Carboniferous back-arc extension in the southern Yili-Central Tianshan Block and its significance to the formation of the Kazakhstan Orocline: insights from the Wusun Mountain volcanic belt

In Central Asia, the Carboniferous is a crucial period in the formation of the Tianshan Belt and associated bending of the Kazakhstan tectonic collage. In order to reveal Carboniferous magmatic events of the region and their tectonic implications, we conducted field investigations, zircon U–Pb dating, whole-rock geochemical and Sr–Nd isotopic studies on the Early Carboniferous Dahalajunshan Formation and Late Carboniferous Yishijilike Formation volcanic rocks of the Wusun Mountain Range (southern Yili-Central Tianshan Block). Volcanic rocks of the Dahalajunshan Formation consist of calc-alkaline basalt, andesite and dacite, yielding new zircon U–Pb ages of ~ 350 Ma. They have positive whole-rock εNd(t) values (+ 0.5 to + 1.6). In contrast, the Yishijilike Formation volcanic rocks dominantly comprise alkaline and calc-alkaline bimodal suites that erupted at ~ 337 Ma to 313 Ma and have higher whole-rock εNd(t) values (+ 2.3 to + 4.3). These two episodes of Carboniferous magmatism were correlated with partial melting of depleted mantle that metasomatized by slab-derived fluids. The late Carboniferous Wusun Mountain magmatic belt shows characteristics of a back-arc system that evolved due to trench retreat relative to the southern margin of the Yili-Central Tianshan Block. This mechanism induced an extensional regime with gradually depleting magma sources. The asymmetric retreat of the paleo-subduction zones of the South Tianshan Ocean and Junggar Ocean relative to the Yili-Central Tianshan Block was hence a vital driving force for the bending of the Kazakhstan Orocline.


Introduction
Accretionary orogens form at intraoceanic and active continental margins due to the subduction of oceanic plates (Cawood et al. 2009). These orogens are commonly affected by oroclinal bending, accretion of magmatic arcs, back-arcs, ophiolitic mélanges and continental fragments (Cawood et al. 2009(Cawood et al. , 2011Johnston 2004;Van der Voo 2004). Based on the rates and dips of the subducting plates, accretionary orogens can be grouped into advancing and retreating types. Advancing accretionary orogens form when convergence of the overriding plate advances more rapidly compared to the subducting plate. This results in compressional forces in the overriding plate. Consequently, advancing orogens generally have thickened crust, develop retro-arc fold-and-thrust belts, and are associated with magmatic rocks derived from enriched magma sources. In contrast, retreating orogens form when the rate of trench retreat progresses faster than convergence of the overriding plate. Retreating orogens can thus be associated with an extensional state, thinner crust, and induce arc break-up or arc-back-arc extensional systems (Collins 2002;Collins et al. 2011;Cawood et al. 2009;Kemp et al. 2009). Back-arc extension is hence generally 1 3 related to retreating orogens and shows a distinctive geological process that records complicated behaviors of the subducted oceanic lithosphere, asthenosphere and upper overriding plate, such as the changing of the oceanic plate's dip, asthenospheric upwelling and/or trench retreat or slab roll-back (Heuret and Lallemand 2005;Pearce and Stern 2006;Schellart and Lister 2004). In some cases, the asthenospheric upwelling may lead to distinct melting sources (e.g., depleted MORB mantle, subcontinental mantle, and subduction component-enriched lithospheric mantle) and produce significant magmatic activity (Gribble et al. 1996(Gribble et al. , 1998Ishizuka et al. 2010;Pearce et al. 2005;Shinjo et al. 1999;Shinjo and Kato 2000;Stern et al. 1990). Large-scale magmatic activity makes the back-arc lithosphere thinner, hotter and weaker; therefore, the back-arc region is easy to extend by the force developed at plate boundaries and may result in ensuing mobile belts (Hyndman and Currie 2011;Hyndman et al. 2005). In addition, the trench retreat or slab roll-back can cause curvature of the subduction zone to form an orocline with diverse geometries (Schellart et al. 2002(Schellart et al. , 2007. Thus, identifying back-arc extension, understanding its magmatic activity and tectonic evolution can help explain the type of the accretionary orogeny that might develop. It can also provide insights into crust-mantle interaction, lithospheric structure and oroclinal bending (Cawood et al. 2009(Cawood et al. , 2011Pearce et al. 2005;Schellart et al. 2007;Stern et al. 2003;Wilhem et al. 2012;Xiao et al. 2018).
The Central Asian Orogenic Belt (CAOB) is one of the largest accretionary orogens in the world. It is situated between the Siberian Craton to the north and the North China-Tarim Cratons to the south ( Fig. 1a; Şengör et al. 1993;Windley et al. 2007;Xiao et al. 2004). The western CAOB is characterized by the horseshoe-shaped Kazakhstan Orocline. Its formation had a significant effect on the architecture of the CAOB (Fig. 1b). The main parts of the Kazakhstan Orocline comprise the so-called external Devonian Volcanic Belt (DVB) and the internal Balkhash-Yili arc (Fig. 1b, c;Bazhenov et al. 2012). The Tianshan Belt is located in the internal Balkhash-Yili arc, and its formation was associated with the consumption of two major oceanic plates (i.e. the Junggar Ocean and the South Tianshan Ocean; Charvet et al. 2007Charvet et al. , 2011Gao et al. 1998Gao et al. , 2009aHan et al. 2011;Wang et al. 2008;Xiao et al. 2013). Formation of the ancestral Tianshan also had a genetic link with the bending of the Kazakhstan Orocline (Abrajevitch et al. 2008;Li et al. 2017aLi et al. , 2018Yi et al. 2015). The closure of the Junggar and South Tianshan Oceans caused the amalgamation of diverse island arcs, seamounts, accretionary prisms and micro-continents, and ended up with the docking of the Junggar Plate to the north and the Tarim Craton to the south (Charvet et al. 2007Gao et al. 1998Gao et al. , 2009aHan et al. 2011;Wang et al. 2008;Xiao et al. 2013). The asymmetric slab roll-back of the Junggar oceanic plate drove the first-stage formation of the Kazakhstan Orocline during the Late Devonian to Early Carboniferous (Li et al. 2017a. The subduction of the South Tianshan oceanic plate during the Late Devonian to Early Carboniferous was almost contemporaneous with the timing of the first-stage bending of the Kazakhstan Orocline (Gao et al. 2009a;Han et al. 2015;Han and Zhao 2018;Huang et al. 2020;Jiang et al. 2014;Li et al. 2017aLi et al. , 2018Long et al. 2011;Xia et al. 2014). However, the subduction evolution of the South Tianshan oceanic plate is still poorly understood. Complex subduction and accretion processes took place during the aforementioned period, producing massive Late Paleozoic magmatism in the western Chinese Tianshan, which is well recorded in a series of widespread Carboniferous volcanic rocks (XBGMR 1993). Previous studies suggested that they were formed in an intracontinental rift setting (Xia et al. 2012;Xia and Li 2020), a continental arc setting Tang et al. 2013;Wang et al. 2007b;Yu et al. 2016;Zhong et al. 2017;Zhu et al. 2009), a post-collisional setting (Chen et al. 2020;Feng and Zhu 2019;Ge et al. 2015;Long et al. 2012;Sun et al. 2008;Xiao et al. 2010) or a back-arc extensional setting Qian et al. 2006;Su et al. 2018;Wang et al. 2018aWang et al. , b, 2019Yan et al. 2015). Due to these controversial interpretations, their tectonic setting, and especially the implication that they have for the formation of the Kazakhstan Orocline, still warrant further elaboration.
In this contribution, we present new zircon U-Pb, wholerock major and trace element, and Sr-Nd isotopic data of the Carboniferous volcanic rocks from the Wusun Mountain Range, in the southern part of the Balkhash-Yili arc (Su et al. 2018). Our data, together with published results, allow us to uncover the nature of the magma source, geodynamic process as well as the formation of the Kazakhstan Orocline in the Late Paleozoic accretionary orogenesis.

