Geochronology and geochemistry of Permian bimodal volcanic rocks from central Inner Mongolia, China: Implications for the late Palaeozoic tectonic evolution of the south-eastern Central Asian Orogenic Belt

Accepted Manuscript Geochronology and geochemistry of Permian bimodal volcanic rocks from central Inner Mongolia, China: Implications for the late Palaeozoic tectonic evolution of the south-eastern Central Asian Orogenic Belt Zhicheng Zhang, Yan Chen, Ke Li, Jianfeng Li, Jinfu Yang, Xiaoyan Qian PII: DOI: Reference:

S1367-9120(17)30014-7 http://dx.doi.org/10.1016/j.jseaes.2017.01.012 JAES 2930

To appear in:

Journal of Asian Earth Sciences

Received Date: Revised Date: Accepted Date:

24 May 2016 11 January 2017 12 January 2017

Please cite this article as: Zhang, Z., Chen, Y., Li, K., Li, J., Yang, J., Qian, X., Geochronology and geochemistry of Permian bimodal volcanic rocks from central Inner Mongolia, China: Implications for the late Palaeozoic tectonic evolution of the south-eastern Central Asian Orogenic Belt, Journal of Asian Earth Sciences (2017), doi: http:// dx.doi.org/10.1016/j.jseaes.2017.01.012

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Geochronology and geochemistry of Permian bimodal volcanic rocks from central Inner Mongolia, China: Implications for the late Palaeozoic tectonic evolution of the south-eastern Central Asian Orogenic Belt Zhicheng Zhanga, b*, Yan Chena, b, Ke Lic, Jianfeng Lid, Jinfu Yanga, b, Xiaoyan Qiana,b

a MOE Key Laboratory of Orogenic Belts and Crustal Evolution, Peking University, Beijing 100871, China b School of Earth and Space Sciences, Peking University, Beijing 100871, China c College of Earth Sciences, Chengdu University of Technology, Chengdu 610059, China d Key Laboratory of Paleomagnetism and Tectonic Reconstruction of Ministry of Land and Resources, Institute of Geomechanics, Chinese Academy of Geological Sciences, Beijing 100871, China * Corresponding author: Zhicheng Zhang Corresponding author’s contact details: Prof. Zhicheng Zhang Department of Geology School of Earth and Space Sciences Peking University, Beijing, China Tel: 86–10–62757287 Fax: 86–10–62751159

E–mail: [email protected] Postal Address: Room 3307, Yifu–2 Building, Peking University, No. 5 Yiheyuan Road, Haidian District, Beijing 100871, China.

Abstract: Zircon U–Pb ages, geochemical data and Sr–Nd isotopic data are presented for volcanic rocks from the lower Permian Dashizhai Formation. These rocks are widely distributed in the south-eastern Central Asian Orogenic Belt in central Inner Mongolia, China. The volcanic rocks mainly consist of basaltic andesite and rhyolite, subordinate dacite and local andesite, and exhibit bimodal geochemical features. The results of zircon U–Pb dating indicate that the volcanic rocks formed during the early Permian (292 Ma to 279 Ma). The mafic volcanic rocks belong to low-K tholeiitic to medium-K calc-alkaline series. These mafic volcanic rocks are also characterised by moderately enriched light rare earth element (LREE) patterns; high abundances of Th, U, Zr and Hf; negative Nb, Ta and Ti anomalies; initial 87Sr/86Sr ratios of 0.70514 to 0.70623; and positive εNd(t) values (+1.9 to +3.8). These features indicate that the mafic volcanic rocks were likely derived from the high-percentage partial melting of subduction-related metasomatised asthenospheric mantle. The felsic rocks show an A-type affinity, with enrichments in alkalis, Th, U and LREEs. The felsic rocks are depleted in Ba, Sr, Nb, Ta and Ti and exhibit moderately LREE-enriched patterns (LaN/YbN = 2.09 to 6.45) and strongly negative Eu anomalies (Eu/Eu* = 0.04 to 0.25). These features, along with the positive εNd(t) values (+2.6 to +7.7) and young TDM2 ages (TDM2 = 435 Ma to 916 Ma), indicate that the felsic rocks were likely derived from a juvenile crustal source that mainly consisted of juvenile mid-ocean ridge basalt-related rocks. The volcanic association in this study and in previously published work widely distributed in central Inner Mongolia. The observations in this study suggest that the lower Permian volcanic rocks formed in an identical tectonic environment. The regional geological data indicate that the bimodal volcanic rocks from the lower Permian Dashizhai Formation in the study area formed in an extensional setting that was likely related to post-collisional delamination. Keywords: zircon U–Pb dating, early Permian, geochemistry, Dashizhai Formation, central Inner Mongolia, tectonic evolution

1. Introduction The Inner Mongolia–Daxinganling Orogenic Belt (IMDOB) (Robinson et al., 1999; Miao et al., 2008; Chen et al., 2012), also known as the Xing’an–Mongolia Orogenic Belt (Ren et al., 1999; Wu et al., 2002; Xu et al., 2015), is traditionally considered the southeastern section of the Central Asian Orogenic Belt (CAOB) (Fig. 1a). The IMDOB is a key area for analysing the history of accretion and collision history between the North China Block (NCB) and the Southern Mongolian microcontinent (Tang, 1990). Numerous studies have been performed over the past three decades to understand the development of the IMDOB (Shao, 1989; Tang, 1990; Chen et al., 2000, 2009; Xiao et al., 2003, 2009; Li, 2006; Jian et al., 2008, 2010, 2012; Cocks and Torsvik, 2013; Kröner et al., 2014; Xu et al., 2013, 2014, 2015). Several models have been proposed for the tectonic evolution of the IMDOB and can be broadly grouped into two types based on the position and timing of the final closure of the Palaeo-Asian Ocean (PAO): (1) the successive accretion of microblocks and magmatic arcs to the northern margin of the NCB and southern margin of the Southern Mongolian microcontinent, which led to the formation of the Solonker suture during the late Permian (Sengör et al., 1993; Chen et al., 2000; Xiao et al., 2003, 2009; Li et al., 2006; Jian et al., 2008, 2010; Miao et al., 2008; Li et al., 2011), and (2) the closure of the PAO along the Xilinhot and Ondor Sum belts before the end of the Devonian (Tang, 1990; Zhao et al., 2013; Cheng et al., 2014; Xu et al., 2013, 2015) or during the Late Devonian to Carboniferous (Shao 1989; Hong et al., 1994; Zhang et al., 2008). Although the IMDOB is commonly considered a typical accretionary orogen, the actual timing of the final closure of the PAO is still under considerable debate (Xiao et al., 2003, 2009; Li, 2006; Chu et al., 2013; Liu et al., 2013; Xu et al., 2015; Fu et al., 2015; Yu et al., 2016). The late Palaeozoic geodynamic setting is the key to determiningthe tectonic evolution of the IMDOB. However, the nature of geodynamic setting remains unclear. Some authors

proposed that an active continental arc setting existed during the late Carboniferous to Permian (Chen et al., 2000; Zhang et al., 2006, 2009; Qing et al., 2012; Liu et al., 2013, 2016; Eizenhöfer et al., 2015a, 2015b; Yu et al. 2016), whereas other authors insisted that the tectonic setting had already transformed into a post–collisional extensional regime by the late Carboniferous (Zhu et al., 2001; Shi et al., 2004; Zhang et al., 2008; Tang et al., 2011; Chu et al., 2013; Li et al., 2014a; Zhang et al., 2014; Chen et al., 2015; Zhang et al., 2015c). These different opinions regarding the late Palaeozoic tectonic setting are attributed to the absence of a comprehensive understanding of the diverse upper Palaeozoic volcanic rocks in central Inner Mongolia. These upper Palaeozoic volcanic rocks are, widely distributed in central Inner Mongolia (Fig. 1b), are the key to understanding the contradictions among these models. The north-eastern Sonid Zuoqi area is one of the regions where upper Palaeozoic volcanic rocks are well exposed. Understanding the eruptive ages and the origin of these volcanic rocks can provide important constraints on the tectonic evolution of the IMDOB during the late Palaeozoic. This paper integrates geochronological, petrological and geochemical data for the upper Palaeozoic volcanic rocks from the north-eastern Sonid Zuoqi area to (1) constrain the extrusive age of the volcanic rocks; (2) determine their magma source and trace their magmatic evolution; and (3) reveal their tectonic implications for the evolutionary history of the IMDOB. 2. Geological background 2.1. Tectonic framework The formation of the IMDOB is generally attributed to a series of complex accretionary– collisional events that culminated in the NCB–Southern Mongolian terranes collision. The IMDOB mainly consists of island arcs, ophiolites, oceanic islands, seamounts, accretionary

wedges, oceanic plateaus and microblocks, and it is comparable to the Circum-Pacific Mesozoic–Cenozoic accretionary orogens (Xiao et al., 2003, 2009; Windley et al., 2007). The IMDOB comprises several roughly east–west-trending tectonic zones in central Inner Mongolia, from south to north, namely the southern early to middle Palaeozoic orogenic belt (SOB), Hunshandake block (Solonker suture zone), northern early to middle Palaeozoic orogenic belt (NOB), Erenhot–Hegenshan ophiolite accretionary belt (EHOB) and Uliastai continental margin (UCM) (Xiao et al., 2003; Jian et al., 2012; Xu et al., 2013; Zhang et al., 2015c) (Fig. 1b). Three additional tectonic belts continue west into the southern portion of Mongolia, including the Nuhetdavaa terrane, Enshoo terrane, and Hutag Uul block (Badarch et al., 2002), which correlate well with the tectonic belts, i.e., the UCM, EHOB and NOB, respectively (Fig. 1b), in Inner Mongolia (Xiao et al., 2009). The Solonker suture zone, which is located between the SOB and the NOB, is more than 900 km long and 60 km wide and is marked by mélanges and remnants of arcs and ophiolites. This suture zone has been interpreted as the final suture zone of the south-eastern CAOB (Xiao et al., 2003). However, others have argued that the Ligurian-type ophiolite characteristics of this zone indicate an extensional tectonic setting (Xu et al., 2014, Song et al., 2015). The NOB is characterised by metamorphic complexes, blueschists, ophiolites, granites and volcanic rocks. A discontinuous metamorphic complex, which is referred to by many researchers as the Xilin Gol Complex (Li et al., 2014b), represents a Precambrian micro– continental block. Xue et al. (2009) revealed that this complex represents a subductionrelated magmatic terrane that developed within a fore-arc environment based on sensitive high-resolution ion microprobe (SHRIMP) zircon U–Pb isotopic data. However, a clear unconformity between Devonian continental molasses deposits, which contain plant fossils,

and mélange, which contains 383 Ma blueschist blocks, indicates the existence of a middle Palaeozoic orogenic belt (Xu et al., 2013). The isotopic ages of igneous rocks from this belt are between 480 Ma and 420 Ma (Chen et al., 2000; Jian et al., 2008; Chen et al., 2016a, 2016b), but a late early Carboniferous thermal event is reflected by undeformed (garnetbearing) granites that intruded the Xilin Gol Complex between 310 Ma and 324 Ma (Chen et al., 2000; Shi et al., 2003; Xue et al., 2009; Hu et al., 2015).. Late Carboniferous epicontinental carbonate deposits with subordinate clastic rocks covered the north-eastern Sonid Zuoqi and adjacent areas (Zhao et al., 2016). The EHOB contains several ophiolitic fragments that consist of dunite, gabbro, sheeted dikes, tholeiitic pillow basalt, radiolarian chert and coral limestone (Tang 1990; Jian et al., 2012; Zhang et al., 2015c). Recently acquired SHRIMP zircon U–Pb ages for the Hegenshan ophiolitic rocks and Eastern Erenhot ophiolitic rocks ranging from 330 Ma to 354 Ma (Jian et al., 2012; Zhang et al., 2015c; Yang et al., 2017) are interpreted to be associated with the formation of a new ocean. An undeformed and unmetamorphosed dioritic porphyry dike that intruded into the Carboniferous strata near the Eastern Erenhot ophiolitic complex exhibits an intrusive age of 313.6 ± 2.9 Ma and provides a possible upper limit for the emplacement of the ophiolite (Zhang et al., 2015c). The Hegenshan ophiolite in the Xiaobaliang area is unconformably overlain by the middle Permian Zhesi Formation, which suggests that the emplacement time of the Hegenshan ophiolite must have occurred before the middle Permian (approximately 280 Ma), most likely between 300 Ma and 335 Ma (Zhou et al., 2015). 2.2. Lower Permian volcanic rocks in central Inner Mongolia The central portion of Inner Mongolia is characterised by widespread upper Palaeozoic volcanic rocks (Figs. 1b and 2). Previous studies indicated that these volcanic rocks mainly formed during the early Permian, with some small amounts formed during the late

Carboniferous (Zhu et al., 2001; Zhang et al., 2008; Li et al., 2014a, 2015; Fu et al., 2015). The lower Permian volcanic rocks are classified under several stratigraphic schemes according to their locations or the researchers (IMBGMR, 1991; Mueller et al., 1991; Shen et al., 2006). In the EHOB, upper Carboniferous to lower Permian volcanic–sedimentary strata were identified in a 1: 200,000-scale geological map as the Baolige Formation (IMBGMR, 1980). The field relationships showed that these volcanic–sedimentary strata unconformably overlie Devonian sedimentary rocks, are intruded by a number of early Permian alkaline granite intrusions, and are overlain by Upper Jurassic volcanic rocks (Jahn et al., 2009; Li et al., 2015). Lower Permian volcanic–sedimentary strata in the EHOB and NOB regions were identified as the Dashizhai Formation, a general name for the lower Permian volcanic rocks in central Inner Mongolia (IMBGMR, 1991). The Dashizhai Formation mainly consists of rhyolites, dacites, andesites, tuffs and volcaniclastic rocks, with varying thickness of approximately 200 m to 1200 m. Volcanic rocks from the Dashizhai Formation in the Xiwuqi area have been relatively well investigated in recent years. Zircon U–Pb ages and geochemical data from the Dashizhai Formation volcanic rocks in the Xiwuqi area show a bimodal distribution in terms of composition, with magmatic emplacement occurring at approximately 280 Ma in an extensional setting that was most likely related to postcollisional delamination (Zhang et al., 2008; Chen et al., 2014). The lower Permian volcanic–sedimentary strata exposed in the SOB and NCB regions are called the Elitu Formation (Figs.1b and 2). This formation consists of lavas and volcaniclastic rocks. The lava units mainly consist of andesites, dacites and rhyolites, and the volcaniclastic units consist of volcanic agglomerates, breccias and tuffs.

