Petrogenesis and tectonic implications of I-type and A-type granitoids in the Dachadaban area, North Qilian Orogenic Belt

1 Introduction

The Qilian orogenic belt, situated in the central segment of China’s Central Orogenic Belt, serves as a critical tectonic junction linking the North China Craton, the West Qinling orogenic belt, and the Tibetan Plateau. Regionally, it is subdivided from north to south into three first-order tectonic units of the North Qilian orogenic belt, the Central Qilian uplift, and the South Qilian orogenic belt (Jiang et al., 2000; Xia et al., 2016; Yu et al., 2021). Among these, the North Qilian orogenic belt is renowned for its well-preserved trench–arc–basin system, composed of a collage of oceanic crustal remnants (ophiolites), island-arc volcanic–intrusive rocks, and accretionary complexes. It is widely regarded as a natural laboratory for investigating the evolution of the Paleo-Asian Ocean (Qilian Ocean), subduction initiation mechanisms, and subsequent continent–continent collision processes (Xia et al., 2003; 2012; Yu et al., 2021; Hui et al., 2025).

Magmatic activity in this region predominantly occurred from the Early Cambrian to the Early Devonian (ca. 512–360 Ma), with the most intense and diverse magmatism concentrated in the Early Paleozoic. Sporadic records also exist from the Mesoproterozoic, Neoproterozoic, and Early Triassic (Xia et al., 2016). The North Qilian orogen exposes a complete suite of ultramafic, mafic, and felsic rocks. Ophiolite belts extend continuously from west to east, including those at Changma, Dachadaban, Langshan (i.e., the southern Corridor Mountains), Lenglongling, Maomaoshan, Laohushan, and Xihuashan. Representative sections include Yushigou, Aoyougu, Jiugequan, Dongcaohe, Baimuxia–Mengang, and Dachadaban. These sections typically consist of lower serpentinized peridotite, middle tholeiitic gabbro and basalt, and upper boninitic volcanic rocks, collectively preserving critical information on the formation, seafloor spreading, and early subduction evolution of the Qilian oceanic crust (Xia et al., 2012; 2016).

Notably, the Dachadaban ophiolite is exceptionally well preserved. Previous studies indicate that its lower section consists of MORB-type tholeiitic basalt (dated at ca. 517 ± 4 Ma), while the upper section comprises typical boninite (ca. 487 ± 9 Ma), clearly documenting a tectonic transition from mid-ocean ridge spreading to subduction initiation during the Late Cambrian to Early Ordovician (Zhang et al., 1998; Meng et al., 2010; Xia et al., 2012). In recent years, with the accumulation of high-precision geochronological and geochemical data, increasing attention has been focused on the Dachadaban ophiolite belt (Meng et al., 2010; Su, 2018; Tian et al., 2023).

Concurrently, the North Qilian orogenic belt hosts widespread syn-subduction volcanic-arc granitoids. Geochronological studies reveal that these intrusions were emplaced between 520 and 446 Ma. Representative plutons include the Keke granite–tonalite complex (512–476 Ma; Wu et al., 2010; Song et al., 2013), the Chaidano granitoid (516–508 Ma; Chen et al., 2014), the Niuxinshan granitoid (477 Ma; Wu et al., 2010), the Yaogou granodiorite (463 Ma; Wu et al., 2010), and the Leigongshan tonalite (453 Ma; Tseng et al., 2009). However, systematic investigations of coeval felsic rocks in the Dachadaban area remain notably insufficient compared to the extensive research on its ophiolites and mafic–ultramafic suites.

As a major component of continental crust, the petrogenesis, spatiotemporal distribution, and source characteristics of granitoids are crucial for understanding continental crustal growth, crust–mantle interaction, and the dynamic evolution of orogenic belts (Zhang et al., 2008; Condie et al., 2011). Modern theories of granitoid genesis generally hold that most granitoids are not purely derived from crustal melting but instead result from integrated crust–mantle interactions of mantle-derived magmas not only provide the thermal energy necessary for partial melting of the crust but also frequently contribute directly to felsic magma generation through magma mixing (Sparks and Marshall, 1986; Bergantz, 1989; Wang, 2004; Wu et al., 2007). Compared to S-type granitoids (primarily sourced from sedimentary protoliths), I-type and A-type granitoids typically contain greater mantle contributions. Their geochemical signatures effectively trace the involvement of deep mantle materials, magmatic evolution pathways, and shifts in tectonic settings. Consequently, composite I–A-type plutons are considered ideal targets for unraveling deep crust–mantle interaction processes.

Therefore, this study focuses on the coexisting I-type and A-type granitoids exposed in the Dachadaban area. By integrating dual-mineral LA-ICP-MS U–Pb geochronology (zircon and apatite) with in-situ trace element analyses (e.g., Zr-in-zircon, Ti-in-zircon, REE in apatite), whole-rock major and trace element geochemistry, and Sr–Nd isotopic systematics, we aim to precisely constrain the crystallization ages of these felsic rocks, elucidate the genetic linkage between I-type and A-type magmas, reconstruct the physicochemical conditions of magma generation (e.g., temperature, oxygen fugacity), and clarify their implications for regional tectonic evolution.

2 Geological setting and petrographic

The North Qilian Orogenic Belt is located between the Central Qilian Block and the southwestern margin of the North China Craton (Alxa Block) and represents a typical Early Paleozoic orogenic belt. It mainly comprises ophiolite suites, subduction–accretion complexes, metamorphic rocks, island-arc volcanic rocks, and back-arc basins (Figure 1). The belt is characterized by widespread syn-orogenic thrust–nappe structures and subduction-related ductile thrust shear zones developed during the main orogenic stage (Xiao et al., 1978). Previous studies suggest that the North Qilian Orogen evolved from a Neoproterozoic lithospheric rift system into a trench–arc–basin system with modern-style plate tectonic characteristics during the Early Paleozoic (Xia et al., 2012; Song et al., 2013).

