearc ophiolite-accretionary complex sequence.The OPS is composed of a sequence of pillow/massive basalt,red and light green siliceous-argillaceous rocks and minor limestone.Construction of structural profiles show that they are in thrust contact and repeated several tens of times.The melange is characterized by typical block-in-matrix fabrics,and duplex,thrust,imbricate and asymmetric structures.Blocks consist of several centimeters-tens of meters of limestone,chert,banded iron formation,graywacke,basalt and blueschist,which are embedded in the strongly deformed mica quartz schist and meta-mafic schist matrices.The trench-fill turbidite is composed of coherent gravel sandstone,graywacke,siltstone and mudstone and is in thrust contact with the OPS sequences.The forearc basin sequence in the north is composed of coherent sandstone-siltstone-mudstone and intruded by a collision-related dioritic pluton.Kinematic markers indicate that all the litho-tectonic units underwent southward thrusting and shearing;coupled with the spatial architecture of the different tectonic units,a northward subduction polarity of the North Qilian back-arc ocean basin is proposed.New detrital zircon U-Pb ages of sedimentary rocks from different litho-tectonic units constrain the maximum depositional ages of the forearc clastic sandstones(-453 Ma),mudstones within OPS(?447 Ma),trench-fill turbidites(?453 Ma),and the schist matrices of melanges(?470 Ma).Together with the zircon ages of igneous ophiolitic rocks and collision-related intrusions,these new geochronological data indicate that the terminal accretion and tectonic stacking of the Laohushan subduction complex occurred between ca.447 and 430 Ma.(2)Detailed geochronology,geochemistry and isotope studies were conducted on the ophiolitic rocks within the Laohushan Complex.New zircon U-Pb ages reveal two-periods of formation of oceanic crust(ca.474 Ma and ca.450 Ma),which are represented by the ages of gabbro(474±5 Ma and 449±5 Ma)and plagiogranite(473±3 Ma and 454-449 Ma).Geochemistry and spinel compositions of peridotites show supra-subduction zone(SSZ)forearc peridotite characteristics with U-shaped REE patterns and high Cr#(53-60)values.Basaltic rocks outcropping both in the subduction complex and ophiolite sheet can be classified into three types.Type 1 basalts from the northern accretionary complex and Type 3 basalts from the ophiolite sheet are normal mid-ocean ridge basalts(N-MORBs),which show LREE-depleted to flat REE patterns without significant depletion in HFSEs and enrichment in LILEs,and their Th-Nb proxies lie in the MORB-ocean island basalt(OIB)array,suggesting that they formed in a subduction-unrelated or subduction zone distal setting.In contrast,type 2 basalts in the southern accretionary complex show MORB-IAT affinities with enriched Th,U,and Pb and negative Nb and Ta anomalies and possess high Th/Yb ratios,suggesting that they formed in a subduction-related setting.All gabbros are characterized by depleted LREE and HFSEs(e.g.,Nb and Ti),enriched LILEs,and positive zircon ?Hf(t)values(+12.6-?+15.3),indicative of subduction-modified mantle source.The younger plagiogranites,which intruded into the ophiolite sheet have high Mg#(53.7-55.0),Sr/Y ratios(39.3-54.4),enriched LREE,and positive ?Hf(t)values(+12.3?+14.8).They were probably produced by water-fluxed partial melting of mafic oceanic crusts.The plagiogranites from the accretionary complex have higher SiO2,lower Mg#(36.4-37.4),consistent with partial melting of back-arc mafic rocks.All plagiogranites possess strong LILE enrichment,and pronounced Nb,Ta and negative Ti anomalies,suggesting a subduction-related origin.Based on the structural,geochronological,geochemical and isotopic data of two episodes of ultramafic-felsic rocks,a geodynamic model involving back-arc spreading and subsequent intra-oceanic subduction is proposed to account for the formation and emplacement of ophiolitic rocks and extinction of the North Qilian back-arc ocean basin.The older gabbro block(ca.474 Ma)and plagiogranite(ca.473 Ma)in the accretionary complex record the early spreading event of the back-arc ocean basin.The younger SSZ-type gabbro(ca.449 Ma)and adakitic plagiogranite(454-449 Ma)from the ophiolite sheet constrains the initial intra-oceanic subduction of the back-arc basin.(3)We document the petrology,mineral compositions,geochronology,geochemistry and phase equilibria modelling to discuss the pressure-temperature(P-T)evolution of the newly-recognized blueschists from the Laohushan accretionary complex in the eastern NQOB,which provide new insights into the spreading and collapse processes of the North Qilian back-arc basin.The Laohushan blueschist blocks were wrapped by mafic schists,epidote schist,and metasedimentary rocks,which constitute a greenschist to blueschist facies melange.Phase equilibria modelling suggests that the blueschist records P-T conditions of?7.5-14.5 kbar/410-465?,corresponding to a thermal gradient of?9.7-16.5?/km;the sodic-amphibole bearing mafic schist matrix has a peak P-T condition of?7.2-9.1 kbar/?462-500?,corresponding to thermal gradients of?15.9-19.5?