2.3. Regional geology and petrography The Permian volcanic–sedimentary strata of this study lie approximately 20 km northeast from the town of Sonid Zuoqi (Fig. 1c), forming a discontinuous E–W-trending belt approximately 40 km long and 10 km wide. Gao and Jiang (1998) found that the Permian strata in the area comprise the lower Permian Dashizhai Formation, the middle Permian Zhesi Formation and the base of the upper Permian Linxi Formation, according to the integration of key unconformable contact relationships and plant and marine fossils. Volcanic rocks from the Dashizhai Formation along two sections were studied in the north-eastern Sonid Zuoqi area. The volcanic rocks are interbedded with terrigenous clastic strata and mainly consist of felsic rocks with minor mafic rocks in section (a). Felsic rocks often occur as thick layers, with thicknesses ranging from a few tens of metres to hundreds of metres (Fig. 3a). The Dashizhai Formation volcanic strata dip approximately 45° to approximately 70° to the south in the northern portion but 82° to the north in the southern portion in geological section (a) (Fig. 3a). The structural geometry of this section cannot be properly defined and is simply considered a monocline. Three rock units exist in section (b), namely, the Dashizhai Formation volcanic rocks, ultramafic rocks and upper Permian sedimentary strata. The ultramafic rocks are in fault contact with the Dashizhai Formation volcanic rocks to the north and are covered by Quaternary sediments to the south. The upper Permian sedimentary strata in this section are mainly sandstones and shales with limestone lenses and dip approximately 72° to approximately 80° to the south. The Dashizhai Formation, which dips 56° to the north, mainly consists of felsic rocks. Fresh volcanic rocks from the Dashizhai Formation were collected along the two geological sections and on several scattered outcrops along dry river valleys for petrological and geochemical analyses (Fig. 1c). Felsic rocks are grey–white in colour and exhibit a

massive structure and a porphyritic texture with K-feldspar phenocrysts up to 1 mm in size. The matrix mainly consists of aphanitic plagioclase, K-feldspar and quartz, which exhibits microcrystalline or cryptocrystalline textures. The mafic rocks are greyish-green in colour, massive and generally display an intergranular–interstitial texture with fine-grained feldspar and pyroxene. These mafic rocks have not been affected by visible hydrothermal alterations. 3. Analytical methods 3.1. Sampling We chose a total of 29 fresh volcanic rock samples from the Dashizhai Formation in the north-eastern Sonid Zuoqi area for major oxide and trace element analyses, 10 samples for isotope analyses and 6 samples for zircon U–Pb dating. The sample locations are shown on the geological map (Figs. 1c and 3). 3.2. Zircon U–Pb dating Zircon grains from six volcanic rocks were mechanically separated from approximately 5 kg to 10 kg samples by crushing and sieving, followed by standard magnetic and heavy liquid separation and purification by handpicking under a binocular microscope. Representative zircon grains were colourless, transparent columnar crystals that were mounted on an epoxy resin disc and ground to approximately half of their original thicknesses. Cathodoluminescence (CL) images were obtained by using the Quanta 200 FEG Scanning Electron Microscope at Peking University after photographing the grains under reflected and transmitted light. These images were used to determine the internal structure and select target sites for the U–Pb isotopic analyses. The samples (i.e., NM10-30, 37 and 43) were dated using the SHRIMP II instrument at the Beijing SHRIMP Center, Chinese Academy of Geological Sciences. The SHRIMP

analytical procedure for zircon was similar to that described by Williams (1998). The details of the analytical procedures were described by Compston et al. (1992) and Song et al. (2002). The primary beam size was approximately 30 µm. Each analytical site was rastered for 2 min to 3 min prior to analysis. Sites for dating were selected based on the CL images. Standard zircon analyses were conducted after every three or four spots on the sample zircons during data collection to maintain precision. The standards SL13 (U = 238 ppm) and Temora-2 (206Pb/238U age = 417 Ma) were used (Williams, 1998; Black et al., 2003). The decay constants that were used for the age calculations were those recommended by the International Union of Geological Sciences Subcommission on Geochronology (Steiger and Jager, 1977). The measured 204Pb was applied as the common lead correction, and data processing was performed by using the SQUID and ISOPLOT program (Ludwig, 2003). The reported ages are 206Pb/238U ages for all the data because they are considered the most reliable measurement for concordant Phanerozoic zircons (Compston et al., 1992). The analytical results are the mean values of five consecutive scans for each analytical spot. The errors are listed as ±1σ. The zircons from the samples NM12-88, 91 and 113 were analysed for U, Th and Pb using laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) at the Key Laboratory of Orogenic Belts and Crustal Evolution, Peking University. This instrument includes a quadrupole ICP-MS (Agilent 7500c) coupled with a 193-nm ArF Excimer laser (COMPexPro 102, Coherent, DE) with an automatic positioning system. The calibrations for zircon analyses were conducted using NIST 610 glass as an external standard and Si as an internal standard. U–Pb isotope fractionation effects were corrected using the Plesovice zircon (337 Ma) as an external standard. The zircon standard 91500 was also used as a secondary standard to monitor deviations in the age measurements/calculations. The isotopic ratios and element concentrations were calculated using the GEMOC Laser ICP-MS’s Total

Trace Element Reduction program (ver. 4.4.2, Macquarie University). The relative probability diagrams were plotted with ISOPLOT (3.0) (Ludwig, 2003). Detailed descriptions of the operating conditions of the laser ablation system and LA-ICP-MS instrument and the data reduction method can be found in Tang et al. (2014). 3.3. Major and trace element measurements All 29 samples were crushed in a specially designed steel crusher and then powdered in an agate mill to a 200 mesh grain size. The whole-rock major element abundances were determined via the X-ray fluorescence technique using a Jarrell-AshICAP 9000SP spectrometer on fused–glass discs at the Key Laboratory of Orogenic Belts and Crustal Evolution, Peking University, Beijing. The analytical uncertainty range was limited to 1%, as monitored through the analysis of the Chinese national standard samples GSR-1, GSR-2 and GSR-3. The losses on ignition (LOI) were determined by the gravimetric method. Analyses of the trace elements were conducted with the VG Axiom multi-collector inductively coupled plasma mass spectrometer (MC-ICP-MS) after the acid digestion of the whole-rock powders (50 mg) in Teflon bombs at the Beijing Research Institute of Uranium Geology, Beijing. The analytical precision of the trace element analyses was better than 5%, except for Nb and Ta, which had precisions of 9% based on analyses of the Chinese national standard samples GSR-1 and GSR-3. 3.4. Sr–Nd isotope analyses Whole-rock Sr and Nd isotopic data for six samples (i.e., NM10-30, 34, 37, 39, 43 and 44) were measured on an ISOPROBE-T thermal ionisation mass spectrometer at the Beijing Research Institute of Uranium Geology, Beijing. The Sr and Nd isotopic compositions of another four samples (i.e., NM12-87, 90, 93 and 113) were measured with the VG Axiom MC-ICP-MS at the Key Laboratory of Orogenic Belts and Crustal Evolution, Peking

University, Beijing. The 87Rb/86Sr and 147Sm/144Nd ratios were calculated from the Rb, Sr, Sm and Nd contents measured using the ICP-MS. The following parameters were used to calculate the initial (87Sr/86Sr)i values, initial Nd isotopic (εNd(t)) values, and Nd model ages: (147Sm/144Nd)CHUR = 0.1967 (Jacobsen and Wasserburg, 1980); (143Nd/144Nd)CHUR = 0.512638 (Goldstein et al., 1984); (143Nd/144Nd)DM = 0.513151 and (147Sm/144Nd)DM = 0.2136 (Liew and Hofmann, 1988); Rb = 1.42 × 10–11 year–1 (Steiger and Jager, 1977); and Sm = 6.54 × 10–12 year–1 (Lugmair and Marti, 1978). 4. Analytical results 4.1. Zircon U–Pb dating results The results of the SHRIMP zircon U–Pb analyses of the samples NM10-30, 37 and 43 are listed in Table 1. The results of the LA-ICP-MS zircon analyses for the other samples (i.e., NM12-88, 91 and 113) are shown in Table 2. The zircons from the rhyolite sample NM10-30 range in size from 50 µm to 120 µm and are mostly sub-euhedral columnar crystals with length-to-width ratios between 1:1 and 3:1. In the CL images, most of the grains show oscillatory zoning. Some grains show grey cores with weak oscillatory growth zoning. A total of 12 analyses were conducted with the SHRIMP II ion microprobe on 12 grains from this sample. All 12 analyses exhibit Th/U ratios between 0.48 and 0.65 and form a coherent group with a weighted mean 206Pb/238U age of 279.4 ± 2.9 Ma [mean square weighted deviation (MSWD) = 0.67] (Fig. 9a). This result is interpreted as the extrusive age of the felsic volcanic rocks from the Dashizhai Formation in the northeastern Sonid Zuoqi area. The zircons from the rhyolite sample NM10-37 are mostly clear, euhedral to subhedral and stubby to elongate prisms. The size of these zircons ranges from 120 µm to 200 µm with

length-to-width ratios between 1.5:1 and 2:1. These zircons commonly exhibit strong magmatic oscillatory zoning in the CL images. A total of 12 analyses were conducted on 12 grains from this sample. The measured U and Th concentrations range from of 135 ppm to 592 ppm and from 55 ppm to 262 ppm, respectively. All the analyses yielded Th/U ratios between 0.34 and 0.56 and a weighted mean 206Pb/238U age of 291.6 ± 2.9 Ma, with an MSWD value of 0.67 (Fig. 9b). The zircon grains from the andesite sample NM10-43 exhibit clear, subhedral to euhedral, and irregularly shaped (stubby to elongate prisms) morphological features. Most of the zircons exhibit distinct oscillatory zoning in the CL images, which indicate that these grains are typical magmatic zircons. Some zircons with cores (e.g., NM10-43-6 and 9) probably have a complicated origin. The Th/U ratios (0.18 to 1.54) of 13 plots (except for the plot of NM10-43-3) revealed the characteristics of magmatic zircons. One grain is a wellrounded xenocryst that yielded a 206Pb/238U age of 1,767 ± 19 Ma. Some zircon grains (spots of NM10-43-6, 8 and 10) are rounded xenocrysts with early to late Palaeozoic ages (approximately 329 Ma to 536 Ma). Three zircon grains (spots of NM10-43-1, 7, 12 and 14) yielded a weighted mean 206Pb/238U age of 308.2 ± 4.4 Ma, which probably reflects inheritance from the previous Carboniferous volcanic rocks in this area (Li et al., 2014a, 2015). NM10-43-3.1 was young, probably because of the effect of a later thermal event. The other 3 data (i.e., NM10-43-4, 5 and 13) from the 12 analyses clustered on or around the concordia line (Fig. 9c) and yield a weighted mean 206Pb/238U age of 288.9 ± 4.7 Ma. This age represents the crystallisation age of the basaltic andesite, which is similar to the zircon age of the rhyolite sample NM10-37 (291.6 ± 2.9 Ma). This finding indicates their contemporaneous emplacement time. A total of 30 zircon grains from the rhyolite sample NM13-88 were analysed by LAICP-MS. Most of the zircons are subhedral to euhedral and irregularly shaped (stubby to

elongate prisms), with oscillatory zoning in the CL images. A cluster of 19 zircon concordant analyses with Th/U ratios from 0.40 to 0.76 yielded a weighted mean 206Pb/238U age of 282.1 ± 1.6 Ma (MSWD = 0.39) (Fig. 9d). This result is interpreted as the extrusive age of the felsic volcanic rocks from the Dashizhai Formation in the Huheaobao Taolega area. The dating results of the other zircons fell outside the concordia line. This result possibly indicates Pb or U loss. The zircons from the rhyolite sample NM13-91 range in size from 50 µm to 150 µm and are mostly sub-euhedral columnar crystals with length to width ratios between 1:1 and 1.5:1. In the CL images, most of the grains show oscillatory zoning. Some grains show grey cores with weak oscillatory growth zoning. A total of 30 zircon grains from the rhyolite sample NM13-91 were analysed by LA-ICP-MS. A cluster of 17 zircon concordant analyses with Th/U ratios from 0.28 to 0.82 yielded a weighted mean 206Pb/238U age of 285.5 ± 2.4 Ma (MSWD = 0.60). This result is interpreted as the crystallisation age of the rhyolite (Fig. 9e). Two zircon grains (spots of NM13-91-2 and 26) yielded 206Pb/238U ages of 307 and 326 Ma, which probably reflect inheritance from the previously emplaced Carboniferous plutons in this area (Chen et al., 2000). The other zircons produced discordant ages with 206Pb/238U dates from 293 Ma to 671 Ma. This finding indicates variable radiogenic Pb loss. The zircons from the rhyolite sample NM13-113 range in size from 50 µm to 150 µm and are mostly sub-euhedral columnar crystals with length-to-width ratios between 1:1 and 1.5:1. In the CL images, most of the grains show oscillatory zoning. Some grains show grey cores with weak oscillatory growth zoning. A total of 30 zircon grains from the rhyolite sample NM13-113 were analysed by LA-ICP-MS. Three zircons produced discordant ages, with the measured 207Pb/238U dates ranging from 318 Ma to 527 Ma (Table 2) because of the marked loss of radiogenic Pb. One zircon grain (spot of NM13-113-24) yielded a 206Pb/238U age of 306 Ma, which probably reflects inheritance from the Carboniferous volcanic rocks in

this area (Li et al., 2014a, 2015). A cluster of 26 zircon concordant analyses with Th/U ratios of 0.33 to 0.63 yielded a weighted mean 206Pb/238U age of 281.2 ± 2.0 Ma (MSWD = 0.93) (Fig. 9f). This result is interpreted as the crystallisation age of the rhyolite. 4.2. Geochemical results for the volcanic rocks All 29 samples show variable LOI (0.55% to 6.80%). This result indicates different degrees of alteration. The amounts of LOI in the mafic rock samples are higher than those in the felsic rock samples. Major and trace element data for the volcanic rocks from the Dashizhai Formation in the north-eastern Sonid Zuoqi area in central Inner Mongolia are presented in Table 3. 4.2.1. Major elements The samples from the lower Permian Dashizhai volcanic rocks in the study area roughly consist of two units, which exhibit a quasi-bimodal geochemical distribution in the total alkalis versus silica diagram (Le Maitre, 2002) (Fig. 4a). The mafic end-member mainly consists of basaltic andesite with minor basaltic trachyandesite. The felsic end-member consists of alkali rhyolite, subalkali rhyolite, and minor dacite. One andesitic sample is also observed, which plots between the two end-members. Andesite exhibits similar geochemical characteristics to the felsic volcanic rocks (discussed in subsequent sections). Thus, this andesite can be assigned to the felsic end-member for convenience. In the Harker plots of major elements (Fig. 5), most of the elements define a roughly correlated evolution trend: Al2O3, Fe2O3T, MgO, CaO, and TiO2 show negative correlations with SiO2, whereas Na2O shows no trend. The negative correlations between SiO2 and most major elements indicate that fractional crystallisation played an important role during the ascent of the magma to the surface.

