Introduction

Accretionary orogens are formed through continuous oceanic lithosphere subduction at the convergent plate margins, which have played an irreplaceable role in continental growth throughout Earth’s history (Cawood et al. 2009; Condie 2007). The Central Asian Orogenic Belt (CAOB, or Altaids), surrounded by the Baltica Craton to the northwest, the Siberian Craton to the north, and the Karakum, Tarim, and North China cratons to the south, is considered as one of the largest Phanerozoic accretionary orogens in the world (Fig. 1; Jahn et al. 2000; Şengör and Natal’in 1996; Şengör et al. 1993; Windley et al. 2007; Xiao et al. 2015a). The CAOB has undergone ~ 800 Ma tectonic evolution from Neoproterozoic to Early Mesozoic (Khain et al. 2002; Kröner et al. 2007; Xiao et al. 2009), and was built by sustaining accretion of subduction–accretion complexes, magmatic arcs, arc-related basins, ophiolites, seamounts, and continental fragments during the subduction and final closure of the Paleo-Asian Ocean (PAO) (Kröner et al. 2007; Safonova et al. 2017; Windley et al. 2007; Xiao et al. 2015a; Zhou et al. 2018c). The South Tianshan, Beishan, Alxa, and Solonker sutures along the southernmost CAOB have been considered as the position for the final closure of the PAO, and a large number of studies have focused on the Late Paleozoic–Early Mesozoic tectonic evolution of these regions to constrain the terminal amalgamation of the CAOB (Liu et al. 2017b; Xiao et al. 2003, 2009, 2013; Zhang et al. 2013c).

Fig. 1
figure 1

Tectonic map of Central Asian Orogenic Belt and its surrounding cratons (modified after Şengör and Natal’in 1996, Xiao et al. 2015a, Song et al. 2018 and Zhang et al. 2014), showing the spatial–temporal distribution characteristics of the main tectonic units in the Central Asian Orogenic Belt. Along the southernmost Central Asian Orogenic Belt, the major tectonic zones include Central Tianshan Arc, South Tianshan Suture Zone, Beishan, Alxa, Solonker Suture Zone and Bainaimiao Arc from west to east

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The Alxa Tectonic Belt, situated between the Beishan orogen in the west and the Solonker suture zone in the east (Fig. 1), contains abundant Late Paleozoic magmatic–sedimentary records and ophiolitic mélanges related to amalgamation of the southern CAOB (Feng et al. 2013; Song et al. 2019, 2018; Zheng et al. 2014). However, the Early Paleozoic tectonic evolutionary history of the Alxa Tectonic Belt is largely unknown. Early Paleozoic strata in the Zhusileng and Hangwula areas in the northern Alxa were considered as passive continental margin deposits (Wu and He 1993). Zheng et al. (2016) reported Silurian intrusions from the Engger Us area, and interpreted them as arc magmatism. Ordovician to Early Devonian arc-related plutons in the southwestern Alxa were considered to be related to southward subduction of the PAO (Liu et al. 2016b). The Early Paleozoic tectonic history of the Alxa Tectonic Belt needs further constraints by comparing with adjacent regions. To the east of the Alxa Tectonic Belt, an Early Paleozoic magmatic arc belt, the Bainaimiao Arc, developed between the Solonker suture zone to the north and the North China Craton to the south (Xiao et al. 2003; Fig. 2). However, intense controversies remain on the subduction polarity and tectonic origin of the Bainaimiao Arc, including the southward subduction of the Paleo-Asian Ocean evidenced by Ondor sum subduction–accretion complexes to the north of the Bainaimiao Arc (Xiao et al. 2003), or the northward subduction of the South Bainaimiao Ocean supported by the Wude–Chegandalai ophiolitic mélanges to the south of it (Zhang et al. 2014). The along-strike tectonic correlation between the Alxa Tectonic Belt and the Bainaimiao Arc is important for a better understanding of the Early Paleozoic tectonic evolution of the eastern segment of the southern CAOB.

Fig. 2
figure 2

Modified from Liu et al. (2016b), Liu et al. (2020), Guy et al. (2014) and Taylor et al. (2013)

A Tectonic map of southeastern part of Central Asian Orogenic Belt and adjacent regions showing situations of the major tectonic units including the Southern Mongolian Collage System, the Northern Accretionary Orogen, the Engger Us Suture Zone, the Solonker Suture Zone, the Alxa Tectonic Belt, the Bainaimiao Arc, the Qilian Orogenic Belt and the North China Craton. B Simplified geological map of Alxa-northern margin of the North China Craton showing major lithostratigraphic composition. The location of the study area of Fig. 3 is indicated.

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The Ganqimaodu area connects the Alxa Tectonic Belt with the Bainaimiao Arc along the southern CAOB (Fig. 2B). It consists mainly of early Paleozoic igneous rocks, (meta)sedimentary rocks and ophiolitic mélanges (Anonymous 1980a, b; Xu et al. 2013; Fig. 3). The tectonic setting of early Paleozoic strata in the Ganqimaodu area is still unclear and is intensely debated. Xu et al. (2013) suggested that the Ordovician–Silurian Ondor sum Group represents the passive margin deposition of the Hunshandake Block and the early–middle Silurian Hadahushu Group deposited in a collision-related foreland basin. However, Xiao et al. (2003) interpreted the Ondor sum Group as subduction–accretion complexes by southward subduction of the PAO. The Hadahushu Group was also suggested as back-arc basin deposition above the south-dipping subduction zone beneath the North China Craton (Tian et al. 2019e). In this study, we integrate field geology with zircon U–Pb–Hf isotopic analyses for (meta)sedimentary rocks from the Ganqimaodu area to determine their provenance and tectonic setting. Our new data combined with previous studies provide invaluable information for the early Paleozoic tectonic framework and accretionary process of the southern CAOB.

Fig. 3
figure 3

modified from Anonymous 1980a, b Xu et al. 2013 and personal field observation). Previous data are from Tian et al. (2019e), Xu et al. (2013), Zhang et al. (2015) and Zhang et al. (2019b)

Detailed geological map of the Ganqimaodu area (

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Geologic background

The Ganqimaodu area

The Ganqimaodu area is located in a key position in the southern CAOB connecting the Southern Mongolian accretionary system to the north, the Alxa Tectonic Belt to the southwest, and the Bainaimiao Arc to the east (Figs. 2B; 3). According to Anonymous (1980a, b), strata in the Ganqimaodu area mainly include: Mesoproterozoic quartzite, marble and garnet schist, Early–Middle Paleozoic Ondor sum Group greenschist-facies clastic rocks, Early–Middle Silurian slightly metamorphic sedimentary rocks (the Hadahushu Group), and a small amount of Carboniferous–Permian strata. Recent detrital zircon U–Pb dating indicate that the Ondor sum Group was deposited in the early Paleozoic, with the lower part at 470–455 Ma and the upper part at 455–415 Ma (Xu et al. 2016). The Silurian Hadahushu Group was suggested to be deposited after early Silurian (~ 438 Ma) (Tian et al. 2019e). Numerous Mesoproterozoic to Late Paleozoic plutons intruded into these strata (Anonymous 1980a; Sun et al. 2013, 2018). Early Paleozoic intrusions in the Ganqimaodu area were considered as products of the southward subduction of the PAO beneath the North China Craton (Xu et al. 2013; Zhang et al. 2019b).

