The Proto-Tethys Ocean has played a significant role in the geological history of Earth. However, ongoing debates persist regarding the timing and polarity of its early subduction. Volcanic rocks associated with iron deposits in the Bulunkuole Complex, West Kunlun Orogen, offer insights into both the complex’s formation age and Proto-Tethys evolution. This study presents newly obtained zircon U–Pb age data (~536 Ma) along with comprehensive whole-rock major and trace element and Sr–Nd–Hf isotope analyses of these volcanic rocks. Our dataset implies that the Bulunkuole Complex partly formed in the early Paleozoic rather than entirely in the Paleoproterozoic, as previously suggested. Geochemically, the volcanic rocks exhibit enrichments in large ion lithophile elements and light rare earth elements, along with depletions in high-field strength elements. They also display elevated initial 87Sr/86Sr values (0.71093, 0.72025) and negative εNd(t) values (−5.13, −6.18), classifying them as continental arc volcanic rocks. These geochemical fingerprints, complemented by zircon εHf(t) values (−12.7 to −1.6), indicate that the parental magmas of the volcanic rocks were produced by partial melting of the lithospheric mantle wedge, which had been metasomatized by subducted sediment-derived melts. The available data, in conjunction with previously published findings, strongly suggest that the Proto-Tethys Ocean subducted southward prior to approximately 536 Ma due to the assembly of Gondwana. Subsequent slab rollback may have resulted in a crustal thinning of 9–25 km during 536–514 Ma. Further shifts in subduction dynamics led to the transition from high-angle subduction to either normal or low-angle subduction, facilitating the formation of a thicker crust ranging from 39 to 70 km between 514 and 448 Ma. This study, therefore, provides valuable insights into the early evolution of the Proto-Tethys Ocean and contributes significantly to our understanding of the tectonic history of the West Kunlun Orogen.

The West Kunlun Orogen (WKO), located along the southwestern margin of the Tarim Craton (Figure 1(a)), occupies a pivotal tectonic junction that connects the domains of central Asia and Tethys [1]. Its evolutionary history is intricately intertwined with the Paleozoic subduction and closure events of the Proto-Tethys Ocean—an oceanic realm that emerged following the breakup of the Neoproterozoic Rodinia supercontinent [2-4]. Through its intricate geological framework and magmatic activities, the orogen offers crucial insights into the tectonic processes that shaped Earth’s ancient oceans [5]. Serving as a significant orogenic belt, the WKO has been at the center of debates concerning the initiation timing, polarity, and process of Proto-Tethys Ocean subduction. These discussions encompass various propositions, including scenarios of encompassing discussions on southward subduction [6, 7], northward subduction [8, 9], and bidirectional subduction beneath the southern Kunlun terrane and the Tianshuihai terrane [10]. Varied proposed initiation times for subduction ranging from late Neoproterozoic to early Paleozoic [3-6]. Within this context, volcanic rocks emerge as pivotal witnesses to the magmatic events that accompanied these tectonic dynamics.

The focus of this study lies in unraveling the ages and petrogenesis of volcanic rocks within the WKO, enriching our understanding of the geodynamic evolution of the Proto-Tethys Ocean. The WKO comprises distinct tectonic units (Figure 1(b)), including the North Kunlun Terrane (NKT), South Kunlun Terrane (SKT), Taxkorgan-Tianshuihai Terrane (TTT), and Karakorum Terrane (KKT). Among these, the TTT’s Bulunkuole Complex garners attention as an enigmatic geological entity. Reputed as either the early Precambrian basement of the TTT or the oldest lithostratigraphic unit within the WKO [11, 12], this complex has gained prominence due to its significant iron belt. With proven Fe reserves of 0.8 billion tons and an estimated resource of 1.556 billion tons (Figure 2) [13, 14], ore exploration not only underscores its economic value but also emphasizes its geological importance.

The Bulunkuole Complex represents an extensive geological unit exposed across the Taxkorgan region along the southwestern margin of the Tarim Craton. It is characterized by the coexistence of volcanic rocks and sediments, closely associated with the iron orebodies, offering a window into Paleoproterozoic, early Paleozoic, and Triassic history through zircon U-Pb ages [6, 11, 15-19]. These volcanic rocks provide a unique avenue to decipher eruption ages and infer the prevailing tectonic setting. However, debates persist regarding their ages, petrogenesis, and tectonic settings [20-22], ranging from Paleoproterozoic intracontinental settings to early Paleozoic subduction-related magmatism, thus emphasizing the necessity of comprehensive investigations.

To address these uncertainties, this study employs zircon U-Pb age data, Lu-Hf isotopes, whole-rock major and trace element data, and Sr-Nd isotopes for volcanic rocks of the Bulunkuole Complex at the Zankan deposit. These analyses aim to unravel the petrogenesis, magma sources, and tectonic settings of these volcanic rocks. Through integration with existing data, we endeavor to refine the formation age of the Bulunkuole Complex, constrain the mineralization age of the Zankan iron deposit, and elucidate potential variations in crustal thickness during the Proto-Tethys subduction.

The Pamir-West Kunlun-Altun Mountains [23-25], forming the southern margin of the Tarim Basin, connect eastward with East Kunlun [26], Qinling [27-29], and Dabie Shan [30], constituting the very large Central China orogenic belt (CCOB) [31]. The CCOB evolved from the Proto- to Paleo-Tethys and took shape through the early Mesozoic continental collision between the united North China-Tarim continent and segments separated from Gondwanaland after long-term multistage subduction-related accretion and terrane amalgamation (Figure 1(a)) [27, 32-36].

The WKO consists of four tectonic units (Figure 1(b)), namely, from north to south, the NKT, the SKT, the TTT, and KTT, with the Kudi, Mazha−Kangxiwa, and Longmucuo-Shuanghu/Karakorum faults forming their boundaries (Figure 1(b)) [1, 37].

