Zircon U-Pb Geochronology and Geochemical Constraints of Tiancang Granites, Southern Beishan Orogenic Belt: Implications for Early Permian Magmatism and Tectonic Evolution

Abstract

The Beishan Orogenic Belt, situated along the southern margin of the Central Asian Orogenic Belt, represents a critical tectonic domain that archives the prolonged subduction–accretion processes and Paleo-Asian Ocean closure from the Early Paleozoic to the Mesozoic. Early Permian magmatism, exhibiting the most extensive spatial-temporal distribution in this belt, remains controversial in its geodynamic context: whether it formed in a persistent subduction regime or was associated with mantle plume activity or post-collisional extension within a rift setting. This study presents an integrated analysis of petrology, zircon U-Pb geochronology, in situ Hf isotopes, and whole-rock geochemistry of Early Permian granites from the Tiancang area in the southern Beishan Orogenic Belt, complemented by regional comparative studies. Tiancang granites comprise biotite monzogranite, monzogranite, and syenogranite. Zircon U-Pb dating of four samples yields crystallization ages of 279.3–274.1 Ma. These granites are classified as high-K calc-alkaline to calc-alkaline, metaluminous to weakly peraluminous I-type granites. Geochemical signatures reveal the following: (1) low total rare earth element (REE) concentrations with light REE enrichment ((La/Yb)_N = 3.26–11.39); (2) pronounced negative Eu anomalies (Eu/Eu* = 0.47–0.71) and subordinate Ce anomalies; (3) enrichment in large-ion lithophile elements (LILEs: Rb, Th, U, K) coupled with depletion in high-field-strength elements (HFSEs: Nb, Ta, P, Zr, Ti); (4) zircon ε_Hf(t) values ranging from −10.5 to −0.1, corresponding to Hf crustal model ages (T_DMC) of 1.96–1.30 Ga. These features collectively indicate that the Tiancang granites originated predominantly from partial melting of Paleoproterozoic–Mesoproterozoic crustal sources with variable mantle contributions, followed by extensive fractional crystallization. Regional correlations demonstrate near-synchronous magmatic activity across the southern/northern Beishan and eastern Tianshan Orogenic belts. The widespread Permian granitoids, combined with post-collisional magmatic suites and rift-related stratigraphic sequences, provide compelling evidence for a continental rift setting in the southern Beishan during the Early Permian. This tectonic regime transition likely began with lithospheric delamination after the Late Carboniferous–Early Permian collisional orogeny, which triggered asthenospheric upwelling and crustal thinning. These processes ultimately led to the terminal closure of the Paleo-Asian Ocean’s southern branch, followed by intracontinental evolution.

1. Introduction

The Central Asian Orogenic Belt (CAOB), recognized as the largest Phanerozoic accretionary orogenic system globally [1,2,3,4,5], is tectonically situated between the Siberian, East European, Karakum, Tarim, and North China cratons (Figure 1a). It originated from prolonged subduction and closure of the Paleo-Asian Ocean. This orogenic belt preserves a comprehensive record of complex tectonic events, including the assembly–breakup of the Rodinia supercontinent [6,7,8,9,10,11], Paleozoic ocean–continent conversion, and multi-stage accretionary orogenesis [12]. Notably, it hosts the most prominent Phanerozoic continental crustal growth zone, distinguished by three key geological characteristics: (1) the preservation of oceanic crustal fragments documenting the evolution of the Paleo-Asian Ocean [13,14]; (2) the development of extensive magmatic activity and deformation–metamorphism records during ocean–continent transition [15,16]; and (3) the formation of multiphase, structurally superimposed accretionary wedge complex systems [5,17]. These attributes establish the CAOB as a critical research target for deciphering the evolution of paleo-ocean–continent configurations and accretionary orogenic dynamics in the Paleo-Asian tectonic domain [18,19,20,21,22].

As a pivotal tectonic unit along the southern margin of the central CAOB, the Beishan Orogenic Belt underwent multi-stage subduction–collision processes from the Early Paleozoic to Mesozoic, preserving complete geological archives of the closure of the southern Paleo-Asian Ocean [40,41]. Current research reveals four major magmatic episodes at ~1.4 Ga, ~0.9 Ga, 470–360 Ma, and 320–220 Ma [8,10,11,12,15,16,17,18,19,25,26,42,43,44,45,46,47,48,49]. Early Permian magmatic rocks exhibit widespread regional distribution in the Beishan Orogenic Belt, yet their genetic mechanisms remain contentious. Some researchers, based on spatiotemporal distribution patterns of magmatic sequences, propose their formation in subduction-related settings at active continental margins [27,49,50]. Conversely, others advocate a rift genesis associated with mantle plume activity or post-collisional extension [51,52,53], as evidenced by mantle-derived magma compositions and high thermal anomaly signatures. This ongoing debate not only hinders precise determination of the Late Paleozoic tectonic regime along the southern Beishan margin but also impacts the reconstruction of Paleozoic geodynamic models for the CAOB [5,22,24,42,54].