The Alxa Tectonic Belt

The Alxa Tectonic Belt (Fig. 2A, B) is a triangle-shaped region separated from the North China Craton to the east by the Langshan fault and from the Qilian Orogenic Belt to the southwest by the Longshoushan thrust. To the north, the Alxa Tectonic Belt connects the Southern Mongolian accretionary system. Three main faults in the Alxa Tectonic Belt are: the Yagan fault, the Engger Us fault, and the Quagan Qulu fault (Wu and He 1993). These faults divide the Alxa Tectonic Belt into four tectonic zones: Yagan, Zhusileng–Hangwula, Shalazhashan, and Nuru–Langshan.

The Yagan Tectonic Zone lies to the north of the Yagan fault. Neoproterozoic strata consisting of marble and metasandstone in the northeast of Ejin Banner were intruded by a Neoproterozoic (~ 889 Ma) highly fractionated potassium-rich granite (Zhang et al. 2016b). The Middle Ordovician intermediate-basic to intermediate-acid volcanic rocks may represent an intra-oceanic arc (Wu and He 1993). There are also thick Late Paleozoic volcanic–sedimentary successions which were emplaced by numerous Carboniferous–Permian granitoids (Zhang et al. 2017b; Zheng et al. 2013).

Between the Yagan fault and Engger Us fault is the Zhusileng–Hangwula Tectonic Zone. This zone contains relatively complete strata from Neoproterozoic to Mesozoic with intense magmatism during the Late Paleozoic (Fei et al. 2019). A few Neoproterozoic granitoids have recently been reported (Wang et al. 2001; Zhou et al. 2013). Early Paleozoic strata composed of shallow-marine clastic rocks were previously considered as passive continental margin deposition, but recent detrital zircon studies suggested that the provenance of Ordovician to Devonian strata might be the northern microcontinents or arcs in the southern CAOB (Chen et al. 2019; Yin et al. 2015).

The Shalazhashan Tectonic Zone, located between the Engger Us fault to the north and the Quagan Qulu fault to the south, is mainly composed of Permian intrusions and Carboniferous–Permian volcanic and sedimentary rocks (Shi et al. 2014, 2018; Yin et al. 2016; Zhang et al. 2013c). A Mesoproterozoic (~ 1.4 Ga) orthogneiss has been reported in this tectonic zone (Shi et al. 2016).

The Nuru–Langshan Tectonic Zone contains numerous Precambrian rocks that comprise orthogneiss and paragneiss, indicating multi-phase magmatic–metamorphic events from Neoarchean to Neoproterozoic (Dan et al. 2012, 2014b; Gong et al. 2016; Shen et al. 2005; Song et al. 2017; Wu et al. 2014; Zhang et al. 2013a). A few metamorphic intrusions from the Langshan area have been reported, including 2.7–2.56 Ga granitic gneisses, 1.71–1.64 Ga metamorphic granites, and 914–908 Ma mafic intrusions (Bao et al. 2019; Liu 2012; Sun et al. 2018; Wang et al. 2016a). The Langshan Group consist of greenschist-facies metamorphosed clastic rocks, carbonate rocks, and interbedded volcanic rocks. Previous studies showed that the Langshan Group was deposited from ~ 1187 to 804 Ma (Hu et al. 2014; Liu et al. 2019a; Tian et al. d). Ordovician–Silurian diorites and granitoid intrusions were interpreted as resulting from southward subduction of the PAO (Dan et al. 2016; Liu et al. 2016b; Teng et al. 2019; Tian et al. 2019a, 2019c; Wang et al. 2015b). A large number of plutons were emplaced in the Carboniferous–Permian, including gabbros, diorites, granodiorites, and granitoids (Dan et al. 2014a; Feng et al. 2013; Liu et al. 2016a, 2017b, 2019b; Song et al. 2019; Tian et al. 2019b; Wang et al. 2015b). Although early Paleozoic strata are lacking in this area, a few late Paleozoic volcanic–sedimentary rocks from the Beidashan area in the southwest of this zone may provide a key constraint for the final subduction processes of the PAO (Song et al. 2018).

Ophiolitic mélanges with block-in-matrix structure are distributed along both the Engger Us and Qugan Qulu faults. Based on geochronological and geochemical studies of exotic blocks within the ophiolitic mélanges, Zheng et al. (2014) suggested that the Engger Us ophiolite represents the Late Paleozoic main suture of the PAO, while Qugan Qulu ophiolite formed in a back-arc basin. The Yagan fault extends about 100 km from west to east in the Alxa region, but most of it is covered by Badain Jaran Deser (Wu and He 1993).

The Bainaimiao Arc

The Bainaimiao Arc (Fig. 2A, B) is situated on the northern margin of the North China Craton bounded by the E–W-trending Bayan Obo-Chifeng fault. The Bainaimiao Arc is dominated by early Paleozoic magmatic and sedimentary rocks with minor amounts of Precambrian rocks. Zircon Hf isotopic data (εHf(t) values vary from − 9.3 to 11.4; two-stage Hf model ages are mainly 0.7–1.29 Ga and 1.39–2.0 Ga) from these early Paleozoic magmatic rocks and Proterozoic detrital zircons (ca. 0.6–1.2 Ga and 1.3–2.0 Ga) from contemporaneous (meta)sedimentary rocks indicate that the Bainaimiao Arc was built on a Precambrian basement (Chen et al. 2020; Zhang et al. 2014; Zhou et al. 2018b, 2020). However, the tectonic affinity of the basement for the Bainiamiao Arc is still controversial. Most scholars considered that the Bainaimiao Arc is a continental arc on the North China Craton related to the southward subduction of the PAO (Xiao et al. 2003; Xu et al. 2013), while others proposed that the basement of the Bainaimiao Arc has an affinity to the Tarim Craton (Zhang et al. 2014; Zhou et al. 2020). The early Paleozoic plutons in the Bainaimiao Arc can be roughly divided into two types based on their petrography and isotopic compositions: intrusions older than ~ 450 Ma are intermediate-basic with positive εHf(t) values, and those younger than ~ 450 Ma are intermediate-acidic with both positive and negative εHf(t) values (Liu et al. 2020). Early Paleozoic volcanic rocks include basalt, andesite, dacite, and tuff, which have been dated at ~ 518–404 Ma (Liu et al. 2003, 2013; Ma et al. 2019; Qian et al. 2017; Zhang et al. 2013b, 2014, 2017a, 2019a; Zhou et al. 2020). The εHf(t) values of these magmatic rocks generally decreased with decreasing age until ~ 415 Ma (Chen et al. 2020). Early Paleozoic strata are composed of Ordovician to Silurian metasedimentary rocks collectively termed the Baoerhantu Group and Bainaimiao Group (Zhang et al. 2014). The early Silurian Xuniwusu Formation represents a set of flysch deposition consisting of clastic rocks with carbonate and tuff interlayers (Zhang et al. 2017a). The latest Silurian–early Devonian Xibiehe Formation was considered as molasse deposition unconformably on the early Paleozoic ophiolitic mélanges, arc-related plutons, and sedimentary rocks of the Bainaimiao Arc (Wang et al. 2020; Zhang et al. 2010).

