Petrogenesis and Geodynamic Implications of the North Kudi Granitoids in the West Kunlun Orogen, NW China

Abstract

The petrogenesis of aluminous A-type granites is a contentious subject. Here, we focused on the North Kudi pluton in the Western Kunlun orogen to investigate the origin and magmatic processes responsible for generating A-type granites. Samples from the North Kudi pluton are metaluminous to weakly peraluminous. K-feldspar granite samples are characterized by high alkali and Cl contents, high HFSE concentrations and FeO/MgO, low F content, negative Ba, Sr, P, Eu, and Ti anomalies, and high magma temperature (>903 °C), showing affinity to aluminous A-type granites. However, their 10,000 × Ga/Al ratios (1.86–3.18) are relatively lower than typical A-type granites. Quartz-monzonite displays similar Sr–Nd isotopic compositions and Ga/Al ratios with the K-feldspar granite but less pronounced negative Sr, P, and Ti anomalies and no discernable negative Ba and Eu anomalies. Fractional crystallization of alkali-rich, Cl-rich, and F-poor magmas can generate some typical geochemical characteristics of A-type granites (e.g., negative Ba, Sr, P, and Eu anomalies) but has little influence on Ga/Al ratios. The enriched Sr–Nd isotopic compositions, high Cl and alkali contents, arc-like geochemical features, and the involvement of slab-derived components, as indicated by high zircon Ce⁴⁺/Ce³⁺ (up to 503), suggest that the North Kudi pluton was possibly derived from partial melting of the metasomatized lithospheric mantle in the time of asthenosphere upwelling during the post-collisional stage. The emplacement of the North Kudi pluton thus indicates the onset of the post-orogenic stage in the Western Kunlun orogen.

1. Introduction

A-type granites are defined initially as anorogenic granites that occur in a rift zone or stable continental block [1,2,3,4]. They can be distinguished from typical calc-alkaline I-type granites by their “mildly alkaline” (alkalic to alkali-calcic) with high F, rare earth element (REE) (except for Eu), and high field strength element (HFSE) concentrations, high FeO/MgO and Ga/Al ratios, but low Ca, Ba and Sr abundances [1,4,5,6,7,8,9,10,11]. Subsequent studies showed that the original definition of “A-type granite” cannot adequately describe the distinct group [6,12], which leads to further divisions. For example, A-type granites have been divided into A₁- and A₂- subgroups, where the former subgroup share similar geochemical characteristics with oceanic-island basalts, whereas the latter commonly occur in post-collisional settings with arc-like geochemical compositions [6,13,14]. The term “A-type granite” has also been applied to some calc-alkalic granitoids with Fe-rich minerals, high Fe^total/(Fe^total + Mg) ratio, and high REE, HFSE, Ga, and Zn concentrations in southwestern Australia, which are metaluminous to weakly peraluminous rather than peralkaline [5]. Subsequently, this type of metaluminous to weakly peraluminous A-type granites in the Lachlan Fold Belt was defined as “aluminous A-type granites” [12]. In addition, A-type granites are also divided into reduced and oxidized subgroups according to the water content and redox state during the formation processes [3,15,16,17]. The oxidized A-type granitic magmas can contain significant amounts of H₂O (≥4 wt.%), while the reduced subtype was suggested to be derived from an H₂O-poor magma source [16,17]. In summary, although A-type granites can be divided into different subtypes according to different classification schemes, there is a consensus that A-type granites were formed under different extensional settings through various mechanisms.

Possible mechanisms responsible for the generation of peralkaline A-type granites include fractional crystallization of mantle-derived alkaline magmas and mixing between mantle-derived and crustal melts [6,18,19,20]. Aluminous A-type granites generally occur at the final stage of post-collisional magmatism and are spatially and temporally associated with post-collisional I-type granites [21,22,23]. Since aluminous A-type granites usually contain high SiO₂ but extremely low MgO and CaO contents and low Cr, Co, and Ni concentrations, some studies suggested that aluminous A-type granites were formed by partial melting of a felsic infracrustal source [12,22,24]. However, the genesis of the aluminous A-type has also been attributed to the partial melting of a slightly metasomatized mantle with subduction contributions [21,25]. The Kudi pluton in the Western Kunlun orogen comprises quartz-monzonite and K-feldspar granite [26,27]. In previous studies, the K-feldspar granite was classified as aluminous A-type granites associated with partial melting of a crustal source consisting of the Precambrian metasedimentary and metaigneous rocks in the Western Kunlun orogen, which marks the onset of the post-orogenic stage [26,27]. However, the K-feldspar granite samples in the literature also demonstrate some geochemical features that are different from typical A-type granites (e.g., relatively lower 10,000 × Ga/Al). In addition, little effort has been made to investigate the genetic relationship between these quartz-monzonites and K-feldspars granite. The temporally and spatially coexisting intermediate granitoids and metaluminous to peraluminous granites with A-type affinities provide an excellent opportunity to investigate the possibility of fractional crystallization of alkaline magmas to generate the geochemical features of typical aluminous A-type granites. In this paper, we presented new and more detailed zircon U–Pb age and Hf isotopic data, whole-rock major and trace element, and Sr–Nd isotopic compositions of quartz-monzonite and K-feldspar granite in the North Kudi pluton, which can not only provide new information on the origin and genesis of the North Kudi pluton but also provide new insights into the tectonic evolution of the Western Kunlun orogen.