Geological background
The Kazakhstan tectonic collage mainly includes the Chingiz Arc, Kokchetav-North Tianshan Block, Balkash-Yili Block, Devonian Volcanic Belt (DVB) and Stepnyak-North Tianshan Block (SNT) (Fig. 1b;Bazhenov et al. 2012;Windley et al. 2007;Xiao et al. 2015). It welded the Tarim Craton in the south by the intervening South Tianshan Belt (Fig. 1b;Windley et al. 2007;Xiao et al. 2015). The western Chinese Tianshan is the southernmost part of the Kazakhstan collage and can be further divided into four tectonic units, being (1) the North Tianshan Belt, (2) the Yili Block, (3) the Central Tianshan Block and (4) the South Tianshan Belt (Fig. 2). These tectonic units are separated by several suture zones and ductile shear zones 1 3 Fig. 1 a Location of the Central Asian Orogenic Belt (modified after Şengör and Natal'in 1996;Şengör et al. 1993); b geological map of the Kazakhstan collage system in the western CAOB (modified after Windley et al. 2007;Xiao et al. 2015); c the Kazakhstan Orocline before and after bending (modified after Bazhenov et al. 2012) ( Fig. 2; Allen et al. 1992;Gao et al. 1998Gao et al. , 2009aHan et al. 2011;Windley et al. 2007;Xiao et al. 2013).
The North Tianshan Belt is a Late Paleozoic accretionary complex related to the subduction of the Junggar oceanic plate. It mainly contains Devonian-Carboniferous volcanic rocks, turbidites and ophiolitic mélanges (Gao et al. 1998;Li et al. 2014). The final terrane amalgamation is estimated to have occurred in the Late Carboniferous (~ 316 Ma) as constrained by 'stitching granites' crosscutting the mélange zone . The South Tianshan Belt is also considered to be an accretionary complex, which resulted from the subduction of the South Tianshan oceanic plate (Gao et al. 1998(Gao et al. , 2009aXiao et al. 2013). Ophiolitic mélanges, numerous granitic intrusions and an ultrahigh pressure metamorphic belt, crop out along the northern margin of the South Tianshan (Gao and Klemd 2003;Gao et al. 2011;Han et al. 2011;Long et al. 2011;Tan et al. 2019;Zhang et al. 2007Zhang et al. , 2013. The ophiolitic mélange belts discontinuously occur in the Heiyingshan, Kumishi, Kulehu, Baleigong and Guluogou areas Han et al. 2011;Jiang et al. 2014;Wang et al. 2011;Yang et al. 2018). The granitoid plutons in the South Tianshan intruded the northern margin of the Tarim Craton and are predominantly composed of syenites, two-mica peraluminous leucogranites and A-type rapakivi granites (Gao et al. 2009a;Gou and Zhang 2016;Gou et al. 2012Gou et al. , 2015Long et al. 2008Long et al. , 2011. The HP/ UHP metamorphic belt is dominated by foliated metapelites, eclogites, blueschists and marbles (Gao and Klemd 2003;Tan et al. 2019;Zhang et al. 2007Zhang et al. , 2013. The Yili Block is a wedge-shaped domain situated in the easternmost part of the Balkhash-Yili Block, while the adjacent Central Tianshan Block is an arc terrane along the southern margin of the Yili Block (Fig. 2). These two tectonic units merged to form the Yili-Central Tianshan Block as a result of the closure of the 'Terskey Ocean' in the Early Paleozoic (Gao et al. 2009b;Han et al. 2011;Wilhem et al. 2012). The basement of the Yili-Central Tianshan Block is composed of Precambrian amphibolitic and granitic gneisses (Chen et al. 1999;Gao et al. 2015;Hu et al. 2000;Huang et al. 2015b;Wang et al. 2014aWang et al. , b,2017Wang et al. ,2020. The sedimentary covers are composed of Cambrian and Ordovician chert, and carbonates that crop out in the Sayram Lake and Guozigou area (Che et al. 1994;Coleman 1989;Gao et al. 1998;Liu et al. 2014a;Wang et al. 2008;XBGMR 1993). Silurian flysch and intercalated calc-alkaline volcanic rocks are distributed along the Borohoro Mountain Range (Wang et al. 1994). Devonian rocks are sporadically exposed in the North Tianshan and composed of conglomerates, sandstones, siltstones, rhyolite and andesite (XBGMR 1993). The Carboniferous volcanic rocks are widespread both at the northern and southern margins of the Yili-Central  Gao et al. 2009a) Tianshan Block, such as in the Borohoro Mountain Range, Keguqin Mountain Range, Tulasu Basin, Awulale Range, Wusun Mountain Range, and the Haerk and Nalati Mountain Range (Fig. 2). These volcanic strata mainly consist of basalt, trachy-andesite, andesite, dacite, rhyolite, pyroclastic rocks and intercalated with minor sandstones, conglomerates and limestones Su et al. 2018;Tang et al. 2013;Wang et al. 2007b;XBGMR 1993;Zhu et al. 2009). The Permian sequences are mainly composed of conglomerate, sandstone and small amounts of bimodal volcanic rocks (XBGMR 1993). Early Carboniferous to Permian plutonic rocks also well developed in the Yili-Central Tianshan Block Gao et al. 2009a;Han et al. 2010;Long et al. 2011;Wang et al. 2009;Xu et al. 2013;Yin et al. 2016;Yu et al. 2018). The Jurassic strata are made up of fluvio-lacustrine deposits, such as conglomerate, sandstone and mudstone (XBGMR 1993).
The Wusun Mountain Range, located in the southern Yili-Central Tianshan block, crops out along piedmont faults in the northern and southern edges of the block, and are separated by the Yili and Zhaosu basins from the North Tianshan Belt and the Haerk-Nalati Mountain Range, respectively (Fig. 3a, Zhang et al. 1999). The Carboniferous volcanic and sedimentary sequences in the Wusun Mountain range are represented by the Early Carboniferous Dahalajunshan Formation (C 1 d), the Akeshake Formation (C 1 a) and the Late Carboniferous Yishijilike Formation (C 2 y). Angular unconformities delineate their contact zones (Figs. 3 and 4e, j;XBGMR 1993;Li et al. 2020). The Dahalajunshan Formation includes a set of intermediate-felsic lavas, sandstones and conglomerates, as well as minor limestones (Fig. 3). It contains complex disharmonic folds, angular folds, intense corrugation textures and ductile faults (Li et al. 2017b). The Akeshake Formation is mainly made up of clastic rocks from lower shallow marine environments, fossiliferous (corals and brachiopods) carbonates, mudstones, tuffaceous sandstones and siltstones (Figs. 3b, c and 4o). The Yishijilike Formation (C 2 y) comprises bimodal volcanic rocks such as basalt, basaltic trachy-andesite, dacite and rhyolite, the felsic rocks are in conformable contact with the basaltic rocks (Figs. 3 and 4k). Its tectonic deformation is very weak, without intense folding (Li et al. 2017b). The Yishijilike Formation is unconformably covered by the Permian Wulang conglomerates and Jurassic lacustrine sediments ( Fig. 3; XBGMR 1993). The granitic intrusions exhibit a wide range of emplacement ages from the Early Carboniferous to the Permian Yu et al. 2018).

Sample description
Our samples were collected from the Dahalajunshan and Yishijilike Formations in the Wusun Mountain Range, and sampling locations are shown in Fig. 3.

