Introduction

The Buqingshan–A’nyemaqen tectonic mélange belt (BTMB) is located at the junction of the East Kunlun, West Qinling and Bayan Har orogenic belt (Fig. 1a, b). The BTMB intersects with the northeastern margin of the Qinghai–Tibet Plateau and the central orogenic system of China and constitutes a significant structural element in which the Proto-Tethyan and Paleo-Tethyan domain are interwoven (Jiang et al. 1992; Bian et al. 1997, 2001; Xu et al. 1996, 2007, 2013; Pei 2001; Wang and Yang 2004; Zhang et al. 2004; Mu et al. 2018; Chen et al. 2020; Li et al. 2020, 2021). Therefore, this region is of great significance to geodynamic research and is well known to be of multistage evolution. It thus provides a natural laboratory for understanding the evolution of the North China Block and the Yangtze Block (Jiang et al. 1992; Xu et al. 1996, 2001; Yin and Zhang 1997; Pan et al. 2012).

Fig. 1
figure 1

a Map showing the macroscopic tectonic framework of Central Orogenic Belt, China; b Simplified tectonic map of western China, showing major tectonic units; Distribution of the Buqingshan Tectonic mélange belt (BTMB); c Distribution of Mengkete quartz diorite (rock mass) in the BTMB, southern margin of the EKOB (geological map modified from Yin and Zhang 2003). NEKB, Northern East Kunlun Block; SEKB, Southern East Kunlun Block; CFEK, Central fault of East Kunlun; the age with “*” is from Yin and Zhang (2003)

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Most researchers consider the BTMB to consist of the residual oceanic crust from the closure of the northernmost branch of Late Paleozoic Paleo-Tethyan Ocean at the southern margin of the East Kunlun orogenic belt (EKOB) (Jiang et al. 1992; Xu et al. 2001; Bian et al. 2001; Zhu et al. 2002; Chen et al. 2001; Zhang et al. 2004; Wang and Yang 2004; Wang et al. 2004; Mo et al. 2007; Yang et al. 2009). However, recent studies have suggested that the BTMB also includes an Early Paleozoic island-arc-type magmatic rocks (Bian et al. 1999a, b, 2007; Li et al. 2017) and ophiolites (Bian et al. 2001, 2004; Liu 2011), indicating that the BTMB underwent subduction and collision during the Early and Late Paleozoic (Zhang et al. 1999, 2000; Liu et al. 2011a, b).

The timing of the closure of the Proto-Tethyan Ocean and the evolution of the EKOB are currently explained by two different models. One model advocates that the closure of the Proto-Tethyan Ocean suggests a northward subduction during the Late Silurian and Devonian (Bian et al. 2004; Liu et al. 2012; Xiong et al. 2015). The alternative model suggests that the Proto-Tethys Ocean did not close in the Early Paleozoic but that the Buqingshan–A’nyemaqen Ocean Basin was closed by the Middle Triassic (Pan et al. 2012; Dong et al. 2018; Pei et al. 2018).

Previous research on the Early Paleozoic magmatic rocks in the region showed that magmatism occurred in two phases. The early phase (Cambrian to Early Ordovician) produced the De Dur’ngoi diorite (Li et al. 2007) and the Manite granodiorite (Li et al. 2017), among other plutonic rocks. The products of the later phase (Early Silurian) include the Yikehalaer, Bairiqiete, and Manite granodiorite, as well as felsic volcanic rocks (e.g., Bian et al. 1999a, b, 2007; Liu 2011; Liu et al. 2011a, b; Li et al. 2014a, b, 2015, 2017). These Early Paleozoic magmatic rocks provided a means of studying the evolution of the Proto-Tethyan Ocean while also presenting the scientific challenge of elucidating which dynamic background might be represented by the two periods of magmatism.

Previous researchers have disputed the age of the plutons found in the Manite area (Qinghai Geological Bureau 1972; Yin and Zhang 2003; Li et al. 2017). It was earlier considered that all of the plutons in the Manite area were formed in the Late Paleozoic (Qinghai Geological Bureau, 1972). Later, Yin and Zhang (2003) divided the Manite plutons into a southern granodiorite pluton and northern quartz diorite pluton, with the southern pluton being formed in the Early Triassic (zircon U-Pb date, 237 Ma) and the northern pluton being formed in the Late Triassic (zircon U-Pb date, 205 Ma). However, Li et al. (2017) dated the southern Manite granodiorite at 487 ± 11 Ma and 479 ± 2 Ma (by zircon U-Pb dating) and proposed that the ongoing subduction of the Proto-Tethyan Ocean in the Buqingshan area during 487–479 Ma formed these Late Cambrian to Early Ordovician island-arc-type granitoids. Field Geological mapping indicates that the northern quartz diorite pluton (the Mengkete) has a fault contact with the surrounding geological units (Fig. 2). Therefore, determination of the formation and the petrogenesis of the Mengkete quartz diorite are of important to understand the evolution of the Proto-Tethyan Ocean and the Paleo-Tethyan Ocean.