The mafic volcanic rocks in the north-eastern Sonid Zuoqi area are characterised by low TiO2 (0.78 wt.% to 1.05 wt.%) and P2O5 (0.14 wt.% to 0.21 wt.%) contents, high Al2 O3 (15.30 wt.% to 19.83 wt.%) content and moderate CaO (3.39 wt.% to 6.76 wt.%), MgO (1.81 wt.% to 5.98 wt.%) and Fe2O3T (6.80 wt.% to 8.29 wt.%) contents, with low to intermediate Mg# (34 to 60). These rocks plot within the subalkaline series on the total alkalis versus silica chemical classification diagram (Fig. 4a). Most of the mafic rocks belong to the medium-K calc-alkaline series, whereas one sample plots in the low-K tholeiitic series on the K2O versus SiO2 diagram (Fig. 4b). All the mafic rocks fall along the boundary between the calc–alkaline and tholeiitic series in the MgO/FeO versus SiO2 diagram (Fig. 6b). These rocks also plot in the field of the calc-alkaline magma series in the AFM diagram (Fig. 6a). The felsic rocks exhibit large variations in SiO2 (65.07 wt.% to 79.95 wt.%) and low MgO (0.01 wt.% to 1.37 wt.%), CaO (0.08 wt.% to 3.32 wt.%), TiO2 and P2O5 concentrations. These felsic rocks exhibit medium to high K2O (2.20 wt.% to 6.52 wt.%) and display an evolutionary tendency from the medium-K calc-alkaline series to the shoshonitic series via the high-K calc-alkaline series (Fig. 4b). These felsic rocks predominantly show peraluminous characteristics, with A/CNK values from 1.03 to 1.73. 4.2.2. Trace elements The mafic rocks from the Dashizhai Formation exhibit high total rare earth element (∑REE) values (81.43 ppm to 97.12 ppm) and exhibit moderate enrichment in light rare earth elements (LREE) ((La/Yb)N = 3.7 to 5.1) and small negative Eu anomalies (Eu/Eu* = 0.84 to 0.93) (Table 3) (Fig. 7a). In the primitive mantle-normalised spider diagram (Fig. 7b), these mafic rocks show moderate enrichment in high field strength elements (e.g., Zr and Hf) and positive Th and U anomalies but negative Nb, Ta and Ti anomalies. Large ion lithophile

elements (LILEs) (e.g., Rb, Ba, Sr and K) do not show co–systematic changes in some samples. This pattern reflects their relative mobilities during post-magmatic processes. The felsic rocks from the Dashizhai Formation exhibit high ratios of Rb/Sr (2.39 to 14.84), K/Rb (151 to 421) and 10000 × Ga/Al (2.02 to 3.47), which are comparable to the global averages of the Rb/Sr (3.52), K/Rb (229) and Ga/Al (3.75) ratios of A-type granites (Whalen et al., 1987). The Th/U (3.87–6.86) and Nb/Ta (10.76–15.56) ratios are close to those expected for the lower continental crust (Rudnick and Gao 2003). Samples of these felsic rocks exhibit uniform REE and trace element patterns. The rocks have medium to high ∑REE values (106 ppm to 453 ppm), with moderate enrichment in LREEs (LaN/YbN = 2.09 to 6.45) and strongly negative Eu anomalies (Eu/Eu* = 0.05 to 0.29) (Fig. 7). Such REE patterns reflect feldspar fractional crystallisation or hydrothermal fluid and end-stage magma interactions (Jahn et al., 2001, 2004). In the primitive mantle-normalised diagram (Fig. 7d), these felsic rocks are enriched in LILEs (e.g., Rb, K, Th, U and LREEs) and depleted in Ba, Sr, Nb, Ta and Ti. 4.2.3. Sr and Nd isotopes Sr and Nd isotopes were analysed for 10 volcanic rocks from the study area. The results are listed in Table 4. The initial isotopic ratios were calculated based on their zircon U–Pb ages. As shown in the εNd(t) versus (87Sr/86Sr)i diagram (Fig. 8), volcanic rocks from the Dashizhai Formation in the study area are characterised by large dispersions in 87Rb/86Sr (0.0812 to 43.6238), (87Sr/86Sr)m (0.705557 to 0.869383), and (87Sr/86Sr)i (0.69497 to 0.70918). Some samples with 87Rb/86Sr > 4 yielded initial Sr values that are unreliable; the initial Sr ratios of these samples are used only for reference (Liégeois and Stern, 2010; Küster et al., 2008). The (87Sr/86Sr)i values for samples with 87Rb/86Sr < 3 range from 0.70390 to 0.70773. The 147Sm/144Nd values range from 0.1295 to 0.1497. The (143Nd/144Nd)m values

range from 0.512601 to 0.512935. Their εNd(t) values fall between +1.9 and +7.7. The Nd model ages (TDM2) of the samples range from 435 Ma to 916 Ma. 5. Discussion 5.1. Timing of the early Permian volcanism in central Inner Mongolia The samples from the Dashizhai Formation in the north-eastern Sonid Zuoqi area showed an overall age range from 291.6 ± 2.9 Ma to 279.4 ± 2.9 Ma. These results indicate that these volcanic rocks were erupted in the early Permian period. The detrital zircons of one sandstone sample in section (a) were predominantly Palaeozoic in age (269 Ma to 538 Ma), and the youngest age peak was at 275 Ma (Chen et al., 2016a). This finding is consistent with the dating results of the volcanic rocks. Coeval volcanic rocks have also been widely recognised by recent geochronological studies in neighbouring areas. The results of geochronological studies from central Inner Mongolia were compiled to understand the spatial and temporal distributions of the early Permian volcanism, and the results are shown in Fig. 2. Zhang et al. (2011) suggested that volcanic rocks from the Baolige Formation formed at 289 Ma to 287 Ma to the north of the study area according to SHRIMP zircon U-Pb dating. The massive basalt in the Hegenshan block provided early Permian ages ranging from 294 Ma to 292 Ma at Hegenshan to the northeast of the study area (Miao et al., 2008; Liu, 2009). The lower Permian volcanic rocks are commonly intercalated with sedimentary rocks, as revealed by outcrops in the Xiwuqi–Linxi area. These rocks have early Permian ages ranging from 281 Ma to 274 Ma (Zhu et al., 2001; Zhang et al., 2008; Liu, 2009; Chen et al., 2014; Wang et al., 2014; Liu et al., 2016). The lower Permian volcanic rocks are chemically characterised as bimodal volcanic rocks, implying an extensional tectonic setting (Zhang et al., 2008; Chen et al., 2014; Liu et al., 2016). In a study of the Sumoqagan Obo Fluorite mineralisation in the southwestern Erenhot area, the selected rhyolite samples from the

hanging wall of the fluorite bodies yielded an average SHRIMP zircon U–Pb age of 276 ± 10 Ma. The rhyolite and the related fluorite deposit formed within an early Permian rift basin in an extensional setting (Nie et al., 2009). The volcanic rocks of the Duolun Formation in the Bilihe goldfield to the east of Ondor Sum formed during the Jurassic period according to 1:200,000 regional field surveys conducted in 1975 (IMBGMR, 1975). New LA-ICP-MS zircon U–Pb geochronological data revealed that these volcanic rocks formed during the early Permian. Two andesitic rocks yielded U–Pb ages of 281 ± 4.3 and 281 ± 12 Ma that are relatively uniform within the analytical error. These volcanic rocks should belong to the Dashizhai Formation (Qing et al., 2012). Nine whole-rock samples of andesite and basalt yielded an Rb-Sr isochron age of 275 ± 5 Ma for the Permian volcanic rocks to the west of Xianghuangqi (Nie et al., 1994). To the north of Damaoqi, SHRIMP zircon U–Pb dating yielded ages of 270.2 ± 2 and 268.0 ± 3.8 Ma for rhyolite and dacite, respectively (Chen et al., 2015). Tao et al. (2003) determined a zircon U–Pb age of 285 ± 2 Ma for basic to intermediate volcanic rocks in the upper Palaeozoic volcanic rocks from eastern Solonker in Inner Mongolia. Chen et al. (2012) reported a SHRIMP zircon U–Pb age of 273.7 ± 1.0 Ma for pillow basalt from upper Palaeozoic volcanic–sedimentary strata in the same area. Generally, these dating results for volcanic rocks from the Dashizhai Formation and other volcanic rocks of the same age showed continuous volcanism in central Inner Mongolia from 290 Ma to 270 Ma. Thus, an important magmatic event occurred in central Inner Mongolia. Spatially, the lower Permian volcanic rocks exhibited a widely broad spatial across central Inner Mongolia and crosscut several major tectonic–morphologic terranes that are rarely associated with clear tectonic structures (Figs. 1c and 2). 5.2. Petrogenesis of the volcanic rocks

The volcanic rocks from the Dashizhai Formation in the north-eastern Sonid Zuoqi area are dominated by felsic rocks with a small amount of mafic rocks. The compositions of these volcanic rocks are bimodal, with predominant rhyolite and dacite, subordinate basalticandesite and local andesite. 5.2.1. Hydrothermal alteration effects The volcanic rocks from the Dashizhai Formation were altered to varying degrees during post-eruption processes, as indicated by the relatively high LOI for mafic rocks (>3%) and by petrographic observations. This process might have affected the elemental behaviour of some incompatible elements because of their enhanced mobility during surface alteration. Zr is immobile during interactions between igneous rocks and hydrothermal fluids and is usually used as a reference to investigate the mobility of other trace elements (Gibson et al., 1982; Pearce et al., 1992). Thus, the mobility of some selected elements was assessed using variation diagrams between the lower Permian volcanic rocks and Zr (the diagrams are not shown in this study). Nd and REEs (e.g., La and Sm) correlated well with Zr for these samples. This finding indicates that alternation processes did not significantly affect the lower Permian volcanic rocks. Variations in these lower Permian volcanic rocks can be ascribed to magmatic processes and used to constrain the geochemical characteristics of the source of the investigated rocks. Meanwhile, LILEs (e.g., Rb, Ba and Sr) and U exhibited a certain correlation with Zr for the mafic rocks. This finding indicates that the lower Permian volcanic rocks were slightly mobilised. Therefore, these lower Permian volcanic rocks are used as subsidiary indicators for magmatic processes. Sr isotopes were also carefully screened because of the distinguishable Rb and Sr mobilities. 5.2.2. Petrogenesis of the mafic rocks

The high SiO2 (51.50 wt.% to 61.81 wt.%) contents and the low to medium Mg# (35 to 60) and V contents (166 ppm to 236 ppm) of the mafic rocks from the Dashizhai Formation indicate that these mafic rocks do not represent a primary magma from the mantle. The high SiO2 contents could have resulted from the fractional crystallisation of mafic minerals or the incorporation of crustal materials during the evolution of the magma. The Harker diagrams (Fig. 5) show that the basic to intermediate volcanic rocks exhibited a fractional crystallisation trend. The mafic volcanic rocks from the Dashizhai Formation belong to the calc-alkaline series (Fig. 4a). These mafic volcanic rocks mainly plot within the mid-K calc-alkaline fields on the K2O versus SiO2 diagram (Fig. 4b). Furthermore, these mafic volcanic rocks are characterised by high abundances of Th, U and Pb and by slightly enriched LREE patterns. These rocks have relatively high La/Nb (1.90 to 2.83) and low La/Ba (0.03 to 0.07) ratios (Fig. 10a), which indicate a subduction-modified continental-lithospheric mantle source (Saunders et al., 1992). The Nd(t) values of the mafic rocks in the north-eastern Sonid Zuoqi area are positive (+1.9 to +3.8), indicating that these rocks may have originated from a metasomatised mantle. Generally, metasomatised mantle sources are attributable to fluids released from subducted slabs (Turner et al., 1992; Gertisser and Keller, 2003) or from sediments in subduction zones (Plank and Langmuir, 1998) because the amount of H2O in the asthenosphere is significantly less than that in the mantle wedge above subduction zones (Green et al., 2010). Furthermore, the Ba/La versus Th/Yb diagram can be used to further constrain the source of this enrichment because these ratios are reliable indicators of potential sediment or fluid contributions to magma source regions (Woodhead et al., 2001). The mafic volcanic rocks in the Dashizhai Formation exhibit constant Th/Yb ratios and variable Ba/La ratios (Fig. 10b), indicating significant fluid enrichment in the source and negligible involvement of sediments (Woodhead et al., 2001; Fu et al., 2015). Recently, Wang et al.

