Introduction

The northern Alxa region is located west of Inner Mongolia, China and links the central part of the southern Central Asia Orogenic Belt (CAOB) to its eastern and western parts (Fig. 1a, b). This is also where the North China Plate, the Tarim Plate and/or the Kazakhstan Plate meet (Fig. 1b; Wang et al. 1994; Wu et al. 1998). Its evolution is related to the development of the Paleo-Asian Ocean and its continental margins (Kröner et al. 2012, 2014; Şengör et al. 1993; Windley et al. 2007; Wilhem et al. 2012; Xiao et al. 2009a, b, 2013). The geological evolution of the area is so complicated that the relationships between the three plates and their Paleozoic evolution remain highly controversial (Wang et al. 1994; Wu et al. 1998; Zheng et al. 2014). Three important ophiolitic belts or sutures are reported in the northern Alxa region (Fig. 1c). From north to south, they include the Yagan Suture, the Engger Us Ophiolitic Belt and the Qagan Qulu Ophiolitic Belt (Wang et al. 1994; Wu and He 1992; Wu et al. 1998). The Engger Us Ophiolitic Belt is regarded as the suture between the North China Plate and the Tarim Plate (Wang et al. 1994; Wu and He 1992; Wu et al. 1998) or the major suture between the northern margin of the Alxa block (NMAB) and the CAOB (Zheng et al. 2014). The NMAB is usually considered to be the western part of the North China Plate (Zhao et al. 2005), but in recent years many authors have suggested that the Alxa Block is not a part of the North China Craton (e.g., Dan et al. 2014; Zhang et al. 2011). The timing of obduction of the Engger Us Ophiolitic Belt and the Qagan Qulu Ophiolitic Belt has not been confirmed (Wang et al. 1994; Wu and He 1993; Zheng et al. 2014), and consequently, the timing of ocean closure represented by them remains controversial.

Fig. 1
figure 1

a The northern Alxa region in China. b The northern Alxa region in the Central Asian Orogenic Belt (after Jahn et al. 2000); CAOB is the white area. c Simplified geological map of the northern Alxa region (after BGGP 1979, 1980, 1981a, b; BGIMAR 1980; BGNHAR 1976, 1980a, b, c, d, 1982a, b). Six tectonic zones from north to south: HZ Huhetaoergai zone, ZZ Zhusileng zone, GZ Guaizihu zone, ZSZ Zongnaishan–Shalazhashan zone, WZ Wuliji zone, YBM Yabulai-Bayinnuoergong zone. References for age data: 1 = this study; 2 = Zheng et al. (2013); 3 = Zhang et al. (2013a); 4 = Yang et al. (2014); 5 = Shi et al. (2014); 6 = Zhang (2013); 7 = Shi et al. (2012); 8 = Geng and Zhou (2012); 9 = Zhang et al. (2013b); 10 = Zhang et al. (2012); 11 = Ran et al. (2012)

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Most of the igneous rocks in the northern Alxa region are granites ranging from Precambrian to Mesozoic in age (Geng et al. 2002; Shi et al. 2014; Wang et al. 1994; Zhang et al. 2013a; Zheng et al. 2013; Yang et al. 2014). Due to the remote environment and tough working conditions, few granites north of the Engger Us Ophiolitic Belt are well studied, which would help to constrain the tectonic interpretation of the northern Alxa region (Wang et al. 1994; Zheng et al. 2013). We present new geochronological and geochemical data from three late Paleozoic granitic plutons in the west of the Huhetaoergai, the Zhuxiaobuguhe and Wudenghan areas (Figs. 2, 3). These data constrain their ages, relationship and petrogenesis and also facilitate our understanding of the adjacent ophiolitic belts and their relationship to the late Paleozoic tectonic evolution of the northern Alxa region.

Fig. 2
figure 2

Huhetaoergai and Zhuxiaobuguhe plutons, northern Alxa region (after BGGP 1980, 1981a)

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Fig. 3
figure 3

Wudenghan pluton, northern Alxa region (after BGGP 1981a, b; BGNHAR 1982a)

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Geological background

Tectonic zones

Six tectonic zones are delimited in the northern Alxa region (Fig. 1c). From north to south, they are the Huhetaoergai, Zhusileng, Guaizihu, Zongnaishan–Shalazhashan, Wuliji and Yabulai-Bayinnuoergong tectonic zones (Wu et al. 1998; Wang et al. 1994; Zhang et al. 2013a). The last unit is interpreted to belong to the northern margin of the Alxa block, whereas the others belong to the CAOB (Shi et al. 2014; Wang et al. 1994; Zhang et al. 2013a; Zheng et al. 2014; Fig. 1). Together the latter three zones comprise a complete trench-arc-basin system of Late Paleozoic age with the ocean represented by the Engger Us Ophiolitic Belt and the back-arc basin represented by the Qagan Qulu Ophiolitic Belt [for more details about the ophiolitic belts, see Zheng et al. (2014)]. The Wuliji granite in the Zongnaishan–Shalazhashan tectonic zone represents the mature arc of this system, similar to today’s Japan island arc (Feng et al. 2013; Wang et al. 1994; Zhang et al. 2013a). The Qagan Qulu Ophiolitic Belt is considered to be the suture formed by the closure of the back-arc basin in the late Paleozoic (Wu and He 1992, 1993; Wu et al. 1998; Zheng et al. 2014).