Fig. 2
figure 2

Simplified geological map of the Mengkete region in BTMB, southern margin of the EKOB (geological map modified from Yin and Zhang 2003)

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In this paper, we focus on Mengkete quartz diorites in the BTMB and present their petrology together with zircon U-Pb-Hf geochronological and geochemical data. Petrogenesis, magma sources, including magma mixing are also discussed with the aim of providing better constraints on the subduction of the Proto-Tethyan Ocean at the southern margin of the EKOB.

Geological setting

The BTMB consists of a discontinuous E–W trending belt that is more than 700 km long and approximately 10–20 km wide (Fig. 1a, b). It stretches from Maqin in the east across Majixueshan and Tuosuohu to Buqingshan and southeast Heicigou (Fig. 1a) to connect with the mafic–ultramafic rocks of Muzitage (Molnar et al. 1987; Burchfiel et al. 1989; Bian et al. 2004, Fig. 1c). To the north, the BTMB is separated from the EKOB and West Qinling orogenic belt by the southern East Kunlun Fault. To the south, it is separated from the Bayan Har orogenic belt by the Changshitou Fault. Forming a suture zone between the Bayan Har and East Kunlun Blocks, the BTMB is a product of the ocean–continent subduction–collision two-phase tectonic evolution during the Early Paleozoic to the Late Paleozoic (Zhang et al. 1999) and belongs to the East Tethyan Ocean tectonic domain (Jiang et al. 1992; Bian et al. 1999a, b, 2001; Chen and Sun 1999; Chen et al. 2001, 2004; Zhu et al. 1999; Yang et al. 2004; Guo et al. 2007; Liu et al. 2011a, b, c; Pei et al. 2018).

The BTMB is composed of the Lower–Middle Permian Maerzheng Formation (P1-2m), which consists of Early Palaeozoic and Late Palaeozoic ophiolites, Palaeozoic rocks, seamount basalts, and limestone (Liu et al. 2011b; Li et al. 2013c, 2014b, 2017; Li et al. 2014a, 2015; Yang et al. 2014; Pei et al. 2015, 2018; Yang et al. 2016; Pei et al. 2017). The Middle Proterozoic Kuhai Group (Pt2K) found in the northern part of the ophiolite belt includes marble, biotite–quartz schist, gneiss, and amphibolite and constitutes the metamorphic basement. A nappe composed of the Upper Carboniferous to Lower Permian Shumenweike Formation (C2P1-2sh), which primarily consists of carbonates with apparent reef affinities (Fig. 2), covers the entire area.

The Mengkete quartz diorite is a composite pluton that outcrops in the Manite area and Mengkete Valley in the Buqingshan area (Fig. 2). It is exposed at the surface as two main blocks oriented in an NNW–SEE direction (Fig. 2), occupying an area of ~ 16 km2. The pluton is in fault contact with the middle to Lower Permian Maerzheng Formation (P1-2m) and the Late Cambrian to Early Ordovician Manite granodiorite (Fig. 2). On the basis of the field geological mapping, we consider that the plutons are allochtonous that form a mélange block within the Maerzheng Formation (Fig. 2). The edge of the intrusive body is strongly gneissic, but both the margin and interior of the block of plutons exhibit a weak deformation (Fig. 3a, b).

Fig. 3
figure 3

a, b Field photographs (viewing toward SE); c, d microscope micrographs (crossed polarizers, 5 times) of the Mengkete quartz diorite in the BTMB, southern margin of the EKOB. Am Amphibole, Pl Plagioclase, Qtz Quartz

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Petrography

The massive to slightly gneissic diorite is gray to dark gray, with medium-to-fine subhedral grains. The constituent phases are plagioclase (60–65%), quartz (10–18%), amphibole (13–20%), K-feldspar (2–5%), and biotite (1–3%). The accessory minerals are mainly magnetite, titanite, apatite and zircon. The feldspar grains are 0.25–5 mm in size, are subhedral, and show obvious polysynthetic twinning (Fig. 3c, d), and they also exhibit brittle deformation. The quartz grains are 0.1–0.3 mm in size and exhibit undulose extinction (Fig. 3d). The amphibole grains are aligned, and there are some quartz crystals enclosed within the amphibole crystals. The alteration of the diorite occurred mainly by the sericitization and kaolinization of the plagioclase and chloritization of the amphibole and biotite.