2. Geological Setting and Sample Descriptions

The Western Kunlun orogen in central Asia links the Pamir syntaxis to the west and the East Kunlun orogen to the east and is considered as the northwestern margin of the Tibetan Plateau [28,29,30]. It is separated by the Meso-Cenozoic Altyn fault [31,32] and contains two important sutures, the Kudi-Qimanyute suture in the north and the Mazha-Kangxiwa suture in the south [27,29]. The Kegang fault separates the North Kunlun Terrane (NKT) and the Tarim Craton (Figure 1). Previous studies suggested that they have a similar Precambrian metamorphic basement, which is overlain by the Devonian molasse and Carboniferous-Permian shallow marine carbonates [28,33,34].

The South Kunlun Terrane (SKT) is separated from the NKT by the Kudi-Qimanyute suture. The Kudi-Qimanyute suture contains a series of ophiolite slices, including the Kudi ophiolite and the Qimanyute ophiolite [27]. The Kudi ophiolite suite comprises ultramafic rocks, volcanic and volcanoclastic rocks, and forearc sediments, representing an obducted ophiolite slice with ages of 490–525 Ma [27,30,32,35]. The Qimanyute ophiolite comprises peridotite, gabbros, diabase sheet, and basaltic pillow lavas. The layered gabbros in the Qimanyute ophiolite were formed at ~526 Ma and were intruded by Late Ordovician granitic rocks [36]. The outcropped Precambrian strata in the SKT contain the Pushou Group and the Sangzhutage Group in the north and the Saitula Group in the South, separated by the Pushou-Menggubao ophiolitic mélange [37]. The Pushou Group comprises metasedimentary rocks in high amphibolite facies, and the Sangzhutage Group consists of green schist-facies metasedimentary rocks [37,38,39]. A granodioritic intrusion intruded into the Sangzhutage Group at ~505 Ma [40]. The Saitula Group comprises gneiss and schist with minor intercalated amphibolite [39].

Figure 1

Simplified geological map of the Western Kunlun orogen (modified from [40]) illustrating the sampling location.

[Figure omitted. See PDF]

The Tianshuihai Terrane (TSHT) and the SKT are separated by the Mazha-Kangxiwa suture, which is mainly composed of arc complexes and a suit of volcanic-sedimentary rocks. The hornblende quartz diorite from the arc complexes in the Mazha-Kangxiwa suture is emplaced at ~338 Ma [41]. There are also some high-pressure (HP) granulite blocks in this suture, which have a protolith age of ~456 Ma and were subjected to an amphibolite-facies metamorphic event at ~177 Ma [42,43]. The TSHT comprises meta-greywackes and limestone, which were considered as a huge accretionary wedge associated with a Late Paleozoic-Early Mesozoic orogenic process in some previous studies [30,40,44]. However, some Paleoproterozoic metavolcanic rocks were discovered recently, which indicates the existence of the Precambrian basement beneath the TSHT [45].

The granitic magma setting in the Western Kunlun orogen provides essential clues for the tectonic evolution of the Proto- and Paleo-Tethys Oceans. Early studies distinguished two granitoid belts parallel to the suture zones in the Western Kunlun orogen, with the Early Paleozoic belt in the north and the Early Mesozoic one in the south [46]. The former contains the Datong complex and the Quekesu, Yirba, North Kudi, and some small intrusions (Figure 1). Recent studies also reported some Early Paleozoic granitoids in the Tianshuihai terrane, such as the Kelule, Nanpingxueshan, Ayilixi, and Warengzilafu plutons [47,48,49].

In this contribution, we focus on the Early Paleozoic North Kudi pluton, which was classified as an A₁-type intrusion in previous studies [26,27,44]. The North Kudi pluton is located 7 km north of the Kudi village. The North Kudi pluton intruded into the Sangzhutage Group in the South Kunlun terrane, which consists of quartz-monzonite and K-feldspar granite (Figure 1). Quartz-monzonite samples are mainly distributed in the northern part of the pluton, whereas the K-feldspar granite primarily occurs in the southern part of the pluton. However, the petrogenesis of these quartz-monzogranites has not been investigated in previous studies. Quartz-monzonite and K-feldspar granite samples from the North Kudi pluton display a massive structure and medium-grained texture. The quartz monzogranites consist of plagioclase (30%–40%), K-feldspar (30%–45%), quartz (15%–20%), and biotite (7%–10%) (estimated under the polarizing microscope) (Figure 2). Accessory minerals include sphene, apatite, zircon, monazite, and magnetite. K-feldspar granites are composed of K-feldspar (60%–70%), plagioclase (8%–15%), quartz (20%–25%), and biotite (3%–5%) with minor magnetite, zircon, apatite, and sphene.

