Introduction

The Xingmeng orogenic belt, situated in the eastern segment of the Central Asian Orogenic Belt (CAOB), is bounded by the Siberian Craton to the north and the North China Craton to the south (Fig. 1a;1,2). It is composed of a collage of microcontinents, seamounts, and remnants of oceanic crust with diverse geological attributes, reflecting a long and complex tectonic history closely linked to the subduction-accretion processes of the Paleo-Asian Ocean (PAO) during the Paleozoic to Mesozoic (Fig. 1b;3,4). Several major ophiolite belts occur in this region, including the Erlianhot-Hegenshan, Jiaoqir-Xilinhot, Wendunmiao-Xilamulun, and Solonker-Linxi belts5,6,7,8, making it one of the most suitable areas for reconstructing the evolutionary history of the PAO. Over recent decades, extensive petrological, geochemical, and geochronological investigations have been conducted throughout the Xingmeng belt4,9,10,11,12, leading to a consensus that the PAO underwent “scissor-like” closure from west to east, culminating in complete suturing along the Solonker suture zone and the final amalgamation of the North China Plate with the Central Asian Orogenic Belt13,14,15,16,17. Despite this general agreement on the closure pattern, the timing of the final closure of the PAO remains debated. Three main hypotheses have been proposed: (1) Some researchers suggest that the PAO closed during the Late Permian to Early Triassic, with continuous bidirectional subduction persisting from the Early Paleozoic through the Late Paleozoic4,15,18. (2) Others argue for a Late Devonian closure, followed by a Late Carboniferous–Permian extensional regime in the region characterized by the development of small oceanic basins and intracontinental rift systems2,13. (3) A third view posits that the main PAO had already closed before the Late Devonian, and that subsequent Late Carboniferous–Permian intracontinental rifting within the Xingmeng orogenic belt led to the formation of limited oceanic basins, which were finally sutured during the Early Triassic2,19,20.

Fig. 1
figure 1

Tectonic location of the Central Asian Orogenic Belt (a) modified (after14) and regional geological map (b) modified (after21).

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Late Paleozoic magmatism is widely developed in the Xilinhot region, recording key stages of the final subduction and closure of the PAO. The tectonic evolution during the Middle to Late Permian is critical to resolving existing controversies surrounding the closure timing of the PAO. Therefore, constraining the tectonic setting of this period is essential for reconstructing the evolutionary history of the PAO in eastern Inner Mongolia. In this study, we present an integrated analysis of zircon U–Pb geochronology, whole-rock geochemistry, and zircon Hf isotopes from the Narendle granodiorite in the Xilinhot region. Combined with previous regional data, our aim is to elucidate the magmatic origin and tectonic environment of these granodiorites and to provide new geological constraints on the timing of the final closure of the PAO based on the Middle Permian tectonic framework of the Xilinhot area.

Geological background and sample characteristics

The study area is situated in the southwestern part of Xilinhot City and is tectonically positioned at the eastern margin of the CAOB, specifically at the southern end of the Xingmeng orogenic belt (Fig. 1a). It is bounded by the Erlianhot-Hegenshan ophiolite belt to the north and the Jiaoqir-Xilinhot ophiolite belt to the south (Fig. 1b;2,9). Previous studies indicate that this region was affected by the Paleozoic evolution of the PAO and its sub-basin, the Hegenshan Ocean. The stratigraphic sequence in the area spans from the Paleoproterozoic to the Quaternary and includes, in ascending order: the Paleoproterozoic Xilinhot Complex (Pt1X.), the Carboniferous Bembatu Formation (C2b), the Lower Permian Shoushangu Formation (P1ss) and Zheshi Formation (P1z), the Jurassic Wanbao (J2w) and Manketouebo (J3mk) formations, and overlying Quaternary deposits9. The region experienced multiple phases of magmatism, with widespread exposure of intrusive rocks from the Hercynian (e.g., Xilinhot Reservoir granite, Baiyingaole quartz diorite), Indosinian (e.g., Xilinhot granite, this study), and Yanshanian (e.g., Xinlin and Longtoushan granites) periods2,9,22,23.