Analytical methods

LA-ICP-MS testing

Two quartz diorite samples (MNT-02, MNT-07) from the Mengkete pluton were used for isotopic dating. The geographic coordinates of the samples determined by GPS were 35°28′52.86″N, 97°34′31.32″E, 4408 meters in height, 35°29′35.40″N, 97°34′31.02″E, 4338 meters in height, respectively. Rock specimens were crushed (80–100 mesh) using conventional methods and minerals were separated by flotation and electromagnetism techniques. Well-formed, crystal-shaped, and transparent zircons were handpicked using a binocular microscope. The zircon grains were mounted on two-sided adhesive tape and fixed with colorless transparent epoxy resin until fully solidified. The surface was polished to expose the interior of the zircons. Cathodoluminescence (CL) microphotographic images were taken with a Cameca electron probe X-ray microanalyser at the Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing. The analysis voltage was 15 kV, and the current was 19 nA. The in situ U-Pb isotopic age analyses of zircons were performed using LA-ICP-MS at the State Key Laboratory of Continental Dynamics, Northwest University, Xi’an. The analysis instruments were an Elan 6100DRC Type Quadrupole Perch Mass Spectrograph and a Geolas 200M excimer laser ablation system (193 nm, Geolas 200M, Lambda Physic). The facula beam diameter of laser ablation was 30 μm, and the depth of laser ablation samples was 20–40 μm. The detailed experimental principles, technological process, and instrumentation parameters used were the same as those reported by Yuan et al. (2003, 2004).

The international standard zircon 91500 was used as an external standard for the calculation of zircon ages. The artificial synthetic silicate glass NIST SRM610, American National Standard Substance Bureau, was adopted as an external standard for element content analysis. 29Si was used as the internal standard element. The isotopic ratio and element content data were analyzed using the GLITTER software (ver. 4.0, Macquarie University). The general lead adjustment was conducted using the Andersen software (Andersen 2002), and ISOPLOT software (3.0 edition, Ludwig 2003) was used for the age calculation and concordia diagrams.

Geochemical analyses

Seven samples were selected for the analyses of major and trace elements. The samples were ground to 200 mesh, and the major and trace elements were measured in the State Key Laboratory of Lithosphere Evolution, Institute of Geology and Geophysics, Chinese Academy of Sciences. The major elements were tested using X-ray fluorescence spectrometry (XRF-1500). To determine the oxide content, a sheet glass made of 0.5 g sample and 5 g lithium tetraborate was tested using the Shimadzu XRF-1500 with a precision of >2–3%. Trace and rare-earth elements (REE) were analysed by ICP-MS (Element II). The samples were prepared using the acid-solubility method, which has an analytic precision of > 10% (according to the national standards GSR-1 and GSR-2); however, the precision is > 5% when the element content is > 10 ppm. The detailed analysis methods were described by Chen et al. (2000, 2002a).

Zircon Lu-Hf isotope analyses

In situ zircon Hf isotopic analyses were conducted using a Neptune MC-ICPMS, equipped with a 193 nm laser, at the State Key Laboratory of Continental Dynamics, Northwest University, China. During the analyses, a laser repetition rage of 10 Hz at 100 mJ was used and spot diameter was 30 lm. Raw counts for 172Yb, 173Yb, 175Lu, 176(Lu + Yb + Hf), 177Hf, 178Hf, 179Hf, 180Hf and 182W were collected and isobaric interference corrections for 176Lu and 176Yb on 176Hf need to be precisely determined. 176Lu was calibrated using the 175Lu value and correction of 176Yb on 176Hf (Iizuka and Hirata 2005; Iizuka et al. 2017). The detailed analytical technique was described by Yuan et al. (2008). The notations of εHf(t), fLu/Hf, TDM1 and TDM2 are defined as the same as those in Wu et al. (2007).

Analytical results

Zirocn U-Pb ages

The zircons in the sample MNT-02 of the Mengkete quartz diorite are euhedral and pale yellow to colorless. In cathodoluminescence images (Fig. 4a), these zircons show oscillatory zoning, which is indicative of a magmatic origin (Belousova et al. 2002; Wu and Zheng 2004; Siebel et al. 2005). The zircon is prismatic in form with lengths ranging from 110 to 220 µm and aspect ratios of 1:1–3:1. The thorium and uranium concentrations obtained from the analyses of 24 zircon samples were within the ranges 60.50–455.89 and 160.25–469.25 ppm, respectively (Table S1, in the Supplemental material), and all of the zircons were found to have high Th/U ratios (0.28–1.00) (Table S1, Fig. 5a). All the Th/U ratios were greater than 0.1, and Th and U were positively correlated. The zircons display left-inclined curves in chondrite-normalized REE patterns (Fig. 5b), indicate fractionation between light and heavy rare-earth elements (LREEs and HREEs) evident from positive Ce anomalies (Ce/Ce* = 1.17–148.77) and negative Eu anomalies (Eu/Eu* = 0.41–0.63) (Table S2). All of the zircon Th/U ratios, crystal shapes, and REE patterns indicate that the zircons are of magmatic origin. Samples from 23 of the sample locations—the exception being MNT-0q62-11—indicated concordant ages. The 206Pb/238U ages range from 450 to 426 Ma (Table S1, Fig. 4a), with an average age of 441.0 ± 2.6 Ma (MSWD = 1.7) (Fig. 6a, b), which implies crystallization Early Silurian for the Mengkete quartz diorite.