Analytical methods
Analytical methods are presented in the Supplementary Materials. Zircon U-Pb dating, whole-rock major and trace element and Sr-Nd isotopic compositions of the volcanic rocks are available in Tables A1-3, respectively.

Zircon U-Pb dating results
The dacite-porphyry sample (C13ZS01) collected from the Dahalajunshan Formation was dated with the zircon U-Pb method. The zircon crystals are dark in color and semitransparent, they exhibit a roughly euhedral prismatic shape with 120-200 μm in length and aspect ratios of 1-3 (Fig. 5a), and their transparent oscillatory zoning together with relatively high Th/U ratios (0.57-1.06) suggest a magmatic origin. Twelve analyses spots yielded consistent concordant ages, and a weighted mean 206 Pb/ 238 U age of 350.3 ± 3.1 Ma (MSWD = 3.5; Fig. 5a) is interpreted as the crystallization age.
A total of five samples were taken from the Yishijilike Formation. From the rhyolite sample C15WS01, twelve zircon grains were analyzed. These zircons are pale in color and dominantly euhedral in shape, with lengths of 60-180 μm and aspect ratios of 1-3. Most of them display well-developed oscillatory zoning on the CL images ( Fig. 5b), indicating an igneous origin. The latter is further corroborated by their high Th/U ratios (0.46-1.29). This data are plotted on the concordia diagram showing a consistent distribution with a weighted average 206 Pb/ 238 U age of 337.7 ± 2.8 Ma (MSWD = 0.31; Fig. 5b), which is interpreted as the crystallization age. Nine zircon grains from another rhyolite sample (C15WS20) also display typical features of magmatic zircon. They yield a weighted mean 206 Pb/ 238 U age of 337.0 ± 3.3 Ma (MSWD = 0.35; Fig. 5c). This age is comparable to that of the sample C15WS01.
Rhyolite sample C15WS36, from the upper part of the Yishijilike Formation, contains pale, semitransparent zircons. Eleven zircon grains were analyzed. These are euhedral prismatic in shape with lengths of 80-130 μm and length/width ratios of 1-3. Their obvious oscillatory zoning together with relatively high Th/U ratios (0.39-0.69) also suggest a magmatic origin. The 11 analyses yield a range of consistent apparent ages and a weighted mean 206 Pb/ 238 U age of 322.8 ± 2.8 Ma (MSWD = 0.35; Fig. 5d). Zircon crystals from the dacite-porphyry sample C15WS63 also show characteristics of igneous zircon, and a weighted mean 206 Pb/ 238 U age of 321.5 ± 4.4 Ma was calculated (MSWD = 2.0; Fig. 5e). This age is similar to the age of the sample C15WS36.
Zircon grains from the dacite-porphyry (C15WS57), intruding the main dacite units, generally show euhedral prismatic shapes, and their sizes vary from 40 to 140 μm, showing different width/length ratios of 1-3. The distinct oscillatory zoning and high Th/U ratios (0.25-1.58) imply an igneous origin. Twelve analyzed spots generate a weighted mean 206 Pb/ 238 U age of 313.2 ± 4.8 Ma (MSWD = 2.1; Fig. 5f). This age is contemporaneous with a rhyolite sample (316.1 ± 2.6 Ma) reported by Cao et al. (2017) in the eastern Wusun Mountain Range.
In summary, the Carboniferous volcanic rocks erupted in four major episodes: the first volcanic group (Dahalajunshan Formation) initiated at an early stage of ca. 350 Ma, and the other three groups (Yishijilike Formation) formed at ~ 337, ~ 322 and ~ 313 Ma, respectively.

Major and trace element contents
Major and trace element contents of the Carboniferous volcanic rocks from all age groups are presented in Table A2. As revealed by the SiO 2 vs. K 2 O + Na 2 O diagram, samples from different episodes exhibit diverse geochemical characteristics. The earliest episode of the Carboniferous volcanism (~ 350 Ma) produced basalt, andesite, dacite and daciteporphyry (Fig. 6a). These rocks have a wide range of SiO 2 content (43.90-70.50 wt%), variable total alkali contents (2.83-7.48 wt%), moderate Fe 2 O 3 T (2.22-15.48 wt%) and low MgO (0.53-5.59 wt%), displaying calc-alkaline features (Fig. 6b). Their total REE contents range from 60 to 197 ppm with moderate LREE enrichment ((La/Yb) N = 5.44-14.85) and a flat HREE pattern ((Gd/Yb) N = 1.42-1.71). They also show slightly negative Eu anomalies (Eu/Eu* = 0.72-1.04) (Fig. 7a). In addition, the ~ 350 Ma samples are also enriched in large ion lithophile elements (LILE; e.g., Rb, Th, U and K) with respect to the high field strength elements (HFSE; e.g., Nb, Ta and Ti) as can be deduced from the trace element spider diagram normalized to the primitive mantle ( Fig. 7b).

Magmatic episodes of the Carboniferous volcanic rocks in the Wusun Mountain Range
According to previous geological mapping, the Dahalajunshan Formation was defined to represent groups of widespread Early Carboniferous volcanic rocks in the western Chinese Tianshan (XBGMR 1993). Although increasing zircon U-Pb isotopic data suggested an unreasonable wide age range for the Dahalajunshan volcanics from 407 to 301 Ma (see Su et al. 2018 for a detailed overview and discussion), our new zircon U-Pb age of the Dahalajunshan volcanic rocks in the Wusun Mountain Range yield ages of ca. 350 Ma, which is consistent with the reported ~ 355-350 Ma ages by Su et al. (2018) for this volcanic sequence. The newly obtained ages of the Yishijilike Formation volcanic rocks document three eruptive episodes, respectively, at ca.

Petrogenesis and magma sources
Our new zircon U-Pb dating results record four volcanic episodes in the Wusun Mountain Range (Fig. 5). The ~ 350 Ma magmatism is characterized by continuous calc-alkaline mafic to more differentiated felsic rocks with low ε Nd (t) values from + 0.5 to + 1.6. However, the later ~ 337 and ~ 322 Ma groups are dominated by bimodal volcanic rocks showing distinctive alkaline features and the ~ 313 Ma magmatism is calc-alkaline dacite-porphyry. Isotopically, all of them have relatively higher ε Nd (t) values (+ 2.3 to + 4.3).

Petrogenesis of the ~ 350 Ma volcanic rocks
The sequential formation of basic to acid rock series is usually controlled by fractional crystallization. The basalt (C13ZS12) contains 5.59 wt% MgO, with a Mg # value of 42, 33 ppm Ni and 129 ppm Cr, which are much lower than typical mantle-derived primary melts (Cr = 300-500 ppm, Ni = 300-400 ppm; Frey et al. 1978) and higher than andesitic and dacitic rocks ( Fig. S1e; MgO, 0.53-2.70 wt%; Mg # , 26-46; Cr, 9-88 ppm; Ni, 4-15 ppm), indicating that they may have experienced crystal fractionation. The latter is also reflected in the Harker diagram (Fig. S1), from basaltic to felsic rocks, where the contents of TiO 2 , Fe 2 O 3 T and CaO decrease with increasing SiO 2 , indicative of fractional crystallization of plagioclase and ferromagnesian minerals.
The ~ 350 Ma volcanic rocks have weakly positive ε Nd (t) values (+ 0.5 to + 1.6) and low initial 87 Sr/ 86 Sr values, suggesting partial melting of a moderately depleted mantle (Rudnick and Gao 2003). Although mantle-derived magmas potentially assimilate crustal materials during their ascent or storage in a magma chamber, the lack of crustal xenoliths and restricted ε Nd (t) values exclude the significant influence of crustal assimilation. In addition, trace elements of the ~ 350 Ma rocks show enrichment in LILEs and LREEs, but depletion of HFSEs (e.g., Nb, Ta, and Ti). This suggests that the magma source was most likely modified by slabderived fluids (Pearce et al. 2005). The typical subductionrelated affinity is also confirmed by the enrichment of Th compared to Nb, and as a result the basaltic rocks plot in the volcanic arc field in the Th/Yb vs. Nb/Yb and Th-Hf-Ta diagrams (Fig. 9a, b). Therefore, the ~ 350 Ma volcanic rocks were probably derived from melts sourced from a depleted mantle with metasomatism of subduction-related fluids, without clear evidence for crustal assimilation.

