1 Introduction

The Altyn orogenic belt (hereafter AOB) is located at the southwest margin of the Central Asian orogenic belt. It is a complex orogenic belt composed of a series of continental blocks, island arcs, and accretionary units, that extends for ~ 1000 km across China, Russia, Kazakhstan and Mongolia (Li et al. 2020). Moreover, this composite orogenic belt comprises geological units of different geological periods, a variety of structural levels and that formed in distinct tectonic environments. The dominant structures within the AOB are those located in the northern margin of the Qinghai-Tibet Plateau, the southeast margin of the Tarim Basin, the western margin of the Qaidam Basin and the Qilian Kunlun orogenic belt (Luo et al. 2009), whereas the southern part of the belt is limited by the giant Altyn sinistral fault (Wu et al. 2016). From north to south, the AOB can be subdivided into five tectonic units: the North Altyn Block, the North Altyn ophiolitic melange belt, the middle Altyn massif, the South Altyn high pressure and ultra-high- pressure (HP–UHP) metamorphic belt, and the Apa-Mangya ophiolite tectonic melange belt (Che and Sun 1996; Wang 1997; Xu et al. 1999; Cui et al. 2002; Liu et al. 2009a, b, 2015; Yang et al. 2012; Wang et al. 2011; Kang et al. 2013; Chen 2018).

The AOB experienced Archean to Paleoproterozoic continental core and crystalline basement formation (Lu and Yuan 2003), plate convergence and collision in the early Neoproterozoic (Qin et al. 2006), plate expansion in the late Neoproterozoic to Early Paleozoic (Liu et al. 1998, 1999), followed by Caledonian plate subduction and collision (Liu et al. 2015; Kang et al. 2016a, b; Wu et al. 2018) and lastly, late Yanshanian large-scale, sinistral faulting (Guo et al. 2008; Wu et al. 2013). As an important part of the northern margin of the Qinghai-Tibet Plateau, the main fault of the southern Altyn Tagh (hereafter South Altyn) forms a principal zone of sinistral faulting in Central Asia. As such, a comprehensive understanding of the formation and evolution history of the South Altyn is of great scientific significance to the division of geological structures in both Northwest China and the Central Asian continent (Wang et al. 2019). For example, this complex tectonic belt owes its formation to the subduction and collision of paleo-plates (or blocks) in the Early Paleozoic and experienced a convoluted tectonic evolution process in the Mesozoic (Chen et al. 1995, 1998; Liu et al. 1996; Zhang et al. 1999a, b, 2001a, b, c; Xu et al. 1999; Cui et al. 1999; Ni et al. 2008; Kang et al. 2013; Li et al. 2015). The Early Mesozoic (Triassic) outcrops in this area experience uplift and denudation, while the Middle and Late Mesozoic (Jurassic to Cretaceous) resulted in rifting and a stage of sedimentation (Huang et al. 2004). In recent years, the South Altyn area has become a hot spot for geologists (Zhao et al. 2018). However, the related research has mainly focused on the high-pressure and ultra-high-pressure metamorphism (Liu et al. 1997, 1998, 2002, 2003, 2004, 2005, 2007a, b, c, 2009a, b, c, 2012; Zhang et al. 1999a, 2001a, b, 2002a, b, 2004, 2005; Zhang and Meng 2005; Cao et al. 2009; Wang et al. 2011), ophiolites (Li et al. 2009; Ma et al. 2009), and intermediate felsic intrusive rock (Zhang et al. 1999a, b, 2001a, b, 2002a, b, 2007; Cao et al. 2010; Tian 2009; Sun et al. 2012; Yang et al. 2012; Kang et al. 2013, 2016a, b; Wu et al. 2014, 2016; Liu et al. 2015; Pan et al. 2016; Wang et al. 2019; Li et al. 2020). These studies have provided systematic scientific evidence for the tectonic evolution of the south margin of Altyn Tagh during the Nanhua Early Paleozoic Ocean and transition. Nevertheless, research relating to magmatism and igneous rocks are limited primarily to the Paleozoic period (262–504 Ma); discussion of Mesozoic igneous activity has largely been ignored. Because of this, our paper aims to study and discuss the nature and significance of Mesozoic granite and diorite at Qiemo County, southern margin of the Altyn sinistral Fault Zone. These investigations include zircon U–Pb dating, whole-rock major and trace element compositions, coupled to Sr–Nd–Pb isotope and zircon Hf isotope studies. Based on the above research, a credible genetic age and origin are reasonably determined.

