Whole-rock major elements were analyzed by using a RIKEN RIX2000 spectrometer with Li-borate glass discs. BCR-2 and GBW07105 were used as external standards, and analytical precision was better than 2%. Trace elements were determined by using an Agilent 7500a inductively coupled plasma mass spectrometry (ICP-MS). Blanks, duplicate samples, and international reference materials (BHVO-2, AGV-2, BCR-2, GSP-2) were routinely analyzed for quality control, and precision generally better than 5%. The results are listed in Supplementary Table S1.

Zircon U-Pb dating and trace elements analyses were performed using an Agilent 7500a ICP-MS coupled with a 193 nm ArF laser ablation system. The laser spot is 24 μm or 32 μm in diameter. Zircon 91,500 was used as an external age standard and GJ-1 as an unknown sample to monitor data quality. NIST 610 was used as an external reference material and ²⁹Si as an internal standard to calibrate trace element concentrations. Raw data reduction was performed using the Glitter (Ver 4.0) program, and concordia diagrams were made using the Isoplot (ver. 4.15; Ludwig, 2012). Detailed results for the zircon U-Pb dating are presented in Supplementary Table S2, and the zircon trace element data are provided in Supplementary Table S3.

Zircon Lu-Hf isotope analysis was performed using a Nu Plasma II multi-collector ICP-MS coupled with a 193 nm ArF excimer laser ablation. Zircon GJ-1 and Mud Tank were used as quality control monitors. Detailed analytical methods and instrument parameters are described in Yuan et al. (2008) and Bao et al. (2023). The recommended ¹⁷⁶Lu/¹⁷⁵Lu ratio of 0.02669 and ¹⁷⁶Yb/¹⁷²Yb ratio of 0.5886 were used to correct ¹⁷⁶Lu and ¹⁷⁶Yb isobaric interferences, respectively (Chu et al., 2002). Standard zircons 91,500 and GJ-1 were used to monitor the instrument condition. A decay constant for ¹⁷⁶Lu of 1.867 × 10⁻¹¹yr⁻¹ (Albarède et al., 2006), and present-day chondritic ratios of ¹⁷⁶Hf/¹⁷⁷Hf = 0.282772 and ¹⁷⁶Lu/¹⁷⁷Hf = 0.0332 (Blichert-Toft and Albarede, 1997) were adopted to calculate ε_Hf(t) values. For the calculation of single-stage Hf model ages (T_DM1), the present-day ¹⁷⁶Hf/¹⁷⁷Hf ratio of 0.28325 and ¹⁷⁶Lu/¹⁷⁷Hf of 0.0384 of the depleted mantle (Griffin et al., 2000) were utilized. Two-stage Hf model ages (T_DM2) were calculated by adopting the average composition of the continental crust, with a mean ¹⁷⁶Lu/¹⁷⁷Hf value of 0.015 as Griffin et al. (2002). The results are presented in Supplementary Table S4.

Whole-rock Sr-Nd isotope analysis was performed using the MC-ICP-MS (Neptune Plus) from Thermo Fisher Scientific, Germany. Detailed analytical procedures are described in Li C. F. et al. (2012). Standard NBS SRM 987 (⁸⁷Sr/⁸⁶Sr = 0.71025) and Shin Etsu JNdi-1 (¹⁴³Nd/¹⁴⁴Nd = 0.512115) for calibration. The present-day chondritic uniform reservoir (CHUR) ratios of (¹⁴³Nd/¹⁴⁴Nd)_CHUR = 0.512638 and (¹⁴⁷Sm/¹⁴⁴Nd)_CHUR = 0.1967 (Jacobsen and Wasserburg, 1980) were adopted to calculate ε_Nd(t) values. Single-stage Nd model ages (T_DM) were calculated with the depleted mantle (DM) evolution model, adopting the present-day values of (¹⁴³Nd/¹⁴⁴Nd)_DM = 0.51315 and (¹⁴⁷Sm/¹⁴⁴Nd)_DM = 0.2137 (DePaolo, 1981). Analytical results are presented in Supplementary Table S5.