The three northern zones of the CAOB are less studied. The Huhetaoergai tectonic zone is argued to be a volcanic arc developed on oceanic crust preserved when an ocean closed during the late Silurian–Devonian (Wang et al. 1994) or Permian (Wu et al. 1998). The passive continental margin represented by the Zhusileng and Guaizihu tectonic zones became an active continental margin in the end of the early Paleozoic and was under extension during the Carboniferous–Permian (Wang et al. 1994; Wu et al. 1998). The Yagan Suture is considered to be an important boundary separating strata of different affinities. Rocks in the fault zone are highly fragmented and some ultramafic rocks are present (Wang et al. 1994; Wu et al. 1998; Fig. 1c), but no other ophiolitic components have been recognized so far.

Precambrian strata in the Huhetaoergai and Zhusileng tectonic zones

In the Huhetaoergai tectonic zone, Mesoproterozoic (Stenian–Estasian) strata in the west of the Western Huhetaoergai pluton are the only Precambrian strata exposed (Fig. 2). It consists of sandstone, thick marble, some silicalite and slate; the base contains purple-red sandstone with fine conglomerate (BGGP 1980). Paleoproterozoic–Neoproterozoic strata are found in the Zhusileng tectonic zone (Fig. 3). A widely distributed early Paleoproterozoic marine regression sedimentary sequence in the Jindouaobao area includes from bottom to top magnesium carbonates, intermediate-acidic volcanic rocks and clastic rocks (BGNHAR 1982a). Mesoproterozoic (Stenian–Estasian) strata are composed of metasiltstone in the lower and upper parts and carbonate in the middle; they are in fault contact and only exposed south of the Wudenghan area (BGGP 1981a). Minor late Paleoproterozoic–early Neoproterozoic (Statherian–Calymmian) neritic carbonates and black siliceous rocks, early Neoproterozoic (Tonian) siliceous dolomites and late Neoproterozoic (Cryogenian) glacial till (ca. 90 m) also occur in the Zhusileng tectonic zone (BGNHAR 1982a; Wang et al. 1994).

Paleozoic strata in the Huhetaoergai and Zhusileng tectonic zones

In the Huhetaoergai tectonic zone, Middle Ordovician strata are the only early Paleozoic strata exposed (Fig. 2). They are divided into lower, middle and upper parts: neritic intermediate volcanic rocks, clastic rocks and intermediate-acidic volcanic rocks, respectively (BGGP 1980). The Middle Ordovician strata are not exposed in the Zhusileng tectonic zone (Figs. 1c, 3). However, it preserves a continuous sedimentary record from Cambrian to Middle Silurian (Wang et al. 1994). Thick carbonates, silicalites and flysch reflect a passive continental margin environment (Wang et al. 1994; Wu et al. 1998).

The Lower Devonian to Lower Carboniferous-lower part of Upper Carboniferous strata are dominated by littoral–neritic volcanic and clastic rocks in the Huhetaoergai tectonic zone (Fig. 2). Devonian strata are well developed in the Zhusileng tectonic zone. Its western part mainly consists of neritic clastic rocks and biohermal limestone, and its eastern part is composed of unstable neritic volcanic and clastic rocks. In the Wudenghan area, an early Carboniferous anticline folds gray-green graywacke and feldspathic graywacke with interlayered gray-green andesitic basalts (Fig. 3; BGGP 1981a). Lower Carboniferous strata including neritic volcanic and clastic rocks also occur in the Zhusileng tectonic zone. Permian strata are widely distributed in both the Huhetaoergai and Zhusileng tectonic zones (Fig. 1c; BGGP 1980, 1981a; BGNHAR 1982a). They lie unconformably on Carboniferous strata (Wang et al. 1994). Lower Permian strata in the Huhetaoergai tectonic zone comprise clastic rocks in the lower part and volcanic rocks with graywacke in the upper part, indicating an unstable neritic environment (BGGP 1980, 1981a). It coincides with the widespread Lower Permian Shuangbaotang Formation in the Zhusileng tectonic zone (BGGP 1981a; BGNHAR 1982a). The lower part of Shuangbaotang Formation (previously the Maihanhada Formation) represents a normal neritic–coastal sedimentary environment and consists of clastic rocks and carbonates (BGMRIMAR 1991; Li et al. 1996). Its upper part (previously the Aqide Formation) comprises neritic clastic rocks and eruptive intermediate-acidic volcanic rocks in an oceanic basin (BGMRIMAR 1991; Li et al. 1996). In addition, the Upper Permian Haersuhai Formation is exposed in both the Huhetaoergai tectonic zone and the Zhusileng tectonic zone (BGGP 1980, 1981a; BGNHAR 1982a) and represents a flysch deposited in coastal, neritic and marine–terrestrial environments. It is in fault contact with the other strata (Wang et al. 1994). The lower part of the Haersuhai Formation comprises coarse clastic rocks, carbonates and intermediate-acidic volcanic rocks, and its upper part is composed of a thick section of fine clastic rocks and pelitic rocks with some limestone.