Building upon the Tiancang 1:50,000 regional geological survey results, this study focuses on Early Permian granites along the southern Beishan Orogenic Belt. Integrated methodologies, including petrology, zircon U-Pb geochronology, in situ Hf isotopic analysis, and whole-rock geochemistry, are systematically employed. By establishing the spatiotemporal framework of magmatic activities and deciphering magma source characteristics and evolutionary processes, coupled with comparative analysis of regional tectono-magmatic events, this research aims to (1) elucidate the Early Permian tectonic environment of the study area and (2) investigate the Paleozoic tectonic evolution and dynamic mechanisms of the southern Beishan Orogenic Belt.

2. Geological Background

The Beishan Orogenic Belt is situated in the central segment along the southern margin of the Central Asian Orogenic Belt (Figure 1a), bounded by the Dunhuang Block to the south and the Mongolian Orogenic Belt to the north. It is separated from the Eastern Tianshan Orogenic Belt by the Xingxingxia strike-slip fault in the west, while its eastern boundary is obscured by the Ruoshui strike-slip fault beneath the Badain Jaran Desert (Figure 1b) [19,42]. The Beishan Orogenic Belt displays a mosaic tectonic architecture comprising five discrete crustal fragments: the Shibanshan, Shuangyingshan–Huaniushan, Mazongshan, Hanshan, and Queershan terranes. These tectonostratigraphic units are progressively segmented from south to north by four ophiolitic mélanges—the Liuyuan, Hongliuhe–Xichangjing, Shibanjing–Xiaohuangshan, and Hongshishan mélanges—which serve as fundamental tectonic boundaries within the orogenic system [8,17,18,19,55]. The Beishan Orogenic Belt is divided into the northern and southern belts by the Hongliuhe–Xichangjing mélange [8,23,26,30,44].

The study area is located in the vicinity of Tiancang Village, approximately 25 km northeast of Dingxin Town in Jinta County, Gansu Province, situated at the southeastern margin of the junction zone between the Shuangyingshan–Huaniushan terrane and Shibanshan terrane (Figure 1b). The faults within the area predominantly exhibit NWW-trending, approximately E-W-trending, and NE-trending orientations. The stratigraphic sequence exposed in the study area, from oldest to youngest, comprises the following: (1) the Mesoproterozoic Changcheng System: Gudongjing Group, dominated by marble, schist, and quartzite; (2) the Jixian System: Pingtoushan Formation, primarily composed of argillaceous rocks, slate, dolomite, limestone, and argillaceous slate; (3) the Neoproterozoic Qingbaikou System: Yemajie Formation, characterized by phyllite, slate, limestone, and fine-grained sandstone; (4) the Lower Carboniferous Hongliuyuan Formation (Upper Paleozoic): mainly consisting of limestone and welded tuff; (5) the Cretaceous Chijinpu Formation (Mesozoic): predominantly containing conglomerate, sandstone, argillaceous siltstone, and marl. In the study area, the exposed intrusive rocks consist of Neoproterozoic porphyritic granodiorite, Early Devonian granodiorite, and Permian granites. Additionally, dike rocks are quite well developed (Figure 2).

3. Sample Description and Petrography

This study focuses on Tiancang Permian granites, predominantly composed of biotite monzogranite, monzogranite, and syenogranite.

The biotite monzogranite plutons, covering approximately 7.5 km², are distributed in eastern Honggoushan and northwestern Qingshizuizidun (Figure 2). The southern pluton intrudes into the Neoproterozoic Yemajie Formation (Qingbaikou System), predominantly comprising gray-white, medium- to fine-grained biotite monzogranite with the following modal composition: plagioclase (30%–35%), K-feldspar (25%–30%), quartz (20%–25%), and biotite (10%). Accessory minerals consist of zircon, magnetite, apatite, and monazite. Petrographic observations reveal pervasive sericitization of plagioclase and partial kaolinization of K-feldspar (Figure 3a–c). The northern pluton exhibits distinct lithological characteristics, intruding into the Paleoproterozoic Gudongjing Group (Changcheng System) while being intruded by Early Permian syenogranite pluton. This gray-red biotite monzogranite displays a medium- to coarse-grained texture with massive structure, mineralogically composed of plagioclase (35%), K-feldspar (30%), quartz (25%), and biotite (10%). The accessory mineral assemblage includes zircon, apatite, monazite, ilmenite, and columbite–tantalite, with notable carbonate alteration of K-feldspar and chloritization of biotite (Figure 3d–f).

The monzogranite pluton is predominantly located north of Honggoushan, exhibiting a nearly east–west orientation and covering an area of approximately 14 km² (Figure 2). This intrusive body is characterized by its pink-red coloration and coarse- to medium-grained texture. The monzogranite pluton has been crosscut by a coeval syenogranite pluton, indicating multiphase magmatic activity during the Early Permian. The monzogranite consists of quartz (30%), plagioclase (30%), K-feldspar (30%), and biotite (10%). Plagioclase is typically argillized and sporadically replaced by sericite, while K-feldspar occasionally encloses plagioclase or replaces it along its margins. Quartz exhibits a corroded granular texture, with some grains displaying irregular margins and graphic intergrowths with K-feldspar. Biotite is partially replaced by chamosite and minor muscovite, and a small number of mineral pores are filled with secondary sericite. Accessory minerals include magnetite and zircon. The rock exhibits well-developed brittle deformation features, such as fissures and cracks, all of which show no significant displacement (Figure 3g–i).

