1 Introduction

The North Tianshan suture zone, the youngest collisional belt in the Tianshan Orogenic Belt, provides a pivotal upper-age for the last closure of the Junggar-Balkhash Ocean (Filippova et al. 2001; Han et al. 2010; Korobkin and Buslov 2011; Windley et al. 2007). However, the timing for the last closure of the Junggar-Balkhash Ocean has long been controversial (Ge et al. 2015; Han et al. 2010; Tang et al. 2010; Xiao et al. 2008). This ocean may have been closed during the Early-Middle Devonian (Han et al. 1997), or Late Devonian–Early Carboniferous (Gao et al. 1995; Han et al. 2010), or Late Carboniferous (Bykadorov et al. 2003; Wang et al. 2017; Yang et al. 2013), and even Permian (Xiao et al. 2008). These varied ages have been derived from petrology and geochemistry of the volcanic rocks (Ge et al. 2015; Liu et al. 2012; Tang et al. 2010; Wang et al. 2017), intermediate to acidic intrusions (Han et al. 2010; Tang et al. 2010; Zhang et al. 2012), and ophiolites (Xu et al. 2005, 2006). So far, no petrologic and geochemical studies have been undertaken on volcaniclastic rocks. Increasing evidence shows that petrology and geochemistry analysis of volcaniclastics is critical for understanding the plate tectonic settings (Manville et al. 2009). In the North Tianshan region, a suite of volcaniclastic rocks intercalated with the volcanic rocks are assigned to the Arbasay Formation (Fig. 1; BGMRXUAR 1993; Liu et al. 2012, 2015). Previously, the Arbasay Formation was interpreted as a continental arc formed in the transition stage from compression to extension (Wang et al. 2017), which restricts the last closure time of the Junggar-Balkhash Ocean (Han et al. 2010), although its age is open for debates (BGMRXUAR 1993, 1999; Liu et al. 2012, 2015; Su et al. 2006a, 2016b; Wang et al. 2017; Yang et al. 2006). The age constraints and correlations of these volcaniclastic strata have long remained unclear due to the paucity of accurate absolute age, the lateral discontinuity of outcrops, and absence of the datable fossils.

Fig. 1
figure 1

a Location map of the study area. b Tectonic map of the southern Junggar Basin (modified after Wang et al. 2016). c Tectonic map of the Tianshan and location of study area. NTS-North Tianshan; CTS-Central Tianshan; STS-South Tianshan (modified after Wang et al. 2017)

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In this paper, we report new zircon U–Pb ages of the volcaniclastic rocks from the upper part of Arbasay Formation in Shichang Region, northern Xinjiang. Based on these newly obtained ages and previously published data, we examined the era of the Arbasay Formation, and made discussion about the last closure time of the Junggar-Balkhash Ocean.

2 Geological setting

The Tianshan tectonic complex, which results from complex accretions of island arcs and amalgamation of continental lithospheric blocks during the late Paleozoic (Carroll et al. 1995; Charvet et al. 2007; Gao et al. 1998; Wang et al. 2007, 2009, 2010), is crucial for understanding the geological evolution of the Central Asia Orogenic Belt (Han et al. 2010; Xiao et al. 2008). This complex is traditionally subdivided into the North Tianshan, Central Tianshan, and South Tianshan orogenic belts (Fig. 1c). The Tarim–South Tianshan and Central Tianshan Blocks collided during the Devonian–Carboniferous (Chen and Shi 2003; Han et al. 2011; Wang et al. 2010). This accretion was followed by the collision of the newly formed Tarim–Central Tianshan Blocks with a series of the late Paleozoic island arcs now forming the North Tianshan (Han et al. 2010; Charvet et al. 2011).

