The North Qilian Orogen witnessed the opening, subduction, and closure of the Proto-Tethys Qilian Ocean and the post-subduction of multiple exhumation events from Late Neoproterozoic to Early Paleozoic. The Early Paleozoic dioritic–granitic magmatic suites, prominently exposed in the eastern North Qilian Orogen, offer valuable insights into the evolution of the Proto-Tethys Ocean. However, their petrogenesis, magma source, and tectonic evolution remain controversial. Here, we investigate the Leigongshan, Zhigou, and Dalongcun intrusions and present geochronological, geochemical, and isotopic data, aiming to refine the comprehension of their timing and petrogenesis, which will contribute to understanding the tectonic evolution of the Proto-Tethys Ocean. Zircon U-Pb dating reveals mean ages of 471–427 Ma for these intrusions, consistent with compiled formation ages of dioritic–granitic intrusions in the eastern North Qilian Orogen, indicating close temporal links with the tectonic evolution of the Proto-Tethys Ocean during the Early Paleozoic. The studied magmatic rocks could be categorized into two major types: granitoids and diorites. The granitoids are majorly I-type granitoids that are generated through partial melting of the mafic lower crust and fractional crystallization at the middle-upper crust, with the involvement of mantle-derived materials. The diorites underwent limited crustal contamination and fractionation of hornblende, plagioclase, and some accessory minerals. They were derived mainly from the mixture of fertile mantle and reworked crustal components, with minor contributions from subduction-related slab fluids and sediment melts. In addition, all the studied Early Paleozoic dioritic–granitic intrusions (ca. 471–427 Ma) formed within subduction-related arc settings. Combined with the tectonic evolution of the Early Paleozoic Qilian orogenic system, we interpret these Cambrian to Silurian dioritic–granitic intrusions as tectonic responses to the subduction (ca. 520–460 Ma) and closure (~440 Ma) of the Proto-Tethys Ocean, whereas the Devonian Huangyanghe intrusion witnessed the final stage of extensional collapse of the Qilian orogenic system at ca. 400–360 Ma.

The Tethyan orogenic belt, a significant continent–continent collisional belt in the world, preserves records of oceanic subduction, continental collision, and extensional collapse [1-3]. This belt is divided into the Proto-Tethys (Early Paleozoic), the Paleo-Tethys (Late Paleozoic-Early Mesozoic), and the Neo-Tethys (Late Mesozoic-Cenozoic) stages [4-6]. Originating from the breakup of the Rodinia supercontinent, the Proto-Tethys continued to expand in the Cambrian [3]. Subsequently, the Proto-Tethys started to shrink and closed during the assembly of North China and Siberia–Kazakhstan Cratons during the Late Silurian [7]. The Qilian orogenic belt is the pivotal segment of the Central China Orogen and witnessed the subduction and collision processes during the closure of the Proto-Tethys Ocean (Figure 1) [8-14]. The Qilian orogenic belt is also divided into the North Qilian subduction–accretion suture, the South Qilian subduction–accretion suture, and the North Qaidam subduction–collision zone, which was formed by the amalgamation of microcontinental blocks in the Proto-Tethys Ocean [15, 16]. These sutures and zones resulted mainly from the formation and evolution of the North Qilian, South Qilian, and North Qaidam Oceans, forming principal branches of the global Early Paleozoic Proto-Tethys Ocean [16]. Specifically, the North Qilian Ocean, situated between the Central Qilian block and the Alxa block, could serve as an excellent natural laboratory for investigating the tectonic evolution of the Proto-Tethys Ocean from the opening, initial subduction, continental subduction–collision, to extensional collapse spanning from the Neoproterozoic to Devonian [16, 17]. Nevertheless, several controversies primarily involve the tectonic evolution of the North Qilian Ocean, such as the subduction polarity: (1) Mariana-type intraoceanic subduction as supported by the northward subduction of the North Qilian Ocean [17-19] and (2) Andean-type continental arc system as suggested by the southward subduction beneath the Qilian block [20-22]. In addition, other diverse tectonic models, including double-sided subduction [23-25] and multiple subduction–accretion [8, 26, 27], have also been proposed to elucidate the subduction polarity of the North Qilian Ocean.

The eastern North Qilian Orogen (Gulang-Jingtai area), composed mainly of Ordovician-Triassic volcanic-sedimentary suites and scattered ultramafic–mafic–intermediate–felsic intrusions, recorded the subduction-related tectonic history of the North Qilian Ocean from Cambrian to Ordovician [28-30]. The suite of volcanic rocks, ultramafics, gabbro, chert, sandstone, and slate that are exposed in the Laohushan area of the central segments of eastern North Qilian Orogen (Figure 2) is identified as a classic ophiolite succession [31, 32]. Moreover, small magmatic intrusions within the Ordovician volcanic rocks have been interpreted as oceanic crustal components [28]. Although some of the previous studies presented geochronological, geochemical, and isotopic data on the Maozangsi, Huangyanghe, Leigongshan, Beilinggou, Peijiaying, Laohushan, and Jingzichuan intrusions in the eastern North Qilian Orogen (Figure 2), the petrogenesis and tectonic evolution of these intrusions remain equivocal, such as I-type granite related to island arc in post-collisional settings [33-36], post-collisional magmatism without arc-related geochemical affinities [37], I-type granite in syn-collisional settings [38, 39], adakitic geochemical affinities related to the subduction consumption of North Qilian Orogen [34, 40], and A-type granite in post-collisional settings [41].

