Origin and evolution of the Bainaimiao arc belt: Implications for crustal growth in the southern Central Asian orogenic belt

Geological Society of America Bulletin

doi: 10.1130/B31042.1 published online 15 May 2014;Geological Society of America Bulletin

Shuan-Hong Zhang, Yue Zhao, Hao Ye, Jian-Min Liu and Zhao-Chu Hu in the southern Central Asian orogenic beltOrigin and evolution of the Bainaimiao arc belt: Implications for crustal growth

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Origin and evolution of the Bainaimiao arc belt: Implications for crustal growth in the southern Central Asian orogenic belt

Shuan-Hong Zhang1,†, Yue Zhao1, Hao Ye1, Jian-Min Liu1, and Zhao-Chu Hu2

1Institute of Geomechanics, Chinese Academy of Geological Sciences, MLR Key Laboratory of Paleomagnetism and Tectonic Reconstruction, Beijing 100081, China2State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences, Wuhan 430074, China

ABSTRACT

Recent results show that evolution of the huge Central Asian orogenic belt can be ex-plained in terms of southwest Pacifi c–style accretion of arcs and microcontinents. A better understanding of the origin and evo-lution of arcs and microcontinents will en-hance our knowledge on the evolution of the Central Asian orogenic belt. As one of the most important early Paleozoic arc systems south to the Solonker suture zone, the origin and evolution of the Bainaimiao arc belt are still not well constrained. New zircon U-Pb geochrono logical and geochemical results on magmatic rocks in the Bainaimiao arc belt in-dicate that the arc was active from 0.52 Ga to 0.42 Ga and can extend to east Siping in NE China. Zircon U-Pb geochronological results of metasedimentary rocks in the Bainaimiao arc belt indicate that they are early Paleozoic in age, not Precambrian as previously re-garded. Detrital zircon analysis of metasedi-mentary rocks and Sr-Nd-Hf geochemical re-sults of magmatic rocks indicate the existence of some Proterozoic basement rocks beneath the Bainaimiao arc, and it was built upon a Precambrian microcontinent that has a tec-tonic affi nity to the Tarim or Yangtze cratons. The Bainaimiao arc is an ensialic island arc characterized by different evolution history and basement compositions from the north-ern North China craton. It was separated by a wide ocean from the northern North China craton during the Cambrian–Ordovician period. Successive northward subduction re-sulted in contraction of the ocean and fi nal accretion of the Bainaimiao arc to the north-ern North China craton during the Late Silurian–earliest Devonian by arc-continent collision. Arc-continent collision could be an important mechanism for continental crustal growth and formation of the huge Central Asian orogenic belt.

INTRODUCTION

The Central Asian orogenic belt is located between the Siberian, North China, and Tarim cratons and has been regarded as the world’s largest site of juvenile crust formation in the Phanerozoic (e.g., Wang and Liu, 1986; Sengör et al., 1993; Jahn et al., 2000; Xiao et al., 2003; Li, 2006; Windley et al., 2007; Wilhem et al., 2012; Kröner et al., 2014). The development and amalgamation of the Central Asian oro-genic belt are related to subduction-accretion processes within the paleo–Asian Ocean from the latest Mesoproterozoic to late Paleozoic (e.g., Sengör et al., 1993; Badarch et al., 2002; Kröner et al., 2007; Windley et al., 2007; Safon-ova et al., 2011; Safonova and Santosh, 2014, and references therein). Formation of the Central Asian orogenic belt was the result of accretion of island arcs, ophiolites, oceanic islands, sea-mounts, accretionary wedges, oceanic plateaus, and microcontinents in a manner comparable with that of circum-Pacifi c Mesozoic–Cenozoic accretionary orogens (e.g., Mossakovsky et al., 1993; Khain et al., 2003; Windley et al., 2007; Safonova et al., 2011).

Arcs are tectonic belts formed by subduction of a plate of oceanic lithosphere beneath another oceanic or continental plate along a subduction zone with high seismic activity characterized by a high heat fl ow with active volcanoes bordered by a submarine trench (Windley, 1995). Arc systems are very important components of accretionary orogens and play important roles in growth of accretionary orogens and mineral endowment (e.g., Cawood et al., 2009; Xiao et al., 2010; Glen et al., 2011). The identifi cation of magmatic arcs in ancient accretionary orogens is thus a keystone for the understanding of the structure and his-tory of orogen genesis (e.g., Rocchi et al., 2011). Magmatic arcs can be identifi ed by geochemical features of primitive volcanic rocks, which can be directly related to the tectonic setting in which they were generated (Pearce and Parkinson, 1993; Pearce and Peate, 1995; Kelemen et al., 2003; Pearce, 2008; Rocchi et al., 2011).

The mechanism of continental crustal growth in the Central Asian orogenic belt is still a subject of intense debate (e.g., Sengör et al., 1993; Xiao et al., 2003; Kovalenko et al., 2004; Windley et al., 2007; Li et al., 2009; Safonova et al., 2011; Wil-hem et al., 2012; Yarmolyuk et al., 2012; Kröner et al., 2014; Safonova and Santosh, 2014, and references therein). Some researchers emphasize the role of long-lived volcanic arcs in evolution of the paleo–Asian Ocean and formation of the Central Asian orogenic belt (e.g., Sengör et al., 1993; Sengör and Natal’in, 1996, 2004; Stampfl i and Borel, 2002; Yakubchuk, 2004, 2008) and propose formation of the belts via continuous strike-slip duplication of one main island arc (the ~7000-km-long Kipchak–Tuva–Mongol arc; Sengör et al., 1993; Sengör and Natal’in, 1996). Others have proposed that the belt was formed by collision of multiple island arcs, oceanic com-plexes, and continental blocks (e.g., Zonenshain et al., 1990; Badarch et al., 2002; Dobretsov et al., 2003) or accretion of island arcs, ophio-lites, oceanic islands, seamounts, accretionary wedges, oceanic plateaus, and microcontinents during the Neoproterozoic to late Paleozoic in a manner comparable with that of circum-Pacifi c Mesozoic–Cenozoic accretionary orogens (e.g., Xiao et al., 2003; Windley et al., 2007).

