The U-Pb zircon isotopic dating results for sample HBC-YM-7 are summarized in Table 1. Cathodoluminescence (CL) imaging of analyzed zircons (Figure 3c) reveals predominantly euhedral to subhedral crystals with well-preserved morphological features, distinct crystal edges, and clear internal structures displaying magmatic oscillatory zoning. The analyzed zircons exhibit U concentrations ranging from 90 × 10⁻⁶ to 1052 × 10⁻⁶ (average 379 × 10⁻⁶) and Th concentrations from 59 × 10⁻⁶ to 392 × 10⁻⁶ (average 206 × 10⁻⁶), yielding Th/U ratios of 0.28–0.82 (mean 0.59), consistent with magmatic zircon characteristics.

A total of 30 zircon spots were conducted, with 11 spots yielding ²⁰⁶Pb/²³⁸U ages ranging from 240 to 256 Ma. The weighted mean ²⁰⁶Pb/²³⁸U age of 252.9 ± 3.2 Ma (MSWD = 1.5; n = 11) defines the emplacement timing of the diorite porphyry in the Daheishan Horst during the Late Permian (Figures 3a,b). Eight analytical spots (HBC-YM-7-02, 04, 06, 08, 09, 25, 27, 28) were excluded due to short signal durations (<20 s, standard acquisition ∼40 s), likely caused by thin zircon domains or laser ablation breakthrough, which could result in spuriously young age values. Five zircon grains yielded older ²⁰⁶Pb/²³⁸U ages of 490, 1,118, 1,598, 2,158, and 2,462 Ma, interpreted as inherited components incorporated during magma ascent. An additional six grains produced anomalously young ²⁰⁶Pb/²³⁸U ages, likely attributable to partial recrystallization, metamorphic overprinting, or contamination from younger lithologies (e.g., Cenozoic sediments) during sample preparation.

The diorite porphyry sample (HBC-YM) from the Daheishan Horst displays characteristic major element geochemistry (Table 2), with SiO₂ contents ranging 54.17–55.65 wt%, TiO₂ 1.31–1.34 wt%, Al₂O₃ 16.98–18.71 wt%, MgO 2.52–3.3 wt%, and CaO 3.18–3.94 wt%. Its alkali geochemistry reveals sodic affinity (Na₂O + K₂O = 7.26–7.77 wt%; Na₂O/K₂O ratio = 2.44–5.63), showing Na-enrichment and K-depletion features. Petrological classification based on standardized diagrams shows that: (1) the TAS (Total Alkali-Silica) diagram (Figure 4a) plots the sample in the basaltic trachyandesite field; (2) the K₂O vs SiO₂ diagram (Figure 4b) indicates a medium-K calc-alkaline affinity, consistent with magma generation in an active continental margin setting.

The trace and rare earth element (REE) concentrations of the diorite porphyry samples from the Daheishan Horst are summarized in Table 2. Whole-rock geochemistry reveals total REE contents ranging from 172.06 to 196.52 ppm, with light REE (LREE: La-Sm) concentrations varying between 132.91 and 152.85 ppm, and heavy REE (HREE: Eu-Lu) contents from 16.89 to 19.04 ppm. These rocks display significant LREE enrichment relative to HREE (LREE/HREE = 7.86–9.05), indicating pronounced fractionation between light and heavy rare earth elements. The chondrite-normalized REE patterns (Figure 5a) demonstrate characteristic right-inclined profiles with steep LREE-enriched slopes and relatively flat HREE segments, indicative of strong LREE/HREE fractionation. Primitive mantle-normalized trace element diagrams (Figure 5b) reveal distinct geochemical signatures: marked enrichment in large-ion lithophile elements (LILEs; e.g., Rb, Th, U, K), along with notable depletion in high-field-strength elements (HFSEs; e.g., Nb, Ta, P, Ti). This geochemical fingerprint, showing remarkable similarity to upper continental crust compositional trends, exhibits diagnostic features comparable to magmatic suites from arc-related environments or active continental margin tectonic settings.

Seventeen in-situ zircon Hf isotope analyses were performed on sample HBC-YM-7 (diorite porphyry) from the Daheishan Horst. Three analytical spots were discarded due to laser-induced fracturing during ablation, leaving 14 valid analyses representing zircon crystallization at ∼252.9 Ma. These yield ¹⁷⁶Hf/¹⁷⁷Hf ratios of 0.282650–0.282933, εHf(t) values of +1.08 to +11.03, single-stage Hf model ages (T_DM1) of 457–856 Ma, and two-stage model ages (T_DM2) of 534–1090 Ma.