The syenogranite pluton is primarily situated east of Honggoushan, extending in a northwest direction and covering an area of approximately 11.0 km² (Figure 2). This pluton intrudes into the Early Devonian granodiorite and is composed of light pink-red, coarse- to medium-grained syenogranite, with some portions displaying intense K-feldsparization and a deep flesh-red coloration. The syenogranite consists of K-feldspar (45%), plagioclase (20%), quartz (30%), and biotite (5%). Plagioclase commonly exhibits weak argillization and scattered sericitization. A small portion of K-feldspar encloses plagioclase and displays strong argillization. Quartz shows a corroded texture, with some grains exhibiting graphic intergrowths with K-feldspar at their margins. Accessory minerals include magnetite, apatite, and zircon (Figure 3j–l).

4. Analytical Methods and Results

Zircons were separated by conventional heavy liquid and magnetic sorting techniques at the Institute of Hebei Regional Geology and Mineral Survey (IHRGMS) in Langfang, Hebei Province. Zircons were subsequently handpicked under a binocular microscope, mounted in epoxy resin, and polished. The Cathodoluminescence (CL) images of zircon were acquiredusing a JSM-IT300 scanning electron microscope system (JEOL Lts., Tokyo, Japan) at the IHRGMS in Langfang, Hebei Province.

Zircon U-Pb dating was performed using a Thermo Scientiffc NEPTUNE multicollector ICP-MS instrument together with an ESI 193 nm FX ArF Excimer LA system at the Isotopic Laboratory of Institute of Geology and Mineral Resources in Tianjin, China. TEMORA zircon [56] was used as an external standard and was analyzed twice every six unknown analyses. Contents of U, Th, and Pb were calculated using NIST610 glass as an external standard. Correction for common Pb was carried out using the method of Andersen (2002) [57]. Detailed operating conditions for the laser ablation system, ICP-MS instrument, and data reduction followed the methods described by Yuan et al. (2003) [58]. Concordia diagrams and weighted mean ²⁰⁶Pb/²³⁸U ages were calculated using the Isoplot program (version 3.0; Ludwig, 2003) [59]. The age data are presented in Supplementary Table S1.

Whole-rock major- and trace-element analyses were carried out at the Regional Geological Survey of Hebei Province, Langfang, China. Major-element concentrations were determined using an ICAP6300 X-ray fluorescence (XRF) spectrometer, with analytical precision generally better than 2%–5%. Trace-element concentrations were measured using a Thermo Scientific XSERIES 2 ICP-MS instrument, with analytical precision typically better than 5%. The major- and trace-element data are summarized in Supplementary Table S2.

Zircon in situ Hf isotopic analysis was carried out using an ArF excimer 193 nm laser ablation system attached to a Neptune Laser Ablation-Multi Collector-Inductively Coupled Plasma Mass Spectrometry (LA-MC-ICPMS) at Wuhan Sample Solution Analytical Technology Co., Ltd., Wuhan, China. A stationary spot used a beam diameter of 44 μm and a laser pulse frequency of 8 Hz. Instrumental conditions and data acquisition protocols were described by Xu et al. (2004) [60]. The standard zircon 91500 was used in this correction to the ¹⁷⁶Yb–¹⁷⁶Hf interference, showing variable ¹⁷⁶Yb/¹⁷⁷Hf ratios of 0.0113–0.0122 with an average of 0.0118 for 91500. The ¹⁷⁶Lu decay constant of λ = 1.867×10⁻¹¹ yr⁻¹ [61] was used to calculate initial ¹⁷⁶Hf/¹⁷⁷Hf ratios. The chondritic values of ¹⁷⁶Hf/¹⁷⁷Hf = 0.0332 and ¹⁷⁶Lu/¹⁷⁷Hf = 0.282772 [62] were used in our calculation of ε_Hf(t) values. The depleted mantle evolution line is defined by (¹⁷⁶Hf/¹⁷⁷Hf)_DM = 0.28325 and (¹⁷⁶Lu/¹⁷⁷Hf)_DM = 0.0384 [63]. An ¹⁷⁶Lu/¹⁷⁷Hf ratio of 0.015 for the mean continental crust was used to calculate T_DMC. The zircon Lu-Hf isotopic data are provided in Supplementary Table S3.