The NKT is composed of three deformed tectonostratigraphic units in ascending sequence: the Precambrian basement, the slightly metamorphosed Ordovician sedimentary succession, and the upper Devonian−Permian volcanic-sedimentary sequence. The Precambrian basement of the NKT includes the Heluostan Complex (tonalite−trondhjemite−granodiorite) in the Akaze–Xuxugou area and sedimentary rocks in the Tiekelike area. The Heluositan Complex has the oldest known age of 2.42–2.27 Ga and was metamorphosed under amphibolite-facies conditions during 2.0–1.8 Ga associated with the assembly of the Columbia supercontinent [38, 39]. The Precambrian sedimentary sequences in Tiekelike mainly comprise metamorphic clastic rocks and volcanic rocks with ages ranging from Paleoproterozoic to Neoproterozoic [40, 41]. A mildly metamorphosed Sinian succession of laminated carbonates, volcanic rocks, shales, marls, and tuffites, interpreted by Mattern and Schneider [2] as a rift sequence, is well exposed around Akaz Pass in the NKT; however, no stratigraphic record of such a rift sequence has been found [42]. The Ordovician strata, unconformably overlying the Precambrian strata, are composed of carbonate and clastic rocks. Silurian to Middle Devonian strata are absent in the NKT. The Late Devonian strata are terrestrial red molasse deposits [2]. The Carboniferous and Permian strata consist of marine-facies clastic sediments and carbonate rocks. Few Paleozoic intrusions crop out in the NKT, such as ca. 470 Ma Aqiang granodiorite [43], ca. 450 Ma adakitic Alaleike granitoids [44], and ca. 430 Ma Buya alkali-granite [45].

Bound by the Oytag-Kudi Suture to the north and the Mazha−Kangxiwa Fault to the south (Figure 1(b)), the SKT shows lithostratigraphic units similar to those of the NKT until the Devonian [2]. Its basement includes greenschist- to amphibolite-facies gneisses, migmatites, mica schists, and marbles. Zircon U–Pb dating of the gneisses yielded an age of 2048 ± 20 Ma [46]. A pre-Devonian stratigraphic gap also exists in the SKT, where there is no stratigraphic record between the Ordovician sedimentary sequence and upper Devonian terrestrial red molasse deposits [2]. The Carboniferous strata consist of a volcanic sequence, and marine clastic rocks and limestone are locally interlayered with basaltic lavas. The Permian strata are composed of a volcanic-sedimentary formation, which sporadically crops out in the southern part of the SKT close to the Mazha-Kangxiwa Fault. Within the SKT, the Kudi ophiolite suite is exposed and represents an obducted ophiolite unit that was thrust from the suture, composed of ultramafic-gabbroic rocks, a volcanic sequence, and marble [1, 42]. The ultramafic-gabbroic rocks form a southward-thrust 3-km-thick slab that contains sheared basal serpentinite, layered/foliated chromite-bearing dunite, harzburgite, clinopyroxenite, and gabbro [1]. The volcanic sequence consists of massive and pillow basalts, basaltic andesites, andesites, boninites, tuffs, welded andesitic breccias, and agglomerates. The ultramafic rocks yield a whole-rock Sm–Nd isochron age of 651 ± 53 Ma [47]; the gabbroic rocks give sensitive high-resolution ion microprobe (SHRIMP) U–Pb zircon ages of 510 ± 4 Ma [48] and 525 ± 9 Ma [49]. The dolerite from the lower part of the volcanic sequence yields a laser ablation inductively coupled plasma mass spectrometry (LA-ICP‒MS) zircon U−Pb age of 500.3 ± 8.0 Ma [50]. In the upper part of the volcanic sequence, basaltic lavas are interlayered with dacite with a SHRIMP zircon U–Pb age of 492 ± 7 Ma [1]. These volcanic rocks are inferred to have formed in a supra-subduction zone setting [51, 52]. The cherts overlying the volcanic sequence contain radiolaria of the Late Ordovician–Silurian age [53]. The marble with Ordovician crinoids is interpreted by Xiao et al. [3] as fragments of a seamount in an accretionary wedge. The early Paleozoic arc is developed in the SKT and comprises granitic plutons, including diorite, tonalite, granodiorite, monzogranite, and syenogranite, with ages of ca. 514–435 Ma, which are close to the Kudi Suture (Figure 1(b)) [1, 42, 53-57]. Furthermore, ca. 421 Ma North Kudi granites in the SKT show an affinity to within-plate granites [42].

The TTT includes the Bulunkuole Complex, an Ordovician–Triassic submarine volcanic-sedimentary sequence, and a Jurassic–Cretaceous sedimentary sequence. The Bulunkuole Complex is mainly composed of greenschist- to amphibolite-facies metamorphic rocks, and their formation ages remain debated, with ages ranging from Neoarchean to early Mesozoic (Figure 3) [12, 15-17, 37, 40, 58]. This complex consists mainly of biotite-plagioclase gneisses, plagioclase-amphibole gneisses, biotite-quartz schists, sillimanite-garnet schists, magnetite quartzite, metamorphic siltstones, marbles, and plagioclase-amphibole schists intercalated with volcanic rocks. High-pressure metamorphic rocks have been discovered in sillimanite-garnet schist and quartzite units of the Bulunkuole Complex [37]. The Ordovician–Triassic volcanic-sedimentary sequence comprises metamorphosed argillaceous clastic rocks, carbonates, siliceous shales, dacites, and andesites. The Jurassic–Cretaceous sequence is characterized by carbonate and clastic sedimentary rocks, consisting of limestone, mudstone, shale, quartz sandstone, and conglomerate. Recently, many subduction-related plutons and volcanic rocks have been reported in the TTT during the early-middle Cambrian (Figure 1(b); ca. 536–514 Ma), such as monzogranite, granitic porphyry, and bimodal volcanic rocks, which formed in response to the evolution of the Proto-Tethys Ocean [5, 10, 21, 59, 60]. Granulite [61], khondalite [62], and high-pressure metamorphic rocks [37, 63, 64] occur in the eastern and western Kangxiwa fault belts, which both formed during the Triassic and are related to the closure of the Paleo-Tethys Ocean. In the Mazha-Kangxiwa area, a tectonic mélange is composed of Triassic sandstones, shales, and abundant Carboniferous-Permian rocks [24, 65, 66]. Furthermore, island arc- or collision-type granites along the Kangxiwa fault belt to the south of Kudi, Mazha, Sanshiliyingfang, and Kangxiwa emerged during 220–180 Ma [67]. Consequently, the Mazha-Kangxiwa fault belt is widely acknowledged as a plate suture [24, 65, 66, 68].

The Zankan iron deposit is in the northern part of the TTT and geographically within approximately 70 km southeast of Taxkorgan County, Xinjiang Province (Figure 2). It yields proven reserves of 1.46 × 108 t Fe with average grades of 28.3%−58.8% and estimated reserves of 1.8 × 108 t Fe [13, 58]. The main lithostratigraphic units in the Zankan orefield are the pre-Devonian Bulunkuole Complex and Silurian Wenquangou Group (Figure 4). The Bulunkuole Complex consists of metavolcanic and metasedimentary rocks intercalated with gypsum layers, which were metamorphosed to greenschist facies (locally up to amphibolite facies). The Silurian Wenquangou Group, unconformably overlying the Bulunkuole Complex, is mainly composed of quartzite, quartz sandstone, and marble.