The Huichizi biotite monzogranite samples have high SiO₂ (68.7–72.0 wt%), Al₂O₃ (15.1–16.8 wt%) and Na₂O (4.64–5.29 wt%), but low K₂O (1.73–2.76 wt%) and MgO (0.58–0.86 wt%) and TiO₂ (0.25–0.33 wt%) contents. They are weakly peraluminous (A/CNK = 1.02–1.07) and belong to calc-alkaline series with high alkali (Na₂O+ K₂O) content (6.62–7.61 wt%) and medium K₂O/Na₂O ratio (0.35–0.57). They plot in the tonalite field on the An–Ab–Or diagram and I-type granite field on Na₂O–K₂O diagram (Figures 3a,c). The granite is relatively enriched in light rare earth elements (LREE) and depleted in heavy rare earth elements (HREE), and displays LREE enrichment patterns (La_N/Yb_N = 29.1–66.0) without obvious Eu anomalies (δEu = 0.90–1.17, Figure 4a). On the primitive mantle-normalized spider diagram (Figure 4b), the granite is rich in large-ion lithophile elements (LILE, Rb, Ba, Sr, K, Pb), but depleted in high-field-strength elements (HFSE, Nb, Ta, Ti). Additionally, the granite has high Sr (391–741 ppm), but low Y (3.99–6.87 ppm) and Yb (0.29–0.52 ppm) contents, resulting in high Sr/Y (60.6–173) and La/Yb (40.5–91.9) ratios.

The amphibolite samples have low SiO₂ (47.8–49.1 wt%), Na₂O (1.27–1.43 wt%), K₂O (1.16–1.27 wt%), P₂O₅ (0.15–0.22 wt%) but high MgO (6.46–6.72 wt%), Fe₂O₃ᵀ (14.9–16.0 wt%), and TiO₂ (2.40–2.72 wt%) concentrations, with Mg# values of 44–46 and K₂O/Na₂O ratios of 0.81–0.94. They display slightly right-slopping REE patterns (La_N/Yb_N = 1.96–2.51) without obvious Eu anomalies (δEu = 0.95–1.06) (Figure 4c). On the primitive mantle-normalized spider diagram, they are relatively rich in Rb, K, U, Ti, and Pb, but depleted in Ba, Th, Sr, Nb, and Ta (Figure 4d), all fall in the fields of Enriched E-MORB and WPB on different trace element discrimination diagrams (Figure 5).

Zircons from the Huichizi biotite monzogranite (22QYH-9) are colorless, prismatic grains of 50–200 μm in length and aspect ratios of 2:1 to 3:1. In cathodoluminescence (CL) images, a few zircons are homogeneous, but most grains consist of a light-grey core with clear oscillatory zoning and a dark-grey homogeneous rim that is similar to the homogeneous grains. Some zircons additionally have bright remnants with oscillatory zoning (Figure 6j). Analyses on oscillatory-zoned cores yield ²⁰⁶Pb/²³⁸U ages of 435–440 Ma (Supplementary Table S2), with a weighted mean ²⁰⁶Pb/²³⁸U age of 438 ± 5 Ma (MSWD = 0.042, n = 11), interpreted as the formation age of the granite. 10 analyses on zircon rims and homogeneous grains yield a weighted mean age of 401 ± 5 Ma (MSWD = 0.17). These spots all have low Th/U ratios (mostly <0.1), supporting a metamorphic/anatectic origin. Analyses on remanent domains yield scattered ages ranging from 805 to 2005 Ma.