GEOLOGICAL FRAMEWORK AND PREVIOUS STUDIES

The Solonker suture zone marks the fi nal clo-sure of the paleo–Asian Ocean between the North China block and the southern Mongolia compos-ite terranes during the Late Permian to earliest Triassic (e.g., Wang and Liu, 1986; Xiao et al., 2003, 2009; Li, 2006; Windley et al., 2007; Wu et al., 2007; Zhang et al., 2007a, 2009a, 2009b; Li et al., 2009; Chen et al., 2009). The Solonker suture zone extends from Solon Obo (Solonker) near the China-Mongolia border via Sonid Youqi to Linxi in Inner Mongolia (e.g., Wang and Liu, 1986; Xiao et al., 2003, 2009; Li, 2006). It may extend eastward across the Songliao Basin to Changchun and Yanjin areas in central Jilin

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GSA Bulletin; Month/Month 2014; v. 1xx; no. X/X; p. 1–26; doi: 10.1130/B31042.1; 17 fi gures; 2 tables; Data Repository item 2014211.

†E-mail: tozhangshuanhong@ 163 .com.

as doi:10.1130/B31042.1Geological Society of America Bulletin, published online on 15 May 2014

Zhang et al.

2 Geological Society of America Bulletin, Month/Month 2014

Province in NE China (e.g., Sun et al., 2004; Jia et al., 2004; Li, 2006; Wu et al., 2007, 2011; Lin et al., 2008). The North China block south to the Solonker suture zone consists mainly of two tec-tonic units, including the North China craton and the Bainaimiao arc belt, which are separated by the E-W–trending Bayan Obo–Chifeng–Kaiyuan fault zone (Fig. 1). The basement of the North China craton is composed of highly metamor-phosed Archean and Paleoproterozoic rocks, which were covered by Mesoproterozoic–Neo-proterozoic, and Cambrian–Ordovician marine clastic and carbonate platformal sediments, Middle Carboniferous to Triassic fl uvial and deltaic sediments, and Jurassic–Cretaceous and younger volcanic and sedimentary rocks.

Different to the North China craton, the Bainaimiao arc belt is characterized by low-grade (greenschist facies–low amphibolite facies) metasedimentary and volcanic rocks and early Paleozoic intrusive rocks consisting mainly of quartz diorite, tonalite, and minor gabbro, granodiorite, and granite. These rock units are unconformably overlain by Late Silurian–earliest Devonian continental molasse or quasi-molasse deposits named as Xibiehe Formation (e.g., Zhang and Tang, 1989; Tang, 1990; BGMRIM, 1991; Su, 1996; Xu et al., 2003; Wang, 2005; Chen and Boucot, 2007; Y.P. Zhang et al., 2010).

The relation between the Bainaimiao arc belt and the North China craton is still highly controversial. The Bainaimiao arc belt has been regarded as a Japan-style island arc (e.g., Hu et al., 1990; Tang, 1990; Tang and Yan, 1993; Jia and Lu, 1999; Gao et al., 2001; Shang et al., 2003; Jia et al., 2003). However, others consider it as a continental arc in an active continental margin of the North China craton (e.g., Xiao et al., 2003; de Jong et al., 2006; Liu et al., 2013) or exotic terrains accreted to the northern margin of the North China craton by tectonism (e.g., Chen et al., 1993; Li, 1997). To resolve these controversies, a better understanding of the compositions and evolution of different rock units and the basement compositions of the arc is needed, which we undertook in this study.

PETROGRAPHY AND SAMPLE DESCRIPTIONS

The Bainaimiao arc belt has been termed as the Ondor Sum–Ongniud Caledonian fold belt (BGMRIM, 1991), Ondor Sum–Ongniud early Paleozoic accreted terrane (Wang and Liu, 1986), Xar Moron accreted fold belt (Zhang and Tang, 1989; Tang, 1990), Baoerhantu-Bainaimiao island-arc belt (Tang and Yan, 1993), Bainaimiao-Baiyinduxi terrains (Chen et al., 1993), Bainaimiao volcanic island arc (Gao et al., 2001), Ondor Sum–Wengniute oro-

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Bainaimiao arc and crustal growth of the Central Asian orogenic belt

Geological Society of America Bulletin, Month/Month 2014 3

genic belt (Li, 2006) or Southern orogen (Jian et al., 2008; Xu et al., 2013). It extends over 1300 km from Baoerhantu north to Bayan Obo via Bater Obo north to Damaoqi, Bainaimiao, Tulinkai, Hadamiao, and Ongniud-Jiefang-yingzi in Inner Mongolia to eastern Siping and Yitong in southern Jilin Province (Fig. 1). The arc belt has been strongly deformed and is not well exposed because intrusion of late Paleo-zoic to Mesozoic plutons and coverage of late Paleozoic to Cenozoic volcanic-sedimentary rocks or sediments. Better exposed areas of the Bainaimiao arc belt include Bayan Obo–Damaoqi , Bainaimiao , Tulinkai, Ongniud-Jiefangyingzi in Inner Mongolia and eastern

Siping in southern Jilin Province, of which the Bayan Obo–Damaoqi , Bainaimiao, and eastern Siping regions were studied in this paper.

Bayan Obo–Damaoqi

The area north to Bayan Obo–Damaoqi is one of the best exposed areas in the Bainaimiao arc belt (Fig. 2). Metamorphic basement rocks are composed of biotite quartz schist, two-mica quartz schist, garnet-bearing biotite quartz schist, stauro-lite biotite quartz schist, metamorphic sandstone, and metamorphic tuffaceous sandstone with inter-calated quartzite, marble, and actinolite schist (Figs. 3H–3L). They were formerly regarded as

Silurian in age (BGMRIM, 1968, 1991); how-ever, recent research considered them as Paleo-proterozoic metamorphic rocks (IMIGS, 2003). Magmatic rocks consist of Ordovician–Silurian diorite, quartz diorite (Figs. 3A and 3G; GSA Data Repository Fig. DR1A1), tonalite (Fig. 3C),

Figure 2. Geological map of Bayan Obo–Damaoqi area of the Bainaimiao arc belt (modifi ed after IMIGS, 2003). NCC—North China craton; LA-ICP-MS—laser-ablation–inductively coupled plasma–mass spectrometry.

