Paleozoic tectonic evolution of the eastern Central Asian Orogenic Belt in NE China

The eastern Central Asian Orogenic Belt(CAOB)in NE China is a key area for investigating continental growth.However,the complexity of its Paleozoic geological history has meant that the tectonic development of this belt is not fully understood.NE China is composed of the Erguna and Jiamusi blocks in the northern and eastern parts and the Xing’an and Songliao-Xilinhot accretionary terranes in the central and southern parts.The Erguna and Jiamusi blocks have Precambrian basements with Siberia and Gondwana affinities,respectively.In contrast,the Xing’an and Songliao-Xilinhot accretionary terranes were formed via subduction and collision processes.These blocks and terranes were separated by the Xinlin-Xiguitu,Heilongjiang,Nenjiang,and Solonker oceans from north to south,and these oceans closed during the Cambrian(ca.500 Ma),Late Silurian(ca.420 Ma),early Late Carboniferous(ca.320 Ma),and Late Permian to Middle Triassic(260-240 Ma),respectively,forming the Xinlin-Xiguitu,Mudanjiang-Yilan,Hegenshan-Heihe,Solonker-Linxi,and Changchun-Yanji suture zones.Two oceanic tectonic cycles took place in the eastern Paleo-Asian Ocean(PAO),namely,the Early Paleozoic cycle involving the Xinlin-Xiguitu and Heilongjiang oceans and the late Paleozoic cycle involving the Nenjiang-Solonker oceans.The Paleozoic tectonic pattern of the eastern CAOB generally shows structural features that trend east-west.The timing of accretion and collision events of the eastern CAOB during the Paleozoic youngs progressively from north to south.The branch ocean basins of the eastern PAO closed from west to east in a scissor-like manner.A bi-directional subduction regime dominated during the narrowing and closure process of the eastern PAO,which led to“soft collision”of tectonic units on each side,forming huge accretionary orogenic belts in central Asia.

Early Paleozoic accretionary orogens along the Western Gondwana margin

Early Paleozoic accretionary orogens dominated the Western Gondwana margin and were characterized by nearly continuous subduction associated with crustal extension and back-arc basin development.The southwestern margin is represented by Famatinian and Pampean basement realms exposed in South America,both related to the protracted Paleozoic evolution of the Terra Australis Orogen,whereas the northwestern margin is mainly recorded in Cadomian domains of Europe and adjacent regions.However,no clear relationships between these regions were so far established.Based on a compilation and reevaluation of geological,paleomagnetic,petrological,geochronological and isotopic evidence,this contribution focuses on crustal-scale tectonic and geodynamic processes occurring in Western Gondwana accretionary orogens,aiming at disentangling their common Early Paleozoic evolution.Data show that accretionary orogens were dominated by high-temperature/lowpressure metamorphism and relatively high geothermal gradients,resulting from the development of extended/hyperextended margins and bulk transtensional deformation.In this sense,retreating-mode accretionary orogens characterized the Early Paleozoic Gondwana margin,though short-lived pulses of compression/transpression also occurred.The existence of retreating subduction zones favoured mantle-derived magmatism and mixing with relatively young(meta)sedimentary sources in a thin continental crust.Crustal reworking of previous forearc sequences due to trenchward arc migration thus took place through assimilation and anatexis in the arc/back-arc regions.Therefore,retreating-mode accretionary orogens were the locus of Early Paleozoic crustal growth in Western Gondwana,intimately associated with major flare-up events,such as those related to the Cadomian and Famatian arcs.Slab roll back,probably resulting from decreasing convergence rates and plate velocities after Gondwana assembly,was a key factor for orogen-scale geodynamic processes.Coupled with synchronous oblique subduction and crustal-scale dextral deformation,slab roll back might trigger toroidal mantle flow,thus accounting for bulk dextral transtension,back-arc extension/transtension and a large-scale anticlockwise rotation of Gondwana mainland.