The investigated pluton is located near Narendle in the Xilinhot region, with its central coordinates at approximately 43°41′26″N and 106°02′32″E (Fig. 2). It is emplaced as a large intrusive stock, covering an exposed area of 18 km², and is crosscut by multiple amphibole-bearing porphyritic dikes14,24. The pluton intrudes Lower Permian Shoushangu Formation (P1ss) sandstones to the north and Paleoproterozoic Xilinhot mafic units (Pt1X.) to the south. To the east, it is nonconformably overlain by Upper Pleistocene basalts and Holocene fluvial sediments25,26. Early Carboniferous quartz diorite and diorite bodies are also exposed in the northern part of the study area.

Fig. 2
figure 2

Geological map of the study area modified (after19) This image was created in ArcGIS Pro (Version:3.0.1. URL: https://www.arcgis.com) and edited in CorelDRAW (Version:24 5.0.731. URL: https://www.coreldraw.com/cn).

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Two granodiorite samples were collected from boreholes 15ZK08 and 15ZK20. The upper sections of both boreholes comprise Quaternary sediments, underlain by granodiorite and granite. The sampled rocks are fresh granodiorite, displaying a light flesh-red color on fresh surfaces, with a porphyritic texture and blocky structure. The primary mineral assemblage consists of euhedral plagioclase (~ 45%), euhedral K-feldspar (~ 20%), anhedral quartz (~ 25%), and suhedral biotite (~ 10%), with minor amounts of hornblende and accessory opaque minerals (Fig. 3a, b).

Fig. 3
figure 3

Cross-polarized light photomicrographs of granodiorite from Narendle. Pl: Plagioclase, Kfs: K-feldspar, Q: Quart, Bt: Biotite, Hbl: Hornblende.

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Methods

Two fresh granodiorite samples were selected for zircon U-Pb dating, one for Lu–Hf isotopic analysis, and seven for whole-rock geochemical analyses. All samples were prepared, coded, and submitted for ing accordingly. Zircon U-Pb and Lu-Hf isotope analyses were conducted at Beijing Yandu Zhongshi ing Technology Co., Ltd. Zircons were separated from crushed rock samples using standard heavy liquid and magnetic separation techniques. Well-formed zircon grains with smooth surfaces, no inclusions, and clear crystal edges were handpicked under a binocular microscope and mounted in epoxy resin. Zircon U-Pb dating was performed using a LA-ICP-MS system (Aurora M90), with a laser ablation spot size of 30 μm and a repetition rate of 10 Hz. The international standard zircon 91,500 was used as the external reference for age calibration, and NIST SRM 610 served as the external standard for elemental concentration, with ²⁹Si as the internal standard27. Data reduction and calculations of isotope ratios and trace element concentrations were performed using the ICP-MS DataCal software28,29,30. Concordia diagrams and weighted mean age plots were generated using Isoplot 3.331.

In situ zircon Lu-Hf isotopic analyses were conducted using a Neptune Plus MC-ICP-MS, with a laser spot diameter of 40 μm and a repetition rate of 8 Hz. The analytical procedures and calibration methods follow those described by Ludwig31. To correct for isobaric interferences of ¹⁷⁶Lu and ¹⁷⁶Yb on ¹⁷⁶Hf, the correction factors ¹⁷⁶Lu/¹⁷⁵Lu = 0.02658 and ¹⁷⁶Yb/¹⁷³Yb = 0.796218 were applied. Additionally, exponential mass fractionation corrections were performed using ¹⁷²Yb/¹⁷³Yb = 1.35274 and ¹⁷⁹Hf/¹⁷⁷Hf = 0.732532,33.