3. Analytical Methods

3.1. Zircon U–Pb Dating and Hf Isotopes

Zircon U–Pb dating and Hf isotope analysis were all conducted at the State Key Laboratory of Isotope Geochemistry, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences (SKLIG-GIG-CAS). An Agilent 7900 ICP-MS (Agilent Technologies, Inc., Santa Clara, CA, USA) is equipped with a Resonetics RESOlution S155 laser-ablation system (LA) to perform zircon U–Pb dating and trace element analyses. Helium was used as a carrier gas [50]. The repletion rate was set to be 8 Hz, laser energy to be 85 mJ, ablating time to be 45 s, and spot size to be 31 μm. International standards TEMORA and NIST SRM 610 were used as the external calibration standard for zircon U–Pb ages and trace element contents. ⁹¹Zr was chosen as the internal standard for zircon trace element analysis with uncertainties of 5% [25]. The standard reference zircon, Qinghu, was also analyzed during our experiments to monitor the dating results, and the mean ²⁰⁶Pb/²³⁸U age of the analyzed Qinghu zircon grains is 158 ± 3 Ma, which is within the recommended value [51]. Since previous studies have already provided zircon SHRIMP U–Pb age data for the K-feldspar granite [26,27], our study only focused on zircon grains separated from a quartz monazite sample. Zircon age and trace element data processing were performed with ICPMSDataCal 7.2 [52]. In situ zircon Lu–Hf isotopic compositions were analyzed on the same zircon domain with a spot size of 44 μm in diameter, which has been previously ablated during LA-ICP-MS zircon dating. The ablation time is 60 s, with an energy density of 20 J/cm² and a repletion rate of 8 Hz [53]. The ¹⁷⁶Lu decay constant is 1.867 × 10⁻¹¹ yr⁻¹ in our calculation of the initial ¹⁷⁶Lu/¹⁷⁷Hf. The εHf(t) was calculated relative to the chondritic reservoir with ¹⁷⁶Lu/¹⁷⁷Hf = 0.0336 and ¹⁷⁶Hf/¹⁷⁷Hf = 0.282725 [54].

3.2. Whole-Rock Major and Trace Elements

Major and trace elements measurements were also analyzed at SKLIG-GIG-CAS. Major element concentrations of our samples were measured via X-ray fluorescence spectrometry (XRF). Sample powders (0.51–0.53 g) were firstly fluxed with Li₂B₄O₇ to make homogeneous glass disks at 1050–1100 °C for XRF analysis. The analytical errors for major elements are better than 1%. The trace element analyses were carried out by ablating fluxed glass disks (with the sample to Li₂B₄O₇ ratio of 1:3) via LA-ICPMS. An Agilent 7500 ICP-MS is coupled with a Resonetics RESOlution M-50 ArF-Excimer laser-ablation system for trace element measurements. The repetition rate is 6 Hz with a laser energy of 80 mJ and a spot size of 69 µm. NIST 610 was used as an external standard, and ²⁹Si as the internal standard for trace element analysis [25]. Data processing is carried out with ICPMSDataCal 7.2 [52]. Fluorite and chlorite content measurement were conducted at the ALS Mineral Laboratory in Guangzhou, China. Sample powder was digested with KOH in a nickel crucible and dissolved in HNO₃ solutions. The solution was analyzed via Metrohm IC 930 ion chromatography to obtain the fluorite and chlorite contents.

3.3. Whole-Rock Sr–Nd Isotope

Whole-rock Sr–Nd isotope compositions were analyzed via a Multi-collector Inductively Coupled Plasma Mass Spectrometry (MC-ICP-MS) at the University of Science and Technology of China after chemical separation and purification in SKLIG-GIG-CAS after careful column chemistry at SKLIG-GIG-CAS. Details about chemical separation and purification procedures were presented in the literature [55]. The international standard, BHVO-2, was used as the standard for isotopic ratio analysis. The mean Sr–Nd isotopic compositions for the standard in our experiments are ⁸⁷Sr/⁸⁶Sr = 0.703609 ± 0.000008 (n = 5) and ¹⁴³Nd/¹⁴⁴Nd = 0.512978 ± 0.000005 (n = 5), which is consistent with the recommended value [55]. The initial isotopic ratios were calculated from the Rb, Sr, Sm, and Nd concentrations obtained via ICP-MS.

4. Results

4.1. Zircon U–Pb Ages and Hf Isotopic Compositions

Zircon grains from the quartz-monzonite sample 87–239 were dated by LA-ICP-MS to obtain the crystallizing age of the North Kudi pluton, and the results are presented in Table S1. Zircon crystals in the sample 87–239 from the North Kudi pluton are pale pink and 150–300 μm in size with length/width ratios of 1.5:1–3:1 (Figure 3). Most zircon grains are characterized by weak or blurred oscillatory magmatic zoning with Th/U ratios ranging from 0.30 to 1.47. Forty-four spot analyses on these zircons yielded a weighted mean ²⁰⁶Pb/²³⁸U age of 414 ± 2 Ma (2σ, MSWD = 1.6) (Figure 4). Interestingly, there are a few zircon crystals with convoluted textures in CL images (Figure 3). For example, four zircon grains have recrystallized rims with Th/U ratios of 0.36–0.69 (Figure 5), and three zircon crystals have dark cores and bright rims in CL images. Seven spot analyses on these recrystallized areas yielded a weighted mean ²⁰⁶Pb/²³⁸U age of 373 ± 8 Ma (2σ, MSWD = 1.6) (Figure 4). When all the analysis spots are potted in the coordinate diagram, the Concordia lower intercept age is 403 ± 8 Ma (2σ, MSWD = 3.0).

