Ordovician Intrusive Rocks from the Eastern Central Asian Orogenic Belt

International Geology Review

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Ordovician intrusive rocks from the eastern Central Asian Orogenic Belt in Northeast China: chronology and implications for bidirectional subduction of the early Palaeozoic Palaeo-Asian Ocean

Hongying Li, Zhiguang Zhou, Pengju Li, Da Zhang, Changfeng Liu, XiaoQi Zhao, Lizhen Chen, Congnan Gu, Tingting Lin & Mengmeng Hu

To cite this article: Hongying Li, Zhiguang Zhou, Pengju Li, Da Zhang, Changfeng Liu, XiaoQi Zhao, Lizhen Chen, Congnan Gu, Tingting Lin & Mengmeng Hu (2016) Ordovician intrusive rocks from the eastern Central Asian Orogenic Belt in Northeast China: chronology and implications for bidirectional subduction of the early Palaeozoic Palaeo-Asian Ocean, International Geology Review, 58:10, 1175-1195, DOI: 10.1080/00206814.2015.1135762

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Download by: [Shanghai Jiaotong University] Date: 17 October 2016, At: 01:24 INTERNATIONAL GEOLOGY REVIEW, 2016 VOL. 58, NO. 10, 1175–1195 http://dx.doi.org/10.1080/00206814.2015.1135762

Ordovician intrusive rocks from the eastern Central Asian Orogenic Belt in Northeast China: chronology and implications for bidirectional subduction of the early Palaeozoic Palaeo-Asian Ocean Hongying Lia,b, Zhiguang Zhoua, Pengju Lib, Da Zhanga, Changfeng Liua, XiaoQi Zhaoa, Lizhen Chenc, Congnan Gud, Tingting Lina and Mengmeng Hua aSchool of Earth Science and Resources, China University of Geosciences (Beijing), Beijing, China; bSchool of Economic and Management, Sichuan University of Science and Engineering, Sichuan Zigong, China; cRegional Geology and Mineral Resources Survey Institute, Langfang, China; dTianjin North China Geological Exploration Bureau, Tianjin, China

ABSTRACT ARTICLE HISTORY The eastern segment of the Central Asian Orogenic Belt is traditionally called the Xing’an Received 26 July 2015 Mongolia Orogenic Belt (XMOB). Ordovician intrusive rocks exposed in the XMOB, from north to Accepted 21 December 2015 south, are the Abaga-East Ujimqin Qi-Duobaoshan belt, the Sonid Zuoqi-West Ujimqin Qi belt, and KEYWORDS – the Damaoqi-Baimaimiao-Tulinkai belt, respectively. Zircon U Pb dating and geochemical data are Intrusive rocks; Ordovician; presented for the intrusive rocks in East Ujimqin Qi and West Ujimqin Qi, Inner Mongolia. The Xing’an Mongolia Orogenic intrusive rocks from East Ujimqin Qi consist of gabbro, diorite, and granodiorite. LA-MC-ICP-MS Belt; East Ujimqin Qi; bidir- zircon U–Pb ages range 446 to 461 Ma. Geochemical data suggest that the gabbros and diorites ectional subduction; Palaeo- from East Ujimqin are a tholeiitic series, both of arc-related and N-MORB (mid-ocean ridge basalt) Asian Ocean signature, indicating a back-arc basin setting. The granodiorites have a shoshonitic series and arc- related signature. Rare earth element (REE) patterns and trace element characteristics suggest gabbros, diorites, and granodiorites are petrogenetically correlated. These intrusive rocks from East Ujimqin Qi have high light REE, Th, and U concentrations, suggesting the effect of middle– upper continental crustal contamination. Major oxides display positive or negative correlations, with increasing MgO or SiO2, indicating that fractional crystallization occurred during magma

evolution. Geochemical data of diorite from West Ujimqin Qi indicate a tholeiitic series, arc-related signature. Zircon U–Pb dating yielded an age of 441.8 ± 1.5 Ma. Integrated with the regionally exposed Mid–Late Ordovician plutons and metasedimentary strata, we concluded that the northward subduction of the Palaeo-Asian Ocean (PAO) that occurred beneath the southern margin of the South Mongolian Micro-continent along the Sonid Zuoqi-Xilinhot gave rise to early Palaeozoic igneous rocks from the Abaga–East Ujimqin Qi–Duobaoshan and the Sonid Zuoqi–West Ujimqin Qi belts. Southward subduction beneath the North China Craton generated the Damaoqi– Baimaimiao–Tulinkai belt. The results support the bidirectional subduction model of the PAO in the early Palaeozoic.

1. Introduction et al. 1993; Khain et al. 2002, 2003; Windley et al. 2007). The eastern CAOB extends across Inner Mongolia and The Palaeo-Asian Ocean (PAO) once existed in the wide Northeast China and is named the Xing’an Mongolia area between the Tarim, North China and Siberia Orogenic Belt (XMOB) (Ren et al. 1980; Shao 1991;Xu Cratons (Dobretsov et al. 1995). Subduction of the PAO et al. 2013, 2015). The long evolutionary history of the and collision between plates, arcs, and micro-continen- CAOB and PAO is documented by multi-stage magmatic tal massifs (Chen et al. 2000, 2001, 2009; Shi et al. 2005; activities that mainly occurred in the early Palaeozoic Jian et al. 2008, 2010) gave rise to the Central Asian (Chen et al. 2000; Shi et al. 2004; Jian et al. 2008; Zhang Orogenic Belt (CAOB) in the Phanerozoic, coeval with et al. 2013; Zhang et al. 2014), late Palaeozoic (Ge et al. crustal accretion (Windley et al. 2007;Geet al. 2007a; 2007a;Wuet al. 2011; Liu et al. 2013; Zhang et al. 2015), Chen et al. 2009; Kröner et al. 2014). The CAOB is a giant and early Mesozoic (Chen et al. 2009;Liet al. 2015a). accretionary orogen, extending from the Urals to the Some possible geodynamic evolutionary models have Pacific Ocean and from the Siberian and East European been proposed to interpret the tectonic evolution of the cratons to the Tarim and North China cratons (Sengör

CONTACT Zhiguang Zhou [email protected] School of Earth Science and Resources, China University of Geosciences (Beijing), Beijing, China Supplemental data for this article can be accessed here. © 2016 Taylor & Francis 1176 H. LI ET AL.