Whole-rock major and trace element analyses were conducted at the Northeast Asia Key Laboratory for Mineral Resources Evaluation. Rock samples were dried, crushed, and ground to 200 mesh prior to analysis. Major elements were determined by glass bead X-ray fluorescence (XRF) spectrometry using a RIX 2100 instrument, with analytical precision and accuracy better than 5%. Trace and rare earth elements (REEs) were measured using inductively coupled plasma mass spectrometry (ICP-MS) with an Agilent 7500a system. The analytical error for REEs was less than 7%, while the error for other trace elements was within 10%.

Analysis of sample results

Major element

The results of whole-rock geochemical analyses for the Narendle granodiorite are presented in Supplementary Table 1. The samples exhibit SiO₂ contents ranging from 68.11 to 71.30%, Al₂O₃ contents from 13.36 to 15.71%, and total alkali (Na₂O + K₂O) contents between 6.11 and 6.77%. The K₂O/Na₂O ratios vary from 0.48 to 0.60, and the Mg# values range from 33.99 to 49.67. In the QAP diagram (Fig. 4a), most of samples plot within the granodiorite field. The aluminum saturation index (A/CNK) values range from 1.10 to 1.36, indicating weakly peraluminous characteristics (Fig. 4b).

Fig. 4
figure 4

Diagram of QAP (a)modified (afterStreckeisen34) and A/NK-A/CNK(b) modified (after Manlar et al.35) for the granodiorite from Narendle. Q: Quartz, A: Alkali Feldspar, P: Plagioclase, A/NK = Al2O3/Na2O + K2O, A/CNK = Al2O3/CaO + Na2O + K2O, oxides recalculated in mole %.

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Trace element

The Narendle granodiorite samples exhibit relatively low total rare earth element (REE) contents, with ΣREE ranging from 70.49 × 10⁻⁶ to 86.22 × 10⁻⁶. The chondrite-normalized REE patterns normalized values after36 display smooth, gently right-sloping curves with weak negative Eu anomalies (Eu* = 0.75–0.92; average = 0.84), suggesting either plagioclase fractionation during crystallization or residual plagioclase in the magma source (Fig. 4a). In terms of trace elements, the samples are characterized by high contents of Ba (325.00 to 407.40 ppm), Sr (177.12 to 286.08 ppm), and Rb (106.38 to 173.92 ppm), and low contents of Nb (4.52 to 5.09 ppm), Ta (0.262 to 0.445 ppm), and Th (6.75 to 8.49 ppm). On the primitive mantle-normalized trace element spider diagram normalization values after36, the samples display a right-dipping, multi-peaked pattern. Compared to the primitive mantle, they show enrichment in Rb, Th, and U, and depletion in Nb, Ta, and Ti (Fig. 4b), reflecting characteristics typical of arc-related magmatism.

Fig. 5
figure 5

Chondrite-normalized REE patterns (a) and primitive mantle-normalized trace elements spider diagram (b) for the granodiorite from Narendle.

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Zircon U-Pb isotope

The results of LA-ICP-MS zircon U–Pb isotopic analyses for the Narendle granodiorite samples (15-g1 and 15-s1) are summarized in Supplementary Table 2. All analyzed zircons display well-preserved bipyramidal prismatic morphologies, and cathodoluminescence (CL) images reveal distinct oscillatory zoning with aspect ratios ranging from 1:1 to 1:2, consistent with magmatic origin.

For sample 15-g1, Th/U ratios range from 0.29 to 0.62. The concordia diagram and weighted mean age plot are shown in Fig. 5a, b, respectively. The zircon grains yield a weighted mean ²⁰⁶Pb/²³⁸U age of 266.4 ± 1.7 Ma (n = 20; MSWD = 1.50). Sample 15-s1 exhibits Th/U ratios of 0.35 to 0.78, and its concordia and weighted mean age diagrams are presented in (Fig. 6c, d). This sample yields a weighted mean age of 255.2 ± 2.0 Ma (n = 26; MSWD = 0.66).