In situ zircon Lu–Hf isotope analyses were performed on the same area, previously dated by LA-ICP-MS. The results are presented in Table S2 and graphically illustrated in Figure 6. Zircon grains from the North Kudi pluton have ¹⁷⁶Hf/¹⁷⁷Hf of 0.282321–0.282472 with corresponding εHf(t) values of −7.39 to −2.00. The recrystallized areas of zircon grains display a narrow range of ¹⁷⁶Hf/¹⁷⁷Hf from 0.282378 to 0.282450 with εHf(t) from −6.41 to −3.70.

4.2. Whole-Rock Geochemistry

The present bulk-rock major and trace element data and those from the literature are all given in Table S3. Samples from the North Kudi pluton are mostly plotted in the field of quartz-monzonite, alkali-feldspar granite, and syenogranite (Figure 7a). The quartz-monzonite samples have SiO₂ contents of 60.9–65.2 wt.%, MgO contents of 1.09–1.91 wt.%, Al₂O₃ contents of 14.7–16.4 wt.% and total alkalis (Na₂O + K₂O) contents of 8.22–9.59 wt.%. They are metaluminous and are mostly plotted in the field of quartz-monzonite (Figure 7). These samples are enriched in K₂O (4.44–5.28 wt.%) with affinity to the shoshonitic series (Figure 8). They display LREE-enriched patterns with slightly negative Eu anomalies (Eu/Eu* = 0.72–0.84). In the primitive mantle-normalized diagram, they show enrichment in Rb, Ba, Th, U, Pb, and LREE and depletion in Nb, Ta, Sr, P, and Ti (Figure 9). Notably, all these quartz-monzonite samples have low 10,000 × Ga/Al ratios ranging from 2.16 to 2.97 but high HFSE concentrations (Zr + Nb + Ce + Y = 555–962 ppm).

K-feldspar granite samples have relatively high SiO₂ contents of 68.1 wt.% to 71.2 wt.% and K₂O contents of 5.48 wt.% to 6.40 wt.% (Figure 8). The MgO contents of these samples vary from 0.28 wt.% to 0.73 wt.%. Total rare earth element concentrations of these samples range from 345 ppm to 674 ppm, and their REE distribution patterns are also LREE-enriched with (La/Yb)_N of 9–25. However, the negative Eu anomalies (Figure 9) are much stronger than that of the quartz monazite samples. The K-feldspar granite samples are also characterized by high Zr concentrations (339–621 ppm) with 10,000 × Ga/Al ratios of 1.86–3.18. In the primitive mantle-normalized trace element diagram, these samples display a distribution pattern similar to the upper crust. They exhibit positive Th, U, K, and Pb anomalies and negative Ba, Sr, P, and Ti anomalies.

Quart monzonite and K-feldspar granite samples from the North Kudi pluton have low F content from 100 ppm to 240 ppm but high Cl content from 2380 ppm to 2580 ppm. In general, the P₂O₅ contents of samples from the North Kudi pluton decrease with increasing SiO₂.

4.3. Sr–Nd Isotopic Compositions

The Sr–Nd isotopic data are given in Table S3. The present data and those from the literature are all shown in Figure 10. The quartz-monzonite samples from the North Kudi pluton (including literature data) have (⁸⁷Sr/⁸⁶Sr)_i of 0.708214 to 0.710415 and εNd(t) of −3.14 to −3.39. K-feldspar granite samples also display a wide variation of (⁸⁷Sr/⁸⁶Sr)_i from 0.708244 to 0.710133 but a narrow range of εNd(t) from −4.32 to −2.27 (Figure 10). In summary, samples from the North Kudi have relatively uniform εNd(t) but a variable Sr isotopic composition (Figure 10).

5. Discussion

5.1. Geochronology of the North Kudi Granitoids

Previous geochronological studies on the North Kudi pluton based on Ar–Ar, single zircon TIMS U–Pb, and zircon SHIMP U–Pb methods yielded a wide span of its formation age from ~384 Ma to ~421 Ma [26,27,44]. Zircon is usually crystallized during the early stage of magma evolution and can survive from most late-stage magmatic, metamorphic, and erosional processes [59]. Thus, the zircon U–Pb age can provide the best estimation of the crystallization age for magmatic rocks [40]. However, the single zircon TIMS U–Pb age is unsuitable for zircons with xenocryst cores because the yield age using the single zircon TIMS U–Pb method will be older than the crystallization age of the pluton.