In this study, we focus on the Leigongshan, Zhigou, and Dalongcun intrusions from the eastern North Qilian Orogen which have not been studied in detail. We collected fresh samples from these intrusions to conduct systematic petrological, geochemical, and zircon U-Pb-Lu-Hf isotopic studies, with major objectives to constrain the emplacement ages, petrogenesis, source characteristics, and magma evolution. Additionally, we compiled published all geochemical, geochronological, and isotopic data of the Early Paleozoic dioritic–granitic intrusions in the eastern North Qilian Orogen and present regional contour maps of zircon U-Pb ages to offer insights into the tectonic evolution of Proto-Tethys Ocean.

2.1. Regional Geology

The Qilian orogenic belt, located at the northeastern margin of the Qinghai-Tibetan Plateau (Figure 1(a)), constitutes an important part of the Central China Orogen [10]. The orogen is bounded by the Qaidam block to the south, the Altyn shear fault to the west, the Alashan block to the north, and the Qinling orogen to the east (Figure 1(a)) [14, 36, 42-45], which consists mainly of the North Qilian accretionary belt, the Central Qilian block, the South Qilian accretionary belt [31, 44, 46, 47], and the North Qaidam ultrahigh-pressure metamorphic belt from north to south [48, 49]. At the northern margin of the Qilian orogenic belt, the North Qilian accretionary belt (or North Qilian Orogen) is sandwiched between the northeastern Alashan and central Qilian blocks, which traced the closure of the ancient Qilian Ocean during the Early Paleozoic (Figure 1(b)) [9, 19, 36, 42, 44, 50, 51]. The North Qinlian accretionary belt is also a typical oceanic suture zone, composed of the south oceanic ophiolitic belt, the central arc-island volcanic complex belt, the north back-arc basin ophiolitic belt, and the Precambrian crystalline basement [52, 53]. The south oceanic ophiolitic belt comprises abyssal sedimentary rocks, basalt, gabbro, and diabase and was formed at ca. 550–496 Ma [40, 54]. The central island arc volcanic complex belt is composed of the calc-alkalic intermediate-felsic igneous rock, basic rock, high-pressure/low-temperature metamorphic rocks, Silurian flysch formations, and Devonian molasse [33, 49]. The north back-arc basin ophiolitic belt is formed at ca. 490–448 Ma and consists of chert, basalt, gabbro, and mantle peridotite [54, 55].

2.2. Eastern North Qilian Orogen

The Gulang-Jingtai area, located at the eastern segment of the Qilian Orogen (Figure 2), represents a crucial domain in the eastern North Qilian Orogen. In this region, the North Qilian Ocean underwent northward subduction from the Early Cambrian to the Middle Ordovician and generated numerous epicontinental arc igneous rocks [28-30]. The Ordovician volcanic rocks and faults show near east–west trend, with extensive exposures of Devonian-Triassic strata [28]. Except for the westernmost Huangyanghe pluton (island arc magmatic rocks) [32] and the northern Peijiaying pluton (Figure 2), other intrusions in this orogen are all hosted in the east–west trending fault zones, including the Leigongshan, Zhigou, Heimajuan, Laohushan, and Jinzichuan plutons from west to east, which occur as dikes or strains [28]. The lithology is mainly dominated by volcanic rocks (Ordovician), ultramafic rocks, gabbro, diabase, basalt, and intermediate-felsic intrusive rocks, with typical geochemical affinities of the oceanic crust [28, 56].

Representative dioritic rocks were collected from surface outcrops in the eastern North Qilian Orogen (Figure 2). Specifically, samples BL-1/1 and BL-1/2 were taken from the western outcrop around the Bolin reservoir within Dalongcun village (N: 37°08′24.93″; E: 103°11′02.20″; H: 3031 m). Samples ZG-2/1 and ZG-2/2 were sampled from Zhigou pluton (N: 37°14′39.75″; E: 102°51′55.48″; H: 2739 m), and samples YHW-3/1 and YHW-3/2 were collected from the Leigongshan pluton in the Yehuwan village (N: 37°15′32.97″; E: 102°48′31.87″; H: 2808 m). These rocks are primarily grayish-green to gray colored and featured by medium- to coarse-grained texture and massive structure (Figure 3). In addition, the rock exposures are sometimes covered by Quaternary sediments and undergo weak alteration (Figure 3(g)). The studied samples could be classified into two groups: diorite (BL-1/2, ZG-2/1, and ZG-2/2) and quartz diorite (BL-1/1, YHW-3/1, and YHW-3/2). Microscopically, the diorites display an assemblage of polysynthetic twinned plagioclase (50%–55%), strongly pleochroic subhedral hornblende (25%–35%), anhedral biotite (5%–10%), and anhedral quartz (5%–10%), with zircon, apatite, and magnetite as accessary minerals (Figure 3(b) and (f)). In contrast, the quartz diorites show subanhedral plagioclase (40%–45%), subhedral hornblende (30%–35%), subhedral pyroxene (5%–10%), anhedral quartz (5%–10%), with zircon, apatite, and titanium-iron oxide as accessary minerals (Figure 3(d) and (h)). Additionally, schiller structure and ophitic texture are observable in some pyroxene grains, along with the reaction rims of hornblende around pyroxene crystals (Figure 3).