Petrogenesis of early Paleozoic granitoids in the North Qinling Orogen,Central China:Implications for crustal evolution in an accretionary orogen

Accretionary orogens are sites of extensive continental crustal growth and modification.The mechanism by which mafic crust is transformed into silicic melts(i.e.,maturation of continental crust)is important for understanding the formation of the continental crust.The North Qinling Orogen(NQO)is a composite orogenic belt and contains an early Paleozoic accretion-dominated orogenic system,which is ideal for investigating continental crustal maturation.We obtained zircon and monazite U–Pb age and O isotope data,zircon Lu–Hf isotope data,and whole-rock major-and trace-element and Sr–Nd isotope data for early Paleozoic granitoids of the NQO.The granitoids are divided into three groups.Group 1 includes the Taiping tonalite(445±3 Ma),the Manziying syenogranite(445±2 Ma),and the Huoshenmiao granodiorite(436±2 Ma).The Taiping and Huoshenmiao plutons have relatively high SiO_(2)contents(68.64–71.67 wt.%)and Na_(2)O/K_(2)O ratios(1.15–4.19),with enrichments in Rb,Ba,Th,and U and depletions in Nb,Ta,P,and Ti,and they are geochemically similar to sodic arc magmas.The Manziying syenogranite is a peraluminous potassic granite with high K_(2)O contents(4.59–5.27 wt.%).Grantioids from Group 1 have similarly depleted Sr–Nd–Hf–O isotopic features(eHf[t]=+5.9 to+8.8;δ^(18)O=3.98‰–5.41‰),indicative of derivation via partial melting of oceanic arc crust,which suggests that partial melting of oceanic arc crust in a subduction system contributes to the generation of continental crust and causes its maturation.Group 2 consists of the Wuduoshan monzogranite(418±2 Ma)and the Sikeshu granodiorite(423±3 Ma).These plutons have relatively high SiO_(2)(65.59–72.06 wt.%),K_(2)O(3.26–4.79 wt.%),and Al2O3(14.65–16.12 wt.%)contents and Sr/Y(33–87)and(La/Yb)N(23–48)ratios.The Wuduoshan monzogranite has positive zircon eHf(t)(+0.4 to+3.1)and uniformδ^(18)O(6.38‰–8.07‰)values,but the Sikeshu granodiorite has more variable isotopic compositions(eHf[t]=-1.9 to+5.0;δ^(18)O=6.37‰–10.60‰).The Wuduoshan monzogranite and Sikeshu granodiorite have similar whole-rock Sr–Nd isotopic compositions to basement rocks of the NQO.These features indicate that the two plutons formed by partial melting of basement rocks(i.e.,subducted into the lower crust)of the North Qinling unit,along with juvenile crustal material.Group 3 is represented by the Xiaguan monzogranite,which formed at 434–430 Ma,and can be subdivided into heavy rare earth element(REE)-depleted and-enriched units.The former has high Sr/Y(56–98)and(La/Yb)_(N)(34–73)ratios and low MgO(0.13–0.24 wt.%),Cr(0.37–1.69 ppm),and Ni(0.32–1.09 ppm)contents,similar to adakites derived from metabasaltic sources.The heavy REE-enriched nature of the Xiaguan monzogranite may reflect modification of its source by melt or fluid.Our results show that partial melting of enriched oceanic arc crust contributed to crustal maturation in an accretionary orogen.The addition of evolved crustal material also facilitated this process;therefore,the basement rocks and crustal thickness should be considered when assessing crustal dynamics in an accretionary orogen.

Accretionary processes and metallogenesis of the Central Asian Orogenic Belt:Advances and perspectives

