A Geochemical and Sr–Nd–Hf–O Isotopic Study of the Early Silurian Shandan Adakites in the Longshoushan Area: Implications for the Collisional Setting of the Proto–Tethyan North Qilian Orogen, Northwest China

Abstract

The North Qilian Orogen experienced a series of late Neoproterozoic to early Paleozoic tectonic events, including the opening and closure of the Proto-Tethyan Qilian Ocean, as well as post-subduction processes. This study investigated the Shandan adakites in the Longshoushan area of the North Qilian Orogen, focusing on zircon U–Pb geochronology, whole-rock geochemistry, and Sr–Nd–Hf–O isotopic compositions. The Shandan adakites yield ages of ca. 446–440 Ma, suggesting they crystallized during the collision between the Alxa and Qilian blocks following the closure of the Proto-Tethyan North Qilian Ocean. High Sr/Y (40.9–117) ratios and enrichments in light rare earth elements indicate that the Shandan adakites were formed by partial melting of thickened magnesian lower crust. They have relatively rich ε_Nd (t) (−7.66 to −6.32), ε_Hf(t) (3.30 to −12.4), and δ¹⁸O (5.34‰–7.52‰). Zircon Hf–O and whole-rock Sr–Nd isotopes confirm significant contributions from the ancient crust and mantle-derived melts, suggesting complex crust–mantle interactions in their magma sources. We propose that the Shandan adakites formed during the (early) post-collisional stage of orogenesis. Based on regional geological evidence and previous studies, we suggest the Alxa and Central Qilian blocks collided during ca. 446–440 Ma, leading to the thickening of the lower crust. After ca. 440 Ma, the tectonic setting of the Northern Qilian Orogen transitioned from a collisional to a post-collisional stage.

1. Introduction

Adakites are characterized by high Sr contents, elevated Sr/Y ratios (>40), and relatively low Y and Yb contents. Adakites have long been debated in igneous petrology, with ongoing discussions about their formation processes under different environmental conditions [1,2]. Recent studies indicate that adakites can form due to (1) interactions between melts derived from subducting oceanic crust and mantle peridotites [3,4,5,6,7]; (2) partial melting of thickened lower crust [8,9,10,11,12]; (3) partial melting of delaminated lower crust [13,14,15]; (4) partial melting of subducted continental crust [16]; and (5) fractional crystallization of tholeiitic magmas [2]. In general, adakites record information on processes such as oceanic subduction, continental collision, and lithospheric extension. Studies of their petrogenesis are important for understanding crust–mantle interactions during subduction and continental crustal growth.

The Qilian Orogenic Belt is a key part of the Central Orogenic Belt in China, which was formed during the early Paleozoic [17,18,19,20]. From north to south, the Qilian Orogenic Belt is divided into the North Qilian Belt, Central Qilian Block, and Southern Qilian Belt, forming a composite orogenic system that has experienced a long and complex history [21] (Figure 1b). The Northern Qilian Belt, along the southwestern margin of the Alxa Block, consists of a series of intrusive plutons along the Longshoushan Fault that comprise the Longshoushan structural belt [22].

The Longshoushan area is located at the southern margin of the Alxa Block and the northern margin of the Northern Qilian Belt. It contains extensive exposures of early Paleozoic intermediate-acid magmatic rocks [23], which are considered to be related to the northward subduction of the North Qilian oceanic lithosphere and subsequent collisional processes [18,23,30]. Previous studies of the ca. 445–403 Ma intermediate-acid magmatic rocks in the Northern Qilian region suggest that magmatism after the Paleozoic closure of the North Qilian Ocean occurred in various tectonic settings, including a compressional environment associated with a collisional orogeny [31], a post-collisional extensional environment [22,31,32,33], and a transitional setting from compression to extension [18,34,35,36]. Moreover, the origins and source regions of the ca. 445–403 Ma intermediate-acid magmatic rocks in the Longshoushan area are also debated. Several differing views have been proposed regarding the origins of the granitic rocks in this region: (1) formation by partial melting of ancient Precambrian crust of the Longshoushan Group, with a minimal contribution from mantle material [30,37]; (2) mixing between adakitic magmas generated by crustal melting and mantle-derived magmas [38,39]; and (3) the silicic magmas being produced by fractional crystallization of intermediate to mafic magmas [40,41]. As such, research on the genesis petrogenesis of the adakites in the Longshoushan area is critical for resolving conflicting models of crust–mantle interaction during the Proto-Tethyan closure.

In this paper, we present zircon U–Pb geochronological, whole-rock geochemical, and Sr–Nd isotope and zircon Lu–Hf–O isotope data for the Shandan adakites from the Longshoushan area. Our aim was to investigate the age, petrogenesis, source, and tectonic significance of these rocks, thereby constraining the tectonic evolution of the Northern Qilian Orogenic Belt.