4.1. Zircon U-Pb Dating and Trace Elements

Zircon grains were separated using standard gravimetric and magnetic separation techniques within the Yu'neng Geological and Mineral Separation Survey Center, Langfang, Hebei Province, China, and then handpicked under one binocular microscope and fixed to an epoxy resin disk for polishing and exposing their interiors. Transmitted, reflected, and cathodoluminescence (CL) images were taken at the Beijing Geoanalysis Co., Ltd., using one microscope and scanning electron microscope (JSM510) equipped with a Gatan CL probe, which was applied to target suitable sites for zircon U-Pb, Lu-Hf isotopic, and trace elemental analyses. Zircon U-Pb dating and trace element analyses were synchronously performed at the Key Laboratory of Mineral Resources in Western China (Gansu Province), Lanzhou University, China, using an Agilent 7900× inductively coupled plasma-mass spectrometer (ICP-MS) combined with the Analyte 193 nm laser ablation system. The analyzed spot size was set as ~30 μm, and the zircon Plešovice 91500 and NIST610 were used as standards to monitor accuracy and to calculate the correction factor for the 207Pb/206Pb ratio. The GLITTER and IsoplotR [57] software were employed to process age and trace element data.

4.2. Zircon Lu-Hf Isotopes

Zircon Lu-Hf isotopes were analyzed using the laser ablation multicollector ICP-MS (LA-MC-ICP-MS) combined with the NEW WAVE 193 nm FX laser at the Tianjin Center, China Geological Survey. The analyzed spot size (50 µm) and laser repetition rate of 10 Hz with 100 mJ were adopted. The Hf model (TDM) and two-stage Hf model (TDM2) ages were calculated based on the 176Lu decay constant (1.865 × 10-11 y-1), the depleted mantle with present-day Hf isotopic ratios (176Hf/177Hf = 0.28325 and 176Lu/177Hf = 0.0384), and the average continent crust (176Lu/177Hf = 0.015), and the detailed analytical procedure was outlined by Geng et al. [58].

4.3. Whole-Rock Geochemistry

Whole-rock geochemical analysis was conducted at the Key Laboratory of Mineral Resources in Western China (Gansu Province), Lanzhou University, China. Major elements were determined using Agilent720Axial ICP Optical Emission Spectrometer, with an analytical uncertainty of less than 1.0%. Trace elements and rare earth elements (REEs) were analyzed using Agilent 7700× ICP-MS, with an analytical uncertainty of less than 10%. The GSP-2 standard was used to monitor the whole analytical procedure and to ensure the accuracy of the analyzed element data. The loss of ignition (LOI) was measured by heating about 500 mg powders in a muffle furnace at 995°C for 2 hours, following the detailed methodology noted by Qian et al. [59].

4.4. Contour Mapping of Zircon U-Pb Ages

Zircon U-Pb mean ages of dioritic–granitic intrusions (e.g., Maozangsi, Huangyanghe, and Leigongshan) in eastern North Qilian Orogen were adopted for contour mapping. The inverse distance weighted interpolation method in Geostatistical Analyst modeling of ArcGIS software was employed for contour mapping. All age data were input into the ArcGIS software and then grouped into several class breaks in the ArcMap module of the ArcGIS software, following the detailed mapping procedure adopted by Yang et al. [60].

5.1. Zircon U-Pb Dating and Trace Elements

Zircon grains from samples BL-1/1, ZG-2/1, and YHW-3/1 are colorless and transparent and show euhedral to subhedral with mostly long- or short-prismatic shapes. They show a size range of 80–250 μm in length and 60–150 μm in width, with aspect ratios of 4:1 to 1:1. Under CL images, except for a few zircon grains with sector zoning texture, most of them show oscillating zoning texture, with xenocrystic cores (Figure 4(a)–(c)).

Sixty-eight zircon grains were analyzed, and the results are listed in the online supplementary Table S2. Twenty-one zircons from sample BL-1/1 show Th and U contents and Th/U ratios of 5.0–42.8, 31.5–255.5, and 0.16–0.33 ppm, respectively. These zircons also yielded two 206Pb/238U spot ages of 444–441 Ma and a 206Pb/238U mean age of 471.2 ± 4.3 Ma (MSWD = 2.6; n = 19; Figure 5(a)), with an age peak at 467 Ma (Figure 5(b)). Thirty zircons in sample ZG-2/1 yielded two 206Pb/238U mean ages of 461.8 ± 7.0 Ma (MSWD = 1.8; n = 8) and 430.7 ± 3.1 Ma (MSWD = 1.5; n = 21), a 206Pb/238U spot age of 502 Ma (Figure 5(c)), and two age peaks at 461 and 430 Ma (Figure 5(d)). These zircons also show Th and U contents in the range of 63.0–532.8 and 138.6–829.7 ppm, respectively, with Th/U ratios of 0.32–0.86. Seventeen zircons from sample YHW-3/1 display Th contents of 63.0–793.9 ppm and U contents of 230.5–3394.3 ppm, with Th/U ratios of 0.10–0.55. These zircons also yielded two 206Pb/238U mean ages of 461.8 ± 3.9 Ma (MSWD = 1.2; n = 9) and 426.7 ± 5.4 Ma (MSWD = 1.4; n = 7) and a 206Pb/238U spot age of 494 Ma (Figure 5(e)), which define two age peaks at 464 and 429 Ma (Figure 5(f)).

On the chondrite-normalized REE patterns, zircon REE data of different diorites in this study show a similar elemental variation trend, characterized by depleted light REEs (LREEs) and enriched heavy REEs (HREEs; Figure 4(d)–(f)), with distinct Ce positive anomalies and weak Eu negative anomalies (online supplementary Table S3). All the analyzed zircon grains plot in the field of magmatic zircon in the (Sm/La)N versus La and Ce/Ce* versus (Sm/La)N diagrams (Figure 6(a) and (b)). Except for the sample BL-1/1 showing mixed geochemical affinities of continental and ocean crust zircons (Figure 6(c) and (d)), zircon grains in the other samples are continental zircons.