In this paper, we show that some zircon grains contain recrystallized rims. The igneous oscillatory zoning is occasionally visible in the cores (Figure 3). These domains with igneous oscillatory zoning and zircon grains with no core–rim textures yielded a weighted mean age of 414 ± 2 Ma, which is indistinguishable from the SHRIMP zircon U–Pb age of the K-feldspar granite (408 ± 7 Ma) within uncertainties [26,27]. The recrystallized rim yields a mean ²³⁸U/²⁰⁶Pb age of 373 ± 8 Ma, which is comparable to the biotite ⁴⁰Ar/³⁹Ar age (384.7 ± 5.2 Ma) and also is within the age span of amphibolite facies metamorphism of basic and acidic dykes at the South Kunlun terrane (380–350 Ma) [60,61]. These imply that the crystallization age of the North Kudi pluton is 414 ± 2 Ma. Then, this pluton is likely subjected to a thermal-tectonic event at ~370 Ma when some magmatic zircon grains experienced partial recrystallization. In summary, there is no discernible difference in the crystallization age for the quartz-monzonite and the K-feldspar granite. Therefore, the best estimate for the formation age of the North Kudi pluton is 414 ± 2 Ma.

5.2. Geochemical Features of A-Type Granite in the North Kudi Granites

Aluminous A-type granites with relatively lower SiO₂ contents (SiO₂ = 65%–72%) in the Lachlan Fold Belt can be distinguished from I-type granites by their high Na₂O + K₂O, FeO^t/(FeO^t + MgO), HFSE, REE (with exception of Eu), Ga and Zn, low Ba, Sr, Eu, Cr and Ni concentrations and high magmatic temperature (>900 °C) [5,7,14,62]. K-feldspar granite samples from the North Kudi pluton are characterized by high alkalis (Na₂O + K₂O = 8.22–9.68 wt.%) and high Zr (339–621 ppm) concentration with Zr + Nb + Ce + Y = 555–962 ppm. In the primitive mantle-normalized spider diagram, K-feldspar granite samples from the North Kudi pluton exhibit evident negative Ba, Sr, P, and Ti anomalies. They are plotted into the field of within-plate granites [26,63], and also display high FeO^t/(FeO^t + MgO), and are mostly plotted into the field of ferroan granitoids [18]. In addition, K-feldspar granite is associated with intermediate quartz-monzonite, which also belongs to the shoshonitic series. As shown in Figure 8, the Zr concentration of samples from the North Kudi pluton increases with the increasing SiO₂ for quartz-monzonite but is inversely correlated with SiO₂ for K-feldspar granite samples. The increasing Zr concentration in quartz-monazite indicates that Zr solubility has not been reached during magma fractionation. Therefore, the minimum estimate of magma temperature is the highest zirconium saturation temperature calculated from the K-feldspar granite samples [64]. In this case, the parent magmas of the North Kudi pluton should be higher than 903 °C. All these features are suggestive of an affinity to A-type granitoids.

According to the classification scheme of [6], K-feldspar granite samples are all plotted in the A₁-subgroup granites, which share similar chemical characteristics with oceanic island basalts. Interestingly, the Y/Nb and Ce/Nb of K-feldspar granite and quartz-monazite samples from the North Kudi pluton are positively correlated, with some samples close to the A₂-subgroup granitoids. Based on the Dall’Agnol’s classification scheme [16,65], these K-feldspar granite samples mostly fall into the field of oxidized A-type granites (Figure 11).

Even though K-feldspar granite samples from the North Kudi pluton share many geochemical characteristics with the A-type granites, they also exhibit some unique features. Firstly, the most typical characteristic of A-type granites is high Ga/Al (>2.6), but most K-feldspar granite samples are characterized by relatively lower Ga/Al (Figure 11a). On the other hand, the F concentration of samples from the North Kudi pluton is also much lower than Lachlan A-type granites (Figure 12). In addition, unlike aluminous A-type granites in southeastern Australia, which are calc-alkalic and are derived from the partial melting of felsic granulite [5], K-feldspar granite samples from the North Kudi pluton is alkalic to alkali-calcic. Therefore, models for the petrogenesis of the North Kudi pluton should also address these geochemical characteristics different from the typical A-type granitoids.

5.3. Petrogenesis of the North Kudi Pluton

Considering the indistinguishable zircon U–Pb ages, similar Sr–Nd isotopic compositions, and similar mineral assemblages of the K-feldspar granite and the quartz-monzonite, there should be some genetic relationships among different rock types from the North Kudi pluton. It is noted that quartz-monzonite samples from the North Kudi pluton display no discernable negative anomalies of Eu and Ba, and thus cannot be classified as A-type granitoids. Compared with the K-feldspar granite, quartz-monazite has lower SiO₂ and higher CaO, Ba, and Sr, which is consistent with relatively low proportions of K-feldspar and high proportions of biotite and plagioclase for quartz-monzonite. As shown in Figure 12, some K-feldspar granite samples deviate from the increasing trend, which indicates that their chemical composition might be partly associated with fractionation crystallization. The decreasing Ba and Sr with increasing SiO₂ for samples from the North Kudi pluton indicate fractionation of both plagioclase and K-feldspar (Figure 8). The relatively more robust negative P anomaly of the K-feldspar granite samples and the inversely correlated SiO₂ and P₂O₅ indicate apatite fractionation. The nearly constant Ta/Nb with decreasing TiO₂ suggests the fractionation of magnetite, but there is also a K-feldspar granite sample with relatively high Ta/Nb, which might be associated with the fractionation of biotite (Figure 13). Therefore, the higher K₂O content and occurrence of negative Ba anomaly and more pronounced negative Sr, P, Eu, and Ti anomalies for these K-feldspar granite samples can be attributed to fractionation crystallization (Figure 13).