2. Geological Setting

The Longshoushan area is located at the southwestern margin of the Alxa Block in the western North China Craton [42]. It is 30 km wide and 300 km long, with its main part occurring in a NW–SE-trending belt that gradually transitions to a nearly E–W orientation in the east (Figure 2). It extends eastward from Hongyashan, and to the west is separated from the Tarim Plate by the Altin Fault. To the south, it is adjacent to the Minle Basin of the Hexi Corridor, from which it is separated by a deep fault at the southern margin of the Longshoushan. To the north, it is bounded by a deep fault between the northern margin of the Longshoushan and the Chaoshui Basin. The Longshoushan area has experienced multiple episodes of superimposed tectonic deformation, with an overall structural trend that trends approximately WNW–ESE. Its main structural features are faults and folds, followed by ductile shear zones. The Longshoushan Fault was active during the early Paleozoic Qilian Orogeny, with its southern segment exhibiting right-lateral motion and trending WNW–ESE.

The Longshoushan area consists primarily of the Late Mesoproterozoic Dunzigou Formation and Neoproterozoic Hanmushan Formation. The Dunzigou Formation consists of an upper unit of calc–silicate marble with meta chert bands, intercalated with quartzite, quartz conglomerate, and hematite lenses, and local occurrences of marble; the middle unit consists of purplish phyllite, quartzite, calc–silicate marble, and tuffaceous phyllite; and the lower unit consists of quartzite interbedded with calc–silicate marble, phyllite, and sericite–quartz schist [43]. The upper Hanmushan Formation, which is Neoproterozoic in age and exposed mainly in the northern part of Shandan County, consists of quartz sandstone, slate, and volcanic rocks that are intruded by early Paleozoic granitoids [44,45]. The Longshou Mountains consist mainly of metasedimentary rocks and granitic gneiss, The magmatic and metamorphic zircons from the granitic gneiss yield ages of 2.04–2.07 Ga and 1.89–1.93 Ga, respectively, confirming that the Longshou Mountains formed mainly in the Paleoproterozoic [46,47]. Voluminous igneous rocks crop out in the Longshoushan area, ranging from ultramafic to mafic compositions, and the granitoid intrusions have multiple phases, with magmatic events recorded from the Paleoproterozoic to Paleozoic. Numerous outcrops of Paleozoic intrusive and volcanic rocks occur in this region. For example, in the central Jiling area, exposures include gabbro, diorite, porphyritic granite, quartz monzonite, and granodiorite in the eastern Hexibao area; peridotite in the Xiaokouzi area; dioritic granite in the western Zhigoumen area; and monzogranite in the Hongshiquan area.

The study area is located ~5 km west of Shandan County in the Hexi Corridor Belt in Gansu Province and 6–10 km north of the Longshoushan Fault (Figure 2).

Figure 2. Simplified geological map of the Longshoushan–Shandan area (modified after Duan et al. [48]).

Figure 2. Simplified geological map of the Longshoushan–Shandan area (modified after Duan et al. [48]).

3. Analytical Methods

All analyses were carried out at the Guangxi Key Laboratory of Hidden Metallic Ore Deposit Exploration, Guilin University of Technology, Guangxi, China.

3.1. Zircon U–Pb Dating and Hf–O Isotopes

We separated zircon grains from three adakites (samples 21SD-280, 21SD-281, and 21SD-285) using conventional heavy liquid and magnetic methods. The zircons were mounted in epoxy resin and polished with 0.25 μm diamond paste. The internal textures of the grains were carefully documented with transmitted and reflected light photomicrographs and cathodoluminescence (CL) images. The CL imaging was performed using a JXA-8230R electron microprobe (JEOL Ltd., Tokyo, Japan).

Zircon U–Pb dating and trace element analyses were carried out simultaneously by laser ablation–inductively coupled plasma–mass spectrometry (LA–ICP–MS) comprising a quadrupole ICP–MS instrument (Agilent 7900; Agilent Technologies, Santa Clara, CA, USA) coupled to a GeoLas HD 193 nm ArF excimer laser (Lambda Physik, Göttingen, Germany) with an automatic positioning system. The laser spots were 24 μm in diameter. Each analysis included 20–30 s of background acquisition (i.e., a gas blank) followed by 50 s of data acquisition (U, Th, ²⁰⁴Pb, ²⁰⁶Pb, ²⁰⁷Pb, and ²⁰⁸Pb) from the sample. Trace element contents were calibrated using the NIST 610 glass and Si as external and internal standards, respectively. Isotopic fractionation was corrected using the Plešovice zircon as an external standard. ICPMSDataCal [49] was used to calculate trace element contents and U–Pb isotopic ratios. Concordia ages and diagrams were obtained with Isoplot/Ex [50].