The Dachadaban area, the focus of this study, is located approximately 20 km northeast of Sunan County, Gansu Province, and constitutes an important part of the northern ophiolite belt of the North Qilian. Early studies identified a typical ophiolitic assemblage in this area, including lower serpentinized peridotite, tholeiitic gabbro–basalt sequences, and upper boninitic pillow lavas. Subsequent investigations revealed the coexistence of MORB-type basalt and boninite, indicating that the Dachadaban ophiolite records a complex history of oceanic crust formation and evolution (Zhang et al., 2008).

In addition to ophiolites, granitoids are also widely distributed in the Dachadaban area. The pluton investigated in this study crop out in the northwestern part of Dachadaban. Structurally, they are located at the southeastern margin of the Chaidano batholith and occur as intrusive bodies that are spatially closely associated with the ophiolitic units in the area. Based on petrographic characteristics, the pluton can be divided into three lithological types of monzodiorite, granodiorite, and monzogranite.

Monzodiorite (Figures 2a,b) is mainly composed of plagioclase, amphibole, and biotite, with minor K-feldspar and quartz. Plagioclase occurs as subhedral to euhedral prismatic crystals, commonly showing polysynthetic twinning and variable degrees of sericitization and epidotization. Amphibole forms euhedral prismatic crystals with typical amphibole cleavage and displays local preferred orientation, producing a weak gneissic fabric. Biotite occurs as brown flakes and is locally altered to chlorite. Accessory minerals include magnetite, zircon, apatite, and titanite. Overall, the rock shows moderate magmatic differentiation and minor late-stage hydrothermal alteration.

Granodiorite (Figures 2c,d) is dominated by plagioclase, quartz, amphibole, and K-feldspar, with a relatively low content of biotite. Plagioclase is mostly subhedral and exhibits slight sericitization and epidotization. Quartz occurs as anhedral grains, locally elongated and flattened, and displays subgrain structures indicative of dynamic recrystallization. Amphibole and biotite are locally aligned, forming a weak gneissic fabric. The accessory mineral assemblage is similar to that of the monzodiorite. Petrographically, the rock is intermediate to felsic in composition.

Monzogranite (Figures 2e,f) is mainly composed of K-feldspar, plagioclase, and quartz, with minor biotite. K-feldspar typically occurs as anhedral grains and commonly forms a characteristic monzonitic texture with plagioclase; local kaolinization is observed. Plagioclase is widely sericitized. Quartz occurs as elongated grains with preferred orientation and shows undulose extinction and subgrain development. Biotite is commonly replaced by muscovite, forming pseudomorphic textures. Accessory minerals are mainly magnetite and zircon. Overall, the rock displays characteristics of a highly differentiated felsic intrusion.

3 Analytical methods

A total of six samples were collected from the study area, including three monzodiorites, two granodiorites, and one monzogranite. These samples were used for thin section preparation, whole-rock powdering, and mineral separation (zircon and apatite) for petrographic observation, whole-rock geochemical analysis, and mineral geochronology and in-situ geochemical analyses. All sample preparation and analytical work were carried out at Nanjing Shunke Testing Technology Co., Ltd.

3.1 Whole-rock major and trace element analyses

Samples were crushed and ground to 200 mesh for whole-rock chemical analyses. Major elements were determined using inductively coupled plasma optical emission spectrometry (ICP-OES; Agilent 5110), whereas trace elements were measured by inductively coupled plasma mass spectrometry (ICP-MS; Thermo Fisher Scientific iCAP RQ). International and Chinese standard reference materials were used for calibration and quality control during the analytical procedures (Jochum and Nohl, 2008). Geochemical reference materials of USGS: basalt (BCR-2, BHVO-2), andesite (AVG-2), granodiorite(GSP-2) were treated as quality control. Measured values of these reference materials were compared with preferred values in GeoReM database (Jochum and Nohl, 2008; http://georem.mpch-mainz.gwdg.de). Deviation were better than 20% for trace elements between 0.5 and 5 ppm, better than 10% for these between 5 and 50ppm, and better than 5% for these exceeded 50 ppm.

3.2 Zircon and apatite U–Pb geochronology and trace element analyses

Zircon and apatite grains were separated using conventional heavy liquid and magnetic separation techniques, followed by handpicking under a binocular microscope. Selected grains were mounted in epoxy resin and polished to expose crystal interiors.

Zircon U-Pb isotopic ratios and trace element concentrations were simultaneously determined using laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). The analytical system consisted of a 193 nm ArF excimer laser ablation system (RESOlution LR, Australian Scientific Instruments) coupled with a quadrupole ICP-MS (Thermo iCAP RQ). Prior to analysis, the instrument was tuned using line scans on NIST SRM 610 glass to achieve ²³⁸U signal intensities >40,000 cps, Th/U >98%, and ThO/Th <0.4% to ensure data quality.

Laser ablation was performed with a spot diameter of 30 μm, a repetition rate of 6 Hz, and an energy density of ca. 4.5 J/cm². Data acquisition was conducted in peak-hopping mode, with 15 s of background collection followed by 45 s of sample signal acquisition for each analysis. Zircon U–Pb ages were calibrated against the Plesovice zircon standard, whereas trace element concentrations were quantified using NIST SRM 610 and NIST SRM 612 as external standards. Raw data reduction was carried out offline using ICPMSDataCal v10.9 (Liu et al., 2010).

Major element compositions of apatite were determined using an electron probe microanalyzer (EPMA; JEOL JXA-8230). Analytical conditions included an accelerating voltage of 15 kV and a beam current of 10 nA, with peak counting times of 10 s and background counting times of 5 s. All data were corrected using the ZAF procedure.

Apatite U–Pb isotopic ratios and trace element concentrations were also analyzed by LA-ICP-MS using the same laser ablation system and ICP-MS as for zircon. Laser conditions included a spot diameter of 38 μm, a repetition rate of 6 Hz, and an energy density of ca. 4.0 J/cm². Apatite U–Pb data were corrected using the MAD method, and trace elements were quantified using NIST SRM 610 and 612 as external standards. All raw data were processed and calibrated using IOLITE 4 software.