PAO in the Phanerozoic. Sengör et al.(1993) considered a et al. 2008;Liet al. 2012a; Qin et al. 2013; Liu et al. 2014; single and simple magmatic arc (Kipchak arc) was frag- Shi et al. 2014; Zhang et al. 2014), which contributes to mented along its strike by strike-slip faulting during the establishing a framework for early Palaeozoic igneous accretionary process. Differential rotation of Baltica and rocks and a discussion of the tectonic evolution of the Siberia resulted in the duplication of the arc by strike-slip PAO. However, magmatism in the Uliastai Active faulting, and finally gave rise to closure of the ocean by Continental Margin is a response to the northward sub- the late Carboniferous and the formation of CAOB. duction of the PAO beneath the South Mongolia Micro- Yakubchuk (2002, 2004) increased the number of arcs continent but is poorly constrained due to the scarcity and back-arcs to modify the Kipchak model. Filippova of geochemical and geochronological data from early et al.(2001) proposed an archipelago model to interpret Palaeozoic igneous rocks. In this article, we present formation of the CAOB. Windley et al.(2007) modified the zircon U–Pb geochronological and geochemical data archipelago model by applying ridge–trench interaction from the Late Ordovician intermediate-basic intrusive to account for the growth of the CAOB. Xiao et al.(2015) rocks of East Ujimqin Qi and West Ujimqin Qi. In addi- proposed that the CAOB was constructed by three col- tion, an overview of published data from Ordovician lage systems that were finally rotated into two oroclines, igneous is presented along with a discussion of their and their terminal amalgamation was in the end-Permian geochronological and tectonic significance. to Middle Triassic. Specifically, regarding the evolution of the eastern 2. Tectonic outline CAOB in the early Palaeozoic, Xu and Chen (1997) identified a subduction-collision orogenic belt along Five tectonic units can be identified in the eastern Erdaojing to Honger south of Sonid Zuoqi, and he CAOB, including the North China Craton, Southern therefore proposed that a northward subduction of Orogenic Belt, Palaeo-Asian Ocean, Arigin Sum-Xilin the PAO occurred in the middle Palaeozoic. Xiao et al. Hot Block, Northern Orogenic Belt, and Uliastai Active (2003) created a model with north-directed subduction Continental Margin (Figure 1). that occurred before the Cambrian, resulting in the formation of the Ulan arc, followed by Mid-Ordovician- 2.1. North China Craton Permian bidirectional subduction beneath the South Mongolian Micro-continent to the north and the North The North China Craton is one of the oldest cratons in China Craton to the south. Jian et al.(2008) modified the world, with a metamorphic basement as old as the bidirectional subduction model, named the 3.8 Ga (Zhai and Santosh, 2011 and references therein). Southern and the Northern orogens, and proposed a The northern margin of the NCC was strongly influ- time scale for the SSZ (supra-subduction zone) ophiolite enced by southward subduction during early life cycle from chronological data of igneous rocks in Palaeozoic, as indicated by magmatism (Liu et al. 2003, the eastern CAOB. Final collision of the bidirection sub- 2014; Jian et al. 2008;Liet al. 2012a). duction systems occurred during the late Permian to Early Triassic and gave rise to the Solonker suture that 2.2. Southern Orogenic Belt marks the final closure of the Palaeo-Asian Ocean between the North China Craton and the South The Southern Orogen forms a typical arc-trench com- Mongolian Micro-continent (Xiao et al. 2003; Windley plex north of the North China Craton, and is mainly et al. 2007; Zhang et al. 2014). Xu et al. (2013) also composed of the Bainaimiao arc belt and Ondor Sum argued for the bidirectional subduction model and pro- subduction–accretion complex. The Bainaimiao arc belt posed a new tectonic framework, including the North extends from Baoerhantu north to Bayan Obo via Bart China Craton, Southern Orogenic Belt, Hunshandake Obao north to Damaoqi, Bainaimiao, Tulinkai, Ongniud- Block, Northern Orogenic Belt, South Mongolia Micro- Jiefangyingzi in Inner Mongolia to Siping and Yitong in continent, and Southern Margin of Ergun Block from Jinlin (Zhang et al. 2014). It was separated by a wide north to south, and concluded that the bidirectional ocean from the northern North China Craton during the subduction–collision accretionary process was the Cambrian–Ordovician (Hu et al. 1990; Xiao et al. 2003; dominant geodynamic feature for the eastern CAOB Jian et al. 2008; Zhang et al. 2014). It is composed of during the early to middle Palaeozoic. metavolcanic rocks, intrusive rocks, and greenschist Abundant chronological and geochemical data of facies-low amphibolite facies metasedimentary rocks Ordovician subduction-related igneous rocks to the (Jian et al. 2008; Liu et al. 2014; Zhang et al. 2014), and south of the Solonker Suture Zone have been reported unconformably overlain by late Silurian–Early Devonian (Xu and Chen 1997; Xiao et al. 2003;Xuet al. 2003b; Jian continental molasse or quasi-molasse strata of the INTERNATIONAL GEOLOGY REVIEW 1177

Figure 1. (a) Simplified tectonic map of CAOB (modified from Xu et al. 2013); (b) simplified geological sketch map of Late Ordovician intrusion in East Ujimqin, Inner Mongolia, modified from Jian et al. (2008). The black box shows the location of the study areas. Data from: 1. Zhang et al. 2014; 2. Zhang et al. 2013; 3. Liu et al. 2014; 4. Li et al. 2015a; 5. Zhang et al. 2015; 6. Nie et al. 1995; 7. Gu et al. 2012; 8. Xu 2005; 9. Chen et al. 2000; 10. Shi et al. 2004; 11. (Liu et al. 2003; 12. Jian et al. 2008; 13. Li et al. 2012a; 14. Chu et al. 2013; 15. Zhao et al. 2012; 16. Qin et al. 2013; 17. Ge et al. 2011; 18. Li et al. 2011; 19. Li et al. in press; 20. this paper.