1GSA Data Repository item 2014211, Geo chrono-logical, geochemical and Hf isotopic data, photo-micro graphs, zircon CL images of the Early Paleozoic magmatic and sedimentary rocks in the Bainaimiao arc, and relative probability density plot of zircon ages from the Precambrian microcontinents and Neopro-terozoic to Paleozoic arc terranes in the Central Asian orogenic belt, is available at http:// www .geosociety .org /pubs /ft2014 .htm or by request to editing@ geosociety .org.


Zhang et al.


A

D

G

J

M

P Q R

N O

K L

H I

E F

B C

Figure 3. Representative out-crop photos of different rock units in the Bainaimiao arc belt. (A) Early Silurian quartz dio-rite from Bayan Obo–Damaoqi . (B) Strongly deformed Middle Silurian alkali feldspar gran-ite from Bayan Obo–Damaoqi. (C) Early Ordo vi cian tonalite from Bayan Obo–Damaoqi. (D) Early Ordovician andesite from Bayan Obo–Damaoqi . (E) Cambrian dacite from Bayan Obo–Damaoqi. (F) Late Ordovician dacite from Bayan Obo–Damaoqi . (G) Late Ordo vician quartz diorite from Bayan Obo–Damaoqi. (H) Early Silurian garnet-bearing biotite quartz schist from Bayan Obo–Damaoqi. (I) Early Silurian tonalite in-truded into Late Ordovician to Early Silurian schist. (J) Late Ordovician to Early Silu-rian schist from Bayan Obo–Damaoqi . (K) Early Silurian metamorphic tuff from Bayan Obo–Damaoqi. (L) Late Ordo-vician to Early Silurian meta-morphic sandstone from Bayan Obo–Damaoqi. (M) Early Silu-rian granite from Bainaimiao. (N) Early Silurian tonalite from Bainaimiao. (O) Late Ordovi-cian to Early Silurian biotite quartz schist from Bainaim-iao. (P) Cambrian to Ordovi-cian metamorphic sandstone from Gongzhuling, Siping, NE China. (Q) Strongly deformed Late Ordovician tonalite from Gongzhuling, Siping, NE China. (R) Strongly deformed Early Silurian quartz diorite from Gongzhuling, Siping, NE China.




granodiorite , and minor granite (Fig. 3B; Fig. DR1B [see footnote 1]) and Cambrian–Ordo-vician andesite (Fig. 3D; Fig. DR1C [see foot-note 1]), spilite, dacite (Figs. 3E–3F; Fig. DR1D [see footnote 1]), and andesitic tuff, named the Baoerhantu Group, which are unconformably overlain by the Late Silurian–earliest Devonian Xibiehe Formation. Ophiolitic mélanges exist near Wude and Harihade-Chegen dalai in the southern margin of the arc belt (Fig. 2). The Wude ophiolitic mélange is several hundred meters to 2 km wide and 20 km long and is com-posed of different structurally contacted rock units including oceanic serpentinite, layered gabbro, diopsidite, deep-water chert, fl ysch-like siltstone, andesite, spilite, andesitic basalt, quartzite, two-mica schist, quartz diorite, and diorite (Figs. 4A–4C; Jia et al., 2003; Shang et al., 2003). The Harihade-Chegendalai ophio litic mélange is 15 km long and consists of ophio litic peridotite, pyroxene peridotite, gabbro, basalt, diabase, deep-water chert, and Silurian musco-vite leptynite, quartzite, marble, plagio clase amphibole schist, and mica schist (Figs. 4D–4F; Shao, 1986, 1989, 1991; Tang, 1992). All these rocks were intruded by late Paleozoic to Triassic granitic plutons and are overlain by late Paleo-zoic to Cenozoic strata (Fig. 2). Sixteen sam-ples, including three quartz diorites (samples 08480–1, 08557–1, 07130–1; Fig. DR1A [see footnote 1]), four tonalites (samples 08500–1, 08502–1, 08515–2, 08558–1), two dacites (samples 08556–1, 08505–1; Fig. DR1D [see footnote 1]), one andesite (sample 08504–1; Fig. DR1C [see footnote 1]), one alkali feld-spar granite (sample 08487–1; Fig. DR1B [see footnote 1]), one muscovite granite (sample 08513–1), one monzogranite (sample 08522–1), one metamorphic tuffaceous sandstone (sample 09224–3; Fig. DR1E [see footnote 1]), and two metamorphic sandstones (samples 09226–1, 09225–1; Fig. DR1F [see footnote 1]) from the metamorphic basement rocks, were collected for geochronological and geochemical analyses in this study.

Bainaimiao

The Bainaimiao area is another one of the best exposed areas in the Bainaimiao arc belt (Fig. 5). Rock units in the Bainaimiao area include the Bainaimiao Group, the Xuniwusu Formation, and the Naqing (or Xibiehe) Forma-tion (Fig. 6). The Bainaimiao Group, which is considered as Mesoproterozoic (e.g., Nie et al., 1991; Chen et al., 1993; Li et al., 2002) or Ordo-vician–Silurian (e.g., Hu et al., 1990; BGMRIM, 1991; Zhang et al., 2013) in age, is composed of a greenschist-facies metamorphosed volcanic-sedi mentary sequence including andesitic tuff,

rhyolitic-dacitic tuff, tuffaceous sandstone, and tuffaceous siltstone with intercalated andesitic basalt, tuff breccia, marlstone, sandstone, silt-stone, and copper layers. Ordovician–Silurian dioritic-granitic plutons or intrusions are common within the Bainaimiao Group (Figs. 3M–3N). The Xuniwusu Formation is a mid–Late Silurian metamorphic fl ysch sequence composed mainly of pebbly sandstone, sandstone, siltstone, and mudstone. The Naqing (or Xibiehe) Formation is a latest Silurian–Early Devonian molasse-type sediment consisting mainly of conglomerate and sandstone, which unconformably overlies the Ordovician–Silurian strata and plutons. Ten sam-ples, including four tonalites (samples 08413–1, 08417–1, 08429–1, 070813–6; Fig. DR1G [see

footnote 1]), three quartz diorites (samples 08404–1, 08409–1, 070813–7; Fig. DR1H [see footnote 1]), one diorite (sample 08432–1), one muscovite granite (sample 08406–1) from the Ordovician–Silurian dioritic-granitic plutons and one biotite quartz schist of the Bainaimiao Group (sample 08411–1; Fig. DR1I [see footnote 1]), were collected for geochronological and geo-chemical analyses in this study.

