Introduction

Although volumetrically minor in the upper continental crust, pegmatites host abundant rare metal mineralization (e.g., Li, Be, Nb, and Ta) (London, 2018). Pegmatites and associated mineralization are generally thought to originate from extreme differentiation of a parent granite system, as they have granitic compositions and high fluxing components (Thomas et al. 2000; Barnes et al. 2012; London, 2018) or from formation by low degrees of partial melting of schists involving muscovite dehydration melting under amphibolite facies metamorphic conditions during compressional deformation of orogenic belts (Stewart, 1978; Henderson and Ihlen, 2004; Chen et al. 2020).

Pegmatites with minor beryllium mineralization (the Wudaogou Silurian pegmatites) have been discovered recently in the southern East Kunlun Orogen (EKO). A genetic model for these pegmatites has not yet been established, and the relationship between the pegmatites and co-existing granites remains unclear. In this paper, we present detailed field relationships, zircon U–Pb age, trace element, and Lu–Hf isotope data, and whole-rock major and trace element data for samples of pegmatites and host rock (granodiorite) from the Wudaogou area of the southern EKO. Results of the study allow us to (1) establish a genetic model for the Wudaogou pegmatites, (2) constrain the genetic links between the granitic pegmatites and associated granodiorites in this area, and (3) determine the tectonic environment of the magmatism that formed the pegmatites and granodiorites in the context of the closure of the Proto-Tethys Ocean in the EKO.

Geological setting and sample descriptions

The EKO is located south of the Qaidam Block and north of the Qiangtang-Songpan Block (Fig. 1a). The orogen is divided into three parts by the North Kunlun Fault and Central Kunlun Suture (Jiang et al. 1992); i.e., the North Kunlun Belt (NKB), Central Kunlun Belt (CKB), and South Kunlun Belt (SKB). Precambrian strata in the EKO have been highly metamorphosed and/or deformed, and they are dominated by the Paleoproterozoic Jinshuikou Group in the NKB and CKB (Li et al. 2008) and by the Mesoproterozoic–Neoproterozoic Kuhai and Wanbaogou groups in the SKB (Liu et al., 2016; Xu et al. 2016; Zhang et al. 2018; Wu et al. 2019). The EKO also contains abundant Permian–Triassic flysch successions and early Paleozoic and Middle–Late Triassic granitoid rocks (Liu et al. 2005; Wang et al. 2007, 2012; Chen et al. 2013).

Fig. 1
figure 1

Simplified geological map of part of a EKO, b Kunlun region, and c Wudaogou area. QT-SP, Qiangtang-Songpan Block; QDM.B, Qaidam Block; QLB, Qilian Block; ALT.F, Altun Tagh Fault; NKL.F, North Kunlun Fault; CKL.F, Central Kunlun Fault; SKL.F, South Kunlun Fault; ①, North Kunlun Belt; ②, Central Kunlun Belt; ③, South Kunlun Belt

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The study area is located in the central SKB (Fig. 1b). The geology of the area comprises mainly Ordovician–Silurian Nachitai Group rocks, together with Silurian volcanic sedimentary formations and minor exposures of the Cambrian Shasongwula Formation and Triassic flysch successions (Fig. 1b). The Nachitai Group crops out in the study area and is composed predominantly of basaltic andesite, dacite, and rhyolite with island arc affinity, as well as pyroclastic rocks and turbidites interbedded with minor limestones (Dong et al. 2018).

The Wudaogou pegmatites are found in the north and east of the Wudaogou pluton in the Kunlunhe area of the SKB, where they intrude Silurian granitoids and the Nachitai Group (Fig. 1c). The dikes are gray white and sub-vertical, with widths of 0.2 to 9 m (Fig. 2a, b). The obtained sample (WDP) has pegmatitic grains measuring mainly 1–3 cm in length (and up to a maximum of 17 cm) and is composed mainly of microcline (40 vol.%), oligoclase (28 vol.%), quartz (24 vol.%), and muscovite (6 vol.%) (Fig. 2c), small amounts of garnet, tourmaline, apatite, and topaz, and trace amounts of beryl, chalcopyrite, tetrahedrite, and limonite.