4.1. Zircon U-Pb Geochronology Results

Zircons from two biotite monzogranites (TW8835, PM101TW9), one monzogranite (PM103TW92), and one syenogranite (PM103TW79) were selected for LA-ICP-MS U-Pb dating. Cathodoluminescence (CL) images of the zircons, along with the dating positions marked by white solid circles, are presented in Figure 4. Zircons from all four samples are predominantly euhedral to subhedral, displaying prismatic morphologies with lengths of 100–300 µm, widths of 50–100 µm, and aspect ratios ranging from 1.3:1 to 3.5:1. They are transparent, colorless to pale brown, and exhibit well-defined magmatic oscillatory zoning (Figure 4). The analyzed zircons have variable uranium (58–667 ppm) and thorium (164–733 ppm) contents, with Th/U ratios ranging from 0.21 to 0.92, consistent with a magmatic origin [64].

Nineteen zircons from sample TW8835 yield a weighted mean age of 274.4 ± 2.1 Ma (MSWD = 2.2), with an average Th/U ratio of 0.58 and ²⁰⁶Pb/²³⁸U ages ranging from 267 Ma to 285 Ma. On the U-Pb concordia diagram, the analytical points plot on or near the concordia line (Figure 5a). This suggests that the U-Pb isotopic system of the zircons has remained closed since their formation, with no significant loss or addition of U or Pb isotopes, confirming the reliability of the results. Twenty zircons from sample PM103TW92 yield a weighted mean age of 274.1 ± 1.3 Ma (MSWD = 1.01), with ²⁰⁶Pb/²³⁸U ages ranging from 268 Ma to 280 Ma and an average Th/U ratio of 0.59 (Figure 5b). Similarly, twenty zircons from sample PM103TW79 yield a weighted mean age of 276.0 ± 2.4 Ma (MSWD = 3.1), with ²⁰⁶Pb/²³⁸U ages ranging from 267 Ma to 284 Ma and an average Th/U ratio of 0.52 (Figure 5c). For sample PM101TW9, twenty zircons yield ²⁰⁶Pb/²³⁸U ages ranging from 272 Ma to 284 Ma, with a weighted mean age of 279.3 ± 1.7 Ma (MSWD = 1.5; Figure 5d). The Th/U ratios for this sample range from 0.21 to 0.63, with an average of 0.38.

Based on the CL characteristics and Th/U ratios of the zircons, the four weighted mean ages are interpreted as the crystallization ages of the granites, consistent with a magmatic origin. The timing of granite crystallization is constrained to between 274.1 Ma and 279.3 Ma, corresponding to the 320–220 Ma magmatic event (one of four major episodes).

4.2. Whole-Rock Major- and Trace-Element Results

The geochemical analysis results of 15 granite samples collected from the study area are presented in Supplementary Table S2.

Biotite Monzogranite: Four samples of biotite monzogranite exhibit high contents of SiO₂, ranging from 71.17 to 75.02 wt.%, K₂O (3.17–4.46 wt.%), and Na₂O (3.51–4.07 wt.%). The concentrations of Al₂O₃ (13.15–14.64 wt.%) and CaO (1.23–2.49 wt.%) are moderate, while the abundances of FeO* (1.36–2.91 wt.%) and MgO (0.36–0.61 wt.%) are low. Additionally, their Rb contents span from 86.9 to 148 ppm, with relatively low Rb/Sr ratios of 0.54–1.49. These samples fall within the high-K calc-alkaline to calc-alkaline series (Figure 6a) and are weakly peraluminous, with A/CNK values ranging from 1.00 to 1.05 (Figure 6b). They are calc-alkalic (Figure 6c) and magnesian, except one sample (Figure 6d).

Monzogranite: Six samples of monzogranite have high SiO₂ contents (71.16–74.75 wt.%), K₂O (3.63–3.93 wt.%), and Na₂O (3.96–4.36 wt.%). The levels of Al₂O₃ (12.99–14.10 wt.%) and CaO (0.74–1.58 wt.%) are moderate, and the amounts of FeO* (1.60–2.48 wt.%) and MgO (0.29–0.44 wt.%) are low. Their Rb contents are in the range of 101–147 ppm, with relatively moderate Rb/Sr ratios of 1.14–2.66. These samples are high-K calc-alkaline and range from metaluminous to weakly peraluminous, with A/CNK values of 0.99–1.07 (Figure 6a,b). They are calc-alkalic (Figure 6c), ferroan (Figure 6d), and high-fractionated (Figure 6e).

Syenogranite: Five samples of syenogranite are enriched in SiO₂, showing high and narrowly distributed values (74.59–76.58 wt.%), along with high K₂O (3.92–4.19 wt.%) and Na₂O (4.03–4.16 wt.%). They have low concentrations of Al₂O₃ (12.52–12.85 wt.%), CaO (0.37–1.01 wt.%), FeO* (0.63–1.58 wt.%), and MgO (0.13–0.27 wt.%). Their Rb contents range from 110 to 127 ppm, with relatively high Rb/Sr ratios of 2.34–4.02. These samples are high-K calc-alkaline and range from metaluminous to weakly peraluminous, with A/CNK values of 0.99–1.06 (Figure 6a,b). They are calc-alkalic (Figure 6c) and high-fractionated (Figure 6e).