2 Regional geological background and sample petrological characteristics

The protracted evolution of the AOB includes this having experienced ancient Archean crust formation and multi-stage magmatic activities, strong transformation, and intermediate-mafic magmatism during the Paleoproterozoic (2.5–1.8 Ga), Neoproterozoic (1.0–0.8 Ga) collisional orogeny and large-scale magmatism (Wang et al. 2006, 2011; Liu et al. 2009a, b, c), as well as, complex, structural belt formation by subduction and collision of ancient plates (or blocks) in the Early Paleozoic, that were later transformed by a Mesozoic–Cenozoic sinistral fault system. The South Altyn is located between the southern Altyn sinistral Fault and the southern margin fault of Altyn (Liu et al. 1998; Wang et al. 1999), it differs from the Sulu-Dabie ultra-high pressure metamorphic belt which represents an area of deep subduction collision within the Yangtze Craton (Suo et al. 2004).

The present study area is located in the complex rock of the Ananmanya tectonic belt (Fig. 1), mainly comprising old metamorphic rocks (500–1000 Ma; granite and granite gneiss; Liu et al. 2007a, 2015; Lu et al. 2008; Wang et al. 2008; Song et al. 2012; Fan et al. 2019), such as the Paleo-Proterozoic Altyn Group, middle Proterozoic Bashkorgan Group, and the Neoproterozoic Solcuri Group, as well as, Mesozoic-Jurassic, Cenozoic-Paleogene, Neogene and Quaternary systems. Mafic–ultramafic rocks and intermediate-felsic rocks of the Jinning, Caledonian, Hercynian, and Yanshanian periods are very well developed in the study area, and they are distributed in a beaded pattern along the southern edge of the South Altyn, forming relatively large rock units. In addition, intermediate-felsic rocks are mainly distributed in the southern part of the main fault zone.

Fig. 1
figure 1

a Tectonic divisions of west China (Liu et al. 2012), b geological and tectonic map of the Altyn Tagh Orogen (Liu et al. 2012), and c geological map of the southeastern Altyn

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Samples for this study were collected at outcrop from Chimo County, South Altyn in the Xinjiang Uygur Autonomous Region, China (Fig. 1). The diorite (sample 17A-44-1-10) has a semi-autochthonous medium-fine-grained equigranular structure. Its mineral composition includes plagioclase, K-feldspar, hornblende, biotite, and minor quartz (< 5 %) (Fig. 2). Locally, diorite is commonly observed interlayered with marble (~ 4.0 m). The sample 17A-44-1 is selected for zircon separation. In contrast, the granite (e.g., porphyritic granite, K-feldspar granite, and gneissose granite) (sample 17A-45-1-12) has a coarse-grained equigranular structure, and its mineral composition includes semi-autochthonous quartz (45–50 %, 0.2–1.0 cm), autochthonous- semi-autochthonous K-feldspar (35–40 %) and plagioclase (5–10 %) (Fig. 2). Some outcrops of the porphyritic granite have obvious mylonitization and are cut by mafic intrusions (dykes), whereas the gneissose granites have xenoliths within of dark gabbro. Both the dark inclusions and granites have suffered deformation, and each contains tourmaline. And the sample 17A-45-3 is selected for zircon separation.