As one of the largest Phanerozoic orogens in the world,the Central Asian Orogenic Belt(CAOB)is a natural laboratory for studies of continental dynamics and metallogenesis.This paper summarizes the research progresses of the accretionary processes and metallogenesis of the CAOB since the Peopled Republic of China was founded,and puts forward the prospect for future research.During the early period(1950s-1970s),several geological theories were applied to explain the geological evolution of Central Asia.In the early period of China's reform and opening-up,the plate tectonics theory was applied to explain the evolution of the northern Xinjiang and Xingmeng regions,and the opinion of subduction-collision between Siberian Kazakhstan and China-North Korea-Tarim plates was proposed.The idea of the Solonker-Yanbian suture zone was established.In the 1990s,the study of the CAOB entered a period of rapid development.One school of scholars including geologists from the former Soviet Union proposed a multi-block collision model for the assemblage of the CAOB.In contrast,another school of scholars,led by a Turkish geologist,Celal Sengor,proposed that the Altaids was formed through the growth and strike-slip duplicates of a single island arc,and pointed out that the Altaids is a special type of collisional orogen.During this period,Chinese geologists carried out a lot of pioneering researches on ophiolites and high-pressure metamorphic rocks in northern China,and confirmed the main suture zones accordingly.In 1999,the concept of"Central Asian metallogenic domain"was proposed,and it became one of the three major metallogenic domains in the world.Since the 21st century,given the importance for understanding continental accretion and metallogenic mechanism,the CAOB has become the international academic forefront.China has laid out a series of scientific research projects in Central Asia.A large number of important scientific research achievements have been spawned,including the tectonic attribution of micro-continents,timing and tectonic settings of ophiolites,magmatic arcs,identification and anatomy of accretionary wedges,regional metamorphism-deformation,(ultra)high-pressure metamorphism,ridge subduction plume-plate interaction archipelagic paleogeography and spatio-temporal framework of multiple accretionary orogeny,continental growth accretionary metallogenesis,structural superposition and transformation etc.These achievements have made important international influences.There still exist the following aspects that need further study:(1)Early evolution history and subduction initiation of the Paleo-Asian Ocean;(2)The accretionary mechanism of the extroversion Paleo-Asian Ocean;(3)The properties of the mantle of the Paleo-Asian Ocean and their spatiotemporal distribution;(4)The interaction between the Paleo-Asian Ocean and the Tethys Ocean;(5)Phanerozoic continental growth mechanism and its global comparison;(6)Accretionary metallogenic mechanism of the Central Asian metallogenic domain;and(7)Continental transformation mechanism.

Refnement of the supercontinent cycle with Hf, Nd and Sr isotopes

The combined use of Hf,Nd and Sr isotopes is more useful in understanding the supercontinent cycle than the use of only Hf isotopic data from detrital zircons.Sr and Nd seawater isotopes,although not as precise as εNd and εHf distributions,also record input from ocean ridge systems.Unlike detrital zircons where sources cannot be precisely located because of crustal recycling,both the location and tectonic setting often can be constrained for whole-rock Nd isotopic data.Furthermore,primary zircon sources may not reside on the same continent as derivative detrital zircons due to supercontinent breakup and assembly.Common to all of the isotopic studies are geographic sampling biases reflecting outcrop distributions,river system sampling,or geologists,and these may be responsible for most of the decorrelation observed between isotopic systems.Distributions between 3.5 and 2 Ga based on εHf median values of four detrital zircon databases as well as our compiled εNd database are noisy but uniformly distributed in time,whereas data between 2 and 1 Ga data are more tightly clustered with smaller variations.Grouped age peaks suggest that both isotopic systems are sampling similar types of orogens.Only after 1 Ga and before 3.5 Ga do we see wide variations and significant disagreement between databases,which may partially reflect variations in both the number of sample locations and the number of samples per location.External and internal orogens show similar patterns in εNd and εHfwith age suggesting that both juvenile and reworked crustal components are produced in both types of orogens with similar proportions.However,both types of orogens clearly produce more juvenile isotopic signatures in retreating mode than in advancing mode.Many secular changes in εHf and εNd distributions correlate with the supercontinent cycle.Although supercontinent breakup is correlated with short-lived decreasing εHf and εNd （≤ 100 Myr） for most supercontinents,there is no isotopic evidence for the breakup of the Paleoproterozoic supercontinent Nuna.Assembly of supercontinents by extroversion is recorded by decreasing εNd in granitoids and metasediments and decreasing εHf in zircons,attesting to the role of crustal reworking in external orogens in advancing mode.As expected,seawater Sr isotopes increase and seawater Nd isotopes decrease during supercontinent assembly by extroversion.Pangea is the only supercontinent that has a clear isotopic record of introversion assembly,during which median εNd and εHf rise rapidly for ≤ 100 Myr.Although expected to increase,radiogenic seawater Sr decreases （and seawater Nd increases） during assembly of Pangea,a feature that may be caused by juvenile input into the oceans from new ocean ridges and external orogens in retreating mode.The fact that a probable onset of plate tectonics around 3 Ga is not recorded in isotopic distributions may be due the existence of widespread felsic crust formed prior to the onset of plate tectonics in a stagnant lid tectonic regime,as supported by Nd and Hf model ages.