Siping-Yitong, NE China

The Siping-Yitong area east to the Songliao Basin in NE China is possibly the east exten-sion of the Bainaimiao arc belt in the northern margin of the North China block (Fig. 1). Arc

A B

C D

E F

Figure 4. Field photos of ophiolitic mélanges between the Bainaimiao island arc and the North China craton. (A–B) Ultramafi c blocks within the Wude ophiolitic mélange. (C) Mar-ble blocks within the Wude ophiolitic mélange. (D) Ultramafi c-mafi c rocks within the Hari-hade-Chegendalai ophiolitic mélange as indicated by exploratory trenches (~1.5 m wide). (E) Strongly deformed gabbros within the Harihade-Chegendalai ophiolitic mélange. (F) In-tensely carbonatized ultramafi tes within the Harihade-Chegendalai ophiolitic mélange.


Zhang et al.


magmatic rocks in the Siping-Yitong area con-sist of Cambrian–Silurian granitic plutons and the Late Ordovician to Early Silurian Fangniu-gou volcanic or volcanic clastic rocks (e.g., Jia, 1988; 1994; Jia and Lu, 1999). Arc plutons are composed mainly of diorite, quartz diorite (Fig. 3R; Fig. DR1J [see footnote 1]), tonalite (Fig. 3Q), and granodiorite. Although these granitic plutons were considered as Cambrian–Silurian in age (e.g., BGMRJP, 1988, 2001; Jia, 1988, 1994; Jia and Lu, 1999), their intrusive ages have not been well constrained. The Fangniugou volcanic or volcanic clastic rocks consist mainly of basalt, basaltic andesite, andesite, andesitic tuff, dacite, rhyolite, rhyolitic tuff, and tuffa-ceous sandstone, and they are unconformably overlain by the Late Silurian–earliest Devonian Zhangjiatun (Xibiehe) Formation (Jia and Lu, 1999). Early Silurian graptolite fossils are very common in the siltstone and shale layers within the Fangniugou volcanic or volcanic clastic rocks (e.g., BGMRJP, 1988, 1997; Jia, 1994). The arc magmatic rocks have been strongly damaged, with only minor outcrop because of

abundant intrusions of late Paleozoic to Meso-zoic magmatism (e.g., Wu et al., 2011, and ref-erences therein) and covering by late Paleozoic to Cenozoic strata (Fig. 7). A tonalite (sample 09341–2) and a quartz diorite (sample 09344–1; Fig. DR1J [see footnote 1]) from the Cambrian–Silurian granitic plutons near Huanglingzi vil-lage east to Gongzhuling were collected in order to better understand the compositions and evo-lution of the arc system.

The global positioning system (GPS) loca-tions and main features of all samples used for geochronological, geochemical, and isotopic analysis in the paper are listed in Table 1.

RESULTS

Zircon U-Pb Geochronology

Magmatic RocksResults of laser-ablation–inductively coupled

plasma–mass spectrometry (LA-ICP-MS) U-Pb analyses of zircons from the intrusive and volcanic rocks from different areas in the

Bainaimiao arc belt are listed in Table DR1 (see footnote 1) and illustrated on concordia dia-grams in Figure 8. Fifteen to 30 zircon grains from each sample were analyzed, and most of them were concordant. Some old inherited zir-con grains with 207Pb/206Pb ages from 2628 ± 16 Ma to 1322 ± 21 Ma exist in samples from Bayan Obo–Damaoqi (08515–2) and Bainaim-iao (08406–1, 08429–1, 08432–1). All zircons used for weighted mean calculation are char-acterized by euhedral prismatic morphology, oscillatory zoning in cathodoluminescence (CL) images (Fig. DR2 [see footnote 1]), and high Th/U ratios (Table DR1 [see footnote 1]), indicating they are magmatic in origin. There-fore, their ages refl ect the emplacement ages of plutons (intrusions) or eruption ages of vol-canic rocks.

(1) Bayan Obo–Damaoqi. Three quartz diorite samples (07130–1, 08480–1, 08557–1) were dated and yield weighted mean 206Pb/238U ages of 453 ± 2 Ma (Fig. 8A), 436 ± 4 Ma (Fig. 8B), and 445 ± 3 Ma (Fig. 8J), respectively. Three tonalite samples (08500–1, 08502–1,

Figure 5. Geological map of Bainaimiao and adjacent areas of the Bainaimiao arc belt (modifi ed after BGMRIM, 1991). LA-ICP-MS—laser-ablation–inductively coupled plasma–mass spectrometry.




08515–2) yield weighted mean 206Pb/238U ages of 473 ± 2 Ma (Fig. 8D), 470 ± 3 Ma (Fig. 8E), and 433 ± 6 Ma (Fig. 8H), respectively. The alkali feldspar granite yielded a relatively young weighted mean 206Pb/238U age of 429 ± 4 Ma (Fig. 8C). One andesite sample from the Baoerhantu Group (08504–1) yielded a weighted mean 206Pb/238U age of 474 ± 5 Ma (Fig. 8F). Two dacite samples (08505–1, 08556–1) yielded weighted mean 206Pb/238U ages of 518 ± 3 Ma (Fig. 8G) and 445 ± 4 Ma (Fig. 8I), respectively.

(2) Bainaimiao. Three quartz diorite samples (070813–7, 08404–1, 08409–1) were dated and yielded weighted mean 206Pb/238U ages of 421 ±

2 Ma (Fig. 8K), 439 ± 3 Ma (Fig. 8L), and 430 ± 3 Ma (Fig. 8N), respectively. Three tonalite samples (08413–1, 08417–1, 08429–1) yielded weighted mean 206Pb/238U ages of 439 ± 5 Ma (Fig. 8O), 430 ± 3 Ma (Fig. 8P), and 433 ± 3 Ma (Fig. 8Q), respectively. One muscovite granite sample (08406–1) yielded a weighted mean 206Pb/238U age of 432 ± 5 Ma (Fig. 8M). One diorite sample (08432–1) yielded a weighted mean 206Pb/238U age of 436 ± 4 Ma (Fig. 8R).