3.3 Sr–Nd isotopic analyses

Strontium and neodymium separation and purification were conducted in the ultra-clean laboratory at Nanjing Shunke Testing Technology Co., Ltd., using conventional two-column ion-exchange chromatography. Approximately 150 mg of sample powder and standard materials (e.g., BCR-2) were dissolved in sealed Savillex beakers using a mixture of HClO₄ and HF on a hotplate for 3 days. Strontium and rare earth elements (REE) were first separated using cation exchange columns (AG50W resin), followed by further purification using a second cation exchange column (P507 resin).

Sr and Nd isotopic ratios were measured using a Nu Plasma II multi-collector ICP-MS (MC-ICP-MS). Analytical procedures followed those described by Jahn et al. (1996). Instrumental mass fractionation was corrected using ⁸⁶Sr/⁸⁸Sr = 0.1194 and ¹⁴⁶Nd/¹⁴⁴Nd = 0.7219 for internal normalization. The international standards NIST SRM 987 (Sr) and JNdi-1 (Nd) were used for external drift correction. USGS geochemical reference materials (BCR-2, BHVO-2, AGV-2, RGM-2, etc.) were analyzed as quality control samples and yielded results consistent with published values within analytical uncertainties (Weis et al., 2006).

4 Results

4.1 Whole-rock major and trace element characteristics

Six samples from Dachadaban were selected for whole-rock major and trace element analyses, including three monzodiorites, two granodiorites, and one monzogranite.

As shown in Supplementary Table 1, the loss on ignition (LOI) values range from 1.8 to 3.74 wt%, indicating that some samples have undergone alteration (Figure 2). The granodiorites and monzogranite are silica-rich (SiO₂ = 63.85–73.78 wt%), with moderate Al₂O₃ contents (13.92–14.01 wt%) and total alkalis (Na₂O+ K₂O) of 6.16–7.55 wt%. In contrast, the monzodiorites exhibit relatively lower SiO₂ (53.35–54.40 wt%) and total alkalis (5.83–6.44 wt%). Notably, the Al₂O₃ contents of the granodiorites and monzogranite (13.98–14.01 wt%) are significantly lower than those of the monzodiorites (18.40–19.37 wt%).

In the TAS diagram (Figure 3a), samples from Dachadaban plot in the fields of granite, granodiorite, and monzodiorite, consistent with microscopic identification results (Figure 2). The SiO₂ vs. K₂O diagram (Figure 3b) shows that the Dachadaban granitoids (granite and granodiorite) belong to the high-K calc-alkaline series, whereas the Dachadaban diorites fall within the shoshonitic series. In the A/CNK vs. A/NK diagram (Figure 3c), the granitoids (granite and granodiorite) exhibit significant variation, plotting in both metaluminous and peraluminous fields, while the Dachadaban diorites plot in the metaluminous to subaluminous fields. Harker diagrams of major oxides (Figures 4a–f) display coherent evolutionary trends with granitoids from adjacent regions (Wu et al., 2010; Chen et al., 2014), showing negative correlations of Al₂O₃, Fe₂O₃ ^T, TiO₂, CaO, MnO, and P₂O₅ with increasing SiO₂.Primitive mantle-normalized trace element spider diagrams (Figure 5a; Supplementary Table 1) show remarkably similar patterns among all samples, suggesting derivation from a common magmatic system. The rocks are enriched in large-ion lithophile elements (LILEs; e.g., Rb) and high-field-strength elements (HFSEs; e.g., Th, Zr, Hf), but depleted in Ba, Sr, Nb, Ta, and Ti, which is characteristic of arc-related granitoids. Nb/Th ratios range from 0.91 to 2.15 (mean = 1.47), and Nb/Ta ratios from 9.30 to 20.04 (mean = 14.15), which are close to typical upper continental crust values (Nb/Th ≈ 2–3; Nb/Ta ≈11–12) and markedly lower than those of mantle-derived melts (Nb/Th > 15; Nb/Ta ≈17–22; Sun and McDonough, 1989; Rudnick and Gao, 2003), supporting a dominant crustal contribution to the magma source. This signature is consistent with coeval granitoids in neighboring areas (Wu et al., 2010; Chen et al., 2014).

Chondrite-normalized rare earth element (REE) patterns (Figure 5b; Supplementary Table 1) show ΣREE values of (221–293 ppm) for granodiorites and monzogranite, and (190–317 ppm) for monzodiorites, all displaying a right-inclined “seagull-type” profile. The patterns are characterized by strong light REE (LREE) enrichment and weak heavy REE (HREE) fractionation. Eu anomalies (δEu = 0.58–0.84; mean = 0.64) suggest either residual plagioclase in the source region or significant plagioclase fractionation during magma evolution. These anomalies are less pronounced than those observed in granitoids from adjacent regions (Wu et al., 2010; Chen et al., 2014).

4.2 Zircon U–Pb geochronology

To constrain the emplacement age of the Dachadaban plutons, zircon U–Pb dating was performed on two monzodiorite samples (DCDB-05, DCDB-07) and one monzogranite sample (DCDB-10) using LA-ICP-MS. Cathodoluminescence (CL) images (Figures 6a–c) reveal euhedral to subhedral zircons with well-developed oscillatory zoning, indicative of a magmatic origin. Analyses targeted pristine, inclusion-free, crack-free zircon domains with clear internal zoning. The weighted mean age of the eight monitor standards (Tanz, Supplementary Table 1) is 565.1 ± 7.2 Ma (MSWD = 0.07), which is consistent with the recommended age of 566.16 ± 0.77 Ma.

U–Pb results (Supplementary Table 2) are as follows, For sample DCDB-05 (monzodiorite), after excluding nine discordant analyses (<90% concordance), six concordant spots yield a weighted mean ²⁰⁶Pb/²³⁸U age of 504 ± 12 Ma (MSWD = 0.87) (Figure 6a). Sample DCDB-07 (monzodiorite) of the thirteen concordant analyses give an age of 505.7 ± 4.3 Ma (MSWD = 0.14) (Figure 6b). Sample DCDB-10 (monzogranite) of nine concordant analyses yield 504.3 ± 3.2 Ma (MSWD = 0.16) (Figure 6c), while two older inherited cores record ages of 527 Ma and 719 Ma, respectively.