Xibiehe and Xuniwusu formations (Zhang et al. 2010). controversial. Wang (1985) reported ages of 630 Ma Some researchers considered that the Bainaimiao arc (Rb-Sr) for pillow lava and 535.8–626 Ma (K-Ar) for was a Japan-style island arc (Jia et al. 2003; Shang amphibole in metamorphic gabbro, and based on an et al. 2003), whereas Zhang et al.(2014) considered it integration of contact relations, metamorphism, ophio- to be an ensialic island arc, and others considered it a lite suite, and palaeontology, he constrained the age of continental arc (Xiao et al. 2003; De Jong et al. 2006). the Ondor Sum Group to late Precambrian–middle The metamorphic rocks had been considered Silurian. Some authors reported ages of 961 ± 66 Ma Palaeoproterozoic (IMIGS, 2003) or Mesoproterozoic (Sm-Nd) and 624 ± 110 Ma (Rb-Sr) for metavolcanic (Hu et al. 1990; Nie et al. 1991); however, high-precision rocks of the Ondor Sum Group (Zhang and Wu 1999). zircon U–Pb dating constrains its formation to Peng (1984) discovered early Cambrian radiolarians and Ordovician to Silurian (Gu et al, 2012, Liu et al. 2014; microfauna in abyssal sedimentary rocks of the Ondor Zhang et al. 2014). The Ordovician igneous rocks in the Sum Group, and combined with an Rb-Sr age of 6.3 Ga Southern Orogen are a product of southward subduc- of pillow lava beneath the abyssal sediment, he tion of the PAO (Hu et al. 1990; Jia et al. 2003; Shang regarded the Ondor Sum Group as an early Cambrian et al. 2003; Xiao et al. 2003; Shi et al. 2013; Liu et al. ophiolite suite. Recent published data constrained the 2014). Ondor Sum Group to Cambrian–Ordovician from ages The Ondor Sum subduction–accretion complex con- of 497–477 Ma for the ophiolite and 479 Ma for plagi- sists mainly of blocks of pillow basalts, gabbro, diabase, oclase (Jian et al. 2008), 458 Ma for adakitic dacite (Liu tuff, metasandstone, chert, pelagic sediments, and lime- et al. 2003), and 470 Ma for metamorphic andesite (Li stone (Wang, 1985; Xiao et al. 2003; Shi et al. 2013), and et al. 2012a). has been considered an ophiolite suite (Tang 1992; Jian et al. 2008). Based on field investigation and published chronological data, Li et al.(2012a) proposed that the 2.3 Arigin Sum–Xilin Hot Block Ondor Sum Group included not only ophiolite but also the lithological association of an intra-ocean island arc. The Arigin Sum–Xilin Hot Block scatters in the North However, the age of the Ondor Sum Group remains Orogen as a small outcrop, and is an independent 1178 H. LI ET AL. tectonic unit represented by the Arigin Sum and Xilin The Baiyanbaolidao pluton probably extends eastwards Hot groups. The Arigin Sum Group outcrops in Airgin to Xilin Hot and West Ujimqin Qi. Sum and consists of mica quartzite, sericite quartz schists, gneisses, and gneissic granite. A youngest peak age of 1180 Ma of mica quartzite was reported 2.5. Uliastai Active Continental Margin by Xu et al. (2013). In addition, Airgin Sum is adjacent The Uliastai Active Continental Margin (UACM) extends to Hutag Uul Block in Mongolia to the west, where along the northern border of Inner Mongolia from et al Yarmolyuk . (2005) reported an age of 952 Ma Chagan Obo to East Ujimqin Qi (Uliastai) (Xiao et al. from two-mica gneissic granite. The above dating 2003). Recent research has revealed that the presence results suggest that there is a Precambrian basement of a Late Ordovician Geri Obao pluton (449 ± 3 Ma) in in the Airgin Sum area. The Xilin Hot Group is dis- the middle part of the UACM (Zhao et al. 2012), an island membered from the previous Xinlin Gol Complex, arc represent by Duobaoshan volcanic, and a back-arc which outcrops discontinuously within a late basin in the eastern UACM (Figure 1). Ordovician strata et al Carboniferous arc accretionary zone (Liu . 2013; exposed in the UACM include Tongshan, Duobaoshan, et al Li . 2014a) along Honger, Xilin Hot, and West and Wubinaobao formations. The Tongshan Formation Ujimqin Qi. The previous Xilin Gol Complex not only is comprised of silty slate, phyllites, sandstone, and lime- – contains metamorphic rocks dated at 1025 1286 Ma stone, defining a littoral facies. The Duobaoshan et al (Xu . 1996; Hao and Xu 1997), but also granite Formation consists of calc-alkaline andesites, rhyolites, et al aged 458 Ma (Ge . 2011) and some late spilite-keratophyre, tuff, and interbedded tuffaceous et al Carboniferous intrusive rocks (Ge . 2011). sandstone. The Wubinaobao Formation comprises slate, et al Accordingly, Ge .(2011) divided the Xilin Gol sandstone, and limestone lense, and these are suc- Complex into three major geological units: (1) a ceeded by Silurian argillaceous siltstone, metasandstone, Mesoproterozoic volcanic-sedimentary rock unit, (2) siltstone, and metaconglomerate. Neoproterozoic basic-ultrabasic rocks, (3) and Ordovician–early Carboniferous intermediate-acid intrusions. The Mesoproterozoic volcanic-sedimentary 3. Geological setting and sampling rock unit, the Xilin Hot Group, is the residual base- 3.1. East Ujimqin Qi (Uliastai) ment of a Precambrian micro-continent (Xu et al. 1996; Hao and Xu 1997;Zhuet al. 2004;Geet al. 2011). East Ujimqin Qi is situated, tectonically, in the UACM (Figure 1). The oldest unit exposed in the study region is the Early Ordovician Wubinaobao Formation that is 2.4 Northern Orogenic Belt intruded by Late Ordovician and Early Cretaceous intru- The northern orogen forms a N-dipping thrust belt (Xiao sions. Skarnization occurred along the contacts of the et al. 2003) and consists of the Airgin Sum-Erdaojing Ordovician strata with the plutons, and siltstone is subduction–accretion complex, the Baiyanbaolidao TTG altered to hornfels and limestone to marble. Late pluton (Jian et al. 2008). Ordovician intrusive rocks in East Ujimqin include The Arigin Sum–Erdaojing subduction–accretion gabbro, diorite, and granodiorite. Early Cretaceous complex is composed of limestone, chert, sandstone, intrusions are dominated by granite. Late Ordovician dolomite, dunite, harzburgite, lherzolite, tholeiitic gabbro outcrops west of Chaobuleng and basalt, gabbro, and blushcist (Zhang and Wu 1999;Xu Barunburgastai. Granodiorite is exposed near et al. 2001, 2015;Liet al. 2014a) and is unconformably Barunburgastai with diorite distributed along the peri- overlain by Devonian conglomerate (Xu et al. 2001). An meter of the granodiorite pluton (Figure 2). Combined Ar-Ar age of 383 ± 13 Ma has been obtained from Na- with published chronological and geochemical data of amphibole of the blueschist in Naomuhunni, Sonid the gabbros (Li et al. 2015, in press), we collected sam- Zuoqi (Xu et al. 2001). ples of diorite (PM111–3TW) and granodiorite (PM111- The Baiyanbaolidao pluton consists of deformed 7TW) to reveal the chronological, petrological, and geo- diorite, quartz diorite, tonalite, and granodiorite, with chemical features of Ordovician intrusions in East emplacement ages ranging from 498 to 461 Ma (Chen Ujimqin Qi. et al. 2001; Shi et al. 2004, 2005; Jian et al. 2008). Some Gabbros are medium-coarse grained (Figure 4a, b), authors consider that the tonalites from the pluton are with gabbroic texture and massive structure; they com- adakites (Shi et al. 2004); however, Chen (2002) prise plagioclase (55 vol.%) and altered pyroxene (35 regarded these as typical island arc magmatic rocks. vol.%). Plagioclases are largely 2–5 mm in size, with INTERNATIONAL GEOLOGY REVIEW 1179

Figure 2. Simplified geological map of Late Ordovician intrusive rocks in East Ujimqin Qi. Data of gabbro samples are from Li et al. 2015 (in press). zoisitization. Accessory minerals consist of actinolite (<5 3.2. West Ujimqin vol.%), chlorite (<1 vol.%), epidote (<1 vol.%), and apa- West Ujimqin is situated, tectonically, in the Xilin Hot tite (<1 vol.%). Block scattered in the Northern Orogen (Figure 1b). The Diorites are subhedral, medium-grained, with mas- most ancient geological unit in this area is the Xilin Hot sive structure and comprise plagioclase (65–70 vol.%), Group (Figure 3). The Xilin Hot Group is in fault contact hornblende (20–30 vol.%), and alkali feldspar (3–7 with the Upper Ordovician strata and early vol.%) (Figure 4c). Plagioclases have a diameter of Carboniferous volcanic rocks. Ordovician diorite, early 2–5 mm and are subhedral tabular with sericitization. Permian granites, and quartz diorite and numerous Hornblendes are 13 mm in size, columnar or grained, later granitic and diorite veins have intruded into the with general actinolization. Alkali-feldspars are 0.5– Xilin Hot Group. 4 mm in diameter and are xenomorphic granular. The Diorite from West Ujimqin was collected from the accessory minerals comprise quartz and ferritized mag- previous Xilin Gol Complex. The sample is grey–green netite, with minor actinolite and ouralite-filled fractures. in colour and shows a fine-grained lepidogranoblastic Granodiorites comprise phenocryst (10 vol.%) and texture and banded structure (Figure 4e), composed of matrix (90 vol.%), with porphyritic-like texture and massive hornblende (50%), plagioclase (35%), and quartz (8%), structure (Figure 4d). The matrix is medium to fine grained with epidote (4%) and magnetite (1%) as secondary and of massive, locally cataclastic structure and includes minerals. Most hornblendes are up to 0.2–0.3 mm in plagioclase (50 vol.%), alkali feldspar (15 vol.%), quartz (18 size, with some 0.4–0.8 mm. Plagioclases are 0.2– vol.%), hornblende (5 vol.%), biotite (2 vol.%), and a small 0.5 mm in size, with strong sericitization. Quartz and portion of magnetite. The phenocrysts are composed of epidote are 0.1–0.2 mm in size (Figure 4f). plagioclase (8 vol.%) and alkali feldspar (2 vol.%). 1180 H. LI ET AL.