The North Tianshan Orogenic Belt is located in south of the Junggar Basin (Fig. 1), and is usually interpreted as a continental arc distributed over the northern part of the Central Tianshan Block since the southward subduction of the paleo-Junggar-Balkhash Ocean during the Paleozoic (Gao et al. 1998; Wang et al. 2007, 2017, 2018). The intermediate-acidic igneous rocks are widely distributed in the North Tianshan Mountain areas (Gao et al. 1998), characteristics of typical calc-alkaline series and continental arc-like geochemical compositions (Wang et al. 2017). Along the northern margin of the North Tianshan, both volcanic and volcaniclastic rocks are widely distributed in the northern areas of the Bayinggou ophiolite belt (Fig. 1). These rocks were primitively assigned to the Lower Permian Arbasay Formation, which is typically exposed in the Jingou River (Fig. 1). The Arbasay Formation unconformably overlies the Upper Carboniferous Qianxia Formation which is characterized by island arc volcanic rocks, and was overlain by the Late Permian–Early Triassic coarse clastic rocks (Figs. 2 and 3). The overlying strata contacted the Arbasay Formation with faults (Figs. 2a and 3). Lithologically, the Arbasay Formation volcanics show typical basalts–andesites–dacites–rhyolites volcanic series, and are dominated by andesites with minor constituents of basalts and rhyolites (Fig. 2b, c). The volcaniclastic rocks include pink tuff (ignimbrite), greyish volcanic breccia, greyish brown tuffaceous sandstone, and variegated volcanic agglomerate (Figs. 4 and 5). Intermediate-acid and basic igneous rocks dominated the lower part of the Arbasay Formation (Fig. 4a, b, d, e), and andesite and basalt with interbeds of tuff, tuffaceous sandstone, volcanic breccia, and volcanic agglomerate predominated the upper part of the formation (BGMRXUAR 1993; Wang et al. 2017; Figs. 4a, c, f–i and 5). Conspicuously, the lower part of the Arbasay Formation mainly consists of volcanic rocks, while the upper part is principally composed of volcaniclastic rocks with minor volcanic rocks (Fig. 2b, c).

Fig. 2
figure 2

a Geological map of the Arbasay Formation in the Shichang Region with logged section marked (age data from Liu et al. 2012, 2015; Wang et al. 2017). b Measured section along the Jingou River, with sampling locations. c Lithological column of the Upper Carboniferous Arbasay Formation

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Fig. 3
figure 3

Comparison of stratigraphic contact relation of the Arbasay Formation in the Shichang Region

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Fig. 4
figure 4

Field photographs of the Arbasay Formation. a Panoramic photos of the Arbasay Formation; telegraph pole is 10 m high. b General characteristics of the lower part of Arbasay Formation. c General characteristics of the upper part of Arbasay Formation. d Claret-colored andesite, Lower Arbasay Formation, hammer (28 cm long) shows the scale. e Greyish-green basalts, Lower Arbasay Formation, pencil is 20 cm (scale) long. f–i The tuffs intercalated with the volcanic and volcaniclastic rocks; Upper Arbasay Formation, person (scale) is 1.7 m tall

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Fig. 5
figure 5

Representative photographs of volcaniclastic rocks in the upper part of Arbasay Formation. a–c Greyish volcanic breccia and agglomerate; hammer (scale) is 28 cm long, person (scale) is 1.7 m tall, alpenstock (scale) is 1 m long. d Pink tuff, the deformation is caused by faults and folds; hammer (scale) is 28 cm long. e–g Greyish brown tuffaceous sandstone with cross beddings; pencil (scale) is 20 cm long. h Volcaniclastic rocks are conformable contact with each other; person (scale) is 1.7 m tall

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3 Methods and data

Zircon grains were separated from the volcaniclastic rocks of the Arbasay Formation for U–Pb dating. They were extracted using heavy-liquid and magnetic techniques, and further purified by hand-picking under a binocular microscope. They were set in an epoxy mount which was polished, and then vacuum-coated with a layer of 50 nm high-purity gold. Cathodoluminescence (CL) images for zircons were undertaken in order to examine the internal structure of individual grains. Zircon LA–ICP–MS U–Pb isotope analyses were performed at MOE Key Laboratory of Orogenic Belts and Crustal Evolution, School of Earth and Space Sciences, Peking University. Detailed analysis procedures were similar to those described by Yuan et al. (2004). The common lead was corrected using the method of Andersen (2002). The age calculation was finished by ISOPLOT 3 (Ludwig 2003).

