Dirk S Schmeller

Mark Blooi

An Martel

Trenton W J Garner

Matthew C Fisher

Fre ´de ´ric Azemar

Frances C Clare

Camille Leclerc

Lea Ja

Michelle Guevara-Nieto

Adeline Loyau

Frank Pasmans

Microscopic Aquatic Predators Strongly Affect Infection Dynamics of a Globally Emerged Pathogen

placed particular emphasis on host-derived barriers to infection and disease. This focus neglects important extrinsic determinants of the host/pathogen dynamic, where all barriers to infection should be considered when ascertaining the determinants of infectivity and pathogenicity of wildlife pathogens [START_REF] Anderson | Coevolution of hosts and parasites[END_REF][START_REF] Anderson | Infectious Diseases of Humans[END_REF][START_REF] Day | Parasite transmission modes and the evolution of virulence[END_REF]. Those pathogens with free-living stages, such as fungi causing catastrophic wildlife declines on a global scale [START_REF] Fisher | Emerging fungal threats to animal, plant and ecosystem health[END_REF], must confront lengthy exposure to environmental barriers before contact with an uninfected host [START_REF] Studer | Effects of ultraviolet radiation on the transmission process of an intertidal trematode parasite[END_REF][START_REF] Morley | Thermodynamics of miracidial survival and metabolism[END_REF][START_REF] Leisner | Intraguild predation provides a selection mechanism for bacterial antagonistic compounds[END_REF][START_REF] Chavez-Dozal | Predation response of Vibrio fischeri biofilms to bacterivorus protists[END_REF]. Hostile environmental conditions therefore have the ability to decrease the density of infectious particles, reducing the force of infection and ameliorating the impact as well as the probability of establishing an infection [START_REF] Perez-Heydrich | Population-level influence of a recurring disease on a long-lived wildlife host[END_REF]. Here we show that, in nature, the risk of infection and infectious burden of amphibians infected by the chytrid fungus Batrachochytrium dendrobatidis (Bd) have a significant, site-specific component, and that these correlate with the microfauna present at a site. Experimental infections show that aquatic microfauna can rapidly lower the abundance and density of infectious stages by consuming Bd zoospores, resulting in a significantly reduced probability of infection in anuran tadpoles. Our findings offer new perspectives for explaining the divergent impacts of Bd infection in amphibian assemblages and contribute to our understanding of ecosystem resilience to colonization by novel pathogens.

Results

We investigated the infection dynamics of one of the most devastating wildlife pathogens, Batrachochytrium dendrobatidis (hereafter Bd) [START_REF] Fisher | Global emergence of Batrachochytrium dendrobatidis and amphibian chytridiomycosis in space, time, and host[END_REF]. Although Bd is associated with species declines and mass mortalities of amphibians worldwide, prevalence varies significantly at local and regional scales [START_REF] Bosch | Climate change and outbreaks of amphibian chytridiomycosis in a montane area of Central Spain; is there a link?[END_REF][START_REF] Bosch | Evidence of a chytrid fungus infection involved in the decline of the common midwife toad (Alytes obstetricans) in protected areas of central Spain[END_REF][START_REF] Walker | Factors driving pathogenicity vs. prevalence of amphibian panzootic chytridiomycosis in Iberia[END_REF]. Even a single highly susceptible host species, such as the European midwife toad Alytes obstetricans, can exhibit strong variation in the prevalence of infection across small geographic scales. Mortality in this species owing to chytridiomycosis correlates positively with altitude, which is due at least in part to the effects of environmental temperature [START_REF] Bosch | Climate change and outbreaks of amphibian chytridiomycosis in a montane area of Central Spain; is there a link?[END_REF][START_REF] Walker | Factors driving pathogenicity vs. prevalence of amphibian panzootic chytridiomycosis in Iberia[END_REF][START_REF] Garner | Climate change, chytridiomycosis or condition: an experimental test of amphibian survival[END_REF]. However, this does not explain why sites with equivalent temperature regimes can still exhibit substantial variation in prevalence and mortality associated with infection [START_REF] Walker | Factors driving pathogenicity vs. prevalence of amphibian panzootic chytridiomycosis in Iberia[END_REF], or why Bd-positive sites were found to be more similar to each other than would be expected based on chance [START_REF] Strauss | Why does amphibian Chytrid (Batrachochytrium dendrobatidis) not occur everywhere? An exploratory study in Missouri ponds[END_REF]. We first used water sampled at amphibian breeding sites located in the Pyrenees with known histories of presence of Bd in the sentinel amphibian host species, A. obstetricans, to experimentally examine the effect of water and the aquatic microbial community on the probability of infection. The majority of these sites (n = 23) contain populations of A. obstetricans that exhibit low prevalence (<5%) or complete lack of infection (minimum sampling size = 30 individuals) and an absence of mortality in A. obstetricans across up to 6 years of field sampling, while a smaller number of populations (n = 9) have consistently exhibited high prevalence (usually R90%) over time (up to 10 years). Mass mortalities of recently metamorphosed A. obstetricans were observed over the same time span at those sites exhibiting 97% to 100% Bd prevalence (see Table S1 available online).

Dynamics of Motile and Immotile Bd Zoospores in Environmental Water

We investigated whether unfiltered environmental water affected the motility of Bd zoospores based on the prevalence of the source of the water in past years (experiment 1). We found that the number of motile zoospores varied significantly and in accordance with observed patterns of infection at the source of the water (generalized linear mixed model [GLMM]; F 1,244 = 14.18; p < 0.001). Motile zoospores decreased as early as 2 hr after exposure to water from low-prevalence sites, while the number of motile zoospores in cultures that were exposed to water from high-prevalence sites only declined 33 hr after exposure (Figure 1). Counts of immotile zoospores did not differ significantly between low-and high-prevalence sites (GLMM; F 1,244 = 0.35; p = 0.554; Figure S1). To determine whether loss of motility was associated with zoospore death (experiment 2), we assayed the viability of zoospores exposed for 24 hr to the two types of water using quantitative PCR. Zoospore survival was significantly greater in water from sites with high prevalence of infection (GLM; F 1,23 = 4.65; p = 0.042). Water from high-prevalence sites also contained significantly reduced numbers of protozoans and microscopic metazoans compared to water from low-prevalence sites (generalized linear model [GLM]; F 1,23 = 8.14; p = 0.004, Figure S2). Furthermore, the number of protozoans and microscopic metazoans was significantly and negatively correlated with the observed Bd prevalence in 2012 (n = 25; rs = 20.430; p = 0.033) and was positively correlated with the reduction in the number of viable Bd zoospores (n = 25; rs = 0.682; p < 0.001; Table S1). Bd prevalence in 2012 was positively correlated with the altitude of the site (n = 32; rs = 0.779; p < 0.001), whereas water acidity and conductivity had no apparent effect on zoospore survival (Table S1). The best model explaining the reduction in viable zoospores in our experiment included both the number of protozoans and microscopic metazoans and altitude, but not pH (F 1,23 = 8.267; p = 0.002, Akaike information criterion [AIC] 43.087), and Bd prevalence in 2012 was best explained by altitude (F 1,23 = 18.976; p < 0.001, AIC 180.765; Table S1). Filtering out microorganisms with 0.45 mm cellulose acetate syringe filters (experiment 3) significantly reduced the ability of water from low-prevalence sites to cause mortality of Bd zoospores (GLMM; F 1,231 = 91.95; p < 0.001; Figure S3). Our observations in the first set of experiments suggested that the process through which Bd zoospore viability is affected is determined not by water quality but rather by the resident aquatic microfauna.

Challenge Experiments in Environmental Water

We directly tested the infectivity of zoospores in each different water type by exposing uninfected A. obstetricans tadpoles to zoospores in water from the two prevalence categories (<5% and >90% prevalence) and also zoospores in water from low-prevalence sites that had been heat treated (boiled for 15 min) to kill resident microfauna (experiment 4). Infection was significantly greater 8 days after exposure in tadpoles that had been exposed in either water from high-prevalence sites or heat-treated water from low-prevalence sites (F 2,93.1 = 17.14; p < 0.001; Figure 2). Although prevalence in tadpoles exposed in water from low-prevalence sites increased 12 days postexposure, infections of these animals were significantly weaker than infections of animals exposed in water from high-prevalence sites (post hoc pairwise test, p = 0.041) and trended in the same direction as experiments using heat-treated low-prevalence water (Figure 2).