The Zankan iron field has five orebodies (Figure 4), among which the No. 1 orebody is the largest and accounts for 72.01% of the total ore reserves. Orebodies occur as bedded or near-bedded shapes parallel to their host rocks (Figure 5). The majority of orebodies are hosted in the biotite-quartz schists, plagioclase-amphibole schists, and amphibole-quartz schists of the Bulunkuole Complex, although some orebodies are spatially associated with the volcanic rocks (figures 4 and 5).

The majority of volcanic rocks are observed in the mining area (Figure 6(a)), a part of the Bulunkuole Complex (Figure 6(c)). Volcanic rocks, locally overlying iron orebodies (Figure 4), are conformable with iron orebodies (Figure 6(b)), and several of them are altered into iron ores. These rocks show typical porphyritic texture (Figures 6(d)–6(f)). The phenocrysts mainly include 20–40 vol.% hornblende, plagioclase, and quartz (Figure 6(d)–6(f)). The groundmass is composed of quartz, plagioclase, and biotite. Accessory minerals are apatite, magnetite, and zircon. These volcanic rocks have generally undergone alterations to various degrees. Some of the plagioclases have undergone sericitization (Figure 6(e)), but polysynthetic twinning can usually be preserved (Figure 6(e)). Minor quartz phenocrysts exhibit euhedral and occasional hexagonal and etched textures (Figure 6(f)). We collected several volcanic rocks for geochronological and geochemical analyses. The sampling sites of volcanic rocks for geochronological analyses are indicated in Figure 4, and samples ZKA0-1 and ZKA1-3 are in the No. 2 orebody roof and the contact of the No. 3 orebody footwall and host rocks, respectively.

4.1. Zircon U–Pb Dating and Trace Element Analysis

Zircon grains were separated using conventional heavy liquid and magnetic techniques. Representative zircon grains were handpicked and mounted in epoxy resin discs and then polished and coated with gold. Transmitted and reflected light photomicrographs and cathodoluminescence (CL) imaging were used to reveal the internal structures of these zircon grains.

Zircon U–Pb and trace elements were analyzed by using LA-ICP‒MS at the State Key Laboratory of Ore Deposit Geochemistry, Institute of Geochemistry, Chinese Academy of Sciences (CAS), in Guiyang. The detailed analytical procedure can be found in Liu et al. [69, 70]. A GeoLasPro laser ablation system (Lamda Physik, Gottingen, Germany) and Agilent 7700× ICP‒MS (Agilent Technologies, Tokyo, Japan) were combined for the experiments. The 193 nm ArF excimer laser, homogenized by a set of beam delivery systems, was focused on a zircon surface with a fluence of 10 J/cm2. The ablation protocol employed a spot diameter of 44 µm at a 5 Hz repetition rate for 40 seconds (equal to 200 pulses). Zircon 91500 was used as an external standard to correct instrumental mass discrimination and elemental fractionation. Zircons GJ-1 and Plešovice were treated as quality controls for geochronology. The lead concentration of zircon was externally calibrated against NIST SRM 610 with Si as the internal standard and Zr as the internal standard for other trace elements [70]. Offline selection and integration of background and analytic signals and time-drift correction and quantitative calibration for trace element analyses and U–Pb dating were performed by ICPMSDataCal [69, 70]. The analytical results are reported with 1σ error. The weighted mean U–Pb ages (with 95% confidence) and concordia plots were processed using ISOPLOT 3.0 [71]. The age data given in the figures and discussion are 207Pb/206Pb ages for grains older than 1.0 Ga and 206Pb/238U ages for younger grains.

4.2. Zircon Lu–Hf Isotopes

In situ zircon Lu–Hf isotope analyses were undertaken on the same spots previously dated by LA-ICP‒MS using a Neptune multicollector ICP‒MS (MC-ICP‒MS) equipped with a GeoLas-2005 laser ablation system at the State Key Laboratory of Continental Dynamics in Northwest University, Xi'an. The detailed analytical procedure followed that described in Yuan et al. [72].

The interference of 176Lu on 176Hf was corrected by measuring the intensity of interference-free 175Lu using the recommended 176Lu/175Lu ratio of 0.02669 [73] to calculate 176Lu/177Hf. Similarly, the isobaric interference of 176Yb on 176Hf was corrected by using a recommended 176Yb/172Yb ratio of 0.5586 [74] to calculate 176Hf/177Hf ratios.

The initial 176Hf/177Hf ratios were calculated with reference to the chondritic uniform reservoir (CHUR) at the time of zircon growth from magmas. The decay constant for 176Lu and the chondritic ratios of 176Hf/177Hf and 176Lu/177Hf used in the calculations were 1.865 × 10-11 y-1 [75] and 0.282772 and 0.0332 [76], respectively.

The εHf(t) value is calculated using the U–Pb age of an individual grain. Plots of zircon U–Pb age versus εHf(t) are used to display the spread of εHf(t) values relative to the CHUR and depleted mantle (DM) evolution lines. Magmas is dominated by a mantle-derived component plot near the DM line, whereas magmas derived from older crustal rocks plot near or below the CHUR line.

Single-stage Hf model ages (TDM1) are calculated relative to the DM with a present-day 176Hf/177Hf ratio of 0.28325 and 176Lu/177Hf ratio of 0.0384. Two-stage Hf model ages (TDM2) are calculated by referring to a 176Lu/177Hf value of 0.015 for average continental crust [77, 78] and an fLu/Hf value of −0.55 for average continental crust [79]. The fLu/Hf ratio equals (176Lu/177Hf)S/(176Lu/177Hf)CHUR−1, where (176Lu/177Hf)CHUR = 0.0332, and the (176Lu/177Hf)S value is obtained from sample analysis.

4.3. Major and Trace Element Analyses

All major and trace element studies were carried out at the Institute of Geochemistry, CAS, in Guiyang. The major oxides in the samples were measured by using X-ray fluorescence spectrometry on fused glass beads. The analytical uncertainty was less than 5%. Trace elements (including rare earth elements) were determined by ICP‒MS, with an analytical precision better than 10% relative standard deviation. We used standards OU-6, AMH-1, and GBPG-1 as reference materials. The procedure for the trace elements followed that described in detail by Qi and Gregoire [80].

