Carboniferous back-arc bimodal rocks in West Kunlun during northward subduction of the Paleo-Tethys

ABSTRACT The widespread occurrence of Carboniferous Wuluate Formation volcanic rocks in the West Kunlun is closely related to the evolution of the Paleo-Tethys but has received little attention. In this study, we conducted detailed fieldwork on the Wuluate Formation in West Kunlun and performed zircon U‒Pb dating, whole-rock major and trace element analysis, whole-rock Sr-Nd isotope analysis, and in situ zircon Hf isotope analyses. The Wuluate Formation is characterized by a bimodal volcanic suite. Zircon U‒Pb dating indicates that the rhyolites erupted at ca. 348–353 Ma, which is coeval with the zircon U‒Pb dating result of 354 Ma for the basalt. The basalts show a tholeiitic magmatic evolution trend with flattened REE patterns and depleted whole-rock Sr-Nd and zircon Hf isotopes, indicating geochemical affinity to back-arc basin basalt (BABB). Trace element modelling results indicate that the primitive basaltic compositions fit well with the products of the DMM + sediments + H2O source under melting conditions of asthenospheric temperature and thin lithosphere thickness, and the REE modelling results show that the basalts underwent 40% to 95% crystallographic differentiation of olivine, monazite, hornblende, plagioclase, and magnetite. The silicic rocks are calcic to calcic-alkaline and meta/peraluminous. They exhibit flattened REE patterns and slightly enriched zircon Hf isotopes, which could originate from Precambrian metaigneous sources of West Kunlun. The Kungai bimodal suite is likely to have formed in a back-arc setting corresponding to the northward subduction of Paleo-Tethys. The subduction of Paleo-Tethys had already occurred, and a back-arc basin had developed before the Early Carboniferous.


Introduction
The Paleo-Tethys was an extensive Paleozoic-Mesozoic Ocean between Gondwana and the European and Asiatic Hunic terranes that formed as the Rheic and Proto-Tethys subducted under these terranes along the northern margin of Gondwana or orogenic collapse due to closure of the Proto-Tethys (Von Raumer and Stampfli 2008;Zhao et al. 2018;Dong et al. 2020;Metcalfe 2021).Numerous terranes (e.g.Armorica, Tarim, Qaidam, and Qiangtang) broke away from the Gondwana continent and drifted northward, spreading throughout the expanse of the Paleo-Tethys (Zhao et al. 2018;Wu et al. 2020).Meanwhile, a significant number of small, complex, and dispersed microcontinental blocks (e.g.North Qinling, Central Altyn, West Kunlun, and East Kunlun) were scattered throughout the Paleo-Tethys during the Paleozoic-Mesozoic era (Zhao et al. 2018;Dong et al. 2020;Sui 2021;Wang et al. 2022).Despite their importance, these microcontinental blocks have frequently been overlooked in previous reconstructions of Paleo-Tethys evolution (Meert 2003;Metcalfe 2009).A detailed study of the tectono-magmatic events occurring in these microblocks can provide new insight into their properties and the evolution of the Paleo-Tethys.
The West Kunlun Orogenic Belt (WKOB) occupies a crucial position along the tectonic junction between the Asian and Tethyan domains, providing critical clues to the evolution of the Paleo-Tethys.The WKOB contains two major magmatic pulses in the Early Paleozoic and Early Mesozoic (Figure 1).Although the lateral pulse of Triassic magmatic activity well documented the event linked to closure of the Paleo-Tethys (Liao et al. 2010;Jiang et al. 2013;Liu et al. 2015), its dynamic evolution of early-stage remains unclear.Yuan (1999) and Xiao et al. (2002) have previously suggested that the lack of magmatic activity during the period of 350 to 280 Ma in the WOKB was a consequence of the cessation of subduction of Paleo-Tethys Ocean.However, a recent investigation by Tang et al. (2021) proposed that the Paleo-Tethys oceanic basin in the WOKB actually opened approximately 340 Ma, based on the analysis of igneous rocks from the western Kunlun-Pamir region.Further research is necessary to elucidate the early-stage geological evolution of the Paleo-Tethys in the WOKB.Basaltic rocks, which could provide important constraints on the initial stages of oceanic crust formation and the subsequent tectonic processes that occurred during the ocean's early development.
The Carboniferous Wuluate Formation is the most widespread volcanic-sedimentary suite and contains a giant bimodal suite (up to 1950 m).Bimodal volcanic rocks commonly occur in extensional environments related to postcollisional, intraplate or back-arc rifting (e.g.Chen et al. 2018;Wang et al. 2021).However, previous studies have mainly focused on Carboniferous granitoids (Jiang et al. 2008;Li et al. 2009;Ji et al. 2018;Wang et al. 2022).In this paper, we report a comprehensive dataset of zircon U -Pb ages and Hf isotopes, bulk-rock chemical compositions and Sr -Nd isotopes of bimodal suites from the Kungai area, SKT (Figures 1 and 2).These new data allow us to address the tectonic evolution of the WKOB during the Early Carboniferous and have significant implications for delineating the geodynamic evolution and subduction of the Paleozoic Tethys.