Hafnium isotopic analyses of the same zircon grains were conducted with an ArF excimer LA system coupled to a Neptune Plus multi-collector (MC)–ICP–MS (Thermo Scientific, Waltham, MA, USA) with a laser beam diameter of 44 μm, repetition rate of 6 Hz, and beam energy density of 6 J/cm². During data acquisition, the GJ-1 zircon standard was analyzed as an external standard to check the data quality.

Zircon O isotopic compositions were measured with a Cameca IMS-1280-HR secondary ion mass spectrometer (CAMECA, Gennevilliers, France) at the SKLaBIG, GIG-CAS, Guangzhou, China. The detailed analytical procedures were similar to those described by Li et al. [51] and Yang et al. [52]. The ¹³³Cs⁺ primary beam was accelerated at 10 kV with an intensity of ~2 nA and a beam size of ~20 μm. A normal incidence electron gun was activated to compensate for sample charging during the analysis. A mass resolution of 2500 was obtained in multi-collection mode. A nuclear magnetic resonance (NMR) probe was used for magnetic field control. ¹⁸O and ¹⁶O were measured in two off-axis Faraday cups (H1 and L2), respectively. Each analysis took <3 min, consisting of pre-sputtering (30 s), automatic secondary beam centering (60 s), and integration of O isotopes (4 s/cycle × 16 cycles).

Measured ¹⁸O/¹⁶O ratios are reported as δ¹⁸O in per mil (‰) relative to the O isotopic composition of Vienna Standard Mean Ocean Water with (¹⁸O/¹⁶O)_VSMOW = 0.0020052 [53]. The instrumental mass fractionation factor (IMF) for zircon was obtained using the zircon standard Penglai [54] (δ¹⁸O = 5.31‰). Fourteen measurements of the Qinghu zircon standard yielded a weighted-mean δ¹⁸O value of 5.6 ± 0.1‰ (2σ), which is within error of its reported value of 5.4 ± 0.2‰ [55].

3.2. Whole-Rock Data

Four unaltered samples were selected and crushed into small chips. Any chips that were altered or included secondary veins were removed and the remaining chips were soaked in 4 N HCl for 30 min to leach any alteration minerals. The rock chips were then powdered to <200 mesh using an alumina ceramic shatter box.

Major element compositions were determined with a ZSX Primus II X-ray fluorescence spectrometer. The loss-on-ignition (LOI) value of each sample was measured before the major element analyses.

Trace element analyses were undertaken with an Agilent 7500CX ICP–MS instrument (Agilent Technologies, Santa Clara, CA, USA) using the acid dissolution method of Liu et al. [56]. First, ~50 mg of sample powder was placed in a bomb and 0.5 mL of purified HNO₃ and 1.0 mL of HF were added. The bomb was then placed in a high-pressure tube at 190 °C for 48 h. After evaporation, 0.5 mL of purified HNO₃ was added to the bomb, and this step was repeated. Subsequently, 4 mL of purified 4 N HNO₃ was added to the bomb, which was then placed in the high-pressure tube at 170 °C for 4 h. Finally, the solution was diluted 1000 times, and 10 ppb of Rh was added as an internal standard to correct for instrument drift.

The Sr–Nd isotopic compositions were determined using the method of Zhang et al. [57]. The sample powder was dissolved in a Savillex Teflon beaker, Eden Prairie, MN, USA, by adding 2 mL of 22 N HF and 1 mL of 8 N HNO₃ and then heated to 120 °C on a hot plate for 5–7 d. After drying, 3 mL of HNO₃ was added to dissolve the sample again. The Sr was separated using SR-B50-A (100–150 µm) resin. Rare earth elements (REEs) were separated with AG 50-X8 cation exchange resin, and Nd was purified with HDEHP resin. The blanks contained <300 pg Rb and Sr, and <100 pg Sm and Nd. Mass fractionation was corrected by assuming an ⁸⁸Sr/⁸⁶Sr ratio of 8.375209 and a ¹⁴⁶Nd/¹⁴⁴Nd ratio of 0.7219. Instrument stability was monitored using NBS-987 and JNdi-1 standard solutions for the Sr and Nd isotopic compositions, respectively. Repeated analyses of NBS-987 and JNdi-1 yielded a mean ⁸⁷Sr/⁸⁶Sr ratio of 0.710294 ± 0.000016 (n = 40; 2 SD) and mean ¹⁴³Nd/¹⁴⁴Nd ratio of 0.512081 ± 0.000008 (n = 40; 2 SD), respectively.