Collectively, these data indicate that the Dachadaban granitoid suite was emplaced at ca. 505 Ma, during the Late Cambrian.

4.2.1 Apatite U–Pb geochronology

Apatite grains appear gray to dark in CL images (Figures 6d–f), exhibiting subhedral to euhedral, predominantly prismatic to tabular morphologies (length of 100–150 μm; width of 50–80 μm). Clean, unaltered grains were selected for U–Pb analysis. Tera-Wasserburg concordia plots yield lower intercept ages of 512.7 ± 7.8 Ma (MSWD = 1.5) for DCDB-05 (Figure 6d); 511.0 ± 13.0 Ma (MSWD = 2.2) for DCDB-07 (Figure 6e) respectively (Supplementary Table 3).

4.3 Mineral compositions of zircon and apatite

4.3.1 Zircon trace elements

Trace element data were acquired simultaneously with U–Pb isotopes (Supplementary Table 4). Some zircons display core–rim structures (Figures 6a–c), suggesting multiple magmatic pulses. Monzodiorite zircons (DCDB-05, DCDB-07) contain 318–1,470 ppm U with Th/U ratios of 0.13–0.45. Monzogranite zircons (DCDB-10) have lower U contents (185–544 ppm) and variable Th/U (0.10–0.97). Zircon Ti concentrations average 3.52 ppm in monzodiorites (average of 0.78–7.42 ppm) and 11.8 ppm in monzogranite (range of 1.83–17.7 ppm), indicating higher crystallization temperatures for the felsic intrusion.

4.3.2 Apatite major and trace elements

In-situ EPMA analyses (Supplementary Table 5) show that apatite from monzodiorites (DCDB-05, DCDB-07) is compositionally homogeneous, with CaO = 54.7–55.4 wt% and P₂O₅ = 41.1–42.8 wt%. Chlorine is low (0.05–0.19 wt%), whereas fluorine is elevated (2.00–2.87 wt%; mean ≈2.40 wt%). Minor REE oxides (La₂O₃ of 0–0.18 wt%; Ce₂O₃ of 0–0.61 wt%) are detected in some grains.

LA-ICP-MS trace element data (Supplementary Table 6) reveal Sr = 281–342 ppm, Zr = 0.67–9.71 ppm, Y = 411–1,457 ppm (yielding Y/Zr > 100), Nb ≤ 1.81 ppm, Ta ≤0.16 ppm, and Th/U = 2.0–4.3. Chondrite-normalized REE patterns are LREE-enriched with δEu = 0.35–0.75 and Ce/Ce* ≈1 (0.92–1.08). Zr/Hf ratios range from 11 to 83, and Nb/Ta ratios are generally low (0.2–5), though localized Nb enrichment yields values up to 10–11 in a few spots. Overall, apatite from DCDB-05 and DCDB-07 shows consistent geochemical signatures (Figure 6f), reflecting magmatic homogeneity.

4.4 Whole-rock Sr–Nd isotopes

Five samples were analyzed for Sr–Nd isotopes (Supplementary Table 7). Excluding the monzogranite, the monzodiorites and granodiorites display ⁸⁷Sr/⁸⁶Sr = 0.7326–0.7443 and ¹⁴³Nd/¹⁴⁴Nd = 0.5119–0.5121. The monzogranite has a higher ⁸⁷Sr/⁸⁶Sr (0.8136) but similar ¹⁴³Nd/¹⁴⁴Nd (0.5120). At t = 504 Ma, initial ⁸⁷Sr/⁸⁶Sr values range from 0.7205 to 0.7286. The monzodiorites yield ε_Nd(t) = −6.4 to −7.3, corresponding to two-stage Nd model ages (T_DM2) of 1751–1824 Ma. The granodiorites and monzogranite have more evolved signatures (ε_Nd(t) = −7.4 to −9.7; T_DM2 = 2019–1835 Ma). These isotopic characteristics are consistent with those of coeval granitoids in adjacent regions (Figure 7; Wu et al., 2010; Chen et al., 2014).

5 Discussion

5.1 Timing of emplacement

The North Qilian Orogenic Belt hosts widespread, multi-phase, and compositionally diverse Paleozoic granitoid intrusions, which serve as a key archive of the region’s tectono-magmatic evolution. Zircon U–Pb dating of intermediate to felsic intrusions from the Dachadaban area yields crystallization ages of 504 ± 12 Ma for monzodiorite, 505.7 ± 4.3 Ma for granodiorite, and 504.3 ± 3.2 Ma for granite. Apatite U–Pb analyses on the monzodiorite give ages of 512.7 ± 7.8 Ma and 511.0 ± 13.0 Ma. Although minor differences exist between zircon and apatite U–Pb ages, their 2σ confidence intervals overlap, indicating that the Dachadaban granitoids were emplaced during a single magmatic event spanning ca. 512 to 504 Ma. The slightly older apatite ages likely record early-stage crystallization or thermal perturbations within the magmatic system.

These results are in excellent agreement with coeval regional intrusions, such as the Chaidano granite (516–505 Ma), Kekeli plagiogranite (ca. 512 Ma), quartz diorite (ca. 501 Ma), and Yemaqu granite (ca. 508 Ma, Wu et al., 2010; Chen et al., 2014), collectively documenting a regional magmatic flare-up in the North Qilian during the Late Cambrian. Previous studies suggest that this magmatism occurred within an arc–active continental margin setting under sustained subduction of the Qilian Ocean. For example, the Zhamash Alaskan-type mafic complex (513 ± 4.5 Ma) represents a typical subduction-related island-arc environment (Liu et al., 2005; Tseng et al., 2009), whereas the coeval intermediate to felsic intrusions likely reflect magmatic differentiation and thermal reorganization within crustal magma reservoirs at varying depths beneath the arc.