4.4. Whole-Rock Sr–Nd Isotope Analyses

Whole-rock Sr–Nd isotope analyses were performed at the State Key Laboratory of Isotope Geochemistry, Guangzhou Institute of Geochemistry, CAS. For Sr and Nd isotope analyses, powders of samples were dissolved in HF-HClO4 at 200°C for 1 week. The isotope measurements were performed on a Neptune Plus multicollector mass spectrometer equipped with nine Faraday cup collectors and eight ion counters. Details of the Sr and Nd isotope analytical methods were similar to those of Waight et al. [81], Yang et al. [82], and Chernyshev et al. [83]. The normalizing factors used to correct the mass fractionation of Sr and Nd during the measurements were 86Sr/88Sr = 0.1194 and 146Nd/144Nd = 0.7219. Analyses of standards NIST SRM 987 and Shin Etsu JNdi-1 over the measurement period provided 87Sr/86Sr = 0.710280 ± 6 (2σ, n = 15) and 143Nd/144Nd = 0.512087 ± 2 (2σ, n = 18), respectively. Analytical errors for Sr and Nd isotope ratios are given as 2σ.

The ratios of 87Rb/86Sr and 147Sm/144Nd were calculated using the Rb, Sr, Sm, and Nd concentrations obtained by ICP‒MS and the measured isotope ratios of the samples. (87Sr/86Sr)i represents the initial 87Sr/86Sr ratio when the rock formed, using λRb = 1.42 × 10−11 a−1 [84]. εNd(t) was calculated using (143Nd/144Nd)CHUR = 0.512638, (147Sm/144Nd)CHUR = 0.1967 [85], and λSm = 6.54 × 10−12 a−1 [86]. TDM was calculated using (143Nd/144Nd)DM = 0.51315 and (147Sm/144Nd)DM = 0.2137 [87]. The (87Sr/86Sr)i and εNd(t) values were calculated for t = 536 Ma.

5.1. Zircon Morphology and U–Pb Ages

Sixty-three zircon grains from two samples were dated using the LA-ICP‒MS U–Pb technique. The zircon crystals are mostly subhedral to euhedral and colorless to light purple, with lengths ranging from approximately 60 to 360 µm and length/width ratios from 1:1 to 3.6:1 (Figure 7). The CL investigation (Figure 7) shows that most zircon crystals have oscillatory zoning, and a few grains have inherited cores and zircon rims or show evidence of erosion. All zircons have Th/U ratios ranging from 0.21 to 1.66 (Table 1), indicating that the analyzed zircons are of magmatic origin [88]. According to the CL images and obtained ages, the zircon grains can be divided into two classes, that is, early Paleozoic zircons and inherited pre-Paleozoic zircons.

As shown in Figure 8, the sixty-three analyzed zircons are all concordant within uncertainties (Figure 8), with individual ages widely ranging from 2446 ± 52 to 525 ± 7 Ma. The youngest 206Pb/238U ages from the early Paleozoic (n = 47 in total) obtained from samples ZKA0-1 and ZKA1-3 are considered to reflect the eruption age of the volcanic rocks, yielding coeval concordia ages of 536.4 ± 4.0 Ma (n = 23, Mean Squared Weighted Deviation,(MSWD )= 0.64; Figure 8(a)) and 536.8 ± 3.4 Ma (n = 24, MSWD = 0.32; Figure 8(b)), respectively. The two oldest ages (2446 ± 19 and 2387 ± 52 Ma) indicate that the region might have had Paleoproterozoic materials in the magma source. The other fourteen ages fall in a range from 1150 to 566 Ma.

5.2. Zircon Trace Elements

Early Paleozoic zircon grains from ZKA0-1 have wide ranges of U (477, 2080 ppm) and Th (144, 2178 ppm) contents, with Th/U ratios ranging from 0.28 to 1.06 (Table 1). They display significant light rare earth elements (LREE) depletions with (La/Yb)N = 0.00001–0.02253, strong negative Eu anomalies with Eu/Eu* = 0.027–0.142, and positive Ce anomalies with Ce/Ce* = 1.26–93.7 (Table 2; Figure 9(a)), which are features typical of magmatic zircons [89]. Inherited zircons with pre-Paleozoic ages share similar features with Early Paleozoic magmatic zircons. Their U and Th contents vary between 147 and 1292 ppm and 79.8 and 1334 ppm, respectively, with corresponding Th/U ratios of 0.34–1.03 (Table 2). The (La/Yb)N values range from 0.0003 to 0.01458, showing intense heavy rare earth elements (HREE) enrichment in chondrite-normalized rare earth elements (REE) patterns. Pronounced negative Eu anomalies (Eu/Eu* = 0.028–0.193) and positive Ce anomalies (Ce/Ce* = 1.29–208) are also evidenced (Figure 9(a)).

The analyses on Early Paleozoic zircons from ZKA1-3 have relatively low U and Th contents of 202–1105 and 77–688 ppm, respectively, but their Th/U ratios (0.22, 0.62) are parallel to those of ZKA0-1. The chondrite-normalized REE patterns are enriched in HREEs ([La/Yb]N = 0.00002–0.04323) and have pronounced negative Eu anomalies (Eu/Eu* = 0.040–0.441) and positive Ce anomalies (Ce/Ce* = 1.18–47.4; Figure 9(b)). However, the inherited zircons with pre-Paleozoic ages show relatively more pronounced negative Eu anomalies (Eu/Eu* = 0.076–0.376) and positive Ce anomalies (Ce/Ce* = 1.51–153). The (La/Yb)N values range from 0.00004 to 0.01864, and the Th/U ratios vary between 0.21 and 1.66.

Zircon grains from ZKA0-1 and ZKA1-3 display similar chondrite-normalized REE patterns, that is, low LREE/HREE ratios and strong negative Eu anomalies, indicating that all these volcanic rocks underwent similar fractionation processes.

5.3. Zircon Hf Isotopes

Forty of sixty-three zircon grains from two samples were analyzed for Hf isotopes (Table 3). These grains have low 176Lu/177Hf ratios (176Lu/177Hf = 0.000608–0.001648), indicating that the postcrystallization accumulation of radiogenic Hf was limited.

The early Paleozoic magmatic zircons with a 206Pb/238U age of ca. 536 Ma have negative εHf(t) values of −12.7 to −1.6 and two-stage Hf model ages (TDM2) of 2296–1598 Ma, suggesting that these zircons are crystallized from magmas that were derived mainly from the partial melting of recycled old crust (Table 3).

The pre-Paleozoic zircons have variable εHf(t) values and TDM2 ages (Table 3; Figure 10). One zircon grain with a Paleoproterozoic age has an εHf(t) value of −3.5 and a TDM2 age of 3171 Ma. One late Mesoproterozoic zircon with an age of 1150 Ma yields a high positive εHf(t) value (11.7) and a TDM1 age of 1201 Ma, indicating a late Mesoproterozoic juvenile crustal source. Zircons aged 998–569 Ma show a broad range of εHf(t) values from −25.8 to 5.7, and TDM2 ages range from 3239 to 1209 Ma, indicating that the Neoproterozoic zircons are a binary mixture of melts derived from a DM source and from ancient continental crust.