All εHf(t) values are positive (>0), with data points plotting between the depleted mantle evolution curve and the chondritic uniform reservoir (CHUR) reference line (Figures 6a,b). This signature distinctly differs from the negative Hf isotopes characteristic of the Yanshan Orogen but aligns with Phanerozoic rocks in the eastern Xing’an-Mongolia Orogenic Belt (Yang et al., 2007). Complete analytical data are provided in Table 3.

Whole-rock Sr–Nd isotopic analyses of HBC-YM samples are presented in Table 4 and Figure 6c. The samples exhibit significant Rb–Sr compositional variations. Sample HBC-YM-1 (Rb = 118 ppm, Sr = 83.3 ppm, Rb/Sr = 1.42) shows an anomalously low initial ⁸⁷Sr/⁸⁶Sr ratio (0.690555), suggesting possible analytical disturbance given its high Rb/Sr ratio. In contrast, samples HBC-YM-2 and HBC-YM-3 display lower Rb/Sr ratios (0.025–0.077) and geochemically plausible initial (⁸⁷Sr/⁸⁶Sr)_i values (0.704601–0.705469), consistent with mantle-derived signatures. These samples yield positive εNd(t) values (+5.15 to +5.24) with two-stage Nd model ages (T_DM2) of 606–598 Ma.

Previous studies have extensively investigated the geochronology of magmatic rocks within the Daheishan Horst. Early researchers, based on regional stratigraphic correlation, proposed that the magmatic emplacement was contemporaneous with the metamorphic volcanic rocks of the Taoshan Formation (420.3 ± 6.47 Ma; Jiang, 2014), suggesting an Early-Middle Silurian formation age (Peng et al., 1979; Jia, 1990; Wu, 1994; Zhao, 1996; Wang and Su, 1996). Additionally, Gao (1985) inferred a Late Paleozoic origin through integrated stratigraphic analysis and whole-rock Rb-Sr dating methodology. Recent advances in zircon U-Pb geochronology (Figure 1b) have demonstrated that these magmatic rocks were emplaced during the Early to Late Permian (293–252.9 Ma; Han et al., 2017; Cao, 2013; Song, 2018), thereby challenging the conventional perspective of Early Paleozoic formation. The crystallization age of ∼252.9 Ma obtained from our HBC-YM sample further corroborates previous findings, confirming its genetic affiliation with Late Paleozoic magmatic activity.

Previous studies have demonstrated that subduction-related magmatic rocks along the southern segment of the Permian Daheishan Horst were emplaced during 283–253 Ma (Han et al., 2017; Song, 2018; Mou, 2022). This temporal framework contrasts with the northern segment where subduction-generated magmatism occurred earlier (293–276 Ma; Shi et al., 2020), indicating distinct temporal patterns of magmatism between the two segments and a potentially longer duration of subduction in the southern domain (Figure 1b). This study reports, for the first time, the identification of subduction-affinity magmatic rocks dated at ∼252.9 Ma in the northern Daheishan Horst. This discovery extends the magmatic activity window of the northern segment to 288–252.9 Ma, revealing substantial temporal overlap with the southern segment’s magmatic record (283–253 Ma). The coeval magmatic manifestations across both segments imply that the Daheishan Horst likely experienced a unified tectono-magmatic event during the Permian period.