Multi-phase tectono-magmatic events recorded by regional ophiolites indicate a complex oceanic evolution history of the North Qilian Ocean. Granitoid magmatism between ca. 510 and 500 Ma occurred after subduction initiation (ca. 520 Ma) but prior to final ocean closure (ca. 445 Ma), marking a critical tectonic transition. Compared to the initial subduction stage, magmatism during this interval was more voluminous and compositionally diverse, accompanied by elevated crystallization temperatures and evolving oxygen fugacity, reflecting significant thermal reworking of the sub-arc lithosphere. Therefore, the emplacement age of the Dachadaban intrusions (504–512 Ma) most likely records a major magmatic pulse associated with the transition of the North Qilian subduction system from its early establishment toward a mature arc–back-arc configuration, rather than an isolated tectonic event.

5.2 Petrogenesis and source characteristics

The source composition of granitoids has long been a focal topic in geological research. Variations in magma sources are among the primary factors responsible for the geochemical diversity observed in granitic rocks (Wu et al., 2007). For instance, S-type granitoids are typically peraluminous, whereas I-type granitoids are generally metaluminous. Moreover, magmatic processes are governed by both internal and external parameters such as temperature, pressure, water content, and crystal fraction, which collectively influence magma dynamics and evolution (Cassidy et al., 2018). This study integrates in situ zircon and apatite geochemistry, whole-rock geochemistry, and Sr–Nd isotopic data to provide multi-scale constraints on the nature of the magma source.

5.2.1 Apatite geochemistry and magma source characteristics

Geochemical compositions and mineral assemblages of granitoids are often modified by magmatic differentiation and post-magmatic hydrothermal alteration, which may obscure their primary source signatures. However, recent studies have demonstrated that apatite geochemistry provides robust indicators of magma source properties (Sha and Chappell, 1999; Laurent et al., 2017; Zhang et al., 2020). Apatite trace element compositions are primarily controlled by their host magmas (Sha and Chappell, 1999), although processes such as fractional crystallization or hydrothermal fluid interaction can significantly modify their geochemical signatures (Chen et al., 2016; Zhang et al., 2020). In their study of granitoids from the Lachlan Fold Belt, Australia, Sha and Chappell (1999) documented distinct trace element differences in apatite from granitoids of different petrogenetic types. Apatite in I-type granitoids exhibits right-inclined rare earth element (REE) patterns (Figure 6f), whereas apatite in S-type granitoids shows relatively flat REE patterns. In our samples, the apatite REE patterns closely resemble those typical of I-type granitoids (Figure 6f). This similarity is consistent with the presence of hornblende, a diagnostic mineral of I-type suites, in the granitoid rocks (Figure 2). This preliminary evidence suggests an I-type affinity for the intermediate rocks.Furthermore, apatite trace element ratios effectively constrain granitoid source characteristics. On the La/Sm vs. Th/U and Y vs. Ce discrimination diagrams for granitoid types (Figures 8a,b), the dioritic samples predominantly plot within the I-type field. Additionally, on the recently proposed apatite-based genetic classification diagram for global I-, S-, M-, and A-type (ISMA) granitoids (Kieffer et al., 2024), apatite from the Dachadaban granitoids falls within the IM domain (interpreted as mafic I-type granitoids and mafic volcanic rocks) (Figure 8c). Similarly, in the Sr/Y vs. Eu/Eu* vs. (REE + Y)/10,000 ternary diagram (Figure 8d), apatite compositions lie within the mafic field. Collectively, these apatite geochemical features indicate that the studied rocks largely inherited the signature of an I-type, arc-derived magmatic source.

5.2.2 Zircon thermometry and oxygen fugacity evolution

Zircon saturation thermometry provides a robust method for estimating zircon crystallization temperatures. Using the recommended parameters for rutile-undersaturated granitoid systems (TiO₂ = 0.5; Ferry and Watson, 2007), we calculated zircon saturation temperatures for the Dachadaban intrusive suite. The results show that zircons from the monzodiorites (samples DCDB-05 and DCDB-07) exhibit a broad crystallization temperature range of 608 °C–784 °C (average of 692 °C), whereas zircons from the monzogranite (DCDB-10) display an even wider range of 659 °C–880 °C (average of 826 °C). Miller et al. (2003)demonstrated an inverse relationship between zircon inheritance and zircon saturation temperature, classifying North American granitoids into “hot” types (average T = 837 °C; little to no inherited zircon) and “cold” types (average T = 766 °C; high inheritance, typically ca. 50%). In this framework, the granitoids in our study area record a transition from “cold” to “hot” conditions, implying the input of substantial external heat to sustain elevated magmatic temperatures. The redox state of magmas can also be inferred from Ce and Eu anomalies in zircon (Ballard et al., 2002). Applying the method of (Ballard et al., 2002), the monzodiorite samples (DCDB-05/07) yield Eu_N/Eu_N values of 0.31–0.55 (mostly >0.4). On the Ce_(IV)/Ce_(III) versus Eu_N/Eu_N diagram (Figure 9), these samples show significant variation in Ce_(IV)/Ce_(III), with most values from DCDB-07 exceeding 1.1, indicating relatively oxidizing magmatic conditions. These conditions are likely associated with crustal melting or mantle-derived input (Ballard et al., 2002; Chen et al., 2016). In contrast, the monzogranite (DCDB-10) exhibits lower Eu_N/Eu_N ratios (0.11–0.31, mostly <0.2) and limited Ce_(IV)/Ce_(III) variation (0.13–0.58), suggesting minimal oxidation of Ce to Ce⁴⁺ and thus formation under relatively reduced (low-f_O2) conditions. These features are similar to those of the A-type granites of Qianlishan (Chen et al., 2016). The contrasting geochemical signatures between zircon and apatite in the Dachadaban granitoids likely reflect an evolution of the regional magmatic system from relatively oxidizing toward more reduced conditions. This decreasing trend in oxygen fugacity may not be solely attributable to magmatic differentiation but could instead reflect changes in the nature of deep-seated heat sources and melting regimes. For example, it may represent a transition from a fluid-fluxed, subduction-modified system toward a drier, lower-crustal dehydration melting environment, resulting in reduced volatile content and diminished oxidation capacity. Such a shift may reflect a response to deeper tectonic reorganization rather than internal evolution within a single magma chamber.