Fig. 2
figure 2

Representative cathodoluminescence (CL) images for zircon in the rocks from the Xinjiang Uygur Autonomous Region of South Altyn

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3 Analytical procedures

3.1 U–Pb dating by laser-ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) methods

Zircon from five of the investigated Xinjiang Uygur Autonomous Region samples was separated using conventional heavy liquid and magnetic techniques. Representative zircon grains were then hand-picked under a binocular microscope before being mounted in an epoxy resin disc, polished, and then coated with gold, before the analysis. Individual crystals were studied using optical microscopy techniques and under cathodoluminescence (CL) to aid in characterization and to reveal any internal features. CL imaging (Fig. 2) and the U–Pb analyses were undertaken by LA-ICP-MS methods at the State Key Laboratory of Continental Dynamics, Xi’an, China. The analytical procedures used were those as described in detail in Harris et al. (2004); a spot diameter of 29 μm was used. U–Th–Pb ratios and absolute abundances were determined by reference to replicate measurements of a standard TEMORA zircon and the NIST 610 glass standard (Figs. 3, 4).

Fig. 3
figure 3

The corresponding LA-ICP-MS U–Pb concordia diagrams for the rocks studied from the Xinjiang Uygur Autonomous Region of South Altyn

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Fig. 4
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Representative photomicrographs of the studied granitic rock from the Xinjiang Uygur Autonomous Region of South Altyn. Key: Q: quartz, Pl: plagioclase, Bi: biotite, Hb: hornblende, Kfs: K-feldspar

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3.2 Major elemental and trace elemental analyses

Whole-rock major element compositions were determined using analytical Axioms-advanced X-ray fluorescence (XRF) spectrometer at the State Key Laboratory of Ore Deposit Geochemistry (SKLODG), Guiyang, China with an analytical precision of better than 5 %. Trace element compositions were determined by Inductively-coupled plasma mass-spectrometry (ICP-MS) utilizing a Perkin-Elmer ELAN DRC-e instrument at the SKLODG. Prior to analysis, powdered samples (50 mg) were dissolved in high-pressure Teflon bombs, using an HF + HNO3 acid attack for 48 h, at a temperature of ~ 190 °C (Qi et al. 2000). Signal drift was monitored during the analysis by reference to an Rh internal standard. GBPG-1, OU-6, GSR-1, and GSR-3 standards were additionally used for analytical quality control with a determined analytical precision of better than 5 %.

3.3 Sr–Nd–Pb isotopic analyses

For Rb–Sr and Sm–Nd isotope analyses, sample powders were spiked with mixed isotope tracers, following dissolution with HF + HNO3 acids (in Teflon bombs). The isotopes were separated by conventional cation-exchange techniques. Isotopic measurements were performed using a Finnigan Triton Ti thermal ionization mass spectrometer (TIMS) at the SKLODG. Procedural blanks yielded concentrations of < 200 pg for Sm and Nd, and < 500 pg for Rb and Sr, respectively. The mass fractionation corrections for Sr and Nd isotopic ratios were based on 86Sr/88Sr = 0.1194 and 146Nd/144Nd = 0.7219. Analysis of the NBS987 and La Jolla standards yielded the following results: 87Sr/86Sr = 0.710246 ± 16 (2σ), and 143Nd/144Nd = 0.511863 ± 8 (2σ), respectively. Prior to Pb isotopic analysis, Pb was separated and purified by conventional cation-exchange techniques, using diluted HBr as an eluent. Analysis of the NBS981 standard yielded mean values for 204Pb/206Pb of 0.0896 ± 15, 207Pb/206Pb of 0.9145 ± 8, and 208Pb/206Pb of 2.162 ± 2.

3.4 In-situ zircon Hf isotopic analysis

In-situ zircon Hf isotopic analyses were conducted using a Neptune multi-collector system (MC-ICP-MS), equipped with a 193 nm laser, at the Institute of Geology and Geophysics, Chinese Academy of Sciences in Beijing, China. During the analysis, a laser repetition rate of 10 Hz at 100 mJ was used with spot sizes of 32 and 63 μm. Raw count rates for 172Yb, 173Yb, 175Lu, 176(Hf + Yb + Lu), 177Hf, 178Hf, 179Hf, 180Hf, and 182W were collected and isobaric interference corrections for 176Lu and 176Yb on 176Hf were determined precisely.176Lu was calibrated using the 175Lu value and a correction was made to 176Hf. The 176Yb/172Yb value of 0.5887 and mean βYb value obtained during Hf analysis on the same spot were applied for the interference correction of 176Yb on 176Hf (Iizuka and Hirata 2005). Details of the analytical techniques employed are described in Xu et al. (2004) and Wu et al. (2006). During the analysis, the determined 176Hf/177Hf and 176Lu/177Hf ratios of the standard zircon (91500) were 0.282300 ± 15 (2σn, n = 24) and 0.00030, respectively, which are similar to the commonly accepted 176Hf/177Hf ratio of 0.282302 ± 8 and 0.282306 ± 8 (2σ) measured using the solution method (Goolaerts et al. 2004; Woodhead et al. 2004).