Fig. 6
figure 6

Zircon U-Pb isotopic concordia (a,c) and weighted mean age diagrams (b,d) for the Narendle granodiorite.

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Zircon in situ hf isotopes

In situ Lu-Hf isotopic analyses were conducted on the same zircon domains previously dated by U–Pb methods from granodiorite sample 15-g1 (Supplementary Table 3). The analyzed zircon grains exhibit ¹⁷⁶Hf/¹⁷⁷Hf ratios ranging from 0.282849 to 0.282934. Corresponding εHf(t) values are uniformly positive, ranging from + 8.5 to + 11.4 (Fig. 7a, b), indicating a juvenile crustal source. The single-stage Hf model ages (tDM1) range from 451 to 567 Ma, while the two-stage model ages (tDM2) span from 561 to 748 Ma.

Discussion

Petrogenesis and magam source of granodiorite

The Narendle granodiorite investigated in this study belongs to the weakly peraluminous, high-K calc-alkaline. The dominant mafic mineral is biotite, with minor amounts of hornblende. No diagnostic minerals indicative of A-type or S-type granitoid-such as albite, garnet, or alkaline mafic minerals-were observed in thin sections. In the granite classification diagrams for magma genesis (Fig. 7a, b), the samples plot within the undifferentiated I-, S-, and M-type granitoid fields.

Geochemically, the granodiorite is characterized by relatively high Fe2O3t contents (2.55–4.66 wt%), total alkali contents (Na₂O + K₂O) ranging from 6.11% to 6.77%, and low MgO (1.11–1.52 wt%) and Mg# values (34.0–49.7). It also shows low 10,000×Ga/Al ratios (1.77–2.43), moderate Zr + Nb + Ce + Y contents (180–211 ppm), and low zircon saturation temperatures (T(Zr) = 780–803 °C). In addition, low K₂O/Na₂O and Rb/Sr ratios further support the interpretation that the Narendle granodiorite represents a relatively undifferentiated I-type granitoid.

Fig. 7
figure 7

Discrimination diagrams of tectonic setting for the Narendle granodiorite.

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The Lu-Hf isotopic system in zircon is a robust tracer of magma source characteristics due to its high closure temperature and resistance to isotopic resetting during late-stage magmatic evolution37. Zircons from the Narendle granodiorite exhibit high εHf(t) values of + 8.5 to + 11.4 and young two-stage Hf model ages (tDM2) ranging from 561 to 748 Ma, suggesting derivation from a juvenile crustal source. Geochemical proxies further support this interpretation. The granodiorite displays elevated Nb/Ta ratios (11.32–17.79; mean = 15.72), moderate Rb/Sr ratios (0.49–0.98; mean = 0.64), and relatively high Th/U ratios (1.47–3.30; mean = 2.77), comparable to the compositional signatures of the juvenile lower crust in eastern China38,39,40,41. These features collectively indicate that the magma originated from the partial melting of newly formed mafic lower crustal material, which was generated through the metasomatism of the mantle wedge by fluids/melts derived from the subducting slab.

Tectonic setting and geological significance

The Narendle granodiorite is situated within the northern subduction-related orogenic domain of the bidirectional subduction system of the PAO. Petrographic and geochemical data indicate that the granodiorite is enriched in large-ion lithophile elements (LILEs) such as Rb, Sr, and K, as well as high-field-strength elements (HFSEs) such as Th and U, while it is distinctly depleted in HFSEs including Nb, Ta, and Ti. The samples also exhibit relatively low Sr contents and high Y and Yb contents, which are significantly different from the high Sr and low Y and Yb characteristics of granitoids formed in a post-collisional delamination setting. Furthermore, the granodiorite samples have relatively high U/Th ratios (0.3–0.7), Rb/Nb ratios (22.2–38.5), and Th/La ratios (0.41–0.49) features that are distinctly different from those of granites formed in delamination or rift (mantle plume-influenced) settings16,17,42. These geochemical signatures indicate the involvement of partial sedimentary end-members (i.e., fluids released by the dehydration of the subducting slab) rather than mantle end-members, collectively reflecting geochemical signatures characteristic of arc-related magmatism in a subduction zone setting43,44,45. Tectonic discrimination diagrams (Fig. 8) further support this interpretation, as all samples plot within the volcanic arc granite (VAG) field, suggesting that the Narendle granodiorite was emplaced in an active oceanic plate subduction environment.