4 Results

The zircon U–Pb analytical results were listed in Additional file 1 and CL images were shown in Fig. 6. Almost all of these zircons are 80–240 μm in size and have euhedral crystals. The CL images show strong oscillatory zoning (Fig. 6), combined with their high Th/U ratios (0.36–1.61; Additional file 1), indicative of magmatic origin (Corfu et al. 2003).

Fig. 6
figure 6

Cathodoluminescence (CL) images of zircons separated from 15JG-09, 15JG-14, 15JG-18 and 15JG-28. Scale bar in each photomicrograph is 50 μm long

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Sample 15JG-09 is tuffaceous sandstone obtained from the upper part of Arbasay Formation. 59 analyses of U–Pb dates give a range of 206Pb/238U ages scattering between 287 and 355 Ma (Additional file 1). 206Pb/238U age spectrum exhibits a major peak of 308 Ma which correlates well with the major eruption time of the Arbasay volcanic rocks in the Jingou River (Figs. 7 and 8a; Liu et al. 2012; Wang et al. 2017).

Fig. 7
figure 7

Zircon U–Pb Concordia and age spectrum diagrams for the four samples

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Fig. 8
figure 8

206Pb/238U age spectrum of the a volcanic rocks, b tuffaceous sandstones, c tuffs and d all the samples. Data are from Liu et al. (2012, 2015); Wang et al. (2017) and this text

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Sample 15JG-14 is a tuff (ignimbrite) derived from the upper part of Arbasay Formation. Total 20 zircons give two 206Pb/238U age populations, with a weighted mean age of 304 ± 2 Ma (n = 13, MSWD = 0.97) and 315 ± 3 Ma (n = 5, MSWD = 0.12), respectively (Fig. 7).

Sample 15JG-18 from the upper part of Arbasay Formation yields 61 age points with sufficient concordance (Additional file 1). Except for one inherited zircon with age of 387 ± 4 Ma, the rest analyses give ages scattering from 287 Ma to 326 Ma. The 206Pb/238U age spectrum has a major peak of 308 Ma, which was well consistent with the age of contemporaneous volcanism recorded in the Arbasay Formation (Figs. 7 and 8a).

The tuff sample 15JG-28 was collected from the upper part of Arbasay Formationin. Total 20 zircons yield a 206Pb/238U weighted mean age of 310 ± 3 Ma (MSWD = 2.9) (Fig. 7), which is interpreted as the depositional age of the formation. Other four zircons have ages ranging from 323 Ma to 333 Ma, representing the inherited magmatic zircons from previous volcanic rocks.

5 Discussion

5.1 Age constraint for the Arbasay Formation

Both LA–ICP–MS and SHRIMP U–Pb zircon dating approaches have been applied to calibrate the age of Arbasay volcanic rocks and tuffs (Table 1). The different types of volcanic rocks (andesite, dacite and rhyolite) of the Arbasay Formation reflected the volcanism of the Late Carboniferous (308–305 Ma; Fig. 8a). However, the age of the volcaniclastic rocks in the upper part remains controversial, which led to the uncertainties of the age of Arbasay Formation. In order to further confirm the age of Arbasay Formation, four volcaniclastic rocks (two tuffs and two tuffaceous sandstones) were collected for zircon LA–ICP–MS U–Pb dating. Two tuffaceous sandstone samples, Sample 15JG-09 and Sample 15JG-18, show similar unimodal peaks at 308 Ma, pointing to one single source. Combination of nearly automorphic crystal fragments, angular detritus, and principally andesitic volcanic fragments (Fig. 5) implies that the contemporaneous Arbasay volcanic rocks (ca. 308 Ma; Fig. 8b) are likely the principal provenance for these two tuffaceous sandstone beds. Moreover, the pyroclastic flow deposits are also developed and interlayered with the volcaniclastic rocks (Fig. 5). This means that sandstones and volcaniclastic rocks of the Arbasay Formation were likely deposited during the syn-sedimentary processes. The minimum age group of the detrital zircons from Sample 15JG-09 and Sample 15JG-18 are 308 ± 2 Ma and 309 ± 3 Ma, respectively, indicating that their sedimentary ages should be younger than 308 Ma (Fig. 8b; Gehrels 2014).