Sample descriptions

Wudenghan pluton

The Wudenghan pluton in the Zhusileng tectonic zone covers about 120 km2 and was emplaced in the core of a late Carboniferous anticline (Fig. 3). Plutonic lithofacies are not well developed and consist mainly of biotite granodiorite and biotite monzogranite. As the whole pluton is highly weathered, especially the granodiorite, only the biotite monzogranite was studied. The monzogranite is medium-grained (3–5 mm). The modal mineralogy (Fig. 4a, b) includes plagioclase (25–35 %, mostly altered to sericite and partly altered to epidote), perthitic feldspar (25–40 %, partly altered to clay minerals), quartz (30–40 %), mafic minerals (8–15 %, mainly biotite, altered to chlorite) and minor muscovite. Zircon, monazite, apatite, hematite and magnetite are the main accessory minerals.

Fig. 4
figure 4

Field photographs and microphotographs of late Paleozoic granites in the northernmost Alxa region. a, b Mineralogy of the Wudenghan monzogranite, left plane light, right cross-polars; c, d Mineralogy of the Huhetaoergai granodiorite, left plane light, right cross-polars; e, f Zhuxiaobuguhe granodiorite; g, h Mineralogy of the Zhuxiaobuguhe granodiorite, left plane light, right cross-polars

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Huhetaoergai pluton

The Huhetaoergai pluton is in the Huhetaoergai tectonic zone and represents a batholith covering about 80 km2. In the southeast, it intrudes Mesoproterozoic sandstone (Fig. 2). Banded structure and weakly developed gneissic fabric are present. The banded structure comprises 2- to 3-cm-wide light and dark parallel laminations. Generally, coarse quartz, plagioclase and feldspar comprise the light bands and fine biotite and plagioclase form the dark bands. Most minerals are aligned parallel to the bands. Elongate marble xenoliths parallel to the gneissic schistosity or laminations are also present (BGGP 1980).

The pluton is heterogeneous with variable composition and irregular lithofacies. However, it is predominantly fresh red biotite granite and gray biotite granodiorite. The biotite granite is the Neoproterozoic in age and intruded by the biotite granodiorite (Zhang et al. 2015). In this study, we mainly focus on the biotite granodiorite. It has a porphyritic-like texture. Most phenocrysts (8–24 mm) are euhedral–subhedral plagioclase (25–35 %). The matrix (<4 mm) includes plagioclase (35–45 %, partly altered to sericite and epidote), quartz (20–35 %, some recrystallization has occurred), biotite (15–20 %, partly altered to chlorite) and perthitic feldspar (10–15 %; Fig. 4c, d). Zircon, titanite, apatite, hematite and magnetite are the main accessory minerals.

Zhuxiaobuguhe pluton

The Zhuxiaobuguhe pluton is to the west of the Huhetaoergai pluton in the Huhetaoergai tectonic zone (Fig. 2) and is larger than the Huhetaoergai pluton. Its northern part intrudes Middle Ordovician dacite and rhyolite. No clear evidence for recrystallization or thermal metamorphism is found in the wallrock, but the pluton has a well-developed foliation (Fig. 4e, f). Plutonic lithofacies are not well developed. Marble xenoliths have similar structures to those in the Huhetaoergai pluton (BGGP 1980).

The Zhuxiaobuguhe pluton consists mainly of porphyritic hornblende–biotite granodiorite (Fig. 4g, h). The phenocrysts (mostly 4–10 mm, some can reach 20 mm) include euhedral–subhedral plagioclase (10–30 %). The matrix (0.1–3 mm) includes plagioclase (40–50 %, mostly altered to sericite and epidote), quartz (15–25 %, with pronounced abundant recrystallization), hornblende (5–15 %, some altered to uralite and actinolite), biotite (10 %, mostly altered to chlorite) and perthitic feldspar (<5 %). Large subhedral titanite (about 1 mm) is abundant. Zircon, apatite, hematite and magnetite also occur as accessory minerals.

Analytical results

See “Analytical methods of electronic supplementary material” for a description of the analytical methods. For each pluton, 3–4 samples representing the batholiths’ principal compositional phases were selected for major and trace element analyses. The results are given in “Table S1 of electronic supplementary material”. After geochemical analyses, a representative sample of each pluton was selected for geochronology. One sample (AB10-48) was dated by LA-ICP-MS and two samples (W08-170 and AB10-30) by secondary ion mass spectrometry (SIMS). Analytical results are shown in “Tables S2 and S3 of electronic supplementary material”, respectively. Integrating crystal shapes, textures and high Th/U ratios of the zircons, the majority of grains are presumed to be magmatic (Belousova et al. 2002; Corfu et al. 2003). A few analyses showing apparently old (AB10-48-31, n4158-02 and 04) or discordant (n4159-08) ages, analyses on cracks (n4159-05 and 15) or inclusions (n4159-14), or partly off the crystal (n4158-13, n4159-02, 03, 04 and 10), are excluded from the age calculations. The isotopic results are summarized in “Table S4 of electronic supplementary material”. Determination of “initial” conditions is based on the geochronology. Nd model ages have been calculated with f Sm/Nd of −0.2 to −0.6.