4 Results

4.1 Zircon U–Pb dating

Clean, prismatic grains of euhedral zircon in samples 17A-44 and 17A-45 series display evident oscillatory zoning, suggesting that these were the products of a crystallizing magma. A total of 17 zircon grains provided a weighted mean 206Pb/238U age of 238.8 ± 1.1 Ma (1σ) (95 % confidence interval, MSWD = 3.6) for 17A-44 (Table 1), whereas 17 zircon grains from sample 17A-45 gave a weighted mean 206Pb/238U age of 238.0 ± 1.5 Ma (1σ) (95 % confidence interval, MSWD = 5.2) (Table 1). These determinations are the best estimates for the crystallization ages of the investigated intermediate and felsic intrusive rocks from the Xinjiang Uygur Autonomous Region. No inherited zircon characteristics were observed in the investigated sample populations.

Table 1 LA-ICP-MS U–Pb isotopic data for zircon from the studied rocks of Xinjiang Uygur Autonomous Region of South Altyn
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4.2 Major and trace elements

Whole-rock geochemical data for the studied rocks are presented in Tables 2 and 3. The diorite samples exhibit a fairly narrow range of compositions (Table 2); each is situated within the alkaline field in terms of the total alkali-silica diagram (Fig. 5). By contrast, the granite samples exhibit a relatively wide range of compositions (Table 2). While all of the granite samples also fall into the alkaline field in terms of the total alkali-silica diagram (Fig. 5a), they additionally reside within the shoshonitic series field in terms of a plot of Na2O versus K2O (Fig. 5b) and are metaluminous (A/CNK = 0.7–1.0; Fig. 5c) in terms of aluminum saturation (Maniar and Piccoli 1989; Ji et al. 2016). Moreover, all samples studied are characterized by light rare earth element (LREE) enrichment and heavy rare earth element (HREE) depletion, with a narrow range of Eu/Eu* (0.73–1.05) and high (La/Yb)N ratios (61–169) (Table 3 and Fig. 6a, b). On primitive mantle-normalized trace element diagrams, the studied rocks show enrichment in LILEs (i.e., Rb, Ba, Sr, K), Th, U and Pb, and depletion for HFSEs (i.e., Nb, Ta, Hf, and Ti) (Fig. 6b).

Table 2 Major oxides (wt %) of the studied rocks from Xinjiang Uygur Autonomous Region of South Altyn
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Table 3 The trace elements analysis results (ppm) for the studied rocks from Xinjiang Uygur Autonomous Region of South Altyn
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Fig. 5
figure 5

Classification of the granitic rocks from the Xinjiang Uygur Autonomous Region on the basis of: a the total-alkali versus SiO2 (TAS) diagram. All the major element data have been recalculated to 100 % on a LOI-free basis (Middlemost 1994; Le Maitre 2002); b K2O versus Na2O diagram, showing the alkaline association to be shoshonitic (Middlemost 1972); and (c) Al2O3/(Na2O + K2O) molar versus Al2O3/(CaO + Na2O + K2O) molar plot (Maniar and Piccoli 1989)

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Fig. 6
figure 6

a Chondrite-normalized REE diagrams: b primitive mantle-normalized trace element distribution spiderdiagrams. The normalization values are from Sun and McDonough (1989)