Geological background
The West Kunlun orogenic belt (WKOB) is located between the Tarim Basin to the north and the Karakunlun (Qiangtang) Terrane to the south (Figure 1a).The WKOB is subdivided from north to south into three major tectonic units: the Northern Kunlun Terrane (NKT), the Southern Kunlun Terrane (SKT), and the Tianshuihai Terrane (TST) by the Oytag-Kudi suture and Mazha-Kangxiwa suture, respectively (Figure 1a) (Mattern and Schneider 2000;Xiao et al. 2005).The NKT consists of a Precambrian basement covered by the Devonian red molasse and Carboniferous-Permian shallow marine carbonate rocks, which is considered an uplifted part of the Tarim Block (Mattern and Schneider 2000;Zhang et al. 2013Zhang et al. , 2019;;Liu et al. 2014;Wang et al. 2014Wang et al. , 2015)).Widespread Early Paleozoic magmatic rocks are exposed in the SKT with two major pauses of silicic rocks (533-420 Ma and 251-223 Ma, respectively) across the whole block, two ophiolites (Kudi and Pushou) in the central part, and a Carboniferous-Permian volcanic-sedimentary basin in the northwestern part (Figure 1a) (Yuan et al. 2003;Jiang et al. 2013;Liu et al. 2014;Yin et al. 2020;Wu et al. 2021).The SKT has a Precambrian basement including the Kulangnagu Group, Saitula Group, and Sangzhutage Group (Pan 1996;Wang 2004;Wang et al. 2020).Early studies considered the SKT to be an integral part of the Tarim Block (Matte et al. 1996) or an exotic block aggregated to the Northern Kunlun Terrane during the Early Paleozoic (Yuan et al. 2002;Xiao et al. 2003), which shares a similar Precambrian basement with the NKT and the Tarim Block (Cui et al. 2007).Recently, Zhang et al. (2018b) argued that the basement of the SKT was deposited during 600-480 Ma, representing a massive Early Paleozoic accretionary wedge.
The collision of the WKOB with the southern margin of the Tarim Block resulted in the Caledonian orogenic belt in the late Early Paleozoic (Wang et al. 2020).However, a suite of oceanic sedimentary sequences representing transgression developed in the Early late Paleozoic, with the Kangxiwa-Mazha-Waqia forming in the PaleoTethys (Zhang et al. 2021).Previous studies have proven that the subduction of the Kangxiwa oceanic crust towards the West Kunlun Block formed a series of sedimentary basins in the back-arc and arc magmatism in the Kangxiwa-Mazha belt (388-324 Ma, Li et al. 2006;Liu et al. 2014;Kang et al. 2015).The Oytag-Kurliang basin is located along the northern margin of the West Kunlun massif, including the Oytag marine volcaniclastic sedimentary basin and the Kuerliang intracontinental rifting sedimentary basin (Figure 1a).The late Paleozoic strata in this basin mainly consist of the lower Carboniferous Wuluate Formation and the upper Carboniferous-lower Permian Tahaqi Formation, which are characterized by oceanic lavas and pyroclastic and clastic rocks (Yun et al. 2015;Zhang et al. 2021).
The TSHT and SKT are separated by the Mazha-Kangxiwa suture (Figure 1a).The TSHT is composed of metagraywackes and limestone, which are in fault contact with the accretionary complex (Zhang et al. 2018a).The nature of the TSHT is currently a topic of debate, with differing perspectives on whether it is part of a large accretionary wedge formed during a late Paleozoic-Early Mesozoic orogenic event (Xiao et al. 2003(Xiao et al. , 2005;;Yin et al. 2020) or a distinct terrane with a Precambrian basement (Ji et al. 2011;Hu et al. 2016;Zhang et al. 2018a).