Fig. 4
figure 4

Cathodoluminescence (CL) images of representative zircon grains from the Mengkete quartz diorite in the BTMB, southern margin of the EKOB. a Sample MNT–02; b sample MNT–07. The yellow solid and red dashed circles show the locations for U-Pb and Lu-Hf isotope analysis, respectively. Their corresponding 206Pb/238U ages (yellow digits) and εHf(t) values (red digits) are also shown

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Fig. 5
figure 5

Th–U contentand and chondrite-normalized REE patterns for zircon grains from Mengkete quartz diorite in the BTMB, southern margin of the EKOB (chondrite data data for normalization taken from Sun and McDonnough 1989) (a and b, sample MNT–02; c and d, sample MNT–07)

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Fig. 6
figure 6

LA–ICP–MS zircon U–Pb concordia diagram of the Mengkete quartz diorite in the BTMB, southern margin of the EKOB (a and b, sample MNT–02; c, d and e, sample MNT–07)

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The zircons in the sample MNT-07 of the Mengkete quartz diorite are euhedral and pale yellow to colorless. In cathodoluminescense images (Fig. 4b), these zircons show oscillatory zoning, which is again indicative of a magmatic origin (Belousova et al. 2002; Wu and Zheng 2004; Siebel et al. 2005). The thorium and uranium concentrations for the 30 zircon samples were within the ranges 69.96–463.19 and 152.46–677.44 ppm, respectively, and all of the zircons were found to have high Th/U ratios (0.44–1.00). All of the Th/U ratios were greater than 0.1, and Th and U were positively correlated (Table S1, Fig. 5c). The zircons display left-inclined curves in chondrite-normalized REE patterns (Fig. 5d) and indicate fractionation between LREEs and HREEs evident from positive Ce anomaly (Ce/Ce* = 1.29–68.38) and negative Eu anomaly (Eu/Eu* = 0.23–0.75) (Table S2). All of the zircon Th/U ratios, crystal shapes, and REE patterns indicate that the zircons are of magmatic origin. Samples from 29 of the sample locations—the exception being MNT-07-007—indicated concordant ages. These ages can be subdivided into two groups: one group (corresponding to 5 of the zircon cores) yielded 206Pb/238U ages ranging from 495 to 465 Ma (Fig. 4b) with a weighted mean age of 474 ± 15 Ma (MSWD = 2.5) (Fig. 6c, d), representing zircons captured from the wall rock. The other group (corresponding to 24 of the samples in zircon rims) yielded 206Pb/238U ages ranging from 450 to 417 Ma (Table S1, Fig. 6c) with a weighted mean age of 435.9 ± 3.9 Ma (MSWD = 0.52) (Fig. 6e); this age corresponds to the crystallization age of the Mengkete quartz diorite. Therefore, we suggest that the Munkte quartz diorite was formed between 441.0 and 435.9 Ma, during the Early Silurian.

Geochemcial characteristics

The SiO2 content of the Mengkete quartz diorite samples is variable (56.63–65.22%) (Table S3). In a total alkali–silica diagram (Fig. 7a), most of the samples fall into the diorite field and have a low alkali contents (4.91–5.59%) and K2O/Na2O ratios of 0.17–0.31. All of the samples can be classified as medium-K calc-alkaline and are characterized by a medium K2O content and high Al2O3 (and low FeO/(FeO + MgO) ratios (16.09–17.79% and 0.49–0.53, respectively) (Fig. 7b).

Fig. 7
figure 7

a TAS diagrams (after Le Maitre 2002 Middlemost 1994). b K2O–SiO2 diagrams (after Rickwood 1989) of the Mengkete quartz diorite in the BTMB, southern margin of the EKOB

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The trace-element results for the Mengkete quartz diorites indicate low REE contents (47.61–163.08 ppm with an average of 87.53 ppm), fractionation between LREEs and HREEs (LREE/HREE = 3.58–11.53 with an average of 7.08), and enrichment in LREEs with depletion in HREEs (Fig. 8a). All of the diorites have weakly positive Eu anomalies (Eu/Eu* = 0.96–1.32) (Fig. 8a). LREE fractionation is also indicated by the (La/Sm)N values (1.20–5.65 with an average of 3.20). The HREE depleted is probably caused by the residual garnet and hornblende (Patino-Douce and Johnston 1991), which have La/Yb ratios ranging from 3.37 to 20.98 (average: 11.41), (La/Yb)N ratios ranging from 2.42 to 15.05 (average: 8.19), and (Gd/Yb)N ratios ranging from 1.42 to 2.00 (average: 1.64).

Fig. 8
figure 8

a Chondrite-normalized rare earth element (REE) patterns, b primitive mantle-normalized incompatible element distribution patterns for Mengkete quartz diorite in the BTMB, southern margin of the EKOB (chondrite data and primitive mantle data for normalization taken from Sun and McDonnough 1989)

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The Mengkete quartz diorites are characterized by high Sr, low Y, and low Yb, with Rb/Sr ratios of 0.05–0.09, Ra/Ba ratios of 0.05–0.11, and high K/Rb ratios (118.98–143.23). A primitive mantle-normalized spidergram (Fig. 8b) indicates that the diorites are enriched in LILEs (such as Rb, Th, and Ba) and depleted in HFSEs (such as Nb, Ta, Zr, Hf, and Ti). The relatively enriched Zr and depleted Nb, Ta, and Ti indicate that a crustal rock is a possible source of hese diorites (Green et al.1987; Green 1995; Barth et al. 2000). The curves of the spidergrams and the REE patterns are identical across all samples, which is indicative of their cognate source rock.