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4.3 Sr–Nd–Pb isotopes

Sr, Nd, and Pb isotopic data for 14 representative rocks from this study are presented in Tables 4, 5 and Figs. 7, 8a, b. The diorite samples exhibit a wide range in (87Sr/86Sr)i values of between 0.7062 to 0.7090 and wide variation in εNd (t) values, from − 9.1 to − 11.3. The granite samples similarly exhibit a wide range in (87Sr/86Sr)i values of between 0.705 to 0.7114 and wide variation in εNd (t) values, from − 8.8 to − 10.5. These data are suggestive of source areas with slight enrichment. The investigated diorite rocks display relatively constant Pb isotopic ratios of: (206Pb/204Pb)i = 16.61–17.88, (207Pb/204Pb)i = 15.56–15.58 and (208Pb/204Pb)i = 37.47–38.04. The investigated granite units also display relatively constant Pb isotopic ratios of: (206Pb/204Pb)i = 17.03–17.76, (207Pb/204Pb)i = 15.45–15.55 and (208Pb/204Pb)i = 37.10–37.69.

Table 4 Sr-Nd isotopic compositions for the granite and diorite rocks in this study
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Table 5 Pb isotopic compositions for the rocks in this study
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Fig. 7
figure 7

Initial 87Sr/86Sr versus εNd (t) diagram for the rocks studied from the Xinjiang Uygur Autonomous Region of South Altyn, China

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Fig. 8
figure 8

208Pb/204Pb (a) and 207Pb/204Pb (b) versus 206Pb/204Pb diagrams for the rocks studied from the Xinjiang Uygur Autonomous Region, China. Fields for I-MORB (Indian MORB) and P&N-MORB (Pacific and North Atlantic MORB), OIB, NHRL and 4.55 Ga geochron are after Barry and Kent (1998), and Hart (1984), respectively

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4.4 Zircon Hf isotope analysis

The results for zircon Hf isotope analyses in the studied samples are listed in Table 6. Twenty-five spot analyses were obtained for sample 17A-44, yielding very uniform εHf (t) values of between −8.7 and −11.2, which correspond to TDM2 model ages of between 1814  and 1976 Ma (Figs. 9, 10). Twenty-five spot analyses were obtained for sample 17A-45; they show a lower range of εHf (t) values of between −14.5 and −18.7, corresponding to TDM2 model ages of between 2182  and 2440 Ma (Figs. 9, 10).

Table 6 Zircon Hf isotopic compositions of the rocks in this study
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Fig. 9
figure 9

Age versus εHf (t) plot for the zircons from the rocks studied from the Xinjiang Uygur Autonomous Region, China

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Fig. 10
figure 10

Histograms of zircon εHf (t) values and two-stage Hf model ages for the investigated granite and diorite rocks in this study

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5 Discussion

As one of the most widely distributed rock types and an important sign of continental crustal growth, intermediate-felsic igneous rock, especially granite, is an excellent window and research object in studies of the growth and tectonism of continental crust (Xiao et al. 2005). For example, based upon the study of granites in the Lachlan fold belt of Australia, the classification of the S-type, I-type, A-type, and M-type granites were proposed (Chappell and White 1974; White and Chappell 1983). A-type is used to describe felsic rocks, which in addition to appearing in anorogenic tectonic settings, are more alkaline. A-type granites appear to be polygenetic, with no single process accounting for them all. Such magmas can form through melting of the lower crust under conditions that are usually extremely dry, or in the fractionation of basaltic magma. M-type covers those granites that derive from mafic or intermediate magmas, generally sourced from the mantle. These are rare, usually occurring only in oceanic crust within an ophiolite suite, and mostly associated with basalt and meta aluminous plagioclase granite. In general, the aluminous saturation index (A/CNK; Maniar and Piccoli 1989) is used to delineate the boundary between I-type (igneous protolith) granite and S-type (sedimentary protolith) granite. Rocks with an A/CNK of > 1.1 are strongly peraluminous, and typically belong to the S-type granite; a value for A/CNK of < 1.1 is weakly aluminous and representative of I-type granite.