Wudenghan pluton

Major and trace elements

Wudenghan biotite monzogranite has high SiO2 contents (>75 wt%) and low Al2O3 (12.27–13.09 wt%), CaO (0.44–0.67 wt%) and MgO (0.09–0.21 wt%) with low Mg# [0.14–0.27; Mg2+/(Mg2+ + Fe2+), Irving and Green 1976]. All samples are metaluminous (A/NCK < 1.1; Fig. 5a) and ferroan [Fe# = FeOT/(FeOT + MgO) = 0.88–0.94] (after Frost et al. 2001). The total alkali content (Na2O + K2O) is 8.23 wt% on average, and all samples contain more K2O than Na2O (Na2O/K2O = 0.71–0.78). Using the classification of Irvine and Baragar (1971), the Wudenghan monzogranite represents the high-K calc-alkaline series (Fig. 5b). Chondrite-normalized REE patterns (Fig. 6a) show enrichment of LREE [(La/Yb)N = 4.5–6.3] and strongly negative Eu anomalies (δEu = 0.27–0.31). N-MORB-normalized trace element patterns (Fig. 6b) are enriched in LREE, Rb, U and K and depleted in Ba, Nb, Ti and Zr.

Fig. 5
figure 5

Major element characteristics of the northernmost Alxa granites. a Al2O3/(CaO + Na2O + K2O) versus Al2O3/(Na2O + K2O) diagram (after Shand 1943); b SiO2–K2O diagram (fields from Rickwood 1989); c SiO2 versus Na2O + K2O diagram (after Irvine and Baragar 1971)

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Fig. 6
figure 6

Normalized REE and trace elements patterns for the Wudenghan (a, b), Huhetaoergai (c, d) and Zhuxiaobuguhe (e, f) samples. Chondrite and N-MORB normalization values from Sun and McDonough (1989)

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Geochronology

Zircon grains from this granite (AB10-30) are euhedral, clear to pink and transparent with aspect ratios (AR) of 1:1–3:1. Some grains have inclusions. In CL images, most show oscillatory or broad zoning (Fig. 7a). One core was found (n4159-03). Th/U ratios = 0.38–0.59. Seven analyses were concordant and combined to yield a concordia age of 383 ± 3 Ma (MSWD = 2.3; Fig. 8a), which is interpreted as the crystallization age for sample AB10-30.

Fig. 7
figure 7

Cathodoluminescence (CL) images of zircons from the Wudenghan monzogranite (a), the Huhetaoergai pluton (b) and the Zhuxiaobuguhe granodiorite (c). All scale bars 50 μm

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Fig. 8
figure 8

U–Pb concordia plot for the Wudenghan monzogranite (a), the Huhetaoergai granodiorite (b) and the Zhuxiaobuguhe granodiorite (c). Unfilled symbols excluded from final synthesis. Plots generated using Isoplot (Ludwig 2003)

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Nd and Sr isotopes

The Wudenghan monzogranite has low initial 87Sr/86Sr of 0.704719–0.706113 and slightly negative ε Nd(t) of −0.2 to −0.1. They indicate Neoproterozoic-age components when intrusive age is plotted against ε Nd(t) (Fig. 9a). Single-stage model ages (T DM1) are 1097–1123 Ma.

Fig. 9
figure 9

Isotopic data. a ε Nd(t) versus intrusive age; b ε Nd(t) versus (87Sr/86Sr)i ratios [lower crust and upper crust fields from Wu et al. (2003)]

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Huhetaoergai pluton

Major and trace elements

The Huhetaoergai biotite granodiorite has relatively lower SiO2 (64.42–71.37 wt%) and TiO2 (0.43–0.67 wt%) but much higher Al2O3 (14.88–17.95 wt%), CaO (2.87–4.20 wt%) and MgO (1.24–1.78 wt%) with high Mg# (0.49–0.53). Na2O/K2O ratios for the granodiorite are 1.9 on average with lower total alkali content (5.52–6.34 wt%) than the Huhetaoergai granodiorite. It is metaluminous to peraluminous (A/NCK = 1.07–1.12; Shand 1943; Zen 1988; Fig. 5a) and magnesian (Fe# = 0.69–0.71) and belongs to the medium-K calc-alkaline series (Fig. 5b, c). Chondrite-normalized REE patterns (Fig. 6c) show enrichment of LREE [(La/Yb)N = 4.2–11.9] and positive to slightly negative Eu anomalies (δEu = 0.91–1.25). N-MORB-normalized trace element patterns (Fig. 6d) show that the samples are enriched in Rb, K, Pb and Sr and depleted in Nb and P.

Geochronology

Zircon grains from this granite (AB10-48) are euhedral, clear to pink and transparent. AR ranges from 1:1 to 5:1. Some have inclusions and inherited cores. Most analyses represent idiomorphic grains with oscillatory zoning throughout the crystal (Fig. 7b). Th/U = 0.09–3.45. One core was found and a slightly older age of 806 ± 22 Ma was obtained (AB10-48-31). Twenty-eight analyses combine to yield a concordia age of 356 ± 3 Ma (MSWD = 1.3; Fig. 8b), which is interpreted as the crystallization age for sample AB10-48.