LA-ICP-MS zircon U-Pb dating and Hf isotopes
Laser ablation inductively coupled plasma-mass spectrometry (LA -ICP -MS) was performed for zircon U -Pb dating by using a GeoLas Pro coupled to an Agilent7700× ICP-MS at the Key Laboratory for the study of focused Magmatism and Giant ore Deposits, MNR, Xi'an Center of Geological Survey, China Geological Survey.Each analysis incorporated a background acquisition of approximately 10 s (gas blank) and a data acquisition for 40 s from the selected samples.The counting time for U, Th, 204 Pb, 206 Pb, 207 Pb and 208 Pb is >25 ms.U -Pb isotope fractionation effects were corrected using zircon 91,500 as an external standard (Wiedenbeck et al. 1995).The off-line selection and integration of background and analyte signals, time-drift correction, quantitative calibration for trace element analyses, and U-Pb dating were performed by Glitter 4.4.Concordia diagrams and weighted mean calculations were carried out using Isoplot 3.0 (Ludwig 2003).Details of the instrument conditions and data acquisition procedures were similar to those described by Li and Wang (2015).The U -Pb zircon data are given in Supplementary Table S1.
In situ zircon Lu -Hf isotopic studies were conducted using a Geolas Pro laser ablation system coupled with a Neptune multicollector ICP -MS at the Key Laboratory for the Study of Focused Magmatism and Giant Ore Deposits, MNR, at the Xi'an Center of Geological Survey, China Geological Survey.Details of the instrumental conditions and data acquisition procedures are similar to those described by Hou et al. (2007).For the studies, a stationary laser ablation point with a beam width of 30 m was used.Before being injected into the ICP-MS plasma, the helium-bearing ablated aerosol was mixed with argon in a mixing chamber.All Hf analyses for U -Pb laser ablation were performed at the same locations.Zircon GJ-1 was used as the reference standard and yielded a weighted mean 176 Hf/ 177 Hf ratio of 0.282030 ± 40 (2SD) during this study.The zircon Hf isotope data are listed in Supplementary Table S2.

Whole-rock major and trace elements
Prior to grinding samples for the bulk rock composition examination of host rocks and enclaves, worn rims were removed.After sample powder was melted using Axios mAX (4 kW) equipment, major element compositions were measured using an X-ray fluorescence spectrometer (XRF) at the Xi'an Center for Geological Survey, China Geological Survey.Using references BHVO-2, W-2A, and GSP-2, the precision and accuracy of the XRF analysis were determined to be less than 1%.Trace element compositions were measured by digestion of samples in a HF-HNO3 solution in steel jacketed Teflon bombs at 190°C for 48 hours, followed by inductively coupled plasma-mass spectrometry (ICP -MS) analysis using an SX-II ICP -MS.The accuracy, verified against the United States Geological Survey (USGS) BHVO-2 and AGV-2 references, was less than five percent.Similar information on the data collection process is provided by Zhu et al. (2020).Whole-rock major and trace element data of Kungai volcanic rocks and standards are presented in Supplementary Table S3.

Whole-rock Sr-Nd isotopic compositions
The Sr-Nd isotope analyses were performed on a Neptune Plus MC -ICP-MS (Thermo Fisher Scientific) at the Wuhan Sample Solution Analytical Technology Co., Ltd., Wuhan, China.The Sr and Nd isotopic compositions were determined in static mode on the Nu Plasma MC -ICP-MS.Sr and Nd (and other rare earth elements) were separated/ concentrated using standard chromatographic columns with AG50 W-X8 and HDEHP resins following the procedure of Gao et al. (2004).The measured 143 Nd/ 144 Nd and 87 Sr/ 86 Sr ratios were normalized to 0.7219 and 0.1194, respectively.External reproducibility of the isotopic measurements was estimated by repeated analyses according to international standards.The Sr -Nd isotope data are listed in Supplementary Table S4.