5.1 The source of the Xinjiang Uygur Autonomous Region magmas

The rocks investigated from Xinjiang Uygur Autonomous Region are characterized by the following isotopic compositions: high (87Sr/86Sr)i = (0.7062–0.7114), (206Pb/204Pb)i = (16.74–17.88), (207Pb/204Pb)i = (15.51–15.58), and (208Pb/204Pb)i = (35.36–38.04), negative εNd (t) and εHf (t) values of (− 8.8 to − 11.3, and −8.7 to −11.2), and high (La/Yb)N ratios of 61–169 (see Table 3, 4, 5, 6; Figs. 6a, b, 7, 8a, b, 9, 10), implying that they were derived from a relatively enriched magma source area. In addition, the diorite and granite samples have negative εHf (t) (− 8.7 to − 11.2, and − 14.5 to − 18.7) relating to an older two-stage model age (1.8–2.0 Ga, 2.2–2.4 Ga), which indicates that the rocks likely derived from an ancient crustal source (Taylor and McClennan 1985; Wu et al. 2007). This is further supported by their higher SiO2 contents (50.27–72.96, Table 2). Although the rocks studied have similar REE, trace element and isotopic characteristics, there are also some important differences. Such characteristics indicate that the granite and diorite investigated from Xinjiang Uygur Autonomous Region may have two different sources or origins.

To decipher if and how mantle materials may have participated in the genetic process of these rocks requires some explanation. In general, there are two ways for mantle material to influence a source area: 1. The mantle-derived components can provide heat input to induce partial melting of crustal materials and thereby produce a spectrum of magma compositions, depending upon the heterogeneity of source materials. As such, felsic magma may directly be linked at its source to the formation of diorite (Griffin et al. 2002; Kemp et al. 2007; Zhao et al. 2010, 2012). 2. The lower crust was formed through the underplating of mantle-derived components, and then partially melted to form diorite under the influence of later thermal events (Jahn et al. 2000; Wu et al. 2006; Zheng et al. 2007). Generally, the two-stage model ages of igneous rocks are quite different from their metamorphic ages; the possibility of the second mode of genesis thus is plausibly ruled out in this study.

5.2 Fractional crystallization

On Harker plots (Fig. 11), with increasing SiO2 content, MgO, TiO2, and Fe2O3 decrease, which shows a typical magmatic mixing or fractional crystallization (e.g., K-feldspar and hornblende) evolutionary trend. Moreover, Na2O does not change greatly with an increase of SiO2, but Al2O3 and CaO decrease as SiO2 increases. The negative anomalies observed for Eu and Sr in the investigated rocks (Table 3, Fig. 6a, b) indicate that plagioclase crystallization was important during magma evolution. However, the fractional crystallization of plagioclase in granite is relatively weaker than for diorite, reflecting its lower Sr contents (Table 3, Fig. 6a, b).

Fig. 11
figure 11

SiO2 versus MgO, TiO2, and Fe2O3 plots for the rocks studied from the Xinjiang Uygur Autonomous Region, China

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5.3 Origins

There are clear negative correlations between MgO, TiO2, Fe2O3 and SiO2 contents (Fig. 11a–c) for the Xinjiang Uygur Autonomous Region diorite and granite samples as studied, indicating that the separation and crystallization of mafic minerals (mainly amphibole and to a lesser extent biotite) accompanied their evolution (Wang 2010; Wang et al. 2010). This observation is further supported by Th enrichment and Nb–Ta depletion in the investigated intermediate-felsic igneous rocks (Fig. 6b; Wu et al. 2001). In addition, these rocks are characterized by high Rb, Ba, Th, U, K, and LREE contents (Table 2; Fig. 6a, b), high to very high Zr/Hf ratios (133–3477), low Mg# values (29–46, Table 2), and depletion in Nb, Ta, Ti, and HREE (Fig. 6). Generally, the experimental petrology theory shows that the CaO/Na2O ratio can be used to distinguish the characteristics of intermediate to felsic magmatic rocks (Kang et al. 2016a, b). The CaO/Na2O ratio for the investigated diorite samples falls between 0.95 and 1.31, indicating that the original rock (as melted) should be clastic rock with a small proportion of mudstone. By contrast, except for samples, 17A-45-6 and 17A-45-8, the relatively high CaO/Na2O (0.55, 0.54), the high CaO/Na2O ratio (0.15–0.27) of the granite indicates that the original rock should be a feldspar poor and clay-rich mudstone. Moreover, the investigated Xinjiang Uygur Autonomous Region rocks have high zircon contents and determined saturation temperatures of (T = 731–909 °C), which suggests that the zircon in the parent magmas reached saturation. Such a temperature range likely represents the initial magma temperature of their parental magmas (Miller et al. 2003; Zhao 2010). Further, in the SiO2 versus TiO2 temperature diagrams (Fig. 12), the determined temperature of both magmas is lower than 900 °C.