5.4. Whole-Rock Geochemistry

The major and trace element compositions of the early Paleozoic volcanic rocks from the Bulunkuole Complex are shown in Table 4. All the volcanic rocks have experienced various degrees of alteration, as shown by their high loss-on-ignition (LOI) values ranging from 0.93 to 9.62 wt.% (mostly <5 wt.%). Their major oxide contents (recalculated to 100% in a volatile-free state) vary widely, with SiO2 contents ranging from 53.60 wt.% to 77.65 wt.%, Al2O3 contents ranging from 9.97 wt.% to 14.75 wt.%, MgO contents ranging from 0.20 wt.% to 2.90 wt.%, CaO contents ranging from 0.48 wt.% to 8.89 wt.%, and Fe2O3(t) contents ranging from 0.62 wt.% to 12.25 wt.%. The relatively high total alkali values (K2O + Na2O, 4.61–7.28 wt.%) and variations in K2O/Na2O ratios (0.03, 0.76 wt.%) could be the result of hydrothermal alteration. Thus, the K2O–SiO2 and/or total alkalis versus silica diagrams conventionally used to classify igneous rocks are not suitable in our case. However, some high-field strength elements (HFSE: Zr, Hf, Nb, Ta, Ti, and Y), Th, and REEs apart from Eu and La are considered immobile during low-temperature alteration processes [90, 91]; thus, Zr/TiO2 versus Nb/Y and TFeO/MgO versus SiO2 diagrams are applied instead in this study. On the Zr/TiO2 versus Nb/Y diagram [92], all the analyzed samples are plotted in the andesite and dacite fields (Figure 11(a)). On the TFeO/MgO versus SiO2 diagram (Figure 11(b)) [93], the samples mainly plot in the calc-alkaline field, with a small number of samples plotting in the tholeiitic field. This phenomenon suggests that magmas erupted through the continental arc because magmas that traverse the continental arc are more calc-alkaline than those that traverse island arcs [94]. Although the trace element contents of the volcanic rocks are largely variable, their primitive mantle-normalized trace element patterns are mostly similar to each other (Figure 12(a)), indicating that they were derived from the same magma source. They are enriched in large ion lithophile elements (LILEs) (such as Rb, Th, and U) and depleted in HFSEs (Nb, Ta, and Ti), similar to arc volcanic rocks but not normal midocean ridge basalt (N-MORB) and ocean island basalt (OIB) [90, 95] (Figure 12(a)). In the Ba/Nb-La/Nb and Th/Yb-Nb/Yb diagrams (Figures 11(c) and 11(d)), most of the samples plot in the volcanic arc field. On the chondrite-normalized REE diagram (Figure 12(b)), these samples are characterized by strong LREE enrichment, with (La/Sm)N values of 1.41–3.46 and moderately negative Eu anomalies (Eu/Eu* = 0.40–0.71).

The analytical results for whole-rock Sr–Nd isotopes are given in Table 5. The studied volcanic rocks from the Bulunkuole Complex have 87Rb/86Sr values from 0.4574 to 1.2969, with initial 87Sr/86Sr ratios ranging from 0.71093 to 0.72025. Their εNd(t) values range from −5.13 to −6.18, with DM Nd model ages (TDM[Nd]) of 1.73–2.64 Ga.

6.1. Age of the Bulunkuole Complex and Zircon Sources

The age of the Bulunkuole Complex remains a subject of ongoing debate, with estimates spanning from approximately 2700 to 221 Ma [12, 15-17, 20, 37, 40, 58]. The lithostratigraphic units within the Bulunkuole Complex share similarities with those found in the southwestern Pamir. Notably, researchers from the former Soviet Union assigned the Bulunkuole Complex to the Paleoproterozoic based on zircon U-Pb and whole-rock Rb-Sr ages of 2130–2700 Ma obtained from metamorphic rocks in the southwestern Pamir [11]. Sun et al. [12] reported a U-Pb upper intercept age of 2772 ± 393.2 Ma for biotite-plagioclase gneiss from the Bulunkuole Complex in the Bulunkou area. Furthermore, single-zircon U-Pb dating of metarhyolite from the Dabudaer area yielded an age of 2481 ± 14 Ma [20].

Alternatively, Zhang et al. [40] used LA-ICP-MS U-Pb dating techniques and obtained inherited zircon ages of 2200–600 Ma for sillimanite-biotite-plagioclase gneiss and sillimanite-garnet-biotite schist in Taxkorgan County. This suggests a late Neoproterozoic to early Paleozoic timeframe for the formation of the Bulunkuole rocks. Yang et al. [37] dated sillimanite-garnet-biotite gneiss and garnet-amphibole gneiss from eastern Taxkorgan County, revealing protolith ages younger than 253 ± 2 and 480 ± 8 Ma, respectively. This supports the notion that the sillimanite-garnet schist-quartzite unit is a late Paleozoic unit rather than part of the Paleoproterozoic Bulunkuole Complex. Additionally, Yang [17] reported zircon U-Pb ages of <296 Ma for mica quartzite and 251–221 Ma for high-pressure granulite in eastern Taxkorgan County, suggesting a Triassic origin for the Bulunkuole Complex.

Li et al. [18, 19] indicated that detrital zircons from the Jiertieke biotite-quartz schist yielded ages of 547–528 Ma, implying that Jiertieke Fe-bearing sedimentary rocks were deposited during the early Cambrian. Moreover, Zhang et al. [7] obtained zircon U-Pb ages of 519–508 Ma for amphibolite and metavolcanic rocks from the Bulunkuole Complex in the Zankan-Laobing iron deposit area. This sheds light on the depositional age of the Bulunkuole Complex in the NE section of the Pamir, placing it within the middle to late Cambrian. Gao et al. [21] reported a zircon U-Pb age of 521.3 ± 3.3 Ma for dacites from the Bulunkuole Complex in the Taaxi area. Notably, the U-Pb ages of 536.4 ± 4.0 and 536.8 ± 3.4 Ma obtained from the volcanic rocks of the Bulunkuole Complex from the Zankan deposit in this study, along with geochronological data for dacite porphyry from the Zankan deposit [22], biotite-quartz schist from the Laobing and Ziluoyi deposits [15-17], and amphibolite from the Taaxi, Zankan, and Mokaer deposits [96] (Figure 3), collectively provide strong evidence that certain iron-bearing rocks within the Bulunkuole Complex were formed during the middle to late Cambrian.