Sampling and petrography
The Lower Carboniferous Wuluate Formation we investigated is exposed around Kungai Mountain in the Oytage marine volcaniclastic sedimentary basin in the northwestern SKT of West Kunlun Figures 1(a,  b).The Wuluate Formation exhibits little change and is unconformably overlain by the lower Permian Maerkusaishan Formation, Lower Cretaceous Kezilsu Group, and Pliocene Atushi Formation (Figure 1c).In the Oytage marine volcaniclastic sedimentary basin, abundant volcanic rocks are exposed in the Carboniferous Wuluate Formation with a thickness of approximately 1950 m (Figure 1b).The Wuluate Formation mainly consists of (a) ~1270 m basalts and metabasalt in the lower part, (b) ~130 m limestone and marble/silicate in the middle part, and (c) ~560 m meta-andesite and dacite/rhyolite tuff.

Whole-rock geochemical data
The Kungai silicic rocks have a loss on ignition (LOI) of less than 2 wt.%.However, the basaltic rocks have high loss on ignition (LOI) values that range from 2.06 to 6.04 wt.% (Supplementary Table S3).This result and petrographic observations suggest that the basaltic rocks have undergone varying degrees of alteration.However, rare earth elements (REEs) (e.g.La and Yb) and high field strength elements (HFSEs) (e.g.Th and Nb) exhibit linear correlations in the case of basalts, indicating that these elements were not disturbed significantly by alteration (not shown).Mobile elements (e.g.K and Rb) show roughly linear correlations with HFSEs, implying that their original signatures have been remarked upon by alteration.Thus, only immobile elements are used in the following discussion for the Kungai basaltic rocks.The basaltic rocks exhibit subparallel chondritenormalized rare earth element (REE) patterns (Figure 6a) and primitive mantle-normalized trace element patterns (Figure 6b).They show flat REE patterns with (La/Yb) N = 0.7-1.3[N means chondritenormalized, values of chondrite are from Sun and Mcdonough (1989)] (Figure 6a).They are enriched in LILEs (e.g.Th and U) and relatively depleted in Nb and Ti (Figure 6b).Four basaltic samples have ( 87 Sr/ 86 Sr) t ranging from 0.70356 to 0.70486 and ε Nd (t) from 6.6 to 7.1 (Supplementary Table S4).

Carboniferous magmatic rocks exposed in the West Kunlun Belt
Carboniferous magmatic rocks in the WKOB are mainly exposed at Waqia-Mazha in the south, which is closely related to the Mazha-Kangxiwa melange belt and the Oytag marine volcaniclastic sedimentary basin in the north (Figure 1).In the Mazha area, the calc-alkaline basaltic to silicic volcanic and intrusive rocks in the Halamilanhe Group were interpreted as arc magmas with a diorite yielding a sensitive high-resolution ion microprobe (SHRIMP) zircon U -Pb age of 338 ± 10 Ma (Li et al. 2006).The Carboniferous volcanic-sedimentary suites in the Oytag marine volcaniclastic sedimentary basin include the Wuluate Formation, Kuerliang Group, Yishake Group, Talong Group in Oytag, north Kudi, and Aqiang (e.g.Yun et al. 2015;Ji et al. 2018).The scarcity of available zircon U-Pb age data not only greatly hindered the matching of these Carboniferous magmatic rocks in different places but also hampered the tectonic evolution of the West Kunlun Belt during the late Paleozoic.The geochronological data reported here indicate that the Wuluate Formation from the Kungai area erupted ca.352 Ma (Figure 3).In the Xiaolebulong area, slightly younger silicic volcanic rocks (ca.332.9 Ma) in the Wuluate Formation have been identified by LA -ICPMS zircon U -Pb dating (Yun et al. 2015).These available zircon U -Pb age data suggest that the Wuluate Formation likely formed in the Early Carboniferous.