5.2.3 Whole-rock geochemical signatures and granite classification

Combined with the observed increase in zircon saturation temperatures from intermediate to felsic compositions, we infer that both the mode of thermal input and the depth of melting likely changed during magmatism in the study area. Specifically, the early-stage magmas (represented by DCDB-05 and DCDB-07) were relatively oxidized and hydrous, possibly reflecting continuous fluid or melt flux from an active subduction system. In contrast, the late-stage magma (DCDB-10) exhibits high-temperature, reduced, and anhydrous characteristics, consistent with a dehydration-dominated melting regime in the deep crust.The prevailing classification scheme for granites divides them into four main types of S-type (derived from sedimentary protoliths), I-type (derived from igneous protoliths), M-type (mantle-derived), and A-type (formed in non-orogenic settings, Whalen et al., 1987). A-type granites are geochemically characterized by high SiO₂ and alkali contents, elevated (Na₂O+ K₂O)/CaO and FeO^T/MgO ratios in major elements, and enrichment in high-field-strength elements (HFSE) along with high concentrations of (Zr + Nb + Ce + Y) and elevated Ga/Al ratios in trace elements (Figure 10; Eby, 1990; 1992). The Dachadaban monzogranite, together with granites from adjacent regions (Wu et al., 2010; Chen et al., 2014), exhibits high SiO₂, high alkalis, low CaO, and low MgO, features consistent with the major-element composition typical of A-type granites (Whalen et al., 1987). In terms of trace and rare earth elements, the samples display δ_Eu values of 0.60–0.84, indicating a distinct negative Eu anomaly. Chondrite-normalized REE patterns show an overall right-inclined “seagull-shaped” profile, with slight depletion in heavy REEs (HREE) relative to light REEs (LREE). On primitive mantle–normalized spider diagrams, the rocks are enriched in Rb, Th, U, Zr, and Hf but markedly depleted in Nb, Sr, Ba, Ti, and P, characteristics consistent with highly fractionated A-type granites (Whalen et al., 1987; Eby, 1990). In discrimination diagrams such as Zr vs. 10,000Ga/Al, (Na₂O+ K₂O) vs. 10,000Ga/Al, and (Na₂O+ K₂O)/CaO vs. (Zr + Nb + Ce + Y), the samples plot within the A-type field or in transitional zones between I- and A-types. Additionally, the relatively high zircon saturation temperature (ca. 826 °C) and low oxygen fugacity are also consistent with high-temperature, A-type–affinity magmas. Therefore, the Dachadaban monzogranite is classified as A-type, whereas the broader suite of granitoids in the Dachadaban area, together with regional granites (Wu et al., 2010; Chen et al., 2014), exhibits transitional I–A-type characteristics. A-type granites were initially regarded as granites formed in non-orogenic, alkaline, and water-poor conditions. However, subsequent studies have shown that they can also form during the post-orogenic lithospheric extension stage (Eby, 1990). Based on this, A-type granites can be further subdivided into two subtypes of A1 and A₂. A1-type granites are typically associated with intraplate or hotspot settings, whereas A₂-type granites are commonly linked to the late stages of subduction systems or the extensional phases in the later evolution of orogenic belts (Eby, 1992). The primary trace-element discriminants between A1 and A₂ types are the Y/Nb and Rb/Nb ratios of A1 granites exhibit relatively low ratios, whereas A₂ granites show higher values (Eby, 1992). In the Rb/Nb vs. Y/Nb discrimination diagram (Figure 11a) and the Nb–Y–Ce ternary diagram (Figure 11b), the Dachadaban granitoids plot almost entirely within the A₂ field, indicating that they likely formed in an extensional tectonic setting related to an orogenic belt.