Fig. 2
figure 2

a and b Wudaogou pegmatites; c thin section of Wudaogou pegmatite; and d Wudaogou granodiorite. Kf, K-feldspar; Pl, plagioclase; Q, quartz; Bi, biotite; Ms, muscovite

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The host rock of the pegmatite dikes is granodiorite (sample WDG), which consists of plagioclase (57 vol.%), K-feldspar (22 vol.%), quartz (16 vol.%), biotite (3 vol.%), and muscovite (1 vol.%), along with accessory minerals (1 vol.%) of zircon, rutile, and apatite. Plagioclase occurs as subhedral tabular grains (1–3 mm) displaying obvious sericitization. K-feldspar appears as grains generally 0.5–1.0 mm in size, has perthitic texture, and contains irregular cracks (Fig. 2d).

Analytical methods

Zircon U–Pb dating and Hf isotope and whole-rock geochemical analyses were carried out at Yanduzhongshi Geological Analysis Laboratories, Beijing, China. Zircons were handpicked, mounted in epoxy resin disks, and then polished until all mineral grains were approximately sectioned in half. Transmitted and reflected light and cathodoluminescence (CL) images were taken of the analyzed zircons to constrain their origins and to identify dating sites.

U–Pb dating and trace element analysis

Zircon U–Pb dating and trace element analyses were conducted simultaneously by laser ablation–inductively coupled plasma–mass spectrometry (LA-ICP-MS). Helium was used as a carrier gas and argon as a make-up gas. Each time-resolved analysis involved 20–30 s of blank signal and 40 s of sample signal acquisition. Offline data processing was conducted with ZSkits software, including selection of sample and blank signals, drift corrections, and calculation of elemental contents, U-Th-Pb isotope ratios, and ages. Isotopic fractionation correction was carried out by analysis of zircon standard 91,500. After 5–10 sample analyses, zircon 91,500 was analyzed twice along with one analysis of Plešovice zircon. Weighted mean ages were calculated and concordia diagrams plotted using Isoplot/Ex_ver3 software (Ludwig, 2003), and correction for common lead was made based on Andersen (2002). Ages obtained for zircon standards 91,500 and Plešovice are consistent with their recommended values (Wiedenbeck et al. 1995). Elemental compositions of the zircons were calibrated against multiple reference materials (BCR-2G and BIR-1G) and by internal standardization. The preferred values for element contents in the USGS reference glasses were taken from the GeoReM database (http://georem.mpch-mainz.gwdg.de/).

Major and trace element analyses

Fresh whole-rock samples were broken into small pieces in a steel jaw crusher and then powdered to 200 mesh in an agate mill. H2O+, CO2, and FeO contents were determined by gravimetric, volumetric, and titrimetric methods, respectively, and the other major elements were analyzed by X-ray fluorescence (XRF) spectrometry. Relative standard deviations (RSDs) for the major elements are < 1%. Trace elements were analyzed using an Agilent 7500a ICP-MS instrument. The sample preparation and digestion methods for ICP-MS analyses were the same as those described by Liu et al. (2008). The RSDs estimated from repeated analyses of three standard reference materials (G-2, AGV-1, and GSR-3) are < 5% for rare earth elements (REEs) and 5–12% for other elements. Detailed analytical procedures have been described by Qi et al. (2000).