Petrogenesis of the ~ 337 and ~ 322 Ma bimodal volcanic rocks
Petrogenesis of the basaltic rocks All the 337 and 322 Ma basaltic rocks have low silica, high Mg, positive ε Nd (t) values (+ 2.3 to + 4.3) and low initial 87 Sr/ 86 Sr ratios (0.7042-0.7069), indicating that they originated from a depleted mantle source (Rudnick and Gao 2003). As shown, the homogeneous Sr-Nd isotopic compositions and the lack of crystal xenoliths exclude an influence of crustal contamination.
Generally, the lithospheric mantle is cold and conductive, it has low ε Nd (t) values and high initial 87 Sr/ 86 Sr values because of its long isolation from the convective mantle  Lebas et al. 1986); b AFM diagram (after Irvine and Baragar 1971) and interaction with magmas (McDonough and McCulloch 1987). In contrast, the asthenospheric mantle is hot and convective, and isotopically depleted (high ε Nd (t) values and low initial 87 Sr/ 86 Sr rates). The magma derived from the asthenospheric mantle consequently has a depleted or primitive mantle affinity in both chemical and isotopic compositions. The basaltic rocks of the Yishijilike Formation have high ε Nd (t) values (+ 2.3 to + 4.3) and low ( 87 Sr/ 86 Sr) i ratios (0.7042-0.7068; Fig. 8), displaying typical asthenospheric properties (Saunders et al. 1992 1990). In our study, the Zr/Nb, Sm/Nd, Ta/Yb and La/Yb ratios of the Yishijilike Formation basaltic rock range from 17-31, 0.23-0.28, 0.08-0.20 and 3.88-8.44, respectively. These ratios are similar to MORB characteristics. In the Nb/Yb vs. Zr/Yb diagram, all the basaltic rocks plot within the MORB-OIB array, also indicative of an asthenospheric mantle source (Fig. 9c).
The trace element compositions of our basic rock samples indicate that the asthenospheric mantle source must have experienced mantle metasomatism. The enrichments of LILEs and LREEs, and remarkable negative Nb, Ta, Ti anomalies are in agreement with subduction-related mantle metasomatic features (Pearce and Peate 1995). These features are well demonstrated by the Th/Yb versus Nb/Yb and Th-Hf-Ta diagrams, in which the calc-alkaline basaltic rocks plot in the continental arc field and display arc-type basalt features (Fig. 9a, b). The depleted mantle-derived primary magma probably experienced significant crystal fractionation when forming the ~ 337 and ~ 322 Ma basaltic rocks because their Cr (< 80 ppm) and Ni contents (8-17 ppm) are much lower than typical primary mantle-derived magmas (e.g., Cr = 300-500 ppm, Ni = 300-400 ppm; Frey et al. 1978). Furthermore, the negative correlations between TiO 2 , Fe 2 O 3 T , MgO and SiO 2 are compatible with fractionation of ferromagnesian minerals (Fig. S1a, c, e), and the moderately negative Eu and Sr anomalies imply the fractionation of plagioclase.
Yb, Gd and Dy generally share similar partition coefficients in spinel, while garnet has a higher partition coefficient for Yb compared to Gd and Dy (Green 2006;Johnson 1994). Therefore, partial melting in the spinel stability field only leads to slight fractionation of Dy/Yb and Gd/ Yb, whereas partial melting of garnet-facies mantle may cause notable fractionation of Dy/Yb and Gd/Yb. The ~ 337 and ~ 322 Ma basaltic rocks show flat HREE patterns (Fig. 7c, e, g), and their low Dy/Yb and Gd/Yb ratios also plot on the melting curve for spinel peridotite (Fig. 9d), suggesting that the variable degrees (< 10%) of partial melting occurred at a relatively shallow depth, mostly within the spinel stability field.
Summarizing, we suggest that the ~ 337 and ~ 322 Ma basaltic rocks from the Wusun Mountain Range probably were derived from low degrees (< 10%) of partial melting of a depleted spinel-facies asthenospheric mantle, metasomatized by slab-derived fluids, which was also accompanied by crystal fractionation of ferromagnesian minerals and plagioclase, and without crustal contamination.

Petrogenesis of felsic rocks
The formation of felsic rocks in the bimodal suites is a long-standing, enigmatic petrogenetic issue (Bonnefoi et al. 1995). The felsic rocks of the Wusun Mountain Range show affinity to A-type granites because of their high silica and alkali contents, low MgO, CaO, TiO 2 values and high concentrations of Ga, Zr, Nb, Ce and Y ( Fig. 9e; Whalen et al. 1987). Additionally, they can be associated with A 2 -type granites (Eby 1992) based on their relatively high Y/Nb ratios (2.98-4.53) (Fig. 9f).
The crust in the western Chinese Tianshan contains fragments of ancient Precambrian crystalline basement as well as juvenile crustal material. The Precambrian crystalline basement in the western Chinese Tianshan exhibits low and negative ε Nd (t) values (−1.9 to −9.1) (Chen et al. 1999;Hu et al. 2000;Wang et al. 2014a), while the A-type felsic rocks in the Wusun Mountain Range have relatively high ε Nd (t) values (+ 3.2 to + 4.2). The ε Nd (t) values of the felsic magmas should be negative or near-zero when  Table A4. Elemental values of primitive mantle and chondrite, N-MORB, E-MORB and OIB are from Sun and McDonough (1989); Data in the fields of the Okinawa Trough BABB and the Mariana trough are from Shinjo et al. (1999) and Pearce et al. (2005), respectively ◂ they would have been generated by the partial melting of the ancient crustal material. Hence the partial melting of Precambrian crystalline basement can be ruled out. Moreover, the ε Hf (t) values of the Late Devonian to Late Carboniferous felsic rocks in the southern Yili Block range from −8.19 to + 8.16 with variable ancient T DM C values (0.84-1.81 Ga) (Huang et al. 2020), indicating that the partial melting of juvenile crust alone is also not possible to explain the observed signatures. Magma mixing between crustal and mantle sources for the generation of the A-type magmas, where the mantle serves both as a heating source and an important contributor, could be a feasible explanation (Foland and Allen 1991;Frost et al. 1999). However, the ε Nd (t) values of the Devonian to Early Carboniferous volcanic rocks in the Yili-Central Tianshan Block range from −5.2 to + 6.2 with an average value of + 2.1 (Table A3), and the crustal components have negative ε Nd (t) values. Magma mixing of crustal-derived felsic magma and mantle-derived basic magma, therefore, could not have produced the A-type magma with such high ε Nd (t) values (+ 2.3 to + 4.3). Hence, magma mixing can also be excluded as sole answer. Given all the above, fractional crystallization seems to be the most reasonable cause to explain the petrogenesis of these felsic rocks. The lack of mafic microgranular enclaves in the felsic rocks, and homogeneous Sr-Nd isotopic compositions with the coeval mafic rocks indicate that they share the same magma source with insignificant crustal contamination (Figs. 4h and 8). This conclusion is also corroborated by the significant negative correlations between SiO 2 and TiO 2 , Al 2 O 3 , Fe 2 O 3 T , CaO and MgO in the Harker diagrams (Fig. S1) that make a case for crystal fractionation of plagioclase and ferromagnesian minerals. In addition, although the argument against the fractional crystallization model is the absence of intermediate rock compositions (Pin and Paquette 1997), some studies have indicated that magma replenishment coupled with high eruption rates, minimized crustal ponding and limited residence time in the magma chamber can fractionate the magma with higher SiO 2 contents (Bonnefoi et al. 1995;Brophy 1991;Geist et al. 1995;Thompson 1972). This process is probably the formation process of the bimodal volcanic rocks.