5.2. Zircon Lu-Hf Isotopes

Thirty zircon grains used for U-Pb dating were analyzed for Lu-Hf isotopes, and the results are given in online supplementary Table S3. The data show 176Hf/177Hf ratios ranging from 0.282438 to 0.282936. Zircons from sample BL-1/1 display the highest εHf(t) values of 11.4–15.8 Ma and the youngest TDM2 of 729–442 Ma (Figure 7(a)–(b)). Zircons from sample ZG-2/1 show εHf(t) values ranging from −2.7 to 4.1 and the oldest TDM2 of 1580–1141 Ma (Figure 7(a)–(b)). Zircons from sample YHW-3/1 have positive εHf(t) values (4.2–7.6), with TDM2 from 1171 to 970 Ma (Figure 7(a)–(b)).

5.3. Whole-Rock Geochemistry

The whole-rock geochemical data including major and trace elements of the samples analyzed in this study are given in the online supplementary Table S4.

The diorites show SiO2 contents of 52.93–65.57 wt.%, Al2O3 contents of 14.78–17.86 wt.%, TiO2 contents of 0.60–1.47 wt.%, MnO contents of 0.11–0.25 wt.%, Fe2O3 contents of 5.47–14.75 wt.%, MgO contents of 1.98–5.40 wt.%, CaO contents of 4.07–8.57 wt.%, Na2O contents of 2.28–4.35 wt.%, K2O contents of 0.62–1.55 wt.%, and P2O5 contents of 0.07–0.12 wt.%. The Mg# and LOI values of the diorites range from 37.30 to 56.29 and 1.44–2.88 wt.%, respectively. In the TAS diagram (Figure 8(a)), the rocks are divided into two groups: high silicic granodiorites (SiO2: 63.54–65.57 wt.%; BL-1/1, YHW-3/1, and YHW-3/2) and low silicic gabbroic diorites (SiO2: 52.39–56.74 wt.%; BL-1/2, ZG-2/1, and ZG-2/2). The rocks show calc-alkaline (Figure 8(b)) and metaluminous to peraluminous features, with A/CNK ratios in the range of 0.74–1.06 (Figure 8(c)). In comparison with the other Early Paleozoic dioritic–granitic intrusions in the eastern North Qilian Orogen, except sample BL-1/1 and a few Laohushan samples that plot in the gabbro field, other samples show compositional variation from diorite to granite (Figure 8(a)). Furthermore, almost all the rocks in this region are classified as calc-alkaline and metaluminous to weakly peraluminous rocks, although a few samples from the Huangyanghe and Zhigou intrusions are alkaline and strongly metaluminous (Figure 8(c)). The A/CNK ratios show a wide variation range of 0.22–1.19, with mostly less than 1.10 and only five samples showing values more than 1.10 (range: 1.12–1.19; online supplementary Table S4).

The diorites show total REEs (ΣREE) contents ranging from 45.38 to 139.50 ppm, with enrichment in LREEs and depletion in HREEs, and weak Eu negative anomalies (Eu/Eu*: 0.60–1.15; Figure 9(a)–(b)). The chondrite-normalized REE patterns show two groups: samples YHW-3/1 and YHW-3/2 display high HREE/LREE (8.69–23.45) and (La/Yb)N (10.83–37.94) ratios, and the other diorites are characterized by the low HREE/LREE ratios of 1.47–4.21 and (La/Yb)N ratios (0.69–3.79). In the primitive mantle normalized patterns (Figure 9(c)–(d)), the studied diorites show enrichment of Rb, K, and Pb as well as negative anomalies at Ta, Nb, and Ti, indicating subduction-related geochemical affinities [61, 62]. Except for samples BL-1/1 and BL-1/2 showing flat patterns, the other Early Paleozoic dioritic–granitic intrusions have similar variation patterns and are enriched LREEs and depleted HREEs in chondrite normalized REE patterns (Figure 9(a)–(b)). Moreover, the Beilinggou, Leigongshan (YHW-3/1, YHW-3/2), and Laohushan intrusions display weak Eu negative to positive anomalies (Eu/Eu*: 0.79–1.15), the Maozangsi intrusion mostly shows strong Eu negative anomalies (Eu/Eu*: 0.56–0.91), a few samples of this intrusion have Eu positive anomalies (Eu/Eu*: 1.13–1.24), other intrusions (e.g., Zhigou, Huangyanghe, and Dalongcun) all display strong Eu negative anomalies (Eu/Eu*: 0.43–0.96; Figure 9(a)–(b)). In the primitive mantle normalized patterns (Figure 9(c)–(d)), the Huangyanghe, Beilinggou, Maozangsi, and Laohushan intrusions also show consistent variation patterns, characterized by positive anomalies at Th, Pb, and Nd and negative anomalies at Ba, Nb, P, and Ti.

5.4. Contour Mapping of Zircon U-Pb Ages

The compiled geochronological, geochemical, and isotopic data of the Early Paleozoic dioritic–granitic intrusions from the eastern North Qilian Orogen are given in online supplementary Table S5.