In situ Hf isotope analysis of zircon by LA-ICP-MS

Zircon Lu–Hf isotope analyses were carried out in situ using a NWR193 LA microprobe (Elemental Scientific Lasers) coupled to a Neptune multi-collector ICP-MS instrument. Instrumental conditions and data acquisition methods have been described by Wu et al. (2006). A stationary spot was used for the analyses, with a beam diameter of 40 μm. Helium was used as a carrier gas to transport the ablated sample from the LA cell to the ICP-MS torch via a mixing chamber where argon was introduced. To correct for isobaric interferences of 176Lu and 176Yb on 176Hf, 176Lu/175Lu and 176Yb/173Yb ratios of 0.02658 and 0.796218, respectively, were used (e.g., Chu et al. 2002). The isotopic ratios of Yb were normalized to 172Yb/173Yb = 1.35274 (e.g., Chu et al. 2002) and Hf isotope ratios to 179Hf/177Hf = 0.7325 using an exponential law. The mass bias behavior of Lu was assumed to follow that of Yb; mass bias correction protocols have been described by Wu et al. (2006). Zircon standards 91,500 and Plešovice were used as reference standards during the period of analysis. Initial 176Hf/177Hf ratios and εHf(t) values were calculated with reference to the chondritic uniform reservoir (CHUR) of Blichert-Toft and Albarede (1997) at the time of zircon growth from the magma. Single-stage Hf model ages (TDM1) were calculated relative to the depleted mantle with present-day 176Hf/177Hf = 0.28325 and 176Lu/177Hf = 0.0384 (e.g., Griffin et al. 2000).

Results

Zircon U–Pb dating and trace element data

Zircon grains from sample WDP are 100–300 μm long, irregular granular, opaque, and dark in color. CL images show that most zircons have spongeous texture and lack oscillatory zonation (Fig. 3). Zircons from the Wudaogou granodiorite are light brown to dark gray in color, subhedral–euhedral, and elongated to stubby in shape and have lengths of 90–250 µm with aspect ratios of 1:1–3:1. Oscillatory zonation is common in zircons, indicating a magmatic origin (Fig. 4b).

Fig. 3
figure 3

Cathodoluminescence of Wudaogou pegmatite zircons (WDP sample)

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Fig. 4
figure 4

Zircon concordia diagrams of the Wudaogou pegmatite and the related granodiorite

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Zircons from the pegmatite contain contents of Th (5–27 ppm) and U (1685–3415 ppm) that yield Th/U ratios mostly between 0.002 and 0.013. In comparison, zircons from the granodiorite have Th contents of 1–392 ppm and U contents of 32–2971 ppm, with Th/U ratios of 0.01–2.25 (Table 1).

Table 1 LA-ICP-MS zircon U–Pb analyses for the Wudaogou pegmatite and the related granite (WDP and WDG) from the SKB
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Zircon U–Pb data for the two samples from the study area yield weighted 206Pb/238U mean ages of 425.8 ± 2.2 Ma (MSWD = 0.17, n = 18) for the pegmatite and 427.7 ± 2.9 Ma (MSWD = 0.082, n = 12) for the granodiorite (Fig. 4a, b).

REE contents of zircons from the Wudaogou pegmatite are listed in Table 2. The total REE contents vary from 524 to 1301 ppm, with a mean of 820 ppm. Chondrite-normalized REE patterns (Fig. 5) exhibit marked depletion in light REEs (LREEs) and enrichment in heavy REEs (HREEs). Most of the zircons have positive Ce anomalies (Ce/Ce* = 1.11–8.20), except for two analysis points with Ce/Ce* = 0.95 and 1.06, and moderate to large negative Eu anomalies (Eu/Eu* = 0.05–0.34), similar to those of typical magmatic zircons (Fig. 5) (Hoskin and Ireland, 2000; Hoskin, 2005). It is generally considered that negative Eu anomalies indicate zircon crystallization during plagioclase growth, as Eu2+ is preferentially incorporated into plagioclase (Hoskin, 2005). Strong enrichment in HREEs indicates that garnet growth did not occur during zircon crystallization (Sun et al. 2016).

Table 2 Rare earth elements concentrations (ppm) of zircons from the Wudaogou pegmatite
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Fig. 5
figure 5

Chondrite-normalized REE patterns of zircons of the Wudaogou pegmatite (normalization values after Sun and McDonough (1989)). Typical magmatic zircon and hydrothermal zircon data are after Hoskin and Ireland (2000) and Hoskin (2005)

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Whole-rock major and trace element data