(3) Siping-Yitong, NE China. Tonalite sam-ple 09341–2 yielded a weighted mean 206Pb/238U age of 446 ± 3 Ma (Fig. 8S). The quartz diorite sample yielded a weighted mean 206Pb/238U age of 438 ± 4 Ma (Fig. 8T).

Metasedimentary RocksThe LA-ICP-MS U-Pb analyses of zircons

from the metasedimentary rocks in Bayan Obo–Damaoqi and Bainaimiao areas are listed in Table DR2 (see footnote 1) and plotted on con-cordia diagrams and probability density plots in Figure 9.

(1) Bayan Obo–Damaoqi. Fifteen spots on 15 zircon grains from a metamorphic tuffa-ceous sandstone (sample 09224–3) were ana-lyzed, and most of them are concordant (Fig. 9A). Except for one discordant analysis (08), the other analyses yield three peaks at 440 Ma, 453 Ma, and 466 Ma, respectively, on a probability density plot (Fig. 9B). Euhedral prismatic morphology, oscillatory zoning in CL images (Fig. DR3 [see footnote 1]), and high Th/U ratios (1.32–0.70; Table DR2 [see footnote 1]) indicate a magmatic origin for these zircons. Therefore, the youngest peak at 440 Ma may represent the age of the metamor-phic tuffaceous sandstone, and the other two peaks at 453 Ma and 466 Ma probably refl ect inherited ages from the early magmatism. Our new zircon U-Pb results indicate that sedimen-tation of the metamorphic tuff in Bayan Obo–Damaoqi occurred during the Early Silurian at ca. 440 Ma.

Sixty spots on 60 zircon grains from a meta-morphic sandstone (sample 09225–1) within the metamorphic basement rocks were analyzed, and most have concordant ages (Fig. 9C). On a probability density plot (Fig. 9D), they exhibit a main peak at 458 Ma and several minor peaks at 622 Ma, 960 Ma, 1158 Ma, and 3109 Ma, respectively. These zircons are characterized by high Th/U ratios (2.34–0.23; Table DR2 [see footnote 1]) and oscillatory zoning in CL images (Fig. DR3 [see footnote 1]), indicating they are magmatic in origin. Therefore, sedi-mentation of these rocks occurred later than ca. 458 Ma, probably during the Late Ordovician to Early Silurian.

Sixty spots on 60 zircon grains from another metamorphic sandstone (sample 09226–1) within the metamorphic basement rocks were analyzed, and most of them are concordant (Fig. 9E). Except for two discordant analyses (10, 15), the remaining analyses exhibit main peaks at 464 Ma, 499 Ma, and 534 Ma and several minor peaks at 612 Ma, 845 Ma, 986 Ma, 1106 Ma, 1254 Ma, and 1607 Ma, respectively, on probability density plots (Fig. 9F). Several old zircons exhibit 207Pb/206Pb ages at 1.7–1.8 Ga, 2.4 Ga, and 2.6–2.7 Ga, respectively. High Th/U ratios (2.74–0.11; Table DR2 [see footnote 1]) and oscillatory zoning in CL images (Fig. DR3 [see footnote 1]) indicate that these zircons are magmatic in origin. Therefore, sedimentation of these rocks occurred later than ca. 464 Ma,

A BFigure 6. Stratigraphic column of the Bainaimiao arc belt with emphasis on their source rocks (modifi ed after Hu et al., 1990; BGMRIM, 1991; IMIGS, 2003).


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probably during the Late Ordovician to Early Silurian.

(2) Bainaimiao. Forty-eight spots on 48 zircon grains from a biotite quartz schist (sample 08411–1) of the Bainaimiao Group in Bainaimiao were analyzed, and most have con-cordant ages (Fig. 9G). On a probability den-sity plot (Fig. 9H), they exhibit main peaks at 446 Ma, 469 Ma, 706 Ma, and 981 Ma and sev-eral minor peaks at 591 Ma, 889 Ma, 1134 Ma, 2174 Ma, and 2954 Ma, respectively. Most of zircons are characterized by high Th/U ratios (2.90–0.11; Table DR2 [see footnote 1]) and oscillatory zoning in CL images (Fig. DR3 [see footnote 1]), indicating they are magmatic in origin. Although four zircon grains (03, 21, 23, 40) are characterized by low Th/U ratios of 0.01–0.03, they exhibit oscillatory zoning in CL images (Fig. DR3 [see footnote 1]), indicating they are also magmatic in origin. Therefore,

sedimentation of these rocks occurred later than ca. 458 Ma, probably during the Late Ordovi-cian to Early Silurian.

Major- and Trace-Element Compositions

Major- and trace-element compositions of 24 magmatic rock samples are listed in Table DR3 (see footnote 1). They are characterized by variable contents of SiO2 (58.4–78.0 wt%), high contents of Al2O3 (12.8–19.7 wt%), CaO (0.4–6.7 wt%), and Na2O (1.9–5.6 wt%), and low total alkalis (Na2O + K2O = 3.0–8.2 wt%). Mg# values of granite are from 12.94 to 51.18, and those of other rocks are from 39.75 to 64.49. In the total alkali (K2O + Na2O) versus silica (SiO2) (TAS) classifi cation diagram from Le Bas et al. (1986) and Middlemost (1994), most of them fall into fi elds of diorite and granodio-rite; others fall into fi eld of granite (Fig. 10A).

All of them are plotted in the subalkaline fi eld (Fig. 10A). According to the classifi cation of Peccerillo and Taylor (1976), they belong to the calc-alkaline or high-K calc-alkaline series (Fig. 10B). An A/NK versus A/CNK diagram (not presented) indicates that most of the diorite and granodiorite samples are weakly peraluminous or metaluminous (A/CNK < 1.1 and A/NK > 1). Most of the granite samples are strongly peralu-minous in composition, with A/CNK > 1.1. The lithological association, mineral assemblage, and geochemical compositions of the diorite and granodiorite rocks as classifi ed in the TAS diagram indicate that they are I-type granitoids (Chappell and White, 1992). However, most of the granite rocks are S-type (Chappell and White, 1992), which is supported by existence of minor garnet in some granite samples.