(2015) reported that slab-triggered wet upwelling produces large volumes of melt, which may form in the hydrous mantle transition zone. This phenomenon explains the arc-like signatures that are observed in some large-scale intracontinental magmas (Xia, 2014; Wang et al., 2015). As mentioned previously, the distinctive features of the mafic rocks (e.g., negative Nb– Ta anomalies) (Fig. 7b) might have resulted from continental contamination or might have been derived from the subduction-related sources (Rudnick and Gao, 2003; Xia, 2014; Wang et al., 2015). However, all the investigated mafic rocks are characterised by positive Zr–Hf anomalies (Fig. 7b). Rocks that are affected by continental contamination exhibit similar anomalies because crustal materials are relatively rich in these two elements (Xia, 2012). Therefore, crustal contamination played a significant role during the evolution of the magma. The formation of these mafic rocks can be explained by the reduction and melting of mantle wedge materials because of fluids from subducted materials. These magmas caused crustal remelting during their ascent. These mafic rocks can also be explained as the result of the melting of the residual depleted mantle wedge, which was affected by subduction fluids during a late thermal event. Zhang et al. (2008) proposed that these volcanic rocks were produced by high-percentage partial melting of metasomatised asthenospheric mantle in the spinel facies field at depths shallower than 60 km to 70 km. Generally, the mafic volcanic magma originated from the partial melting of depleted mantle that had been metasomatised by subduction related fluids. Moreover, the basaltic andesite, basaltic trachyandesite, and andesite were produced by the fractional crystallisation of the basaltic magma. 5.2.3. Origin of the felsic rocks The felsic rocks mainly include rhyolite and dacite, based on the spatio-temporal distribution of the volcanic rocks from the Dashizhai formation in the north-eastern Sond

Zuoqi area. Such a large-scale, short-duration eruption of felsic magma could not have been produced by the fractional crystallisation of mantle-derived magma and must reflect considerable involvement of continental crust. As discussed previously, these felsic rocks show an A-type affinity, with typical enrichments in silica and alkalis; distinct depletions in Ba, Sr, Nb, Ta, Ti and P; high Rb/Sr, K/Rb and 10,000 × Ga/Al ratios; and fractionated REE patterns with strongly negative Eu anomalies. Furthermore, most of the felsic rocks fall into the A-type granite field on the discrimination diagrams (not shown). A number of hypotheses have been proposed to explain the origin of these felsic rocks: (1) crustal dehydration and anatexis caused by mantle-derived mafic magmas with distinct isotopic compositions (Roberts and Clemens, 1993; Guffanti et al., 1996; Zhu et al., 2012); (2) the reaction of mantle-derived mafic magma with crustal rocks (Foland and Allen, 1991; Frost and Frost, 1997; Frost et al., 1999); and (3) the extensive fractional crystallisation of mantle-derived basaltic melts, coupled with crustal contamination (Bonin, 2004). The tectonic position of A-type felsic rocks has also long been debated because these rocks can form in post-orogenic and anorogenic settings (Whalen et al., 1987, 1996; Eby, 1992; Nédélec et al., 1995; Pitcher, 1997). The felsic rocks in the north-eastern Sonid Zuoqi area bear a close resemblance to those associated with the Variscan post-collisional volcanism in Italy in the K2O + Na2O versus SiO2 diagram of Cortesogno et al. (1998) (Fig. 4a). Furthermore, these felsic rocks exhibit similar trace element compositions to some felsic volcanic rocks and A-type granitic rocks from other post-collisional orogenic belts (Nédélec et al., 1995; Whalen et al., 1996; Patiño Douce, 1997). All these felsic rocks fall within the post collisional fields in the Rb versus Y + Nb and Nb versus Y plots, as shown in Fig. 11 (Pearce et al., 1984). This finding is also consistent with their distribution in the discriminant diagrams of Al2O3 versus SiO2 (Maniar and Piccoli, 1989) and R2 versus R1 (Batchelor and

Bowden, 1985). The young TDM2 ages (TDM2 = 435 Ma to 916 Ma) indicate that these rocks were likely derived from a recently formed crustal source that mainly consisted of early juvenile mid-ocean ridge basalt (MORB)-related rocks with positive Nd isotopic compositions (εNd(t) = +2.6 to +7.7) (Zhang et al., 2008). The Nd isotope data support their generation from the melting of a predominantly juvenile mantle component with subordinate recycled ancient crust. Wu et al. (2002) used a simple two-component mixing model to discuss the origin of the Phanerozoic A-type granites in north-eastern China. This model envisions that these rocks were derived from mixing between a predominantly juvenile crustal component and an old Precambrian crustal component. Following this model, the proportions of juvenile and ancient crustal components that were involved in the genesis of the felsic volcanic rocks can be estimated. The results showed that a juvenile crustal source that resulted from mantle-derived magmatic underplating played a predominant role in the genesis of the felsic volcanic rocks (Fig. 12). Furthermore, the Eu/Eu* ratios (0.05 to 0.09) of some of the felsic rocks (such as samples 12-37–40, NM12-87–90 and NM12-112–113) indicate that the rocks experienced hydrothermal fluid and end-stage magma interactions at higher temperatures. Furthermore, the Nb/Ta (10.76 to 15.56) and Zr/Hf (22.61 to 30.74) ratios are similar to those of the Woduhe and Baerzhe granites, which experienced extensive magmatic differentiation (Jahn et al., 2001, 2004). 5.3. Tectonic implications Tectonic implications of the different types of upper Palaeozoic volcanic rocks in central Inner Mongolia have been investigated in recent decades (Tang et al., 1990; Nie et al., 1994; Zhu et al., 2001; Zhang et al., 2008, 2011; Chen et al., 2014; Li et al., 2014a, 2015; Chen et al., 2015). Several mechanisms have been proposed to explain the tectonic setting of the upper Palaeozoic volcanic rocks in central Inner Mongolia and adjacent regions in northeastern China. These mechanisms include (1) post-orogenic lithospheric extension related to

post-collisional delamination (Zhang et al., 2008; Chen et al., 2014; Li et al., 2014a), (2) postsubduction extension followed by slab breakoff (Zhang et al., 2011), (3) partial melting of a mantle wedge by the northward or southward subduction of the PAO (Xiao et al., 2003; Yu et al., 2016), and (4) continental rifting that developed in response to broadly distributed regional tension (Tang, 1990). The Dashizhai Formation mainly consists of felsic volcanic rocks that are interlayered with minor mafic to intermediate volcanic rocks in the north-eastern Sonid Zuoqi area. The present data, when combined with previous studies, show continuous volcanism in central Inner Mongolia from 290 Ma to 270 Ma (Fig. 2). An obvious question is whether all these volcanic rocks formed in a common tectonic setting. Several lines of evidence indicate that the lower Permian volcanic rocks formed in an identical tectonic environment (e.g., Zhang et al., 2008, 2011; Li et al., 2014a, 2015; Li et al., 2014b). Except for the molasse deposits along the NOB and SOB, Upper Devonian to lower Carboniferous (359 Ma to 318 Ma) sediments are absent from most of central Inner Mongolia (Figs. 1 and 2). Upper Carboniferous volcanic–sedimentary strata directly covered the lower Palaeozoic arc magmatic rocks, lower Palaeozoic ophiolitic mélange and pre-Carboniferous strata. The regional absence of Upper Devonian–lower Carboniferous strata indicates a genetic link between the closure of the PAO and regional uplift during the Late Devonian (Jian et al., 2012; Cheng et al., 2014; Zhao et al., 2016). The base of these upper Carboniferous strata is marked by brecciated limestones with brecciated sandstone blocks from 2 cm to 10 cm in length. This feature indicates an erosional basal contact. An inland sea was proposed to have been the main palaeogeographic feature for central Inner Mongolia during the late Carboniferous (Zhao et al., 2016). However, Chen et al. (2001) and Liu et al. (2013, 2016) proposed a subduction-related setting based on geochemical studies of

granitoids from the Sonid Zuoqi area and the subduction–accretion complex in the Xiwuqi area. The negative Nb, Ta and Ti anomalies in the volcanic rocks from the Dashizhai Formation can also be considered signatures of island arc magmas. The mantle source was previously enriched in LILEs and LREEs by distinct metasomatic agents (e.g., fluids and silicic or carbonatite melts) prior to magma generation. These early Palaeozoic subduction processes were probably responsible for the metasomatism of central Inner Mongolia’s subcontinental lithospheric mantle source (Xu et al., 2013). Therefore, the geochronological and geochemical data in this study and regional geological data indicate that the lower Permian volcanic rocks in central Inner Mongolia formed during post-collisional crustal extension. According to Liégeois (1998a, 1998b), the post-collisional stage is a complex period that includes oblique collision, the subduction of small oceanic plates, large movements along shear zones and lithospheric delamination and rifting that are accompanied by a continuous or episodic extensional regime. Numerous ancient/modern analogues exist for post-collisional tectonism in certain locations, such as the Cenozoic Tibetan Plateau (Turner et al., 1996), the late Neoproterozoic East African orogeny (Küster and Harms, 1998), the late Palaeozoic East Tianshan belt (Chen et al., 2011), and the late Palaeozoic Alpine Mountains (Köksal et al., 2004). All the lower Permian volcanic rocks in central Inner Mongolia and adjacent areas formed in an extensional environment (Nie et al., 1994; Zhu et al., 2001; Zhang et al., 2008, 2011; Chen et al., 2014; Chen et al., 2015; Li et al., 2015). This phenomenon is evidenced by the presence of early Permian A-type granites, alkali rhyolites (Hong et al., 1994; Jahn et al., 2009; Hao, 2012; Jiang et al., 2013; Li et al., 2015; Zhang et al., 2015b; Tong et al., 2015), coeval pillow basalt (Wang et al., 2014) and the development of extensional basins, such as the small Ondor Sum oceanic basin (Zhang et al., 2014; Song et al., 2015). This extensional

environment is related to the upwelling of the asthenosphere due to delamination of the thickened crust. In fact, most of the CAOB, including north-eastern China, was characterised by extension during the early Permian (Chen et al., 2011; Ma et al., 2015). A newly created Red Sea-like ocean basin might have formed in central Inner Mongolia during the early Permian (Chen et al., 2012; Luo et al., 2016). Large amounts of mafic pillow lava volcanics and marine siliceous rocks were developed (Wang et al., 2014; Song et al., 2015), which is also consistent with the increasing water depth during the early Permian (Shao et al., 2014; Eizenhöfer et al., 2015a, 2015b; Zhao et al., 2016). The small oceanic basin finally closed during the early Mesozoic and was accompanied by a regional metamorphism, locally reaching blueschist facies, and granitic intrusions (Chu et al., 2013; Wang et al., 2014; Zhang et al., 2015a). 6. Conclusions The following conclusions can be drawn from our geochronological and geochemical data on the volcanic rocks from the lower Permian Dashizhai Formation in the north-eastern Sonid Zuoqi area and previously published data in central Inner Mongolia: (1) New zircon U–Pb dating results indicate that the volcanic rocks in the north-eastern Sonid Zuoqi area in central Inner Mongolia formed during the early Permian (292 Ma to 279 Ma). The volcanic rocks in this study and in the previously published work showed a broad spatial distribution in central Inner Mongolia. (2) The mafic rocks possessed similar geochemical characteristics to subduction-modified continental lithospheric mantle sources. This condition indicates that these mafic rocks might have originated from the partial melting of lithospheric mantle that had been metasomatised by subduction-related fluids. The felsic rocks exhibited similar trace element compositions to

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Wilde, S.A., 2015. Final amalgamation of the Central Asian Orogenic Belt in NE China: Paleo-Asian Ocean closure versus Paleo-Pacific plate subduction — A review of the evidence. Tectonophysics 662, 345–362. Williams, I.S., 1998. U–Th–Pb geochronology by ion microprobe. In: Mckibben, M.A., Shanks, W.C., Ridley, W.I. (Eds.), Applications of Microanalytical Techniques to Understanding Mineralizing Processes. Review in Economic Geology, 7, pp.1–35. Windley, B.F., Alexeiev, D., Xiao, W.J., Kröner, A., Badarch, G., 2007. Tectonic models for accretion of the Central Asian Orogenic Belt. Journal of the Geological Society of London 164, 31–47. Woodhead, J.D., Hergt, J.M., Davidson, J.P., Eggins, S.M., 2001. Hafnium isotope evidence for ‘conservative’ element mobility during subduction zone processes. Earth and Planetary Science Letters 192, 331–346. Wu, F.Y., Sun, D.Y., Li, H., Jahn, B.M., Wilde, S., 2002. A-type granites in northeastern China: Age and geochemical constraints on their petrogenesis. Chemical Geology 187, 143–173. Xia, L.Q., 2014. The geochemical criteria to distinguish continental basalts from arc related ones. Earth-Science Reviews 139, 195–212. Xiao, W.J., Windley, B.F., Hao, J., Zhai, M.G., 2003. Accretion leading to collision and the Permian Solonker suture, Inner Mongolia, China: termination of the Central Asian Orogenic Belt. Tectonics 22, 1069–1088. Xiao, W.J., Windley, B.C., Huang, B.C., Han, C. M. Yuan, C. Chen, H. L. Sun, M. Sun, S. Li, J.L., 2009. End–Permian to mid–Triassic termination of the accretionary processes of the southern Altaids: implications for the geodynamic evolution, Phanerozoic

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Figure captions Fig. 1. (a) Tectonic framework of the central-eastern segment of the Central Asian Orogenic Belt (CAOB; modified after Jahn, 2004). (b) Geologic sketch map of the North China– Mongolia area (modified after Jian et al., 2008, 2010; Miao et al., 2008; Heumann et al., 2012; Xu et al., 2013; Zhang et al., 2015c; Chen et al., 2016a). Abbreviations: NCB, North China Block; SOB, southern orogenic belt; SSZ, Solonker suture zone; HB, Hunshandake block; NOB, northern orogenic belt; EHOB, Erenhot–Hegenshan ophiolite belt; UCM, Uliastai continental margin; EGFZ, eastern Gobi fault zone. (c) Schematic geological map of the north-eastern Sonid Zuoqi area, central Inner Mongolia (modified from IMBGMR, 1980). Fig. 2. Distribution of lower Permian volcanic rocks in central Inner Mongolia, showing the sample locations and isotopic ages of the volcanic rocks (based on IMBGMR, 1991). Fig. 3. Geological sections of the lower Permian volcanic-bearing strata in the north-eastern Sonid Zuoqi area, with some photographs of field outcrops and rock specimens.