The Southern Mongolian accretionary system

The Southern Mongolian accretionary system here refers to the domain that lies to the south of the Main Mongolian Lineament (Fig. 2A). It is divisible into four zones from north to south: the Lake zone, the Gobi-Altai zone, the Trans-Altai zone, and the South Gobi zone. The Lake zone is mainly composed of Precambrian basement with Early Cambrian ophiolitic mélanges and eclogites and Cambrian-Ordovician arc-related magmatic rocks (Lehmann et al. 2010). The Gobi-Altai zone has a Cambrian crystalline basement unconformably overlain by a thick Early–Middle Ordovician sedimentary–volcanic sequence, followed by a Silurian–Devonian passive margin deposition which was intruded by Devonian–Permian Japan-type magmatic arc (Badarch et al. 2002; Lehmann et al. 2010). The Trans-Altai zone, separated from the Gobi-Altai zone by the Trans-Altai fault to the north and bounded by the Gobi-Tianshan fault to the south, largely consists of dismembered ophiolitic mélange containing serpentinized peridotite, gabbro and pillow basalt, overlain by Late Silurian–Early Devonian radiolarian cherts, andesitic and basaltic volcanic rocks and Middle Devonian to Carboniferous volcanic–sedimentary rocks (Badarch et al. 2002; Guy et al. 2014). The South Gobi zone occupies the southernmost part of Mongolia, separated from the Trans-Gobi zone by the Gobi-Tianshan fault. Badarch et al. (2002) classified this zone as a cratonal terrane with a Precambrian basement. Now it is considered as the South Mongolian Microcontinent evidenced by some Neoproterozoic granitoids or granitic gneisses (Wang et al. 2001; Yarmolyuk et al. 2005; Zhang et al. 2016b). Ordovician to Silurian siliceous clastic rocks, Early Devonian to Carboniferous volcanic and sedimentary rocks deposited extensively above the basement (Lehmann et al. 2010). However, Taylor et al. (2013) found that all so-called Precambrian metamorphic rocks formed from Carboniferous to Triassic based on zircon U–Pb dating, and argued that the Southern Mongolia is a Paleozoic arc rather than a microcontinent.

Field geology, sampling and petrology

To investigate the early Paleozoic accretionary history of the southern CAOB, a systematic study including field geology and zircon U–Pb–Hf isotopic analyses were carried out for the early Paleozoic rocks in the Ganqimaodu region. The locations for sampling are indicated in Figs. 3 and 4.

Fig. 4
figure 4

modified from Anonymous (1980a,, b), Tian et al. (2019e) and our own observation). Field and microscopic photographs of these sedimentary rocks in the study area are shown: (a, b) sample 20ZH23, quartz sandstone; (c, d) sample 20ZH24, quartz sandstone; (e, f) sample 20ZH16, metamorphosed quartz sandstone; (g, h) sample 20ZH13, chlorite–quartz schist; (i, j) sample 20ZH27, chlorite–quartz schist. All microscopic photos are under cross-polarized light. Q, quartz; Bt, biotite; Chl, chlorite

Stratigraphic column of the Early Paleozoic strata in the Ganqimaodu area (

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The Hadahushu Group, unconformably overlying the Ondor sum Group (Xu et al. 2013), can be divided into three parts based on lithologic assemblages (Anonymous 1980a; b; Fig. 4): a lower part of conglomerate, slate, thin-bedded sandstone and siltstone, and limestone lenticles along with coral fossils; a middle part of interbedded sandstone and phyllite; an upper part of slate, limestone and siltstone. Ophiolitic mélanges are found within the lower part of the Hadahushu Group where limestone and basic volcanic rock blocks were wrapped in the (meta)sedimentary rocks (Fig. 6a, b, d). A ~ 180 m long measured section (Fig. 5) of the Hadahushu Group comprises interbedded sandstone, siltstone and mudstone. Three samples were collected from the lower part of the Silurian Hadahushu Group. Samples 20ZH23 (Fig. 4a, b) and 20ZH24 (Fig. 4c. d) are quartz sandstones from the cross section described above (Fig. 5). These sandstones mainly consist of quartz and biotite. Sample 20ZH16 (Fig. 4e, f) is a metamorphosed quartz sandstone and mainly composed of fine-grain quartz and biotite with NW dipping foliation.

Fig. 5
figure 5

Field features and sketch of stratum section of the Hadahushu Group flysch deposits. The location is indicated in Fig. 3

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According to Anonymous (1980a, b), the Ondor sum Group in the Ganqimaodu area are characterized by greenschist intercalated with quartzite and basic volcanic rock, and are divisible into the lower and upper parts with slightly lithologic variations (Fig. 4). The lower part primarily consists of chlorite–quartz schist, sericite–quartz schist and quartzite, and the upper part comprises metasandstone with small limestone lens. Ophiolitic mélanges also exist widely in the lower part of the Ondor sum Group strata, which contain abundant blocks such as ultramafic rocks, basic volcanic rocks, limestones, pillow basalts, and siliceous rocks (Fig. 6c, f, g, h). In addition, the Ondor sum Group is deformed strongly with a foliation dipping NNW (Fig. 6e). Two chlorite–quartz schists (Sample 20ZH13, Fig. 4g, h; and 20ZH27, Fig. 4i, j) mainly consisting of fine-grained quartz and minor chlorite are taken from the lower part of Ondor sum Group.

Fig. 6
figure 6

Field characteristics of ophiolitic mélange within the early Paleozoic strata in the Ganqimaodu area. a limestone block within quartz schist and folded together; b blocks of the basic rocks in quartz schist; c block of carbonatation ultramafic rock in chlorite–quartz schist; d limestones as blocks within metasandstone matrix; e intense deformed strata of the Ondor sum Group, quartz veins developed along the foliation and folded; f block-in-matrix structure: basic rocks as the blocks and mica quartz schist as the matrix; g pillow basalt within the strata of the Ondor sum Group; h red siliceous rocks encased within pillow basalt. The locations of all field photographs are shown in Fig. 3

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Analytical methods

Zircon U–Pb dating

Zircon grains were separated by conventional magnetic and heavy liquid methods and hand-picked under a binocular microscope. Then these grains were randomly selected to mounted on adhesive tape, followed by enclosing in epoxy resin and polishing to about one third to one half of their thickness. To understand the origin and internal structure of these zircons and to select the most suitable target spots for U–Pb dating and Hf isotopic analyses, Cathodoluminescence (CL) imaging was operated with a scanning electron microscope at the Institute of Geology and Geophysics, Chinese Academy of Sciences (IGGCAS) in Beijing.

Zircon U–Pb dating was carried out at the IGGCAS using an Agilent 7500a Quadrupole-ICP-MS with a GeoLasHD 193 nm ArF excimer laser-ablation system. The diameter of laser ablation spot is 32 μm with the density of energy of 4.0 J/cm2 at 71.8 mJ output of energy. Each spot analysis comprised 20 s of gas background followed by 50 s of sample ablation. Zircon 91,500 was used as external standard for age calculation, which measured twice every ten analyses. NIST SRM 610 and ARM were used as external standard for concentration calculation and another reference zircon SA01 was analyzed as an unknown to monitor the quality of age date. The more instrumental settings and detailed analytical procedures can be found in Wu et al. (2007) and Xie et al. (2008). Isotopic concentrations and ratios were calculated using the GLITTER program (Macquarie University). Common Pb was corrected according to the method proposed by Andersen (2002). The age calculations and plotting of Concordia diagrams were made using ISOPLOT 3.0 (Ludwig 2003), and DensityPlotter 7.2 was used to generate probability density and kernel distributions (Vermeesch 2012). In this study, 207Pb/206Pb ages are reported for zircons older than 1000 Ma, while 206Pb/238U ages are reported for zircons younger than 1000 Ma.