Zhuxiaobuguhe pluton

Major and trace elements

Major elements of the Zhuxiaobuguhe hornblende–biotite granodiorite have similar characteristics to the Huhetaoergai biotite granodiorite. SiO2 contents range from 62.96 to 69.95 wt%, Al2O3 from 15.18 to 17.23 wt%, MgO from 1.29 to 2.31 wt% with high Mg# (0.49–0.53), TiO2 (0.47–0.78 wt%) and CaO (2.66–5.46 wt%). All samples are metaluminous (A/NCK = 0.94–1.05; Fig. 5a) and magnesian (Fe# = 0.68–0.72). The total alkali content is between 5.37 and 6.54 wt%, and all samples contain more Na2O than K2O (Na2O/K2O = 2.11–3.13). Geochemically, the Zhuxiaobuguhe granodiorite represents a medium-K calc-alkaline pluton (Fig. 5b, c). Chondrite-normalized REE patterns (Fig. 6e) show higher enrichment of LREE [(La/Yb) N  = 9.9–22.3] and weakly negative Eu anomalies (δEu = 0.77–0.96). N-MORB-normalized trace element patterns (Fig. 6f) show enrichments of LREE, Rb, K and Nd and depletions in Ba, Nb, Ti and P.

Geochronology

Zircon grains from this granite (W08-170) are euhedral, clear to pink and transparent with AR of 5:1. Some grains have inclusions. Most crystals have broad oscillatory zoning. Neither zircon cores nor old inherited ages were obtained (Fig. 7c). The Th/U varies from 0.74 to 1.73. Eleven analyses combine to yield a concordia age of 286 ± 2 Ma (MSWD = 0.95; Fig. 8c), which is interpreted as the crystallization age for sample W08-170.

Nd and Sr isotopes

The Zhuxiaobuguhe granodiorite has initial 87Sr/86Sr of 0.708370–0.708462 and negative ε Nd(t) of −2.0 to −1.1 (Table S4). They indicate Neoproterozoic-age component with T DM1 = 1000–1013 Ma (Fig. 9a).

Discussion

Genesis of Late Paleozoic granites in the northernmost Alxa region

Late Devonian Wudenghan pluton

This pluton was previously considered to be Late Paleozoic in age (BGGP 1981a). Our new SIMS zircon U–Pb age from the biotite monzogranite (AB10-30) is 383 ± 3 Ma. From its mineralogical and geochemical characteristics, such as the absence of alkaline minerals, high SiO2 contents (>75 wt%) and K2O (>4.5 wt%), low Na2O/K2O and Ga/Al ratios, low MgO (<0.3 wt%), the Wudenghan monzogranite is interpreted to represent a highly fractionated high-K calc-alkaline rock (e.g., Sylvester 1989; Whalen et al. 1987). It is metaluminous, with A/CNK < 1.1 and A/NK > 1, suggesting that it represents an I-type or A-type granitoid rather than S-type (Chappell and White 1992). The low Fe/Mg, Ga/Al and concentrations of Zr, Nb, Ga, Y, Ce, Sr and CaO further indicate that it is not an alkaline (A-type) granite but rather a fractionated felsic granite (Whalen et al. 1987). This is also supported by the striking depletions in Ba, Nb, Eu and Ti (Fig. 6b). Negative Nb–Ti anomalies are interpreted to relate to fractionation of Ti-bearing phases (ilmenite, titanite, rutile, etc.), negative P anomalies to result from apatite separation with strongly negative Eu anomalies requiring extensive fractionation of plagioclase and/or K-feldspar. The fractionation of K-feldspar would produce negative Eu–Ba anomalies. Therefore, the Wudenghan granite can be described as an evolved I-type granite, possibly representing a fractionated melt(s) derived from a parental calc-alkaline magma.

Calc-alkaline, I-type granitoids are usually generated by (I) partial melting of mafic to intermediate igneous sources, (II) advanced assimilation and fractional crystallization of mantle-derived basaltic parental magmas, or (III) mixing of mantle-derived magmas with crustal-derived materials (e.g., De Paolo 1981; Li et al. 2007; Sisson et al. 2005; Hildreth and Moorbath 1988). Highly fractionated I-type granitoids can be formed by mixing of mantle wedge basaltic melts with associated induced crustal melts (Zhu et al. 2009). Plutons formed via mixing of different proportions between these two distinctive chemical and isotopic end-member compositions can only generate the relatively narrow isotopic and chemical composition of the Wudenghan pluton from homogeneous mixing and emplacement in a single melt batch. Based on the criteria employed by Watson and Harrison (1983), the zircon saturation temperature (T Zr) calculated for the Wudenghan pluton yields values of 766–779 °C (average 772 °C). This pluton lacks inherited zircon, reflecting zircon-undersaturated melt compositions. Thus, T Zr represents a minimum estimate of magma temperature (Miller et al. 2003). The low initial Sr isotopic ratios (0.704719–0.706113) combined with only slightly negative ε Nd(t) values (−0.2 to −0.1) for the Wudenghan pluton indicate a predominantly juvenile crustal source and only a minor older crustal component(s). Therefore, the Wudenghan pluton seems more likely to be generated from mantle-derived basaltic parental magmas with assimilation and fractional crystallization.