Fig. 4. Classification diagrams for the volcanic rocks from the Dashizhai Formation: (a) plot of total alkalis versus silica (Le Maitre, 2002) and (b) plot of K2O versus SiO2. Field boundaries after Peccerillo and Taylor (1976). Fig. 5. Harker variation diagrams for major oxides versus SiO2 contents for the volcanic rocks in the Dashizhai Formation from the north-eastern Sonid Zuoqi area, central Inner Mongolia. Fig. 6. (a) AFM diagram (after Irvine and Baragar, 1971) and (b) FeOT/MgO–SiO2 diagram (after Miyashiro, 1974) of the mafic volcanic rocks in the Dashizhai Formation from the north-eastern Sonid Zuoqi area, central Inner Mongolia. C, calc-alkaline series; T, tholeiitic series. Fig. 7. Chondrite–normalised REE patterns and primitive mantle-normalised spider diagrams for the volcanic rocks in the Dashizhai Formation from the north-eastern Sonid Zuoqi area, central Inner Mongolia. (the normalisation values are from Sun and McDonough,1989). Fig. 8. Plot of the initial εNd(t) and (87Sr/86Sr)i values for the volcanic rocks in the Dashizhai Formation from the north-eastern Sonid Zuoqi area, central Inner Mongolia. Fig. 9. Concordia diagrams of the zircon U–Pb ages for volcanic rocks in the Dashizhai Formation from the north-eastern Sonid Zuoqi area, central Inner Mongolia. Fig. 10. Plots of (a) La/Ba versus La/Nb and (b) Th/Yb versus Ba/La. In (a), the reference fields for OIB, MORB and high U/Pb mantle source (HIMU) are from Saunders et al. (1992) and those for DM are from McKenzie and O’Nions (1991). CLM, continental lithospheric mantle. In (b), MORB and the arrows are discussed by Woodhead et al. (2001). Fig. 11. (a, b) Rb versus Y+Nb and Nb versus Y tectonic discrimination diagrams of Pearce et al. (1984). VAG, volcanic arc granites; WPG, within-plate granites; COLG, collisional

granites; ORG, oceanic ridge granites. (c) Tectonic discriminant diagram of granitoids (after Maniar and Piccoli, 1989). (d) Plot R2 versus R1 (after Batchelor and Bowden, 1985), R1 = 4 Si − 11(Na + K) − 2(Fe + Ti), R2 = 6 Ca + 2 Mg + Al. Fig. 12. Nd(t) versus TDM2 diagram for the felsic volcanic rocks in the Dashizhai Formation from the north-eastern Sonid Zuoqi area. The curve representing the mixing proportion between the two components, i.e., juvenile crust and ancient crust, is from Wu et al. (2002). Table captions Table 1 SHRIMP Zircon U–Pb data for the volcanic rocks in the Dashizhai Formation from the north-eastern Sonid Zuoqi area. Table 2 LA-ICP-MS zircon U–Pb data for the volcanic rocks in the Dashizhai Formation from the north-eastern Sonid Zuoqi area. Table 3 Major and trace element data for the volcanic rocks in the Dashizhai Formation from the north-eastern Sonid Zuoqi area. Table 4 Rb–Sr and Sm–Nd isotopic compositions for the volcanic rocks from the Dashizhai Formation in the north-eastern Sonid Zuoqi area.

a

Permian sedimentary rocks in China /Permian vocalno-sedimentary strata in Mongolia Early Permian volcanic rocks

Siberia tral Cen

50°N

ian

Bel

Orogenic

40°N

Fig. 1b

Precambrian basement

116°E

Dongwuqi

Early to mid-Palaeozoic strata

112°E

100°E

90°E

an

ag

Ch

o Ob

lt

fau

EHOB

114°E

Xiwuqi

110°E

120°E

FZ

EG

N

44°N

a

dav

et uh

e

ran

er at

ult

ot fa

110°E

Enshoo terr

46°N

UCM

Metamorphic complex 46°N

Carboniferous strata

118°E

China

NorthEChina 110°0′

Tarim 80°E

Mongolia

Palaeozoic Granite

t

As

Ultrumaific rocks

Fig. 1c

Xilinhot

nh Ere

NOB

Sonidzuoqi Erenhot

ane

Linxi

Hushandake desert

ock Hutag Uul bl

Ondor Sum Xianghuangqi

Zhengxiangbaiqi

SSZ/HB ne

ault Zo

oron F

正篮旗

Linxi

fault

Xar M

Sonidyouqi

Solonker

44°N

SOB ult

–Chifeng fa

Bayan Obo

42°N

NCB

Damaoqi

0

100km

118°E

b 112°E

110°E

114°E

113° 45’

Cenozoic sediments Late Early Middle Permian sedimentary rocks Early Permian volcanic -sedimentary rocks

116°E 114° 00’

5 km

NM12-86 NM12-87 NM12-88 (b) NM12-89

(a) NM10-39 NM10-40

Late Triassic granite Ultramafic rock

NM10-37 NM10-38

NM12-111 NM12-112 NM12-113

c

NM12-90 NM12-91 NM12-92

Fault NM10-34

Sample location 44° 00’

07NM-65 07NM-66 07NM-67 07NM-68 07NM-69 07NM-70

NM10-43 NM10-44

NM10-41 NM10-42

Huheaobao Taolegai

NM10-30 NM10-31 NM10-33

44° 00’

Bagerenhuduge

07NM-64

Changte 113° 45’

114° 00’

42°N

Upper Permian sedimentary rocks Middle Permian sedimentary rocks Early Permian volcanic rocks Late Carboniferous calstic rocks

90km

0

(Ar-Ar plateau age, Miao et al., 2008)

287 ± 3 Ma 289 ± 3 Ma

294 ± 30 Ma

(LA-ICP-MS zircon U-Pb, Liu et al., 2009)

(SHRIMP zircon U-Pb, Zhang et al., 2011)

Late Carboniferous-early Permian volcanic rocks Devonian detrital rocks

46°N

292.85 ± 0.63 Ma

Dongwuqi

291.6 ± 2.9 Ma 288.9 ± 4.9 Ma 279.4 ± 2.9 Ma

(SHRIMP zircon U-Pb, this study)

Cambrian-Silurian Metamorphic rocks Paleozoic Granite

Xiwuqi

281.2 ± 2.0 Ma 285.5 ± 2.4 Ma 282.1 ± 1.6 Ma

gabbro

Ultrumaific rocks

(LA-ICP-MS zircon U-Pb, this study)

Metamorphic complex

44°N Xilinhot

Precambrian basement

271 ± 8 Ma 276 ± 10 Ma

(SHRIMP zircon U-Pb, Nie et al., 2009)

Sonidzuoqi Linxi

279 ± 3 Ma

Erenhot

273.7 ± 1.0 Ma

(SHRIMP zircon U-Pb, Zhang et al., 2008)

274.1 ± 2.8 Ma

281 ± 3 Ma

(LA-ICP-MS zircon U-Pb, Liu, 2009) (SHRIMP zircon U-Pb, Zhang et al., 2008）

(SHRIMP zircon U-Pb, Chen et al., 2012)

285 ± 11 Ma

277.9 ± 4.2 Ma

(SHRIMP zircon U-Pb, Chen et al., 2014)

(TIMES zircon U-Pb, Tao et al., 2003)

Sonidyouqi

281 ± 4.3 Ma 281 ± 12 Ma

277 ± 3 Ma

(LA-ICP-MS zircon U-Pb, Qing et al., 2012)

Solonker

Xianghuangqi

270.2 ± 2.7 Ma 268.0 ± 3.8 Ma

Zhengxiangbaiqi

272 ± 11 Ma 277 ± 14 Ma (Rb-Sr isochron age, Zhu et al, 2001)

正篮旗

(Sm-Nd isochron age, Nie et al.,1994)

North China Block 112°E

114°E

42°N

118°E

275 ± 5.2 Ma

Damaoqi

(SHRIMP zircon U-Pb, Chen et al., 2015） 110°E

(LA-ICP-MS zircon U-Pb, Wang et al., 2014)

116°E

NM10-34

(a)205°

NM10-30 NM10-31 NM10-33

NM10-41 NM10-42

NM10-43 NM10-44

NM10-37 NM10-38

1 km NM10-39 NM10-40

173°∠52°

170°∠70°

330°∠82°

168°∠45°

Rhyolite

Shale

Basaltic andesite

Tuffaceous Sandstone sandstone

(b) 140° 250 m

NM12-89 NM12-90 NM12-91 NM12-92 NM12-93 NM12-86 NM12-88 NM12-87 153°∠80°

Ultramafic Unconsolidated Fault sediments rocks 326°∠56°

142°∠72°

14 Na2O + K2O(wt.%)

12

7

(a) foidite

10 8 6 4 2 0 40

tephriphonolite phonotephrite

tephrite (ol < 10%) basaltic trachybasanite (ol > 10%)trachy- andesite basalt alkali basalt picrobasalt

subalkali basalt

50

basaltic andesite

Mafic volcanic rocks

trachyte

trachyandesite

(b)

Felsic volcanic rocks

phonolite

5

alkali rhyolite

(trachydacite ifq > 20%)

6

K2O (wt.%)

16

Shoshonite Variscan post-collisional volcanism in Italy

4 h-K

Hig

3

andesite

60 SiO2(wt%)

dacite

subalkali rhyolite

ium

Low-K tholeiitic

0 80

45

e

alin

-alk

alc -K c

Med

1

Variscan post-collisional volcanism in Italy

70

2

ine

lkal

-a calc

50

55

60

65

SiO2(wt.%)

70

75

80

2

Al2O3(wt.%)

TiO2(wt.%)

Felsic volcanic rocks Mafic volcanic rocks

1

0 6

Fe2O3T(wt.%)

CaO(wt.%)

7

20 19 18 17 16 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 6

5 4 3 2 1 7

5

5

Na2O(wt.%)

MgO(wt.%)

6

4 3 2

4 3

1 0 50

60

SiO2(wt.%)

70

80

2

50

60

70 SiO2(wt.%)

80

FeOt

75 Mafic volcanic rocks

SiO2(wt.%)

Tholeiitic

70 65 Tholeiitic 60 55 Calc-Alkaline 50

Calc-Alkaline (a)

(b)

45 Na2O+K2O

MgO

0

1

2 3 FeO/MgO

4

5

200

500

10 5

Sample/Chondrite

1000

Sample/Primitive Mantle

Sample/Chondrite

100

(b)

07NM-69 07NM-70 NM10-43 NM10-44

100

Mafic Rocks

La

Ce

Pr

Nd

(c)

Sm

Eu

Gd

Tb

Ho

Dy

NM10-30 NM10-31 NM10-33 NM10-34 NM10-37

100

Tm

Er

Lu

Yb

NM10-38 NM10-39 NM10-40 NM10-41 NM10-42

10 Section (a) Felsic Rocks

1

Sample/Chondrite

1000

La

Ce

Pr

Nd

(e)

Eu Tb Ho Tm Lu Sm Gd Dy Er Yb NM12-86 NM12-87 NM12-88 NM12-89 NM12-90

100

NM12-91 NM12-92 NM12-111 NM12-112 NM12-113

10 Section (b) Felsic Rocks

1

La

Ce

Pr

Nd

Eu Tb Ho Tm Lu Sm Gd Dy Er Yb

10

1 Rb Th K Ta Ce Nd Zr Eu Y Lu Ba U Nb La Sr Sm Hf Ti Yb 1000 (d)

Sample/Primitive Mantle

07NM-64 07NM-65 07NM-66 07NM-67 07NM-68

100 10 1 0.1

1000

Sample/Primitive Mantle

(a)

Rb Th K Ta Ce Nd Zr Eu Y Lu Ba U Nb La Sr Sm Hf Ti Yb (f)

100 10 1 0.1

Rb Th K Ta Ce Nd Zr Eu Y Lu Ba U Nb La Sr Sm Hf Ti Yb

10

DM

NM12-87 NM12-93

MORB

5

NM10-44 OIB

NM10-43 NM10-34

εNd(t)

0

NM10-30 NM10-37

-5

-10 EMI

EMII

-15

-20

0.700

0.705

Sr/86Sri

87

0.710

0.715

296

0.047

0.046

0.048

Pb/238U

272

0.041 0.22

264 260 0.24

0.26

0.28

280

305 295 285 275 265

268 0.042

0.30

270

0.32

0.34

0.042 0.22

0.36

Pb/238U

0.10

600

(c) Sample NM10-43 Andesite

0.047

500

Pb/238U

0.046

Pb/238U

400

206

3 spots Pb/ U age: 286.4 ± 3.8 Ma ~ 292.3 ± 5.2 Ma

200

207

0.3

0.056

207

0.5

Pb/238U

0.7

(e) Sample NM12-91 Rhyolite

350

Pb/238U

310 0.048

206

Weighted mean 206Pb/238U age: 285.5 ± 2.4 Ma (n=17) MSWD = 0.60

0.044

0.042 0.22

282

274 0.26

0.4

0.8

Pb/238U

207

0.30 207

0.34

Pb/238U

(f) Sample NM12-113 Rhyolite 300 Weighted mean 206Pb/238U age: 281.2 ± 2.0 Ma (n=26) 290 MSWD = 0.93

0.046

280

0.044

305 295 285 275

260

275 0.0

286

278

0.042

285

270

0.42

290

272

270

295

0.38

292

276

305

290

0.42

280

0.048

330

0.38

284

0.045

0.043

0.9

0.34

296

268

0.1

0.052

Pb/238U

Weighted mean 206Pb/238U age: 282.1 ± 1.6 Ma (n=19) 288 MSWD = 0.39

238

0.02

0.040

0.30

(d) Sample NM12-88 Rhyolite

0.044

300 0.04

Pb/238U

0.26

206

0.06

206

315 305 295 285 275

0.044

207

0.08

290

0.046

206

276

0.043

206

(b) Sample NM10-37 310 Rhyolite 206 238 Weighted mean Pb/ U age: 291.6 ± 2.9 Ma (n=12) 300 MSWD = 0.87

280 0.044

206

Pb/238U

0.045

0.050

(a) Sample NM10-30 292 Rhyolite 206 238 Weighted mean Pb/ U age: 288 279.4 ± 2.9 Ma (n=12) 284 MSWD = 0.67