Contour mapping of zircon U-Pb mean ages define the lower age domains around Huangyanghe intrusion, with ages varying from 402 to 383 Ma (Figure 10(a)). Other Early Paleozoic dioritic–granitic intrusions within the north of the east–west trending fault zone show median age domains with mean ages ranging from 456 to 424 Ma (Figure 10(a)). These dioritic–granitic intrusions (e.g., Leigongshan, Zhigou, and Laohushan) that hosted in the east–west trending fault zone show older age domains and yielded zircon U-Pb mean ages of 471–426 Ma (Figure 10(a)). In comparison, the southern intrusions are collectively older than those outcropped in the north part of the eastern North Qilian Orogen.

6.1. Age Interpretation and Temporal Links with Proto-Tethys Ocean

In general, magmatic zircon shows oscillating zoning texture and is different from the core-rim or weak zoning texture of metamorphic zircon grains [63]. Although Th/U ratios in zircon are difficult to precisely characterize the genetic types of zircons, the Th/U ratios of magmatic zircon are commonly higher than those in metamorphic zircon and more than 0.1 [64]. In addition, magmatic zircon is also characterized by steep slopes from LREEs to HREEs on chondrite-normalized REE patterns, with strong Eu negative anomalies and Ce positive anomalies [65, 66]. In this study, the studied zircons all show oscillating zoning texture under CL images (Figure 4), with Th/U ratios of 0.10–0.86 (Figure 5). These zircons also indicate a right steep slope in REE patterns featuring strongly Ce positive anomalies and moderate to weak Eu negative anomalies (Figure 4). Taken together, these features suggest that the analyzed zircons are magmatic grains, as also attested by their plots in the field of magmatic zircon in zircon genetic discrimination diagrams (Figure 6(a)–(b)). Consequently, it could be interpreted that the newly obtained zircon U-Pb ages from the diorite and quartz diorite suggest multiple magma emplacements during the Early Paleozoic (471–427 Ma; Figure 5).

The zircon U-Pb data in this study show 206Pb/238U spot ages in the range of 502–417 Ma and 206Pb/238U mean ages of 471–427 Ma (Figure 5). When integrating the zircon U-Pb spot ages of this study with those compiled Early Paleozoic dioritic–granitic intrusions (e.g., Maozangsi, Huangyanghe, and Leigongshan) from the eastern North Qilian Orogen, the age data collectively vary from 527 to 372 Ma, with two prominent age peaks at 459 and 429 Ma (Figure 10(b)). The 206Pb/238U mean ages of these intrusions are in the range of 471–383 Ma (Figure 10(c)). In addition, contour mapping of zircon U-Pb mean ages of these intrusions also show variation characterized by younger ages in the north and older ages in the south of the eastern North Qilian Orogen (Figure 10(a)). The emplacement ages of these intrusions mainly cluster in Cambrian–Devonian suggesting continuous magmatic activities that span about ~100 Ma. Yu et al. [16] recently reviewed the tectonic evolution of the Qilian orogenic system and proposed that the Qilian orogenic system, including the Proto-Tethys Qilian Ocean, underwent opening at 580–550 Ma, subduction during ca. 520–460 Ma, closure with subsequent deep continental subduction (455–430 Ma), multistage exhumation of the deeply subducted continental slab (425–400 Ma), and final extensional collapse at ca. 400–360 Ma. Therefore, the age data (471–383 Ma) of these intrusions suggests close temporal linkage with the tectonic evolution (520–455 Ma) of the Proto-Tethys Qilian Ocean and the post-collisional Qilian orogenic evolution during the Middle Paleozoic (455–383 Ma).

6.2. Petrogenesis and Magma Source

6.2.1. Diorites

As depicted in the TAS diagram (Figure 8(a)), the investigated rocks can be classified into two groups. The first group consists of granitoids (quartz diorites) and plots into the diorite and granodiorite fields, whereas the second group comprises diorites and falls within the gabbroic diorite field (Figure 8(a)).

The diorite sample shows zircon εHf(t) values ranging from −2.7 to 4.1 (Figure 7(a)–(b)) with TDM2 of 1580–1141 Ma, indicating magma primarily sourced from the mantle with minor contributions from reworked ancient crustal compositions. Similarly, the adjacent Laohushan diorites display (87Sr/86Sr)i of 0.7058–0.7071, negative εNd(t) values (−1.1 to −0.3), and two-stage Nd model ages of 1.26–1.20 Ga [35], which further support the involvement of reworked crustal components in the magma source. The Mg# values of studied diorites and those in published works from Zhigou and Laohushan intrusions range from 32.87 to 75.93 (mean = 51), most of them exceeding 40 (online supplementary Table S5). This implies the major contribution from mantle-derived magma in the source. In addition, the diorites show Nb/Yb ratios in the range of 0.63–1.51, significantly higher than enriched mid-ocean ridge basalt (0.02) [67], indicative of a fertile mantle source [68]. Furthermore, the Zr/Nb ratios of these diorites (range: 4.23–43.02) are much higher than those of ocean island basalt (OIB) (5.8) [67], and the Nb/Ta ratios (5.31–21.87) are lower than the subchondritic Nb/Ta ratios (<17.5) [69]. These observations collectively rule out an OIB-derived origin and suggest a mantle source metasomatized by low-degree melts.