Results of major and trace element analyses are given in Table 3. All major element contents were normalized to a 100 wt.% on a volatile-free basis. The Wudaogou pegmatite samples (WDP-1, WDP-2, WDP-3, WDP-4) are characterized by high SiO2 (73.61–76.10 wt.%), Al2O3 (13.73–16.06 wt.%), and K2O + Na2O (8.38–9.72 wt.%) and low total Fe2O3 (0.58–0.63 wt.%), MgO (0.10–0.16 wt.%), and CaO (0.65–0.93 wt.%) contents. The Wudaogou granodiorite samples (WDG-1, WDG-2, WDG-3) have contents of SiO2 = 72.31–75.65 wt.%, Al2O3 = 14.43–14.92 wt.%, K2O + Na2O = 7.80–8.19 wt.%, total Fe2O3 = 1.19–2.18 wt.%, MgO = 0.31–0.62 wt.%, and CaO = 0.33–1.48 wt.%, similar to those of the pegmatites. Data for the WDP and WDG samples plot in the subalkaline series field in a total-alkali-silica (TAS) classification diagram (Fig. 6a) and belong to the medium-K and shoshonite series (Fig. 6b). The pegmatite and granodiorite have relatively high A/CNK values (1.04–1.24, mean = 1.14), suggesting that they have a peraluminous nature (Table 3).

Table 3 Composition of major (wt%) and trace elements (ppm) of the Wudaogou pegmatite (WDP) and granodiorite (WDG)
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Fig. 6
figure 6

a Total alkali-silica diagram (after Wilson (1989)). b SiO2 versusK2O for the studied rocks (continuous line is after Peccerillo and Taylor (1976); dotted line is after Middlemost (1985))

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The Wudaogou pegmatite samples (WDP-1 to WDP-4) have very low total REE contents, ranging from 5.39 to 8.08 ppm. The LREE/HREE and LaN/YbN ratios for the WDP samples are 2.81–6.35 and 3.73–12.01, respectively. The samples display positive Eu anomalies (Eu/Eu* = 1.47–2.75) (Fig. 7a). In N-MORB-normalized diagrams (Fig. 7b), the WDP samples are enriched in Rb, U, Sr, P, and Hf and depleted in Th, Nd, Zr, and Ti. Chondrite-normalized REE patterns of the Wudaogou granodiorite samples (WDG-1 to WDG-3) show REE enrichment (ΣREE = 115.62–194.17 ppm) and pronounced LREE enrichment (LREE/HREE = 7.50–17.11), with (La/Yb)N = 9.35–35.53. These samples exhibit negative Eu anomalies (Eu/Eu* = 0.35–0.49) and are enriched in Rb, Th, U, Nd, Zr, and Hf and depleted in Nb, Sr, P, and Ti. Negative P, Sr, and Ti anomalies may indicate fractional crystallization of apatite, plagioclase, and Ti-bearing phases (e.g., rutile, ilmenite, and titanite), respectively.

Fig. 7
figure 7

Chondrite-normalized REE pattern (a) and primitive mantle-normalized spider diagrams (b) from pegmatites of Wudaogou area

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Zircon in situ Hf isotope data

In situ zircon Lu–Hf analyses undertaken during this study used the same analysis spots as used for zircon U–Pb dating, with results for both two samples given in Table 4. Zircons from the Wudaogou pegmatite yield a narrow range of initial 176Hf/177Hf (0.282407–0.282489) values, with calculated εHf(t) values from − 3.89 to − 0.79 (Fig. 8a, b) (mean of − 2.26) and TDM2 ages of 1664–1465 Ma.

Table 4 Zircon in situ Hf isotope composition of the pegmatites
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Fig. 8
figure 8

Plot showing variations in Hf(t) values and inferred crystallization ages for the magmatic zircons analyzed during this study. CHUR, chondritic uniform reservoir. The 176Lu/177Hf values of the mafic lower crust and the average continental crust are from Kemp et al. (2006)

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Zircons from the Wudaogou granodiorite have uniform initial 176Hf/177Hf ratios (0.282299–0.282594) and calculated εHf(t) values that range from − 7.53 to 2.73 (Fig. 8a, b) (mean of − 2.48). Their TDM2 ages range from 1897 to 1242 Ma and are indicative of crustal contamination by Paleoproterozoic–Mesoproterozoic rocks. The Wudaogou pegmatite and granodiorite samples of the present study and four granitic samples from Chen et al. (2021b) have similar εHf(t) values (Fig. 8a), which suggests that they were probably derived from the same source.