4. Results

4.1. Petrography

In this study, four representative adakite samples were collected from the Shandan area in the central–eastern Longshoushan Belt. The adakite samples include two granodiorites and two potassic granites. The granodiorite samples are gray–white or pink, with a medium–coarse-grained porphyritic texture and massive structure (Figure 3 and Figure 4). The phenocrysts consist of euhedral plagioclase (50–60 vol.%) that is ~3 mm in size and has poorly developed multiple twinning. The groundmass is mainly subhedral–anhedral plagioclase (5 vol.%; 1–2 mm), K-feldspar (10 vol.%; 2–3 mm), quartz (20 vol.%; 1–2 mm), and biotite (5 vol.%; 0.5–1.0 mm). The biotite is brown, flaky, and shows pronounced pleochroism. The potassic granite is pinkish red and contains phenocrysts of euhedral K-feldspar (50–60 vol.%; 2–3 mm) in a groundmass of subhedral–anhedral plagioclase (5 vol.%; ~1 mm), K-feldspar (5 vol.%; 1–2 mm), quartz (25 vol.%; 1–2 mm), and biotite (5 vol.%; 0.5–1.0 mm).

4.2. Zircon U–Pb Geochronology and Hf–O Isotopic Compositions

The zircon U–Pb age data for the Shandan adakites are presented in Table S1. In these samples, the zircon grains are predominantly euhedral, with elongated columnar to short and stubby shapes. The grains are relatively large, with sizes of 100–200 μm, and exhibit clearly defined oscillatory zoning. All 50 analyses showed Th/U = 0.07–1.32, which is typical of magmatic zircons [58]. The contents of Th (144.12–15702.57 ppm) and U (1917.97–16,843.40 ppm) in SD-285 were larger than those of Th (318.89–4286.21 ppm) and U (327.12–4966.33 ppm) in SD-280 and SD-281. They have negative Eu anomaly (Eu/Eu* = 0.04–0.90). Zircon U–Pb dating of the three adakite samples (SD-280, SD-281, and SD-285) yielded weighted-mean ages of 444.3 ± 2.3 Ma (2σ; n = 14; MSWD = 0.18), 446.2 ± 2.8 Ma (2σ; n = 11; MSWD = 0.34), and 440.2 ± 1.9 Ma (2σ; n = 25; MSWD = 0.18), respectively. These represent the crystallization ages of the representative magmas (Figure 5).

In situ zircon Lu–Hf isotopic analyses were undertaken on adakite samples SD-280, SD-281, and SD-285 (Table S2). The initial ¹⁷⁶Hf/¹⁷⁷Hf ratios are 0.282145–0.282591. The corresponding ε_Hf(t) values vary from 3.30 to −9.57, with an average of −2.01, and the calculated crustal model ages (T_DM2) are 2.04–1.22 Ga, indicating a magma source dominated by ancient crustal materials (Figure 6a). In situ zircon O isotope data for the Shandan adakite samples SD-280 and SD-285 are listed in Table S2. The adakite samples have a range of δ¹⁸O values (5.34 ± 0.15‰ to 7.52 ± 0.51‰) with corresponding variations in zircon ε_Hf(t) values (+2.48 to −12.4) (Figure 6b). The zircon crustal Hf model ages are 2214–1271 Ma.

4.3. Whole-Rock Major and Trace Elements

The SiO₂ contents of the Shandan adakites are 66.8–73.2 wt.%. In a Na₂O + K₂O–SiO₂ diagram, the samples lie in the granodiorite and granite fields (Figure 7a). The Na₂O contents are 1.91–4.24 wt.% and K₂O contents are 2.29–7.72 wt.%, with K₂O/Na₂O = 0.54–4.05. In a K₂O–SiO₂ diagram, the samples lie mainly in the medium- to high-K series fields (Figure 7b). The MgO contents of the adakites are 0.26–1.57 wt.%, with Mg# = 41–49. The A/CNK values are 0.92–1.09. In an A/NK–A/CNK diagram, the samples lie in the nearly peraluminous to metaluminous fields (Figure 7c). In a (Na₂O + K₂O − CaO)–SiO₂ diagram, the samples exhibit both alkaline–calcic and calc–alkalic affinities (Figure 7d).

The total REE (∑REE) contents of the adakites are 28.9–218 ppm. On a chondrite-normalized REE diagram, the samples are LREE enriched and heavy REE depleted, with (La/Yb)_n values of 4.05–37.8, similar to continental crust (Figure 8a). In addition, the adakites exhibit positive Eu anomalies (Eu/Eu* = 1.07–5.01). On a primitive-mantle-normalized trace element diagram, the adakites are enriched in large-ion lithophile elements (e.g., Rb, Ba, Th, U, and Pb), indicating the magma source was rich in minerals such as K-feldspar and mica, and are depleted in high-field-strength elements (e.g., Nb, Ta, and Ti), similar to island arc rocks associated with subduction zones and the continental crust (Figure 8b). The major and trace element data are listed in Table S3.