However, Mg# (32.87-75.93) of diorites (including literature data) are notably lower than typically those of mantle-derived primary melts (Mg#: 73–81), suggesting possible varying degrees of fractional crystallization. This is also attested by the Harker diagrams of major elements (Figure 11(a)–(e)), where the negative correlations between FeOt, MgO, and SiO2 indicate the fractionation of hornblende, the negative correlations between TiO2, Al2O3, P2O5, and SiO2 as well as the Eu, P, and Ti anomalies (Figures 9 and 11) confirm the fractionation of titanite, ilmenite, plagioclase, and apatite. The Rayleigh fractional crystallization model is used here to constrain magmatic evolution, and the detailed parameters in the modeling are listed in Table 1. Since the sample BL-1/2 diorite has the most mafic composition (lowest SiO2 and highest MgO contents, Figure 11(d)), it was utilized as the starting composition. Modeling curves (Figure 12(a)–(b)) illustrate that the diorite samples plot between plagioclase, K-feldspar, and amphibole fractional lines, indicating that the evolution of dioritic magma is primarily controlled by fractions of hornblende and feldspar, a consistency reflected in the elemental variation diagrams (Figure 11).

Furthermore, crustal contamination usually arises when mantle-derived magma incorporates components from continental crust during ascent from mantle sources [70]. These values of these compiled Early Paleozoic intrusions predominantly surpass those of crustal rocks (La/Nb = 1.77–2.90) and differ from those of primitive mantle (La/Nb < 1) [71-73]. Moreover, the Ti/Zr ratios (range: 9–218; mean = 52) and Ti/Y ratios (106–337; mean = 227) for all these diorites are largely higher than those of crustal rocks (Ti/Zr < 30 and Ti/Y < 200) [71]. These geochemical features collectively suggest that crustal contamination is minimal. This is also attested by the Nb/U ratios (1–14), predominantly lower than 9 but close to 5 (online supplementary Table S5), which contrasts with the values of upper continental crust (Nb/U close to 9) [61] and aligns with those of subduction sediments (Nb/U close to 5) [74]. The possibility of subduction-related sediment melt input is also considered, which is further supported by plots depicting mixed characteristics of slab fluids and sediment melts in diagrams of Ba/La versus Th/Nd and Ba/Th versus Th/Yb (Figure 13(a)–(b)). Thus, we deduce that the diorites from the eastern North Qilian Orogen involve a mixture of fertile mantle and reworked crustal materials and underwent fractionation of hornblende, feldspar, and certain accessory minerals.

6.2.2. Granitoids

Usually, granitoids (quartz diorite) can be divided into I-, S-, M-, and A-types. Specifically, I-, S-, and M-type granitoids are defined mainly based on their magma source, with I-types originating from igneous protoliths, S-types from sedimentary protoliths, and M-types from the melting of subducted oceanic crust or overlying mantle [75-77]. A-type granitoids refer to alkaline and anorogenic and are sourced from recycled and dehydrated continental crust and mantle [78-81]. In addition, I-type granite has geochemical features of Na2O > 3 wt.%, Rb/Sr < 1, and Eu/Eu* of 0.8–1.1, whereas S-type granite with Na2O < 3 wt.%, Rb/Sr > 1, and Eu/Eu* < 0.6 [82]. Our studied quartz diorites are notably different from A-type granitoid (Figure 8(d)–(e)) due to the absence of sodic ferromagnesian minerals, the lower Zr contents [83, 84] and FeOt/MgO ratios [85], and higher Sr contents than the typical A-type granitoids [85, 86]. The exclusion of M-type granite is supported by distinct mineral and geochemical characteristics from oceanic plagiogranite [76]. Several lines of evidence could confirm the I-type geochemical signature of these granitoids: (1) the absence of Al-rich minerals and the occurrence of amphibole and pyroxene (Figure 3), (2) A/CNK values are mostly below 1.1 (Figure 8(c)), (3) the geochemical affinities largely consistent with I-type granite (Na2O > 3 wt.%, Rb/Sr < 1, and Eu/Eu* of 0.8–1.1), (4) the geochemical plots within the field of I-type granitoid field (Figure 8(f)), and (5) the variation trend of I-type granite in elemental discrimination diagrams (Figure 11(e) and (f)). Coupled with geochemical signatures, the granitoids in the eastern North Qilian Orogen predominantly are further classified into I-type calc-alkaline characteristics (Figure 8(b)–(f)).

Calc-alkaline I-type granites often formed via (a) fractional crystallization of mantle-derived basaltic magma [87-90], (b) reworking of sedimentary materials modified by mantle-like magma [91], or (c) the partial melting of mafic lower crust with or without the addition of mantle-derived mafic magma [92, 93]. The granitoids (quartz diorite) were not derived from the dioritic magma as revealed by several lines of evidence: (1) on the La/Sm versus La and Zr/Nb versus Zr diagrams (Figure 13(c)–(d)), the studied quartz diorites along with compiled granitoids are identified to have not undergone significant fractional crystallization, but partial melting may exert a key role, and (2) based on the modeling results (Figure 12(a)–(b)), the sample BL-1/1 shows similar compositions to BL-1/2, yet their major elemental compositions differ significantly, indicating that quartz diorite cannot be derived from the fractional crystallization of dioritic magma. Therefore, the possibility of dioritic magma evolution through fractional crystallization can be excluded.