Effects of fractional crystallization and crustal contamination
Primary magmas that are in equilibrium with mantle olivine are generally Mg-rich with Mg# > 72 (Niu and O'Hara 2008).The primitive Kungai basalts have high MgO contents (9.0-16.4wt.%), high Mg# (65.6-71.4) and high Cr (266-1280 ppm) and Ni abundances (91.5-232 ppm), which could only undergo minor fractionation of olivine (Lee et al. 2009).The composition of the Kungai basalts exhibits a trend of increasing total FeO with decreasing Mg#, showing a typical iron-rich (also called tholeiitic) liquid line of descent (LLD) (Figure 5c) (Grove et al. 2003;Villiger et al. 2007).The positive correlations between Mg# and Ni and between Mg# and Cr indicate that mafic minerals (olivine and pyroxene) are the major mineral phases of fractional crystallization Figures 5(d, e).The negative correlation between Mg# and Al 2 O 3 proves the fractional crystallization of plagioclase (Figure 5b).However, the weak Eu anomalies (Eu* = 0.79-1.00) in the Kungai basalts suggest that the   (Winchester and Floyd 1977).
plagioclase crystallized under conditions more oxidizing than QFM + 2, leading to low Eu 2+ /Eu 3+, or that the proportion of plagioclase may not have exceeded 10% during fractional crystallization (Aigner-Torres et al.

2007
).In addition, the subparallel REE and trace element patterns, as shown by the negative correlation between incompatible elements (e.g.La in olivine, pyroxene, and plagioclase) and Mg#, also suggest that the Kungai basalts mainly result from fractional crystallization of olivine, pyroxene and plagioclase (Figure 5f).Furthermore, Sample S07-14 was selected to represent the basaltic parent magma using 25% olivine + 20% monazite + 25% hornblende + 25% plagioclase + 5% magnetite as the crystallographically differentiated phase.The REE patterns of the basalts matched the simulated results of S07-14 crystallographic differentiation, showing that it underwent 40% to 95% crystallographic differentiation (Figure 7).Therefore, it is suggested that the basalts of the Kungai basalts have undergone extensive crystallographic differentiation of olivine, monazite, hornblende, plagioclase and magnetite.
The continental crust is characterized by enriched LILEs and depleted HFSEs with enriched Sr -Nd isotopes (Rudnick and Gao 2003).Thus, crustal contamination will significantly modify the Th/Nb ratio and Sr -Nd isotopes.The Kungai basalts show no linear relationship between (Th/Nb) PM (PM means primitive mantle-normalized, values of PM are from Sun and Mcdonough 1989) and Mg# (Figure 8).This result, in combination with their uniformly depleted Sr-Nd isotopes, indicates little or no contribution from the Precambrian basement in West Kunlun (Figure 8).It should be noted that, the (Th/Nb) PM of the Kungai basalts is higher than that of mantle-derived ocean island basalt (OIB) or mid-ocean ridge basalt (MORB) (<1, Sun and Mcdonough 1989), suggesting that the negative Nb anomaly in the Kungai basalts is likely inherited from their source.In summary, the whole-rock elemental and isotopic characteristics of the Kungai basaltic rocks are mainly controlled by fractional crystallization with possibly minor crustal contamination.