Zircon Lu–Hf isotopic analysis

Zircon in situ Hf isotope analysis was carried out using a Resolution SE 193 laser-ablation system attached to a Thermo Fisher Scientific Neptune Plus MC-ICP-MS at Beijing ZKKY Technology Co., Ltd. Instrumental conditions and data acquisition protocols were described by Hou et al. (2007). Lu–Hf isotopic compositions were analyzed for zircons near the same locations where U–Pb analyses were carried out. A beam diameter of 44 μm was used for all the samples. The repetition rate was 10 Hz and the density of energy was 8 J/cm2. As the carrier gas, Helium was used to transport the ablated material mixed with Argon from the laser-ablation cell to the MC-ICP-MS torch by a mixing chamber. Yb isotope ratios were normalized to 173Yb/172Yb = 1.35274 and Hf isotope ratios were normalized to 179Hf/177Hf = 0.7325 using an exponential law for instrumental mass bias correction (Wu et al. 2006). Due to the 176Lu/177Hf ratios in zircon is usually less than 0.002, the isobaric interference of 176Hf isomers is mainly caused by 176Yb, so the mean βYb value and the 176Yb/172Yb ratio of 0.5887 were applied for the Yb correction (Iizuka and Hirata 2005; Wu et al. 2006). Zircon international standard GJ-1 was used as the reference standard, with a weighted mean 176Hf/177Hf ratio of 0.282006 ± 21 (2σ, n = 16). The measured 176Lu/177Hf ratios and the 176Lu decay constant of 1.867 × 10–11 yr−1 proposed by Söderlund et al. (2004) were used to calculate initial 176Hf/177Hf ratios. The present chondritic values of 176Hf/177Hf = 0.282772 and 176Lu/177Hf = 0.0332 (Blichert-Toft and Albarède 1997) were used for the calculation of εHf(t) values. Model ages (TDM, TDMc) were calculated by the current 176Hf/177Hf = 0.28325 and 176Lu/177Hf = 0.0384 of depleted mantle (Griffin et al. 2000) and the average continental ratios of 176Lu/177Hf = 0.015 reported by Griffin et al. (2002).

Results

Samples from Hadahushu group

Sample 20ZH23: Zircons from this sample are generally euhedral and prismatic in shape with oscillatory zones (Fig. 7A), indicating a magmatic origin. Ninety zircons were randomly selected for U–Pb dating, and 65 grains of them have concordant ages (Fig. 8A, B; Table S1). The Th/U ratios of these zircons range from 0.17 to 2.32 except for two grains (0.04 and 0.07). Approximately one third of the concordant ages are early Paleozoic ranging from ~ 421 to ~ 524 Ma with a prominent peak at 446 Ma. Precambrian zircon ages can be fall into five populations: 591–934 Ma (peak ages at 586 Ma and 910 Ma), 1010–1360 Ma (peak at 1243 Ma), 1423–1650 Ma (peak at 1465 Ma), 1714–2234 Ma (peak at 1825 Ma), and another two grains have Archean ages of 2524 Ma and 2732 Ma.

Fig. 7
figure 7

Cathodoluminescence (CL) images for representative zircons from samples in this study. The white and yellow circles are analysis situs of U–Pb and Lu–Hf isotope, respectively. The white numbers are U–Pb ages with 2σ uncertainty, and the yellow numbers are εHf(t) values

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Fig. 8
figure 8

U–Pb concordia diagrams and Kernel Density Estimate (KDE) for detrital zircons from the (meta)sedimentary rocks in this study

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Twenty grains with concordant ages were analyzed for Lu–Hf isotopic compositions (Fig. 10; Table S2). The εHf(t) values of early Paleozoic zircons range from -23.9 to + 5.9, with the two-stage Hf model ages (TDMc) ranging from 1036 to 2923 Ma. The Neoproterozoic to Paleoproterozoic zircons yield mostly positive εHf(t) values from 0.7 to + 13.4, and only four of them have negative εHf(t) values from − 1.7 to − 12.2.

Sample 20ZH24: zircons from this sample have subrounded to prismatic shape with clear oscillatory zones and their length–width ratios ranging from 1:1 to 3:1 (Fig. 7B). Ninety zircons were selected randomly for U–Pb isotopic analyses of which 83 grains yield concordant ages from 418 to 2882 Ma (Fig. 8C, D; Table S1). Their Th/U ratios vary from 0.16 to 1.26 except one zircon of 0.09. Twenty-eight (30%) of them are early Paleozoic ages from 418 to 532 Ma with a prominent peak at 445 Ma. Two youngest zircons have ages of 418 ± 4 Ma and 421 ± 4 Ma. Precambrian zircons are grouped as: 563–608 Ma (peak at 580 Ma), 833–1673 Ma (peak at 1068 Ma), 1730–1944 Ma (peak at 1765 Ma), 2233–2467 Ma, and 2560–2882 Ma.

Lu–Hf isotopic analyses were performed on twenty-three zircons with concordant ages (Fig. 10; Table S2). Most early Paleozoic zircon grains (~ 70%) yield negative εHf(t) values from -30.3 to -6.4, and two grains have positive εHf(t) values of + 3.7 and + 3.4. The TDMc model ages of these early Paleozoic grains range from 1180 to 3327 Ma. The Precambrian zircons are also dominated by negative εHf(t) values from − 44.7 to − 1.0 except for three grains with εHf(t) values from + 0.2 to + 5.9. More than half of Precambrian zircons have Archean TDMc model ages.

Sample 20ZH16: Most zircon grains show clear oscillatory zones with subrounded to prismatic shapes (Fig. 7C). Ninety grains were analyzed of which 84 grains yield concordant ages (Fig. 8E, F; Table S1). Except two spot of 0.06 and 0.07, their Th/U ratios range from 0.15 to 2.89. Paleozoic zircons occupy 17.9% with ages from 427 to 523 Ma with a prominent age peak at 470 Ma. Precambrian zircons are abundant in this sample and are grouped as: 573–998 Ma (peak at 953 Ma), 1064–1368 Ma (peak at 1112 Ma), 1410–2059 Ma (peak ages at 1458 Ma, 1761 Ma, and 1986 Ma), 2286–2497 Ma (peak at 2380 Ma), and three grains yield Archean ages of 2721 Ma, 2802 Ma and 3079 Ma.

Lu–Hf isotopic compositions were analyzed for twenty-eight zircon grains with concordant ages (Fig. 10; Table S2). Early Paleozoic zircons yield mostly positive εHf(t) values from + 1.9 to + 5.8, and one grain with εHf(t) value of -7.0. These early Paleozoic grains have TDMc ages ranging from 1085 to 1895 Ma. The Precambrian zircons have εHf(t) value of − 10.7 ~  + 8.8. The TDMc ages of Precambrian zircon grains range from 1275 to 4000 Ma and two grains have Archean model ages.