Table 1 Lithologies, locations and geochronological data of the Arbasay Formation in the Shichang Region, North Tianshan, NW China
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In addition, two tuffs samples were analyzed to further constrain the age of the Arbasay Formation in the studied interval. These quickly deposited tuffs (tephra layers) are regionally compared (Fig. 4). Thus, the tephra layers are excellent key horizons for dating of associated non-volcanic sedimentary sequences (Manville et al. 2009). Sample 15JG-14 from the upper part of Arbasay Formation clustered around two groups (315 ± 3 Ma and 304 ± 2 Ma). As the tuffs not only appear before the eruption (“pre-eruptive” units), but also after the eruption (“post-eruptive” units), the older age of 315 ± 3 Ma therefore is interpreted to represent the commencing timing of the Arbasay volcanism, and the younger age of 304 ± 2 Ma for the demise time of the phreatomagmatic activity (Manville et al. 2009; Ross et al. 2005; White et al. 2009). The Arbasay Formation therefore was likely deposited from 315 Ma to 304 Ma after excluding the influence of the inherited zircons. Furthermore, the tuff (Sample 15JG-28) of the upper part of the Arbasay Formation gave a weighted mean age of 310 ± 3 Ma when the inherited zircon ages are irrespective. This age correlates well with the major volcanism of the Arbasay Formation (“syn-eruptive” units), which again suggested that their peak volcanism occurred during the Late Carboniferous (310–307 Ma; Fig. 8c). Stratigraphically, the tuffs conformably contact the volcanic rocks and volcaniclastic rocks (Fig. 2b, c), which are interlayered with the volcanic agglomerate, volcanic breccia, tuffaceous sandstone, and basalt (Fig. 4f–i). Therefore, the age of the tuff can represent the depositional age of the volcaniclastic rock, and further constrains the age of basalt, i.e., the basalt was also erupted during the Late Carboniferous.

Clearly, the volcanism indicated by various types of volcanic rocks primarily took place in ca. 308 Ma (Fig. 8a). Zircon U–Pb dating analyses of the sandstone samples suggest that the volcaniclastic rocks were likely formed by the syn-sedimentary volcanism (Fig. 8a, b). The age distributions of tuffs constrained that volcanism took place between 315 and 304 Ma, and the major explosion occurred in ca. 307 Ma (Figs. 7 and 8c). Our results, in combination with the published data, indicate that the Arbasay Formation in the Shichang Region was deposited during the Late Carboniferous (Fig. 8d).