In the chondrite-normalized REE diagrams (Figure 7a), all samples display enrichment in LREE, with (La/Yb)_N ratios varying from 3.26 to 11.39. Four samples of biotite monzogranite show weak negative Eu anomalies (Eu/Eu* = 0.58–0.71). Six samples of monzogranite have moderate negative Eu anomalies (Eu/Eu* = 0.52–0.68), and the sample Pm103YQ92 has distinct negative Ce anomalies (Ce/Ce* = 0.52). Five samples of syenogranite show distinct negative Eu anomalies (Eu/Eu* = 0.47–0.63) and negative Ce anomalies (Ce/Ce* = 0.46–0.71) (Supplementary Table S2). In the primitive mantle—normalized spider diagrams (Figure 7b), all samples display pronounced enrichment in large-ion lithophile elements (LILEs, e.g., Rb, Th, U, K, and Pb), indicative of crustal contamination or a crustal source. Conversely, they exhibit relative depletion in high-field-strength elements (HFSEs, e.g., Nb, Ta, P, Zr, and Ti), likely attributable to the crystallization of zircon, titanite/rutile, and monazite/apatite. The distinct negative Ce anomalies for some samples likely suggest the involvement of few oceanic sediments in their parental magma source. The conspicuous negative Sr and Ba anomalies may indicate plagioclase separation and fractional crystallization during the melting and magma evolution processes.

4.3. In Situ Zircon Hf Isotope Results

The in situ zircon Hf isotopic analyses of the samples for which U-Pb ages were determined are presented in Supplementary Table S3. Given the relatively large size of the laser ablation pits, these analyses were carried out on the same zircon grains but at different sites (white dotted circles, Figure 4). Hf isotope analyses on magmatic zircon rims from the samples show ¹⁷⁶Lu/¹⁷⁷Hf and ¹⁷⁶Hf/¹⁷⁷Hf ratios of 0.0011–0.0033 and 0.2823–0.2826, respectively.

Twelve zircon grains were analyzed from the biotite monzogranite (sample TW8835), which has a weighted mean ²⁰⁶Pb/²³⁸U age of 274.4 ± 2.1 Ma. Their ε_Hf(t) values span from −2.2 to −0.6, and the crustal model ages (T_DMC) range from 1.44 to 1.34 Ga. In the case of the monzogranite (sample PM103TW92), eleven analyses with a ²⁰⁶Pb/²³⁸U age of 274.1 ± 1.3 Ma result in ε_Hf(t) values ranging from −3.7 to −0.1. The corresponding crustal model ages (T_DMC) are between 1.54 and 1.30 Ga. Nineteen zircon grains from the syenogranite (sample PM103TW79), with a weighted mean ²⁰⁶Pb/²³⁸U age of 276.0 ± 2.4 Ma, exhibit ε_Hf(t) values from −4.3 to −0.1, and the crustal model ages (T_DMC) are 1.57–1.30 Ga. Finally, nineteen analyses of zircon grains from the biotite monzogranite (sample PM101TW9) with a ²⁰⁶Pb/²³⁸U age of 279.3 ± 1.7 Ma yield ε_Hf(t) values from −10.5 to −6.3, corresponding to crustal model ages (T_DMC) of 1.96–1.70 Ga (Supplementary Table S3, Figure 8). Such low ε_Hf(t) values and Proterozoic T_DMC ages suggest that the source material for the Tiancang granites likely incorporated a substantial proportion of ancient, highly radiogenic-depleted crustal components, which had been isolated from the mantle for a long time.

5. Discussion

5.1. Emplacement Age of Magmatism

Previous studies have not precisely constrained the emplacement ages of granites in the study area. In this investigation, four high-precision LA-ICP-MS zircon U-Pb ages were obtained: the biotite monzogranite at Qingshizuizidun (southern area) yields an age of 279.3 ± 1.7 Ma, while the biotite monzogranite, monzogranite, and syenogranite from Honggoushan (northern area) yield ages of 274.4 ± 2.1 Ma, 274.1 ± 1.3 Ma, and 276.0 ± 2.4 Ma, respectively (Figure 2 and Figure 5). These results collectively define an emplacement timeframe of 279–274 Ma (Early Permian) for the granites in this region. Regional geological surveys reveal that within the adjacent 1:50,000-scale Qingshanpo and Diwandongliang sheets, volcanic rocks of the Early–Middle Permian Shuangputangian are exposed. The lithological assemblage is predominantly composed of dacite–rhyolite associations (284–274 Ma, unpublished data), with locally occurring basalt interlayers. Further north, coeval gabbroic intrusions (275 Ma, unpublished data) are exposed in the Waizuishan and Shahongshan sheets, intruding into the Lower Carboniferous Hongliuyuan and Baishan formations [70].