Both the diorite porphyry samples analyzed in this study and the Daheshen Formation magmatic suite in the Huadian area (Figure 1b; Cao et al., 2012) exhibit a calc-alkaline affinity. Whole-rock geochemical analyses reveal that these samples share remarkable geochemical congruity: (1) Chondrite-normalized rare earth element (REE) patterns exhibit consistent enrichment in light rare earth elements (LREEs) coupled with depletion in heavy rare earth elements (HREEs), manifesting characteristic right-inclined REE distribution patterns (Figure 5a); (2) Primitive mantle-normalized trace element spider diagrams (Figure 5b) display significant enrichment in large-ion lithophile elements (LILEs; e.g., Rb, Th, U, K) alongside pronounced depletion in high-field-strength elements (HFSEs; Nb, Ta, P, Ti), displaying distinct geochemical signatures that are markedly different from those of OIB, E-MORB, and N-MORB basalts. Notably, previous petrogenetic studies have established that the Daheshen Formation magmatic rocks possess diagnostic geochemical fingerprints of active continental margin settings, interpreted as products of PAO plate subduction (Cao et al., 2012). This geochemical coherence, particularly the characteristic LILE/HFSE fractionation patterns and calc-alkaline affinity, strongly suggests that the Daheishan Horst diorite porphyry likely originated through similar subduction-related processes within an active continental margin framework. These two magmatic suites are both interpreted as Permian arc-related products developed along the Solonker–Xar Moron–Changchun–Yanji suture zone, offering important petrogenetic constraints for understanding the closure of the Paleo-Asian Ocean.

In-situ zircon Hf isotopic analyses for the diorite porphyry samples yield εHf(t) values of +1.08 to +11.03, with a mean value of +6.04 and median of +6.4. Corresponding two-stage Hf model ages (T_DM2) range from 534 to 1090 Ma. These values exhibit strong consistency with the Hf isotopic spectrum of Phanerozoic igneous rocks in the Central Asian Orogenic Belt (CAOB) (Figures 6a,b; Yang et al., 2007; Cao, 2013), indicating derivation of the magmatic source primarily from: a subduction-fluid-metasomatized depleted mantle or Neoproterozoic–Early Paleozoic juvenile crust accreted during CAOB evolution. Although subordinate analyses show lower εHf(t) values (down to +1.08), potentially reflecting minor assimilation of ancient crustal materials or inherited zircon effects, these contributions do not overprint the dominant juvenile isotopic signature.

Combined with whole-rock Sr–Nd isotopic signatures (⁸⁷Sr/⁸⁶Sr)_i = 0.704601–0.705469, εNd(t) = +5.15 to +5.24, (T_DM2) = 606–598 Ma), the data points cluster proximal to the mantle array. This further corroborates that the magmas were dominantly generated by partial melting of a depleted mantle source, with negligible contamination from ancient continental crust.

Critically, post-magmatic hydrothermal alteration has affected the studied diorite porphyry samples, potentially compromising the accuracy of their initial ⁸⁷Sr/⁸⁶Sr ratios. To validate the robustness of the observed signatures, we compiled Sr-Nd data from pristinely preserved Late Permian igneous rocks along the southern suture zone of the Daheishan Horst (Song, 2018). Collectively, these datasets demonstrate coherent depleted mantle-like Sr-Nd isotopic compositions (Figure 6c), confirming a regionally depleted mantle source beneath the Daheishan Horst.

Previous studies have demonstrated that the relationship between La/Sm ratios and La concentrations is a key geochemical indicator for evaluating rare earth element (REE) fractionation behavior. This method is widely used to distinguish magmatic processes controlled by fractional crystallization from those dominated by partial melting (Allègre and Minster, 1978; Zhang, 2024). Diagnostic patterns emerge in La/Sm-La systematics: (1) Horizontally aligned data points (constant La/Sm ratios with variable La concentrations) typically indicate melt differentiation through fractional crystallization from a common parental magma; (2) Positive correlations between La/Sm and La abundances suggest melt generation via progressive partial melting of a compositionally homogeneous source. In the La/Sm-La discrimination diagram (Figure 7b), the studied samples exhibit a co-varying increase in La/Sm ratios and La concentrations. This geochemical trajectory demonstrates enhanced light rare earth element (LREE) fractionation and precludes fractional crystallization as the dominant process. The observed compositional variability in diorite porphyry samples is therefore attributed to either variable degrees of partial melting or inherent geochemical heterogeneity within the source region.

This is a key geochemical indicator of continental crustal materials and subducted sediments, and is widely used to constrain the petrogenesis and tectonic settings of basaltic systems. In active continental margin settings, typical samples plot in the high-Th (>5 ppm) and intermediate-to-low Ta-Hf/3 (0–2) field (Wood et al., 1979). In the Th-Ta-Hf/3 discrimination diagram (Figure 7a), the studied diorite porphyries from the Daheishan Horst exhibit elevated Th contents (6.21–11.66 ppm) with moderate Ta-Hf/3 ratios, geochemically consistent with intermediate-acid volcanic rocks in the southern segment of this belt (Han et al., 2017; Song, 2018). All samples fall within the calc-alkaline basalt series field, which is characteristic of subduction-related arc environments, indicating typical magmatic affinities of ocean-continent subduction zones.