The Narendle granodiorite lacks the characteristics of forearc granitoids, specifically the low SiO₂ and high Mg# values typical of fore-arc accretionary mantle wedges—thus ruling out the possibility of it being a fore-arc granite. Furthermore, the absence of contemporaneous bimodal volcanic rocks and continental or shallow-marine sedimentary rocks eliminates the likelihood that this granite formed in a back-arc extensional setting. Geochemically, the Narendle granodiorite is characterized by distinct fractionation between light and heavy rare earth elements (LREEs and HREEs), high Th/La and Rb/Nb ratios, and intense depletion of Nb and Ta. These features indicate intense fluid release from the subducting slab. Spatially, the pluton is situated between the Erlianhot-Hegenshan (fore-arc ophiolite belt) and Jiaoqir-Xilinhot ophiolite belts (oceanic crust remnant belt), corresponding to a typical main arc zone. Meanwhile, the Early Permian Shoushangu Formation (P₁ss) marine sandstones distributed to the north of the pluton are consistent with the sedimentary characteristics of the main arc zone. In summary, the Narendle granodiorite is a volcanic arc granite (VAG), and its subdivided sub-environment should be categorized as the main arc setting.

Fig. 8
figure 8

Tectonic discrimination diagram for the Narendle granodiorites modified (after46).

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The study area is tectonically situated between the Hegenshan Ocean to the north and the PAO to the south, and its magmatic activity was influenced by the evolutionary histories of both oceanic domains. Zircon U-Pb dating of the Hegenshan ophiolite yields ages of 333–354 Ma47,48. In addition, the overlying Gegenaobao Formation (323 ± 3 Ma) is in localized eruptive unconformity with the ophiolitic sequence49, constraining the upper limit of ophiolite emplacement to ~ 323 Ma. This implies that the Hegenshan Ocean had closed by the Late Carboniferous. Consequently, the subduction environment in the Xilinhot region during the Middle Permian should be attributed to the northward subduction of the PAO. This also confirms that the Paleo-Asian Ocean had not yet closed during the Middle Permian but remained in an active stage of ongoing subduction and slab rollback.

The timing of the final closure of the eastern segment of the PAO remains debated, with three main interpretations proposed in previous studies.

  1. (1)

    Based on the development of stable passive margin sedimentary successions during the Devonian in northeastern China, some researchers suggest that the PAO closed during the Middle to Late Devonian-Early Carboniferous.

  2. (2)

    Based on the two sequences of Early Permian basaltic volcanism, i.e., from the high-degree melting of the asthenospheric mantle and the slightly later, low degree of melting of the lithospheric mantle, it is believed that the PAO was closed in the Late Carboniferous-Early Permian50.

  3. (3)

    Based on geochronological data from collisional granitoids in southeastern Inner Mongolia, proposes that final closure occurred during the Late Permian to Early Triassic20.

A systematic review of major magmatic events in the Xilinhot region (Table 1) reveals that they can be categorized into six age groups: Early Carboniferous, Late Carboniferous, Early Permian, Middle Permian, Triassic, and Jurassic–Cretaceous. These, in combination with the Narendle granodiorite examined in this study, provide a framework for reconstructing the Late Paleozoic tectono-magmatic evolution of the PAO in the region. Widespread arc-type magmatism associated with subduction occurred during the Late Carboniferous, exemplified by the Xilinhot Reservoir granodiorite (~ 317 Ma), the Baiyingaole quartz diorite in Xiwuqi (323 Ma), the Meilaotewula O-type adakites (305.6 Ma), and the volcanic rocks of the Benbatu Formation (C₂b)21,51,52,53,54. These rocks display I-type or A-type geochemical signatures, consistent with subduction-related magmatism derived from partial melting of a metasomatized mantle wedge. This suggests that the PAO remained in a stage of active subduction through the Late Carboniferous.