Samples from Ondor sum group

Sample 20ZH13: Almost all zircons from this sample are euhedral and prismatic with clear oscillatory zones (Fig. 7D), indicating a proximal magmatic source. Ninety zircons were analyzed for U–Pb isotopes of which 78 grains yield concordant ages with Th/U ratios of 0.12–2.55. (Fig. 8G, H; Table S1). About 42% of them are Paleozoic ages from 404 to 524 Ma with a peak at 423 Ma, in accordance with a weighted mean age of 421 ± 3 Ma of fifteen clustered ages of Paleozoic grains. The youngest grain yielded an age of 404 ± 7 Ma. Precambrian zircons are grouped as 573–1493 Ma (peaks at 615 Ma and 1172 Ma) and 1525–2058 Ma (peak at 1588 Ma), and the remaining two grains have ages of 2440 Ma and 2646 Ma.

Lu–Hf isotopic analyses were performed on 27 zircon grains with concordant ages (Fig. 10; Table S2). The εHf(t) values of early Paleozoic zircons range from -5.5 to + 9.1, and TDMc ages of them range from 849 to 1751 Ma. Zircons with Mesoproterozoic to Paleoproterozoic ages have εHf(t) values from − 6.0 to + 7.2 with TDMc ages from 1493 to 3203 Ma.

Sample 20ZH27: Most zircons from this sample are euhedral and prismatic with clear oscillatory zones (Fig. 7E). Ninety zircon grains were randomly selected for U–Pb dating and 68 of them yield concordant ages (Fig. 8I, J; Table S1). All analysis spots have Th/U ratios from 0.12 to 1.59. About 1/3 are Paleozoic ages from 415 to 523 Ma with a peak at 441 Ma. The ages of Precambrian zircons range from 543 to 2753 Ma, which yield four major peaks at 959 Ma, 1600 Ma, 1950 Ma and 2564 Ma.

Twenty-seven zircon grains with concordant ages were analyzed for Lu–Hf isotopic compositions (Fig. 10; Table S2). These early Paleozoic zircons have mostly positive εHf(t) values from 0.0 to + 6.9, and three grains exhibit negative εHf(t) values from − 1.8 to − 3.4. The two-stage Hf model ages (TDMc) of these early Paleozoic zircon grains range from 982 to 1639 Ma. Two Neoproterozoic grains have negative εHf(t) values of − 6.9 and − 11.9. Zircons with Paleoproterozoic to Archean ages yield almost all positive εHf(t) values except for a εHf(t) value of − 1.7. Precambrian grains have TDMc ages ranging from 1480 to 3018 Ma, of which four zircons have Archean model ages.

Discussion

Constraints on depositional ages

The maximum depositional age (MDA) of a sedimentary unit can be assessed by the youngest detrital zircon age, and it could be closed to the real depositional age in some situations (Dickinson and Gehrels 2009; Gehrels 2014). According to Dickinson and Gehrels (2009), the youngest detrital zircon age of a sedimentary rock can be measured by diverse ways such as: the youngest single detrital grain age (YSG), the weighted mean age of youngest cluster of two or more zircon ages overlapping at 1σ uncertainty (YC1σ (2 +)), or the youngest graphical detrital zircon age peak (YPP). In this paper, we regard the YC1σ (2 +) as the MDAs for all samples, and the YSG and YPP are considered as the reference data for the MDAs (Fig. 8; Table 1).

Table 1 Detrital zircon ages and maximum depositional age constraints for these (meta)sedimentary rocks from the study area
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Due to a lack of biostratigraphic data, the age of the Ondor sum Group strata in the study area was roughly estimated to Neoproterozoic based on metamorphic grade and regional geological correlations (Anonymous 1980a). However, the depositional age of the Ondor sum Group in the Tulinkai area to the east of the study area was constrained as Cambrian to Middle Silurian (Li et al. 2012). Xu et al. (2016) suggested that the Ondor sum Group from the western Inner Mongolia was mainly deposited in the early Paleozoic based on stratigraphic correlation. Two chlorite–quartz schists (samples 20ZH13 and 20ZH27) from the Ondor sum Group in the study area yield MDAs of 412.3 ± 5.5 Ma and 417.7 ± 2.3 Ma, respectively (Table 1), which constrain the strata to early Devonian.

The coral fossils including Petraea sp., Orthophyllum sp. and Propora sp. from limestone blocks within the lower part of the Hadahushu Group strata were reported, and their ages are constrained to early–middle Silurian (Anonymous 1980a, b). Moreover, Tian et al. (2019e) also suggested that the Hadahushu Group formed in the early–middle Silurian based on detrital zircon ages. In this study, a meta-quartz sandstone (sample 20ZH16) has a MDA of 435.3 ± 5.9 Ma calculated by a weighted mean age of four youngest grains overlapping at 1σ uncertainty. Another two quartz sandstones (samples 20ZH23 and 20ZH24) have MDAs of 424.8 ± 5.0 Ma and 419.5 ± 2.8 Ma (Table 1). Our new data indicate that the depositional age of the Hadahushu Group should be later than late Silurian. Field relationship shows that the Hadahushu Group unconformably overlies the Ondor sum Group in the Ganqimaodu area. Therefore, we suggest that the Hadahushu Group was also deposited in early Devonian.

Provenance analyses

All samples from the Ondor sum Group and the Hadahushu Group have similar detrital zircon age patterns which contain prominent early Paleozoic ages with extensive Precambrian ages (Fig. 8). The early Paleozoic grains were likely derived from nearby subduction-related plutons consisting of quartz diorites and granitoids in the Ganqimaodu area (Xu et al. 2013; Zhang et al. 2019b; our unpublished data; Fig. 3). This is also consistent with the shapes of the zircon grains which are euhedral and prismatic with distinct oscillatory zones (Fig. 7). Precambrian zircons of the Hadahushu Group occupy a greater proportion than those of the Ondor sum Group (Fig. 8). It may suggest that more input of exposed Precambrian basement with continuous consumption of the early Paleozoic plutons. Sample 20ZH16 from the Hadahushu Group has a great deal of Precambrian zircons (about 82%) with a prominent Neoproterozoic peak (~ 953 Ma). The detrital zircon age spectrums of samples 20ZH23 and 20ZH24 are very similar, whose Precambrian zircons take an amount of about 70%. For the Ondor sum Group strata, early Paleozoic zircons of sample 20ZH13 have a larger proportion than those of sample 20ZH27. Precambrian zircons of sample 20ZH13 have a prominent peak at 1588–1680 Ma, while those of sample 20ZH27 are relatively scattered with a few minor peaks at ~ 959 Ma, ~ 1600 Ma, ~ 1950 Ma and ~ 2564 Ma (Fig. 8H, J). All detrital zircon ages from the Ondor sum Group and Hadahushu Group in this study are pooled together respectively. The Ondor sum Group has age peaks at ~ 425, ~ 966, ~ 1590, ~ 1930 and ~ 2564 Ma, while the Hadahushu Group yields a series of peaks at ~ 443, ~ 586, ~ 960, ~ 1473, ~ 1765 and ~ 2480 Ma (Fig. 9). These two sets of strata display similar detrital zircon age spectrums with some slight differences between early Mesoproterozoic and late Paleoproterozoic. Based on the above comparison for the detrital zircon ages, we suggest that sediments of these two sets of strata are much more likely from the same source.