Petrogenesis of the ~ 313 Ma dacite-porphyry
The ~ 313 Ma dacite-porphyry, along with contemporaneous rhyolites reported by Cao et al. (2017) in the eastern Wusun Mountain Range, reflects another magmatic event in the Late Carboniferous. Although the dacite-porphyry may not represent an additional bimodal volcanic suite given that it lacks basaltic components, it does exhibit similar geochemical and isotopic features to the ~ 337 and ~ 322 Ma felsic rocks (Figs. 6, 7, 8, 9e, f and S1). Therefore, they may share an analogous mode of petrogenesis, implying that the dacite-porphyry might have been derived from partial melting of asthenospheric mantle with insignificant crustal contamination.

Tectonic implications
Two sub-parallel magmatic belts occur in the southern Yili-Central Tianshan Block, i.e., the Wusun Mountain Magmatic Belt and the Haerk-Nalati Magmatic Belt ( Fig. 2; Su et al. 2018) separated by the Zhaosu basin. Both belts are mainly composed of Carboniferous magmatic rocks ( Fig. 2; Gao et al. 2009a;Gou et al. 2012;Huang et al. 2015aHuang et al. , 2020Long et al. 2011;Ma et al. 2014;Xu et al. 2013;Yu et al. 2016;Zhu et al. 2009Zhu et al. , 2010Yin et al. 2016;Zhang et al. 1999). Some studies suggest that these two Carboniferous magmatic belts constitute magmatic arcs and were  Table A3 controlled by the subduction of the South Tianshan oceanic plate (Zhu et al. 2009(Zhu et al. , 2010Yu et al. 2016;Xu et al. 2013 and references there in). However, in our study we argue that the Wusun Mountain Magmatic Belt is a back-arc edifice based on the specific structural characteristics and rock associations. The Carboniferous volcanic rocks in the Wusun Mountain Belt are grouped in the Dahalajunshan Formation and Yishijilike Formation. The Dahalajunshan Formation displays complicated disharmonic and angular folds, intense corrugation textures and ductile faults. However, the tectonic  Table A4 deformation of the Yishijilike Formation is very weak and lacking folding. These contrasting structural characteristics suggests that the tectonic stress changed from a compression to an extension in the Wusun Mountain Magmatic Belt, and this transition illustrates the back-arc extension in the Wusun Mountain area (Li et al. 2017b). Concerning the rock associations, the Dahalajunshan Formation comprises calc-alkaline mafic to felsic volcanic rock series, while the Yishijilike Formation consists of bimodal volcanic rocks. The Dahalajunshan Formation volcanic rocks probably formed in an initial back-arc extension setting (Su et al. 2018). Next, we will discuss the formation mechanism and tectonic setting of the bimodal volcanic rocks of the Yishijilike Formation.
Bimodal volcanic rocks are generally related to extensional regimes and can be formed in various geodynamic settings (Wang et al. 2000), such as continental rifts (Pin and Marini 1993;Wilson 1989), oceanic islands (Geist et al. 1995), incipient back-arc spreading (Hochstaedter et al. 1990a, b), post-orogenic extensional settings (Coulon et al. 1986), intra-oceanic arcs (Brouxel et al. 1987) and mature islands/active continental margins (Frey et al. 1984;Pin and Paquette 1997). Considering that the Wusun Mountain magmatic belt was built inside the Yili-Central Tianshan Block with widespread continental crustal basement (Hu et al. 2000), the bimodal volcanic rocks are unlikely to have formed in oceanic islands or intra-oceanic arc settings. Furthermore, both the basaltic and felsic rocks of the bimodal volcanics have the same Sr-Nd isotopic compositions, hence excluding the possibility of formation during continental-breakup where the basaltic and felsic rocks come from different magma sources and thus contain distinct Sr-Nd isotopic compositions (Griffiths and Campbell 1990;Wilson 1989). Finally, the absence of intermediate rocks does not support a mature arc setting (Frey et al. 1984;Pin and Paquette 1997). As a result, the Yishijilike bimodal volcanics were most likely formed in a back-arc extension or post-orogenic extensional setting.
Here the key issue to determine the geological setting of the Yishijilike bimodal volcanic rocks lies in the timing of the final collision between the Tarim Craton and the Yili-Central Tianshan Block. It has been proposed that the South Tianshan Belt between these two blocks was formed by the northward subduction and accretion of the South Tianshan oceanic plate. The Akeyazi HP/UHP metamorphic belt with relevant ophiolitic remnants delineate the suture zone (Gao and Klemd 2003;Gao et al. 1998;Han et al. 2011;Long et al. 2011;Qian et al. 2009;Xiao et al. 2013). Recently, a series of studies documented that the closure of the South Tianshan Ocean as well as the final collision between the Yili-Central Tianshan Block and the Tarim Craton happened in the end of Carboniferous. A first argument for this is that the aforementioned HP/UHP metamorphic rocks yielded peak metamorphic ages of ~ 327-313 Ma Li et al. 2011;Liu et al. 2014b;Su et al. 2010;Yang et al. 2013), which was followed by their rapid exhumation and post-orogenic magmatic events. It is hence thought that this timing constrains the final amalgamation Klemd et al. 2005;Wang et al. 2007c). Second, the youngest age group of the ophiolitic remnants along the South Tianshan mélange belt were constrained at ~ 320 Ma (Han and Zhao 2018 and references therein), implying that the subduction process of the South Tianshan Ocean should have lasted at least until the Late Carboniferous. Third, the Late Carboniferous (327-310 Ma) detrital zircon from the South Tianshan Belt are likely to record subduction-related magmatic events instead of post-orogenic magmatism (Han et al. , 2016Ren et al. 2011). Therefore, it is reasonable to interpret the formation of the Yishijilike bimodal volcanic rocks to be the result of back-arc extension rather than a post-orogenic extension setting.
Not only back-arc basin basalts (BABB), but also bimodal volcanic rocks can be formed in a back-arc extensional region (Clift et al. 1995;Gribble et al. 1998;Hawkins et al. 1990;Shinjo et al. 1999;Shinjo and Kato 2000;Stern et al. 1990Stern et al. , 2003. The BABB is commonly generated by the decompressional melting of MORB-like mantle with addition of subduction components (Gribble et al. 1998;Hawkins et al. 1990;Stern et al. 1990). Thus, the BABB, especially those that form in the initial back-arc extension stage, generally display strong arc-like features with high concentrations of LILEs and low concentrations of HFSEs (Hawkins and Melchior 1985;Stern et al. 1990Stern et al. , 2003. At the later stage of back-arc extension, both sub-alkaline BABB which serves as a magma-type intermediate between MORB and islandarc basalts (IAB), and alkaline BABB may appear (Fryer et al. 1981;Gill 1976;Ishizuka et al. 2009). The volcanic rocks of the Wusun Mountain Range are enriched in LILEs, depleted in HFSEs (especially Nb, Ta, Ti), and have high Th/Yb and Nb/Yb ratios. Hence, they exhibit strong arc-like features and point to a subduction-related setting (Figs. 7 and 9a,b). In addition, their REE contents show values that typically lie between OIB and N-MORB, and are similar to a BABB signature (Fig. 7a, c, e, g;Fryer et al. 1981;Gill 1976). On the other hand, their lithology changes from calc-alkaline arc-type series to alkaline bimodal volcanics. The characteristics of felsic rocks of the bimodal volcanic rocks are comparable to A 2 -type granites, indicative of an extensional setting (Fig. 9e, f; Eby 1992). The mafic rocks have relatively high Zr contents and Zr/Y ratios, which are identical to the mafic rocks typical of a within-plate setting (Fig. 10a). Moreover, their low Ti and V/Ti ratios, medium Y and La/Nb ratios are highly comparable to those of BABB (Fig. 10b, c, d). As a result, all the evidence, including the lithologic characteristics, major and trace element features of the Yishijilike volcanic rocks in the Wusun Mountain Range suggest their formation in a back-arc extensional setting.
Finally, crustal thinning and asthenospheric upwelling frequently take place under an extensional tectonic setting. The change of calculated Moho depth can provide crucial evidence for the evaluation of crustal thinning. Based on the consensus that arc magmatism is generated by decompressional melting of the mantle wedge, Mantle and Collins (2008) suggested that the trace element chemistry of arc basalts and Moho depth show good correlation with maximal Ce/Y ratios. Specifically, for rocks having 44-53 wt% SiO 2 , > 4 wt.% MgO and < 4 wt.% LOI, the empirical correlation equation is Ce/Y = 0.3029e 0.0554Dm , where Dm gives Moho depth. For the Wusun Mountain Range the Early Carboniferous basaltic rocks (> 350 Ma) indicate Moho depths between 26 and 41 km (with an average of 32 km), while the Moho depths during the formation of the younger volcanics from our study area (after 337 Ma) are 19-29 km (with an average of 24 km) (Fig. 11). In addition, the crustal thickness can also be tracked by the Sr/Y ratios of the intermediate rocks Chiaradia 2015;Profeta et al. 2015). Only limited samples with their SiO 2 values ranging from 55 to 68 wt%, MgO contents between 1% and 4%, and Rb/Sr ratios in the range of 0.05-0.2, can be used to calculate the crustal thickness, using the following equation: dm = 1.11Sr/Y + 8.05 (where dm represents the crustal thickness or depth to Moho; Chapman et al. 2015; Profeta  (Shervais 1982); c Y versus La/Yb diagram (Floyd et al. 1991); d Zr versus V/Ti diagram (Woodhead et al. 1993). IAT island arc tholei-ite; N-MORB normal mid-ocean ridge basalt; BABB back-arc basin basalt; WPB within-plate basalt; OFB ocean floor basalt. Symbols used are identical to those in Figs. 6 and 8 Fig. 11 Age versus Moho depth diagram (Mantle and Collins 2008;Profeta et al. 2015). Symbols used are identical to those in Figs. 6 and 8. Data sources are listed in Table A4 et al. 2015). Here we collected published data of intermediate rocks from the arc edifice (Southern Yili-Central Tianshan Block) to make a comparison with the back-arc region (data sources can be found in Table A4). The crustal thickness of the arc (average 43 km) is found to be thicker than the back-arc, while the former shows insignificant changes throughout the entire Carboniferous (Fig. 11). Therefore, this demonstrates that the Wusun Mountain area experienced a significant crustal thinning process. This crustal thinning causes asthenospheric upwelling and hence decompressional melting, with magma from more depleted sources. The ε Nd (t) values of the ~ 350, ~ 337, ~ 322 and ~ 313 Ma volcanic rocks show a rising trend with younging ages, representing an increase of the depleted mantle constituents and a decrease of the enriched crustal materials in the source (Fig. 8), which is consistent with the crustal thinning process. In contrast, the ε Nd (t) values of the Haerk-Nalati arc do not alter significantly, suggesting that their magma sources experienced little or no changes, which is in line with an invariant crustal thickness (Figs. 8 and 11).
In summary, we conclude that a back-arc extensional tectonic setting was responsible for the generation of the volcanic rocks of the Wusun Mountain Range (Yishijilike Formation).