Discussion

Genesis of zircons from the Wudaogou pegmatite

The studied pegmatite sample has high zircon U contents (mostly > 2000 ppm). High-U zircons may yield a higher-than-expected U–Pb apparent age (the “high-U effect”) and can also be influenced by the radioactive Pb loss effect yielding a lower-than-expected apparent age (Li and Chou, 2016). Therefore, it can be difficult to reliably determine the age of high-U zircon. However, in the present study, 206Pb/238U ages of analytical spots of pegmatite sample do not show systematic variation with respect to U contents (Fig. 9a). Therefore, the possible influence of high U on the determined zircon U–Pb ages of the studied pegmatite can be disregarded.

Fig. 9
figure 9

Discrimination diagrams of a 206Pb/238U vs. U contents and b ( Sm/La) N vs. La for zircons from Wudaogou pegmatites (after Hoskin (2005))

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Chondrite-normalized REE patterns of the Wudaogou pegmatite samples, which show positive Ce anomalies (Ce/Ce* = 0.95–8.20) and negative Eu anomalies (Eu/Eu* = 0.05–0.34) (Fig. 5), exhibit characteristics of magmatic zircons, whereas features observed in CL images and the very low Th/U ratios (Th/U = 0. 002–0. 0133) differ from those of magmatic zircons (Hoskin and Black, 2000). The origin of zircons can also be determined by a (Sm/La)N-La diagram (Hoskin, 2005). According to Fig. 9b, all analysis points of Wudaogou pegmatite zircons fall in the transition region between magmatic zircons and hydrothermal zircons, indicating that the zircons were formed into the transition stage between magmatic zircons and hydrothermal zircons.

Genetic model for the Wudaogou pegmatite

The Wudaogou pegmatites occur as dike swarms sharply bounded by granodiorite (Fig. 2a, b) and the Nachitai Group (Fig. 1c) and are confined to a region within 1–1.5 km of the inner or outer contact zone between the dikes and host rocks.

The LA-ICP-MS zircon U–Pb ages of the Wudaogou pegmatite and granodiorite are 425.8 ± 2.2 Ma and 427.7 ± 2.9 Ma, respectively, suggesting that the pegmatite formed at the simultaneously time as or slightly later than the granodiorite and that the pegmatite dikes represent further magmatic evolution relative to the granodiorite by extreme differentiation (e.g., Hulsbosch et al. 2014; London, 2018).

In terms of geochemistry, the Wudaogou pegmatite and granodiorite are similar in composition, with characteristics of high silica, enrichment in alkalis, and peraluminosity. REE contents of the pegmatite are much lower than those of the granodiorite, suggesting crystallization differentiation of accessory minerals, such as apatite, during magmatic evolution. The distribution coefficients of these accessory minerals in the residual melt were high, which would have influenced the contents of REE elements (Fujimaki, 1986; Mahood and Hildreth, 1983; Yurimoto et al. 1990). Figure 10 shows that the variation in REE content of the pegmatite may have been caused by crystal fractionation of apatite and allanite.

Fig. 10
figure 10

(La /Yb) N vs. La plot of the Wudaogou pegmatites. Apatite distribution coefficient is after Fujimaki (1986); zircon and allanite distribution coefficient is after Mahood and Hildreth (1983); distribution coefficient of monazite is after Yurimoto et al. (1990). Pl, plagioclase; Kf, potassium feldspar; Bt, biotite; Aln, allanite; Mnz, monazite; Ap, apatite; and Zrn, zircon

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Zircon Hf isotopes can be reliably used for inferring geological evolution and for tracing magmatic rock source (Wu et al. 2007). εHf(t) values of the studied Wudaogou pegmatite show a narrow range of negative values ranging from − 3.89 to − 0.79 (Fig. 8a, b) (mean of − 2.26). The negative εHf(t) values indicate that the magmatic source of the pegmatite dikes comprised melted crustal materials (Hawkesworth and Kemp, 2006), and the narrow range of values suggests that the magmatic source region was simple. εHf(t) values of the Wudaogou granodiorite range more widely from − 7.53 to 2.73 (Fig. 8a, b) (mean of − 2.48). Combining our data with those from Chen et al. (2021b), the Wudaogou pluton as a whole has εHf(t) values of − 10.62 to 2.73, which suggest consistent εHf(t) values of granite and pegmatite.