Zircon in-situ Lu-Hf isotopic compositions

To determine the magma source for the Mengkete quartz diorite, the Lu-Hf isotopic compositions of a set of representative zircon samples (MNT-07, ~ 435.9 Ma) were analyzed. The beam positions are shown in Fig. 4b, and the analytical results are listed in Table S4.The 176Yb/177Hf ratios of the samples range from 0.018122 to 0.066622. The 176Lu/177Hf ratios are smaller than 0.002 (range: 0.000598–0.001919), indicating that only a small amount of radiogenic Hf accumulated after the zircons crystallized (Griffin et al. 2000). Therefore, the initial 176Hf/177Hf ratio can also be used to represent the final 176Hf/177Hf ratio—that is, the ratio when the zircons initially formed (Wu et al. 2007). The fLu/Hf values ranged from -0.94 to -0.98 (average: -0.97). The in-situ Lu-Hf isotopic compositions of the zircons in the Mengkete quartz diorites are homogenous and have similar εHf (t) values (7.79–13.02) (Fig. 9a) and TDM2 model ages (1130–657Ma) (Fig. 9b).

Fig. 9
figure 9

a Plot of εHf (t) vs. U-Pb ages and b Histogram of εHf (t) for zircons from studied diorite (the values used for constructing the depleted mantle and crust reference evolution lines are after Griffin et al. 2000, 2002)

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Discussion

Petrogenesis and source characteristics

The Mengkete quartz diorites have La/Nb ratios greater than 1.0 (range 1.33–5.53). These are different from the values that mantle-derived magma would have (DePaolo and Daley 2000), indicating that these quartz diorites formed in crust-derived magma. The depleted Nb, Ta and Ti also indicate that the source was dominated by crustal materials. The Rb/Sr ratios range from 0.05 to 0.09 (average: 0.06, closer to the average crustal value of 0.24, Taylor and McLennan 1985), again supporting the hypothesis that the quartz diorites are associated with crust-derived magma.

The magma derived from the partial melting of basaltic rocks in the crust can form a basaltic and metaluminous granitic magma (Wolf and Wyllie 1989; Beard and Lofgren 1991; Johannes et al. 2003; Sisson et al. 2005). The high K/Rb, high Sr/Y, low Rb/Sr, and low Rb/Ba ratios obtained for the diorites indicate that the source rocks were depleted in Rb and enriched in Sr and Ba, whereas the low Rb/Sr and Rb/Ba ratios and high Mg# values indicate that the source rocks consisted of mafic rocks, metabasalt, or mafic metagreywackes (Altherr et al. 2000; Liegeois 1998). In a plot of Al2O3/(MgO + FeOTotal) vs CaO/(MgO + FeOTotal) (an AMF-CMF diagram) (Fig. 10a), all of the samples plot in the field that corresponds to the melting of basaltic rocks. In an SiO2–Mg# diagram (Fig. 10b), all of the samples plot in or lie close to the field corresponding to adakite melts, also indicating a mafic source rock. According to plots of La/Sm vs La and Zr/Sm vs Zr plots (Fig. 10c, d, respectively), all of the samples were affected by partial melting during the evolution of the magma. This is also supported by the slightly positive Eu anomalies and LREE-HREE fractionation (Fig. 8a), which both indicate that the Mengkete quartz diorites were formed by partial melting of mafic crustal rock.

Fig. 10
figure 10

a AMF–CMF diagram (after Sylvester 1998); b SiO2–Mg# diagram (the curves of mantle assimilation–fractional crystallization (AFC) and crustal AFC are after Stern and Kilian 1996); c Zr/Sm–Zr diagram; d La/Sm–La diagram (after Allègre and Minster 1978) for Mengkete quartz diorite in the BTMB, southern margin of the EKOB

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Zircon Hf isotope analysis is an important means of determining the source area for granite (Wu et al. 2007). We found that εHf(t) for zircons from the Mengkete quartz diorites was dominated by positive values ranging from +7.79 to +13.02 (Fig. 9a), with corresponding TDM2 model ages ranging from 1130 to 657 Ma. In the εHf(t)-t diagram, all of the samples plot above the chondritic uniform-reservoir evolution line and below the depleted-mantle evolution line, indicating that the petrogenesis involves young components. There are two possible ways for young components to participate in the petrogenesis of granite. One way involves the mixture of the felsic magma formed by the partial melting of the mantle source with the lower crustal material (Griffin et al. 2002; Belousova et al. 2006; Kemp et al. 2007); in the other, mantle-derived magma underplating causes partial melting of the crustal material (Jahn et al. 2000; Wu et al. 2006; Zheng et a1. 2007). Values of εHf(t) for the Mengkete quartz diorite are positive and do not vary by more than 6 ε units. The Hf isotope values are relatively uniform, possibly indicating partial melting of the crustal material to form granitic magma. These values are also consistent with the geochemical characteristics of control by a crustal source.