Fig. 9
figure 9

Comparison of detrital zircon populations from the Ondor sum Group and Hadahushu Group in the study area

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When pooling all the data from early Devonian strata in the Ganqimaodu area together, they yield a prominent early Paleozoic peak at ~ 452 Ma and subordinate Precambrian age peaks at ~ 587 Ma, ~ 961 Ma, ~ 1120 Ma, ~ 1482 Ma, ~ 1761 Ma, and ~ 2560 Ma (Fig. 11C). To the east of the study area, the Bainaimiao Arc, has a very similar detrital zircon spectrum with early Devonian strata from the Ganqimaodu area (Fig. 11B; age peaks at ~ 457 Ma, ~ 578 Ma, ~ 975 Ma, ~1103 Ma, ~ 1567 Ma, ~ 1864 Ma, and ~ 2478 Ma). A lot of early Paleozoic (ca. 420–500 Ma) arc-related igneous rocks with both positive and negative εHf(t) values (−9.3 to 11.4) developed in the Bainaimiao Arc, indicating juvenile crust growth and ancient crust reworking (Chen et al. 2020; Zhang et al. 2014; Zhou et al. 2018b). This is consistent with early Paleozoic detrital zircons from early Devonian strata in the study area which also have both positive and negative εHf(t) values (Figs. 10 and 12B). It seems to indicate that not only these intrusions from the Ganqimaodu area but also magmatic rocks from the Bainaimiao Arc have been the main source for these early Paleozoic detrital zircons from the early Devonian strata. There are distinct changes for εHf(t) values of Mesoproterozoic to Late Paleoproterozoic zircons from the early Devonian strata in the Ganqimaodu area: an increasing εHf(t) values of zircons from late Paleoproterozoic to middle Mesoproterozoic with a decreasing εHf(t) values of zircons from middle Mesoproterozoic to late Mesoproterozoic. These changes are also consistent with εHf(t) values of contemporaneous zircons from the Bainaimiao Arc (Fig. 12B). Our results show that the Ganqimaodu area may represent the western part of the Bainiamiao Arc, where the early Paleozoic arc-related magmatic rocks and Proterozoic basement were the main sources for these early Devonian strata in the study area. Recently, the Bainaimiao Arc was suggested as an exotic block with respect to the North China Craton and has a tectonic affinity to the Tarim Craton based on detrital zircon ages and Hf isotopes comparison (Zhang et al. 2014; Zhou et al. 2020; Fig. 11F and Fig. 12A). Plenty of ~ 889–980 Ma Neoproterozoic granitoids or granitic gneisses were reported in the Southern Mongolian accretionary system to the north of the study area (Demoux et al. 2009; Wang et al. 2001; Zhang et al. 2017b; Zhou et al. 2013b; Fig. 11A), which may provide detritus to the early Devonian sedimentary rocks in the Ganqimaodu area. The sediments of the early to middle Paleozoic strata from the north Alxa (Zhusileng–Hangwula Tectonic Zone) were also considered to be derived from the northern microcontinent of the CAOB such as Central Tianshan, South Beishan or South Mongolia (Chen et al. 2019; Yin et al. 2015; Fig. 11E).

Fig. 10
figure 10

The diagram of Zircon U–Pb age versus εHf(t) of the early to middle Paleozoic sedimentary rocks in the study area

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Fig. 11
figure 11

Comparison of the age populations of the zircons from: A Southern Mongolia (Demoux et al. 2009; Rojas-Agramonte et al. 2011; Wang et al. 2001; Zhang et al. 2016b; Zhou et al. 2013a); B Bainaimiao Arc (Chen et al. 2020; Zhang et al. 2014, 2017a; Zhou et al. 2020); C Early Devonian strata of the Ganqimaodu area (Tian et al. 2019e; this study); D Nuru–Langshan Tectonic Zone (Bao et al. 2019; Dan et al. 2012, 2014b, 2016; Geng and Zhou 2010; Gong et al. 2016; Hu et al. 2014; Liu et al. 2019a, 2016b; Peng et al. 2010; Song et al. 2017; Sun et al. 2013; Teng et al. 2019; Tian et al. 2019c; Wang et al. 2015b, 2016b; Xiao et al. 2015b; Zhang et al. 2013a); E Zhusileng–Hangwula Tectonic Zone (Chen et al. 2019; Yin 2016; Yin et al. 2015); F Tarim Craton (Rojas-Agramonte et al. 2011; Zhu et al. 2021); G North China Craton (Liu et al. 2017a; Rojas-Agramonte et al. 2011; Wang et al. 2015a; Zhou et al. 2020)

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Fig. 12
figure 12

Comparison of U–Pb age versus εHf(t) for Early Devonian (meta)sedimentary rocks in the Ganqimaodu area with adjacent tectonic units and cratons. Date sources: A Tarim Craton (He et al. 2014a, 2014b; Li et al. 2015; Zheng et al. 2020); B Bainaimiao Arc (Chen et al. 2020; Wang et al. 2016a; Zhang et al. 2014; Zhou et al. 2018a, 2020) and Early Devonian strata of the Ganqimaodu area (this study); C Nuru–Langshan Tectonic Zone (Bao et al. 2019; Dan et al. 2012, 2014b, 2016; Gong et al. 2016; Liu et al. 2016b, 2019a; Teng et al. 2019; Tian et al. 2019c); D North China Craton (Liu et al. 2017a; Wang et al. 2015a; Zhou et al. 2020)

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A few detrital zircon age peaks of the Nuru–Langshan Tectonic Zone are also similar to those of early Devonian strata from the Ganqimaodu area (Fig. 11C, D). However, it is largely excluded as the source by comprehensive analysis for our data, based on following reasons. First, although the early Paleozoic magmatic rocks also developed in the Nuru–Langshan Tectonic Zone, almost all magmatic zircons from these rocks have negative εHf(t) values (Dan et al. 2016; Liu et al. 2016b; Teng et al. 2019; Tian et al. 2019a; Fig. 12C). They are different from early Paleozoic detrital zircons of these early Devonian (meta)sedimentary rocks from the Ganqimaodu area which have both positive and negative εHf(t) values (Fig. 10). Second, Neoproterozoic magmatic events from the Nuru–Langshan Tectonic Zone occurred mainly between ~ 800 Ma and ~ 930 Ma, represented by ~ 805 Ma and ~ 817 Ma acid volcanic rocks from the Langshan area (Peng et al. 2010), and ~ 910 Ma and ~ 930 Ma granite plutons from the Bayinnuoergong area (Dan et al. 2014b). However, these magmatic zircons are lacking in early Devonian sedimentary rocks from the Ganqimaodu area. The prominent Neoproterozoic age peaks (~ 587 Ma, ~ 961 Ma) of these early Devonian strata in the study area are inconsistent with those (peaks at ~ 806 Ma, ~ 898 Ma) of the Nuru–Langshan Tectonic Zone (Fig. 11C, D). Third, abundant late Mesoproterozoic (Stenian) zircons with both positive and negative εHf(t) values are detected from the Ganqimaodu early Devonian sediments, whereas contemporaneous zircons from the Nuru–Langshan Tectonic Zone are relatively few and almost all of them have positive εHf(t) values (Fig. 12B, C). Besides, a significant Paleoproterozoic age peak of the Nuru–Langshan Tectonic Zone is at ~ 2038 Ma, which is also absent in the early Devonian strata from the study area (Fig. 11C, D). The εHf(t) values of Mesoproterozoic to middle Paleoproterozoic zircons from the Nuru–Langshan Tectonic Zone have an increasing tendency with decreasing age, similar to the evolutionary trend of those of the North China Craton (Fig. 12C, D). However, it is different from the variation trend of εHf(t) values of Mesoproterozoic to middle Paleoproterozoic zircons from the Ganqimaodu area and the Bainaimiao Arc (Fig. 12B). In addition, the North China Craton is excluded as source for these early Devonian strata in the study area based on the missing of early Paleozoic to Neoproterozoic magmatic events (Fig. 11G).