Mantle sources and melting conditions
The Kungai basaltic rocks exhibit slightly depleted LREE, lower TiO 2 /Yb, Nb/Yb, Zr/Y, and Nb/Y, and depleted Sr-Nd isotopes [( 87 Sr/ 86 Sr)t = 0.70356-0.70486and ε Nd (t) = 6.6-7.1] and zircon Hf isotopes [ε Hf (t) = 7.6-12.2],suggesting that they are mainly from a depleted mantle source (Figures 6a, 7 b, 9a, and 9b).However, the enriched LILEs and inherited negative Nb anomaly indicate the involvement of enriched components in their source, possibly from recycled/subduction sediments and/or slab fluids (Stracke et al. 2003 is generally represented using the ratio between a mobile element and Th as an immobile element (e.g.Ba/Th).La/Sm is a proxy for sediment incorporated into the mantle source (Hermann and Rubatto 2009;Labanieh et al. 2012).The varied Ba/Th and La/Sm ratios indicate that hydrous fluid and sediment both influenced the signatures of the Kungai basaltic rocks (Figure 9c).Based on the information on the mantle source, trace elements of the Kungai basaltic rocks are used to quantitatively determine the melting conditions.The trace elements of primitive magmas during hydrous mantle melting vary as a function of temperature, water content and melt productivity with depth (Mckenzie and O'nions 1991;Katz et al. 2003;Kimura and Kawabata 2014;Kimura 2017).Based on a thermodynamic fractional adiabatic melting model of depleted MORB mantle (DMM) and parameterized P -T-H 2 O -F melting relationship of mantle peridotite (Mckenzie and Bickle 1988;Katz et al. 2003), forward and reverse trace element models were carried out by using the adiabatic mantle melting simulator HAMMS1 (Kimura and Kawabata 2014).This method allows us to estimate the melting conditions of primitive basalts based on trace element signatures Figures 10(a, b).The results of this program show that the primitive Kungai basaltic rocks fit well with source components of (98.87-99.54wt.%) DMM + (0.06-0.15 wt.%) H 2 O + (0.4-1.0 wt.%) sediment under mantle potential temperatures of 1280-1330°C and lithospheric thicknesses of 50-60 km (Figure 10c) (Supplementary Table S5).Such results suggest that the Kungai basaltic rocks may have been generated by slab fluids and sedimentmodified DMM under asthenospheric mantle temperatures and a thin lithosphere.
Trace element ratios of incompatible elements, such as Nb/Y, Th/Yb and Ta/Yb, will insignificantly change during partial melting and fractional crystallization and therefore reflect differences in source compositions (Fitton et al. 1997;Condie 2005).Back-arc basin spreading is always associated with subduction, resulting in the geochemistry of back-arc basalts exhibiting both MORBlike and arc-like characteristics (Stern 2002;Stern et al. 2006;Xia and Li 2019).The Kungai basaltic rocks exhibit flat REE patterns (Figure 6a) and depleted Sr-Nd and Hf isotopes (Figures 7b and 12) but enriched LILEs and negative Nb anomaly (Figure 6b), which is similar to the back-arc basalts in the Mariana Trough (Figures 9a,   (Tamura et al. 2011).Precambrian rocks in the WKOB (Liu et al. 2015).Normalization data are from Sun and Mcdonough (1989).
9b, and 9d).Such a result is also consistent with the results of trace element modelling.The water contents in the source of Kungai primitive magmas (0.06-0.15%) are similar to BABB (0.024-0.50%), which are significantly higher than MORB (e.g.0.012%; Dixon et al. 2004) but lower than the island arc basalt (IAB) (e.g.0.25-1.00%;Dixon et al. 2004).As a result, the water content in the primary basalts could reach 1.3 wt.%, which is similar to BABB in the southwest Pacific and Mariana Trough (0.2-2.0%; Danyushevsky et al. 1993;Stolper and Newman 1994) (Supplementary Table S5).The modelling results can further be proven by the evolution of the Kungai basalts following a tholeiic trend of increasing FeO rather than a calc-alkaline trend of increasing SiO 2 (Figure 5), indicating that the parental magma is not 'wet' (>2%) (Mandler et al. 2014).

Petrogenesis of the Kungai silicic rocks
The Kungai silicic rocks are calcic to calcic-alkaline, silicasaturated meta/peraluminous (Frost and Frost 2008) Figures 11(a, b).The distribution of SiO 2 content in Kungai volcanic rocks is generally bimodal with geochemical groupings at 45.2-52.5 and 63.8-76.9wt.% (Figure 4).The generation of such a compositional gap (Daly gap) is a subject of much debate.Hypotheses focus on the origin of silicic rocks, which are variably interpreted as a result of partial melting of mafic precursors and/or crust (e.g.Suneson and Lucchitta 1983;Mahoney et al. 2008;Colón et al. 2018) or fractional crystallization of basaltic melts with possible crustal contamination (e.g.Sisson and Grove 1993;Annen and Sparks 2002).Although the silicic rocks show similar flattened REE patterns to the basaltic rocks, the zircon Hf isotopes of the silicic rocks [Ɛ Hf (t) =-3.1 -−0.1] are significantly lower than those of basaltic rocks (7.6-12.2) (Figure 12).
As shown in the Harkar diagrams, the major and trace elements in silicic rocks are not correlated with those of basaltic rocks, proving that silicic rocks cannot be produced by fractional crystallization of basaltic rocks (Figure 5a).

Tectonic setting
The tectonic background of the West Kunlun region during the Carboniferous period has been a subject of debate due to the complex tectonic evolution history and the influence of multiple tectonic events  (Liu et al. 2015),SCLM-derived Cambrian mafic rocks (Liu et al. 2019),Triassic silicic rocks (Jiang et al. 2013;Liu et al. 2015), and Cambrian to Silurian silicic rocks (Wang et al. 2017;Liu et al. 2019).
in the area.Some researchers suggest that West Kunlun was in an island arc environment during the Carboniferous period and may have undergone intraoceanic subduction (Deng 1989; Jiang et al.   (Pearce 1982).Data sources: PM, OIB, N-MORB, E-MORB (Sun and Mcdonough 1989), Upper Crust (Rudnick and Gao 2003).