Early Carboniferous Huhetaoergai pluton

This pluton was previously considered to be early Paleozoic (BGGP 1980) or late Neoproterozoic in age (689 Ma, TIMS zircon U–Pb; Wang et al. 1994). Our new LA-ICP-MS zircon U–Pb age (AB10-48) is 356 ± 3 Ma. It represents the age of the Huhetaoergai biotite granodiorite pluton and resolves this controversy.

The high Na2O/K2O ratio and high Mg# (0.51 on average) indicate that the Huhetaoergai granodiorite represents an unfractionated calc-alkaline magma (Whalen et al. 1987). Smaller geochemical anomalies (Fig. 6) also suggest that fractionation was not extensive. Separation of feldspar from the Huhetaoergai granodiorite magma appears minor, as indicated by positive to slightly negative Eu anomalies. Positive Rb and Pb anomalies indicate the addition of a continental crustal component. The slightly high Mg# is consistent with input of primary mantle-derived melt (Kelemen et al. 1995). The zircon saturation temperature (T Zr) calculated for the Huhetaoergai pluton is 737–779 °C (average 754 °C); the pluton has inherited zircons, indicating zirconium saturation, and therefore, the T Zr is considered to be a maximum initial magma temperatures at the source (Miller et al. 2003). According to Miller et al. (2003), the Huhetaoergai pluton represents a “cold” granite (T Zr < 800 °C) which appears to form at temperatures too low for dehydration melting involving biotite or hornblende and probably requires fluid influx. Experiments and modeling indicate a water-rich fluid phase appears to be the only mechanism for inducing large-scale melting at T < 800 °C (Chappell et al. 2004; Miller et al. 2003). As early Carboniferous mafic rocks have not been recognized in the Huhetaoergai area before now, the suggestion of water-rich fluid in its source is enigmatic.

Early Permian Zhuxiaobuguhe pluton

The Zhuxiaobuguhe and Huhetaoergai plutons were previously regarded as a single pluton separated by Upper Jurassic and Quaternary strata (BGGP 1980; Wang et al. 1994). SIMS zircon U–Pb dating of the Zhuxiaobuguhe pluton (W08-170), however, gives an age of 286 ± 2 Ma. As no significant compositional diversity occurs in the Zhuxiaobuguhe pluton, we interpret this age to represent the age of the entire Zhuxiaobuguhe pluton. Therefore, the Zhuxiaobuguhe pluton was emplaced at 286 ± 2 Ma and is not contiguous with the Huhetaoergai pluton.

Similar to the Huhetaoergai granodiorite, the Zhuxiaobuguhe granodiorite represents unfractionated calc-alkaline melt (Whalen et al. 1987). Slightly negative initial ε Nd (−2.0 to −1.1) and moderate initial 87Sr/86Sr (0.708370–0.708462) indicate of probably the crust–mantle interactions. The arc-like geochemical signature, such as enriched LREE coupled with negative Nb–Ta–Ti anomalies, is easily explained if inherited from the paleo-volcanic arc in the Huhetaoergai tectonic zone. Therefore, the Zhuxiaobuguhe pluton is most likely derived from partial melting of mafic to intermediate igneous arc sources.

It is notable that the Yagan pluton of similar age (283 ± 2 Ma, SIMS zircon U–Pb) also occurs in the Huhetaoergai tectonic zone (Fig. 3; Zheng et al. 2013). The Yagan monzogranite represents unfractionated metaluminous to peraluminous high-K calc-alkaline granodiorite melt. It has higher SiO2 (66.96–70.71 wt%) and total alkali contents (Na2O + K2O = 7.24–9.19 wt%), with lower TiO2 (0.21–0.29 wt%), CaO (2.07–2.43 wt%) and Na2O/K2O (1.04–1.34). The chondrite-normalized REE patterns show less enrichment of LREE [(La/Yb)N = 5.0–11.0] and more negative Eu anomalies (δEu = 0.59–0.77). Compared with the Zhuxiaobuguhe granodiorite, the Yagan monzogranite has lower initial ε Nd(t), similar initial 87Sr/86Sr (Fig. 9b; Table 1) and an older T DM1 age (1162–1821 Ma; Zheng et al. 2013). Consistent with the Zhuxiaobuguhe pluton, the T Zr of the Yagan pluton is also lower than 800 °C (743–768 °C, average 754 °C) with inherited zircon. Therefore, we suggest that early Permian granites in the Huhetaoergai tectonic zone were generated from partial melting of mafic lower crust of the paleo-volcanic arc.