The Narendle granodiorite and coeval granitoids in the region both exhibit positive εHf(t) values, relatively young two - stage model ages, and geochemical characteristics including a calc-alkaline affinity and significant depletion in Nb and Ta elements38,39,55,56. These features are indicative of a subduction - related tectonic setting. Together with the Nb - rich basalts in Xiwuqi and the marine - deposited Shoushangu Formation48 this further indicates that the PAO had not closed by the Middle Permian and that subduction was still ongoing. Provenance analysis of the Upper Permian Linxi Formation reveals that its lower part was sourced from Middle Permian volcanic rocks of the Dashizhai Group and coeval magmatic activity, whereas detritus from the North China cratonic basement appears only in the upper section. Detrital zircon U-Pb dating constrains the maximum depositional age of the Linxi Formation to the end of the Late Permian57,58.

During the Early Triassic, widespread S-type and I-type granitoids were emplaced, including the Shuangjingzi two-mica granite (238 Ma), Xinlin granite (241 Ma), Zhuoshanzi granite (246 Ma), Jiantun granite (249 Ma), and Longtoushan granite (246 Ma). Geochemical data indicate that these granites originated from partial melting of thickened crustal material and formed in a syn-collisional tectonic regime59,60, providing direct evidence for continental collision following oceanic closure. Additionally, the Lower Triassic Liujiagou formations (maximum sedimentary age of 250 ± 6.0 Ma,61) and Shanggou formations are characterized by red beds of shallow lacustrine to terrestrial facies62,63, marking a regional transition from marine to continental depositional environments. Collectively, these data indicate that the final suturing of the North China Plate and the Central Asian Orogenic Belt occurred during the Late Permian to Early Triassic, marking the complete closure of the PAO and the onset of continent-continent collision.

Table 1 Age statistics of main magmatic rocks in Xilinhot area.
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To summarize, the Hegenshan Ocean basin in the northern part of the Xilinhot region is interpreted to have closed during the Middle to Late Permian, whereas the PAO to the south remained in a stage of active subduction and slab consumption until its final closure along the Solonker Suture Zone during the latest Permian to Early Triassic(255 Ma ~ 250 Ma).

Conclusions

  1. (1)

    Zircon U-Pb isotopic analyses of the Narendle granodiorite yield weighted mean ages of 266.4 ± 1.7 Ma and 255.2 ± 2.0 Ma, indicating that the granite was emplaced during the Middle to Late Permian.

  2. (2)

    The Narendle granodiorite is a high-K calc-alkaline, enriched in Rb, Th, and U and depleted in Nb, Ta, and Ti, displaying geochemical signatures typical of subduction-related magmatism. Its positive εHf(t) values (+ 8.5 to + 11.4) and young two-stage Hf model ages (tDM2 = 561–748 Ma) suggest derivation from juvenile lower crustal material, which was formed by subduction slab-derived fluids/melts metasomatizing the mantle wedge.

  3. (3)

    Integrating regional ophiolitic remnants, bimodal volcanic rocks, back-arc basalts, stratigraphic data, and the Late Permian zircon U-Pb age of the Narendle granitoid in this study, we infer that the Paleo-Asian Ocean remained in a subduction regime during the Middle-Late Permian, with the minimum age of this subduction constrained to 255 Ma. The Paleo-Asian Ocean ultimately closed between 255 Ma and 250 Ma. These findings provide important geological constraints on the closure time of the eastern Paleo-Asian Ocean and contribute to a better understanding of the tectonic evolution of the region.