Zircons from the amphibolite (22QYH-6) are colorless and prismatic, with lengths of 100–300 μm and aspect ratios of 2:1. Most zircons show core-rim structures in CL images. The cores are grey-black and show mottled or fir-tree structures, whereas the rims are grey-black to grey-white and homogeneous. Some grains additionally have a grey-white, narrow outer rim at the outermost part and inherited domains with oscillatory zoning are also observed in a few grains (Figure 6a). The inherited zircons have high rare earth elements (REE) content (200–1,577 ppm; Supplementary Table S3) and Th/U ratios (0.05–0.53, mostly >0.1). They display heavy REE (HREE)-enriched (Lu_N/Gd_N = 6.68–41.9) REE patterns with remarkable Eu anomalies (δEu = 0.06–0.33), similar to typical magmatic zircons (Figure 6c). 19 analyses on inherited zircons yield ²⁰⁶Pb/²³⁸U ages of 601–855 Ma (Supplementary Table S2) and eight of them form a cluster on the concordia diagram, yielding a weighted mean ²⁰⁶Pb/²³⁸U age of 810 ± 21 Ma (MSWD = 2.0, n = 8, Figure 6b), representing the protolith age of the amphibolite. In contrast, 34 analyses domains without oscillatory zoning have low Th/U ratios (mostly <0.1, except three of 0.16–0.19) and display flat HREE patterns (Lu_N/Gd_N = 1.31–12.3) without obvious negative Eu anomalies (δEu = 0.91–1.40, Figure 6c), indicating their metamorphic origin. They yield ²⁰⁶Pb/²³⁸U ages of 402–498 Ma and form three clusters on the concordia diagram (Figure 6b), with weighted mean ages of 492 ± 8 Ma (MSWD = 0.19, n = 9), 452 ± 4 Ma (MSWD = 0.36, n = 22) and 404 ± 10 Ma (MSWD = 0.054, n = 3). The age of 492 Ma comes from grey-black mottled cores and has the lowest ΣHREE (16.9–47.9 ppm). The 452 Ma ages were obtained from grey-black overgrowth rims around older cores or from fir-tree sector-zoned cores, and show higher ΣHREE (22.1–55.0 ppm). The age of 404 Ma is from grey-white rims or separate grey-white grains, its ΣHREE (37.5–56.3 ppm) resembles the 452 Ma group and is slightly higher than the 492 Ma group (Figure 6c).

Zircons from the leucosome (22QYH-7) are colorless, prismatic grains (50–150 μm in length; aspect ratios 2:1 to 3:1). They are homogeneous and greyish black in CL images with scarce bright cores (Figure 6d). Analyses on homogeneous grains show that they have extremely high U (1,529–4,367 ppm) and REE (738–1,341 ppm) contents, but low Th (28.0–156 ppm) contents, resulting low Th/U ratios of 0.01–0.05. They exhibit HREE-enriched (Lu_N/Gd_N = 54.4–132) REE patterns with obvious negative Eu anomalies (δEu = 0.20–0.88, except one shows a positive Eu anomaly, probably contaminated by plagioclase inclusions), similar to typical anatectic zircons (Chen et al., 2013; Figure 6f). Zircon U-Pb dating yields ²⁰⁶Pb/²³⁸U ages of 398–408 Ma (Supplementary Table S2), giving a weighted mean ²⁰⁶Pb/²³⁸U age of 402 ± 4 Ma (MSWD = 0.12, n = 14; Figure 6e), interpreted as the leucosome formation age. Two analyses on bright-luminescent cores show that they have obviously lower U (58.0–472 ppm) and Th (7.00–298 ppm) contents, but higher Th/U ratios (0.63 and 0.13), and ²⁰⁶Pb/²³⁸U ages of 605 Ma and 712 Ma were yielded, indicating an inherited origin.

Zircons from the felsic gneiss (22QYH-8) are colorless and prismatic grains with lengths of 50–200 μm. Most of them consist of a grey core with oscillatory zoning and a black homogeneous rim (Figure 6g). The cores have high Th/U ratios (0.09–0.69) and exhibit left-slopping REE patterns with remarkable Eu anomalies, typical of magmatic origin. 16 analyses form a cluster on concordia diagram, giving a weighted mean ²⁰⁶Pb/²³⁸U age of 866 ± 8 Ma (MSWD = 0.41). The rims have low Th/U ratios of 0.02–0.08, indicative of metamorphic origin. 16 analyses on rims form three clusters on concordia diagram (Figure 6h), yielding weighted mean ²⁰⁶Pb/²³⁸U ages of 496 ± 9 Ma (MSWD = 0.069, n = 5), 452 ± 8 Ma (MSWD = 0.2, n = 6), and 400 ± 7 Ma (MSWD = 0.16, n = 5), interpreted as three distinct metamorphic events in the felsic gneiss.

Seven analyses were conducted on core domains with clear oscillatory zoning of zircons from the Huichizi granites. They have ¹⁷⁶Lu/¹⁷⁷Hf ratios of 0.000657–0.001416 and ¹⁷⁶Hf/¹⁷⁷Hf ratios of 0.282340–0.28708 (Supplementary Table S4), corresponding to ε_Hf(t) values of (−6.01 to +7.38, most are positive) and two-stage Hf model ages (T_DM2) of 949–1795 Ma, when calculating at t = 438 Ma (Figure 7a).