The Early Permian magmatism in the Beishan region exhibits distinct spatiotemporal correlations. In the southern belt, from west to east, the following plutons are documented: the Wufengshan (275 Ma) and Dongdaquan (279–277 Ma) intrusions in the Liuyuan area [30], the Yinaoxia pluton (282 Ma) [34], and the Bandaoshan intrusion (285 Ma) [35]. The Lower Permian felsic volcanic sequences display systematic age gradients: rhyolites from Dushan (281 Ma) [38], Ganquan (294 Ma) [71], Shanhujing (292 Ma) [38], and Yujingzi spherulitic rhyolite (273 Ma) [72] to Shahongshan rhyolite (295 Ma) [38]. Early Permian granitoids have also been reported in the northern Beishan [26,37,39]. Regional comparisons indicate that this magmatic episode in western Xinjiang is characterized by coeval ultramafic–mafic rocks (295–270 Ma) [27,73] and felsic volcanic rocks (~280 Ma) [74], whereas the eastern Tianshan–Beishan junction zone features widespread mantle-derived mafic–ultramafic complexes (300–269 Ma) [52,75] and granitic intrusions (298–270 Ma) [76,77]. These spatiotemporal patterns confirm that the Early Permian (~300–270 Ma) represents a critical tectono-magmatic phase in the Beishan Orogenic Belt and the southern Central Asian Orogenic Belt.

5.2. Petrogenesis and Nature of Magma Source

The Early Permian granites investigated in this study belong to the high-K calc-alkaline to calc-alkaline series, characterized by metaluminous to weakly peraluminous compositions (A/CNK < 1.1). Petrographic observations reveal the absence of hornblende, primary Al-rich minerals (e.g., garnet, muscovite), and alkaline minerals (e.g., aegirine, arfvedsonite), precluding classification as typical I-, S-, and A-type granites. Geochemical analyses show high SiO₂ (71.16–76.58 wt.%), elevated alkalis (K₂O + Na₂O = 6.92–8.40 wt.%), and high differentiation indices (DI = 81.82–94.67), indicative of extensive fractional crystallization. Experimental petrology studies [78] demonstrate that apatite solubility in metaluminous to weakly peraluminous magmas decreases with increasing SiO₂, whereas the opposite trend occurs in strongly peraluminous systems. This differential behavior has been widely utilized to discriminate between I-type and S-type granites [79,80,81,82]. The weakly peraluminous nature (A/CNK < 1.1), low P₂O₅ (<0.09 wt.%), and pronounced negative correlation between P₂O₅ and SiO₂ observed in our samples align with the evolutionary trends of I-type granites [Figure 6f].

Although the samples are plotted within the A-type and I-type granite fields in major element discrimination diagrams (Figure 6c,d), their trace element signatures (10,000 Ga/Al = 2.06–2.89, Zr = 114–208 ppm, Nb = 14.1–42.4 ppm, Ce = 14.9–79.0 ppm, Y = 18.3–50.3 ppm) deviate significantly from classical A-type granite criteria (10,000 Ga/Al > 2.6, Zr > 250 ppm, Nb > 20 ppm, Ce > 100 ppm, Y > 80 ppm) [83], exhibiting transitional features between A-type and I-type granites in composite discrimination diagrams (Figure 9a–d). Recent studies [84,85,86,87] suggest that highly fractionated granites (SiO₂ > 72 wt.%) may mimic A-type geochemical signatures due to convergence toward eutectic compositions, despite fundamental differences in formation temperatures: A-type granites typically form at high temperatures (>800 °C) [88,89], whereas fractionated granites crystallize at notably lower temperatures (mean ~764 °C) [90]. Calculated zircon saturation temperatures for our samples [91] (biotite monzogranite: 759–786 °C; monzogranite: 788–802 °C; syenogranite: 766–787 °C; mean 783 °C), combined with high SiO₂ (>71.16 wt.%), elevated DI (mean 91), and low Zr/Hf ratios (23–34), collectively support classification as I-type granites, with apparent A-type signatures attributed to intense fractionation (Figure 9e,f).

Zircon ε_Hf(t) values from the Honggoushan pluton (northern sector) and Qingshizuizidun pluton (southern sector) range from –4.3 to –0.1 and –10.5 to –6.3, respectively (Figure 8a,b), corresponding to crustal model ages of 1.57–1.30 Ga and 1.96–1.70 Ga (Figure 8c,d). These data suggest derivation primarily from partial melting or remelting of ancient crustal materials. The ~400 Myr gap in mean Hf model ages between the northern (1.4 Ga) and southern (1.8 Ga) granites implies contributions from Paleoproterozoic versus Mesoproterozoic crustal basements. The high silica content and marked depletions in Ba, Nb, Ta, Sr, P, Ti, and Eu (Figure 7b) indicate substantial fractional crystallization. Depletions in Nb, Ta, and Ti suggest the separation of Ti-rich phases (e.g., ilmenite/rutile), while P depletion reflects apatite and/or monazite fractionation. Negative Eu anomalies and Sr-Ba depletions point to plagioclase and/or K-feldspar crystallization. Thus, the granites likely originated from mantle-derived magma-induced anatexis of ancient crust, followed by hybridization and extensive fractional crystallization.