4.4. Whole-Rock Sr–Nd Isotopic Compositions

Strontium–Nd isotopic analyses were conducted on four Shandan adakite samples. The samples have ⁸⁷Sr/⁸⁶Sr = 0.707867–0.710006, and age-corrected (⁸⁷Sr/⁸⁶Sr)_t ratios of 0.706468–0.706535 (Figure 9). The adakites have low ε_Nd(t) values, ranging from −7.66 to −6.32. The Sr–Nd isotopic characteristics are typical of crust-derived granites, and the granites have two-stage model ages (T_DM2) of 1.81–1.70 Ga, consistent with Paleoproterozoic crustal reservoirs in the North China Craton [62]. The whole-rock initial Sr–Nd isotopic compositions of the Shandan adakites are listed in Table S4.

5. Discussion

5.1. Petrogenesis

The Shandan adakites have relatively high SiO₂ (66.8–73.2 wt.%) and CaO (1.85–3.49 wt.%) contents, and differentiation index values (DI = 90.57–96.96), indicating the magmas experienced fractional crystallization. In Harker diagrams, all samples display well-defined linear relationships between SiO₂ and MgO, Fe₂O₃T, TiO₂, MnO, and P₂O₅. The negative correlations amongst these elements suggest the adakites experienced fractional crystallization of biotite, apatite, and Fe–Ti oxides, while the negative correlation between MnO and SiO₂ indicates that Mn-bearing minerals also fractionated. In the process of crystallization differentiation, hornblende is preferentially combined with MREE [64]. When magmatic composition evolves to feldspathic composition, hornblende differentiation produces a stable or reduced (Dy/Yb) _N ratio (Figure 10f), which indicates that Shandan adakites are separated and crystallized by biotite and hornblende at the same time, which is similar to the characteristics of high Mg Adakite. According to the classification criteria of adakites, Sr/Y > 40, Yb < 1.9, and La/Yb > 20 were used as diagnostic parameters [65]. Shandan adakites have high Sr/Y ratios (40.9–117) and Y contents (7.67–15.8). They are characterized by low Yb (0.98–1.50) and high La/Yb ratios (5.65–52.75, mean 31.81), and lie in the adakite field on the Sr/Y-Y diagram. (Figure 11a). Moreover, the adakites have high SiO₂ contents and moderate Mg# values and lie in the adakite field on a SiO₂–Mg# diagram (Figure 11b).

Existing petrogenetic models for adakites include (1) interactions between melts derived from subducting oceanic crust and mantle peridotites [3,4,5,6]; (2) partial melting of thickened lower crust [9,10,11,12,15,66]; (3) partial melting of delaminated lower crust [13,14,15]; (4) partial melting of subducted continental crust [16]; and (5) fractional crystallization of tholeiitic magma [2]. We now evaluate these potential adakite formation mechanisms in terms of the studied samples.

Figure 11. Plots of (a) Sr/Y versus Y (modified from Defant et al. [65]) and (b) Mg# versus SiO₂ (modified from Zhao et al. [67]). The geochemical data of Maozangsi High-Mg adakitic rocks are from Yu et al. [61], while those of quartz diorites from Mengjiadawan (MJDW) and granodiorites from Mengjiadawan (MJDW), Lianhuashan (LHS), and Yangqiandashan (YQDS) in the North Qilian Orogenic Belt and the southern margin of the Alxa Block are from Zhang et al. [23].

Figure 11. Plots of (a) Sr/Y versus Y (modified from Defant et al. [65]) and (b) Mg# versus SiO₂ (modified from Zhao et al. [67]). The geochemical data of Maozangsi High-Mg adakitic rocks are from Yu et al. [61], while those of quartz diorites from Mengjiadawan (MJDW) and granodiorites from Mengjiadawan (MJDW), Lianhuashan (LHS), and Yangqiandashan (YQDS) in the North Qilian Orogenic Belt and the southern margin of the Alxa Block are from Zhang et al. [23].