Effect of Different Microorganisms on Bd Infection Rate

To investigate whether specific microorganisms could be responsible for the observed patterns, we exposed Bd zoospores to 14 freshwater protozoans and microbial metazoans and again quantified zoospore viability (experiment 5). Two of these species (Paramecium aurelia and Lecane stichaea) were isolated from Pyrenean water samples, and 12 were sister species of microorganisms commonly found in Pyrenean lakes. The viability of Bd zoospores varied substantially when exposed to different microorganisms (20.04 6 0.19 log 10 genomic equivalent [GE] for the species associated with the smallest reduction in viable Bd zoospores [Stentor coeruleus] to 2.02 6 0.36 log 10 GE for the species associated with the greatest reduction in viable zoospores, the rotifer Notommatidae spp.; Figure 3). To determine the mechanism underlying this pattern, we observed the interactions between fluorescently stained zoospores and six microorganisms used in experiment 5: two that had the weakest impact on zoospore viability (Dileptus anser and S. coeruleus), two that had the greatest impact (P. caudatum and Notommatidae spp., experiment 6), and the two species isolated from Pyrenean sites (P. aurelia and L. stichaea). Our observations suggest that the process through which viability is affected is due at least in part to ingestion of zoospores (Figure S4). Interestingly, In our final experiment, we tested whether microorganisms with different impacts on zoospore viability determined the probability of infection in the predicted manner. We exposed tadpoles (Discoglossus scovazzi) to Bd zoospores in water containing one of the three presumed predatory microorganisms (P. aurelia isolated from the Pyrenees, P. caudatum, or the rotifer Notommatidae spp.; experiment 7). The presence of microorganisms significantly affected the probability of infection (GLMM; F 1,119 = 34.41, p < 0.0001; Figure 4A; F 2,43 = 11.93, p = 0.003; Figure 4C). None of the tadpoles exposed with Notommatidae spp. developed infections, 3 of 15 tadpoles were infected in the presence of P. caudatum, and 16 of 60 tadpoles exposed with the Pyrenean P. aurelia were infected (Figure 4). P. aurelia isolated from the Pyrenees also reduced significantly the strength of infection (GE) compared to control tadpoles (GLMM; F 1,66 = 18.82, p = 0.012; Figure 4D), whereas this was not the case between control tadpoles and those housed with lab-reared P. caudatum treatments (GLMM; F 1,8 = 2.03, p = 0.757; Figure 4B).

Discussion

The ability of microorganisms to forage on Bd zoospores has been postulated but never explicitly shown and then linked to both field conditions and experimentally derived patterns of infection [START_REF] Buck | Predation by zooplankton on Batrachochytrium dendrobatidis biological control of the deadly amphibian chytrid fungus?[END_REF][START_REF] Hamilton | Daphnia in tadpole mesocosms: trophic links and interactions with Batrachochytrium dendrobatidis[END_REF]. Here we demonstrate that ciliate and rotifer microorganisms are effective consumers of Bd zoospores in the Pyrenean mountain lakes, reducing the number of free-swimming, infectious zoospores. The presence of these microorganisms also reduces the probability of infection in two amphibian species that are highly susceptible to Bd. Due to the dose-dependent impact of infection by Bd on host life-history traits, decreasing their infection burden will reduce the impact of Bd on larval development and probably postmetamorphic survival [START_REF] Briggs | Enzootic and epizootic dynamics of the chytrid fungal pathogen of amphibians[END_REF]. Bd infection may result in host mortality only when a threshold density of sporangia (infection intensity) is reached [START_REF] Briggs | Enzootic and epizootic dynamics of the chytrid fungal pathogen of amphibians[END_REF], implying that control may be achieved by limiting the number of Bd zoospores. Our study raises hope that the rate and intensity of Bd infection in amphibian populations can be manipulated by natural means, and that appropriate methods of natural augmentation of predatory microorganisms will significantly decrease the adverse effects of chytridiomycosis on amphibians and ecosystems.

The results of our experiments show that both the prevalence and intensity of infection in larvae of A. obstetricans are site dependent and correlate with the presence of indigenous predatory microorganisms in the water; we experimentally confirmed this latter pattern in a second susceptible species (D. scovazzi). We were able to show that rotifers and ciliates reduce the number of Bd zoospores in the environment and that this reduction might occur in several different ways, such as concomitant predation, predation of freeliving stages, or passive consumption and filtration [START_REF] Johnson | When parasites become prey: ecological and epidemiological significance of eating parasites[END_REF]. Predatory microorganisms such as the ciliates P. caudatum and P. aurelia and the rotifers Notommatidae spp. and L. stichaea appear to have a higher foraging efficacy for Bd zoospores, ingesting Bd zoospores more efficiently than planktonic species with different foraging strategies, for example D. anser and S. coeruleus. Indeed, it is likely that only a few typical freshwater plankton species may be unable to ingest Bd zoospores. Microorganisms able to prey on Bd zoospores are expected to be numerous, as aquatic environments are rich with species of chytrid that utilize zoospores as a dispersal unit and therefore represent a rich source of potential nutrition [START_REF] Sime-Ngando | Hidden diversity among aquatic heterotrophic flagellates: ecological potentials of zoosporic fungi[END_REF]. This observation has been confirmed by laboratory-based studies showing that planktonic Daphnia species can consume Bd [START_REF] Buck | Predation by zooplankton on Batrachochytrium dendrobatidis biological control of the deadly amphibian chytrid fungus?[END_REF][START_REF] Hamilton | Daphnia in tadpole mesocosms: trophic links and interactions with Batrachochytrium dendrobatidis[END_REF]. Therefore, we suggest that many indigenous plankton species are preadapted to preying on Bd zoospores, as these are similar in size and form to endemic zoosporic aquatic fungi that likely form a key nutritional component of the microfaunal plankton diet [START_REF] Kagami | The parasitic chytrid, Zygorhizidium, facilitates the growth of the cladoceran zooplankter, Daphnia, in cultures of the inedible alga, Asterionella[END_REF].

Environmental factors have been identified that covary with the prevalence of infection and chytridiomycosis [START_REF] Bosch | Climate change and outbreaks of amphibian chytridiomycosis in a montane area of Central Spain; is there a link?[END_REF][START_REF] Walker | Factors driving pathogenicity vs. prevalence of amphibian panzootic chytridiomycosis in Iberia[END_REF]. Most prominently, lower temperature regimes at higher altitudes are associated with higher Bd infection probability [START_REF] Walker | Factors driving pathogenicity vs. prevalence of amphibian panzootic chytridiomycosis in Iberia[END_REF]. Temperature may act to modify prevalence through direct and indirect pathways, either by directly influencing host immunity and pathogen growth rates or by indirectly influencing the activity of microorganisms across infected sites. This goes some way toward explaining why both prevalence of infection and mortality are more common in the Pyrenees when environmental temperatures are very low, and when laboratory estimates of Bd growth rates and zoospore production indicate that infection should be rare [START_REF] Walker | Factors driving pathogenicity vs. prevalence of amphibian panzootic chytridiomycosis in Iberia[END_REF][START_REF] Woodhams | Life-history trade-offs influence disease in changing climates: strategies of an amphibian pathogen[END_REF]. Environmental factors may also explain the composition, density, and dynamics of the planktonic communities across seasons [START_REF] Bartumeus | Factors shaping diversity patterns in pelagic rotifer assemblages of high mountain lakes (Pyrenees)[END_REF], with sites of high Bd prevalence being an enemy-free space for Bd, allowing the fungus to infect suitable host species rapidly and with a high intensity. Additional support for the generality of our findings comes from recent studies showing that Bd-positive ponds are more similar to each other than would be expected based on chance [START_REF] Strauss | Why does amphibian Chytrid (Batrachochytrium dendrobatidis) not occur everywhere? An exploratory study in Missouri ponds[END_REF] and that the ''dilution effect'' hypothesis may apply to the amphibian-Bd system [START_REF] Venesky | Linking manipulative experiments to field data to test the dilution effect[END_REF], suggesting links between pond ecology and local-scale epidemiological dynamics. More detailed ecological studies are now needed to better link abiotic variables to the composition, density, and dynamics of the planktonic communities and the outcome of the host/pathogen dynamic.