Table 1 Summary of northernmost Alxa granitic plutons
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Tectonic implications

Wudenghan pluton

All samples from the Wudenghan monzogranite have highly fractionated calc-alkaline, volcanic arc and post-collisional characteristics (Pearce 1996; Whalen et al. 1987). Almost all highly fractionated granites converge in major and trace element geochemistry to resemble each other, no matter which tectonic environment they form in. It is very difficult to use highly fractionated melts to infer tectonic environments. Furthermore, arc-like geochemical signatures, such as enriched LREE coupled with negative Nb–Ta–Ti anomalies, can be inherited from any source with subduction zone chemistry.

An additional complication from the sedimentary record inhibits understanding whether the late Devonian Wudenghan pluton represents a post-collisional granite or volcanic arc granite and hinges on the interpretation of the Wotuoshan Formation in the Zhusileng tectonic zone. The Wudenghan pluton intrudes the Zhusileng tectonic zone (Figs. 1c, 3) which represents a passive continental margin from Cambrian to Middle Silurian time (Wang et al. 1994; Wu et al. 1998). On the one hand, Wang et al. (1994) thought the ocean between the Zhusileng tectonic zone and Huhetaoergai tectonic zone closed during late Silurian–Devonian time because the Middle Devonian Wotuoshan Formation in the Zhusileng tectonic zone represented a marine molasse. However, it is highly debated whether the Wotuoshan Formation actually represents a molasses: The lower part of the Wotuoshan Formation is composed of conglomerate and pebbly coarse sandstone, the middle part includes varicolored calcic feldspathic quartz sandstone, calcic tuffaceous sandstone and siltstone with thin-layered limestone, and the upper part includes medium-coarse feldspathic quartz sandstone. On the other hand, Wu et al. (1998) suggested that Devonian deposition in the Zhusileng tectonic zone implied a stable littoral environment and that during the Carboniferous the Zhusileng tectonic zone changed from a passive to an active margin associated with the northward subduction of the oceanic crust represented by the Engger Us ophiolitic belt.

Whether the late Devonian Wudenghan pluton represents a post-collisional granite or volcanic arc granite is crucial for constraining the tectonic environment of Devonian Zhusileng tectonic zone. If the Wudenghan pluton formed in post-collisional setting, it is difficult to explain the source of relatively juvenile mantle input [indicated by higher ε Nd(t) (−0.2 to −0.1) and very low (87Sr/86Sr) t  = 0.704719–0.706113]. As the early Carboniferous Huhetaoergai pluton also formed in a volcanic arc environment (see “Huhetaoergai pluton” section) and the oceanic crust represented by the Yagan Suture did not seem to subduct southward during Devonian time, we suggest that the late Devonian Wudenghan pluton formed by the northward subduction of the oceanic crust represented by the Engger Us ophiolitic belt. This is also supported by the early–middle Paleozoic rock distributions in the Northern Alxa region (Fig. 1c), which probably indicate southeastward younging and migration of arc magmatism from Ordovician to Devonian time. Therefore, the Wudenghan pluton is more likely to have formed during subduction–accretion processes rather than in a post-collisional setting. It provides new evidence that the basin between the Zhusileng tectonic zone and Huhetaoergai tectonic zone was not closed before Late Devonian time (Fig. 10).

Fig. 10
figure 10

a–f Schematic tectonic evolution of the northernmost Alxa region. See text for details. Based on data from this study, Wang et al. (1994), Wu et al. (1998), Zhang et al. (2013a) and Zheng et al. (2013)

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Huhetaoergai pluton

The early Carboniferous Huhetaoergai granodiorite has trace element compositions associated with a volcanic arc granite (Pearce 1996; Pearce et al. 1984). This is consistent with the sedimentary record in the Huhetaoergai tectonic zone (BGGP 1980) which also reflects a volcanic arc environment during the early Carboniferous. The Lower Carboniferous strata with continuous deposition of acidic volcanic rocks in the lower part and clastic rocks in the upper part represent a littoral-neritic depositional setting. It is only exposed in the extreme north near Mongolia. The lower part of Upper Carboniferous strata, in contact with Lower Carboniferous strata by a fault, also represents a littoral environment. From bottom to top, they include fine-grained arkosic graywacke interbedded with intermediate volcanic rocks, intermediate-acidic volcanic rocks and quartz sandstone. The fault contact is exposed between the bottom and middle layers. In addition, the uppermost Upper Carboniferous strata have not been recognized in the Huhetaoergai tectonic zone until now. The geochemistry of the Huhetaoergai granodiorite combined with the sedimentary history of the region indicates that the Huhetaoergai area is likely to represent a volcanic arc setting during early Carboniferous time, and the oceanic basin was still open at that time (Fig. 10). This is also supported by studies of the linked Mongolia terranes. The Huhetaoergai tectonic zone extends to the northeast into Mongolia as the Hashaat Terrane, which is interpreted as fragments of island arcs of the Paleo-Asian Ocean system (Badarch et al. 2002 and references therein). It occurs between the back-arc basin represented by the Atasbogad Terrane and the cratonal block represented by the Tsagaan Uul Terrane of southern Mongolia. The Jindouaobao gneiss (Wang et al. 2001) testifies that the Zhusileng tectonic zone is the continuation of the Tsagaan Uul Terrane, and they may represent a part of the South Gobi microcontinent (Badarch et al. 2002). Lamb and Badarch (2001) pointed out that the Devonian strata of southern Mongolia represented components of two arc systems, including a trapped ocean basin that developed into a back-arc basin (e.g., the Atasbogad Terrane, Badarch et al. 2002). The Carboniferous arc was probably a continuation of the Devonian arc system and was built in part upon Devonian strata (e.g., the Hashaat Terrane, Badarch et al. 2002). Consistent with studies in southern Mongolia, the early Carboniferous Huhetaoergai tectonic zone reflects a volcanic arc environment. Therefore, the Huhetaoergai pluton is interpreted to represent a volcanic arc granite.