265 1.2

1.6

0.040

0.1

0.2

207

0.3

Pb/238U

0.4

0.5

1.0

15

(a)

(b)

Mafic volcanic rocks

10

HIMU

0.1

Th/Yb

La/Ba

MORB

OIB

Sediment involvement

5

0.01 0.1

Subductionmodified CLM

1

La/Nb

10

0

Fluid involvement

MORB

0

10

20

30

Ba/La

40

50

1000

2000 1000

Post-COLG

Syn-COLG WPG

WPG

100

Rb

Nb

100

10

VAG

1

ORG

10

100

Y+Nb

1

1000

1

IAG+CAG+CCG

R2

Al2O3

1500

14 13

POG

12

10

100

Y

1 - Mantle Fractionates 2 - Pre-Plate Collision 3 - Post-Collision Uplift 4 - Late-Orogenic 5 - Anorogenic 6 - Syn-Collision 7 - Post-Orogenic

2000

15

11

10

1000

2500

17 16

Post-COLG

ORG

Felsic volcanic rocks

1

VAG+ Syn-COLG

10

2

1000

3 4

500

RRG+CEUG

1

6

5

70 71 72 73 74 75 76 77 78 79 80

SiO2

0

0

500

1000

7

1500

R1

2000

2500

3000

10

Juvenile crust

5

90

Felsic volcanic rocks

A-type granites in NE China

εNd(t)

80

0

70

I-type granites in NE China

60 50

-5

40 30 20

-10

Old crust

-15 -20

10

0

500

1000

1500

TDM2 (Ma)

2000

2500

Table 1 SHRIMP Zircon U–Pb data for the volcanic rocks in the Dashizhai Formation from the north-eastern Sonid Zuoqi area. Concentrations (ppm) Spots

206

Pbc(%)a)

206

Pb* a)

Age (Ma)

U

Th

232

Th/238U

206

Pb/238Ub)

1σ

206

Pb/238Uc)

Ratios 1σ

206

Pb/238Ud)

1σ

207

Pb*/235Ub)

± %

206

Pb*/238Ub)

error ±%

NM10-30-1

1.57

20.8

526

273

0.54

285.1

5.0

285.1

4.9

286.2

5.4

0.3233

7.0

0.0452

1.8

0.26

NM10-30-2

2.2

16.4

428

233

0.56

275.5

4.9

277.5

4.8

278.1

5.3

0.2743

9.3

0.0437

1.8

0.19

NM10-30-3

1.23

18.9

485

241

0.51

282.2

4.8

282.2

4.8

282.4

5.3

0.3194

5.8

0.0447

1.8

0.30

NM10-30-4

1.67

23.4

613

325

0.55

275.4

4.7

276.9

4.7

277.7

5.1

0.2843

7.5

0.0436

1.7

0.23

NM10-30-5

0.97

22.8

580

321

0.57

286.2

4.9

287.1

4.8

287.1

5.3

0.3099

6.5

0.0454

1.7

0.27

NM10-30-6

1.57

15.4

396

189

0.49

280.3

4.9

281.9

4.9

281.3

5.3

0.2897

7.9

0.0444

1.8

0.23

NM10-30-7

0.23

25.5

671

420

0.65

278.9

4.6

279.1

4.6

279.7

5.1

0.3119

2.8

0.0442

1.7

0.61

NM10-30-8

0.25

23.7

632

399

0.65

274.6

4.6

274.4

4.6

276.1

5.1

0.3132

3.4

0.0435

1.7

0.51

NM10-30-9

0.49

20.2

530

281

0.55

278.9

4.7

278.8

4.7

279.8

5.1

0.3195

3.7

0.0442

1.7

0.46

NM10-30-10

0.66

21

565

296

0.54

271.3

4.6

271.8

4.7

273.4

5.1

0.2979

4.1

0.043

1.7

0.43

NM10-30-11

0.43

18

471

246

0.54

278.5

4.7

277.5

4.7

279.2

5.2

0.3337

3.8

0.0442

1.7

0.45

NM10-30-12

0.19

15.2

407

191

0.48

274

4.8

273.1

4.8

273.6

5.2

0.3265

4.2

0.0434

1.8

0.43

NM10-37-1

1.04

10.1

261

110

0.44

282.5

5.9

281.3

5.9

282.4

6.3

0.3417

7.0

0.0448

2.1

0.30

NM10-37-2

1.36

13.5

336

136

0.42

289.8

5.2

291.2

5.2

291.3

5.5

0.3054

8.1

0.046

1.8

0.23

NM10-37-3

2.17

5.4

135

51

0.39

288.2

5.8

287.9

5.7

289.3

6

0.3345

13.2

0.0457

2.1

0.16

NM10-37-4

0.36

24.3

592

197

0.34

299.9

5.0

299.1

5.0

300.1

5.3

0.3578

2.9

0.0476

1.7

0.58

NM10-37-5

1.39

13.9

342

140

0.42

294.4

5.2

295.4

5.3

297.4

5.6

0.3181

9.2

0.0467

1.8

0.20

NM10-37-6

2.17

5.4

136

73

0.56

285.4

5.7

286.8

5.5

287

6.2

0.2981

13.1

0.0453

2.1

0.16

NM10-37-7

0.67

18.4

457

262

0.59

292.6

5.0

292.2

5.0

294

5.5

0.3401

4.2

0.0464

1.7

0.42

NM10-37-8

1.81

6.1

153

63

0.43

288.7

5.5

288.7

5.5

292.2

5.9

0.3293

10.8

0.0458

2

0.18

NM10-37-9

0.82

14.2

356

151

0.44

289.8

5.0

289.9

5.0

291.8

5.4

0.3275

5.4

0.046

1.7

0.33

NM10-37-10

1.33

12.1

300

114

0.39

293.3

5.2

294.8

5.1

295.5

5.5

0.3079

7.4

0.0465

1.8

0.24

NM10-37-11

0.64

17.5

433

199

0.47

294.2

5.0

294.4

5.0

295.1

5.3

0.3308

4.3

0.0467

1.7

0.40

NM10-37-12

1.04

7.9

194

72

0.38

297.5

5.5

296.1

5.5

299.6

5.9

0.3653

6.7

0.0472

1.9

0.28

NM10-43-1

2.01

5.4

122

59

0.5

307.6

4.8

308.8

4.7

308.2

5.1

0.3438

12.0

0.0506

1.9

0.16

NM10-43-2

0.63

9.0

187

167

0.92

339

4.6

338.5

4.7

338.6

5.5

0.4191

5.5

0.0559

1.8

0.33

NM10-43-3

1.45

6.4

189

10

0.06

235.4

3.5

233.6

3.5

233.5

3.5

0.3035

11.0

0.0385

1.9

0.17

NM10-43-4

0.72

10.9

268

189

0.73

286.4

3.8

285.8

3.8

286

4.3

0.3484

4.3

0.0471

1.8

0.40

NM10-43-5

1.61

5.7

137

204

1.54

289.8

4.4

288.4

4.4

289.6

5.8

0.3682

10.5

0.0476

1.9

0.18

NM10-43-6

0.65

17

233

155

0.69

506.7

6.5

508.2

6.6

506.7

7.3

0.6410

3.9

0.0847

1.7

0.45

NM10-43-7

1.32

10.5

238

110

0.48

308.6

4.4

311.1

4.3

310.8

4.6

0.3214

9.5

0.0508

1.8

0.19

NM10-43-8

11.92

39.9

657

251

0.39

376.5

6.5

377.8

5.2

367

8.7

0.4410

20.4

0.0623

2.1

0.10

NM10-43-9

0.12

115.2

410

70

0.18

1767.5

19.0

1753.8

21.1

1767.1

19.5

5.1804

1.8

0.3267

1.7

0.95

NM10-43-10

0.64

70.5

908

328

0.37

536.3

6.2

537

6.3

536.6

6.6

0.7073

2.8

0.0899

1.6

0.59

NM10-43-11

1.75

8.5

180

62

0.35

327.9

4.9

329.3

4.8

329.2

5.1

0.3688

10.2

0.0541

1.9

0.19

NM10-43-12

1.34

8.2

185

95

0.53

308.4

4.3

309.3

4.3

310.6

4.6

0.3512

8.0

0.0508

1.8

0.23

NM10-43-13

1.34

7.7

185

95

0.53

292.3

5.2

293

5.2

294.4

5.6

0.3325

8.0

0.0481

2.1

0.26

Notes: a) Pbc and Pb* indicate the common and radiogenic portions, respectively; b) Common Pb corrected using measured

204

Pb; c) Common Pb corrected by assuming 206Pb/238U-207Pb/235U age-concordance; d) Common Pb corrected by assuming 206Pb/238U-208Pb/232Th age-concordance.

Table 2 LA-ICP-MS zircon U–Pb data for the volcanic rocks in the Dashizhai Formation from the north-eastern Sonid Zuoqi area. Concentration/ppm

Isotopic- ratio

Age/Ma

Point No.