We here show the importance of predation in controlling infections in larvae of two amphibian species and provided direct evidence that zoospore ingestion is the mechanism through which infection is modified [START_REF] Johnson | When parasites become prey: ecological and epidemiological significance of eating parasites[END_REF]. Development of methods that facilitate natural augmentation of predatory microorganisms as a form of Bd biocontrol may hold promise as a field mitigation tool that lacks the downsides associated with introducing nonnative biocontrol agents, such as the use of antifungal chemicals or release of nonnative skin bacteria into the environment, or the reliance of unpredictable environmental temperature to ''cure'' infections [START_REF] Becker | Towards a better understanding of the use of probiotics for preventing chytridiomycosis in Panamanian golden frogs[END_REF][START_REF] Doddington | Context-dependent amphibian host population response to an invading pathogen[END_REF]. However, before biocontrol can be safely attempted, additional study is required.

Figure 1 .

 1 Figure 1. Dynamics of Bd Zoospores under Lab Conditions in Water of High-and Low-Prevalence Sites Relative counts of viable zoospores (counts for each water sample were standardized relative to counts at t = 0) exposed to water from high-prevalence (C) versus low-prevalence sites (B). Controls are counts of viable zoospores in treatments where distilled water was added to the culture. Error bars indicate SEM across water samples in the same prevalence group. *p % 0.05, **p % 0.01, ***p % 0.001 by post hoc pairwise test.

Figure 2 .

 2 Figure 2. Infection Probability and Strength in Different Water Treatments Impact of water source and heat treatment on infection strength (A) and infection probability at 5, 8, and 12 days postexposure in A. obstetricans tadpoles (B). Different bar shading indicates different water types and treatments. Strength of infection is the uncorrected qPCR estimate of the average number of zoospores (genomic equivalents, GE) for each water category. Pairwise p values from Tukey's post hoc tests are shown.

Figure 3 .

 3 Figure 3. Reduction of Zoospore Viability Associated with the Presence of 14 Protozoan/Metazoan Species Reduction of viability represented as log 10 GE of viable Bd zoospores after 24 hr + 1 hr SD. Error bars indicate SEM of three replicates per species. Two species were bred from samples stemming from our Pyrenean sites, the Lecanidae rotifer Lecane stichaea and the Parameciidae ciliate Paramecium aurelia.

Figure 4 .

 4 Figure 4. Challenge Experiment under the Influence of Microfauna Number of infected tadpoles (A and C) and intensity of infection (B and D) of Discoglossus scovazzi tadpoles exposed repeatedly to Bd zoospores and cohoused with Notommatidae spp. (A), Paramecium caudatum (A and B), or P. aurelia from the Pyrenees (C and D) or housed in the absence of microorganisms.

Acknowledgments

All animal experiments were conducted in compliance with national laws of Germany and Belgium. Financial support to D.S.S., M.C.F., F.C.C., T.W.J.G., and A.L. was provided by the ERA-NET BiodivERsA through the project RACE. M.B. was supported by a Dehousse grant provided by the Royal Zoological Society of Antwerp. Joyce E. Longcore kindly provided an isolate of Bd (JEL423) used in this study. D.S.S. and A.L. would like to thank Maı ¨te Ármengaud, Frederic Rodriguez, and Jean-Luc Probst for their support.

Supplemental Information

Supplemental Information includes four figures, one table, and Supplemental Experimental Procedures and can be found with this article online at http://dx.doi.org/10.1016/j.cub.2013.11.032.


Zhao Pan

Fang Junqin

Xu Bei

Chen Yan

Faure Michel

Early Paleozoic tectonic evolution of the Xing-Meng Orogenic Belt: constraints from detrital zircon geochronology of western Erguna-Xing'an Block, North China

Keywords: Xing-Meng Orogenic Belt, Early Paleozoic, detrital zircon, Erguna-Xing'an Block, Inner Mongolia

To better constrain the Early Paleozoic tectonic evolution of the southwestern Arigin Sum-Xilinhot-Xing'an Block, we applied detrital zircon U-Pb dating on the Ordovician to Devonian sedimentary strata along the southeastern China-Mongolia border. Most of the zircons from all five sedimentary samples display fine-scale oscillatory growth zoning and Th/U ratios higher than 0.1, indicating a magmatic origin. All these five Ordovician-Devonian samples display the similar age distribution patterns with age groups at ~440 Ma, ~510 Ma, ~800 Ma, ~950 Ma, with some Meso-to Paleo-proterozoic and Neoarchean grains. This age distribution pattern is similar to those from adjacent blocks in the southeastern Central Asian Orogenic Belt. Taking previous tectonic studies into consideration, bidirectional provenances from the Erguna-Xing'an Block and Baolidao Arc were suggested.

Consequently, a new model was proposed to highlight the Early Paleozoic tectonic evolution of the southern margin of Arigin Sum-Xilinhot-Xing'an Block. The collision between Erguna-Kerulen Block and Arigin Sum-Xilinhot-Xing'an Block occurred before the Late Cambrian, with Late Cambrian granitic and mafic intrusions along both sides of the suture zone. After that, sedimentary basins developed in the Erguna-Xing'an Block during the Ordovician-Silurian accompanying with coeval magmatic intrusions, which provided provenance for sedimentary basins together with the Late Cambrian intrusions. Meanwhile, the northward subduction of the Paleo-Asian Ocean beneath the Arigin Sum-Xilinhot-Xing'an Block was from ~490 Ma to ~420 Ma, producing the Baolidao Arc magmatism along its southern margin, which also made contribution for the provenance. The eastward successive closure of marine sedimentary basins on the Arigin Sum-Xilinhot-Xing'an Block took place at the Early Devonian to the west and Late Devonian to the east, due to the pre-Late Devonian collision between the Songliao-Hunshandake Block and the Arigin Sum-Xilinhot-Xing'an Block.

Introduction

The Xing-Meng Orogenic Belt (XMOB), located to the southeast of Central Asian Orogenic Belt (CAOB; Fig. 1a), recorded the accretionary history among several main blocks, such as North China Craton (NCC), Songliao-Hunshandake Block (SHB), Airgin Sum-Xilinhot-Xing'an Block (AXXB) and Erguna Block (EB)(Fig. 1a; [START_REF] Wu | The Hulan Group: Its role in the evolution of the Central Asian Orogenic Belt of NE China[END_REF][START_REF] Zhou | Early Paleozoic metamorphic rocks of the Erguna block in the Great Xing'an Range, NE China: Evidence for the timing of magmatic and metamorphic events and their tectonic implications[END_REF]. These blocks experienced multi-stage of amalgamation during the Paleozoic. Numerous studies have been carried out in the past two decades, however, no consensus about the tectonic evolution of XMOB could have been reached (e.g. [START_REF] Xiao | Accretion leading to collision and the Permian Solonker suture, Inner Mongolia, China: termination of the Central Asian Orogenic Belt[END_REF][START_REF] Wu | Geochronologyof the Phanerozoic granitoids in northeastern China[END_REF][START_REF] Zhou | Early Paleozoic metamorphic rocks of the Erguna block in the Great Xing'an Range, NE China: Evidence for the timing of magmatic and metamorphic events and their tectonic implications[END_REF]Zhang et al., 2012;[START_REF] Xu | Middle Paleozoic convergent orogenic belts in western Inner Mongolia (China): framework, kinematics, geochronology and implications for tectonic evolution of the Central Asian Orogenic Belt[END_REF]. The collision between EB and AXXB is considered to be occured before the Late Cambrian, as orogenic related Late Cambrian alkaline granites, granotoids and gabbros were identified from both sides of the Xinlin-Xiguitu suture zone, representing a post-collisional setting (Fig. 1a; [START_REF] Sorokin | Early Paleozoic granitoids in the Eastern Margin of the Argun' Terrane, Amur area: first geochemical and geochronologic data[END_REF]2011;[START_REF] Wu | Discovery ofthe Early Paleozoic post-collisional granites in northern margin of theErguna massif and its geological significance[END_REF][START_REF] Imbgr | Geological map of China, Dongwuzhumuqinqi sheet (L50C003002[END_REF][START_REF] Ge | Emplacement age of the Tahegranite and its constraints on the tectonic nature of the Ergun block in thenorthern part of the Da Hinggan Range[END_REF][START_REF] Wu | Geochronologyof the Phanerozoic granitoids in northeastern China[END_REF].