In light of distinct occurrences, rock associations, and geochemical features observed in various areas, it is prudent to treat the Bulunkuole Complex as a classification rather than a coherent stratigraphic sequence. For clarity, the Bulunkuole Complex in the WKO can be categorized into three types based on their respective formation ages (Figure 3). Notably, the predominant age estimates for the Bulunkuole Complex are clustered around ca. 2481, 540–444, and 251–221 Ma, delineating three distinct groups of strata: Paleoproterozoic, early Paleozoic, and Triassic. The Paleoproterozoic strata encompass old high-grade metamorphic rocks, such as magnetite quartzite and plagioclase-amphibole gneisses, which might constitute the crystalline basement of the TTT. These Paleoproterozoic inherited zircons found in the volcanic rocks of this study correspond to the development of the protocontinental crust of the Tarim Craton.

The early Paleozoic strata are primarily composed of plagioclase-amphibole schists, amphibole-quartz schists, and biotite-quartz schists intercalated with volcanic rocks, serving as host rocks for the Fe deposits in the Taxkorgan area. Geochronological and geochemical data for the volcanic rocks presented in this study suggest that the early Paleozoic strata were formed within a continental arc environment caused by the subduction of the Proto-Tethys Ocean. In contrast, the Triassic strata consist of mica quartzite and high-pressure metamorphic rocks, which may form a subduction-collision complex associated with the closure of the Paleo-Tethys Ocean [17, 37].

The ages of zircons from volcanic rocks in the Bulunkuole Complex reported in this study cluster into four ranges (Table 1; Figure 8): 2600–2300 (n = 2), 1200–700 (n = 5), 650–550 (n = 9), and 540–430 Ma (n = 47). Among the sixteen inherited zircons, two grains yielded the oldest ages of 2446 ± 19 and 2387 ± 47 Ma, aligning with Paleoproterozoic magmatic events crucial to the development of the protocontinental crust within the Tarim Craton [97] and the age distribution in the basement of the WKO [39, 98]. Notably, Paleoproterozoic lithologies are present in the WKO, exemplified by the Heluositan Complex and parts of the Bulunkuole Complex [20, 39, 98]. The zircon population dating back to 1200–700 Ma coincides with the Rodinia supercontinent, a record seen in the WKO [99, 100]. Late Mesoproterozoic volcanic rocks and middle Neoproterozoic intrusive plutons have been identified in the WKO [97, 99, 100]. The 650–550 Ma age group may correspond to a late Neoproterozoic magmatic event. This period coincides with the separation of the WKO from the Tarim Craton, leading to the formation of the Proto-Tethys Ocean. While rift-affiliated late Neoproterozoic igneous rocks are limited in the WKO, a whole-rock Sm-Nd isochron age of 651 ± 53 Ma for ultramafic rocks within the Kudi ophiolite suite suggests the inception of Proto-Tethys subduction [47]. Additionally, early Paleozoic zircons’ εHf(t) values range from −12.7 to −1.6 (Table 3; Figure 10(a)), indicative of crustal material as the primary source for these magmas. This aligns with early Paleozoic granitoids of arc affinity distributed throughout the WKO, further supporting the concept of continental arc magmatism induced by Proto-Tethys Ocean subduction [1, 42].

In conclusion, the ages of zircons from volcanic rocks within the Bulunkuole Complex are consistent with various magmatic episodes within the WKO. This alignment underscores the likelihood that these zircons were mainly sourced from the WKO itself.

6.2. Timing of Volcanic Eruption and Associated Mineralization

The LA-ICP‒MS zircon U‒Pb ages of the volcanic rocks obtained in this study range between 536.4 ± 4.0 and 536.8 ± 3.4 Ma (Figure 8), which are identical within error to the zircon U‒Pb ages of 533 ± 10 and 527.4 ± 9.0 Ma for the dacite porphyry from Lin [22]. Hence, it is suggested that the eruption timing of the volcanic rocks from the Bulunkuole Complex at Zankan occurred in the early Cambrian.

Previous research reported that the orebodies and ores at Zankan exhibit sedimentary features [101]; that is, orebodies are shaped as beds and generally parallel to their host rocks, showing the nature of stratiform or stratabound deposits, and ores display laminated to banded structures. The ores show a unique mineral association of magnetite, pyrite, and anhydrite with variable ratios. The iron-ore geochemical data suggest that the Zankan deposit was derived from seawater and hydrothermal fluids, with minor terrigenous clastic components [102-104]. The in situ trace elements for magnetite indicate that magnetite grains have much higher Al + Mn and Ti + V concentrations and lower Ni/(Cr + Mn) ratios than those from banded iron formations (BIFs) [102, 105, 106]. These features support the interpretation of Zankan as a volcanogenic-sedimentary iron system [15, 16, 107-109], rather than a Precambrian BIF, such as the Algoma or Superior type, which predominantly formed between 3.8 and 1.8 Ga [110, 111]. Field observations further strengthen this interpretation, as the volcanic rocks exhibit conformable relationships with the iron orebodies (Figure 6(b)). Specifically, samples ZKA0-1 and ZKA1-3 are located at the roof of an iron orebody and at the contact between an iron orebody and the host rock, respectively. Thus, these data indicate that volcanic activity was associated with iron metallogenesis, and the Zankan deposit was considered to have an early Paleozoic age (ca. 536 Ma) similar to that of volcanic rocks. This age also overlaps with those Fe deposits hosted in the Bulunkuole Complex, such as Laobing, Ziluoyi, Taaxi, Mokaer, and Jiertieke (547, 500 Ma) [15-19, 96].

6.3. Petrogenesis of the Volcanic Rocks

The volcanic rocks from the Bulunkuole Complex display enrichments in LILEs (Th and U) and LREEs ([La/Sm]N = 1.55–2.33) and depletions in HFSEs (Nb, Ta, Zr, Hf, and Ti) relative to primitive mantle values. These characteristics are typical of arc volcanic rocks; thus, it is of vital importance to determine contributions from different components within the arc system, for example, the lithospheric mantle wedge, subducted sediments, and subducted oceanic crust (slab), in terms of fluids, melts, and contamination.

The immobility of Th during fluid activity and its incompatibility during the partial melting of subducted sediments/slabs [112-114] are noteworthy. Consequently, melts derived from subducted sediments would exhibit elevated Th/Ce ratios and reduced Sr/Th ratios relative to fluids arising from subducted sediment dehydration. Notably, the volcanic rocks from the Bulunkuole Complex display higher Th/Ce ratios ranging from 0.06 to 0.45 (average 0.15; Figure 13(a)), as well as lower Sr/Th ratios spanning from 1.75 to 9.67 (average 5.24), in contrast to those observed in the southern Yidun volcanic rocks [115]. The evident subhorizontal trend in the Sr/Nd versus Th/Yb diagram (Figure 13(b)) further suggests that partial melts rather than fluids from the subducted component substantially contributed to the parental magmas.