Figure 3. Simplified geological map of Late Ordovician diorite in West Ujimqin Qi.

4. Analytical methods performed at Tianjin Institute of Geological and

Mineral Resource. Laser ablation was performed by 4.1 Whole-rock geochemical analyses using 193 nm laser ablation system, with the spot dia- Whole-rock geochemical analyses were performed at meter of 35 μm, and the ablation depth of 20-40μm. the laboratory of Regional Geology and Mineral Zircon GJ-1 was used as the external standard for U–Pb Resources Survey Institute in Langfang, Hebei, China. dating and was analysed twice every 8 analyses. Glass Major oxides were analysed by X-ray fluorescence ana- sample NIST612 was used as external standard for cal- lysis (XRF). Analytical precision is generally better than culating the content of Pb, U and Th. The specific 5% for all the major elements. Trace element and rare measuring procedure has been described by Geng earth element (REE) abundances were measured using et al.(2012). The data were processed by using inductively coupled plasma mass spectrometry (ICP-MS) ICPMSDataCal (Liu et al. 2008) and ISOPLOT (Ludwig following the technique of Gao et al.(2003), with analy- 2003). tical precision for most elements better than 5%; for Zr, Hf, Nb, and Ta, values were within 10% of certified 5. Analytical results values. 5.1. Zircon U–Pb dating 5.1.1. Samples from the UACM 4.2. Zircon U–Pb dating The CL images for zircons of Late Ordovician intrusive Zircon crystals were extracted from fresh rock samples rocks collected from the East Ujimqin Qi back-arc basin by conventional heavy liquid and magnetic separation in the UACM are presented in Figure 5. The zircon U–Pb technique and then hand-picked under a binocular data can be seen in Supplementary Table 1 (see http:// microscope at the lab of Regional Geology and dx.doi.org/10.1080/00206814.2015.1135762 for supple- Mineral Resources Survey Institute in Langfang, Hebei. mentary tables), and they are plotted on concordia Cathodoluminescence (CL) images were taken to reveal diagrams in Figure 6. the internal texture of all zircons after they were affixed Sample PM111-3TW (E: 118°40′56″, N: 46°34′14″)is on expoxy resin at Beijing Gao Nian Ling Hang pale grey-green to dark grey medium-grained diorite, Technology Co. Ltd. LA-MC-ICP-MS dating was with Th/U ratios ranging from 0.55 to 0.9. Eighteen INTERNATIONAL GEOLOGY REVIEW 1181

Figure 4. Outcrop and microscopic photos of intermediate-basic rocks from East Ujimqin Qi and West Ujimqin Qi, Inner Mongolia. Qz, quartz; Pl, plagioclase; Hb, hornblende; Aug, augite.

analyses were performed and yielded a weighted mean 206Pb/238U age of 447.9 ± 1.6 Ma (MSWD = 2.0). Sample PM111-7TW (E: 118°41′06″, N: 46°34′42″)is grey porphyritic medium- to fine-grained granodiorite. Ages of 19 zircons clustered to the concodia line and yielded ages of 438–451 Ma, with a weighted mean 206Pb/238U age of 445.8 ± 1.7 Ma (MSWD = 1.13).

5.1.2. Sample from the Xilin Hot Block Sample D3019 was collected from West Ujimqin Qi in the Xilin Hot Block. Zircons from sample D3019-3TW are colourless and with few inclusions. They are 80–130 μm in size and mostly euhedral columnar crystals, with clear oscillatory zoning in CL images. Among 21 analyses of 17 grains, 15 analyses form a coherent group with a Figure 5. CL images for zircons of Late Ordovician intrusive weighted mean 206Pb/238U age of 441.8 ± 1.5 Ma rocks in East Ujimqin Qi and West Ujimqin Qi. (MSWD = 0.91), representing the emplacement age of 1182 H. LI ET AL.

Figure 6. Concordia diagrams for zircons of Late Ordovician intrusive rocks in East Ujimqin Qi and West Ujimqin Qi. the diorite. In addition, the remaining zircon grains yield characterized by lower Mg# (58.2–65.4) and enrichment – five Precambrian ages which were interpreted as inher- in Ti (TiO2 =0.61 1.4%, average 1.23%). The Na2O content ited zircons from the Xilin Hot Group. is much higher than K2O, with total alkali values (K2O+ Na2O) varying from 0.51 to 4.4% (σ = 0.10–2.77), confining the rocks to tholeiite and calc-alkali series, which is in 5.2. Geochemistry accord with results in the K2O–SiO2 diagram (Figure 7d). 5.2.1. Major elements Samples of diorites from East Ujimqin Qi have con-

The results of whole-rock element analyses of Late tents of SiO2 (average 59.55%), MgO (0.66–5.7%), Na2O Ordovician intrusive rocks from East Ujimqin Qi and (3.83–4.86%), K2O (0.31–4.57%), Al2O3 (10.29–16.57%, West Ujimqin Qi are listed in Supplementary Table 2. average 14.59%), and TiO2 (0.69–1.02%). Diorites Samples from East Ujimqin are plotted in the fields of diverge into two groups according to σ values, 1.38– gabbro, diorite, quartz monzonite, and the boundary 1.76 and 4.33–4.4, respectively. Two samples (PM111- between monzonite and quartz monzonite, and one 3YQ1, PM111-3YQ2) have higher contents of K2O and sample from West Ujimqin Qi is plotted in the field of belong to alkali series, but the other two belong to calc- gabbroic diorite in the SiO2 vs. K2O+ Na2O diagram alkali series. The diorite sample from West Ujimqin Qi (Figure 7a). These samples are plotted in the fields of has contents of SiO2 = 53.04%, MgO = 7.06%, granodiorite, diorite, gabbroic diorite, and gabbroic dia- Na2O = 3.17%, K2O = 0.56%, Al2O3 = 16.32%, and base (Figure 7b) in the Nb/Y vs. SiO2 diagram, which is TiO2 = 0.74% (σ = 1.38). almost consistent with results in the QAPF diagram Granodiorites from East Ujimqin Qi have relatively