After that, the new formed Erguna-Xing'an Block (EXB) involved into the Paleozoic accretionary process. From the Late Cambrian, the Paleo-Asian Ocean subducted beneath the AXXB to the north and the NCC to the south, with two arc magmatic belts developed [START_REF] Liu | SHRIMP dating of adakites in the Tulingkai ophiolite, Inner Mongolia: evidence for the Eraly Paleozoic subduction[END_REF][START_REF] Jian | Time scale of an early to mid-Paleozoic orogenic cycle of the long-lived Central Asian Orogenic Belt, Inner Mongolia of China: implications for continental growth[END_REF][START_REF] Xu | Middle Paleozoic convergent orogenic belts in western Inner Mongolia (China): framework, kinematics, geochronology and implications for tectonic evolution of the Central Asian Orogenic Belt[END_REF][START_REF] Zhang | Magmatic and metamorphic development of an early to mid-Paleozoic continental margin arc in the southernmost Central Asian Orogenic Belt, Inner Mongolia, China[END_REF].

High presure metamorphic blueschist was identified from both northern and southern belts, with 40 Ar-39 Ar ages of 383±13 Ma and 453-446 Ma, respectively [START_REF] Xu | Primary study on petrology and geochronology of the blueschist in Sonid Zuoqi, northern Inner Mongolia[END_REF][START_REF] Tang | Regional metamorphism and tectonic evolution of the Inner Mongolia suture zone[END_REF][START_REF] De | Ordovician 40 Ar/ 39 Ar phengite ages from the blueschist-facies Ondor Sum subductionaccretion complex (Inner Mongolia) and implications for the Early Paleozoic history of continental blocks in China and adjacent areas[END_REF]. By structural geology and sedimentary investigations, two unconformities were described, with the ductilely deformed Ondor Sum Group and ophiolitic mélange unconformably overlain by the Late Devonian molasse along the northern suture and the Late Silurian molasse along the southern one [START_REF] Tang | Tectonic development of Paleozoic fold belts at the north margin of the Sino-Korean craton[END_REF][START_REF] Xu | Middle Paleozoic convergent orogenic belts in western Inner Mongolia (China): framework, kinematics, geochronology and implications for tectonic evolution of the Central Asian Orogenic Belt[END_REF]. It means that the amalgamation of EXB, SHB and NCC occurred before the end of Devonian, without wide ocean existing in between, which confirmed by recent paleomagnetic study [START_REF] Zhao | Did the Paleo-AsianOcean betweenNorth China Block and Mongolia Block exist during the late Paleozoic? First paleomagnetic evidence from central-eastern Inner Mongolia, China[END_REF] and palaeontological study [START_REF] Zhou | Discovery of Early-Middle Permian cathaysian flora in Manduhubaolage area, Dong Ujimqin Qi, Inner Mongolia, China and its geological significance[END_REF][START_REF] Xin | Stratigraphic Subdivision and Isotope Geochronology Study on the Baoligaomiao Formation in the East Ujimqin County, Inner Mongolia[END_REF]. However, different arguments have been put forward to consider that the final collision occurred in the Late Permian based on the geochemistry studies on the Carboniferous-Permian magmatic rocks [START_REF] Chen | Two contrasting Paleozoic magmatic belts in northern Inner Mongolia, China: petrogenesis and tectonic implications[END_REF][START_REF] Xiao | Accretion leading to collision and the Permian Solonker suture, Inner Mongolia, China: termination of the Central Asian Orogenic Belt[END_REF][START_REF] Zhang | Contrasting late Carboniferous and late Permian-middle Triassic intrusive suites from the northern margin of the north China craton[END_REF]. Therefore, the tectonic evolution between EXB and SHB is still controversal, especially the south margin of EXB.

In this study, we carried out detrital zircon geochronological investigation on the Ordovician-Devonian strata in the northwestern part of the AXXB (Fig. 1), where investigations remain rare. Form these new data, we try to decipher the sedimentary responses to the collision between the EB and AXXB, and the superposed influence of the subsequent collision between EXB and SHB.

Geological setting

The northeastern China is divided into several blocks, namely Erguna Block, Airgin Sum-Xilinhot-Xing'an Block, Songliao-Hunshandake Block and Jiamusi Block from north to south [START_REF] Wu | The Hulan Group: Its role in the evolution of the Central Asian Orogenic Belt of NE China[END_REF][START_REF] Xu | Middle Paleozoic convergent orogenic belts in western Inner Mongolia (China): framework, kinematics, geochronology and implications for tectonic evolution of the Central Asian Orogenic Belt[END_REF]Fig. 1a). These blocks share the similar Paleo-to Neo-Proterozoic basement, which was revealed by both geochronological studies of magmatic massifs and detrital zircon dating of metasedimentary rocks [START_REF] Wang | Geochronological evidence for existence of the south Mongolian microcontinent: a zircon U-Pb age of granitoid gneisses from the Yagan-Onch Hayrhan metamorphic core complex on the Sino-Mongolian border[END_REF][START_REF] Wu | Geochronologyof the Phanerozoic granitoids in northeastern China[END_REF][START_REF] Li | Detrital zircon U-Pb ages, Hf isotopesand tectonic implications for Palaeozoic sedimentary rocks from the Xing-Meng orogenic belt, middle-east part of inner Mongolia, China[END_REF]Han et al., 2011;Zhang et al., 2012 and references therein;[START_REF] Wu | Zircon U-Pb ages of the metamorphic supracrustal rocks of the Xinghuadukou Group and granitic complexes in the Argun massif of the northern Great Hinggan Range, NE China, and their tectonic implications[END_REF]Sun et al., 2013a;b;[START_REF] Tang | Geochronology and geochemistry of Neoproterozoic magmatism in the Erguna Massif, NE China: Petrogenesis and implications for the breakup of the Rodinia supercontinent[END_REF]. To the west, according to the new tectonic subdivision of Mongolia (Tomurtogoo, 2013), the Erguna Block might connect with the Kerulen Block (Erguna-Kerulen Block, EKB), the Airgin Sum-Xilinhot-Xing'an Block connected with the Totoshan Block at the southmost of Mongolia (Fig. 1a; [START_REF] Wang | Geochronological evidence for existence of the south Mongolian microcontinent: a zircon U-Pb age of granitoid gneisses from the Yagan-Onch Hayrhan metamorphic core complex on the Sino-Mongolian border[END_REF][START_REF] Xu | Middle Paleozoic convergent orogenic belts in western Inner Mongolia (China): framework, kinematics, geochronology and implications for tectonic evolution of the Central Asian Orogenic Belt[END_REF]Zhou et al., 2013). However, some researchers suspect the existence of AXXB due to the lack of precambrian basement and some Paleozoic zircons identified from geochronological studies of the Xilinhot complex [START_REF] Wu | The Hulan Group: Its role in the evolution of the Central Asian Orogenic Belt of NE China[END_REF][START_REF] Xue | The Xilingele complex from the eastern part of the Central Asian-Mongolia Orogenic Belt, China: Products of Early Variscan orogeny other than ancient block: Evidence from zircon SHRIMP U-Pb ages[END_REF][START_REF] Chen | Zircon U-Pb ages of the Xilinhot Metamorphic complex and blueschist, and implications for tectonic evolution of the Solonker suture[END_REF]. In the recent studies, [START_REF] Ge | Dissolution and supracrustal rocks dating of Xilin Gol Complex, Inner Mongolia, China[END_REF] isolated the late Mesoproterozoic supracrustal rocks from the Xilinhot complex, which was intruded by ~740 Ma metagabbro, indicating the existence of precambrian basement.

Meanwhile, Mesoproterozoic granitic gneisses were recently reported from the east of Sunidzuoqi with zircon U-Pb age of 1516-1390 Ma, considered as the basement of AXXB (Sun et al., 2013b).

The study area located along the southeastern part of China-Mongolia border, belonging to the northwestern margin of the AXXB, which was separated from the northern EKB by the Xinlin-Xiguitu suture zone (XXS)(Fig. 1a). To the south, it was separated from Songliao-Hunshandake Block by the Suzuoqi-Xilinhot-Heihe suture zone (SXHS), which was thought to be formed during the Late Devonian-Early Carboniferous based on the Late Devonian unconformity and Carboniferous post-collisional intrusions (Fig. 1; Zhang et al., 2012;[START_REF] Xu | Middle Paleozoic convergent orogenic belts in western Inner Mongolia (China): framework, kinematics, geochronology and implications for tectonic evolution of the Central Asian Orogenic Belt[END_REF].