Five granite samples were selected for whole-rock Sr–Nd isotope analyses. The result shows that this granite is relatively homogeneous and has initial ⁸⁷Sr/⁸⁶Sr ratios [(⁸⁷Sr/⁸⁶Sr)_i] of 0.705160–0.706258 and ε_Nd(t) values ranging from +0.05 to +1.13, which correspond to Nd model ages of 953–1,020 Ma, when calculating at t = 438 Ma (Figure 7b).

As a dating mineral with extremely high stability, zircon can provide reliable genetic constraints on dating results through its internal textures and trace element characteristics, even when the host rock has undergone intense retrogression and the peak-stage mineral assemblages have been completely erased. It has played a critical role in the identification and in-depth study of the UHP metamorphic rocks in the NQTB (Wang et al., 2014; Gong et al., 2016; Chen et al., 2019). In this study, zircon dating yield three metamorphic ages (496–492 Ma, ca. 452 Ma, 404–400 Ma) for both amphibolite and felsic gneiss in the composite lenticle hosted in Huichizi biotite monzogranite (Figures 6b,h). The first metamorphic age (496–492 Ma) is predominantly obtained from the core domains of metamorphic zircons, while the latter two ages (ca.452 Ma and 404–400 Ma) are from the two overgrowth rims (Figure 6a). Notably, metamorphic zircon domains from the amphibolite exhibit flat HREE patterns without visible Eu anomalies, typical of eclogite-facies metamorphic zircon (Rubatto, 2002; 2017). The three metamorphic ages and the corresponding zircon REE patterns from the amphibolite in the composite lenticle are fully consistent with the UHP peak metamorphic age (ca. 500 Ma), the two stages of overprinting metamorphic ages (ca. 470–450 Ma and 420–400 Ma) of the Qinling Complex in the NQTB, and their corresponding zircon REE signatures (Gong et al., 2016; Liao et al., 2016; Wang et al., 2016; Tang et al., 2022; Bai et al., 2025). Additionally, considering the following two lines of evidence: (1) the amphibolite and felsic gneiss in the composite lenticle exhibit an occurrence similar to that in Qinling Complex; and (2) the amphibolite in the composite lenticle has a comparable protolith age (810 Ma) and geochemical signatures (WPB affinity) with (retrograde) eclogites in the Qinling Complex, we infer that the rocks in the composite lenticle in the Huichizi biotite monzogranite are retrograde eclogite-facies metamorphic rocks of the Qinling Complex.

Zircon dating yielded a crystallization age of 438 ± 5 Ma and a metamorphic age of 401 ± 5 Ma for the Huichizi granite (Figure 6j). This crystallization age is younger than the first two metamorphic ages (ca. 496–492 Ma and 452 Ma) of amphibolite and felsic gneiss in the composite lenticle. But the metamorphic age of the granite is consistent within errors with the third metamorphic ages (404 ± 10 Ma and 400 ± 7 Ma, respectively) of amphibolite and felsic gneiss and the anatectic ages (402 ± 4 Ma) recorded by the leucosome within the amphibolite. These results indicate that the composite lenticle was trapped by the Huichizi granite after two episodes of metamorphism at ca. 496–492 Ma and 452 Ma; subsequently, both the composite lenticle and the granite underwent metamorphism and anatexis at ca. 400 Ma.

Geochemical data show that the Huichizi granite is an I-type granite (Figure 3c) and exhibits adakitic signatures of high Sr (391–741 ppm), low Y (3.99–6.87 ppm) and Yb (0.29–0.52 ppm) contents and high Sr/Y (60.6–173) and La/Yb (40.5–91.9) ratios (Figure 8).