(Figure 8). However, closer observation of specimens coherent contents of SiO2 (64.64–64.78%). Three sam- and microscopic identification results confirm that the ples have σ ratios varying from 2.57 to 3.27 and belong quartz monzonite is actually granodiorite and the gab- to calc-alkali series. All granodiorites are enriched in broic diorite is confined to diorite. alkali (K2O = 3.73–4.31%, Na2O = 3.75–4.33%), and Eight gabbro samples from East Ujimqin Qi have rela- Al2O3 (15.71–16.42%, average 16.06%) concentrations, tively uniform contents of SiO2 (41.94–52.16%) and P2O5 but poor in MgO (0.82–1.12%, average 0.96%), with A/ (0.07–1.76%), but inconsistent Al2O3 (7.15%–17.52%) and CNK ratios varying from 0.84 to 1.08, mainly confined to MgO (4.71–14.25%) contents. These samples are granodiorite to peraluminous types (Figure 7c). INTERNATIONAL GEOLOGY REVIEW 1183

Figure 7. (a) Total alkali vs. silica diagram; (b) Nb/Y vs. SiO2 diagram; (c) A/NK-A/CNK diagram; (d) SiO2-K2O diagram. Data of gabbros from East Ujimqin Qi are from Li et al. 2015 (in press); data of Abaga Qi are from Zhao et al. (2012) and Shao (1991); data of Sonid Zuoqi are from Xu et al. (2003b), Zhang and Jian (2008), and Jian et al. (2008); data of Tulinkai are from Liu et al. (2003) and Jian et al. (2008); data of Zhengxiangbaiqi are from Qin et al. (2012).

Figure 8. QAPF diagram of diorite and granodiorite from East Ujimqin Qi. 1184 H. LI ET AL.

Figure 9. MgO vs. other oxides for the Late Ordovician intrusive rocks in East Ujimqin Qi.

For the samples from East Ujimqin Qi, the major Diorites from East Ujimqin Qi have REE contents of –6 –6 oxides SiO2,Al2O3, and Na2O show negative correlation 26.46 × 10 and 295.64 × 10 , (La/Yb)N values of 1.33– with increasing MgO content, while CaO, Fe2O3, and 9.05, (La/Sm)N values of 1.21 and 3.21, (Gd/Yb)N values TiO2 show positive correlation in MgO vs. other oxides of 0.59 and 1.9 (average 1.49), and δEu values of 0.17– diagrams (Figure 9), suggesting that fractional crystal- 0.56 (average 0.4). Diorite samples display REE patterns lization occurred during the process of magma similar to the gabbros of the second and third groups evolution. (Figure 10c). The diorite sample from West Ujimqin Qi has low REE content (57.37 × 10–6), minor positive Eu δ 5.2.2. Trace elements anomaly ( Eu = 1.11), slightly LREE-enriched pattern ((La/Yb) = 2.11, (La/Sm) = 1.66), and a ratio of (Gd/ Based on the REE contents of gabbros from East Ujimqin N N Yb)N = 0.9. Qi, samples can be divided into three groups (Figure 10a). – Granodiorites from East Ujimqin Qi have inconsis- The first group (PM108 1YQ1, PM108-2YQ1) is character- −6 – – tent REE concentration, ranging from 41.58 × 10 to ized by ΣREE = 337.72 × 10 6–432.13 × 10 6, (La/Yb) 206.26 × 10−6,(La/Yb) =0.78–9.94 (average 4.4), = 19.99–20.3, (La/Sm) =4.07–4.14, (Gd/Yb) =2.24– N N N N high fractionation between LREEs and HREEs, high 2.25, and La/Nd = 1.76–3.72. Chondrite-normalized REE fractionation in LREEs ((La/Sm) =2.13–2.88) but patterns of the first group of gabbros are enriched in N weak fractionation in HREEs ((Gd/Yb) =0.26–2.0), LREEs, depleted in heavy REEs (HREES), with weak Eu- N and marked Eu-negative anomalies (δEu = 0.18–0.46) positive anomalies (δEu = 1.06–1.07). The second group (Figure 10e). (PM110-7YQ1, PM110-9YQ1) also displays characteristics Characteristics of major elements and REEs reveal of weak fractionation between LREEs and HREEs ((La/Yb) correlation in regard to petrogenesis between inter- N =4.88–9.68), but the REE concentration is relatively low – – mediate and basic intrusive rocks from East Ujimqin (ΣREE = 208.05 × 10 6–234.23 × 10 6)comparedwiththe Qi. As depicted in Figure 10a, the first group of gabbros first group of gabbros. Besides, gabbro samples of the (PM108-1YQ1, PM108-2YQ1) (age 461.1 Ma) is character- second group are characterized by marked Eu-negative ized by enrichment in LREEs, depletion in HREEs, and anomalies (δEu = 0.44–0.5), significant fractionation in Eu-positive anomalies. LREE contents decrease as the LREEs ((La/Sm) =2.81–2.87) but weak in HREEs, with N gabbros (PM110-7YQ1, PM110-9YQ1) becoming ratios of (Gd/Yb) =0.84–1.46 and La/Nd = 1.63–2.33. N younger (452.5 Ma) and Eu shows negative anomalies. Gabbros of the third group (PM108-3YQ1, PM108-5YQ1, This tendency is more obvious for the third group of PM108-8YQ1, PM110–6YQ1) present features of low REE – – gabbros (PM108-3YQ1, PM108-5YQ1, PM108-8YQ1, content (ΣREE = 21.22 × 10 6–70.28 × 10 6), weak fractio- PM110-6YQ1), whose LREE contents decrease sharply nation between LREEs and HREEs ((La/Yb) =1.33–2.03), N with marked Eu negative anomalies, but HREE content significant fractionation in LREEs ((La/Sm) =1.68–2.83) N decreases only slightly. This trend implies that basic and HREEs ((Gd/Yb) = 0.33–0.43), pronounced Eu-nega- N magma (generated ca. 461 Ma) underwent fractional tive anomalies (δEu = 0.23–0.4), and La/Nd = 0.78–1.7. Six crystallization of the REE- and Eu-enriched minerals to gabbros have in common La/Nd>1.4, suggesting that the generate the latter two groups of younger gabbros. parental magma was generated in an island arc (Zhao Chondrite-normalized REE patterns of diorites and 2007). INTERNATIONAL GEOLOGY REVIEW 1185

Figure 10. (a), (c), (e), (g): Chondrite-normalized REE diagrams (Boynton 1984); (b), (d), (f), (h): primitive mantle/MORB-normalized spider diagram (Sun and McDonough 1989) of Late Ordovician intrusive rocks. granodiorites are similar to the second and the third LREEs (La, Ce, Nd, and Sm) increase with age groups of gabbros, respectively, suggesting that grano- (Figure 10b, d). PM-normalized trace element patterns diorites, diorites, and gabbros originated from the same of diorites also show two different patterns that are source. similar to the latter two groups of gabbros In the PM-normalized trace element spidergrams (Figure 10b, d), suggesting that granodiorites, diorites, (Figure 10b) of the intermediate-basic intrusive rocks and gabbros originated from the same source. The from East Ujimqin Qi, the first group of gabbros dis- diorite sample from West Ujimqin Qi shows positive plays depletion in K, P, and Ti but enrichment in Ba,U, anomalies in LILEs (K, Rb, and Ba) and negative Zr, Hf, and Y. The second and third groups share anomalies in Nb, Ta, Hf, and Ti. In mid-ocean ridge similar features, both being depleted in Ba, K, Nb, Sr, basalt (MORB)-normalized trace element spidergrams, and P, except for total REE content. The contents of granodiorites from East Ujimqin Qi are depleted in Sr, large-ion lithophile elements (LILEs) (Ba and Sr) and Ba, Nb, Ta, and Ti (Figure 10f). 1186 H. LI ET AL.