Within arc systems, high La/Nb ratios and low Nb/U ratios can indicate the interaction between subducted components and the overlying mantle wedge [116]. Except for one measurement, all other volcanic rock samples exhibit elevated La/Nb ratios (3.0, 18.9) and reduced Nb/U ratios (0.91, 4.15) compared with primitive mantle (La/Nb = 2.58; Nb/U = 33.95), lower crust (La/Nb = 1.60; Nb/U = 25), and upper crust (La/Nb = 0.96; Nb/U = 4.44) values (Figures 13(c) and 13(d)), implying a metasomatic mantle source.

Andesitic rocks formed via partial melts of subducted sediments typically feature higher Y and Yb contents relative to those from slabs. The volcanic rocks from the Bulunkuole Complex exhibit substantial Y concentrations (ranging from 20.7 to 104 ppm) and Yb concentrations (ranging from 2.41 to 11.3 ppm), comparable to those found in Yingaguan andesitic rocks (Y = 13.21–28.03 ppm; Yb = 1.48–2.44 ppm), indicating a minor contribution from oceanic crust/slab-derived components [117]. Additionally, Tatsumi [118] introduced a (La/Sm)N versus Ba/Th plot as an effective discriminator for assessing the relative contributions of altered oceanic crust and subducted sediments to magma generation [117, 119]. Notably, the volcanic rocks from the Bulunkuole Complex display elevated (La/Sm)N ratios and a narrow range of Ba/Th ratios (Figure 13(e)), indicating a substantial input of sediment-derived components. Most data points for volcanic rocks from the Bulunkuole Complex align within the domains of modern marine sediments or the upper continental crust in the Th/La versus Th diagram (Figure 13(f)), further reinforcing the incorporation of sediment-derived components. These plots collectively suggest a prominent role of sediment-derived components in magma production, outweighing oceanic crust/slab components.

Two-stage Hf model ages (TDM2) of early Paleozoic magmatic zircons span from 2296 to 1598 Ma (Figure 10(a)) and closely overlap the ages of inherited zircons (Table 1; Figure 10(b)) found in the volcanic rocks. This implies that the parental magmas might have originated from Paleoproterozoic and Mesoproterozoic crust. Additionally, inherited zircons with ages of 998–569 Ma (Figure 10(b)) exhibit TDM2 values mainly corresponding to the Paleoproterozoic and Mesoproterozoic, mirroring those of early Paleozoic magmatic zircons (Figure 10(b)). As such, the Neoproterozoic crust potentially contributed to the parental magmas. The volcanic rocks’ initial 87Sr/86Sr values (ranging from 0.71093 to 0.72025) and εNd(t) values (−5.13 to −6.18) corroborate the involvement of crustal materials. The integration of crustal materials into parental magmas may occur through direct mixing within the magma source region or via crustal contamination during magma ascent [120]. Nonetheless, the negative Zr–Hf anomalies evident in the spider diagram (Figure 12(a)) largely negate the influence of crustal contamination during magma ascent [121]. Moreover, the consistent SiO2 and MgO contents with εNd(t) values (Figures 13(g) and 13(h)) suggest limited contamination from the upper crust during magma ascent. Consequently, the elemental variations and isotopic features of volcanic rocks from the Bulunkuole Complex more plausibly reflect the magma source rather than crustal contamination during magma ascent.

In summary, the volcanic rocks from the Bulunkuole Complex likely originate from partial melting of the lithospheric mantle wedge, which had been metasomatized by subducted sediment-derived melts.

6.4. Crustal Thickness Estimates and Tectonic Implications

The Western Kunlun Orogen recorded the assembly of Gondwana supercontinent as revealed by 533–420 Ma subduction-collision belt [5, 43] and 525–510 Ma Kudi ophiolites [48, 49, 122, 123]. Our study focused on estimating crustal thickness in the WKO using the Sr/Y proxy for subduction-related volcanic and granitic rocks aged between 536 and 448 Ma. The Sr/Y proxy method, which relies on measuring Sr and Y concentrations in intermediate to felsic rocks, has been proven effective in quantifying crustal thickness variations in Phanerozoic orogens [124-127].

We filtered highly differentiated samples with SiO2 content exceeding 75 wt% from both this analysis and previously published data [5, 22, 42, 56, 128-130], as these samples may not reflect fractionation processes occurring near the base of the crust and thus might not accurately record variations in crustal thickness [131]. By employing the Sr/Y proxy method introduced by Chapman et al. [124], we reconstructed the crustal thickness for two time intervals: 536–514 and 514–448 Ma. Our results indicate that the crustal thickness during 536–514 Ma ranged from 9 ± 0.4 to 25 ± 9 km, contrasting with the thicker crust of 39 ± 8 to 70 ± 5 km (with one data reaching 90 km) observed during 514–448 Ma (Figure 14). It is worth noting that the crustal thickness of 9–25 km during 536–514 Ma slightly exceeds that of normal oceanic crust (5.0, 8.5 km) [132], yet it remains less than the average continental crust thickness of 41.0 ± 6.2 km [133]. This crustal variation is also similar to the Pliocene and younger magmatic arcs, such as the South Sandwich arc (11.8 ± 0.1 km), Marianas arc (14.5 ± 1 km), Kurile arc (18.3 ± 0.9 km), Aleutian arc (18.9 ± 4.4 km), Tonga arc (20.0 ± 3.0 km), and New Britain arc (22.5 ± 6.5 km) [124].

Previous studies have shown that the disintegration of Rodinia resulted in the opening of the Proto-Tethys Ocean and the subsequent drift of the southwestern Kunlun terrane from the northwestern Kunlun terrane since late Neoproterozoic, and the closure of the Proto-Tethys Ocean was triggered by the amalgamation of Gondwana around 431–420 Ma [134-136] However, the timing of the Proto-Tethys subduction remains nebulous due to the scarcity of relevant information and data [137, 138]. Our zircon U-Pb dating for the arc-related volcanic rocks yielded ages of 536 Ma. Therefore, it is reasonable that the initiation subduction of the Proto-Tethys oceanic slab occurred prior to 536 Ma.

Regarding the subduction polarity of the Proto-Tethys Ocean in the WKO belt, there have baeen different viewpoints, including northward subduction [8, 9], southward subduction [5-7, 42], and two-sided subduction along the southern Kunlun terrane [139]. In our study, the geochemical estimates of crustal thickness systematically increase from southwest to northeast (Figure 14), which aligns closely with the model proposing southward subduction of the Proto-Tethys Ocean beneath the southern Kunlun terrane (see below). This conclusion is supported by the early Cambrian lithofacies paleogeographic pattern at the southern margin of Tarim and the WKO (Figure 15(a)) [18, 140, 141]. Additional compelling support comes from the observation of the accretionary wedge in the southern Kunlun and Tashikorgan terranes during the early to middle Cambrian period, which has been interpreted as a product of the southward subduction of the Proto-Tethys Ocean [6, 7].