Geodynamic implication: back-arc extension in the Yili-Central Tianshan Block
The subduction of oceanic lithosphere at convergent plate boundaries generally shows a one-sided subduction pattern, while the subducted slab sinks, the overriding plate moves horizontally (Gerya et al. 2008). Based on the kinematic framework and the resulting geological characteristics at convergent plate boundaries, resulting accretionary orogens can be subdivided into retreating and advancing types (Cawood and Buchan 2007;Cawood et al. 2009). For the retreating types, the upper plate moves away from the downgoing plate, and long-term extension (e.g. back-arc basin) will develop. Reversely, when the overriding plate moves towards the downgoing plate, advancing-type orogens are formed and result in foreland fold-and-thrust belts as well as in crustal thickening (Cawood and Buchan 2007;Cawood et al. 2009). Regarding the retreating-type orogens, the oceanic plate will be older, colder and denser when it moves away from a spreading plate. As the oceanic plate is denser than the surrounding mantle, it will subduct under the upper crust and will also have a tendency to roll back because of the negative buoyancy (Schellart and Lister 2004). The rollback of an oceanic plate will give rise to the extension of the overriding continental plate, cause crustal thinning and produce back-arc magmatism. This latter will result in mantle decompressional melting and upwelling as well as potentially some crustal melting under a high geothermal gradient.
Meanwhile, the subduction of the oceanic plate will also generate arc magmatism by partial melting of the metasomatized mantle wedge (Gerya et al. 2008;Hall et al. 2003;Ishizuka et al. 2011). The progressive magmatic activity will make the back-arc lithosphere hotter, weaker and easier to extend along the continental marginal plate boundaries such as in the case of the circum-Pacific orogens (Collins 2002;Hyndman and Currie 2011;Hyndman et al. 2005).
Late Paleozoic magmatic activity in the western Chinese Tianshan is thought to be controlled by the subduction processes of both the Junggar and South Tianshan oceanic plates (Gao et al. 1998(Gao et al. , 2009aHan et al. 2011Han et al. , 2015Xiao et al. 2013). In this context, the Carboniferous magmatism has been extensively studied with various hypotheses. It was considered to develop in an intracontinental rift setting (Xia et al. 2012;Xia and Li 2020), a continental arc setting Tang et al. 2013;Wang et al. 2007b;Yu et al. 2016;Zhong et al. 2017;Zhu et al. 2009), a postcollisional setting (Chen et al. 2020;Feng and Zhu 2019;Ge et al. 2015;Long et al. 2012;Sun et al. 2008) or a back-arc extensional setting Qian et al. 2006;Su et al. 2018;Wang et al. 2018aWang et al. , b,2019Yan et al. 2015) on the basis of geochemical features of different studied targets. These complex Carboniferous magmatic rocks were subdivided into four main belts, from north to south, as the Keguqinshan-Tulasu, Awulale, Wusun Mountain and Haerk-Nalati domains, according to their geographical distribution characteristics (e.g. Su et al. 2018). The formation mechanism of each magmatic belt should be carefully treated with the comprehensive consideration of its spatial-temporal distribution and associated tectonic events. With regard to the magmatic belt of the Wusun Mountain Range, there are two main hypotheses as regard to its petrogenesis. One hypothesis states that it is the southward subduction of the Junggar oceanic plate and the coeval back-arc extension in the Yili-Central Tianshan Block that led to its formation . This hypothesis is based on the top-to-the north ductile shearing deformation observed in the Kekesu-Akeyazi-Atbashy HP-UHP metamorphic complex Wang et al. 2010). However, other authors insisted that the building of the Wusun Mountain Belt was the result of the northward subduction of the South Tianshan oceanic plate Su et al. 2018;Zhu et al. 2009), which is supported by the evidence that: (1) a LP/ HT metamorphic belt ) and a HP/UHP metamorphic belt constitute a paired metamorphic belt at the northern margin of the South Tianshan Belt (Gao and Klemd 2003;Hegner et al. 2010;Klemd et al. 2011;Tan et al. 2019;Zhang et al. 2007); (2) Late Devonian to Late Carboniferous arc calc-alkaline rocks are widely distributed in the Haerk-Nalati range (Gao et al. 2009a;Long et al. 2011;Xu et al. 2013;Zhu et al. 2009;Yin et al. 2016); and (3) the northern part of the Tarim Craton was regarded as a Paleozoic passive continental margin (Windley et al. 1990;Xiao et al. 2013). On the other hand, the Wusun Mountain Belt is situated to the north of the Haerk-Nalati igneous belt at distances of up to 100 km, and the latter has similar emplacement ages of calc-alkaline arc magmatism ( Fig. 2; Gao et al. 2009a;Long et al. 2011;Zhu et al. 2009 and references therein), it would be unrealistic if these two near-parallel magmatic belts were two distinct arc edifices formed under a same subduction scenario. Given all that, we argue that the Carboniferous arc-back-arc system in the southern part of the Yili-Central Tianshan Block was probably controlled by northward subduction of the South Tianshan oceanic plate (e.g., Bao et al. 2018;Su et al. 2018).
The extension in the Wusun Mountain back-arc region was probably due to the trench retreat of the subducting South Tianshan oceanic plate during the Late Devonian to Late Carboniferous. Trench retreat commonly causes crustal thinning in a retreating-type accretionary orogen (Cawood et al. 2009). Therefore, changes in crustal thickness provide crucial evidence to refine the subduction-accretion process. In the case of Wusun Mountain volcanic rocks, the decreasing Ce/Y ratios evidence a crustal thinning process in the Yili-Central Tianshan Block (Fig. 11). Further, slab rollback or trench retreat is always accompanied by the underplating of juvenile mantle-derived materials, with increased ε Hf (t) values (Kemp et al. 2009). Although ε Hf (t) data from the Wusun Mountain magmatic belt is scarce, the ε Hf (t) values of the Southern Yili Block display an increase from the Late Devonian to Late Carboniferous (Huang et al. 2020). In addition, the ε Hf (t) values have a linear relationship with the ε Nd (t) values of magmatic rocks in the western Chinese Tianshan (ε Hf = 1.36ε Nd + 2.95; Huang et al. 2020). Based on the positive linear relationship, the ε Hf (t) values of volcanics in the Wusun Mountain should also gradually increase due to the ε Nd (t) values increase with younger ages (Fig. 8). Therefore, the decreasing Ce/Y ratios and increasing ε Nd (t) values might indicate that the South Tianshan oceanic plate rolled back and the trench retreated during the Late Devonian to Late Carboniferous.
A tentative model is proposed here to illustrate the tectonic processes concerning the evolution of Carboniferous arc-back-arc system along the southern margin of the Yili-Central Tianshan Block (Fig. 13). In the Latest Devonian to Early Carboniferous, the northward subduction of South Tianshan oceanic plate under the Yili-Central Tianshan Block produced the Haerk-Nalati magmatic arc ( Fig. 13b; Gao et al. 2009a;Long et al. 2011;Xu et al. 2013;Zhu et al. 2009 and references therein). During this period, the South Tianshan oceanic plate whose density was denser than the underlying mantle, had a tendency to roll back due to the negative buoyancy. This ultimately resulted in the back-arc extension in the north of Haerk-Nalati range. The early-stage back-arc extension (362-350 Ma) caused partial melting of continental crust and triggered granitic intrusion as well as decompressional melting of the depleted mantle, which had been metasomatized by subduction-related fluids to generate the arc-like Dahalajunshan volcanic rocks in the Wusun Mountain Range (Figs. 13b and 3c;Bao et al. 2018;Su et al. 2018). This process is comparable to the modern analog of the northern Mariana Trough, where the early back-arc basin basalts display strong arc-like characteristics (Stern et al. 1990). The continuous extension led to the opening of a limited back-arc basin, in which the Akeshake Formation limestones were deposited and covered the Dahalajunshan Formation strata (Figs. 13b, 3c and 4e, o). The extension lasted until the Late Carboniferous (later than ~ 337 Ma) with the occurrence of increased decompressional melting of asthenospheric mantle, resulting in the formation of the Yishijilike bimodal volcanics (Figs. 13b and 3c). This scenario resembles the formation process of the present-day Okinawa Trough (Shinjo et al. 1999;Shinjo and Kato 2000). The massive magmatism affected the geothermal gradient and thus the strength regime of the back-arc region in the southern Yili-Central Tianshan Block, by making it thinner, hotter and weaker. Consequently, as a product of backarc extension, the Wusun magmatic belt developed its thin lithosphere (Figs. 11 and 13b). This process is similar to most of the mobile belts located in current or recent back-arc regions on the Earth (Hyndman and Currie 2011;Hyndman et al. 2005).