The oldest strata of the study area are Ordovician in the age, consisting of Lower-Middle Ordovician Wubinaobao and Tongshan formations, Middle Ordovician Duobaoshan Formation, and Middle-Upper Ordovician Luohe Formation (Fig. 2).

Except for the Duobaoshan Formation, which is mainly made of basaltic andesite, the other formations are mainly composed of fine-grained clastic sediments. The Silurian strata are rare in the study area, only Upper Silurian Woduhe Formation was identified from the Narenbaolige and eastern West Ujimqin area (Fig. 2), characterized by Late Silurian Tuvaella fauna bearing clsatic rocks and carbonates [START_REF] Su | The distribution and entironment of Tuvaella fauna[END_REF][START_REF] Imbgmr | Regional geology of Inner Mongolian Autonomous Region[END_REF][START_REF] Wang | Tectonic implication of the Late Paleozoic stratigraphic distribution in Northeast China and adjacent region[END_REF]. The well distributed Lower-Middle Devonian Niqiuhe Formation (Fig. 1b) mainly consists of clastic rocks from conglomerate to siltstone with several limestone interlayers, and displays large differences of sedimentary facies from western to eastern part (Fig. 2). It is characterized by its abundant shallow marine fauna, such as brachiopoda, bivalve, coral and trilobita [START_REF] Imbgmr | Regional geology of Inner Mongolian Autonomous Region[END_REF]. The Middle-Upper Devonian strata were mainly distributed in the west of West Ujimqin, namely Middle-Upper Devonian Tarbaget Formation and Upper Devonian Angeryinwula Formation (Fig. 2). Both of these two formations are mainly composed of fine-grained clastic rocks (Fig. 2). However, corals and brachiopoda were found in the the Tarbaget Formation, while plant fossils were identified in the Angeryinwula Formation, indicating a regression related depositional facies transition from shallow marine to continental facies [START_REF] Imbgr | Geological map of China, Dongwuzhumuqinqi sheet (L50C003002[END_REF]. The Upper Carboniferous-Lower Permian Baoligaomiao/Gegenaobao Formation, a ca. 5000 m-thick sequence, unconformbly covered the underlying strata. It is composed of conglomerate, coarse sandstone and sandstone at the lower part and volcanic and volcano-clastic rocks at the upper part. The abundant flora found therein and the geochemical signatures of volcanic rocks reveal the continental deposition [START_REF] Imbgmr | Regional geology of Inner Mongolian Autonomous Region[END_REF][START_REF] Li | Lithostratigraphy of Inner Mongolia Autonomous Region[END_REF]Zhang et al., 2011). The volcanic rocks are coeval or slightly earlier than the Early Permian alkaline granite, together, they constitute an Early Permian rift-related alkaline magmatic belt [START_REF] Hong | The Permian alkaline granites in central Inner Mongolia and their geodynamic significance[END_REF]1996;[START_REF] Jahn | Peralkaline granitoid magmatism in the Mongolian-Transbaikalian Belt: Evolution,petrogenesis and tectonic significance[END_REF].

Sampling

Five sandstone-siltstone samples were collected from three localities for detrital zircon U-Pb dating. The detailed sampling information is described below.

Two samples were collected from the northwest of Chaganaobao County (Fig. 1b). Sample 100716-33 (GPS: 44°32'33" N, 111°47'52" E) is siltstone from ca. 4500 m-thick Lower-Middle Ordovician Wubinabo Formation (Fig. 2). It is mainly composed of dark green siltstone with penetrative schistosity (Fig. 3a), with grey sandstone interlayers (Fig. 2). Sample 100826-01 (GPS: 44°30'88" N, 111°57'39" E) is red coarse sandstone from the Lower-Middle Devonian Niqiuhe Formation (Fig. 2).

Following the underlying conglomerate (Fig. 3f), the red coarse sandstone is considered as near shore deposition with plenty of lithic grains can be observed (Fig. 3b).

Two samples were collected from the northwest of Narenbaolige County (Fig. 1b). Sample 100719-20 (GPS: 44°56'50" N, 114°20'14" E) was collected from a sandstone layer at the bottom of the Middle-Upper Ordovician Luohe Formation (Fig. 2). It was grayish-green in color with high propotion of basaltic lithics observed in the thin section (Fig. 3c), which might come from the underlying Duobaoshan basaltic andesite (Fig. 2). Sample 100720-20 (GPS: 45°11'15" N, 114°39'18" E) was collected from the lower part of the Lower-Middle Devonian Niqiuhe Formation (Fig. 2). This sandstone is gray in color with wave bedding and parallel bedding developed.

Lithic grains, polycrystal and monocrystal quartz constitute the main clastics of this sandstone (Fig. 3d).

Sample 100723-23 (GPS: 45°36'38" N, 116°44'06" E) was collected from the Lower-Middle Devonian Niqiuhe Formation in the northwest of West Ujimqin County (Fig. 1b). It is yellowish fine-grained sandstone with trilobita and brachiopoda fossils found nearby, indicating stable sedimentation and long distance transportation of provenance (Fig. 3e).

Analytical procedures

Zircons were separated using conventional heavy liquid and magnetic techniques before handpicking under a binocular microscope at the Langfang Regional Geogogical Survey, Hebei Province, China. Handpicked zircons were photographed under transmitted and reflected light under optical microscope before cathodoluminescence (CL) imaging using a Quanta 200 FEG Scanning Electron Microscope in Peking University in order to investigate the internal textures and choose potential target sites for U-Pb analyses. Zircon U-Pb dating was carried out using an Agilient 7500c ICP-MS instrument coupled with a 193-nm ArF Excimer laser with the automatic positioning system at the Key Laboratory of Orogen and Crust Evolution, Peking University. Calibrations for zircon analyses were carried out using NIST 610 glass as an external standard and Si as internal standard. U-Pb isotope fractionation effects were corrected using zircon Plesovice (337 Ma) as external standard. Zircon standard 91500 is used as a secondary standard to supervise the deviation of age measurement/calculation. Isotopic ratios and element concentrations of zircons were calculated using GLITTER (ver. 4.4.2, Macquarie University).

Concordia ages and diagrams were obtained using Isoplot/Ex (3.0) [START_REF] Ludwig | User's Manual for Isoplot 3.0: A Geochronological Toolkit for Microsoft Excel Berkeley Geochronology Center[END_REF].

The common lead was corrected using LA-ICP-MS Common Lead Correction (ver.

3.15), followed the method of [START_REF] Andersen | Correction of common lead in U-Pb analyses that do not report 204 Pb[END_REF]. The analytical data are presented on U-Pb Concordia diagrams with 2s with 23.15), followed were weighted means at 95% confidence levels [START_REF] Ludwig | User's Manual for Isoplot 3.0: A Geochronological Toolkit for Microsoft Excel Berkeley Geochronology Center[END_REF].

U-Pb zircon dating results

Chaganaobao area

Lower-Middle Ordovician siltstone (Sample 100716-33)

The zircon grains are euhedral to subhedral, ranging from 30-50 μm in width and 50 to 100μm in length with fine-scale oscillatory growth zoning (Fig. 4a). The Th/U ratios range from 0.12 to 1.44, indicating magmatic origin (e.g. [START_REF] Rubatto | Zircon trace element geochemistry: partitioning with garnet and the link between U-Pb ages and metamorphism[END_REF][START_REF] Corfu | Atlas of zircon textures[END_REF]. A total of 75 zircons were analyzed and 12 of them were discarded because of high discordance(Supplementary Table 1). The 63 concordant analyses yield apparent ages ranging from 440±6 Ma to 2534±12 Ma and fall into three main age populations: 440-577 Ma (n=30) with a peak at 491 Ma, 766-956 Ma (n=25) with peaks at 827 and 950 Ma, 1019-1598 Ma (n=5) without peak age, respectively (Figs. 5a and5b). In addition, one grain yielded an age of 697±8 Ma and two grains yielded Archean ages of 2530±12 and 2534±12 Ma (Supplementary Table 1).

Lower-Middle Devonian Coarse Sandstone (Sample 100826-01)

The zircon grains are euhedral to subhedral, ranging from 50-150 μm in width and 100 to 250 μm in length with well developed oscillatory growth zoning (Fig. 4b).