However, the pronounced heterogeneity in zircon Hf isotopes within regional samples (Figure 8a) necessitates an open-system process to account for significant variations in ¹⁷⁶Hf/¹⁷⁷Hf ratios [92]. Given that Hf isotopic ratios remain unchanged during partial melting or fractional crystallization, this heterogeneity likely reflects mixing between mantle-derived melts with radiogenic Hf and crustal sources with less radiogenic Hf [93]. Analogous to global examples [79,92,93,94,95,96], we interpret the regional zircon Hf isotopic variability as evidence of magma mixing between mantle-derived and crustal end-members. The regional occurrence of coeval mantle-derived rocks (gabbros, basalts) [34,49,97,98] further supports mantle contributions.

5.3. Tectonic Setting of Early Permian

The Early Permian granites exhibit characteristics of highly fractionated I-type granites. In tectonic discrimination diagrams (Figure 10), data clusters within the late orogenic and post-collisional fields correlate with regional tectonic evolution. Recent studies associate such granites with post-orogenic extension, involving either (1) decompression melting of asthenospheric mantle coupled with crustal assimilation [89] or (2) melting of juvenile lower crust triggered by underplated mafic magmas [99]. Magmatic records from the southwestern Central Asian Orogenic Belt [100,101,102] confirm widespread Permian post-collisional granitoid activity. Regional evidence includes (1) 310–272 Ma A-type granite–rhyolite associations and 290–268 Ma mafic dyke swarms [38], indicative of Early Permian crustal extension, and (2) the Shibanshan–Daqishan–Shenluoshan rift system, initiated in the Early Carboniferous and peaking in the Early Permian with bimodal volcanic assemblages [103,104] and rift–basin sedimentary sequences [105].

Integrated regional magmatic-sedimentary evidence—including coeval granite–basalt associations (310–272 Ma), diabase dyke swarms (290–268 Ma), rift-related sedimentation, and crust–mantle interaction signatures—confirms a continental rift tectonic setting during the Early Permian.

5.4. Paleozoic Tectonic Evolution of Southern Beishan Orogenic Belt

5.4.1. Subduction–Collision Processes

The Beishan Ocean Basin initiated as early as the Early Cambrian (Figure 11), supported by three lines of evidence: (1) Among the four ophiolitic mélanges in the Beishan Orogenic Belt, the Hongliuhe–Xichangjing Ophiolitic Mélange represents the oldest ophiolite suite, characterized as a supra-subduction zone (SSZ) type. Gabbro from the Hongliuhe Ophiolite in the western segment yields a SHRIMP zircon U-Pb age of 528–516 Ma [108,109], while plagiogranite and gabbro from the Xichangjing Ophiolite in the eastern segment record crystallization ages of 536–496 Ma [110,111]. (2) The Lower Cambrian Shuangyingshan Formation unconformably overlies the Neoproterozoic Xichangjing Group glacial diamictite, with a paleo-weathering crust and overlying phosphatic conglomerate at the unconformity surface. The glacial diamictite in the Beishan region correlates with adjacent blocks, suggesting that the Beishan area was likely part of the Rodinia supercontinent [55]. (3) Stratigraphic features of the Cambrian sequence reveal that the Lower Cambrian Shuangyingshan Formation comprises thin-bedded gray-black limestone and minor marble, with abundant bioclasts in the limestone indicating shallow marine to littoral environments. In contrast, the Middle–Upper Cambrian Xishuangyingshan Formation is dominated by grayish-green chert interbedded with thin limestone, reflecting deep-sea chemical sedimentation. The transition from the Shuangyingshan to Xishuangyingshan Formation records a deepening of the depositional environment [55].

The subduction of the paleo-ocean basin persisted throughout the Early Paleozoic. Direct evidence includes the following: (1) Nb-enriched basalts with an age of 451 Ma [49] and adakitic granitoids with ages of 442–424 Ma [48,49,112] in the Liuyuan area of the southern Beishan Orogenic Belt, along with coeval mantle-derived intermediate-acid magmatic rocks dated at 449–423 Ma [16,113,114]. (2) The discovery of the Gubaoquan–Liuyuan eclogite belt [115] further corroborates deep subduction processes. The metamorphic ages of 465 Ma for the Gubaoquan eclogite [115] indicate that the subducted slab had reached eclogite-facies depths by this time [116].

5.4.2. Post-Collisional Extension

During the Early Devonian, post-collisional granitoids developed in the southern Beishan Orogenic Belt (Figure 11), exemplified by Shuangfengshan A-type granite (415 Ma) and Huitongshan K-feldspar granite (397 Ma). These intrusions formed in a post-orogenic extensional setting or the late syn-collisional stage [28,112]. Key constraints include (1) the emplacement age of undeformed granite in the Hongliuhe Ophiolite (405 ± 5 Ma) [108], marking the closure of the ocean; (2) post-orogenic molasse deposition of the Upper Devonian Dundunshan Group [117]; and (3) Pre-Changchengian–Ordovician thrust–nappe structures in the Pingtoushan area [118], indicative of intense N–S compression during the late Silurian–early Devonian. Combined magmatic and structural-sedimentary records confirm that ocean closure and collisional orogeny culminated by the end of the Early Paleozoic, transitioning into a post-collisional extensional regime. During the Late Devonian to Early Carboniferous, there was a significant difference in magmatism between the two regions (the Shibanshan terrane showed no magmatic activity) (Figure 11), indicating that the subduction–collision closure of the branch of the Paleo-Asian Ocean basin likely occurred along the northern margin of the Shuangyingshan block, while the Shibanshan and Shuangyingshan blocks had already amalgamated into a single unit prior to this period.