The magma source is of critical importance in understanding the origin of granite. Since zircon generally crystallizes at the early stage of magma evolution, zircon can hold the original information about the “primitive” magma [40,66]. The low εHf(t) (from −7.39 to −2.00) of zircon grains indicates an enriched magma source, such as sediments, continental crust, or an enriched mantle source [25]. The nearly constant εNd(t) with increasing (⁸⁷Sr/⁸⁶Sr)_i for K-feldspar granite and quartz-monzonite samples from the North Kudi pluton precludes the possibility that the North Kudi pluton is derived from the partial melting of the Precambrian metaigneous (amphibolite) and metasedimentary (mica-quartz schist) rocks (Figure 10b) because there are significant differences in εNd(t) for the Precambrian metaigneous and metasedimentary rocks [58]. It is unlikely that the intermediate to felsic magma is derived from a well-mixed source at the lower crust [20,50]. In addition, aluminous A-type granites derived from a lower crustal quartz-feldspathic source are calc-alkalic, and partial melting experiments of quartz-feldspathic crustal rocks at low pressures can only produce high SiO₂ melts [18,67,68,69]. The higher εHf(t) relative to the basement in the Western Kunlun orogen is also against a crustal source of the North Kudi pluton. As shown in Figure 10a, the Sr–Nd isotopic data of samples from the North Kudi pluton lies along the lithospheric mantle array. There are also several lines of evidence indicating an oxidized metasomatized lithospheric mantle source of the North Kudi pluton. Firstly, the Ce⁴⁺/Ce³⁺ ratios of zircon grains from the North Kudi pluton span a wide range from 19 to 583 for ~414 Ma zircon domains (Figure 5). A previous study showed that the ~446 Ma Sanshili pluton contains some old inherited zircon cores from its crustal endmember with lower Ce⁴⁺/Ce³⁺, but the involvement of metasomatized mantle wedge-derived magmas makes zircon domains with the age of ~446 Ma with high Ce⁴⁺/Ce³⁺ (>300) ratios [40]. Similarly, the existence of high Ce⁴⁺/Ce³⁺ (>300) in both ~414 Ma zircon domains indicates that the magma source of the North Kudi pluton contains some oxidized components. Secondly, the chemical compositions of the K-feldspar granite samples are primarily plotted in the field of oxidized A-type granite, which implies a relatively high water content of the magma source of the North Kudi pluton (Figure 11) [16]. Finally, the negative Nb and Ta anomalies of samples from the North Kudi pluton may be inherited from the magma source because the Ta/Nb remains constant with TiO₂, which is against the fractionation of Ti-bearing minerals (Figure 13). Combined with the arc-like geochemical characteristics (enriched LILE and depleted HFSE), we suggest that the magma source of the North Kudi pluton seems to be metasomatized by the oxidized H₂O-rich components from the subducting slab.

Samples from the North Kudi pluton have relatively lower F concentration than typical A-type granites in the Lachlan Fold Belt but are enriched in Cl (2380–2580 ppm). The high Cl concentrations in the parental magma of the North Kudi pluton facilitate the fractionation crystallization of plagioclase [5,7,70]. However, the Ga/Al ratio remains unchanged during fractionation (Figure 13), because although plagioclase fractionation can remove Al from the parental magma, Ga can also be removed due to the low abundance of GaF3− 6 complex in the parental magma. This might be partly responsible for the relatively low Ga/Al of the K-feldspar granite samples from the North Kudi pluton.

Fluorite is mainly retained in phengite and apatite during plate subduction, which cannot be released during the early stage of plate subduction [25,71]. In contrast, Cl is hosted in chlorine minerals, which are released at a relatively shallower depth in subduction zones [72,73,74]. Partial melting of the magma source metasomatized by alkali-rich, high Cl, and oxidized components would produce the observed geochemical features in the North Kudi pluton. Although samples from the North Kudi pluton are all falling into the field of A₁-type granites, the positively correlated Ce/Nb, Yb/Ta, and Yb/Nb can also be accounted for by the involvement of subduction contributions. Combined with the high K₂O and Cl contents, the slab-derived components may contain sediment-derived melts [66,75,76], because the breakdown of phengite will also cause high F concentrations in the metasomatized magma source. On the other hand, considering that their geochemical compositions are similar to A₁-type granites, the lithospheric mantle source might be only slightly metasomatized by slab-derived components, or the subduction signatures were diluted by successive injections of asthenosphere-derived melts [77]. The geochemical affinity of K-feldspar granite to A-type granites, especially the negative Ba, Sr, Eu, P, and Ti anomalies, is associated with fractional crystallization.