The majority of zircons display high Th/U ratios ranging from 0.24 to 1.06, indicative of magmatic origin. However, 7 grains show Th/U ratios less than 0.1 (0.08-0.09), which may indicate their metamorphic origin [START_REF] Rubatto | Zircon trace element geochemistry: partitioning with garnet and the link between U-Pb ages and metamorphism[END_REF]. All of 75 analyses are concordant, ranging from 449±7 to 923±16 Ma. Two main age populations are defined at 487-527 Ma (n=40) with a peak at 508 Ma, and 536-608 Ma (n=30) with a peak at 576 Ma (Figs. 5c and5d). Meanwhile, five zircons yielded ungrouped ages of 449, 678, 794, 892 and 923 Ma (Supplementary Table 1; Figs. 5c and5d).

Baiyintuga area

Middle-Upper Ordovician Siltstone (Sample 100719-20)

The majority of zircon grains are subhedral to subrounded, ranging from 30 to 100 μm in width and 60-200 μm in length (Fig. 4c). Most zircon grains display oscillatory growth zoning, with a few showing inherited cores and rims (Fig. 4c). All zircons except one (0.03 for #1920-01 with an age of 2081±19 Ma) give high Th/U ratios from 0.24 to 1.47, indicating magmatic origin. Amonga totalof 75 analyses, 69 concordant analyses yielded ages ranging from 428±6Ma Ma to 2092±17 (Supplementary Table 1; Figs. 5e and5f), and fell into one main age population at 428-547 Ma (n=63), with two peaks at 442 Ma and 511 Ma (Figs. 5e and5f).

Meanwhile, six zircons yielded ungrouped ages of 753, 940, 1282, 1461, 2081 and 2092 Ma.

Lower-Middle Devonian Sandstone (Sample 100720-20)

Most of zircon grains are subhedral to subrounded, ranging from 40 to 80 μm in width and 50-200 μm in length (Fig. 4d). Most zircon grains display oscillatory growth zoning, while a small proportion shows inherited cores and rims (Fig. 4d).

Except two grains with Th/U ratio less than 0.1, all other zircons show high Th/U ratios from 0.11 to 1.09, indicating magmatic origin. Among a total of 75 U-Pb analyses, 67 of them are concordant and yielded ages ranging from 453±6 Ma to 2227±14 Ma (Supplementary Table 1; Figs. 5g and5h). The grains fall into two main age populations at 453-581 Ma (n=49) with two peaks at 459 Ma and 512 Ma, and 669-917 Ma (n=17) with peak at 794 Ma (Figs. 5g and5h). In addition, one grain yielded ages of 2227±14 Ma.

West Ujimqin Lower-Middle Devonian Sandstone (Sample 100723-23)

The zircon grains are euhedral to subhedral, ranging from 50-100 μm in width and 80 to 150 μm in length with oscillatory growth zoning (Fig. 4e). Except one grain showing Th/U ratio of 0.04, all other zircons show high Th/U ratios from 0.13 to 0.91, indicating magmatic origin. Among a total of 74 analyses, 8 analyses were discarded because of high discordance, and one grain (#2325-20 with age of 214±3) was also discarded (Supplementary Table 1). The 65 concordant analyses yielded ages ranging from 394±6 Ma to 2018±45 Ma (Supplementary Table 1; Figs. 5i and5j). Broadly, the grains fall into three age populations at 394-495 Ma (n=37) with a peak at 435 Ma, 505-546 Ma (n=15) with peak at 511 Ma, and 778-1078 Ma (n=11) with a small peak at 959 Ma, respectively (Fig. 5j). Meanwhile, two zircon yielded ages of 624 and 2018 Ma (Supplementary Table 1; Figs. 7i and7j).

Discussion

Constraint on the depositional age of studied Ordovician-Devonian strata

The depositional ages of the investigated Ordovician to Devonian formations were determined using association of fossils and regional lithology correlation. Due to the poorly preserved fossils (especially for Ordovician strata) and lithological variations from region to region, the stratigraphic division of the Ordovician-Devonian stratain this area is not precise. Our detrital zircon data provide new constraint for their age assignment.

The youngest zircon age of the Lower-Middle Ordovician Wubinaobao

Formation siltstone is 440±6 Ma, but, defined by just one grain. Consequently, we use here the youngest grouped zircon age of 461±6 Ma defind by 14 grains (Supplementary Table 1) to constrain the depositional age of the Wubinaobao Formation. This age drops into the Middle Ordovician, and is therefore consistent with the former fossil constraint [START_REF] Imbgmr | Regional geology of Inner Mongolian Autonomous Region[END_REF]. For the Middle-Upper Ordovician Luohe Formation, the youngest grouped zircon age is 428±6 Ma, constraining the depositional age to be younger than the Early Silurian, and is slightly younger than the previous assignment [START_REF] Imbgmr | Regional geology of Inner Mongolian Autonomous Region[END_REF]. The abundant fossils found within the Niqiuhe Formation have well constrained its Early-Middle Devonian age, and our detrital zircon data from Chaganaobao (449±7 Ma for the youngest zircon age), Baiyintuga(453±6 Ma for the youngest zircon age) and West Ujimqin (394±6 Ma for the youngest zircon age) are consistent with this age assignment. So, the depositional age of the Niqiuhe Formation should be of the Early-Middle Devonian.

Provenance of Ordovician and Devonian strata

The detrital zircon dating results presented above give us new clue to trace the provenance of these sediments and decipher the tectonic evolution. All these five Ordovician-Devonian samples display similar distributions in zircon age probability diagrams, characterized by predominant Early Paleozoic age groups at ~440 Ma and ~510 Ma, and subordinate Neoproterozoic age groups at ~570 Ma, ~800 Ma and ~950 Ma, respectively, with only few grains of pre-1.0 Ga (Figs. 5 and6a). It indicates relatively stable provenance for this belt from the Ordovician to the Devonian.

The most striking feature of zircon age distribution for this study is the predominant peak age at ~510 Ma (Fig. 6a). This significant age group was well documented from the EKB [START_REF] Wu | Discovery ofthe Early Paleozoic post-collisional granites in northern margin of theErguna massif and its geological significance[END_REF]2012) and AXXB (Figs. 6b and 6c;Li et al., 2010;Han et al., 2011) by geochronological studies of granotoids and clastic rocks, which was considered as the orogenic related magmatic event related to the collision between the EKB and AXXB [START_REF] Wu | Discovery ofthe Early Paleozoic post-collisional granites in northern margin of theErguna massif and its geological significance[END_REF]. Furthermore, this age group has also been revealed by zircon studies of igneous rocks from northwestern-central Mongolia (Fig. 6d;Rojas-Agramonte et al., 2011), central Mongolia (Demoux et al., 2009a), and southern Mongolia (Demoux et al., 2009b). Most of the ~510 Ma zircons display Th/U ratios higher than 0.1 (Supplementary Table 1), which implies that these zircon grains came from the Cambrian magmatic rocks exposed on the EKB, AXXB or/and the Mongolian blocks. Moreover, we noticed that 4 zircons from Sample 100826-01 and 1 zircon from Sample 100723-23 present Th/U ratios less than 0.1 and U-Pb age of 497-517 Ma (Supplementary Table 1), showing the same feature with zircons from Pan-African events in the northeastern margin of the EKB and AXXB [START_REF] Zhou | Early Paleozoic metamorphic rocks of the Erguna block in the Great Xing'an Range, NE China: Evidence for the timing of magmatic and metamorphic events and their tectonic implications[END_REF][START_REF] Zhou | The crustal accretion history and tectonic evolution of the NE China segment of the Central Asian Orogenic Belt[END_REF].