The studied quartz diorites (BL-1/1, YHW-3/1) display εHf(t) values of 4.2–15.8 and TDM2 in the range of 1171–442 Ma (Figure 7(a)–(b)), suggesting source contributions from mantle or juvenile crust components. This can be correlated with the reworked materials of the Precambrian basement rocks and the Early Paleozoic intrusions in the Qilian Orogen [94-96]. Additionally, Mg# values can discriminate whether mantle-derived materials were incorporated during magma evolution [97]. The initial Mg# value of melt that formed through the partial melting of lower crustal mafic rocks is typically lower than 40, but it will increase when mantle-derived components are added [98]. The Mg# values of these studied rocks range from 37.30 to 42.80 (online supplementary Table S4), suggesting mantle input in the source. Moreover, the samples are also plotted within the fields of partial melts from metabasaltic to metatonalitic sources, as well as amphibolites (Figure 12(c)–(d)), which provide further evidence for the partial melting of the mafic lower crust, with contributions from mantle materials that play a crucial role in the formation of granitoids. The common magma source is also supported by other granitic intrusions in the eastern North Qilian Orogen. For example, the Maozangsi granodiorite [41, 54] and Huangyanghe K-feldspar granite [41] show similar isotopic and geochemical features (online supplementary Table S5, Figure 12(c)–(d)), which indicate that the partial melting of the mafic lower crust and mantle-derived magma materials that are the major magma source for the Early Paleozoic granitoid in the eastern North Qilian Orogen. Consequently, it is proposed that the granitoids (quartz diorites) were formed by the partial melting of mafic lower crustal rocks with contributions of mantle-derived mafic magma.

In addition, the role of fractional crystallization during magma evolution should also be considered. As illustrated in the elemental variation diagrams (Figure 11), the fractionation of ferromagnesian minerals (e.g., hornblende and biotite; Figure 3), accessory minerals (e.g., apatite, titanite, and ilmenite), and feldspar (plagioclase and K-feldspar) [59, 99] occurred. The presence of Eu, P, and Ti anomalies in the normalized patterns (Figure 9) also potentially supports the fractionation of feldspar, apatite, and Fe-Ti oxides. Therefore, we conclude that these granitoids from the eastern Qilian Orogen formed through the partial melting of the mafic lower crust, followed by fractional crystallization at the middle-upper crust, also including minor input of some mantle-derived materials.

6.3. Tectonic Evolution of the Early Paleozoic Proto-Tethys Ocean

All the dioritic–granitic intrusions in the eastern North Qilian Orogen show distinctly negative anomalies at high field strength elements (HFSEs; e.g., Nb, Ta, and Ti; Figure 9), indicative of subduction-related components input [100]. In addition, nearly all the La/Nb ratios of these intrusions vary from 1.54 to 16.10 (mean = 4.31), with only 1 sample (0.78) falling below 1 (online supplementary Table S5), consistent with the values of arc magmas (La/Nb > 1) [72]. This is also confirmed by the plots that most geochemical plots of these intrusions fall within the field of typical arc rocks (Figure 13(e)–(f)), suggesting arc-related affinity within subduction settings. In general, arc magmas are associated with the partial melting of mantle wedge induced by fluids released from the oceanic crust during subduction [99, 101], implying the incorporation of subduction-related components. However, a few geochemical data from Maozangsi and Leigongshan (YHW-3/1, YHW-3/2) intrusions are plotted in the adakite field (Figure 13(e)–(f)). Castillo [102] argued that adakite covers a range of arc rocks varying from the primary slab melt, to slab melt hybridized by peridotite, to melt derived from peridotite metasomatized by slab melt, and formed in arc settings involving components of subducted sediments. Accordingly, we infer that the Maozangsi and Leigongshan intrusions possibly involved slab and sediment melts through subduction in an arc setting. In addition, the granitoids from the eastern North Qilian Orogen all fall into the field of volcanic arc granite (Figure 14(a)), whereas the diorites plot in arc-related fields (volcanic arc basalt; island arc tholeiite; Figure 14(d)), further confirming the arc-related settings. In the Nb/Zr versus Zr diagram (Figure 14(d)), almost all the dioritic–granitic intrusions show subduction-related tectonic settings. However, a few geochemical data from the Huangyanghe intrusion display post-collisional geochemical features (Figure 14(b) and (c)). When coupled with the published zircon U-Pb ages of the Huangyanghe intrusion (402–383 Ma; Figure 10), we correlate the post-collisional settings with the final extensional collapse (400–360 Ma) of the Qilian orogenic system [16].

Whole-rock geochemical data of the dioritic–granitic intrusions in eastern Qilian Orogen are enrichment at LREEs and large ion lithophile elements and depleted at HFSEs and HREEs (Figure 9). These features and zircon geochemical data suggest mixed features of continental crust and oceanic crust (Figure 6(c)–(d)). We thus infer that these zircons (502–417 Ma) inherited the geochemical affinity from earlier subduction-related components in the Proto-Tethys Qilian Ocean [16, 103]. Furthermore, we also propose that the Early Paleozoic dioritic–granitic magmatism mainly formed within subduction-related arc settings, which responded to the Early Paleozoic tectonic evolution of the Qilian orogenic system. Numerous published studies have linked the Early Paleozoic Qilian orogenic system to the Proto-Tethys Ocean [6, 9, 36, 104] and proposed that the Qilian orogenic system formed through the oceanic subduction and the amalgamation of continental blocks in response to the Early Paleozoic evolution of Proto-Tethys Ocean [10, 36, 105]. Specifically, the North Qilian, South Qilian, and North Qaidam Oceans, which are important parts of the Qilian orogenic system, formed during the rifting from Rodinia supercontinent and made up the major branches of the global Early Paleozoic Proto-Tethys Ocean [16]. This is also attested by the southern Yushigou ophiolite that was the relict of the Proto-Tethys oceanic lithosphere [17]. Despite ongoing debates primarily focusing on the timing of tectonic stages, the Qilian orogenic system is thought to have undergone opening (580–550 Ma), subduction (520–460 Ma), and closure (~440 Ma) of the Proto-Tethys Qilian Ocean [16, 17], as well as subsequent deep continental subduction (440–430 Ma), multistage exhumation of deeply subducted continental slab (425–400 Ma), and final extensional collapse at ca. 400–360 Ma of the Qilian orogenic system [16]. Previous studies identified that these Late Neoproterozoic to Early Paleozoic ophiolite suites (e.g., Yushigou, Dachadaban, and Jiugequan) [106-109] are the relicts of ancient oceanic lithosphere which recorded oceanic opening, slab subduction, and oceanic closure [110] and also interpreted as the oceanic lithosphere of ancient Qilian Ocean [111]. Nevertheless, the subduction polarity and related tectonic evolution of the North Qilian Ocean are still controversial issues.