2008
).On the other hand, Chen et al. (2007) analysed the lithofacies characteristics and tectonic palaeogeography of the Carboniferous in the West Kunlun orogenic belt and proposed that the Kunlun orogenic belt was generally in an active marginal rift.
Research on Tianpulu-Aqiang volcanic rocks and Oyitake granite suggested that West Kunlun was in a passive continental margin rift environment of the Tarim Plate during the late Paleozoic (Bian et al. 2002).Meanwhile, Gao et al. (2015) and Ji et al. (2018) proposed that the Carboniferous volcanic rocks were the result of back-arc extension during subduction of the Paleo-Tethys.The development of bimodal volcanic rocks in the Carboniferous Wuluate Formation and the features of BABB-type basalts suggest that during the Carboniferous period, the SKT was in a back-arc basin setting induced by northward subduction of the Paleo-Tethys.Bimodal volcanic rocks commonly occur in extensional environments related to postcollisional, intraplate or back-arc rifting (e.g.Chen et al. 2018;Wang et al. 2021).The Kungai basaltic rocks exhibit geochemical affinity to the BABB (Figures 9a,9b Felsic rocks: This study Yun et al. (2015) Volcanic rocks in the Wuluate Formation: associated siliciclastics and tuffs, suggesting eruption in a deep-sea environment (Ji 2005;Yun et al. 2015).In the central SKT, a suite of dark and thinly bedded carbonaceous mudstone and siltstone assemblages developed from the Carboniferous to lower Permian along the Qiaerlong area, and the area around Gezi is dominated by carbonate rocks interbedded with bioclastic and oolitic limestone (Ji et al. 2018).In the eastern SKT, radiolarian bedded chert intercalations are present in the A'qiang volcanic rocks, suggesting that the Carboniferous volcanic rocks of this area were deposited in a slope-deeper basin environment (Ji 2005;Gao et al. 2015).The deep and shallow depositional environment, enriched in terrestrial margin clastic material, is different from that of oceanic basins and is similar to the back-arc basin setting (Ji et al. 2018).Therefore, combining geochemical data with regional geological characteristics, we suggest that the Kungai volcanic rocks were deposited in a different environment from the oceanic basin as a set of back-arc basin volcanic rocks.

Implications for the evolution of WOKB
Initial subduction of the Proto-Tethys predates the Cambrian and generated massive subductionassociated calc-alkaline granites, gabbro, and basalts (Zhang et al. 2018b;Liu et al. 2019;Sui 2021).The Devonian fluvial-lacustrine molasse formation of the Chizilav Formation suggests that significant ocean-land transformations and collisional orogeny occurred during this time in the SKT (Chen et al. 2007).Considering the exposure of the 420-405 Ma Kudi A-type granite slab in an intraplate extensional environment (Yuan et al. 2002;Xiao et al. 2005) and the clastic zircon dating results (Wang et al. 2020), it is inferred that the closure of the Proto-Tethys between the South Kunlun and Tarim blocks occurred between 431 and 420 Ma (Figure 13a).The collision with the Qaidam Block also occurred at the same time in East Kunlun (Qi 2015).The Tanshuihai Terrane, on the other hand, has few reports of magmatism from this period and may still be located on the northern margin of the Gondwana continent (Figure 13a) (Sui 2021).
There is a general consensus that West Kunlun and adjacent areas experienced subduction of the Paleo-Tethys during the late Paleozoic (Xiao et al. 2002(Xiao et al. , 2005;;Rembe et al. 2021;Tang et al. 2021).The geochronology of the Wuluate Formation volcanic rocks indicates that subduction of the Paleo-Tethys had already occurred and that a back-arc basin had developed before the Early Carboniferous.A subsequent question is the polarity of this subduction.It is unlikely that this back-arc basin formed by the southward subduction of the Proto-Tethys.This is because the northward drifting of the SKT collapse onto the Tarim terrane ended the evolution of the Proto-Tethys in the Devonian (Chen et al. 2007;Wang et al. 2020).Much more likely, the back-arc basin was formed by the northward subduction of the Paleo-Tethys (Figure 13b).
In the Eastern Kunlun region of the marginal zone, the Late Devonian -Early Carboniferous magmatic activity was seldom exposed, with only a few occurrences of intermediate-silicic volcanic rocks (364-343 Ma;Xu et al. 2020) and sporadic granitic intrusions (360-345 Ma;Wang 2017;Wu et al. 2019).Although the tectonic setting of this period remains a subject of debate, most researchers argue that it represents an active continental margin (Wang 2017;Dong et al. 2018Dong et al. , 2021;;Xu et al. 2020).Furthermore, it has been suggested that the Eastern Kunlun shifted from compression to extension environments during the Carboniferous to Permian (Dong et al. 2018(Dong et al. , 2021)).The Tuokuzidaban Group, which is widely distributed in the Eastern Kunlun, is thought to be a product of northward subduction of the Paleo-Tethys (Xu et al. 2020).While, the Dagangou Formation and the Shiguaizi Formation developed within this area may have formed in an extensional backarc environment (Wang 2017).
During the Early Carboniferous, palaeogeographic reconstructions indicate that the East Qiangtang block was adjacent to the Yangtze craton and the Cathaysian continent in the northern margin of the Paleo-Tethys Ocean (Li et al. 2009;Zhu et al. 2013) (Figure 13b).Zhang et al. (2018) proposed that an   (Dong et al. 2010;Pullen et al. 2011).The rollback of the southward-subducted Paleo-Tethys lithosphere induced upwelling and decompressional melting of asthenosphere, developing a back-arc system that eventually led to the northward drift of numerous microterranes (e.g.Lhasa Terrane, Qiangtang Terrane, etc.) from the northern margin of the Gondwana continent into the Paleo-Tethys (Zhu et al. 2013).The Tianshuihai Terrane at the northern margin of the Gondwana continent may also have drifted northward, culminating in the Late Triassic collage of the SKT (Sui 2021).