5.4. Geodynamic Implications

The Early Paleozoic evolution of the Western Kunlun orogen remains controversial due to the lack of HP (high pressure) and UHP (ultra-high pressure) metamorphic rocks. The debates mainly focus on whether the Proto-Tethys exists and whether the Kudi and Qimanyute ophiolite represents the remnants of Proto-Tethys [29,78]. Magmatic rocks can be produced at subduction, syn-collision/accretion, and post-collision/accretion stages of an orogeny, which are of critical importance in understanding magmatic evolution in response to different geodynamic processes [40]. A recent study recognized three episodes of more intensive magmatism at 530–500 Ma, 480–470 Ma, and 445–430 Ma associated with three lithospheric extension events [40]. The last extensional event is related to the closure of the Proto-Tethys at ~442 Ma when slab-derived adakites disappeared in the Western Kunlun orogen, and adakitic rocks were mainly derived from the thickened lower crust. The intermediate-felsic plutons in the Taer region are emplaced at ~432 Ma, which is sourced from the middle and lower crust (30–50 km) with a high melting temperature (876 °C) under an extensional setting [79]. The ~414 Ma North Kudi pluton is formed during the final extensional event in the Western Kunlun orogen and is the latest granitoid intrusion in the Early Paleozoic granite belt. After the closure of the Proto-Tethys, slab break-off or post-collisional delamination cause upwelling of the asthenosphere, which then causes partial melting of a slightly metasomatized lithospheric mantle or a metasomatized lithospheric mantle domain diluted by asthenosphere-derived melts (Figure 14). Fractional crystallization of the metasomatized mantle-derived, alkali-rich, chlorine-rich, and fluorine-poor primary magmas can generate the quartz-monzonite. As a result of continuous magma evolution, K-feldspar granite samples display some typical geochemical features of A-type granites (e.g., negative Ba, Sr, P, and Eu anomalies). Therefore, the intrusion of the North Kudi pluton implies the onset of the post-collisional stage. This conclusion is also supported by the occurrence of subsequent lamprophyre dikes in the Kudi region during the post-collisional extension [61]. In addition, the zircon grains from the North Kudi pluton preserved an age group of 370–380 Ma. The age span is indistinguishable from the Ar–Ar age of the pluton and coincident with the amphibolite facies metamorphic age (380–350 Ma) of basic and acidic dykes in the SKT [28]. This implies that the western part of the Western Kunlun orogen may be subjected to a thermal-tectonic event at ca. 370–380 Ma. Further studies are required to determine the geological significance of such a thermal-tectonic event.

6. Conclusions

The North Kudi pluton was emplaced at 414 ± 2 Ma, which is likely subjected to a later thermal-tectonic event at ~370 Ma. Samples from the North Kudi pluton are all metaluminous to weakly peraluminous and characterized by high FeO^t/(FeO^t + MgO). K-feldspar granite samples have high SiO₂ (68.1–71.2 wt.%), Na₂O + K₂O (Na₂O + K₂O = 8.22–9.68 wt.%), high Zr (339–621 ppm) concentration with Zr + Nb + Ce + Y = 555–962 ppm, and enriched Sr–Nd isotopic compositions with (⁸⁷Sr/⁸⁶Sr)_i ranging from 0.708244 to 0.710133 and εNd(t) from −4.3 to −2.27. They also exhibit evident negative Ba, Sr, P, Eu, and Ti anomalies. Their decreasing Zr levels with increasing SiO₂ suggests that the temperature of their parent magma should be higher than 903 °C. These features lead the K-feldspar granite to be classified as an A-type granite in the literature. However, K-feldspar granite samples are Cl-rich and F-poor and show relatively low 10,000 × Ga/Al (10,000 × Ga/Al = 1.86–3.18) relative to typical A-type granites.

Quartz-monzonite samples from the North Kudi pluton share similar Sr–Nd isotopic compositions with the K-feldspar granite samples and display relatively lower SiO₂ (60.9–65.2 wt.%), total alkalis (Na₂O + K₂O) contents (8.22–9.59 wt.%), similar Ga/Al, less pronounced negative Sr, P, and Ti anomalies, and no discernable negative Ba and Eu anomalies. The higher K₂O content and occurrence of negative Ba anomaly and more pronounced negative Sr, P, Eu, and Ti anomalies of the K-feldspar granite samples can be attributed to fractionation crystallization. The nearly constant Ga/Al with magma evolving is caused by the low F concentration because Ga cannot be significantly fractionated from Al during plagioclase fractionation in F-poor magmas. The enriched isotopic compositions, arc-like geochemical features, the involvement of oxidized compositions as indicated by high zircon Ce⁴⁺/Ce³⁺ (up to 503), geochemical affinity to oxidized A-type granites, and high Cl and alkali contents suggest that these granitoids were possibly derived from a slightly metasomatized lithospheric mantle or a metasomatized lithospheric mantle domain diluted by melts derived from the upwelled asthenosphere. The emplacement of the North Kudi pluton thus indicates the onset of the post-collisional stage in the Western Kunlun orogen.