(1) Adakites formed by partial melting of subducted oceanic crust typically have relatively unradiogenic Sr isotopic compositions and high ε_Nd(t) values. However, the adakites in the Shandan area have enriched Sr–Nd isotopic compositions of (⁸⁷Sr/⁸⁶Sr)_i = 0.706468–0.706535 and ε_Nd(t) = −7.66 to −6.32, which are inconsistent with derivation by melting of oceanic crust. Moreover, adakitic melts produced by partial melting of subducting oceanic crust tend to react with mantle peridotite during ascent, resulting in elevated MgO contents. In contrast, the Shandan adakites have low MgO contents, further excluding their formation by the melting of the oceanic crust. Furthermore, compared with adakitic granite–diorite complexes from Niuxinshan in northern Qilian [68], which were formed by the melting of the oceanic crust, the Shandan adakites have lower TiO₂ and MgO contents (Figure 12). (2) Magmas generated by partial melting of delaminated lower crust inevitably interact with the mantle during ascent, and the resulting adakites typically have relatively high contents of compatible elements (e.g., Cr and Ni) and MgO. In contrast, the Shandan adakites have relatively low Cr, Ni, and MgO contents, unlike the late Silurian (423 Ma) high-Mg adakites produced by partial melting of the delaminated lower crust in the eastern Qilian Orogen [69]. Furthermore, compared with adakites formed by the melting of the delaminated lower crust, the Shandan adakites have relatively low TiO₂ contents (Figure 12). (3) Both fractional crystallization of mantle-derived tholeiitic magma and fractional crystallization and coupled assimilation of ancient crust can produce adakites, typically as a product of high-pressure crystallization of island arc magmas at the top of the upper mantle [3]. However, these processes generally form intermediate dioritic rocks [70] and are associated with coeval, regionally extensive mafic–ultramafic magmatism. In contrast, the adakites in the Shandan area have high SiO₂ contents (Figure 12), and there are no evidence of widespread coeval mafic–ultramafic rocks. Moreover, there is no continuous differentiation trend between the Shandan quartz diorites and adakites, and thus the adakites were not derived by fractional crystallization of quartz diorite (Figure 10). Therefore, the formation by fractional crystallization of tholeiitic magma and assimilation of ancient crust can be ruled out. (4) Adakites derived by partial melting of subducted continental crust are characterized by K₂O > 3 wt.% [16], whereas two of the Shandan adakite samples have K₂O < 3 wt.% (2.29–2.80 wt.%), thereby excluding the partial melting of subducted continental crust as a viable mechanism for their generation.

In TiO₂–SiO₂ and MgO–SiO₂ diagrams (Figure 12), the Shandan adakites lie in the field for partial melting of the thickened lower crust. Furthermore, on a Sr–Nd isotope diagram, the Shandan adakites lie in the field corresponding to the melting of the magnesian lower crust of the North China Craton (Figure 9). In addition, the Shandan adakites have similar major element compositions as the adakitic granites from Dachaidan, which were formed by partial melting of the thickened lower crust (Figure 12). The zircon saturation temperatures of the Shandan adakites are 674–835 °C, indicating they were the products of high-temperature partial melting, similar to the formation of A-type granites under high-temperature conditions (>800 °C) [71]. Based on the above considerations, we propose that the Shandan adakites were generated by high-temperature partial melting of thickened, magnesian lower crust as a result of underplating by mantle-derived magmas.

Figure 12. Plots of TiO₂ versus SiO₂ and MgO versus SiO₂ (modified from Wang et al. [72]). Data for the Niuxinshan granodiorite in northern Qilian are from Wu et al. [68]; data for the Dachaidan adakitic granite in northern Qilian are our unpublished data.

Figure 12. Plots of TiO₂ versus SiO₂ and MgO versus SiO₂ (modified from Wang et al. [72]). Data for the Niuxinshan granodiorite in northern Qilian are from Wu et al. [68]; data for the Dachaidan adakitic granite in northern Qilian are our unpublished data.

5.2. Mantle Source

Having established a crustal origin, we now evaluate mantle contributions to the adakitic magma. The Shandan adakites have negative ε_Nd(t) values (−7.66 to −6.32) and negative Nb and Ta anomalies, with trace element patterns similar to those of continental crust, indicative of a magnesian lower crustal source. Igneous rocks with similar U–Pb ages, but different ε_Hf(t) values, are generally considered to have formed from magmas generated by mixing from different source regions [73,74]. The negative zircon ε_Hf(t) values suggest that ancient continental crust had a significant role in the magma formation. Positive ε_Hf(t) values for granites can be interpreted as representing mantle-derived magmas or newly formed crust [70]. The ε_Hf(t) values of the Shandan adakites range from +3.30 to −12.4, indicative of contributions from older crustal material and also enriched mantle components, suggesting the Shandan adakites formed by crust–mantle interactions. Continental crust generally has high La/Nb (~2.5) and low Ce/Pb (~4) ratios [75], and there is no significant correlation with SiO₂. The Ce/Pb and SiO₂ diagrams (Figure 13a) show that the samples are mainly influenced by mantle sources, and the influence of crustal mixing is limited. Further constraints on the mantle source can be obtained from the La/Ba versus La/Nb diagram, in which the Shandan adakites have high La/Nb (0.46–5.41) and low La/Ba (0.00–0.03) ratios, which is similar to the characteristics of high-Mg adakite and subcontinental lithospheric mantle melts affected by subduction (Figure 13b [76]). Therefore, the adakites were likely derived from a relatively enriched continental lithospheric mantle source region. Zircon δ¹⁸O values for mantle-derived magmas are 5.3 ± 0.3‰ [51]. The Shandan adakites have zircon δ¹⁸O = 5.34‰–7.52‰, which is similar to or higher than typical mantle-derived zircon values, indicating the parental magma of the Shandan adakites formed by partial melting due to the interactions between thickened magnesian lower crust and continental lithospheric mantle.