The magmatic rocks from the Bainaimiao arc belt display total rare earth element (REE)

Figure 7. Geological map of Siping-Yitong area of the Bainaimiao arc belt (modifi ed after BGMRJP, 2001). LA-ICP-MS—laser-ablation–inductively coupled plasma–mass spectrometry.




contents from 11.8 to 179.0 ppm (Table DR3 [see footnote 1]), and chondrite-normalized REE patterns for these samples are shown in Figure 11. Most samples exhibit similar REE patterns, characterized by light (L) REE enrichment (LaN/YbN = 2.12–24.52), no sig-nifi cant Eu anomaly (EuN/EuN

* = 0.74–1.43), and fl at heavy (H) REE patterns. One granite sample (08487–1) exhibits a signifi cant nega-tive Eu anomaly (EuN/EuN

* = 0.12; Fig. 11D), and another granite sample (08513–1) and a tonalite sample (08515–2) are characterized by signifi cant positive Eu anomalies (EuN/EuN* = 4.81–5.12; Figs. 11B and 11D).

The trace-element compositions of most samples are characterized by high contents of Sr and Ba, low contents of Y and Yb, and high Sr/Y ratios (10.1–224.1). On primitive mantle–normalized diagrams (Fig. 12), they display depletion in Nb, Ta, P, and Ti and enrichment in Rb, Ba, K, and Sr. In most cases, there is no depletion in Zr and Hf on primitive mantle–nor-malized diagrams.

Sr-Nd Isotopic Data

Whole-rock Sr-Nd isotopic data of the mag-matic rocks are listed in Table DR4 (see foot-note 1) and plotted in Figure 13. Most mag-matic rocks from the Bainaimiao arc belt are

characterized by low initial 87Sr/86Sr ratios of 0.70492–0.70765, high negative to positive εNd(t) values of –4.9 to 5.2, and Nd isotopic depleted mantle (TDM) model ages of 0.78–2.39 Ga. The Early Cambrian dacite (sample 08505–1) and Early Silurian tonalite (sample 08515–2) from the Bayan Obo–Damaoqi area are characterized by high initial 87Sr/86Sr ratios of 0.71031–0.71064, low negative εNd(t) values of –8.1 to –8.2, and Nd isotopic TDM model ages of 1.89–3.07 Ga.

Zircon Lu-Hf Isotopic Data

Magmatic RocksZircon Lu-Hf data of the magmatic rocks are

listed in Table DR5 (see footnote 1) and plot-ted in Figure 14. Synmagmatic zircons from the Early Cambrian dacite (sample 08505–1) in Bayan Obo–Damaoqi display low initial 176Hf/177Hf ratios from 0.282045 to 0.282157, low negative εHf(t) values from –14.2 to –10.3, old Hf isotopic TDM model ages from 1.53 Ga to 1.71 Ga, and TC

DM (crustal model ages) from 2.14 Ga to 2.38 Ga. Synmagmatic zir-cons from the Ordovician samples in Bayan Obo–Damaoqi (07130–1, 08500–1, 08502–1, 08504–1, 08556–1, 08557–1) are character-ized by high 176Hf/177Hf ratios from 0.282554 to 0.282815, positive εHf(t) values from 2.3 to

11.4, young Hf isotopic TDM model ages from 0.61 Ga to 1.04 Ga, and TC

DM from 0.70 Ga to 1.29 Ga. Synmagmatic zircons from the Early–Middle Silurian samples in Bayan Obo–Dam-aoqi (08480–1, 08487–1, 08515–2) display 176Hf/177Hf ratios from 0.282243 to 0.282516, high negative to low positive εHf(t) values from –9.3 to 0.5, Hf isotopic TDM model ages from 1.10 Ga to 1.44 Ga, and TC

DM from 1.39 Ga to 2.00 Ga. From the Late Ordovician to Early Silurian, there is a signifi cant decrease of zircon εHf(t) values in the magmatic rocks from Bayan Obo–Damaoqi (Fig. 14A).

Most synmagmatic zircons from magmatic rocks in Bainaimiao exhibit variable 176Hf/177Hf ratios from 0.282386 to 0.282700, high negative to positive εHf(t) values from –4.4 to 7.1, Hf iso-topic TDM model ages from 0.77 Ga to 1.21 Ga, and TC

DM from 0.97 Ga to 1.69 Ga. Similar to magmatic rocks in Bayan Obo–Damaoqi, there is a signifi cant decrease of zircon εHf(t) values from the Late Ordovician to Early Silurian (Fig. 14B).

Synmagmatic zircons from magmatic rocks in Gongzhuling, Siping, NE China, display 176Hf/177Hf ratios from 0.282451 to 0.282692, high negative to positive εHf(t) values from –1.6 to 7.0, Hf isotopic TDM model ages from 0.79 Ga to 1.14 Ga, and TC

DM from 0.99 Ga to 1.53 Ga (Fig. 14B).

TABLE 1. SAMPLE LOCATIONS AND MINERAL COMPOSITIONS OF THE EARLY PALEOZOIC MAGMATIC AND METASEDIMENTARY ROCKS IN THE BAINAIMIAO ARC BELT