Based on the detailed investigation of Niuxinshan granitoids in the North Qilian Orogen, Yang et al. [22] argued that the southward subduction of the Early Paleozoic North Qilian Ocean beneath the central Qilian Block might be a more reasonable scheme to interpret its subduction polarity. This southward subduction generated an Andean-type continental arc, fostering coeval arc-related magmatism from the Cambrian to Silurian [9, 112]. However, as revealed by the counter map of zircon U-Pb mean ages (Figure 10), there is a northward younging variation trend in the eastern North Qilian Orogen, which contradicts with the southward subduction. This may result from limited age data and magmatic outcrops in the east North Qilian Orogen. In this study, the majority of Early Paleozoic dioritic–granitic intrusions from eastern North Qilian Orogen were identified to have formed under subduction-related arc settings, except for the Huangyanghe intrusion that formed in a post-collisional setting. In conjunction with the Cambrian to Silurian zircon U-Pb mean ages (471–427 Ma; Figure 10(c)), we suggest that these Early Paleozoic dioritic–granitic intrusions in the eastern North Qilian Orogen mainly witnessed the tectonic evolution related to the southward subduction of the Proto-Tethys Qilian Ocean from oceanic to continental subduction (520–430 Ma), whereas the Huangyanghe intrusion (402–383 Ma) might be a tectonic response to the final extensional collapse of the Qilian orogenic system at ca. 400–360 Ma.

Zircon U-Pb geochronology of diorites yields 206Pb/238U mean ages of 471–427 Ma; coupled with geochemical and textural features of magmatic zircon grains, we suggest that these dioritic rocks formed in the Early Paleozoic. Compiled zircon U-Pb mean ages of all the dioritic–granitic intrusions from the eastern North Qilian Orogen show an age variation range from 471 to 383 Ma, indicating close temporal links with the tectonic evolution of the Early Paleozoic Proto-Tethys Qilian Ocean.

The dioritic–granitic intrusions in the eastern North Qilian Orogen are divided into granitoids (quartz diorites) and diorites. The granitoids, identified as I-type granite, underwent partial melting of the mafic lower crust and fractional crystallization at the middle-upper crust, involving mantle-derived magma components input in the source. The diorites underwent limited crustal contamination and fractional crystallization of hornblende, plagioclase, and some accessory minerals. They originated from a mixture of fertile mantle and reworked crust components, with the involvement of subduction-related fluids and sediment.

The Huangyanghe intrusion (402–383 Ma) is an exception that formed in an arc-related post-collisional tectonic setting. In contrast, other Early Paleozoic dioritic–granitic intrusions (471–427 Ma) in the eastern North Qilian Orogen generated under subduction-related arc settings. We interpret the Cambrian to Silurian intrusions as tectonic responses of the southward subduction of the Proto-Tethys Qilian Ocean from oceanic to continental subduction (520–430 Ma), whereas the Huangyanghe intrusion suggests the final extensional collapse stage of the Qilian orogenic system at ca. 400–360 Ma.

The data for this study are available in this manuscript and supplementary material.

The author(s) declare(s) that there is no conflict of interest regarding the publication of this paper.

The editors and referees are appreciated for their constructive suggestions and comments that greatly improve this paper. We thank Dr. Wanfeng Chen and Dr. Xiaoli Yan for their help during the zircon U-Pb dating and trace element analysis, Prof. Hongying Zhou and Dr. Jiarun Tu for their help with zircon Lu-Hf isotopic analysis, and Dr. Conghui Xiong for her help during the whole-rock geochemical analysis. This study was supported by the Second Tibetan Plateau Scientific Expedition and Research Program (2019QZKK0901), the Fundamental Research Funds for the Central Universities (B230201014 and LZUJBKY-2022-42), the Natural Science Foundation of Gansu Province (22JR5RA440), the National Natural Science Foundation of China (42202077), and the Guiding Special Funds of “Double First-Class (First-Class University and First-Class Disciplines)” (Grant No. 561119201) of Lanzhou University, China.

The supplementary materials here contain Table S1 to Table S5. Tables S1–S4 are included in one Word file and Table S5 is supplied in Excel format. Supplementary materials captions are presented as follows. Supplementary Table S1: Zircon U-Pb age data of the studied diorites from the eastern North Qilian Orogen. Supplementary Table S2: Zircon REEs data of the studied diorites from the eastern North Qilian Orogen. Supplementary Table S3: Zircon Lu-Hf isotopic data of the studied diorites from the eastern North Qilian Orogen. Supplementary Table S4: Whole-rock major and trace element data of the studied diorites from the eastern North Qilian Orogen. Supplementary Table S5: Compiled geochronological, geochemical, and isotopic data of the Early Paleozoic dioritic–granitic intrusions from the eastern North Qilian Orogen.

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Supplementary data