Trace element and isotopic data suggest that the Shandan adakites formed by partial melting due to interactions between enriched continental lithospheric mantle and magnesian lower crust. Using a two-end-member (⁸⁷Sr/⁸⁶Sr)_t versus ε_Nd(t) mixing model, we quantitatively estimated the contribution of crustal material to the Shandan adakites (Figure 9), using lower continental crust (LCC [77]) as the crustal end-member and depleted mantle (DM) as the mantle end-member [78,79]. The results indicate the Shandan adakites formed by partial melting of 5%–6% LCC. The lower crust in the North China Craton is mainly composed of Archaean gneiss (TTG). The TTG gneiss in the Beishan complex in the west of the Alashan Block has undergone magmatic crystallization in the late NeoArchaean period and high-pressure metamorphism in the early Paleoproterozoic period, which proves that there is a neoarchaean metamorphic basement in the block [80]. Therefore, TTG gneiss in the western Alxa Block is selected as the crustal end member (~2.5Ga gneiss), and depleted mantle is selected as the mantle end member. Based on zircon Hf isotopes, further estimates of the crustal contribution to the magma suggest the Shandan adakites formed by partial melting and mixing of 5%–15% crustal material with 95%–85% mantle-derived magma (Figure 14).

Figure 13. (a) Ce/Pb versus SiO₂ diagram (continental crust values are from Rudnick et al. [75]), (b) La/Ba versus La/Nb diagram (modified from Saunders et al. [81]). The geochemical data of Maozangsi High-Mg adakitic rocks are from Yu et al. [79], while those of quartz diorites from Mengjiadawan (MJDW) and granodiorites from Mengjiadawan (MJDW), Lianhuashan (LHS), and Yangqiandashan (YQDS) in the North Qilian Orogenic Belt and the southern margin of the Alxa Block are from Zhang et al. [23].

Figure 13. (a) Ce/Pb versus SiO₂ diagram (continental crust values are from Rudnick et al. [75]), (b) La/Ba versus La/Nb diagram (modified from Saunders et al. [81]). The geochemical data of Maozangsi High-Mg adakitic rocks are from Yu et al. [79], while those of quartz diorites from Mengjiadawan (MJDW) and granodiorites from Mengjiadawan (MJDW), Lianhuashan (LHS), and Yangqiandashan (YQDS) in the North Qilian Orogenic Belt and the southern margin of the Alxa Block are from Zhang et al. [23].

The Shandan adakites have low ε_Nd(t) values (−7.66 to −6.32) and high ε_Hf(t) values (3.30 to −12.4), indicating that the Nd–Hf isotope system of the Shandan adakite may show different evolution trends under certain circumstances. Typically, the Lu–Hf and Sm–Nd isotopic systems exhibit a similar behavior during magma evolution and will show a decoupling phenomenon. However, in some samples, the zircon Hf isotope and whole-rock Nd isotope ratios show the same behavior. The Shandan adakites primarily formed by partial melting of the thickened magnesian lower crust, and the Lu–Hf isotopic system may have been modified by variable degrees of lower crustal alteration. The thickened magnesian lower crust was garnet-rich, and thus had a high Lu/Hf ratio, leading to the formation of melts with high ¹⁷⁶Hf/¹⁷⁷Hf ratios [82]. This process leads to the magmas having higher ε_Hf(t) values at a given ε_Nd(t) value, resulting in differences between the Nd–Hf isotopes of the Shandan adakites.

Figure 14. Simulated crustal contamination (the base map is modified from Duan et al. [48]; The Hf isotope evolution of the Alxa basement (2.5 Ga gneiss [80]) was determined using the parameters for the upper crust given by Amelin et al. [83]).

Figure 14. Simulated crustal contamination (the base map is modified from Duan et al. [48]; The Hf isotope evolution of the Alxa basement (2.5 Ga gneiss [80]) was determined using the parameters for the upper crust given by Amelin et al. [83]).

5.3. Geodynamic Processes

The tectonic evolution of the North Qilian Orogenic Belt and Alxa Block involved multiple stages, including oceanic subduction, arc/continent–continent collision, post-collision, and continental subduction, which resulted in the formation of numerous ophiolites and intermediate to silicic igneous rocks. These igneous rocks provide important insights into geodynamic processes and the tectonic evolution, and also document interactions between the lithospheric mantle and continental crust [84]. In the Longshoushan area, located at the northern margin of the North Qilian Orogenic Belt and the southern margin of the Alxa Block, there are extensive exposures of early Paleozoic intermediate to silicic igneous rocks [23,85]. Previous studies have shown that geochemical characteristics can be used to identify the tectonic and geodynamic settings of granites.