6. Discussion outcrops in the UACM, represented by the well- described Duobaoshan Formation volcanic rocks (Yang 6.1. Ordovician igneous activity in XMOB et al. 2014;Wuet al. 2015), which is among the impor- High-precision zircon U–Pb LA-MC-ICP-MS dating tant ore deposits in Northeastern China (Zhao et al. defines emplacement ages of 449.5 ± 2.9, 461.1 ± 1.4, 1997; Zeng et al. 2014; Hao et al. 2015). This belt and 452.5 ± 1.7 Ma for gabbros (Li et al. 2015 in press), extends into Mongolia and connects with Ordovician 447.9 ± 1.6 Ma for diorite, 445.8 ± 1.7 Ma for granodior- volcanic rocks in Gurvansaykhan and Mandalovoo ite from East Ujimqin Qi, and 441.8 ± 1.5 Ma for diorite (Blight et al. 2008; Zhu et al. 2014). The monzonitic from West Ujimqin Qi, constraining the emplacement granite aged 449 ± 3 Ma (Zhao et al. 2012) in Geri age to the Late Ordovician. The ages of these intrusive Obao, north of Abaga Qi, is synchronous intrusive rocks reported in this article overlap, within error, those rock. In addition, Ordovician plutons were also discov- of igneous rocks in XMOB. The geochronological data ered in Eerguna block, considered the product of colli- support the synchronicity at ~450 Ma of the subduc- sion between the Erguna and Xing’an blocks (Ge et al. tion-related magmatic activity in XMOB (Supplementary 2005, 2007b;Wuet al. 2011; Zhou et al. 2011, 2015; She Table 3, Figure 11). et al. 2012; Yang et al. 2014). Three Middle-Late Ordovician subduction-related igneous rocks belts are exposed in XMOB (Xu and 6.2. Tectonic setting Chen 1997; Xiao et al. 2003;Xuet al. 2003b, 2015; Jian et al. 2008; Zhang et al. 2013; Shi et al. 2014). The High-strength field elements are widely used to distin- southern belt extends from Bayan Obo, Bart-Obo, guish the tectonic settings of igneous rocks. As depicted Bainaimiao, and Ondor Sum (Tulinkai) to in Figure 12a, most Late Ordovician gabbros and diorites Zhengxiangbaiqi (Xu et al. 2003b; Liu et al. 2003; Jian from East Ujimqin Qi plot in the volcanic arc basalt (VAB) et al. 2008; Zhang and, 2008;Liet al. 2010; Liu et al. field, while three of them plot in the overlapped field of 2014;Liet al. 2015a), comprising volcanic rocks of the MORB and within plate basalt (WPB). All samples from Bainaimiao (Jian et al. 2008;Liet al. 2012b, 2015b) and West Ujimqin Qi and Zhengxiangbai Qi plot in the field of Baoerhantu groups (Shang et al. 2003) and Bart-Obo VAB. Most samples from Tulinkai plot in the fields of CAB pluton (Liu et al. 2003; Tao et al. 2005;Liet al. 2012b; (continental arc basalt) and VAB. Most samples from Feng et al. 2013), with emplacement/eruption ages XMOB plot in the calc-alkali basalt field in the Th-Hf/3- varying from 440 to 470 Ma (Nie et al. 1995; Liu et al. Ta diagram (Figure 12b), but three samples from East 2003; Tao et al. 2005; Zhang and Jian 2008;Liet al. 2010, Ujimqin Qi plot in the overlapping area of WPB and 2012b, 2015b; Feng et al. 2013; Qin et al. 2013). The N-MORB. In the log-transformed immobile trace element central belt extends from Sonid Zuoqi to Xilinhot–West tectonic discrimination diagrams proposed by Agrawal Ujimqin Qi, represented by subduction-related intrusive et al.(2008)(Figure 13a, 13b), all samples fall within the rocks in Baiyinbaolidao (Chen et al. 2000, 2001; Shi et al. area of island arc basalt (IAB). For acid-intrusive rocks 2004, 2005) and West Ujimqin Qi (Ge et al. 2011) with from East Ujimqin Qi, the bulk of samples from XMOB emplacement ages of 452–483 Ma. The northern belt plot within the field of VAG and syn-GOLG in diagrams Y vs. Nb and Y + Nb vs. Rb (Figure 14). Six gabbros have the common feature of La/Nd > 1.4, indicating an arc-related petrogenesis (Zhao 2007). As mentioned above, gabbros from East Ujimqin Qi are characterized by enrichment in LREE, relative depletion in HREE, with a chondrite-normalized REE pattern similar to island arc igneous rocks. Gabbros of the first group (PM108-1YQ1, PM108-2YQ1) are depleted in Nb, Ta, and Ti, the geochemical hallmark of arc-related igneous rocks (Jian et al. 2010). The other two groups have a relatively flat HREE pattern, enriched in Th, Zr, and Hf and depleted in K, Ba, Ti, indicating a mantle source that is geochemically depleted, similar to the source of depleted N-MORB. Therefore, the gabbros from East Ujimqin Qi possess geochemical features of both arc Figure 11. Histogram for data of Ordovician igneous rocks in and N-MORB igneous rocks. As revealed by previous XMOB. research, back-arc basin basalts are formed by the INTERNATIONAL GEOLOGY REVIEW 1187

Figure 12. (a) Th/Yb vs. Ta/Yb diagram (Pearce 1982); (b) Th-Hf-Ta diagram (Wood 1980); for Ta/Yb vs. Th/Yb diagram, SHO, shoshonite; CAB, continental arc basalt; VAB, volcanic arc basalt; TH, tholeiitic; WPB, within-plate basalt; ALK, alkali series; TR, transpition; MORB, mid-ocean ridge basalt. For the Th-Hf-Ta diagram, (a) normal mid-ocean ridge basalt (N-MORB); (b) enriched mid- ocean ridge basalt (E-MORB) and within-plate basalt (WPB); (c) alkaline within-plate basalt (A-WPB); (d) island arc basalt (IAB: Hf/Th > 3, tholeiitic basalt; Hf /Th < 3, calc-alkaline basalt).