*Pb

Th

U

NM1288 1

62.8

512

852

0.60

0.1321

0.0019

0.9233

0.0136

0.0507

0.0006

0.0301

2

48.2

252

524

0.48

0.1897

0.0026

1.4813

0.0206

0.0567

0.0007

0.0547

3 4

9.7 29.4

78 287

193 572

0.40 0.50

0.0517 0.0523

0.0019 0.0009

0.3164 0.3201

0.0115 0.0058

0.0444 0.0444

0.0006 0.0005

5 6

21.3 9.1

270 83

395 178

0.68 0.47

0.0521 0.0522

0.0014 0.0037

0.3174 0.3188

0.0085 0.0222

0.0442 0.0443

7

31.0

293

590

0.50

0.0529

0.0014

0.3268

0.0085

8 9

27.8 17.5

259 179

534 334

0.48 0.53

0.0519 0.0522

0.0014 0.0011

0.3210 0.3227

0.0088 0.0065

10

137.0

409

780

0.52

0.3830

0.0049

4.1150

11 12

8.1 13.3

71 144

158 248

0.45 0.58

0.0519 0.0521

0.0023 0.0016

0.3201 0.3215

13 14

5.5 23.3

55 316

105 415

0.53 0.76

0.0524 0.0572

0.0030 0.0017

15

25.6

248

487

0.51

0.0522

16 17

6.3 24.4

64 223

121 464

0.53 0.48

0.0522 0.0525

Th/U

207

Pb/206Pb

Pb/206Pb

1

0.0005

2126

0.0009

2740

0.0123 0.0127

0.0004 0.0002

0.0005 0.0006

0.0130 0.0138

0.0448

0.0005

0.0449 0.0449

0.0005 0.0005

0.0536

0.0780

0.0139 0.0097

0.0448 0.0448

0.3234 0.3514

0.0183 0.0106

0.0012

0.3246

0.0029 0.0011

0.3220 0.3279

1

207

Pb/235U

Pb/235U

1

25

664

22

923

271 299

81 41

0.0003 0.0004

289 296

0.0141

0.0003

0.0135 0.0130

0.0003 0.0003

0.0009

0.1337

0.0006 0.0006

0.0131 0.0138

0.0448 0.0446

0.0007 0.0006

0.0077

0.0451

0.0177 0.0069

0.0448 0.0453

1

206

Pb/238U

Pb/238U

1

7

319

4

599

10

8

355

4

1076

17

279 282

9 4

280 280

3 3

248 255

8 5

60 152

280 281

7 17

279 279

3 4

261 277

5 7

326

58

287

6

283

3

283

6

282 295

62 46

283 284

7 5

283 283

3 3

272 261

5 5

0.0021

3844

19

1657

11

484

5

2537

38

0.0005 0.0003

279 290

97 68

282 283

11 7

283 282

4 3

264 276

9 7

0.0122 0.0140

0.0007 0.0003

301 498

125 66

285 306

14 8

283 281

4 3

246 281

15 6

0.0005

0.0132

0.0003

293

53

285

6

285

3

264

6

0.0006 0.0005

0.0127 0.0138

0.0005 0.0003

292 307

122 47

283 288

14 5

283 286

4 3

254 277

9 6

1

208

Pb/232Th

1

207

207

206

208

Pb/232Th

1

18

37.7

82

154

0.53

0.5453

0.0100

6.6281

0.1084

0.0882

0.0013

0.2080

0.0040

4369

27

2063

14

545

8

3819

67

19 20

198.2 29.1

793 283

1205 557

0.66 0.51

0.3694 0.0519

0.0045 0.0010

3.7755 0.3218

0.0478 0.0062

0.0742 0.0450

0.0008 0.0005

0.0990 0.0129

0.0016 0.0003

3789 280

18 44

1588 283

10 5

461 284

5 3

1908 259

29 5

21 22

6.8 13.5

69 118

131 259

0.53 0.46

0.0530 0.0525

0.0046 0.0015

0.3217 0.3278

0.0281 0.0094

0.0441 0.0453

0.0006 0.0006

0.0137 0.0134

0.0005 0.0003

327 307

187 64

283 288

22 7

278 286

4 3

275 269

11 7

23

83.3

325

659

0.49

0.2971

0.0039

2.6348

0.0353

0.0644

0.0007

0.0914

0.0015

3455

20

1310

10

402

5

1767

28

24 25

12.2 68.3

134 357

197 764

0.68 0.47

0.0921 0.2169

0.0023 0.0028

0.5823 1.5738

0.0138 0.0210

0.0459 0.0526

0.0006 0.0006

0.0197 0.0582

0.0005 0.0010

1469 2958

46 21

466 960

9 8

289 331

4 4

395 1144

9 19

26

9.8

84

191

0.44

0.0525

0.0027

0.3211

0.0164

0.0444

0.0006

0.0137

0.0006

306

113

283

13

280

4

276

12

27 28

16.2 14.9

134 96

271 173

0.50 0.56

0.0666 0.1886

0.0014 0.0034

0.4494 1.3725

0.0096 0.0238

0.0489 0.0528

0.0006 0.0007

0.0187 0.0449

0.0004 0.0009

826 2730

44 29

377 877

7 10

308 332

4 4

374 888

8 17

29 30 NM1291 1 2

2.4 123.4

19 794

48 1144

0.40 0.69

0.0520 0.2353

0.0053 0.0031

0.3211 1.9354

0.0328 0.0261

0.0448 0.0597

0.0008 0.0007

0.0136 0.0521

0.0010 0.0009

285 3089

219 21

283 1093

25 9

283 374

5 4

272 1027

20 17

31.6 97.0

414 976

535 1516

0.78 0.64

0.0557 0.0533

0.0015 0.0011

0.3549 0.3811

0.0095 0.0079

0.0462 0.0519

0.0006 0.0006

0.0168 0.0188

0.0003 0.0004

442 342

58 46

308 328

7 6

291 326

3 4

338 377

6 7

3 4

68.3 29.0

558 264

921 565

0.61 0.47

0.1030 0.0518

0.0016 0.0009

0.7617 0.3156

0.0120 0.0054

0.0537 0.0442

0.0006 0.0005

0.0305 0.0149

0.0005 0.0003

1679 278

28 38

575 279

7 4

337 279

4 3

606 299

9 5

5

50.4

839

655

1.28

0.1102

0.0026

0.7312

0.0166

0.0481

0.0006

0.0215

0.0004

1803

41

557

10

303

4

430

8

6 7

29.8 327.3

225 970

578 975

0.39 0.99

0.0539 0.6130

0.0015 0.0073

0.3324 9.2729

0.0093 0.1153

0.0447 0.1098

0.0005 0.0012

0.0168 0.1800

0.0004 0.0027

369 4539

62 17

291 2365

7 11

282 671

3 7

338 3345

8 45

8

25.0

302

446

0.68

0.0518

0.0019

0.3237

0.0121

0.0454

0.0005

0.0160

0.0003

276

83

285

9

286

3

321

7

9

33.0

175

388

0.45

0.1811

0.0028

1.3337

0.0206

0.0534

0.0006

0.0580

0.0010

2663

25

861

9

336

4

1139

19

10 11

6.5 42.0

60 347

122 561

0.49 0.62

0.0519 0.1175

0.0033 0.0022

0.3231 0.8414

0.0201 0.0156

0.0451 0.0520

0.0006 0.0006

0.0161 0.0329

0.0008 0.0006

283 1919

137 33

284 620

15 9

285 327

4 4

323 654

15 11

12

26.3

232

500

0.46

0.0521

0.0017

0.3235

0.0103

0.0451

0.0005

0.0157

0.0004

288

71

285

8

284

3

314

7

13 14

114.5 14.0

1102 110

1881 275

0.59 0.40

0.0724 0.0516

0.0011 0.0026

0.4752 0.3147

0.0075 0.0156

0.0476 0.0443

0.0006 0.0006

0.0213 0.0162

0.0004 0.0006

997 267

31 111

395 278

5 12

300 279

3 4

425 325

7 11

Table 2 (continued) 15

50.0

366

788

0.46

0.0873

0.0015

0.5914

0.0101

0.0492

0.0006

0.0276

0.0005

1367

32

472

6

309

4

551

10

16

7.3

64

137

0.47

0.0515

0.0030

0.3247

0.0191

0.0457

0.0006

0.0156

0.0006

263

130

286

15

288

4

313

13

17

13.1

91

215

0.42

0.0974

0.0032

0.6276

0.0198

0.0467

0.0007

0.0294

0.0008

1575

59

495

12

295

4

586

16

18 19

30.4 22.5

432 193

526 426

0.82 0.45

0.0520 0.0524

0.0016 0.0016

0.3242 0.3287

0.0098 0.0100

0.0453 0.0456

0.0006 0.0005

0.0154 0.0154

0.0003 0.0004

285 301

68 68

285 289

8 8

285 287

4 3

309 310

7 7

20

59.3

664

1083

0.61

0.0533

0.0011

0.3304

0.0069

0.0450

0.0005

0.0158

0.0003

342

46

290

5

284

3

316

6

21 22

14.8 22.5

172 172

255 374

0.68 0.46

0.0550 0.0769

0.0029 0.0017

0.3543 0.5124

0.0182 0.0116

0.0467 0.0483

0.0006 0.0006

0.0167 0.0233

0.0004 0.0005

413 1120

112 44

308 420

14 8

294 304

4 4

336 466

9 10

23 24

44.7 26.2

554 226

724 487

0.77 0.46

0.0571 0.0518

0.0017 0.0024

0.3766 0.3241

0.0111 0.0150

0.0479 0.0454

0.0006 0.0006

0.0181 0.0174

0.0004 0.0005

493 275

64 103

325 285

8 11

302 286

4 4

362 349

7 11

25

7.1

61

132

0.46

0.0522

0.0026

0.3264

0.0163

0.0454

0.0006

0.0169

0.0006

292

111

287

12

286

4

339

12

26 27

40.7 47.8

336 420

704 826

0.48 0.51

0.0541 0.0729

0.0013 0.0013

0.3634 0.4678

0.0086 0.0084

0.0487 0.0466

0.0006 0.0006

0.0185 0.0202

0.0004 0.0004

376 1012

52 36

315 390

6 6

307 293

4 3

371 403

7 8

28

4.3

25

86

0.28

0.0523

0.0047

0.3263

0.0290

0.0453

0.0008

0.0155

0.0016

296

192

287

22

286

5

310

31

29 30

9.0 27.1

71 198

172 514

0.41 0.39

0.0523 0.0526

0.0023 0.0016

0.3264 0.3333

0.0140 0.0104

0.0453 0.0460

0.0006 0.0006

0.0167 0.0166

0.0006 0.0004

299 313

96 69

287 292

11 8

285 290

4 3

334 333

11 8

1 2

16.0 129.5

129 268

303 622

0.43 0.43

0.0522 0.4311

0.0013 0.0056

0.3263 5.0572

0.0078 0.0661

0.0454 0.0851

0.0006 0.0010

0.0159 0.2256

0.0004 0.0037

294 4021

54 19

287 1829

6 11

286 527

3 6

319 4112

8 61

3 4

7.1 16.9

52 192

99 304

0.53 0.63

0.1110 0.0519

0.0089 0.0024

0.7743 0.3241

0.0616 0.0149

0.0506 0.0454

0.0009 0.0006

0.0348 0.0161

0.0016 0.0004

1816 279

139 102

582 285

35 11

318 286

5 3

691 322

31 8

5

16.3

123

315

0.39

0.0518

0.0020

0.3213

0.0125

0.0450

0.0005

0.0161

0.0004

275

86

283

10

284

3

323

9

6 7

9.5 19.0

63 151

185 377

0.34 0.4

0.0521 0.0518

0.0019 0.0016

0.3264 0.3147

0.0117 0.0096

0.0455 0.0441

0.0006 0.0006

0.0149 0.0143

0.0007 0.0005

291 277

80 69

287 278

9 7

287 278

4 4

299 286

14 10

8

70.9

266

730

0.36

0.2234

0.0029

1.7373

0.0232

0.0564

0.0007

0.0897

0.0015

3006

21

1023

9

354

4

1737

28

9 10

2.6 14.2

21 104

49 282

0.42 0.37

0.0521 0.0518

0.0102 0.0014

0.3226 0.3131

0.0629 0.0082

0.0450 0.0439

0.0009 0.0005

0.0181 0.0155

0.0019 0.0004

288 276

394 59

284 277

48 6

284 277

6 3

362 311

38 8

11 12

3.4 10.6

33 93

61 207

0.53 0.45

0.0516 0.0526

0.0104 0.0027

0.3207 0.3159

0.0645 0.0162

0.0451 0.0435

0.0010 0.0006

0.0193 0.0156

0.0017 0.0005

268 313

407 113

282 279

50 13

284 275

6 4

386 313

34 9

13

12.7

89

244

0.36

0.0527

0.0030

0.3287

0.0183

0.0452

0.0006

0.0169

0.0007

318

122

289

14

285

4

338

14

14 15

11.1 14.9

81 151

224 280

0.36 0.54

0.0517 0.0519

0.0034 0.0013

0.3091 0.3216

0.0201 0.0080

0.0434 0.0450

0.0006 0.0006

0.0152 0.0149

0.0007 0.0004

273 280

143 57

274 283

16 6

274 284

4 3

305 299

13 7

16

10.3

74

200

0.37

0.0519

0.0025

0.3204

0.0151

0.0448

0.0006

0.0165

0.0006

279

105

282

12

283

4

330

12

17 18

15.1 26.5

118 225

304 501

0.39 0.45

0.0516 0.0525

0.0021 0.0016

0.3104 0.3244

0.0123 0.0100

0.0437 0.0449

0.0006 0.0005

0.0146 0.0172

0.0005 0.0004

267 305

89 69

275 285

10 8

276 283

3 3

294 345

9 7

19

16.1

167

284

0.59

0.0534

0.0038

0.3371

0.0239

0.0458

0.0007

0.0182

0.0006

344

154

295

18

289

4

365

12

NM12-113

20 21

14.4 16.2

141 122

277 314

0.51 0.39

0.0521 0.0520

0.0028 0.0013

0.3145 0.3233

0.0167 0.0079

0.0438 0.0451

0.0006 0.0006

0.0156 0.0162

0.0004 0.0004

291 286

22

16.4

133

23 24

17.0 41.1

134 216

25 26

11.6 14.1

27 28 29 30

117 56

278 284

13 6

276 284

4 3

314 324

9 8

331

0.4

0.0519

0.0023

0.3086

0.0131

0.0431

0.0007

0.0151

0.0006

318 747

0.42 0.29

0.0540 0.0556

0.0031 0.0015

0.3429 0.3725

0.0193 0.0102

0.0461 0.0486

0.0007 0.0006

0.0169 0.0201

0.0009 0.0006

282

96

273

10

272

4

303

13

372 437

123 60

299 322

15 8

290 306

5 4

339 403

17 11

106 106

222 281

0.48 0.38

0.0523 0.0518

0.0028 0.0017

0.3242 0.3164

0.0175 0.0103

0.0450 0.0443

0.0006 0.0006

0.0151 0.0151

0.0006 0.0005

298 275

119 73

285 279

13 8

284 280

4 3

303 302

12 10

6.1

40

122

0.33

0.0522

0.0026

0.3245

0.0158

0.0451

0.0006

2.8 2.1

19 18

53 39

0.36 0.47

0.0505 0.0518

0.0090 0.0058

0.3130 0.3235

0.0558 0.0358

0.0450 0.0453

0.0009 0.0009

0.0159

0.0008

296

109

285

12

284

4

318

16

0.0218 0.0139

0.0018 0.0012

218 278

369 237

277 285

43 28

284 286

5 5

436 280

35 24

13.7

95

276

0.35

0.0561

0.0027

0.3339

0.0158

0.0432

0.0006

0.0171

0.0005

456

102

293

12

273

3

343

10

Table 3 Major and trace element data for the volcanic rocks in the Dashizhai Formation from the north-eastern Sonid Zuoqi area. sample