5.2.4 Magmatic petrogenesis and geodynamic setting

During continental evolution, specific elements exhibit characteristic migration and enrichment trends. The mobility of these elements can be used to further distinguish the tectonic settings in which granites form (Pearce et al., 1984; Pearce, 1996). In the Yb–Ta diagram (Figure 12a) and the Y + Nb–Rb discrimination diagram (Figure 12b), granitoid samples from the Dachadaban area all plot within the “post-collisional” field. It should be emphasized that this “post-collisional” field in tectonic discrimination diagrams primarily reflects an extensional stress regime in the lithosphere and does not necessarily imply that the magmatism was directly triggered by continental collision. Combined with the high-temperature, water-poor, and transitional I–A-type geochemical characteristics observed in the study area, the A-type–affinity granites are more likely to have formed in a context of lithospheric thinning and thermal reorganization. An increasing number of studies consider the close spatial and temporal association between high-K calc-alkaline I-type and A-type granites as a key indicator of extension-related magmatism (Liégeois, 1998; Eyal et al., 2010), further suggesting that the emplacement of the composite I–A-type granitoids in Dachadaban is closely linked to an extensional tectonic setting. Although A-type granites are widely accepted to form in extensional environments, significant debate persists regarding their petrogenesis and magmatic evolution, particularly concerning the nature of their source regions and the role of mantle-derived magmas (Whalen et al., 1987; Wu et al., 2002). As previously noted, proposed genetic models for A-type granites typically include partial melting of crustal materials, crust–mantle magma mixing, and fractional crystallization of mantle-derived magmas. However, existing studies indicate that fractional crystallization of purely mantle-derived magmas rarely produces typical granitic melts and instead tends to generate peralkaline magmas (Shellnutt and Zhou, 2007; Jahn et al., 2009; Frost and Frost, 2011). The formation of A-type granites generally requires unusually high temperatures, and without an additional heat source such as underplating by mantle-derived basaltic magmas, the continental crust alone cannot attain such high temperatures (Patino Douce, 1999). Zircon Ti-in-zircon thermometry for the Dachadaban A-type granite yields an average crystallization temperature of approximately 826 °C, confirming the occurrence of high-temperature granitic magmatism in the region. Although the Sr–Nd isotopic compositions of these granites are overall similar to those of regional crustal rocks, their positions on the (⁸⁷Sr/⁸⁶Sr)ᵢ–ε_Nd(t) diagram (Figure 7) suggest a mantle component contribution of 20%–30%, significantly higher than the 10%–18% estimated for the neighboring Chaidanuo granitoid suite (Chen et al., 2014). This isotopic difference indicates that mantle-derived magmas in the Dachadaban area not only supplied additional heat but also introduced a minor amount of mafic material into the magmatic system. Therefore, the Dachadaban granitoids are interpreted as predominantly partial melts of Paleoproterozoic crustal sources, with mantle input primarily acting as a thermal trigger accompanied by limited material contamination. This crust–mantle interaction model resembles previously reported hybrid-origin peraluminous granites (Keay et al., 1997; Collins, 1998; Healy et al., 2004; Cai et al., 2011). Compared with coeval I-type granites, the A-type granites exhibit higher formation temperatures and greater degrees of fractionation, suggesting that although both share a similar crustal source, they experienced notably different thermal inputs. Experimental petrology has demonstrated that partial melting of calc-alkaline felsic igneous rocks (e.g., granodiorite) under high-temperature, fluid-absent (dehydration) conditions can generate peraluminous granitic melts with geochemical signatures characteristic of A-type granites (King et al., 1997; Patiño Douce, 1997). The granites in the study area display high total alkali contents, low CaO and Al₂O₃ contents, and relatively low A/CNK values, compositional features closely resembling experimental melts produced by partial melting of granodioritic sources at ca. 950 °C and 4 kbar. Thus, the I-type calc-alkaline intermediate to felsic magmas may have undergone partial melting or extensive fractional crystallization under high-temperature, dehydration conditions, thereby generating granites with A-type affinity. Together, they represent a continuous evolutionary sequence within a single magmatic system evolving from a less fractionated to a highly fractionated end-member. Variations in the ratios of highly incompatible to moderately incompatible elements are commonly used to distinguish between partial melting and fractional crystallization processes (Allègre and Minster, 1978). In the Dachadaban granitoids, the La/Sm vs. La and Ce/Sm vs. Ce diagrams (Figure 13) show only minor variations in elemental ratios with gentle trends, indicating a relatively continuous degree of differentiation among samples. This pattern is more consistent with geochemical behavior controlled by fractional crystallization during magmatic evolution, rather than the generation of multiple discrete melts resulting from varying degrees of partial melting. Moreover, the samples from the study area consistently exhibit relative depletions in Eu, Sr, Ba, Ti, P, and Nb. Previous studies have shown that such elemental anomalies are typically associated with the fractional crystallization of specific minerals (Chappell and White, 1992; Wu et al., 2003). Specifically, depletions in Eu, Sr, and Ba indicate the removal of plagioclase and K-feldspar; negative anomalies in Ti and Nb are linked to the crystallization of Fe–Ti oxides and Ti-bearing minerals (e.g., ilmenite and sphene); and P depletion reflects apatite crystallization. These features are consistent with the pronounced negative correlations between SiO₂ and major oxides such as MgO, TiO₂, and CaO observed in Harker diagrams (Figure 4), collectively indicating that the magma underwent fractional crystallization dominated by feldspars, mafic minerals, apatite, and Ti-rich phases during its ascent and storage. It should be emphasized that although fractional crystallization is a key mechanism controlling magmatic evolution, the granitoids from the study area and those from adjacent regions (Wu et al., 2010; Chen et al., 2014) display remarkable similarities in major and trace element compositions, emplacement ages, and (⁸⁷Sr/⁸⁶Sr)ᵢ–ε_Nd(t) isotopic signatures (Figure 7). This strongly suggests they originated from the same or closely related magmatic system rather than from entirely independent, multi-source melts. Notably, the I-type-affinity intermediate to intermediate-felsic rocks and the highly fractionated, A-type-affinity granites are nearly coeval in age, yet the latter exhibit higher degrees of fractionation, greater alkali enrichment, and lower water contents, implying that they likely represent a continuous evolutionary sequence within a single magmatic system evolving from a relatively less fractionated end-member toward a highly fractionated one. In summary, the granitoid magmas in the study area were predominantly derived from crustal sources. Their geochemical and Sr–Nd isotopic characteristics indicate that mantle-derived components mainly provided heat and contributed a minor amount of material, thereby triggering partial melting of the lower crust, such as granodioritic protoliths. During this process, the generated felsic magmas likely underwent a certain degree of crust–mantle interaction and homogenization at depth, followed by further evolution through fractional crystallization during ascent and emplacement, ultimately producing granitoids with transitional I- to A-type affinities. The systematic changes in magma properties, from relatively oxidized to reduced conditions and from hydrous to relatively anhydrous, do not necessarily signify an abrupt tectonic transition but more likely record adjustments in the regional lithospheric thermal structure and material cycling associated with waning subduction and slab rollback. Specifically, during the tectonic shift from a convergent to a back-arc extensional regime, lithospheric thinning and enhanced heat input promoted dehydration melting of the lower crust, generating high-temperature, relatively water-poor magmas with A-type affinity. This process represents a progressive tectonic adjustment driven by deep-seated geodynamic processes.