Figure 13. Log-transformed immobile trace element tectonic discrimination diagrams for whole-rock samples (Agrawal et al. 2008). For the IAB-CRB+OIB-MORB diagram, DF1 = 0.3518 × Log(La/Th) + 0.6013 × Log(Sm/Th) − 1.3450 × Log(Yb/Th) + 2.1056 × Log(Nb/ Th) − 5.4763 and DF2 = − 0.3050 × Log(La/Th) − 1.1801× Log(Sm/Th) + 1.6189 × Log(Yb/Th) + 1.2260 × Log(Nb/Th) − 0.9944. For IAB-CRB-MORB diagram, DF1 = 0.3305 × Log(La/Th) + 0.3484 × Log(Sm/Th) − 0.9562 × Log(Yb/Th)+ 2.0777 × Log(Nb/Th) − 4.5628 and DF2 = − 0.1928 × Log(La/Th) − 1.1989 × Log(Sm/Th) + 1.7531 × Log(Yb/Th) + 0.6607 × Log(Nb/Th) − 0.4384. IAB: island arc basic rocks; CRB: continental-rift basic rocks; OIB: ocean island basic rocks; MORB: mid-oceanic ridge basic rocks.

Figure 14. Y vs. Nb and Y + Nb vs. Rb (Pearce et al. 1984) diagrams of Late Ordovician granodiorites in the XMOB. upwelling of upper mantle beneath an ocean ridge In XMOB, the Lower-Middle Ordovician Duobaoshan system in a SSZ (Hawkins et al. 1990; Gribble et al. Formation widely outcrops south of the study area. The 1996). Thus, igneous rocks formed in a back-arc basin Duobaoshan Formation is dominated by volcanic rocks, possess the compositional aspects of both MORB and comprising volcanic breccia, agglomerate, intermediate- arc volcanic rocks (Xu et al. 2003a). acid lava, and tuff, embedded with sandstone, pelite, 1188 H. LI ET AL. and marble (Zhu 1986), and is considered to be the product of island arc or active continental margin arc (Yu et al. 1996; Xie 2013;Wuet al. 2015). The Lower- Middle Ordovician Wubinaobao Formation that outcrops in the study area is composed of carbonatite at the bottom, fine-grained metaarkose in the middle (suggesting a near provenance dominated by continental clastics), and andalusite- or sericite-bearing slate at the top, indicating increasing water depth. The typical characteristic of sediment in a back-arc basin is its bidirectional sedimentary provenances. Both the continent and island arc provide clastics for back-arc basins, and lava and volcanic clastic rocks are usually deposited in an arc-proximal setting, while shallow-facies clastics and carbonatite sediment deposit near the continental mar- et al gin (Peng . 1999). The distribution and rock assem- Figure 15. Sm vs. Ce/Sm diagram of Late Ordovician gabbros in blage characteristics of Duobaoshan and Wubinaobao East Ujimqin Qi. Formations are in accord with the sedimentary features of a back-arc basin. Based on an integrated analysis of the geochemical characteristics of Late Ordovician intru- setting. The breaking-off of a subducting plate gave sive rocks and the lithofacies palaeogeography of rise to lithospheric thinning and upwelling of litho- Ordovician strata in East Ujimqin Qi, we infer that the spheric mantle at the back of an island arc, and Late Ordovician intrusive rocks were generated in a generated arc-related magma. Arc-related mafic back-arc basin. magmas may arise directly from the depleted asthe- nosphere mantle and serve as heat sources to crust melting (Holden et al. 1987), thereby generating the 6.3. Petrogenesis tholeiite magma. The initial magma underwent frac- The gabbros from East Ujimqin have large variations in tional crystallization (Coleman and Glazner 1997; chemical composition (SiO2 = 41.94–52.16%, Ratajeski et al. 2001; Wenner and Coleman 2004) MgO = 4.71–14.25). The Mg# value is usually regarded and crustal contamination (DePaolo 1981;Taylor as an indication of magmatic differentiation. Langmuir and McLennan 1985)toformevolvedmagma. et al.(1977) took Mg# values of 60–70 as characteristic Diorite and granodiorite of two different groups dis- of primary magma. Mg# values of Late Ordovician gab- play characteristics similar to the second and third bros vary from 52 to 65, and the contents of major groups of gabbros, respectively, suggesting that T oxides such as CaO and Fe2O3 increase with increase diorite and granodiorite are the evolved products in MgO content while SiO2,Al2O3, and Na2O show of the gabbros. negative linear relation with increase in MgO, representing fractional crystallization during magma evolution, which is seen in the Sm vs. Ce/Sm diagram (Figure 15). 6.4. Tectonic significance Fractional crystallization can be accompanied by crustal assimilation during magma evolution (DePaolo 1981; The northern belt of Late Ordovician arc-related mag- Taylor and McLennan 1985), which explains why sam- matic rocks extends in a NE trend along East Ujimqin Qi, ples have high LREE, Th, and U concentrations. Geri Obao, and Erenhot to Mongolia (Cui et al. 2008; However, unlike the first two groups, gabbros of the Wilhem et al. 2012; Zhao et al. 2012; Qin et al. 2013; third group display a partial melting trend in the Sm vs. Yang et al. 2014; Zhu et al. 2014). Late Ordovician Ce/Sm diagram, suggesting that partial melting contrib- intrusive rocks in East Ujimqin Qi have the geochemical uted to the formation of gabbros. Besides, the first characteristics of both MORB and subduction-related group of gabbros also have a slightly positive Eu anom- IAB, suggesting a back-arc basin tectonic setting. aly and the highest Sr concentration, indicating some Accordingly, the back-arc basin and Duobaoshan island accumulation of plagioclase. arc composed an arc basin system, which verifies that Late Ordovician intrusive rocks in East Ujimqin Qi the southern margin of the South Mongolian Micro- have the compositional aspects of both MORB and continent was an active margin in Late Ordovician arc igneous rocks, suggesting a back-arc basin (Wilhem et al. 2012; Chen et al. 2014). INTERNATIONAL GEOLOGY REVIEW 1189

Figure 16. Conceptual interpretation of bidirectional subduction of the early Palaeozoic Palaeo-Asian Ocean. NCC, North China Craton; SMM, South Mongolian Micro-continent; AXB, Airgin Sum-Xilin Hot Block; PAO, Palaeo-Asian Ocean