Accretionary complex:Geological records from oceanic subduction to continental deep subduction

Accretionary complex was usually formed by offscraping of the subducting crustal material over the trench and thus often referred to as subduction zone mélange.The structure,composition and forming process of accretionary wedges can provide important insights into the evolution history of ocean basin,ocean-continent material cycle,continental accretion and thus contribute to understanding of the origin of plates and the growth of continents.Accretionary complex is characterized by a block-in-matrix structure associated with imbricate thrusts and isoclinal folds,diversified metamorphic types and intense water-rock interactions,which are distinct to the traditional stratigraphy.Since the proposal of the concept of accretionary wedge over a hundred years ago,great progress has been made in a variety of research focuses,such as the identification of the distribution of accretionary complexes,their compositions and formation mechanisms,the affinities of the matrix and igneous rocks,the recognition of the Ocean Plate Stratigraphy(OPS),the reconstruction of oceanic basin,the dynamic background of the tectonic evolution,the relationship between subduction zone and orogenic belt and,in particular,the accretionary complexes in continental subduction zones.These studies have significantly improved our understanding of the plate tectonic theory.Challenges remain in the identification of ancient accretionary complexes,the detailed analysis of accretionary complex zones,the accretion characteristics during continental collision,and the geochemical tracing of water-rock interaction during the accretion.China contains representative orogenic belts and accretionary complex zones in the world,and its geological records provide the best opportunity to make new breakthroughs in understanding of the plate tectonics.

Growth of continental crust in intra-oceanic and continental-margin arc systems: Analogs for Archean systems

Earth’s continental crust has grown and been recycled throughout geologic history along convergent plate margins.The main locus of continental crustal growth is in intra-oceanic and continental-margin arc systems in Archean time. In arc systems, oceanic lithosphere is subducted to the deeper mantle, and together with its overlying sedimentary sequence is in some cases off-scraped to form accretionary prisms. Fluids are released from the subducting slab to chemically react with the mantle wedge, forming mafic-ultramafic metasomatites, whose partial melting generates mafic melts that rise up to form arcs. In intraoceanic arcs, they produce dominantly basaltic lavas, with a mid-crust that includes variably-developed vertically-walled intermediate plutons and higher-level dikes and sills. In continental-margin arcs, different petrogenetic processes cause assimilation and fractionation of basaltic magmas, partial melting/reworking of juvenile basaltic rocks, and mixing of mantle-and crust-derived melts, so they produce andesitic calc-alkaline melts but still have a mid-crust dominated by vertically-walled felsic plutons, which form 3-D dome-and-basin structures, akin to those in some Archean terranes such as parts of the Pilbara and Zimbabwe cratons. Notably, the continental crust of Archean times is dominated by tonalite-trondhjemite-granodiorite(TTG)plutons, similar to that of the mid-crust of these arc systems, suggesting that early continental crust may have formed largely by the amalgamation of multiple arc systems. The patterns of magmatism, in terms of petrogenesis, rock types, duration of magmatic and accretionary events, and the spatial scales of deformation and magmatism have remained essentially the same throughout geological history, demonstrating that plate tectonic processes characterized by subduction and arc magmatism have been in operation at least as long as recorded by the preserved geologic record, since the Eoarchean. However, the early Earth was dominated by accretionary orogens and oceanic arcs, that gradually grew thicker through multiple accretion events to form early continental-margin arcs by 3.5–3.2 Ga, and accretionary orogens. Slab melting and warmer metamorphism was more common in Archean arc systems due to higher mantle temperatures. These early arcs were further amalgamated into large emergent continents by ~3.2–3.0 Ga, allowing large-scale processes such as lithospheric rifting and continental collisions, and the start of the supercontinent cycle. Further work should apply the null hypothesis, that plate tectonics explains the geologic record, to test for differences in the style of plate tectonics and magmatism through time, based on the fundamental difference in planetary heat production and the evolution of rotational dynamics of the Earth-Sun-Moon system.