Adakitic rocks generally can form through partial melting of subducted oceanic crust under eclogite-facies conditions (Defant and Drummond, 1990), partial melting of thickened lower crust (Atherton and Petford, 1993) or delaminated lower crust (Wang et al., 2006), assimilation-fractional crystallization (AFC) of basaltic magmas (Castillo et al., 1999), and crust-mantle magma mixing (Qin et al., 2010). Neither mafic microgranular enclave, nor disequilibrium textures or microstructural signatures of magma mixing (e.g., acicular apatite) have been observed in the Huichizi granite. And most samples exhibit magmatic evolution characteristics dominated by partial melting in the La/Sm-La and La/Yb-La diagrams (Figure 9). These lines of evidence rule out the genetic mechanisms of crust-mantle magma mixing and AFC of basaltic magmas. In addition, the Huichizi granite has low MgO (0.58–0.86 wt%), Cr (4.19–12.3 ppm), and Ni (3.83–6.84 ppm) contents, and low Mg# values (41–44) (Figure 10). These geochemical features indicate that the granitic magma did not undergo material exchange with mantle, thereby ruling out its formation via partial melting of a subducted slab or delaminated lower crust. Therefore, the Huichizi granite is more likely derived from partial melting of thickened lower continental crust.

Experimental studies show that partial melting of both mafic and pelitic rocks under high-pressure conditions (>1.0 GPa, Moyen, 2009) can form adakitic rocks. However, the low K₂O/Na₂O (0.35–0.57) and Rb/Sr (0.07–0.23) ratios of the Huichizi granite clearly differ from melts derived from metapelite (Harris and Inger, 1992; Zeng et al., 2011), indicating a metabasalt partial melting origin. The Huichizi granite is characterized by high Sr, low Y and Yb contents, significant depletion in HREE, Nb, Ta and Ti, with negligible Eu anomalies (Figures 5a,b). These features indicate the source residue contains garnet and rutile but almost no plagioclase, suggesting that the Huichizi granite was derived from partial melting of either thickened mafic lower crust (>1.5 GPa, >50 km, constrained by rutile stability field; Xiong et al., 2005) or eclogite (Figures 8, 10a).

Although the Huichizi granite hosts retrograde eclogite (represented by amphibolite in the composite lenticle), and it has similar zircon Hf and whole-rock Sr-Nd isotopic compositions to those of the HP-UHP eclogites in the NQTB (Figure 7), the following lines of evidence suggest that the Huichizi granite was not formed by decompressional melting of HP-UHP rocks in the NQTB during their exhumation. First, extensive studies demonstrated that decompression melting of deeply subducted crust generally occurs nearly synchronously or within 20–30 Ma after UHP metamorphism, generating tonalites and peraluminous granites with negligible mantle input (Song et al., 2015). In the NQTB, only the Piaochi S-type granite is consistent with syn-collisional magmatism formed either synchronously with UHP metamorphism or during the exhumation, in terms of both formation age (473 Ma; Qin et al., 2014) and geochemical characteristics. However, the Huichizi granite (440–420 Ma) formed at least 50 Ma later than the UHP metamorphism. Second, the Huichizi granite is the largest granite pluton (>300 km² area) in the NQTB and its formation requires a huge volume of mafic rocks, however, the HP-UHP rocks in the NQTB are mainly felsic rocks with only a low proportion of amphibolites/eclogites. Third, the leucosomes and granite veins formed by decompression melting of HP-UHP rocks in the NQTB are potassium-rich and sodium-poor, with negative zircon ε_Hf(t) values (Luo et al., 2018). In contrast, the Huizhizi granite has high Na₂O/K₂O ratios and mainly positive ε_Hf(t) values. Therefore, the Huichizi granite should be derived from partial melting of Neoproterozoic mafic volcanic rocks, which were accumulated beneath the North Qinling terrane (Qin et al., 2015) and similar to protolith of the UHP eclogites (Figure 7).