The 176Lu/177Hf value for sample MNT-7 was less than 0.002, indicating that only a small amount of radiogenic Hf in the zircon after it formed. The sample had an average fLu/Hf value of − 0.97, which is much lower than that of both mafic crust (− 0.34; Amelin et al., 2000) and salic crust (− 0.72; Vervoort et al., 1996). Therefore, the two-stage model age better represents the time when the source rock material was extracted from the depleted mantle (Wu et al. 2007). According to the two-stage granite model age, the Mengkete quartz diorite formed by partial melting of the Meso-Neoproterozoic crustal material.

Tectonic setting

The Mengkete quartz diorites have an SiO2 content of 56.63–65.22%, an MgO content of 2.12–3.23%, a high Sr content (467.89–626.53 ppm), a low Y content (7.79 20.10 ppm), and a low Yb content (0.80–2.03 ppm), all of which suggest an association with adakite (Default and Drummond 1990; Martin 1999). The high Na2O content (3.81–4.77%), low K2O/Na2O ratios (0.17–0.31), high Mg# values (48.92–51.29), slightly positive Eu anomalies, and in some cases high Sr/Y ratios (26.14–63.78) differ from what would be observed for typical crust-derived calc-alkaline granite (Defant and Drmmond 1990; Martin 1999). Altogether, these geochemical signatures; the high Sr, low Y, and low Yb; the Rb/Sr ratios (0.05–0.09), Rb/Ba ratios (0.05–0.11), and high K/Rb ratios (118.98–143.23); the negative Nb, P, and Ti anomalies; and the positive Sr anomalies indicate an affinity with a mafic-magma source in a subduction-related arc environment (Pearce and Norry 1979; McCulloch and Gamble 1991). The relatively enriched Ba and Sr and the relatively depleted Rb, Nb, Ta, P, and Ti also support the idea of an arc-magma source (Fig. 7b).

These results for the Mengkete quartz diorites and those from previous research on coeval magmatic rocks in the Buqingshan area—including the Bairiqiete and Yikehalaer granodiorites and intermediate-to-acidic lava—plot in the volcanic-arc granitoid (VAG) field in a Rb vs (Yb + Ta) diagram (Fig. 11a), thus further supporting an arc environment origin for the diorites. In a diagram of Rb vs (Y + Nb) (Fig. 11b), most of the data fall into the VAG and post-collision granites fields, indicating that these intermediate-to-acidic igneous rocks all formed in the same setting (Liu 2011; Li et al. 2014b). Thus, it can be concluded that the Mengkete quartz diorites are products of Early Paleozoic island-arc magmatism related to Proto-Tethyan subduction and orogenesis.

Fig. 11
figure 11

Tectonic setting discrimination diagrams for Mengkete quartz diorite in the BTMB, southern margin of the EKOB (a, after Pearce et al. 1984; b after Pearce 1996). Mengkete quartz diorite data from this paper; B–Bairiqiete granodiorite data from Li et al. 2014b; BH–Bairiqiete intermediate–acid Lava data from Liu et al. 2011c; Yikehalaer–Yikehalaer granodiorite data from Li et al. 2015. ORG, Ocean Ridge Granites; Post–COLG, Post-Collision Granites; Syn–COLG, Syn-Collision Granites; VAG, Volcanic Arc Granites; WPG Within Plate Granites

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Tectonic significance

The Mengkete quartz diorites (rock mass) formed between 441.0 ± 2.6 Ma and 435.9 ± 3.9 Ma and are products of Early Paleozoic island-arc magmatism related to Proto-Tethyan subduction and orogenesis. The Mengkete quartz diorites are in fault contact with the surrounding pre-Tiassic strata. All of this indicates that the Buqingshan area had a complex tectonic evolutionary history and experienced a series of tectonic events. The different stages of this history are described in detail below.

Stage 1: the beginning of the Neoproterozoic to 516 Ma (Fig. 12a). At present, the Early Paleozoic ophiolites in the BTMB are represented by Delisitan ORB ophiolites (DO, gabbro age is 516.4 ± 6.3Ma–467.2 ± 0.9Ma, Bian et al. 1999a; Liu et al. 2011a, abbreviations explained in Fig. 12), A’nyemaqen Majixueshan ORB ophiolite (gabbro 535 ± 10Ma, Li et al. 2007), and Kuhai oceanic island gabbro (555 ± 9Ma, Li et al. 2007). This is evidence that the Proto-Tethyan Ocean opened in the Early Cambrian. The Early Paleozoic ophiolite in the Buqingshan area is likely to be related to the break-up of the Rodinia supercontinent during late Neoproterozoic to Early Cambrian. This shows that in the Early Paleozoic, the East Kunlun Block, West Qinling Block, and Bayan Hara Block were in a discrete from each other and the southern margin of the East Kunlun was been an open “Buqingshan paleo-ocean basin” (Fig. 12a, Li et al. 2007; Feng et al. 2010; Liu et al. 2011a, b, c; Pei et al. 2018; Yu et al. 2020). A contemporaneous ophiolite was also found in the East Kunlun area, indicating that the Central of East Kunlun Ocean was also opened (e.g., Qingshuiquan ophiolite, 522.3 Ma–507.7 Ma, Yang et al. 1996; Chen et al. 2008; Qingshuiquan-Tatuo ophiolites, 516 Ma–485 Ma, Li et al. 2021; Qushi'ang Ophiolite, 505 Ma–498 Ma, Li et al. 2019b; Kekesha-Kekekete mafic-ultramafic melange belt, 509.4 ± 6.8 Ma, Feng et al. 2010). In addition, the Late Sinian Dundeshaerguole Pluton (DP, 544.8 ± 7.8 Ma, Li et al. 2018a, Fig. 12a) of the Kekesha area also exhibits evidence of the extension-related magmatic events that occurred in the East Kunlun area.