The age peak of ~440 Ma may represent the regional Early Paleozoic magmatic event. Two Early Paleozoic magmatic events have been documented from nearby blocks. After the agglomeration of the EKB and AXXB at the Cambrian, the Early Paleozoic post-collisional magmatic intrusions from ~ 490 to ~ 440 Ma were taken place on the newly formed EXB [START_REF] Wu | Discovery ofthe Early Paleozoic post-collisional granites in northern margin of theErguna massif and its geological significance[END_REF][START_REF] Ge | Emplacement age of the Tahegranite and its constraints on the tectonic nature of the Ergun block in thenorthern part of the Da Hinggan Range[END_REF][START_REF] Sorokin | Early Paleozoic Granitoids in the Lesser Khingan Terrane, Central Asian Foldbelt: Age, Geochemistry, and Geodynamic Interpretations[END_REF][START_REF] Ren | Zircon U-Pb Ages, Hf isotopic Characteristics of the Yongqinglinchang-Shibazhan Granites in the Northern Da Hinggan Mountains, Northeastern China[END_REF][START_REF] Guo | Discussion on the Geological characthristics and Mechanism of rock formation of the Giant Phenocryst adamellite in Southeast Mongolia[END_REF]. To the south, the Early Paleozoic Baolidao arc magmatism at the southern margin of the AXXB was dated at 498-415 Ma [START_REF] Jian | Time scale of an early to mid-Paleozoic orogenic cycle of the long-lived Central Asian Orogenic Belt, Inner Mongolia of China: implications for continental growth[END_REF][START_REF] Xu | Middle Paleozoic convergent orogenic belts in western Inner Mongolia (China): framework, kinematics, geochronology and implications for tectonic evolution of the Central Asian Orogenic Belt[END_REF]. These two kinds of magmatic rocks can both be considered as provenance regions for this zircon group. Notably, only one zircon from the western Chaganaobao Devonian sample (100826-01) presents an age around this age peak, with the main age group at 508 Ma (Fig. 6b), indicating the main provenance for the Devonian strata here is the EXB. However, the percentage of the Early Paleozoic zircon grains increases eastwards (Figs. 6b, 6d and6e). The easternmost Devonian sample from West Ujimqin (100723-23) presents the main age peak at 435 Ma, which is most probably from the southern Baolidao magmatic arc. This eastward tendancy indicates that the contribution of the Baolidao arc to the provenance of Devonian strata increases from west to east.

The Neoproterozoic peaks (800-1000 Ma) are subordinate in our results (Fig. 5), and they can't be used as an effective provenance indicator because this peak age represents the universal basement of nearly all blocks between NCC and Siberia Block (SIB). Neoproterozoic alkaline plutons have been identified from the EKB with the age of 950-737 Ma [START_REF] Wu | Geochronologyof the Phanerozoic granitoids in northeastern China[END_REF][START_REF] Tang | Geochronology and geochemistry of Neoproterozoic magmatism in the Erguna Massif, NE China: Petrogenesis and implications for the breakup of the Rodinia supercontinent[END_REF], which represent an extensive environment [START_REF] Tang | Geochronology and geochemistry of Neoproterozoic magmatism in the Erguna Massif, NE China: Petrogenesis and implications for the breakup of the Rodinia supercontinent[END_REF]. For the eastern Jiamusi Block, genisess and granitoids with the age of 913-777 Ma were reported at its western margin [START_REF] Xie | Zircon SHRIMP U-Pb dating of the amphibolite from "Heilongjiang Group" and the granite in Mudanjiang area, NE China, and its geological significance[END_REF]. This Neoproterozoic basement was also identified from the SHB by detrital zircon dating of Late Neoproterozoic-Cambrian metasedimentary rocks [START_REF] Xu | Middle Paleozoic convergent orogenic belts in western Inner Mongolia (China): framework, kinematics, geochronology and implications for tectonic evolution of the Central Asian Orogenic Belt[END_REF][START_REF] Wang | Precambrian terrane within the Songnen-Zhangguangcai Range Massif, NE China: Evidence from U-Pb ages of detrital zircons from the Dongfengshan and Tadong groups[END_REF]. For the AXXB, ca. 900 Ma gensiss were reported along the China-Mongolia border [START_REF] Wang | Geochronological evidence for existence of the south Mongolian microcontinent: a zircon U-Pb age of granitoid gneisses from the Yagan-Onch Hayrhan metamorphic core complex on the Sino-Mongolian border[END_REF]Zhou et al., 2013). The ca. 950 Ma magmatic events were also recorded in the central and southern Mongolia [START_REF] Kröner | Accretionary growth and crust-formation in the Central Asian Orogenic Belt and comparison with the Arabian-Nubian shield[END_REF]Demoux et al., 2009b). To the northern Tuva-Mongolia Block, a magmatism of the same age was also reported along the southern margin of SIB [START_REF] Kuzmichev | Neoproterozoic (∼800 Ma) orogeny in the Tuva-Mongolia Massif (Siberia): island arc-continent collision at the northeast Rodinia margin[END_REF][START_REF] Gladkochub | Signature of Precambrian extension events in the southern Siberian craton[END_REF]. These universal Neoproterozoic magmatic rocks in these blocks correspond to an important global magmatic event, which may be related to the assemblage and break-up of Rodinia supercontinent [START_REF] Li | Assembly, configuration, and break-up history of Rodinia: a synthesis[END_REF].

In summary, the presented results above indicate bidirectional provenances from the EXB, and the Early Paleozoic Baolidao arc to the south for the Ordovician-Devonian deposition. It provides new clue to decipher the tectonic evolution of this region.

Tectonic implications

A new model was proposed based on our new geochological data to interpreate the Early Paleozoic tectonic evolution of the AXXB with its neighbor blocks.

The collision between EKB and AXXB occurred before the Late Cambrian, followed by Late Cambrian granitic and mafic intrusions [START_REF] Wu | Discovery ofthe Early Paleozoic post-collisional granites in northern margin of theErguna massif and its geological significance[END_REF][START_REF] Ge | Emplacement age of the Tahegranite and its constraints on the tectonic nature of the Ergun block in thenorthern part of the Da Hinggan Range[END_REF][START_REF] Wu | Geochronologyof the Phanerozoic granitoids in northeastern China[END_REF] on both side of Xinlin-Xiguitu suture zone (Fig. 7a). During this period, the southern margin of this new formed block was an passive continental margin, and separated from the southern SHB by the Paleo-Asian Ocean (Fig. 7a).

After that, the new welded Erguna-Xing'an Block (EXB) came into a epicontinental setting, with Ordovician-Silurian sedimentary basin developed accompanied with coeval magmatic intrusions (Fig. 7b;[START_REF] Imbgmr | Regional geology of Inner Mongolian Autonomous Region[END_REF][START_REF] Ge | Zircon U-Pb ages, Hf isotopic characteristics and their implications of the Early Paleozoic granites in the northeastern Da Hinggan Mts., northeastern China[END_REF][START_REF] Sorokin | Early Paleozoic granitoids in the Eastern Margin of the Argun' Terrane, Amur area: first geochemical and geochronologic data[END_REF]2011;[START_REF] Ren | Zircon U-Pb Ages, Hf isotopic Characteristics of the Yongqinglinchang-Shibazhan Granites in the Northern Da Hinggan Mountains, Northeastern China[END_REF][START_REF] Guo | Discussion on the Geological characthristics and Mechanism of rock formation of the Giant Phenocryst adamellite in Southeast Mongolia[END_REF]. The Cambrian (510-485 Ma) orogenic related granitic and mafic intrusions and subsequent Ordovician-Silurian (485-440 Ma) magmatic intrusions provide provenance for the basin deposition (Fig. 7b). Ever since the Paleo-Asian Ocean subducted beneath the southern margin of the welded EXB, the Baolidao arc formed along this margin from ~ 490 Ma to ~ 420 Ma, with the peak at ~ 460 Ma [START_REF] Shi | Zircon SHRIMP dating of K-rich granites in Sonidzuoqi, central Inner Mongolia[END_REF][START_REF] Jian | Time scale of an early to mid-Paleozoic orogenic cycle of the long-lived Central Asian Orogenic Belt, Inner Mongolia of China: implications for continental growth[END_REF][START_REF] Xu | Middle Paleozoic convergent orogenic belts in western Inner Mongolia (China): framework, kinematics, geochronology and implications for tectonic evolution of the Central Asian Orogenic Belt[END_REF]. The Baolidao arc granitoids intruded into the Xilinhot precambrian basement, and extensive thermal metamorphism related to the arc magmatism occurred in the precambian basement (Sun et al., 2013b), which also made contribution for the provenance of the basin deposition (Fig. 7).