Zhuxiaobuguhe pluton

Early Permian granitoids of the CAOB typically have post-collisional signatures (Glorie et al. 2011; Han et al. 1997, 1999; Heinhorst et al. 2000; Wang et al. 2009). Trace element compositions of samples from the Zhuxiaobuguhe pluton are consistent with volcanic arc to post-collisional settings (Pearce 1996; Pearce et al. 1984). Samples from Yagan monzogranite of similar age (283 ± 2 Ma) in the Huhetaoergai tectonic zone are also reported to have both volcanic arc and post-collisional characteristics (Zheng et al. 2013; Fig. 3). Previously, two distinct tectonic environments have been suggested for the Huhetaoergai tectonic zone in the Permian (Wang et al. 1994; Wu et al. 1998). Wu et al. (1998) argued northward subduction of oceanic crust between the Huhetaoergai tectonic zone and the Zhusileng tectonic zone in the Permian resulted in rifting of the Zhusileng tectonic zone. However, though Wang et al. (1994) agreed that in the Permian the Zhusileng tectonic zone was in a rifting stage, they suggested that this was the result of the southward subduction of oceanic crust represented by the Engger Us Ophiolitic Belt and that the Permian Huhetaoergai–Zhusileng tectonic zones together were involved in rifting. Sedimentary investigations show the Permian strata of the Huhetaoergai tectonic zone and Zhusileng tectonic zone may be correlated (BGGP 1980, 1981a; Wang et al. 1994; Wu et al. 1998). In this case, there would not be an ocean between Huhetaoergai tectonic zone and Zhusileng tectonic zone in the Permian. Lamb and Badarch (2001) pointed out that Permian strata record mainly nonmarine basins, as well as the closing of the ocean basin between southern Mongolia and the tectonic blocks of China. Consequently, we conclude that the Permian Huhetaoergai tectonic zone represents a rifting environment rather than an active margin (Fig. 10d).

Tectonic model

To summarize, this study combined with previous investigations into granite genesis (Zheng et al. 2013; Table 1) and the tectonic evolution of the region (Wang et al. 1994; Wu et al. 1998) allows us to develop a new late Paleozoic tectonic scenario for the evolution of the northernmost Alxa region (Fig. 10).

  • Middle Ordovician–Middle Silurian (Fig. 10a): The Paleo-Asian Ocean (now represented probably by the Yagan Suture) started to subduct northward from at least the Middle Ordovician and the volcanic arc represented by the Huhetaoergai tectonic zone formed in the oceanic basin. The Zhusileng tectonic zone represents a passive continental margin (Wang et al. 1994).

  • Late Silurian–early Carboniferous (Fig. 10b): The oceanic basin between the Huhetaoergai tectonic zone and Zhusileng tectonic zone continued subducting northward. In the early Carboniferous, the Western Huhetaoergai granodiorite formed in a volcanic arc. Meanwhile, in the south, the oceanic crust represented by the Engger Us Ophiolitic Belt began subducting northward and resulted in the Late Devonian highly fractionated I-type Wudenghan granite.

  • Late Carboniferous (Fig. 10c): The ocean between the Huhetaoergai tectonic zone and Zhusileng tectonic zone closed in the northern Alxa region and the Yagan Suture formed.

  • Early Permian (Fig. 10d): The Huhetaoergai–Zhusileng tectonic zones entered into a period of extension allowing the Zhuxiaobuguhe and Yagan plutons to be emplaced.

Conclusions

  1. 1.

    The late Devonian Wudenghan monzogranite is a highly fractionated calc-alkaline granite with higher initial ε Nd (−0.2 to −0.1) and low initial 87Sr/86Sr values (0.704719–0.706113). It probably represents a highly fractionated volcanic arc granite formed in response to the northward subduction of the oceanic basin represented by the Engger Us ophiolitic belt.

  2. 2.

    Early Carboniferous Huhetaoergai granodiorite formed in a volcanic arc setting. It formed mainly by the partial melting of lower continental crust. During the early Carboniferous, the oceanic basin between the Huhetaoergai tectonic zone and the Zhusileng tectonic zone still existed.

  3. 3.

    The early Permian Zhuxiaobuguhe pluton is not part of the Huhetaoergai pluton. It is composed of unfractionated calc-alkaline granodiorite with low initial ε Nd (−2.0 to −1.1) and moderate initial 87Sr/86Sr values (0.708370–0.708462). The magma is derived from partial melting of mafic lower crust of the paleo-volcanic arc. The Huhetaoergai tectonic zone entered into the post-collisional extensional stage during early Permian.