In summary, through comparison of the detrital zircon ages and Hf isotopes from the study area and the adjacent tectonic units, we suggest that early Devonian strata from the Ganqimaodu area are the western extension of the Bainaimiao Arc. Early Paleozoic magmatic rocks and Precambrian basement of the Bainaimiao Arc are the main sources for these strata. Also, the Southern Mongolia to the north of the study area is a significant source for them. However, to the south of the Ganqimaodu area, the Nuru–Langshan Tectonic Zone cannot provide detritus for these strata in the study area. Field investigations show that early Devonian strata from the study area are in fault contact with Mesoproterozoic strata from the Nuru–Langshan Tectonic Zone (our unpublished data; Figs. 3 and 4). Thus there might exist a barrier (may be an ocean) to impede the transportation for the materials from the the Nuru–Langshan Tectonic Zone during early Devonian. The South Bainaimiao ocean proposed by Zhang et al. (2014) between the Bainaimiao Arc and the North China Craton may extend to here westward, and consumption of the ocean basin leads to the final collision between the Ganqimaodu area (the western part of the Bainaimiao Arc) and the Nuru–Langshan Tectonic Zone.

Tectonic setting

The detrital zircon age spectra may reflect the tectonic setting of sedimentary basins (Cawood et al. 2012). Based on the age distribution patterns from different types of basins all over the world, Cawood et al. (2012) proposed that the tectonic setting of sedimentary rocks can be distinguished through plotting cumulative proportion curves of the difference values between the crystallization ages (CA) and the depositional age (DA). The sediments that CA–DA smaller than 100 Ma at the youngest 30% belong to convergent setting; while sediments from the collisional setting have CA–DA smaller than 150 Ma at the youngest 5% and greater than 100 Ma at the youngest 30%; others are extensional setting (Fig. 13). All samples in this study are plotted in the Fig. 13, and the depositional ages were defined by the MDAs of these (meta)sedimentary rocks as mentioned above.

Fig. 13
figure 13

Cumulative proportion curves of the difference values between the crystallization ages and the depositional age of the (meta)sedimentary rocks from the Ganqimaodu area. Samples 21B19 and 180,824–32 are from Tian et al. (2019e) and other samples are from this study. A Convergent setting, B Collisional setting, C Extensional setting. Modified after Cawood et al. (2012)

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Almost all samples from the early Devonian strata in this study except for sample 20ZH16 fall into the convergent setting. Two sedimentary rocks from the Hadahushu Group reported by Tian et al. (2019e) also fall into the convergent setting. The results are also consistent with existence of nearby subduction-related diorites with depletions in Nb, Ta in the Ganqimaodu area (Xu et al. 2013; Zhang et al. 2019b; our unpublished data; Fig. 3). Moreover, according to Anonymous (1980a, b), there may be blueschist in the southern part of the study area, which formed in subduction zone setting. This is conflicted with the passive margin deposition and foreland basin deposits proposed by Xu et al. (2013). In addition, provenance analyses of these (meta)sedimentary rocks also indicate that sediments are mainly from a northern unidirectional provenance. It is different from the back-arc basin setting which generally has bidirectional provenance. Ophiolitic mélanges containing blocks of ultramafic–mafic rocks, pillow basalts, siliceous rocks, and carbonate rocks are widespread in the early Devonian strata from the Ganqimaodu area (Fig. 6). The Ondor sum Group, which is considered as matrix of ophiolitic mélanges, has undergone greenschist-facies metamorphism with multi-phase structural deformations. A few basic rocks blocks within the Ondor sum Group have also undergone metamorphism of amphibolite facies (our unpublished data; Fig. 6). The lower part of Hadahushu Group is mainly characterized by a set of thick flysch deposition composed of unmetamorphosed sandstone, siltstone and mudstone (Fig. 5), which may represent a trench-slope basin deposition above deformed accretionary prism (the Ondor sum Group). Therefore, we advocate that the early Devonian (meta)sedimentary rocks in the Ganqimaodu area form parts of an accretionary complex.

Early Paleozoic subduction polarity: southward or northward

Controversy remains for early Paleozoic subduction polarity of the PAO along the Alxa Tectonic Belt and the Bainaimiao Arc. Most authors favor a southward subduction of the PAO in the east segment of the southern CAOB. The early Paleozoic Ondor sum accretionary complexes were suggested as the products of the southward subduction of the PAO beneath the North China Craton (Xiao et al. 2003). The Tulinkai SSZ ophiolites to the south of the Ondor sum area were also considered to be related to southward subduction of the PAO during early Paleozoic (Jian et al. 2008). In addition, Liu et al. (2016b) also proposed the southward subduction of the PAO along the Alxa Tectonic Belt and the northern North China Craton based on comparable arc-related plutons.

However, the Bainaimiao Arc has been suggested as an exotic block relative to North China Craton and a northward subduction of the South Bainaimiao Ocean in the Early Paleozoic was proposed to explain the origin of the Bainaimiao Arc (Zhang et al. 2014). The existence of the South Bainaimiao Ocean is evidenced by a series of early Paleozoic ophiolitic mélanges along the north margin of the North China Craton: the Xiaowulanhua mélange (Liu et al. 2020), the Chegandalai mélange (Hu 2019), the Hongqi mélange (Shi et al. 2013), the Wude mélange (Jia et al. 2003; Shang et al. 2003), and the Ganqimaodu mélange (Xu et al. 2013; Fig. 3 and Fig. 6). Moreover, the northward subduction of the South Bainaimiao Ocean is largely supported by the early Paleozoic passive continental margin for the northern North China Craton which has a widely distributional epicontinental sea deposition without magmatic events (Chen et al. 2015; Zhang et al. 2014; Zhao et al. 2017).

Field data and provenance analyses of (meta)sedimentary rocks from this study show that early Devonian strata in the Ganqimaodu area were parts of an accretionary wedge along the south flank of the Bainaimiao Arc, which support the northward subduction of the South Bainaimiao Ocean. Moreover, our data also imply that the western segment of the South Bainaimiao Ocean must be wide enough so that detritus from the Nuru–Langshan Tectonic Zone cannot be transported to the Ganqimaodu area during early Devonian. Previously the Hadahushu Group from the study area was considered as the western extension of the Xuniwusu Formation in the Bainaimiao area to the east of the study area (Tian et al. 2019e; Xu et al. 2013). Detailed studies for the early Silurian Xuniwusu Formation showed that the Bainaimiao Arc provided almost all detritus for the lower part, while the detritus of the upper part was not only derived from the Bainaimiao Arc but also from the North China Craton (Zhang et al. 2017a). The variation of provenance of the Xuniwusu Formation seems to indicate that the North China Craton were close to the Bainaimiao Arc gradually until they collided with each other. Different from the previous opinion proposed by Zhang et al. (2017a) that the Xuniwusu Formation was an early Paleozoic back-arc basin deposit above the North China Craton, we suggest that the Xuniwusu Formation could be the forearc deposition by the northward subduction of the South Bainaimiao Ocean and was in contact with the passive margin of the northern North China Craton subsequently. Silurian intrusions from the northern Alxa area also indicate that the South Bainaimiao Ocean subducted beneath the Southern Mongolia to the north during early Paleozoic (Zheng et al. 2016). Therefore, it is clear that the northward subduction of the South Bainaimiao Ocean is widespread along the Bainaimiao Arc and the Southern Mongolia.