Fig. 12
figure 12

The SiO2 versus TiO2 temperature diagram for the rocks studied from the Xinjiang Uygur Autonomous Region, China

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The Xinjiang Uygur Autonomous Region granite samples have relatively high SiO2 (65.67–72.96 wt %), Al2O3 (> 14.53 wt %), K2O (> 5.03 wt %), and K2O + Na2O (9.46–11.81 wt %) values (Table 2). By contrast, the studied diorite from this area is characterized by relatively low SiO2, K2O, and K2O + Na2O values (Table 2). The high CaO/Na2O ratio (0.95–1.31) indicates that the crust involved in magma generation should be at less than 30 km depth or crustal thickness (Zhang et al. 2006). The continued thickening of the Earth’s crust in this region of Asia resulted in lithospheric extension and collapse, leading to large-scale upwelling of hot asthenosphere materials. This rise of the asthenosphere can induce crustal melting. The resulting parent melts, after a certain degree of fractional crystallization, may buoyantly rise to be emplaced along with extensional fractures, to coalesce forming a large number of intermediate-felsic intrusions of Mesozoic age in the Xinjiang Uygur Autonomous Region. Thus, we envisage complex tectonism resulted in the conditions necessary for the partial melting of crustal materials, providing the source magmas to the diorite and granite investigated herein.

6 Conclusions

Integrated zircon U–Pb geochronology, whole-rock geochemistry and Sr–Nd–Pb–Hf isotopic studies of a suite of intermediate-felsic igneous rocks from within the Xinjiang Uygur Autonomous Region of South Altyn, China allow us to draw the following conclusions.

  1. 1.

    The diorite and granite rocks from the Xinjiang Uygur Autonomous Region study area were intruded during the Triassic as evidenced in the newly determined zircon U–Pb ages, of 238.8 ± 1.1 and 238 ± 1.5 Ma.

  2. 2.

    All of the investigated rocks have an alkaline affinity. They are enriched in LREE, and select LILE (e.g., Rb, Ba, Sr, K), Th, U, and Pb, and depleted in HFSEs (i.e., Nb, Ta, Hf, and Ti) relative to a primitive mantle. The Xinjiang Uygur Autonomous Region granite and diorite have high initial 87Sr/86Sr ratios (0.7062–0.7114), negative εNd (t) values (− 8.8 to − 11.3), εHf (t) values (−8.7 to −18.7), and relatively constant Pb isotopic ratios [(206Pb/204Pb)i = 16.74–17.884, (207Pb/204Pb)i = 15.51–15.58, and (208Pb/204Pb)i = 35.36–38.04]. These data suggest that the magmas parental to these rock suites were generated by partial melting of the crust.

  3. 3.

    Based upon our findings, we suggest that the investigated Mesozoic granite and diorite rocks from the Xinjiang Uygur Autonomous Region owe their origins to crustal thickening and extensional relaxation, which promoted upwelling of asthenosphere mantle. The uplifted hot mantle caused a rise in the geothermal gradient of the overlying crust and corresponding partial melting of heterogeneous lithologies. The resulting parental magmas of intermediate and felsic composition ascended through the crust to be emplaced as granite and diorite igneous rocks in the Xinjiang Uygur Autonomous Region of South Altyn, China. Such extensional tectonics also promoted crustal thinning and possible rifting, providing important pathways for magma ascent and emplacement.