The granulite-facies metamorphism in Qinling Complex is synchronous with the second episode of granitic magmatism (440–420 Ma) in the NQTB. However, its metamorphic conditions (700 °C–850 °C and 0.8–1.2 GPa; Zhang et al., 2011; Xiang et al., 2012; Liu et al., 2016; Liao, 2018; Zhai et al., 2019; Li et al., 2023; Bai et al., 2025) are significantly lower than the P-T conditions required for the generation of adakites via partial melting of mafic rocks (800 °C–1,000 °C and 1.2–3.0 GPa; Wang et al., 2020). Furthermore, the presence of numerous anatectic leucosomes with formation ages of 490–470Ma (Cheng et al., 2011; He et al., 2018; 2023) and eclogites with amphibolite-facies retrograde ages of 485–475Ma (Wang et al., 2011; Wu and Zheng, 2013; Li et al., 2014; Hu et al., 2020) in the Qinling Complexindicates that the North Qinling UHP Terrane experienced rapid exhumation after peak metamorphism (Wang et al., 2011; Li Y et al., 2012; 2014; Bader et al., 2013; Wu and Zheng, 2013; He et al., 2018; 2023; Hu et al., 2020; Dong et al., 2022; Bai et al., 2025), and reached the middle-shallow crust (Bai et al., 2025) by at least ca.470 Ma. This depth is significantly shallower than the melting depth of the source region of Huichizi granite (>1.5 GPa, >50 km). Therefore, the composite lenticle hosted in the granite should be captured by the ascending magma after eclogite-facies metamorphism at ca. 500 Ma and granulite-facies metamorphism at ca. 450 Ma. This is also consistent with the phenomenon that the first two metamorphic ages recorded by the composite lenticle predate the formation age of the Huichizi granite (438 Ma). All these lines of evidence indicate that the ca. 450 Ma granitic magmatism and granulite-facies metamorphism in the NQTB were likely a tectonic event independent of the earlier (ca. 500 Ma) UHP metamorphism.

Rencently, studies on mafic and felsic granulites with “red-eye socket” textures in the eastern segment of the NQTB have shown that, after undergoing HP-UHP at ca. 500 Ma, the Qinling Complex experience two episodes of granulite-facies metamorphism at ca. 460–448 Ma and ca. 422–421 Ma, respectively (Bai et al., 2025). Based on the recorded prograde metamorphic history by garnet zoning for the first episode granulite-facies metamorphism, as well as the occurrence of amphibole reverse zoning in the Songshugou garnet amphibolites (Li C. F. et al., 2012; Bai et al., 2025) argued that the two episodes of granulite-facies metamorphism are not retrograde metamorphic events associated with exhumation of UHP rocks. Instead, they are more likely related to the subduction of the Shangdan Ocean beneath the exhumed NQT and the subsequent collision following the ocean’s closure. These findings and understandings, combined with our results, indicate that the widely developed 460–420 Ma granulite-facies metamorphism and magmatism in the Qinling Complex represent a tectono-magmatic event associated with the subduction and collision of the Shangdan Ocean.

The Huichizi granite is enriched in LILE (Rb, Ba, Sr, K, and Pb) and depleted in HFSE (Nb, Ta, and Ti), exhibiting characteristics of arc granites. It has high Th/Ta ratios (8.10–31.5), which is similar to igneous rocks formed in active continental margin (Th/Ta = 6–20; Gorton and Schandl, 2000) and oceanic island arc settings (Th/Ta >20–90; Gorton and Schandl, 2000). Its low Ce/Pb (1.67–3.24, 2.23 in average) and Nb/La (0.14–0.35, 0.22 in average) ratios are close to the average values of arc magmatic rocks (Ce/Pb = 3, Nb/La ≤1; Candies, 1989). In the tectonic discrimination diagrams, the Huichizi granite plots into the field of volcanic arc granite (Figure 11). Considering the Sifangtai, Lajimiao, and Fushui complexes with arc basalt geochemical signatures emplaced along the Shangdan Suture Zone during ca.500–420 Ma (Li et al., 1993; Liu et al., 2008; Zhang et al., 2015), the continuous marine sedimentation from the Cambrian to the Silurian, as well as the unconformity between the Middle Devonian and Silurian strata in the SQTB (Dong and Santosh, 2016; Jiang et al., 2019), the Shangdan Ocean did not close until the Early Devonian (Wu and Zheng, 2013; Dong and Santosh, 2016 and references therein; Bai et al., 2025). Therefore, the Huichizi granite should have formed during the subduction of the Shangdan Ocean.