To the south of the South Bainaimiao Ocean, the Nuru–Langshan Tectonic Zone was active continental margin evidenced by early Paleozoic arc-related magmatism (Dan et al. 2016; Liu et al. 2016b; Teng et al. 2019; Tian et al. 2019a). Liu et al. (2016b) argued that early Paleozoic magmatic events from the Nuru–Langshan Tectonic Zone and the Bainaimiao Arc are comparable based on rock associations and geochemical compositions. Actually, there is a distinct difference for εHf(t) values of them: almost all the εHf(t) values of these magmatic rocks from the Nuru–Langshan Tectonic Zone are negative, while arc-related plutons from the Bainaimiao Arc have both positive and negative εHf(t) values (Fig. 12B, C). Thus we suggest that these early Paleozoic magmatic rocks from the Bainaimiao Arc are related to northward subduction of the South Bainaimiao Ocean. There may exist a Paleozoic cryptic suture zone between the North China Craton and the Nuru–Langshan Tectonic Zone (Dan et al. 2016). Detrital zircon age spectrums of the westernmost part of the Nuru–Langshan Tectonic Zone also show a close affinity to the South China Craton rather than the North China Craton (Song et al. 2017). Thus, the Nuru–Langshan Tectonic Zone and the North China Craton were likely to be separated by an ocean during early Paleozoic.

In short, our new data integrated with previous studies support a northward subduction of the South Bainaimiao Ocean which is a branch of the PAO during early Paleozoic. Meanwhile, there was an active continental margin for the Nuru–Langshan Tectonic Zone along the southern rim of the South Bainaimiao Ocean.

Tectonic evolution of the north margin of the North China Craton–Alxa region

Based on the above discussion and previously published data, the following updated tectono-paleogeographic model is proposed for early Paleozoic tectonic evolution along the north margin of the North China Craton–Alxa region (Fig. 14).

Fig. 14
figure 14

Schemas showing the Early Paleozoic tectono-paleogeographic evolutional model for the northern margin of the North China Craton–Alxa region in the Southern Central Asian Orogenic Belt

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To the east of the Gaqimaodu area, the northward subduction of the South Bainaimiao Ocean beneath the Bainaimiao Arc may start at late Cambrian as indicated by 499 ± 2 Ma arc-related meta-volcanic rocks (Zhang et al. 2019a). Chen et al. (2020) suggested that the ~ 518 Ma dacite reported by Zhang et al. (2014) was unrelated to development of the Bainaimiao Arc based on the abrupt change of the εHf(t) values. Numerous early Ordovician to late Silurian arc-related plutons and volcanic–(meta)sedimentary rocks are distributed in the Bainaimiao Arc (Chen et al. 2020; Qian et al. 2017; Zhang et al. 2019a, 2014, 2013b; Zhou et al. 2020). The early Silurian Xuniwusu Formation in the Bainaimiao area is considered as parts of an accretionary wedge by northward subduction of the South Bainaimiao Ocean. In the Ganqimaodu area, Zhang et al. (2015) reported a 475.8 ± 1.6 Ma pillow basalt within the ophiolitic mélanges. Moreover, the early Devonian sediments in this study were considered as flysch in an accretionary wedge by northward oceanic subduction. It indicates that the northward subduction of the South Bainaimiao Ocean was still ongoing during early Devonian in the study area as revealed by the MDAs of the accretionary sediments. The Southern Mongolia were likely an active margin evidenced by Ordovician volcanic and immature clastic rocks (Wu and He 1993) as well as Silurian intrusions (~ 423–434 Ma; Zheng et al. 2016) in the northern Alxa. The ~ 420 Ma peak of detrital zircon ages from the Devonian Yuanbaoshan Formation in the northern Alxa also shows that the Southern Mongolia was an active margin during late Silurian to early Devonian (Yin et al. 2015). All of these geological records indicate a continuous northward subduction of the South Bainaimiao Ocean from late Cambrian to early Devonian (~ 500–410 Ma). To the south of the South Bainaimiao Ocean, the northern North China Craton was possibly a passive continental margin with stable neritic clastic and carbonate deposition (Chen et al. 2015; Zhang et al. 2014), whereas the Nuru–Langshan Tectonic Zone was an active continental margin favored by ~ 460–407 Ma diorites and granites with depletions in Nb, Ta and negative εHf(t) values (Liu et al. 2016b; Teng et al. 2019; Tian et al. 2019a; Wang et al. 2015b). The Nuru–Langshan Tectonic Zone may be separated from the North China Craton by the branch of the South Bainaimiao Ocean. Although the time for final closure of the branch of the South Bainaimiao Ocean between the Nuru–Langshan Tectonic Zone and the North China Craton is still controversial, it should be later than late Devonian (Yuan and Yang 2015; Zhang et al. 2016a).

Previously most scholars suggested that early Devonian molasse deposition of Xibiehe Formation in the Bainaimiao Arc was the product of arc–continental collision (Chen et al. 2020; Xu et al. 2013; Zhang et al. 2014; Zhou et al. 2018b). However, all the detrital zircons of the Xibiehe Formation from the Bayan obo area and Bainaimiao area yield early Paleozoic ages without Precambrian ages (Chen et al. 2020; Wang et al. 2020). The sedimentary rocks from the Xibiehe Formation in the Jiefangyinzi area are dominated by Precambrian zircons, but their detrital zircon ages are different from those of the North China Craton (Ma et al. 2019). It is confusing why the North China Craton cannot provide detritus to the Xibiehe Formation if the molasse deposition is related to collision between the Bainaimiao Arc and the North China Craton. Therefore, the tectonic setting of the Xibiehe Formation needs to be confirmed by further study.

To sum up, we suggest that there exist bidirectional subduction for the western segment of the South Bainaimiao Ocean while the eastern part of it subducted beneath the Bainaimiao Arc to the north during late Cambrian to early Devonian (Fig. 14).

Conclusions

  • (1) Two quartz schists from the Ondor sum Group strata have maximum depositional ages of 412.3 ± 5.5 Ma and 417.7 ± 2.3 Ma, and three (meta)sandstones from the Hadahushu Group strata yield maximum depositional ages of 419.5 ± 2.8 Ma, 424.8 ± 5.0 Ma and 435.3 ± 5.9 Ma. New detrital zircon ages integrated with field data indicate that these two sets of strata were likely deposited in early Devonian.

  • (2) All samples from the early Devonian strata in the Ganqimaodu area have a prominent early Paleozoic age peak with extensive detrital zircons from Neoproterozoic to Archean. These early Paleozoic detrital zircons with both positive and negative εHf(t) values indicate intense magmatism derived from partial melt of the juvenile and recycled ancient crust at that time. The early Devonian strata in the Ganqimaodu area were parts of an accretionary complex containing ophiolitic mélanges. Arc-related plutons and preexisting basement in the Bainaimiao Arc were the main sources for these strata.

  • (3) Our new data supported a northward subduction of the South Bainaimiao Ocean in the Ganqimaodu region. An east–west-trending north-dipping subduction zone existed along the Bainaimiao Arc and the Southern Mongolia during late Cambrian to early Devonian.