Implications for the formation of the Kazakhstan Orocline
The formation of the SW CAOB stands in close relationship with the bending of the Kazakhstan Orocline. The latter is mainly made up of a Devonian Volcanic Belt (DVB) and the Balkhash-Yili arc Li et al. 2018;Windley et al. 2007;Xiao and Santosh 2014;Xiao et al. 2015;Yakubchuk 2017). The bending of the Kazakhstan Orocline is documented by paleomagnetic data (Abrajevitch et al. 2007(Abrajevitch et al. , 2008Levashova et al. 2012;Li et al. 2018;Van der Voo et al. 2006;Yi et al. 2015). The north branch of the orocline has rotated clockwise ~ 112-126° during the Late Devonian to Early Carboniferous and ~ 15-28° between the Late Carboniferous and Late Permian (Abrajevitch et al. 2008;Levashova et al. 2012;Li et al. 2018;Yi et al. 2015). The south branch is characterized by a ~ 39-40° anticlockwise rotation during the Late Carboniferous to Late Permian, while Late Devonian to Early Carboniferous data are lacking Wang et al. 2007a;Yi et al. 2015). Two kinds of bending were suggested to interpret the origin of the Kazakhstan Orocline. One scenario is the buckling of a quasi-linear orogenic belt due to the convergence and collision of the Tarim and Siberia continents (Abrajevitch et al. 2008;Van der Voo 2004;Xiao et al. 2010). The other model argues that asymmetric trench retreat, related to the roll back of the subducting Junggar oceanic plate played an important role (Li et al. 2017aXiao et al. 2018).
The particularity of the south branch of the Kazakhstan Orocline is that its formation was controlled by simultaneous subduction of two oceanic plates (South Tianshan and Junggar oceanic plates) during the Late Paleozoic (Gao et al. 2009a;Han et al. 2010Han et al. , 2011Han et al. , 2015Xiao et al. 2013). Previous models stressed that the south branch was pinned or fixed by the subduction of the Junggar oceanic plate before the Late Carboniferous (Li et al. 2017aYi et al. 2015). However, even though the northward subduction of the South Tianshan oceanic plate during the Late Devonian to Early Carboniferous is coeval with the first-stage bending of the Kazakhstan Orocline (Gao et al. 2009a,b;Han et al. 2015;Huang et al. 2020;Li et al. 2017aLi et al. , 2018 and references therein), its role in the formation of the Kazakhstan Orocline has not received adequate attention (Abrajevitch et al. 2008). Based on the special spatial and temporal distribution characteristics and potential genetic relationships, both the subduction of these two oceanic plates should be taken into consideration to decipher the bending of Kazakhstan Orocline (Huang et al. 2020).
In a subduction scenario, different slab widths control the diverse curvature of subduction zones and their tendency to retreat backwards with time (Schellart et al. 2007). Narrow slabs (≤ 1500 km) retreat fast and develop a concave geometry towards the mantle wedge side (Fig. 12b), while wider slabs (≥ 4000 km) develop a convex geometry with slow trench retreat ( Fig. 12a; Schellart et al. 2007). In addition, the transition from subduction to collision can also cause block rotations and back-arc extension in the upper plate, accompanied with crustal thinning, asthenospheric upwelling and decompressional melting ( Fig. 12c; Gutiérrez-Alonso et al. 2012;Wallace et al. 2005). As for the SW CAOB, the South Tianshan Belt is about 1300 km in length and the quasi-linear DVB and BY are almost three times longer than the subduction zone of the South Tianshan Ocean (Fig. 1b, c). Based on the research of Schellart et al. (2007), the long trench of the Junggar Ocean seems to have retreated towards the opposite direction of the subduction and displays a convex geometry (Fig. 12a, d). Therefore, the rotation should be clockwise in the northern and anticlockwise in the southern sections of the Junggar Ocean trench (Fig. 12d). In contrast, the short trench of the South Tianshan Ocean was concave, and its rotation would have been clockwise in the west, and anticlockwise in the east of the South Tianshan Ocean trench (Fig. 12b, Abrajevitch et al. 2007Abrajevitch et al. , 2008Levashova et al. 2012;Van der Voo et al. 2006;Wang et al. 2007a;Yi et al. 2015). The retreating-type subduction of the South Tianshan oceanic plate in the southern BY is deemed responsible for the back-arc extension in the Wusun Mountain area (Fig. 13a).
During the Late Devonian to Carboniferous progressing subduction of the South Tianshan oceanic plate resulted in plate roll-back, with its narrow trench displaying a concave geometry (Schellart et al. 2007). However, the consequent collision of the Tarim Craton and Yili-Central Tianshan Block eventually made the trench develop in a linear belt rather than a concave one (Figs. 1b, 12d and 13b). Meanwhile, the trench of the Junggar Ocean developed a convex geometry because of its long and asymmetric trench retreat (Li et al. 2017aSchellart et al. 2007;Xiao et al. 2018). In contrast, the wide Junggar Ocean supplied broad space for retreat of its trench, which thus displayed convex geometry (Figs. 12d and 13b;Li et al. 2018;Xiao et al. 2018). The double subduction zones pinned and fixed the south end of the Kazakhstan Orocline, and the trench retreat of the Junggar Ocean dragged the north end to curve (Abrajevitch et al. 2008;Li et al. 2017aLi et al. , 2018. The curvature process is for example similar to the formation of the curved New Hebrides arc in the Cenozoic Schellart et al. 2002Schellart et al. , 2006. The subduction of the New Hebrides slab produced the New Hebrides arc followed by fast slab rollback, causing convex curvature of the New Hebrides arc and the back-arc extension of the North Fiji Basin (Schellart et al. 2002(Schellart et al. , 2006(Schellart et al. , 2007. In analogy with the SW CAOB, during the Late Devonian to Late Carboniferous, the subduction of the South Tianshan oceanic plate and slab roll back produced the Nalati-Haerk arc and the Wusun Mountain back-arc domain in the southern Yili-Central Tianshan Block. The Wusun Mountain and Nalati-Haerk magmatic belts thus constitute an arc-back-arc system in the SW CAOB (Fig. 13b). The fixing at the southern section and curvature of the northern one developed a first-stage curved geometry of the Kazakhstan Orocline ( Fig. 13b; Abrajevitch et al. 2008;Li et al. 2017aLi et al. , 2018. The further bending of the Kazakhstan Orocline in the Permian was associated with the convergence of the Siberian and Tarim cratons, during which strike-slip faults/shear zones cut through the oroclinal structure and the complete consumption of the Junggar oceanic plate in the core of the Kazakhstan Orocline transpired (Li et al. 2017aXiao et al. 2010Xiao et al. , 2015Xiao et al. , 2018Yi et al. 2015).