During the Late Silurian, the magmatic events on the EXB have ceased, and it was dominated by the widespread clastic and carbonate deposits without volcanic component (Fig. 2). The striking feature of these Upper Silurian strata is the Tuvaella funa therein, which can be traced from nearly all the Upper Silurian strata from the EXB [START_REF] Su | The distribution and entironment of Tuvaella fauna[END_REF][START_REF] Imbgmr | Regional geology of Inner Mongolian Autonomous Region[END_REF], marking the stablization of this new formed block (Fig. 7c; [START_REF] Wang | Tectonic implication of the Late Paleozoic stratigraphic distribution in Northeast China and adjacent region[END_REF]. To the south, the subduction of the Paleo-Asian Ocean was still going, but the Baolidao Arc magmatism ceased at the Late Silurian (Fig. 7c), represented by ~ 420 Ma high-K granite [START_REF] Shi | Zircon SHRIMP dating of K-rich granites in Sonidzuoqi, central Inner Mongolia[END_REF][START_REF] Jian | Time scale of an early to mid-Paleozoic orogenic cycle of the long-lived Central Asian Orogenic Belt, Inner Mongolia of China: implications for continental growth[END_REF].

During the Early-Middle Devonian, the marine sedimentary basins on the EXB show significant lateral lithological variations from west to east (Fig. 2). At the Chaganaobao area, the distinctive red basal conglomerate (Figs. 3f and7) represents a regional unconformity, indicating that the sedimentary basin might have been closed before the Early Devonian in this area. However, to the east, the Early-Middle Devonian strata are mainly composed of shallow marine sandstone with abundant of shallow marine fossils, indicating that the marine sedimentary basins still exist.

These variations indicate an Early-Middle Devonian eastaward regression, revealing an eastward successive closure of the marine basin (Fig. 7d). During the Late Devonian, due to the collision between the SHB and the EXB, the Late Devonian molassic deposition uncomformbly overlied the ophiolitic melange [START_REF] Xu | Middle Paleozoic convergent orogenic belts in western Inner Mongolia (China): framework, kinematics, geochronology and implications for tectonic evolution of the Central Asian Orogenic Belt[END_REF]Fig. 7d). This collision also caused the final closure of the marine sedimentary basins on the EXB, as no Late Devonian sediments were found to the west and the plant fossils bearing continental Upper Devonian Angeryinwula Formation occurred to the east (West Ujimqin area) (IMBGR, 2007; Fig. 7d). After the Late Devonian the EXB came into a subaerial setting without sedimentation until the Late Carboniferous continental deposition [START_REF] Imbgmr | Regional geology of Inner Mongolian Autonomous Region[END_REF].

Conclusions

(1) The detrital zircon geochronological study of the five Ordovician-Devonian samples from Erenhot-West Ujimqin (the southeastern part of the China-Mongolia [START_REF] Li | Detrital zircon U-Pb ages, Hf isotopesand tectonic implications for Palaeozoic sedimentary rocks from the Xing-Meng orogenic belt, middle-east part of inner Mongolia, China[END_REF], northeastern Xing'an Block (c; Han et al., 2011), Mongolia (d;Rojas-Agramonte et al., 2011) and Hunshandake Block (e; [START_REF] Xu | Middle Paleozoic convergent orogenic belts in western Inner Mongolia (China): framework, kinematics, geochronology and implications for tectonic evolution of the Central Asian Orogenic Belt[END_REF]. unconformbly overlies the ophiolitic mélange [START_REF] Xu | Middle Paleozoic convergent orogenic belts in western Inner Mongolia (China): framework, kinematics, geochronology and implications for tectonic evolution of the Central Asian Orogenic Belt[END_REF]. Meanwhile, the marine basins on the Erguna-Xing'an Block were closed as no Late Devonian to the west and Late Devonian continental facies sediments to the east.

Figure 1

 1 Figure 1. a. Tectonic sketch map of Northeastern China, modified after Wu et al.,

Figure 2 .

 2 Figure 2. Stratigraphic column of Ordovician to Devonian strata from Chaganaobao,

Figure 3

 3 Figure 3. a-e, Field photographs and photomicrographs of dated strata, with number,

Figure 4 .

 4 Figure 4. Cathodoluminescence (CL) images of selected detrital zircons from each

Figure 5 .

 5 Figure 5. U-Pb concordia and probability diagrams of zircon ages from Ordovician to

Figure 6 .

 6 Figure 6. Comparison of probability plots for the data from southwestern Xing'an

Figure 7 .

 7 Figure 7. Schematic geodynamic evolutionary model of the northwestern margin of

Fig. 1 a

 1 Fig. 1 a. Tectonic sketch map of Northeastern China, modified after Wu et al., (2007), Xu et al., (2013) and Tomurtogoo, (2013). b. Geological map of southwestern margin of Airgin Sum-Xilinhot-Xing'an Block from Erenhot to Dongwuqi, after IMBGMR, (1991).

Fig. 2

 2 Fig. 2 Stratigraphic column of Ordovician to Devonian strata from Chaganaobao, Narenbaolige and West Ujimqin area, showing lithology and sample sites.

Fig. 3

 3 Fig. 3. a-e, Field photographs and photomicrographs of dated strata, with number, age and lithology at the top. f, basal conglomerate of Lower-Middle Devonian Niqiuhe Formation at Chagenaobao area.

Acknowledgments

We thank Shi Guanzhong and Li Ruibiao for their support in the field. This work has been funded by the National Key Basic Research Program of China (2013CB429800), the National Science Foundation of China (40872145 and 41121062), and the China Geological Survey Grant (no. 1212011220906).

border) display the similar age distribution patterns with age groups at ~440 Ma, ~510 Ma, ~800 Ma,~950 Ma, some Meso-to Paleo-proterozoic and Neoarchean grains.

(2) This age distribution display the same patterns with the magmatic periods of Erguna-Xing'an Block. Meanwhile, the youngest age group is well consistent with the Bolidao Arc magmatism at the southern margin of the Erguna-Xing'an Block.

Hence, a bidirectional provenances from the Erguna-Xing'an Block and Baolidao Arc was proposed.

(3) All our data constrain two main Early Paleozoic tectonic events of the Xing-Meng Orogenic Belt: pre-Late Cambrian collision between Erguna-Kerulen Block and Arigin Sum-Xilinhot-Xing'an Block; the Early Paleozoic subduction of Paleo-Asian Ocean and pre-Late Devonian collision between Erguna-Xing'an Block and Songliao-Hunshandake Block.

(4) A new model was proposed to highlight this Early Paleozoic tectonic evolution. The collision between Erguna-Kerulen Block and Arigin Sum-Xilinhot-Xing'an Block occurred before the Late Cambrian, with Late Cambrian granitic and mafic intrusions along both sides of the suture zone. After that, sedimentary basins developed in the Erguna-Xing'an Block during the Ordovician-Silurian accompanying with coeval magmatic intrusions, which provided provenance for sedimentary basins together with the Late Cambrian intrusions.

Meanwhile, the northward subduction of the Paleo-Asian Ocean beneath the Arigin Sum-Xilinhot-Xing'an Block was from ~490 Ma to ~420 Ma, producing the Baolidao Arc magmatism along its southern margin, which also made contribution for the provenance. The eastward successive closure of marine sedimentary basins on the Arigin Sum-Xilinhot-Xing'an Block took place at the Early Devonian to the west and Late Devonian to the east, due to the pre-Late Devonian collision between the Songliao-Hunshandake Block and the Arigin Sum-Xilinhot-Xing'an Block. producing Baolidao Arc at its southern margin. Meanwhile, on the new formed Erguna-Xing'an Block, Ordovician-Silurian magmatic rocks intruded and extensional sedimentary basins developed. The contemporary Baolidao arc magmatic rocks and post-orogenic intrusions provided provenance for the sedimentary basins. c. Late Silurian Tuvaella fauna bearing sediments widespread on the Erguna-Xing'an Block, representing the stabilization of this welded block. The Baolidao arc magmatism might ceased during this period. d. During the Early-Middle Devonian, this belt underwent different evolution from west to east. To the west, the basal conglomerate represents the closure of this basin and cease of northward subduction. While to the east, shallow marine sediments still exists until the Late Devonian, indicating eastward successive closure of the sedimentary basin. As the pre-Late Devonian collision between Songliao-Hunshandake Block and Airgin Sum-Xilinhot-Xing'an Block along the Sunidzuoqi-Xilinhot-Heihe suture zone (SXHS), the Upper Devonian molassic sediments unconformbly overlies the ophiolitic mélange [START_REF] Xu | Middle Paleozoic convergent orogenic belts in western Inner Mongolia (China): framework, kinematics, geochronology and implications for tectonic evolution of the Central Asian Orogenic Belt[END_REF]. Meanwhile, the marine basins on the Erguna-Xing'an Block were closed as no Late Devonian to the west and Late Devonian continental facies sediments to the east.