To gain insights into the tectonic implications of these crustal thickness variations, we examined the spatial patterns and thinning/thickening histories during Proto-Tethys Ocean subduction. We found evidence of crustal thinning and extension during the 536–514 Ma period, which we attribute to the rollback of southward subduction of the Proto-Tethys Ocean slab (Figure 15(b)). Several lines of evidence support this interpretation: (1) the prevalence of intermediate to acid magmas and scarcity of mafic–ultramafic complexes in the WKO during the Early-Middle Cambrian, (2) the derivation of 526 Ma gabbros from a metasomatized asthenospheric mantle source in a forearc setting [6], (3) the formation of 521 Ma gneissic granitic dikes, bimodal volcanic rocks, and 516–512 Ma quartz diorites and gabbros in an extensional setting [21, 123], and (4) the distribution of Early Cambrian magmatism in the TTT compared with sparse occurrences in the southern Kunlun terrane, suggesting a southward migration of the magmatic front in the early Cambrian. Additionally, the rock associations of the Bulunkuole Group from the Taxkorgan-Tianshuihai and southern Kunlun terranes indicate volcano-sedimentary sequences with arc affinities, with formation ages becoming progressively younger from south to north (Figure 3; Reference 7 and this study), further supporting northward migration of magmatism after approximately 530 Ma.

In the subsequent period of 514–448 Ma, our Sr/Y estimates suggest crustal thickening, with estimates ranging from 39 to 70 km, surpassing the average continental crust thickness of 41.0 ± 6.2 km [133]. Similarly, Yin et al. [5] found geochemical evidence from Yierba diorites with adakitic affinities supporting crustal thickening around ca. 513 Ma. We propose that this crustal thickening may be associated with a transition in the subduction angle of the Proto-Tethys Ocean, reverting from high angles to normal or low angles (Figure 15(c)) [5]. Similar transitions in subduction angles have been observed in circum-Pacific tectonic belts, leading to advanced subduction and crustal thickening [142, 143]. The western Kunlun orogenic belt likely experienced more than 22 million years of extension in response to slab rollback.

The Proto-Tethys Oceanic domain, crucial in understanding the tectonic framework during the late Proterozoic to Early Paleozoic, has left some geological records across various segments of the Kunlun orogen. The geochronological evidence suggests that the onset of northward subduction within the East Kunlun Orogen occurred around ~520 Ma, leading to final ocean closure estimated between ~430 and 410 Ma [144]. Early Paleozoic northward subduction is evidenced by a back-arc basin along the Qimantagh-Xiangride mélange zone, where ophiolitic mélanges dating from 486 to 420 Ma occurred in the Heishan, Xiarihamu, Shizigou, Yazidaban-Yaziquan, and Wutumeiren areas [32, 145]. Comparatively, the WKO witnessed earlier southward subduction, beginning prior to ~536 Ma (this study), which concluded not later than ~421 Ma [42], thus revealing a complex interplay of tectonic events across the region. Furthermore, granitoid analyses from the East Kunlun Orogen have provided that crustal thickness during 494–427 Ma was found to range between 45.5 and 62.5 km [146]. This estimation coincides with the thicker crustal dimensions observed in the WKO, ranging from 39 ± 8 to 70 ± 5 km during the 514–448 Ma interval (Figure 14).

Finally, our study provides valuable insights into crustal thickness variations and their tectonic implications in the WKO. The use of the Sr/Y proxy, supported by geochemical and geological evidence, has allowed us to unravel the dynamics of Proto-Tethys Ocean subduction and understand the processes of crustal thinning, extension, and thickening in this region. However, further research incorporating seismic studies, geochronological constraints, and other complementary data will enhance our understanding of the tectonic evolution of the WKO and the adjacent East Kunlun and its significance in the broader context of plate tectonics.

This paper offered a detailed investigation of volcanic rocks in the WKO, focusing on their geochronology, geochemistry, and tectonic implications. The following four contributions can be drawn from our work:

  1. The LA-ICP‒MS zircon U–Pb dating results indicate that these volcanic rocks, which are andesites and dacites, erupted ca. 536 Ma. The volcanic rocks are enriched in LILEs but depleted in HFSEs, which is characteristic of arc magmas. The magmas were partial melting of the lithospheric mantle wedge, which had been metasomatized by subducted sediment-derived melts.

  2. The initiation of subduction for the Proto-Tethys oceanic slab can be inferred to have commenced no later than the Early Cambrian period (>536 Ma), corresponding to the amalgamation of the Gondwana supercontinent. Our investigations reveal that the rollback of a southward subducting Proto-Tethys slab initiated during 536–514 Ma, resulted in crustal thinning, with thickness ranging from 9 to 25 km. Subsequently, a shift from high-angle subduction to either normal or low-angle subduction facilitated an increase in crustal thickness to 39–70 km until 448 Ma.

  3. The Bulunkuole Complex should be divided into three distinct types, rather than treating it as a single unified geological unit, based on new constraints on the formation ages of different constituents. These three types include (1) Paleoproterozoic basement rocks; (2) early Paleozoic volcano-sedimentary sequences related to early Proto-Tethys subduction; and (3) Triassic high-pressure metamorphic rocks linked to Paleo-Tethys closure.

  4. The ages of zircons from the volcanic rocks cluster in four ranges: 2600−2300, 1200−700, 650−550, and 540−430 Ma; these age ranges are consistent with the episodes of tectonic events in the WKO, suggesting a local provenance from the WKO itself.

This work was jointly supported by the National Natural Science Foundation of China (Grant nos. 42172093, U1803242, 41772085, and 41402061), Key Technology Research and Development Program of the Xinjiang Uygur Autonomous Region of China (No. 2022A03010-3), the National Key Technology Research and Development Program of the Ministry of Science and Technology of China (2021YFC2901904, 2021YFC2901805, 2018YFC0604005, and 2015BAB05B04), and the second scientific research project on the Qinghai-Tibet Plateau (SQ2021QZKK0401). We thank Drs. Xiaohui Sun and Ying Zhang assisted with the laboratory work. Careful corrections, relevant comments, and constructive suggestions from Editor Yunpeng Dong, Associate Editor Bo Hui, and two reviewers greatly improved our knowledge and the quality of our paper.

The authors declare that they have no conflicts of interest.

The data used in this study are available in this manuscript.

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