In summary, we conclude that the Wudaogou pegmatite is closely related to the Wudaogou granitic pluton and formed as a result of the late magmatic evolution of the granitic body and the crystallization differentiation of apatite and allanite within the evolving magma.

Tectonic setting of the granitic and pegmatitic rocks

Early Paleozoic subduction-related intrusions have been discovered in the EKO, such as at Xiarihamu, Wulonggou, Nuomuhong (Huxiaoqin), Gelmo Zhiyu, Bairiqiete, Yikehalar, and Xiadawu (from west to east), with ages of 450–435 Ma (Chen et al. 2000; Liu, 2008; Liu et al. 2013; Jiang et al. 2015; Dong et al. 2018). Analysis of a high-Mg diorite–granodiorite complex from the central Kunlun suture zone, with an age of 432 Ma, suggests the cessation of subduction and initiation of collision, as well as slab break-off (Zhang et al. 2014), at around that time. Metamorphic rocks found in the Kunlun HP–UHP belt underwent peak metamorphism at 433–428 Ma in association with the final closure of the Proto-Tethys Ocean in the vicinity of the EKO at ca. 430 Ma (Meng et al. 2013; Qi et al. 2016; Du et al. 2017; Song et al. 2018).

Recently discovered intraplate volcanic rocks with ages of 428–425 Ma in the southern EKO (Chen et al. 2021a) suggest that an intraplate setting existed after ca. 430 Ma. Nearly contemporaneous A- and I-type granitic plutons (425–423 Ma) (Chen et al. 2021b; Norbu et al. 2021) provide further evidence for an extensional tectonic setting throughout the EKO since 428 Ma (Chen et al., 2021a,b). In addition, the Xiarihamu super-large copper–nickel sulfide deposit (Wang et al. 2014) and newly discovered Shitoukengde nickel–copper deposit ( Zhou et al., 2015; Li, 2018; Li et al., 2018; Liu et al., 2018; Zhang et al. 2018; Jia et al., 2020; Norbu et al. 2020) that formed between ca. 425 and 420 Ma also confirmed the same result.

The Wudaogou granitic pluton and its pegmatite have ages of 428–425 Ma, coeval with granites and intraplate volcanic rocks in this area, which suggests that the Wudaogou rocks formed in a post-orogenic setting that was initiated when slab break-off triggered the abrupt cessation of collisional tectonism and rapid uplift (Chen et al. 2021b).

Conclusions

(1) Wudaogou pegmatite dikes have been discovered recently in the southern EKO. Zircon morphology and in situ trace element compositions suggest that zircons from the pegmatite formed in the transition stage between magmatic and hydrothermal zircons. The LA-ICP-MS zircon U–Pb age of the pegmatite is 425.8 ± 2.2 Ma, and that of the granodiorite is 427.7 ± 2.9 Ma, suggesting that the pegmatite was formed simultaneously with or slightly later than the granodiorite.

(2) Both the pegmatite and granodiorite samples have high silica contents, show a peraluminous nature, and belong to the medium-K and shoshonitic series. REE contents of the pegmatite are much lower than those of the granodiorite, most likely caused by crystal fractionation of accessory minerals, such as apatite, during magmatic evolution.

(3) Hf(t) values of the pegmatite range from − 3.89 to − 0.79 (mean =  − 2.26) and those of the granodiorite range from − 7.53 to 2.73 (mean =  − 2.48), suggesting a genetic relationship between the pegmatite and granodiorite that the pegmatite being a more highly differentiated product of the same magma from which the granodiorite was formed. These rocks formed in an extensional tectonic setting after the final closure of the Proto-Tethys Ocean in the EKO.