Fig. 12
figure 12

Tectonic evolution cartoon of Buqingshan–A’nyemaqen Ocean in Paleozoic–Early Mesozoic (modified from Pei et al. 2018) in the southern margin of the EKOB. SEKB, Southern East Kunlun Block; NEKB, Northern East Kunlun Block; CEKB, Central East Kunlun Belt; BHB, Bayan Har Block; BTMB, Buqingshan–A’nyemaqen Tectonic mélange belt; QKO, Qingshuiquan-Kekesha Ophiolite, 522–509 Ma, after Lu et al. (2002), Yang et al. (1996), Feng et al. (2010), Li et al. (2013b). DO-Delisitan Ophiolite, 516.4 Ma, after Liu et al. (2011c); KO-Kekekete Ophiolite, 501 Ma, Li et al. (2013b); OO-Ordovician Ophiolite, 467–450 Ma, after Pei et al. unpublish; EHO-Eastern Haerguole Ophiolite, 332.8 Ma, after Liu et al. (2011c); DP-Dundeshaerguole Plutons, 544.8 Ma, Li et al. (2018a); KP-Kekesha Pluton, 515.2 Ma, after Zhang et al. (2010a); MP-Manite Plutons, 487–479 Ma, after Li et al. (2017); BV-Bairiqiete volcanic, 437.7 Ma, after Liu et al. (2011a); MDP-Mengkete Pluton, 441.0~435.9 Ma, this paper; HNP-Helegang Naren Pluton, 425.0 Ma, after Li et al. (2013a), A-type granite; HS-Haerguole Seamount Basalt age 340.8 Ma, after Yang et al. (2014); CMAP-Late Permian-Middle Triassic Continental margin arc type Plutons, 260–240 Ma, Liu et al. (2004), Liu (2008), Yang et al. (2005), Sun et al. (2009), Ma et al. (2015), Chen et al. (2017), Xiong et al. (2015), Li et al. (2018b). HXP-Helegang Xilikete Pluton, 225 Ma, after Chen et al. (2013a); GP-Gerizhuotuo Pluton, 225.8–224 Ma, after Li et al. (2013c); Liu et al. (2015); KEP-Kekeealong Plutons, 218.3 Ma, Chen et al. (2013b); IBD-intermediate-basic dikes, 205 Ma, after Li et al. (2019a)

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Stage 2: from 516 to 441 Ma (Fig. 12b). As the Buqingshan paleo-ocean basin continued to expand, the Buqingshan ocean basin began to subduct toward the north in the Late Cambrian. The Late Cambrian–Early Ordovician Manite island-arc granite (Fig. 12b, MNP, 487–479 Ma, Li et al. 2017), Deerni island-arc diorite (493 ± 6Ma, Li et al. 2007) formed in the BTMB at this time. At the same time, the intermediate-acid arc igneous complex formed in the north of the Qingshuiquan–Kekesha–Kekekete area in the East Kunlun area (KP, 515 Ma–527 Ma, Zhu et al. 2002; Zhang et al. 2010a; Zhang 2010).