Conclusions
The Wuluate Formation in northwestern West Kunlun is characterized by a bimodal volcanic suite.Zircon U-Pb dating indicates that the rhyolites erupted at

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Figure 3 .
Figure 3. Cathodoluminescence (CL) images of representative zircons and concordia diagrams of the Kungai dolerite and rhyolites.Solid and dashed circles indicate the locations of U-Pb dating and Hf analyses, respectively.

Figure 8 .Figure 9 .
Figure 8. Simulation of thecrystallographic differentiation of trace and rare earth elements.Primitive dolerite S07-14 is selected as the starting composition.
oceanic back-arc basin system present within the Longmuco-Shuanghu-Lancang Paleo-Tethys Ocean (Qiangtang ophiolites) during the Late Devonian to Early Carboniferous.These lines of evidence suggest that microcontinental blocks along the northern margin of the Paleo-Tethys Ocean were likely subject to a northward subduction dynamic mechanism in the Early Carboniferous.The extensional deformation within these regions may be associated with back-arc extension driven by slab rollback.Meanwhile,Murphy et al. (2011) suggest a similar backarc system for the eastern part of the Gondwana continent during the Carboniferous.Studies of calc-alkaline granitoids in the western Qiangtang and southern Lhasa Terrane also suggest that southward subduction of the southern margin of the Paleo-Tethys occurred during the late Devonian approximately 348-353 Ma, which is coeval with the zircon U-Pb dating result of 354 Ma for the basalt.The basaltic rocks show a tholeiitic magmatic evolution trend.They have flattened REE patterns and depleted whole-rock Sr-Nd and zircon Hf isotopes, showing geochemical affinity to BABB.Trace element modelling results indicate that the primitive basaltic compositions fit well with the products of the DMM + sediments + H 2 O source under melting conditions of asthenospheric temperature and thin lithosphere thickness.The silicic rocks are calcic to calcic-alkaline and meta/peraluminous.They exhibit flattened REE patterns and slightly enriched zircon Hf isotopes, which could be from Precambrian metaigneous sources of West Kunlun.The Kungai bimodal suite is likely to have formed in a backarc setting corresponding to the northward subduction of the Paleo-Tethys.The subduction of the Paleo-Tethys had already occurred, and a back-arc basin had developed before the Early Carboniferous.

Figure 13 .
Figure 13.Schematic illustrations of the evolution of the WKOB from the Devonian (a) to Carboniferous (b) (modified from Sui 2021).