Conclusions
(1) Our zircon U-Pb dating results of the volcanic rocks of the Wusun Mountain Belt reveal four significant Carboniferous eruptive episodes in the southern Yili-Central Tianshan Block at ~ 350, ~ 337, ~ 322 and ~ 313 Ma.
(2) Our petrological and geochemical data suggest that the Dahalajunshan Formation (~ 350 Ma) contains basic to acid rock associations that exhibit calc-alkaline features, while the Yishijilike Formation (~ 337, ~ 322  Schellart et al. 2007). c Schematic diagram for collision-induced rotation. CPBM: convergent plate boundary microblock (modified from Wallace et al. 2005). d Rollback-related tectonic model for the Kazakhstan Orocline (modified after Bazhenov et al. 2012;Li et al. 2017aLi et al. , 2018 (1) ~ NW-SE quasi-linear subduction zone in the Early Devonian; (2) schematic illustration of the bending of the Kazakhstan collage system. See text for detailed discussion 1 3 and ~ 313 Ma) comprises bimodal volcanics that contain both calc-alkaline and alkaline rocks. (3) The ~ 350 Ma Dahalajunshan volcanic rocks were derived from a depleted mantle source associated with subduction-related fluids. The ~ 337, ~ 322 and ~ 313 Ma Yishijilike bimodal volcanic rocks originated from low degrees (< 10%) of partial melting of a more depleted spinel-facies asthenospheric mantle source, also with involvement of subduction-related fluids. The felsic series of the bimodal volcanics were formed by fractional crystallization of the coeval basaltic magmas. (4) Available geochemical and geochronological data indicate that the formation of the Carboniferous volcanic rocks of the Wusun Mountain Range was associated with the evolution of an arc-back-arc system resulting Fig. 13 a Simplified tectonic evolution for the back-arc extension (modified after Wallace et al. 2005); b tectonic sketch illustrating the formation processes of the Carboniferous arc-back-arc system and the bending of the Kazakhstan collage system. See text for detailed discussion from the northward subduction of the South Tianshan oceanic plate during the Late Devonian to Late Carboniferous. The retreating-type South Tianshan Orogen played a vital role in the bending of the Kazakhstan Orocline, it pinned and fixed the south end of the Kazakhstan Orocline and accommodated the oroclinal bending.
Acknowledgements This work is financially supported by grants of the NSFC (41872082, 41622205) and the fundamental research funds for the Central Universities (2652018116 and 2652018135). The support provided by the China Scholarship Council (CSC, 201908320260) is appreciated for financing the research stay of the first author in Belgium. Dr. Minjia Sun and Ms. Xiaomei Ma are thanked for their kind assistance during the field work. We appreciate Prof. Xiaoping Xia for generous helps during laboratory analysis. We would like to thank the editor for handling the manuscript, and two anonymous reviewers for their constructive and inspiring comments.
Author contributions KC designed this project and wrote the manuscript, XW and ZB participated in field work and data interpretation. MS, ZH and JDG helped with the writing of the final version of the manuscript. WS wrote the first draft of the paper, following discussion with, and contributions from, all authors. All authors approved the manuscript and agreed to its submission to the International Journal of Earth Sciences.
Funding This work is financially supported by grants of the NSFC (41872082, 41622205) and the fundamental research funds for the Central Universities (2652018116 and 2652018135).
Data availability All data used in this manuscript are found in Appendix Tables 1-4, and all data archiving is underway. We plan to use the EarthChem Library to store our data, and now we temporarily upload a copy of our data as Supporting Information for review purposes.

Conflict of interest
No conflict of interest exists in the manuscript submission, and the manuscript is approved by all authors for publication. I declare on behalf of my co-authors that the work described is original and has not been published previously or under consideration for publication elsewhere, in whole or in part.