In the early Paleozoic, the North Qilian oceanic lithosphere was subducted and the ocean eventually closed, leading to the collision between the Qilian and Alxa blocks [86]. The porphyritic biotite granite, granodiorite, monzogranite, and K-feldspar granite in the Qingshanbao area have a Late Ordovician age (441 Ma), suggesting derivation by melting of the ancient crust of the Paleoproterozoic Longshoushan Formation, with some addition of mantle material. These rocks formed during the collisional orogenic phase in a compressional environment [31]. Wei et al. [87] investigated Late Ordovician (444 Ma) granites from the Hexibao area, which are syn-collision granites, and proposed that northward subduction of the North Qilian oceanic lithosphere occurred at this time, marking the initial collisional phase of the North Qilian Ocean. Chen et al. [88] identified Late Ordovician (447 Ma) quartz monzogranite in the Longshoushan area, and suggested it was the product of magmatism during the subduction of the North Qilian oceanic plate beneath the Alxa Block. Niu et al. [34] identified early Silurian (438 Ma) diorite in the Qingshanbao area, and suggested it formed during the transition from a compressional to an extensional setting. In addition, studies of the 430 Ma Huoshan granite, 427 Ma diabase, 424 Ma Jinfusi S-type granite, and 427–414 Ma Wuwei–Jinchang intrusions [23,89,90,91] suggest that between 435 and 414 Ma, the Alxa Block and Central Qilian–Qaidam blocks transitioned from the syn- to post-collisional stages. Other intermediate to silicic igneous rocks in the North Qilian area include the 516–505 Ma Qaidanu peraluminous granites, 501 Ma Kekeli quartz diorite, 477 Ma Niuxinshan granite, and 457–441 Ma adakitic granites [23,61,90]. Previous studies of the 457–441 Ma adakitic granites in the northern North Qilian Orogenic Belt [23,61,90] indicate that these formed during continent–continent collision [23,61]. The formation of these igneous rocks was closely related to oceanic subduction and subsequent collision. Previous studies have suggested that early Paleozoic magmatism in the Longshoushan area occurred mainly in two phases: Early to Late Ordovician and Late Ordovician to Early Devonian. The Early to Late Ordovician magmatism was primarily associated with the post-collisional stage after the closure of the North Qilian Ocean, while the Late Ordovician to Early Devonian magmatism occurred in a post-collisional setting between the Alxa and Qilian blocks.

Based on previous studies of silicic igneous rocks in the Longshoushan area and northern Qilian region, as well as the 446–440 Ma age of the Shandan adakites obtained in this study (Figure 5), it is likely these rocks formed during the closure of the North Qilian Ocean and collision between the Alxa and Qilian blocks. In Rb versus Y + Nb and Nb versus Y tectonic discrimination diagrams the Shandan adakites lie in the transitional zone between syn-collisional and post-collisional granites, which is similar to that of high-Mg adakite. (Figure 15a,b [92,93]). Therefore, we suggest that the Shandan adakites formed during the collisional to early post-collisional stages between the Alxa and Qilian blocks.

Based on our trace element and Sr–Nd–Hf–O isotope data for the studied adakites, The Shandan adakites have relatively enriched ε_Nd(t), ε_Hf(t) values (3.30 to −12.4) and zircon oxygen isotopes higher than the mantle (δ¹⁸O = 5.34‰–7.52‰), and similar to the subcontinental lithospheric mantle magma in the La/Ba La/Nb diagram. So, we suggest they formed by partial melting resulting from the interaction between the magnesian lower crust and subcontinental lithospheric mantle. The formation of these rocks is attributed primarily to the high-temperature conditions of the thickened magnesian lower crust caused by the underplating of mantle-derived magma. As such, the Shandan adakites likely formed by partial melting induced by heat supplied by asthenospheric mantle upwelling, which triggered interactions between the subcontinental lithospheric mantle and thickened magnesian lower crust (Figure 16). The primary magmas underwent fractional crystallization of biotite, hornblende, apatite, Fe–Ti oxides, and Mn-bearing minerals to form the adakites. The continued subduction of the North Qilian oceanic lithosphere led to ocean closure and collision between the Alxa and Central Qilian blocks at ca. 446 Ma [18]. During 446–440 Ma, the Alxa and Central Qilian blocks collided and the lower crust was thickened. After 440 Ma, the tectonic environment in the Longshoushan area transitioned from the collision to the post-collisional stage.

6. Conclusions

• The Shandan adakites were formed during the collisional to early post-collisional stages between the Alxa and Qilian blocks following the closure of the Proto-Tethyan North Qilian Ocean at ca. 446–440 Ma. The adakites have high Sr/Y (40.9–117) ratios and specific geochemical characteristics, revealing that their genesis is related to the partial melting of the mafic lower crust.
• The relatively enriched ε_Hf(t) (3.30 to –12.4) and δ¹⁸O (5.34‰–7.52‰) of the Shandan adakites indicate that they were formed by the melting of the thickened lower crust and facilitated by mantle-derived heat input, which was accompanied by significant crust–mantle interaction.
• The Alxa and Central Qilian blocks then collided, which caused lower-crustal thickening and a transition to post-collisional tectonics after ca. 440 Ma.