The Ondor Sum-Tulinkai-Kedanshan Ophiolite con- South Mongolia, and its eastward extension can be tains Cambrian–Ordovician ages (Zhang et al. 2003), traced to Xilinhot–West Ujimqin Qi where diorite presenting the Palaeo-ocean in early Palaeozoic, and aged 457.6 Ma (Ge et al. 2011 ) and granite aged this ocean had opened before 508 Ma (Jian et al. 2008; 442 Ma were discovered. In addition, detrital zircons Zhou et al. 2009)(Figure 16a). The northward subduc- from the Xilin Hot Group, with Ordovician ages ranging tion initiation was recorded by the subduction-related from 452 to 463 Ma as metamorphic ages (Li et al. diorite from Hada pluton to the north of Siziwangqi 2011, 2014), recorded the magmatism as Late with an age of 508 ± 10 Ma (Zhou et al. 2009), coeval Ordovician. Late Ordovician northward subduction with the eruption of the Duobaoshan volcano induced the emplacement/eruption of calc-alkaline (Figure 16b). Continuous subduction probably led to igneous rocks in the UACM at ca. 450 Ma (Zhao et al. the break-off of a subducting oceanic plate, giving rise 2012), and gave rise to the emplacement of Geri Obao to the back-arc basin in East Ujimqin Qi that was pluton to the north of Abaga Qi and the emplacement succeeded by sediments of the Wubinaobao of gabbro, diorite, and granodiorite in the East Ujimqin Formation (Figure 16c). The subduction–accretion Qi back-arc basin (Figure 16d). An arc-continent colli- complex (Jian et al. 2008), which is termed a mélange sion event was recorded by high-K granite aged ca. belt by Xu et al. (2013), comprises blocks of ultramafic, 430 Ma (Shi et al. 2005) in Sonid Zuoqi, probably mafic rock, blueschist, dolomite, quartzite, and lime- marking the termination of southward subduction. stone, and extends from Honger, south of The initiation of southward subduction was marked Baiyanbaolidao, and Erdaojing, finally to Airgin Sum by the formation of Tulinkai SSZ ophiolite at ca. 497 Ma in an ENE–WSW trend (Xu et al. 2001;Xuet al. 2013; (Jian et al. 2008)(Figure 16b). This early subduction led Li et al. 2014a)(Figure 16c), marking the suture line of to the emplacement of low-K magmatic rocks with ages the Northern Orogenic Belt. Blueschist from the of ca. 470–475 Ma (Xu 2005; Jian et al. 2008; Zhang et al. mélange has an Na-amphibole Ar/Ar age of 2014). Continued subduction gave rise to the formation 383 ± 13 Ma as a high-pressure metamorphic age (Xu of calc-alkaline magmatism represented by the et al. 2001). Continued subduction gave rise to the Bainaimiao continental margin arc (Zhang 2013; Liu formation of the Baiyanbaolidao arc-pluton at et al. 2014) and synchronous intrusions in Bayan Obo 498–461 Ma (Chen et al. 2001;Shiet al. 2004; Jian and Bart Obao (Liu et al. 2003; Xu et al., 2013; Zhang et al. 2008). Its westward extension to the South et al. 2015). At the same time, there was probably a Mongolian Micro-continent was constrained by the small-scale northward subduction which induced the age of 477 Ma for amphibolite (Jian et al. 2010)in eruption of Ondor Sum volcanic rocks to form an 1190 H. LI ET AL. intra-oceanic arc (Figure 16c). A mélange belt character- Acknowledgement ized by a S-dipping subduction–accretion complex, We are grateful for the constructive comments from two extending from south of Ondor Sum to Hongqi pasture anonymous reviewers. to the north of Bainaimiao continental margin arc, was identified (Tang 1992; Xiao et al. 2003;Xuet al. 2013) (Figure 16d). The mélange belt consists of deformed Disclosure statement serpentinized ultramafic rock, gabbro, quartzite schist, No potential conflict of interest was reported by the authors. and granite (Xu et al. 2013), with blueschist facies metamorphism (Tang 1992; De Jong et al. 2006), suggesting an accretionary wedge in a fore-arc setting and marking Funding the suture line of the Southern Orogenic Belt. To the south of the arc magmatic belt a back-arc basin formed, This study was generously financed by the China Geological Survey [No. 1212010811001]; [No. 1212011220465]; [No. represented by flysch sediment of the Xuniwusu 1212011085490]; [No. 12120114093901]. Formation and molasses sediment of the Xibiehe Formation (Tang 1992; Zhang et al. 2010)(Figure 16d). The mélange, together with arc magmatic belt and References back-arc basin, finally constituted a trench-arc basin Agrawal, S., Guevara, M., and Verma, S.P., 2008, Tectonic dis- system (Tang 1992). crimination of basic and ultrabasic volcanic rocks through Our data, combined with the published data as sum- log-transformed ratios of immobile trace elements: marized above, led us to conclude that the northward International Geology Review, v. 50, p. 1057–1079. subduction of the PAO beneath the southern margin of doi:10.2747/0020-6814.50.12.1057 the South Mongolian Micro-continent along Sonid Blight, J.H.S., Cunningham, D., and Petterson, M.G., 2008, Crustal evolution of the Saykhandulaan Inlier, Mongolia: Zuoqi-Xilinhot gave rise to the northern and central Implications for Paleozoic arc magmatism, polyphase defor- belts of Late Ordovician igneous rocks, and the south- mation and terrane accretion in the southeast gobi mineral ward subduction along Hongqi pasture-Ondor Sum belt: Journal of Asian Earth Sciences, v. 32, p. 142–164. beneath the North China Craton induced the formation doi:10.1016/j.jseaes.2007.10.016 of the southern belt. Thus, two opposing subduction Boynton, W.V., 1984, Geochemistry of the earth elements: in systems are established, which argues for the bidirec- Meteorite studies, Henderson, R., ed., Rare earth element geochemistry: Developments in Geochemistry, volume 2: tional subduction of the PAO during the early Amsterdam, Elsevier, 89–92 p. Palaeozoic. Chen, B., 2002, Characteristics and genesis of the bayan bold pluton in Southern Sonid Zuoqi, Inner Mongolia: Geological Review, v. 48, p. 261–266. (in Chinese with English abstract). Chen, B., Hahn, B.M., Wilde, S., and Xu, B., 2000, Two constrast- 7. Conclusion ing Paleozoic magmatic belts in northern Inner Mongolia, China: Petrogensis and tectonic implications: (1) New zircon U–Pb dating results on intrusive rocks Tectonophysics, v. 328, p. 157–182. doi:10.1016/S0040- from East and West Ujimqin Qi, combined with 1951(00)00182-7 previous published geochronological data, indi- Chen, B., Jahn, B.M., and Tian, W., 2009, Evolution of the cate that three Ordovician igneous belts crop in Solonker suture zone: Constraints from zircon U-Pb ages, the eastern CAOB. Hf isotopic ratios and whole-rock Nd-Sr Isotope composi- (2) Whole-rock compositions and trace element tions of subduction and collision-related magmas and fore- arc sediments: Journal of Asian Earth Sciences, v. 34, p. 245– characteristics of the intrusive rocks from the 257. doi:10.1016/j.jseaes.2008.05.007 Uliastai Active Continental Margin show hall- Chen, B., Zhao, G.C., and Wilde, S., 2001, Subduction-and marks of both MORB and arc-related igneous Collision -related granitoids from southern Sonidzuoqi, rocks, which accords with a back-arc basin Inner Mongolia: Isotopic ages and tectonic implications: – setting. Geological Review, v. 47, p. 361 367. (in Chinese with English abstract). (3) Combined with published data, a bidirectional Chen, M., Sun, M., Cai, K.D., Buslov, M.M., Zhao, G.C., and subduction of the PAO in early Palaeozoic is con- Rubanova, E.S., 2014, Geochemical study of the Cambrian- firmed. The northward subduction beneath the Ordovician meta–sedimentary rocks from the northern southern margin of the South Mongolian Micro- Altai-Mongolian terrane, northwestern Central Asian continent and Xinlin Gol block gave rise to the Orogenic Belt: Implications on the provenance and tectonic – northern and central belts, whereas the southern setting: Journal of Asian Earth Sciences, v. 96, p. 69 83. doi:10.1016/j.jseaes.2014.08.028 belt was generated by the southward subduction Chu, H., Zhang, J.R., Wei, C.J., Wang, H.C., and Ren, Y.W., 2013, of the PAO beneath the North China Craton. A new interpretation of the tectonic setting and age of INTERNATIONAL GEOLOGY REVIEW 1191

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