Additionally, the Nb/Yb ratio is routinely employed to reflect the enrichment-depletion relationships in mantle source regions. The selection of Yb as the denominator minimizes the effects of partial melting and fractional crystallization, enabling reliable retrieval of source mantle compositions through petrogenetic inversions. Geochemical thresholds indicate: Nb/Yb = 0.5–1.5 characterizes transitional regimes between subduction-related depletion (low Nb/Yb) and intraplate enrichment (high Nb/Yb), whereas Nb/Yb < 0.5 reflects extreme depletion in oceanic arc settings. Th/Yb ratios greater than one are considered quantitative indicators of crustal assimilation or subducted sediment input into magmatic sources (Pearce and Peate, 1995). In the Yb-normalized Th-Nb covariation diagram (Figure 7c), samples investigated in this study exhibit Th/Yb ratios of 2.52–3.66 and Nb/Yb values >0.5. These geochemical signatures align with intermediate-acid volcanic rocks from the southern Daheishan Horst (Han et al., 2017; Song, 2018), collectively plotting within the overlapping field between continental arc basalts (CAB) and oceanic arc basalts (OAB), and a pronounced clustering trend toward the continental arc basalt (CAB) domain. It is noteworthy that although both oceanic island arc and active continental margin basalts are genetically associated with subduction-related processes, they exhibit distinct geochemical signatures. Typical oceanic island arc basalts are characterized by low Zr/Y ratios (<3.3) and Ta/Yb ratios (<0.1). In contrast, basalts from active continental margins display relatively elevated La/Zr and Nb/Zr ratios, with a discriminant boundary defined by La × Nb/Zr² = 0.002 (Wood D.A., 1980; Pearce, 1982; Sun et al., 2007; Han et al., 2017). The magmatic rocks from the Daheishan Horst exhibit Zr/Y ratios ranging from 9.7 to 11.5 (>3), Ta/Yb ratios from 0.12 to 0.28 (>0.1), and La × Nb/Zr² values between 0.002 and 0.007 (>0.002), indicating a pronounced continental affinity. These geochemical characteristics rule out the possibility that the samples are oceanic island arc basalts, suggesting instead that they may represent basalts formed in an active continental margin setting or intraplate basalts contaminated by crustal materials. Additionally, the Nb/Zr ratios of the dioritic porphyrite samples analyzed in this study range from 0.018 to 0.052, with one anomalously high value (0.052) and the remaining five samples all falling below 0.02. These values are significantly lower than those typical of intraplate basalts (Nb/Zr >0.04; Wood, 1980), thereby excluding an intraplate basalt origin.

Collectively, the Late Permian diorite porphyry samples from the Daheishan Horst originated from partial melting of a subduction-fluid-metasomatized mantle source, constrained by depleted Hf-Nd isotopes, and were generated through subduction processes beneath an active continental margin.

The tectonic evolution of the eastern PAO during its subduction remains controversial, with two competing models proposed for the SXCYS: 1) a bidirectional subduction-collision closure model (Liu J. et al., 2017; Song, 2018; Jing, 2023) versus 2) a unidirectional subduction model (Cao et al., 2013; Yu et al., 2014; Wang Z. J. et al., 2015). Recent paleomagnetic data suggests that the Late Carboniferous-Early Permian eastern PAO segment along the southern Xing-Meng Orogenic Belt exhibited a trumpet-shaped configuration with eastward-propagating expansion (Zhang et al., 2021). Provenance analysis from the northern margin of the North China Craton further indicates a diachronous closure of the PAO, progressing systematically from west to east (Wang et al., 2019). These findings collectively support a scissor-like closure mechanism for the eastern PAO, characterized by initial closure in the western segment followed by progressive termination eastward (Zhang et al., 2016; Tang et al., 2018; Wang et al., 2019; Tian et al., 2020). In light of these constraints, we propose a tectonic subdivision of the SXCYS into two distinct segments for detailed analysis: (1) the eastern segment (Changchun–Yanji sector), and (2) the western segment (Solonker–Xar Moron River sector), to facilitate a more precise spatiotemporal investigation of the subduction–collision processes along the suture zone.