Stage 3: from 441 to 400 Ma (Fig. 12c). Further subduction within the Silurian Buqingshan–A’nyemaqen Ocean led to the formation of an island-arc magma represented by the Mengkete quartz diorite (441–436 Ma), the Yikehalaer arc granodiorite (YP, 437–402 Ma, Bian et al. 2007; Liu 2011), the Bairiqiete island-arc pluton (BP, 441–439 Ma, Liu 2011, Li et al. 2014b), and the intermediate-to-acidic lava (BV, 437 Ma, Liu et al. 2011c). These island-arc-type magmatic rocks indicate that the Buqingshan–A’nyemaqen Ocean slab continued to subduct northward from the Early Cambrian to the Late Silurian. However, there was little development of the Late Paleozoic magmatic rocks in the Buqingshan and East Kunlun areas (Pei et al. 2018), suggesting that the Late Silurian Buqingshan–A’nyemaqen oceanic ridge spreading activity may have waned and that the oceanic crustal subduction activities may also have waned or stopped. By contrast, the East Kunlun area was affected by the Caledonian orogeny, and the Central of East Kunlun Ocean Basin was closed in the Late Silurian. At this point, the NEKB and SEKB collided (Chen et al. 2002b, 2007; Yin and Zhang 2003; Li et al. 2013b; Pei et al. 2018), which led to the Central of East Kunlun ophiolites thrust in place, accompanied by strong deformation and metamorphism (Meng et al. 2017, Fig. 12c). During the period 425–400 Ma, post-collision plutons, A-type granites, and Maoniushan molasse assemblage in the EKOB were formed (Liu et al. 2012; Xu et al. 2007; Lu et al. 2010; Zhang et al. 2010b; Li et al. 2013a; Chen et al. 2020; Dong et al. 2020; Li et al. 2020; Wang et al. 2022). Most researchers consider that when the Early Paleozoic oceanic basin was closed, the stress regime in the north–south direction transformed from compression to extension. This represented the end of the Early Paleozoic Proto-Tethyan evolution and the beginning of Late Paleozoic Paleo-Tethyan evolution (Yang et al. 2004; Ren 2004; Zhu et al. 2006; Mo et al. 2007; Chen et al. 2007, 2008; Mo 2010; Li et al. 2012; Xiong et al. 2014).

Stage 4: from 400 to 240 Ma (Fig. 12d). Carboniferous island/seamount basalt–limestone assemblages are present in the BTMB. The eastern Haerguole ophiolite (332 Ma) is present in the BTMB along with the Deerni ophiolite (308–345Ma) in the A’nyemaqen area (Chen et al. 2001; Yang et al. 2004, 2005). The presence of these ophiolites indicates that the Buqingshan–A’nyemaqen Ocean still existed in the Early Carboniferous. In the Late Permian, the Buqingshan–A'nyemaqen ocean once again began to subduct under the East Kunlun Block (Yang et al. 2005). Between 240 and 260 Ma, continental margin arc-type plutons (CMAP) caused by the subduction of oceanic crust is distributed in an east–west direction in the East Kunlun area (e.g., Liu et al. 2004; Yang et al. 2005; Sun et al. 2009; Li et al. 2018c; Ma et al. 2015; Chen et al. 2017; Zhao et al. 2019; Kong et al. 2020; Yu et al. 2020; Zhou et al. 2020; Li et al. 2022 Fig. 12d). This stage is akin to Andean continental margin subduction.

Stage 5: from 240 to 205 Ma (Fig. 12e). The Gerizhuotuo diorite with attribute of "stitching pluton" was found in the Buqingshan area (GP, 225–224 Ma, Li et al. 2013c; Liu et al. 2015). The Kekeealong Pluton and Helegang-Xilikete Pluton in the SEKB (KP, Chen et al. 2013a; HXP, Chen et al. 2013b), the Elashan Formation rhyolite, which is high in Nb and Ta, and the gabbro representative of the post-collision extension magmatic rock (Luo et al. 2002; Ding et al. 2011) indicate that the 225 Ma Buqingshan–A'nyemaqen Ocean had closed by this time. The late Late Triassic massive intermediate-basic dikes represent the latest tectonic magmatism in the BTMB (IBD, 205Ma, Li et al. 2019a). During this period, an orogenic collision event finally formed a series of NNE‒SSW striking thrust faults (Fig. 2), and tectonic blocks of different ages and genesis mixed with the Maerzheng Formation flysch turbidite, thus forming the basic tectonic framework of the Buqinshan–A’nyemaqen complex accretive type tectonic mélange belt (Fig. 12e). At the same time, the entire EKOB and main structural framework of the Central Orogenic Belt of China were created (Pei et al. 2018). Therefore, it can be seen that the BTMB at the southern margin of the EKOB records the long-lived history of the subduction and accretion that occurred during the Late Neoproterozoic, Paleozoic, and Early Mesozoic, as well as the final closure of the Buqingshan–A'nyemaqen Ocean in the late Middle Triassic. It also records the evolution of the Proto-Tethys Ocean and Paleo-Tethys Ocean at the southern margin of the EKOB.

Conclusion

From a comprehensive study of the Mengkete quartz diorites in the BTMB based on geochronological, geochemical, and zircon Hf isotopes data, we conclude the following

  • 1. The LA-ICP-MS zircon ages of the Mengkete quartz diorites (rock mass) are between 441.0 ± 2.6 Ma and 435.9 ± 3.9 Ma), indicating that the intrusion formed in the Early Silurian.

  • 2. The Mengkete quartz diorites have high Al2O3, low alkali, and medium K2O contents and belong to the medium-K calc-alkaline series. The zircon Hf isotope compositions gave a TDM2 model age range of 1130–657 Ma, indicating that the diorites are derived from the partial melting of mafic rocks in the Meso-Neoproterozoic lower continental crust.

  • 3. The BTMB in the southern margin of the EKOB had a long-lived subduction and accretion history during the Late Neoproterozoic, Paleozoic, Early Mesozoic and finally closed in the late Middle Triassic. The evolution of both the Proto-Tethyan Ocean and the Paleo-Tethyan Ocean is recorded here.