)egatnecrepemulov(elbmessalarenimniaMepytkcoR)N°(edutitaL)E°(edutignoL.onelpmaSBayan Obo–Damaoqi07130-1 110°12′23.3″ 41°53′47.4″ )%5–%3(tB+)%01(ztQ+)%01(bH+)%01–%5(sfK+)%07–%56(lPetiroidztrauQ08480-1 110°36′35.4″ 41°48′40.1″ )%3(suM+)%01(pE+)%51(lhC+)%2(bH+)%5(sfK+)%02(ztQ+)%05(lPetiroidztrauQ08487-1 110°47′22.9″ 41°49′17.5″ Alkali feldspar granite Kfs (50%) + Qtz (35%) + Pl (10%) + Mus (4%) + Bt (1%) + Grt (1%)08500-1 110°07′04.6″ 41°59′12.8″ )%01(pE+)%01(lhC+)%5(tB+)%02(ztQ+)%55(lPetilanoT08502-1 110°07′12.8″ 41°59′52.5″ )%01(ztQ+)%01–%5(tB+)%51–%01(bH+)%56(lPetilanoT08504-1 110°09′50.3″ 42°00′00.8″ )%5(pE+)%52(bH+)%07(lPetisednA08505-1 110°10′55.1″ 41°59′50.6″ )%01(sfK+)%01(bH+)%51(ztQ+)%56(lPeticaD08513-1 110°32′28.4″ 41°49′56.6″ Muscovite granite Pl (35%) + Qtz (30%) + Kfs (20%) + Ms (15%)08515-2 110°34′11.5″ 41°50′16.1″ )%5(sM+)%51(bH+)%51(ztQ+)%06(lPetilanoT08522-1 110°34′06.1″ 41°55′01.0″ )%1(trG+)%1(tB+)%21(sM+)%53(ztQ+)%02(lP+)%03(sfKetinargoznoM08556-1 109°54′05.4″ 41°59′32.3″ )%5(pE+)%51(ztQ+)%08(lPeticaD08557-1 109°54′11.8″ 41°59′53.8″ )%1(tM+)%5(bH+)%01(tB+)%01(sfK+)%02–%51(ztQ+)%06–%55(lPetiroidztrauQ08558-1 109°55′06.8″ 42°00′14.3″ )%5(pE+)%02(ztQ+)%57(lPetilanoT09224-3 110°29′06.8″ 41°52′05.1″ Metamorphic tuffaceous sandstone Kfs (40%) + Qtz (30%) + Bt (15%) + Ep (15%) + Pl (3%)09225-1 110°26′59.2″ 41°52′29.0″ Metamorphic sandstone Qtz (65%–70%) + Kfs (15%–20%) + Pl (5%–10%) + Bt (10%) + Mus (2%)09226-1 110°27′32.7″ 41°52′26.6″ Metamorphic sandstone Qtz (70%–75%) + Kfs (10%) + Kf (5%–10%) + Bt (10%) + Mus (3%–5%) + Mt (1%)

Bainaimiao070813-6 112°34′37.2″ 42°15′27.9″ )%01(tB+)%51(ztQ+)%02–%51(bH+)%56–%06(lPetilanoT070813-7 112°34′40.3″ 42°14′14.8″ )%1(pE+)%51–%01(lhC+)%51(ztQ+)%01(bH+)%06(lPetiroidztrauQ08404-1 112°43′15.1″ 42°16′49.6″ )%3(tM+)%2(pE+)%51(lhC+)%5(tB+)%01(sfK+)%51(ztQ+)%54(lPetiroidztrauQ08406-1 112°43′19.9″ 42°17′34.4″ Muscovite granite Pl (30%) + Qtz (35%) + Kfs (25%) + Ms (10%)08409-1 112°36′36.0″ 42°12′47.4″ )%5(tB+)%01(sfK+)%51(ztQ+)%51(bH+)%55(lPetiroidztrauQ08413-1 112°31′56.2″ 42°13′24.1″ )%01(lhC+)%5(tB+)%5(sfK+)%51(ztQ+)%51(bH+)%55(lPetilanoT08417-1 112°33′31.7″ 42°14′16.3″ )%3(sfK+)%5(pE+)%01(ztQ+)%01–%5(tB+)%51(bH+)%05(lPetilanoT08429-1 112°37′48.4″ 42°16′13.0″ )%5(tM+)%02–%51(ztQ+)%52(bH+)%04–%03(lPetilanoT08432-1 112°37′27.6″ 42°17′05.9″ )%3(ztQ+)%01(bH+)%51(sfK+)%07(lPetiroiD08411-1 112°33′06.7″ 42°12′58.8″ Biotite quartz schist Qtz (70%) + Bt (15%–20%) + Pl (10%–15%) + Mus (1%) + Ep (1%)

Gongzhuling, NE China09341-2 124°59′40.1″ 43°27′36.7″ )%5–%3(tM+)%5–%3(sfK+)%5(tB+)%51(bH+)%02–%51(ztQ+)%06(lPetilanoT09344-1 124°56′15.6″ 43°27′38.4″ )%01–%5(bH+)%51(tB+)%51(ztQ+)%51(sfK+)%05–%54(lPetiroidztrauQ

Note: Mineral abbreviations: Pl—plagioclase; Bt—biotite; Hb—hornblende; Kfs—K-feldspar; Qtz—quartz; Ms—muscovite; Grt—garnet; Mt—magnetite; Chl—chlorite; Ep—epidote.


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A

D

G

J K L

H I

E F

B C

Figure 8 (on this and following page). U-Pb concordia diagrams for zircons from the magmatic rocks in the Bainaimiao arc belt. Data-point error crosses are 2σ. MSWD—mean square of weighted deviates.




Metasedimentary RocksZircon Lu-Hf data for the Late Ordovician to

Early Silurian metasedimentary rocks in Bayan Obo–Damaoqi and Bainaimiao are listed in Table DR6 (see footnote 1) and plotted in Fig-ure 15. They exhibit a very wide range of Hf isotopic compositions, with 176Hf/177Hf ratios from 0.280546 to 0.282768, εHf(t) values from –19.9 to 10.8, Hf isotopic TDM model ages from 0.69 Ga to 3.68 Ga, and TC

DM from 0.78 Ga to 4.14 Ga. In the εHf(t) versus U-Pb age plot, most of the analyses fall into an area between the 2.6 Ga and 0.7 Ga continental crust evolution lines (Fig. 15A). Early Paleozoic zircons display similar Hf isotopic compositions to those of the synmagmatic zircons from the magmatic rocks in the Bainaimiao arc belt (Figs. 14 and 15B).

There is a trend of signifi cantly increasing εHf(t) values from the Cambrian to Early Ordovician and decreasing εHf(t) values from the Late Ordo-vician to Early Silurian (Fig. 15B), which is very similar to that of the synmagmatic zircons from the magmatic rocks in the Bayan Obo–Damaoqi area.