5.2 Comparisons with coeval deposits in the Toksun–Turpan Region

The Arbasay Formation was first established in the Baiyanggou area of Shichang Region, North Tianshan (BGMRXUAR 1999), Xinjiang, northwestern China. Later, a series of volcanic–sedimentary rocks of the Upper Carboniferous Miulintuokaitaoshan Formation (BGMRXUAR 1993) in the Toksun-Turpan Region were re-assigned to the Arbasay Formation (BGMRXUAR 1999) as well. However, the ‘Arbasay Formation’ in the Toksun–Turpan Region is obviously different from that exposed at its stratotype area in rock types, geochemical characteristics and depositional environment (Gu et al. 2000, 2001; Liang et al. 2011; Wang et al. 2017). Firstly, the rock types in the Shichang Region are dominated by andesite with some interbeds of basalts and rhyolites (Figs. 2b, c and 3). However, the rock types in the Toksun–Turpan Region comprise of volcanic–clastic–carbonate rock association, and the volcanic rock is different from other rocks in having bimodal volcanic with minor constituents of dacite and rare andesitic rock (Liang et al. 2011). Secondly, the volcanic rock in the Shichang Region belongs to the calc-alkaline series and shows arc-like geochemical composition (Liu et al. 2012; Wang et al. 2017). In contrast, the volcanic rock in the Toksun–Turpan Region is moderately enriched in incompatible elements, which is assigned to the alkaline and tholeiite series, showing rift-related geochemical characteristic (Gu et al. 2000, 2001; Liang et al. 2011). Thirdly, the depositional environments of volcaniclastics from these two regions are also different. The Arbasay Formation in the Shichang Region is interpreted to be deposited in continental alluvial fan setting (Liu et al. 2015), while the Arbasay Formation in the Toksun–Turpan Region was marine delta and littoral zone formed probably due to the Bogda Rift during the Late Carboniferous (Gu et al. 2000, 2001; Guo et al. 2014; Jiang et al. 2015; Liang et al. 2011). Moreover, the Arbasay Formation volcanic rock in the Shichang Region is typical of continental volcanic origin (BGMRXUAR 1999; Liu et al. 2015), whereas the same rock in the Toksun–Turpan Region is of marine volcanics sources (Wang 1988; Gu et al. 2000). On the contrary, the volcanic–clastic–carbonate rock association exposed in the Toksun–Turpan Region show high similarity to those of the Aoertu Formation that was established in Urumqi region (Wang 1988) in terms of lithologic and geochemical features.

Accordingly, the “Arbasay Formation” in both Toksun–Turpan and Shichang Regions is lithologically different from one another. The Upper Carboniferous volcanics, clastics, and carbonates of the Toksun–Turpan Region are better assigned to the Aoertu Formation (Wang 1988) than to the Arbasay Formation. The latter unit name is remained to the volcanic–sedimentary rocks in the Shichang Region.

5.3 Implications for the closure timing of the Junggar-Balkhash Ocean

Both geochemical features and tectonic inference suggest that the North Tianshan island arc was likely generated by southward subduction of the Junggar-Balkhash oceanic lithosphere and may represent the collision of the Junggar and Central Tianshan Blocks (Han et al. 2010; Windley et al. 2007; Xiao et al. 2008). However, as aforementioned, the timing for the last consolidation was calibrated to Devonian to Permian. The depositional period of the Arbasay Formation is crucial in confirming the timing for the last closure of the Junggar-Balkhash Ocean (Wang et al. 2017a). On the one hand, the Arbasay Formation was interpreted as the result of sedimentation of a continental arc during the transition stage from tectonic compression to extension (Wang et al. 2017), implying that the Junggar-Balkhash oceanic lithosphere continuously subducted until the Late Carboniferous (Wang et al. 2017). On the other hand, the Permian alkaline igneous rocks and A-type granitoids were regarded as the post-collisional products (Ge et al. 2015; Han et al. 2010; Tang et al. 2010; Wang et al. 2009). Moreover, the Early Permian Taoxigou Group in the Aiweiergou area represents the first conglomerate unit since ebb of marine sediments in the Tianshan tectonic complex (Chen et al. 2015). As a result, the deposition of the Arbasay Formation marks the last closure of the Junggar-Balkhash Ocean during the Late Carboniferous.

6 Conclusions

Regional geological characteristics, LA–ICP–MS and SHRIMP zircon U–Pb dating on the volcanic and volcaniclastic rocks of the Arbasay Formation suggested the age of the Arbasay Formation in the Shichang Region should be redefined as the Late Carboniferous, which marks that the last closure time of the Junggar-Balkhash Ocean was Late Carboniferous. In contrast, the previously called “Arbasay Formation” in the Toksun–Turpan Region should be better assigned to the Aoertu Formation.