6 Tectonic evolution

The North Qilian orogenic belt, situated along the northern margin of the Qaidam–Qilian block, underwent a series of complex tectonic evolutionary stages from the Neoproterozoic to the Paleozoic, including oceanic crustal spreading, subduction of oceanic lithosphere, island arc formation, and back-arc basin development (Zhang et al., 1998; Meng et al., 2010; Song et al., 2013). Existing studies indicate that subduction of the Greater Qilian Ocean commenced at approximately 520 Ma (Xia et al., 2012; Song et al., 2013), initiating early island arc magmatism. An age of ca. 517 Ma is widely regarded as representative of the earliest phase of island arc magmatism in the North Qilian region (Xia et al., 2012). The northern ophiolite belt was emplaced between 517 and 448 Ma (Meng et al., 2010; Xia and Song, 2010; Song et al., 2013), indicating sustained oceanic subduction during this interval and the establishment of a mature arc–back-arc basin system. (Xia et al., 2012). reported tholeiitic basalt–boninite assemblages in a back-arc basin setting formed between 517 and 505 Ma, reflecting enhanced decompression melting within the subduction system and marking the tectonic transition from an immature island arc to a back-arc extensional regime. Although debates persist regarding the polarity of subduction and the timing of arc–continent collision in the North Qilian (Wu et al., 2011; Song et al., 2013; Li et al., 2017; Peng et al., 2017), the spatial distribution of the northern and southern ophiolite belts, together with the geometry of the arc-related magmatic belts, leads most researchers to favor northward subduction of the North Qilian oceanic slab, consistent with a classic “trench–island arc–back-arc basin” tectonic architecture (Song et al., 2013; Chen et al., 2021).The Dacha Daban area hosts a coeval (512–504 Ma) high-K calc-alkaline I–A-type granitoid suite. The close spatial and temporal association between I-type and A-type granites is commonly observed in back-arc extensional settings associated with waning subduction and slab rollback, rather than being restricted exclusively to post-collisional collapse following continent–continent collision (Liégeois, 1998). In tectonic discrimination diagrams (Figure 12), the samples plot within the “post-collisional” field; however, a more reasonable interpretation is that this field reflects a transitional tectonic stage from convergence to extension driven by slab rollback-induced extension of the overriding plate, rather than gravitational collapse following final continental collision. Integrating geochronological and geochemical data, the Dacha Daban pluton was emplaced between 512 and 504 Ma. During this period, elevated zircon saturation temperatures and decreasing oxygen fugacity indicate that the magmatic system evolved from a relatively hydrous, oxidized arc-like environment toward a high-temperature, relatively anhydrous magma system. This transition aligns well with the dynamic sequence of subduction weakening, slab rollback, and back-arc lithospheric extension (Chen et al., 2021). Slab rollback triggered asthenospheric upwelling and lithospheric thinning, leading to basaltic magma underplating that provided sustained thermal input to the lower crust, thereby inducing partial melting of enriched crustal materials. Initially, this produced moderate-temperature (ca. 690 °C–730 °C), metaluminous, calc-alkaline I-type granites. As extension intensified and mantle-derived heat input increased, the magmatic system further heated and dehydrated, ultimately evolving into alkali-rich, water-poor A-type granites. In summary, the granitoid magmas in the study area were predominantly derived from crustal sources. Their geochemical and Sr–Nd isotopic characteristics indicate that mantle-derived components mainly provided heat and contributed a minor amount of material, thereby triggering partial melting of the lower crust, such as granodioritic protoliths. During this process, the generated felsic magmas likely underwent a certain degree of crust–mantle interaction and homogenization at depth, followed by further evolution through fractional crystallization during ascent and emplacement, ultimately producing granitoids with transitional I- to A-type affinities. The systematic changes in magma properties, from relatively oxidized to reduced conditions and from hydrous to relatively anhydrous, do not necessarily signify an abrupt tectonic transition but more likely record adjustments in the regional lithospheric thermal structure and material cycling associated with waning subduction and slab rollback. Specifically, during the tectonic shift from a convergent to a back-arc extensional regime, lithospheric thinning and enhanced heat input promoted dehydration melting of the lower crust, generating high-temperature, relatively water-poor magmas with A-type affinity. This process represents a progressive tectonic adjustment driven by deep-seated geodynamic processes. The coexistence of I-type and A-type granites indicates that the North Qilian region was undergoing a tectonic transition from subduction-related compression to back-arc extension at ca. 512–504 Ma, most likely triggered by slab rollback. Asthenospheric upwelling and lithospheric thinning led to basaltic underplating, which provided sustained heat to the lower crust, inducing high-temperature dehydration melting and generating A₂-type magmas. As illustrated in the geodynamic model (Figure 14), This process represents a progressive tectonic adjustment driven by deep-seated geodynamic processes.

7 Conclusion

• Zircon U–Pb ages of the Dacha Daban diorite, granodiorite, and granite are 504 ± 12 Ma, 505.7 ± 4.3 Ma, and 504.3 ± 3.2 Ma, respectively, with apatite U–Pb ages from the diorite ranging from 512 to 511 Ma. These data indicate that regional magmatism was concentrated at ca. 512–504 Ma, representing a syn-subduction magmatic event related to the evolution of the Qilian Ocean, rather than an isolated or typical post-collisional granite episode.
• The granitoids were predominantly derived from Paleoproterozoic crustal sources and underwent significant fractional crystallization. Mantle-derived components (20%–30%) primarily provided thermal input rather than serving as the main material source. The I-type intermediate to intermediate-felsic rocks and the highly fractionated, alkali-rich, water-poor A₂-type granites show remarkable consistency in geochronology, geochemistry, and Sr–Nd isotopic compositions, suggesting they represent co-genetic end-members of a single magmatic system. The A₂-type granites were likely generated by high-temperature, dehydration melting of calc-alkaline felsic source rocks (e.g., granodiorite) represented by the I-type magmas.
• The coexistence of I-type and A-type granites indicates that the North Qilian region was undergoing a tectonic adjustment process from subduction-related compression to back-arc extension at ca. 512–504 Ma, most likely triggered by slab rollback. Asthenospheric upwelling and lithospheric thinning led to basaltic underplating, which provided sustained heat to the lower crust, inducing high-temperature dehydration melting and generating A₂-type magmas. This process reflects the progressive evolution of the late-orogenic magmatic system from a hydrous, oxidized arc environment to a high-temperature, relatively anhydrous extensional setting, representing a deep-seated geodynamic-driven tectonic adjustment process.

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Supplementary material

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Footnotes

Edited by: Branimir Segvic, Texas Tech University, United States

Reviewed by: Chao Hui, Jilin University, China

Ziheng Han, Chinese Academy of Geological Sciences, China