5.4.3. Intracontinental Stable Stage

Between 360 and 300 Ma, the southern Beishan Orogenic Belt experienced magmatic quiescence (Figure 11). The Upper Devonian Dundunshan Group continental coarse clastics unconformably overlie the Middle Devonian Sangejing Group, collectively forming a Devonian molasse sequence [117]. The Lower Carboniferous Hongliuyuan Formation, dominated by limestone, phyllite, sandstone, and conglomerate, also lacks magmatic signatures. This interval likely represents a continuation of late Early Paleozoic orogenesis followed by tectonic stabilization, with limited magmatism and development of continental deposits.

5.4.4. Intracontinental Rift Stage

In the Late Carboniferous–Permian, the southern Beishan Orogenic Belt underwent extensive magmatism, including mafic–ultramafic complexes, ultrabasic rocks, basic rocks, diorite, A-type granites, and rhyolites [30,53,73]. Geochronological data reveal peak magmatic activity at 290–270 Ma (Figure 11), temporally and spatially correlative with events in the Tarim–East Tianshan tectonic belt [52,108].

Notably, while both the early Late Paleozoic and late Carboniferous–Permian magmatic episodes in the southern Beishan Orogenic Belt have been interpreted as post-collisional, we propose distinct mechanisms based on spatiotemporal patterns: The early Late Paleozoic magmatism reflects post-subduction–collision extension; Late Carboniferous–Permian activity corresponds to lithospheric delamination and asthenospheric upwelling in a continental rift setting. The Early Paleozoic subduction–collision event likely thickened the lithospheric mantle beneath the Beishan rift zone, inducing gravitational instability, delamination, and subsequent asthenospheric ascent. Associated thermal input triggered crustal melting, generating voluminous acidic magmas, including regionally extensive granitoids and felsic volcanic rocks in the southern Beishan Orogenic Belt.

In the Mesozoic–Cenozoic, the Beishan Orogenic Belt developed in an intracontinental setting [18,40,41,103,119]. Some Triassic mafic dykes with E–W, NE–SW, and NW–SE trends occur in this area, while Triassic granitoids are spatially and genetically associated with E–W-trending faults [29]. Several researchers have reported Triassic granitic intrusions (220–240 Ma) in the southern Beishan orogenic belt [29,119]. These Triassic granitoids have high potassium contents and belong to the high-K calc-alkaline and shoshonitic series and are alkalic and alkali-calcic, making them metaluminous to weakly peraluminous granites. They are mainly highly fractionated I-type with transitional features of I- to A-type. The magmatism was most likely triggered by magmatic underplating within a post-orogenic extensional regime following delamination.

Figure 11. A spatiotemporal distribution map of Paleozoic magmatic activities in the southern Beishan Orogenic Belt. Data are from this study and the literature [7,19,27,30,31,34,35,37,38,48,49,52,55,73,74,97,112,113,120,121,122,123,124,125,126,127,128,129,130,131,132].

Figure 11. A spatiotemporal distribution map of Paleozoic magmatic activities in the southern Beishan Orogenic Belt. Data are from this study and the literature [7,19,27,30,31,34,35,37,38,48,49,52,55,73,74,97,112,113,120,121,122,123,124,125,126,127,128,129,130,131,132].

6. Conclusions

The study reaches the following conclusions:

(1)

The emplacement ages of the Tiancang granites (279.3–274.1 Ma) constrain a significant Early Permian magmatic event in the region. These granites exhibit distinct spatiotemporal correlations within the Beishan Orogenic Belt, collectively demonstrating that the Early Permian represents a pivotal tectono-magmatic phase not only in the Beishan Orogenic Belt but also along the southern margin of the Central Asian Orogenic Belt.

(2)

Geochemical characteristics reveal that the Early Permian granites in the study area are predominantly high-K calc-alkaline to calc-alkaline, metaluminous to weakly peraluminous I-type granites. Their petrogenesis involved mantle-derived magma-triggered anatexis of ancient crustal materials, accompanied by subsequent magma hybridization and extensive fractional crystallization processes.

(3)

The integrated analysis of coeval granite–basalt associations, widespread diabase dykes, rift-related sedimentary sequences, and crust–mantle interaction signatures unequivocally demonstrates Early Permian continental rifting in the study area. These proxies collectively record bimodal magmatism, crustal extension, basin evolution, and significant crust–mantle interactions, providing conclusive evidence for extensional tectonics during this pivotal phase.