/km.Zircon and titanite U-Pb dating indicate that the formation age of the protolith of blueschist and mafic schists is ca 493-474 Ma.A sodic amphibole 40Ar/39Ar plateau age of 425±13 Ma probably indicates the exhumation of the blueschist.Our new data,combined with recent detrital zircon ages from the melange matrices,indicate that the blueschist facies metamorphism occurred at ca 450-425 Ma.Geochemical data of blueschists and mafic schist matrix are characterized by significant depletion of Nb,Ta,and Ti,suggesting that the protoliths of blueschist and mafic schist were formed in the North Qilian arc-back-arc setting.Based on regional metamorphic,geochemical and structural data,the Laohushan blueschists in the eastern NQOB probably formed during subduction of the North Qilian back-arc oceanic crust with geothermal gradients of 9.7-16.5?/km,which are significantly higher than those(6-9?/km)in the southern HP metamorphic belt.The higher geothermal gradient of blueschists in the northern belt could be related to the subduction of younger,warmer oceanic crust,and a shorter duration of subduction in the North Qilian back-arc ocean basin.(4)An integrated study of field observations,geochemistry,whole-rock Rb-Sr and Sm-Nd isotopes,and zircon U-Pb ages and Lu-Hf isotopes for the Laohushan mafic to felsic magmatic rocks that were related to initial collision between the Alxa block and the Central Qilian block,was carried out.The Laohushan collision-related magmatic rocks are dominated by quartz diorites(ca.426 Ma),with minor tonalites enclosing dioritic enclaves(ca.430 Ma)and hornblendite xenoliths(ca.448 Ma),and some coeval dolerite dikes(ca.427 Ma)intruded into the accretionary complex.The quartz diorites are characterized by light rare earth element(LREE)-and large ion lithophile element(LILE)-enrichment but have high field strength element(HFSE)-depleted trace element patterns and negative initial ?Hd(-1.6 to-2.9)and positive zircon initial ?Hf(+3.0 to+6.2)values.The dioritic enclaves are also characterized by LREE-enriched and HFSE-depleted patterns and have mostly negative initial ?Nd(-9.2 to+0.03)but positive zircon initial ?Hf(+3.0 to+5.9)values.These geochemical and isotopic features,together with isotopic mixing calculations,suggest that the quartz diorites were likely derived from partial melting of the lower crust dominated by accreted mafic oceanic rocks with minor sediments,whereas the dioritic enclaves originated from underplated mantle-derived magmas mixed with crust-derived melts.The hornblendite xenoliths have high MgO,Cr,and Ni contents,positive Th,U,and Pb anomalies,and negative Nb,Ta,and Ti anomalies.They have negative initial ?Nd(-2.8),near chondritic zircon initial ?Hf(-0.4 to+1.4)values and an Archean Nd model age(TDM=2.74 Ga),suggesting that the hornblendites were likely produced by partial melting of subcontinental lithospheric mantle peridotite that was metasomatized by subduction-related melts beneath the Archean-Proterozoic Alxa block.Partial melting of the lower crust of the early Paleozoic North Qilian orogenic belt is proposed to have occurred in response to slab breakoff and asthenosphere upwelling during the initial stage of collisional orogenesis.This study demonstrates that heterogeneous magma sources,involving accretionary materials(i.e.,accreted oceanic crust and sediments)and various mantle-derived components,were mixed to form the collision-related magmatic rocks.It also highlights the significance of collision-related magmatism in continental growth and stabilization of newly-assembled crusts in accretionary-to-collisional orogens.(5)Combined with comprehensive studies of tectonic,magmatic and metamorphism of the eastern NQOB,a detailed tectonic evolution model is proposed.The northward initial subduction of the North Qilian Ocean led to the formation of the North Qilian arc-forearc complex(?517-487 Ma)and the Laohushan back-arc ocean basin(?490-454 Ma).The southern ophiolite-accretionary complex belt with low T/P high-pressure metamorphic rocks was formed in the forearc of the North Qilian arc,which indicates that the North Qilian Ocean probably closed at?440 Ma.During the late Ordovician,intra-oceanic subduction was initiated in the North Qilian back-arc basin,resulting in the formation of the Laohushan forearc ophiolite(?454-449 Ma)and forearc basin.The OPS accreted in the forearc area forming the Laohushan accretionary complex.The North Qilian back-arc ocean basin was probably closed during the middle Silurian,then collided with the Alxa block,as evidenced by numerous collision-related magmatic rocks and foreland sedimentary sequences.The anatomy of the Laohushan forearc ophiolite-accretionary complex provides sparks new ideas and provides key constraints for the tectonic evolution of the NQOB.Meanwhile,the identification of the overlying forearc oceanic crust and underlying accretionary complexes in the Laohushan Complex demonstrates the complexity of intra-oceanic subduction-forearc accretion processes and ophiolite emplacement mechanisms during accretionary-to-collisional orogenesis,and serves as a general model for accretionary orogens worldwide.The relatively complete overlying forearc oceanic crust sequence(forearc ophiolite)was formed by obduction,while the structurally imbricated ocean plate stratigraphy and melanges were formed by off-scraping and underplating.