A metamorphic age of ca. 400 Ma was obtained from both the Huichizi granite (401 ± 5 Ma, Figure 6k) and its hosted composite lenticle, including amphibolite (404 ± 10 Ma, Figure 6b) and felsic gneisses (400 ± 7 Ma, Figure 6h). A similar anatexis age of 402 ± 4 Ma was also yielded from leucosomes within the amphibolite. This leucosome, composed predominantly of plagioclase (65%–70%) and quartz (25%–30%), is restricted to the interior of the amphibolite, which rules out the possibility of melt injection from the surrounding felsic gneisses or granites. These results demonstrate that the Huichizi granite and the composite lenticle underwent a metamorphism and anatexis event at ca. 400 Ma together. This metamorphism is also observed in the Liangchahe granitic gneiss (Bai, 2024), a small metamorphosed granite located adjacent to the southern side of the Huichizi Granite. Garnet porphyroblasts and garnet megacrysts-bearing leucosome can be observed in the Liangchahe granitic gneiss. Zircon dating yields a protolith age of 441 ± 11 Ma and a metamorphic age of 402 ± 5 Ma for the granitic gneiss, as well as a formation age of 401 ± 5 Ma for the garnet megacryst-bearing leucosome (Bai, 2024). P-T conditions of the Liangchahe granitic gneiss were constrained to 3.8–5.0 kbar and 736 °C–748 °C by phase equilibrium modelling, corresponding to medium-pressure granulite-facies. Ti-in-zircon thermometer (assuming activities of SiO₂ and TiO₂ as one and 0.5, respectively; Ferry and Watson, 2007) yields formation temperatures of 648 °C–733 °C for metamorphic zircons from the Huichizi granite, and 620 °C–768 °C for the anatectic zircons from the leucosome within amphibolite (Supplementary Table S3). These temperatures are consistent with those of the Liangchahe granitic gneiss. All these results, combined with the intense deformation features (Figure 2b) of the Huichizi granite, indicate that the Huichizi granite indeed experienced a medium-pressure granulite-facies metamorphic-anatectic event at ca. 400 Ma. Notably, the ca. 400 Ma metamorphic and anatectic events recorded in the Paleozoic granitoids of the NQTB are reported here for the first time, whereas such events in the Qinling Complex have been extensively documented in recent years (Faure et al., 2008; Bader et al., 2013; Sun et al., 2019; Zhao et al., 2020; Guo et al., 2023; Liu et al., 2024). For instance, Zhao et al. (2020) and Liu et al. (2024) have both identified a medium-pressure high-temperature (MP-HT) granulite-facies metamorphic event at ca.400 Ma (8.5–9.7 kbar and 745 °C–820 °C, 409–395 Ma, Zhao et al., 2020; 790 °C–862 °C and 9.9–11.9 kbar, 410–390 Ma; Liu et al., 2024) in migmatized felsic gneisses of the Qinling Complex in the Weiziping area, and attributed it to thermal relaxation following crustal thickening. Guo et al. (2023) reported a garnet amphibolite of the Qinling Complex in Tianshui area, which experienced high-temperature granulite-facies metamorphism (8.4–9.9 kbar, 869 °C–886 °C) at ca. 410 Ma related to thinning of thickened orogenic crust. Further considered the presence of 415–400Ma undeformed, highly fractionated, K-rich granite (Zhang et al., 2013; Wang et al., 2015), and A₂-type granites in NQTB (Xu et al., 2017), the tectonic regime in the NQTB switched from collisional compression to extension at ca. 400 Ma.

Based on the results of this study and existing research, we summarize the Early Paleozoic tectonic evolution process of the NQTB as follows: The North Qinling Terrane rifted from the northern margin of the Yangtze Block during the breakup of the Rodinia supercontinent and drifted northward. It underwent continental deep subduction and ultrahigh-pressure metamorphism at 500 Ma, followed by rapid exhumation to the middle-shallow crust. Subsequently, continuous northward subduction of the Shangdan Oceanic crust triggered partial melting of Neoproterozoic mafic rocks accumulated beneath the NQT and led to the formation of the Paleozoic granitoids (including Huichizi granite), and also resuted in the contemporaneous granulite-facies metamorphism. The granite trapped a few exhumed retrograde HP-UHP metamorphic rocks of Qinling Complex during magma ascending. At ca. 400 Ma, as the tectonic regime of the NQTB switched from contraction to extension, the Huichizi granite and the composite lenticle, as well as the Qinling Complex experienced medium-pressure granulite-facies metamorphism and anatexis.