Our study also shows that partial melting and fractionation crystallization of these metasomatized mantle-derived, alkali-rich, chlorine-rich, and fluorine-poor magmas can generate some typical geochemical characteristics of A-type granites (e.g., negative Ba, Sr, P, and Eu anomalies) but has little influence on its Ga/Al because Ga cannot be significantly fractionated from Al during plagioclase fractionation in F-poor magmas.

Author Contributions

Conceptualization, K.W.; methodology, H.Y.; software, M.G.; investigation, K.W., M.G. and X.J.; resources, K.W.; data curation, K.W.; writing—original draft preparation, K.W.; writing—review and editing, K.W., M.L. and M.G.; supervision, K.W.; funding acquisition, H.Y. All authors have read and agreed to the published version of the manuscript.

Data Availability Statement

The original contributions presented in the study are included in the Supplementary Materials.

Acknowledgments

We would like to thank three anonymous reviewers for their constructive comments and suggestions on the early version of our manuscript. This study was supported by the National Natural Science Foundation of China (Grants 41903009). We want to thank Ying Liu, Le Zhang, and Xin Li of the Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, for their assistance in experimental work. We also thank Yuquan Zhang of the Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, for providing samples and helpful discussion.

Conflicts of Interest

The authors declare no conflict of interest.

Footnotes

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Figures

Figure 2. Photomicrographs of representative quartz-monzonite (a) and K-feldspar granite (b) samples from the North Kudi pluton. Both quartz-monzonite (a) and K-feldspar granite display medium-grained texture.

Figure 3. Representative cathodoluminescence (CL) images of zircon grains from the North Kudi pluton.

Figure 4. Zircon U–Pb Concordia diagrams for the North Kudi pluton.

Figure 5. Th versus U (a) and Ce4+/Ce3+ versus Eu/Eu* (b) for zircon grains from the North Kudi and the Sanshili pluton in the Western Kunlun orogen. The composition of zircon grains from the Sanshili pluton is from [40].

Figure 6. Zircon Hf isotopic compositions for samples from the North Pluton. The Hf isotopic compositions of the Early Paleozoic granitoids are from the literature [40].

Figure 7. Q’-ANOR normative diagram for samples from the North Kudi pluton [56] (a,b) A/NK versus A/CNK diagram for the North Kudi pluton.

Figure 8. Major and trace element compositions of samples from the North Kudi pluton. MALI versus SiO2 (a), FeOt/(FeOt + MgO) versus SiO2 (b), and ASI versus SiO2 (c) are after [18]. (d–i): Harker diagrams for samples from the North Kudi pluton.

Figure 9. Primitive mantle-normalized trace element (a,c) and chondrite-normalized REE (b,d) distribution diagrams for quartz-monzonite and K-feldspar granite samples from the North Kudi pluton. LCC = lower continental crust, UCC = upper continental crust. Data used for normalization are from [57].

Figure 10. Sr–Nd isotopic compositions for samples from the North Kudi pluton. (b) is a magnified version of (a) to show the nearly constant εNd(t) with increasing (87Sr/86Sr)i. Isotopic compositions for Early Paleozoic gabbros and granitoids are all from the literature [40,58]. The field of Precambrian metasedimentary rocks and basaltic rocks in the Kudi ophiolite and MORB are from [26,58].

Figure 11. Discriminant diagrams to distinguish A granites. Na2O + K2O versus 10,000 × Ga/Al (a) and FeO*/MgO versus Zr + Nb + Ce + Y (b) diagrams [12]; Ce/Nb versus Y/Nb (c) and Yb/Ta versus Y/Nb (d) diagrams after [6]; FeOt/(FeOt + MgO) versus Al2O3 (e) and FeOt/(FeOt + MgO) versus Al2O3/(K2O/Na2O) (f) diagrams after [16]. A1-subgroup granite shares similar geochemical characteristics with oceanic-island basalts (OIB), and A2-subgroup granite occur in post-collisional settings with arc-like geochemical compositions.

Figure 12. F versus SiO2 (a) and La/Sm versus La (b) for samples from the North Kudi pluton. Data of Lachlan A-type granites and I- and S-type granite fields are from [12].

Figure 13. Diagrams illustrating the role of fractional crystallization of the North Kudi pluton. (a) 10,000 × Ga/Al versus SiO2, (b) Al2O3 versus SiO2, (c) Ta/Nb versus Ta, (d) Ta/Nb versus TiO2, (e) Sr versus Eu/Eu* and (f) Zr versus Eu/Eu* diagrams to show the effect of fractional crystallization on the chemical composition variation of samples from the North Kudi pluton.

Figure 14. Schematic diagram showing the formatting process of the North Kudi pluton. Slab break-off or post-collisional delamination cause upwelling of the asthenosphere [80], which causes partial melting of a slightly metasomatized lithospheric mantle and forms the North Kudi pluton.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/min13070941/s1, Table S1: Zircon U-Pb isotopic and compositional data obtained by LA-ICP-MS for samples from the North Kudi pluton; Table S2: Lu-Hf isotope compositions of zircons form the North Kudi pluton; Table S3: Major element (wt.%), trace element (ppm), and Sr-Nd isotopic compositions of samples from the North Kudi pluton.

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