NM12-086

NM12-87

NM12-88

NM12-89

NM12-90

NM12-91

NM12-92

NM12-111

NM12-112

NM12-113

NM10-30

NM10-31

NM10-33

NM10-34

NM10-37

SiO2

79.95

77.41

74.98

75

75.13

75.45

76.42

76.04

75.6

74.88

77.32

78.55

73.06

65.07

76.25

Al2O3

10.54

11.45

12.65

12.52

12.54

12.41

12.14

11.93

11.84

12.61

12.44

12.1

13.61

15.8

12.65

Fe2O3

0.41

1.21

1.57

1.12

1.51

1.5

0.75

1.63

2.43

1.46

0.37

0.37

2.83

4.66

1.19

CaO

0.11

0.16

0.08

0.19

0.12

0.12

0.1

0.15

0.21

0.2

0.18

0.25

0.27

3.32

0.56

MgO

0.21

0.27

0.31

0.42

0.57

0.68

0.53

0.08

0.22

0.25

0.02

0.01

0.61

1.37

0.06

K2O

5.45

5.88

6.17

6.52

5.69

5.46

5.83

6.35

5.83

6.4

3.99

3.85

3.64

3.41

5.34

Na2O

2.6

2.65

3.34

2.59

3.29

3.36

3.19

2.81

2.66

3.01

4.48

3.77

3.76

3.93

3.11

MnO

0

0.01

0

0.01

0

0

0

0.01

0.01

0.01

0

0

0.05

0.07

0.01

TiO2

0.1

0.11

0.19

0.19

0.18

0.17

0.16

0.2

0.15

0.17

0.16

0.16

0.32

0.67

0.16

P2O5

0.04

0.02

0.02

0.04

0.03

0.02

0.02

0.02

0.02

0.03

0.04

0.03

0.06

0.14

0

LOI

0.55

0.76

0.6

1.23

0.84

0.79

0.73

0.69

0.94

0.84

0.86

0.89

1.68

1.41

0.59

Total

99.94

99.91

99.91

99.81

99.9

99.97

99.86

99.91

99.92

99.85

99.86

99.96

99.88

99.85

99.92

Mg#

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

Sc

3.4

3.92

4.17

5.24

4.14

3.97

3.59

3.97

7.76

7.66

3.19

3.2

8.37

12.5

8.43

V

4.63

3.92

7.33

10.7

7.43

9.78

11.4

13.4

8.82

8.12

11.1

11.9

16.3

43.9

8.26

Ga

15.2

17.7

16.8

21

17.3

15.9

13

14.1

20.8

16.2

18.3

18.6

23.2

20.6

18.7

Rb

155

168

150

176

146

139

115

154

122

134

120

121

107

116

129

Sr

19.5

14.3

13.6

22

16.1

16.6

35.3

17.5

16.7

15.8

31.9

33.2

44.7

178

15.7

Y

71.3

48.6

30.1

41.4

38.7

42.9

28.6

33.4

44.5

31.8

65.4

63.7

83

39.8

41.9

Nb

6.79

6.7

6.35

7.14

6.24

6.2

5.96

6.24

8.09

8.35

10

10.3

13.1

8.4

8.53

Ba

105

107

198

220

177

231

219

204

73.2

65.1

595

627

439

583

97.5

La

29.6

27.7

31.6

59.9

46.9

36

15.6

32.4

41.4

36.3

34.3

36.9

33

25.7

26

Ce

87.1

49.6

66

250

111

95.9

38.4

69.8

79.9

77.2

80.2

81.1

75.4

54

62.6

Pr

9.4

7.41

8.56

16.9

13.1

10.3

5.29

9.35

10.3

10.3

10

10.4

9.79

6.87

6.95

Nd

38.4

28.4

32.4

69.3

53.2

41.6

20.5

37.3

39.4

41.7

41.2

44.2

43

28.3

27.7

Sm

9.23

6.06

6.66

16.3

12

9.65

4.49

7.92

8.23

8.79

10.2

10.6

11.2

6.27

6.45

Eu

0.21

0.09

0.18

0.49

0.33

0.48

0.14

0.27

0.13

0.13

0.82

0.85

1.08

1.21

0.13

Gd

7.83

5.41

5.41

11.6

9.47

8.19

3.66

6.31

6.99

6.83

10.1

10.5

11.6

6.6

6.37

Tb

1.91

1.31

1.1

2.17

1.87

1.7

0.84

1.29

1.55

1.38

2.1

2.04

2.45

1.33

1.39

Dy

12.8

8.86

6.3

10.6

10

9.45

5.33

7.25

9.1

7.35

13.4

12.5

16.3

8.34

8.68

Ho

2.47

1.78

1.25

1.76

1.78

1.7

1.1

1.34

1.71

1.39

2.63

2.53

3.3

1.58

1.76

Er

7.6

5.38

4.05

5.47

5.37

5.03

3.69

4.24

5.06

4.39

8.21

7.74

10.4

4.78

5.36

Tm

1.36

0.98

0.76

0.93

0.94

0.85

0.74

0.77

0.83

0.74

1.4

1.32

1.83

0.81

0.91

Yb

8.43

6.26

5.15

6.66

6.52

6

5.12

5.21

5.34

5.2

9.15

8.33

11.3

4.83

5.78

Lu

1.24

0.9

0.78

1.01

0.97

0.86

0.77

0.75

0.78

0.79

1.43

1.27

1.76

0.75

0.94

Ta

0.59

0.61

0.59

0.61

0.57

0.54

0.51

0.51

0.52

0.59

0.84

0.84

1

0.68

0.66

Pb

14.8

23.1

16.7

16.6

16.4

19.9

15.7

16.3

23.3

11.4

7.14

5.05

19.4

16.6

19.9

Th

12.3

13.9

14.3

14.3

12.6

12.8

12.2

10.4

9.72

10.1

14.2

14.6

14.1

11.8

11.9

U

2.71

2.92

2.34

2.2

2.38

2.08

3.15

1.78

2.22

1.69

2.92

3.57

3.76

2.55

2.85

Zr

144

152

214

236

210

208

200

194

213

207

532

532

282

175

283

Hf

6.37

6.6

8.08

8.25

7.77

7.23

7.18

6.85

6.93

7.52

16.8

15.9

9.93

5.56

10

Eu/Eu*

0.08

0.05

0.09

0.11

0.09

0.17

0.11

0.12

0.05

0.05

0.25

0.25

0.29

0.58

0.06

(La/Yb)N

2.52

3.17

4.4

6.45

5.16

4.33

2.19

4.46

5.56

5.04

2.69

3.18

2.09

3.82

3.23

REE

217.58

150.14

170.2

453.09

273.45

227.71

105.67

184.2

210.72

202.49

225.14

230.28

232.41

151.37

161.02

Mg#=MgO/40.3/(MgO/40.3+0.9*Fe2O3/71.84); Eu/Eu*=Eu N/(SmN*GdN)1/2

Table 3 (continued) sample

NM10-38

NM10-39

NM10-40

NM10-41

NM10-42

NM10-43

NM10-44

07NM-64

07NM-65

07NM-66

07NM-67

07NM-68

07NM-69

07NM-70

SiO2

75.88

78.22

78.59

69.74

69.15

54.16

51.5

61.81

54.54

53.49

52.99

53.61

54.74

54.02

Al2O3

13.09

11.75

11.7

14.62

15.77

16.45

19.83

15.3

16.64

16.35

16.79

16.47

16.52

16.76

Fe2O3

1.32

1.3

1.15

4.05

4.32

8.14

7.89

6.8

8.29

8.22

8.39

8.05

7.57

8.13

CaO

0.43

0.08

0.1

2

1.41

5.95

3.67

3.39

4.93

6.4

6.01

6.76

6.37

5.93

MgO

0.04

0.01

0.01

0.29

0.36

4.94

5.98

1.81

4.63

3.4

4.39

4.1

4.23

4.48

K2O

5.24

4.68

4.64

2.23

2.61

0.27

0.81

1.95

0.7

1.5

1.01

1.19

0.59

0.69

Na2O

2.91

3.02

2.9

3.59

3.04

3.46

5.37

3.79

3.09

2.39

2.46

2.49

3.19

2.82

MnO

0.01

0.01

0.01

0.04

0.04

0.11

0.18

0.1

0.12

0.12

0.12

0.12

0.12

0.11

TiO2

0.18

0.12

0.13

0.67

0.73

1.05

0.82

0.78

1.05

0.99

1

1.02

1.02

1.03

P2O5

0.03

0.04

0

0.13

0.15

0.19

0.21

0.14

0.2

0.18

0.18

0.19

0.19

0.2

LOI

0.83

0.71

0.72

2.54

2.31

5.15

3.6

4.01

5.66

6.8

6.51

5.86

5.33

5.67

Total

99.95

99.93

99.93

99.89

99.88

99.86

99.86

99.88

99.85

99.86

99.86

99.85

99.85

99.85

Mg#

-

-

-

-

-

55

60

35

53

45

51

50

53

52

Sc

8.76

4.54

4.34

10.9

13.2

26.9

30

17.6

27.5

25.9

25.9

28.3

25.4

27.8

V

6.97

7.87

8.31

26.8

37.4

225

166

181

227

219

220

236

216

231

Ga

20.3

21.6

20.7

17.4

22.3

18.4

18.5

18.4

20.2

19

19.5

19

18.5

19.9 30.8

Rb

143

187

179

105

143

14.5

34.1

100

35.3

72.2

48.3

53.2

24.6

Sr

16.1

12.6

12.4

148

168

406

372

313

566

422

503

403

471

590

Y

43.5

78.1

54.3

42.1

45.7

26

22.1

23

26.9

24.1

23.5

25.8

23.3

25.8

Nb

8.8

10.2

10

13.3

15.8

6.29

6.15

7.41

6.68

6.5

6.07

6.31

6.44

6.5

Ba

90.3

62.9

78.8

271

385

222

252

497

365

484

364

581

457

420

La

33.9

52.1

39.7

31.7

37.2

15.4

17.4

14.1

15.8

13.9

13.7

14.9

12.8

14.2

Ce

68.8

116

85.9

65.2

76.5

34.3

35.6

30

32.2

31.2

29

32.5

30.4

32.1

Pr

7.99

16.8

10.5

8.31

9.63

4.35

4.46

3.86

4.32

3.94

3.87

4.34

4

4.21

Nd

31.3

70.7

40.4

33.8

38.5

18.7

18.4

16

18.4

16.9

16.1

18.6

16.4

18.1

Sm

6.6

16.6

8.93

7.38

8.33

4.41

3.94

3.77

4.21

4.19

3.73

4.44

4.03

4.4

Eu

0.14

0.37

0.13

1.49

1.74

1.26

1.15

1.1

1.19

1.24

1.08

1.37

1.14

1.31

Gd

6.68

15.8

8.39

7.25

7.92

4.62

4.05

3.73

4.41

4.17

3.88

4.57

4.08

4.41

Tb

1.35

3.03

1.66

1.35

1.5

0.869

0.712

0.6

0.68

0.64

0.61

0.69

0.62

0.67

Dy

8.91

17.8

10.8

8.44

9.21

5.25

4.55

3.52

4.09

3.84

3.72

4.08

3.81

4.01

Ho

1.75

3.19

2.15

1.63

1.77

1.06

0.883

0.75

0.84

0.79

0.79

0.82

0.77

0.82

Er

5.63

9.09

6.8

4.98

5.7

3

2.53

2.22

2.44

2.34

2.24

2.41

2.19

2.42

Tm

0.93

1.54

1.21

0.823

0.947

0.472

0.422

0.31

0.35

0.32

0.32

0.33

0.33

0.35

Yb

5.93

9.33

7.79

5.42

6.01

2.96

2.48

2.12

2.23

2.25

2.07

2.29

2.11

2.25

Lu

0.91

1.44

1.17

0.886

0.97

0.468

0.392

0.31

0.34

0.33

0.32

0.32

0.3

0.33

Ta

0.68

0.89

0.89

1.03

1.19

0.511

0.53

0.6

0.56

0.5

0.49

0.52

0.5

0.52

Pb

15

31.2

18.1

15.2

18.2

7.48

4.08

9.87

10.5

10.9

11.2

7.29

8.1

8.46

Th

11.9

17.5

16.6

16.9

20.2

7.57

8.13

6.85

7.58

7.15

7.17

7.35

7.01

7.31

U

2.49

2.63

2.42

3.52

4.6

1.87

2

2.95

2.18

1.97

1.85

1.86

1.72

1.85

Zr

309

277

260

606

711

284

235

305

356

329

330

336

334

334

Hf

11.4

11.9

11.5

16

18.9

7.35

6.18

7.3

8.15

7.67

7.91

7.95

8.07

7.97 0.91

Eu/Eu*

0.06

0.07

0.05

0.62

0.66

0.85

0.88

0.90

0.84

0.91

0.87

0.93

0.86

(La/Yb)N

4.1

4.01

3.66

4.2

4.44

3.73

5.03

4.77

5.08

4.43

4.75

4.67

4.35

4.53

REE

180.82

333.79

225.53

178.66

205.93

97.12

96.97

82.39

91.5

86.05

81.43

91.66

82.98

89.58

Mg#=MgO/40.3/(MgO/40.3+0.9*Fe2O3/71.84); Eu/Eu*=EuN/(SmN*GdN)1/2

Table 4 Rb–Sr and Sm–Nd isotopic compositions for the volcanic rocks from the Dashizhai Formation in the north-eastern Sonid Zuoqi area. (87Sr/86Sr)m

2σ

(87Sr/86Sr)i

Sm (ppm)

Nd (ppm)

10.93

0.746383

0.00001

0.7027

10.2

41.2

178

1.89

0.711444

0.000009

0.7039

6.27

129

15.7

23.99

0.800373

0.000011

0.70104

NM10-39

187

12.6

43.62

0.869383

0.000013

NM10-43

14.5

406

0.103

0.705557

0.000013

NM10-44

34.1

372

0.265

0.707312

NM12-87

168

14.3

33.18

NM12-93

1.61

56

NM12-90

146

16.1

NM12-113

134

15.8

Sample No.

Rb (ppm)

Sr (ppm)

NM10-30

120

31.9

NM10-34

116

NM10-37

87

Rb/86Sr

147

Sm/144Nd

(143Nd/144Nd)m

2σ

εNd(0)

fSm/Nd

(143Nd/144Nd)i

εNd(t)(Ma)

TDM1(Ma)

TDM2(Ma)

0.1497

0.512704

0.000007

1.3

-0.24

0.512429

3.0

1062

826

28.3

0.1339

0.512657

0.000007

0.4

-0.32

0.512411

2.6

942

843

6.45

27.7

0.1408

0.512669

0.000008

0.6

-0.28

0.51241

2.9

1006

849

0.69497

16.6

70.7

0.1419

0.512691

0.000006

1

-0.28

0.51243

3.3

974

818

0.70514

4.41

18.7

0.1426

0.512722

0.000007

1.6

-0.28

0.51246

3.8

918

772

0.000008

0.70623

3.94

18.4

0.1295

0.512601

0.000009

-0.7

-0.34

0.512363

1.9

994

916

0.843743

0.000019

0.70918

6.06

28.4

0.1354

0.512729

0.000017

1.8

-0.31

0.512476

4.0

819

734

0.081

0.708062

0.000014

0.70773

9.58

46.8

0.1299

0.512725

0.000019

1.7

-0.34

0.512482

4.1

773

713

-

-

-

-

12

53.2

0.1431

0.512935

0.000019

5.8

-0.27

0.512668

7.7

465

435

-

-

-

-

8.79

41.7

0.1337

0.512678

0.000011

0.8

-0.32

0.512432

3.0

899

809

Notes: εNd(t) = [(143Nd/144Nd)i/(143Nd/144Nd)CHUR.i–1] 104; m represents determined values; i represents initial values after age correction; age corrections were based on their zircon U-Pb ages.

Highlights

The bimodal volcanic rocks in the north-eastern Sonid Zuoqi area erupted at 292–279 Ma. Mafic rocks were derived from metasomatised asthenospheric mantle. Felsic rocks originated from juvenile crustal source. The formation of this bimodal suite in an extensional setting was related to post-collisional delamination.

1000

2000 1000

Post-COLG

Syn-COLG WPG

NM10-43 NM10-44

NM10-37

VAG

NM10-39 NM10-40 170°∠70°

330°∠82°

1

1

ORG

10

100

Y+Nb

1

1000

16

1

IAG+CAG+CCG

Shale

Tuffaceous Sandstone sandstone

153°∠80°

Ultramafic Unconsolidated Fault sediments rocks 326°∠56°

142°∠72°

1500

14

R2

Basaltic andesite

Al2O3

Rhyolite

NM12-89 NM12-90 NM12-91 NM12-92 NM12-93 NM12-86 NM12-88 NM12-87

13

POG

12 11 10

100

Y

1 - Mantle Fractionates 2 - Pre-Plate Collision 3 - Post-Collision Uplift 4 - Late-Orogenic 5 - Anorogenic 6 - Syn-Collision 7 - Post-Orogenic

2000

15

(b) 140° 250 m

10

1000

2500

17

168°∠45°

Post-COLG

ORG

Felsic volcanic rocks 173°∠52°

VAG+ Syn-COLG

10

10

NM10-38

1 km

Nb

100

NM10-41 NM10-42

WPG

100

Rb

NM10-34

(a)205°

NM10-30 NM10-31 NM10-33

2

1000

3 4

500

RRG+CEUG

1

6

5

70 71 72 73 74 75 76 77 78 79 80

SiO2

0

0

500

1000

7

1500

R1

2000

2500

3000