DISCUSSION

Ages of Rock Units in the Bainaimiao Arc Belt in the Southern Central Asian Orogenic Belt

A summary of zircon U-Pb ages of the mag-matic and metasedimentray rocks from the Bainaimiao arc belt is listed in Table 2, and U-Pb

ages of magmatic rocks are plotted in a proba-bility density plot in Figure 16. As stated above, pre–late Paleozoic rocks in the Bainaimiao arc belt consist of low-grade (greenschist facies–low amphibolite facies) metamorphic sedimen-tary and volcanic rocks and early Paleozoic intrusive rocks, and these are unconformably overlain by Late Silurian–earliest Devonian continental molasse or quasi-molasse deposits named the Xibiehe Formation. However, ages of the metamorphic rocks in the Bainaimiao arc belt are highly controversial and have been considered as Paleoproterozoic (IMIGS, 2003), Mesoproterozoic (e.g., Hu et al., 1990; Nie et al., 1991; Li et al., 2002), or Ordovician to Silurian (e.g., BGMRIM, 1968, 1991; Zhang et al., 2013). Our new zircon U-Pb dating results

M

P

S T

Q R

N O

Figure 8 (continued ).


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on the metamorphic sedimentary rocks in Bayan Obo–Damaoqi (sample 09225–1 and 09225–2) and the Bainaimiao group (08411–1) indicate they are early Paleozoic in age, not Paleo-proterozoic (IMIGS, 2003) or Meso protero-zoic (e.g., Hu et al., 1990; Nie et al., 1991; Li et al., 2002), as previously regarded, which is confi rmed by recent zircon U-Pb results of the metamorphic volcanic rocks of the Bainaimiao group (Gu, 2012; Zhang et al., 2013; Liu et al., 2013). These results clearly indicate that there are no outcropped Precambrian basement rocks in the Bainaimiao arc belt in the southern Cen-tral Asian orogenic belt.

Magmatic rocks from the Bainaimiao arc belt exhibit main peaks at 472 Ma, 453 Ma, 450 Ma, 446 Ma, 438 Ma, and 427 Ma and minor peaks at 518 Ma, 499 Ma, 479 Ma, and 417 Ma (Fig. 16). Earlier magmatism consists mainly of tonalite, diorite, quartz diorite, granodio-rite, granodiorite, minor gabbro, andesite, and dacite; granite only exists during the later stages (<430 Ma). The earliest magmatism, as repre-sented by Early Cambrian dacite in the Bayan Obo–Damaoqi area with a zircon U-Pb age of 518 ± 3 Ma, indicates initiation of the Bainaim-iao arc magmatism and subduction during the Early Cambrian. The latest magmatism, as represented by an Early Devonian undeformed pegmatite dike emplaced into the Ordovician to Silurian metamorphic rocks in the Bainaimiao area with a zircon U-Pb age of 411 ± 8 Ma (Zhang et al., 2013), indicates termination of the Bainaimiao arc prior to the Early Devonian.

Except for the early Precambrian and early Paleozoic detrital zircons similar to those from the arc magmatism in the Bainaimiao arc belt, there are abundant late Mesoproterozoic–Neoproterozoic (1.25–0.60 Ga) detrital zir-cons from the metasedimentary rocks (Fig. 9; Table 2). Xenocrystic zircons with similar U-Pb ages exist in a quartz monzodiorite dike in the Bainaimiao area (H.F. Zhang et al., 2009). These detrital and xenocrystic zircon ages are very similar to those from the Neoproterozoic to Paleozoic arc terranes and microcontinents in the Central Asian orogenic belt (e.g., Shi et al., 2003; Xue et al., 2009; Demoux et al., 2009a; Li et al., 2011; Rojas-Agramonte et al., 2011; Levashova et al., 2010, 2011; Yang et al., 2011; Kozakov et al., 2012; Jiang et al., 2012; Kröner et al., 2011, 2014; Fig. DR4 [see foot-note 1]), but they are very different from those of the Cambrian to Permian strata in the North China craton (e.g., Cope et al., 2005; Darby and Gehrels , 2006; Yang et al., 2006). Therefore, source areas of the early Paleozoic metasedi-mentary rocks in the Bainaimiao arc belt are likely some Protero zoic microcontinents in the Central Asian orogenic belt instead of the

A B

C D

E

G H

F

Figure 9. U-Pb concordia diagrams and probability density plots of zircons from the meta-sedi mentary rocks in the Bainaimiao arc belt. Data-point error crosses are 2σ. 206Pb/238U ages were used for zircons younger than 1.0 Ga, and 207Pb/206Pb ages were used for those older than 1.0 Ga in probability density plots.




North China craton. Since the north side of the Bainaimiao arc belt was surrounded by oceans during the early Paleozoic period (Figs. 17C–17D), as suggested by many previous studies (e.g., Wang and Liu, 1986; Hsu et al., 1991; Tang and Yan, 1993; Robinson et al., 1999; Xiao et al., 2003; de Jong et al., 2006; Li, 2006; Li et al., 2009), the source areas of the early Paleozoic metasedimentary rocks are located in the Bainaimiao arc belt itself, indicating exis-tence of some Proterozoic microcontinents in the Bainaimiao arc belt that were probably cov-ered by post–early Paleozoic rocks.

East Extension of the Bainaimiao Arc Belt in NE China

Although most researchers believe that the Changchun-Yanji suture zone, central Jilin Prov-ince, in NE China east to the Songliao Basin, is the eastern extension of the Solonker suture zone in central Inner Mongolia (e.g., Wang and Liu, 1986; Li, 1998, 2006; Jia et al., 2004; Sun et al., 2004; Wu et al., 2007, 2011; Lin et al., 2008; Deng et al., 2009; Li et al., 2009, 2013; Y.J. Liu et al., 2010), others have proposed that it may connect with the Heihe-Neijiang suture

west to the Songliao Basin (e.g., X.Z. Zhang et al., 2012). The “early Paleozoic” plutons and metamorphic sedimentary and volcanic rocks in the Siping-Yitong-Huadian-Dunhua area south to the Changchun-Yanji suture zone (e.g., BGMRJP, 1988) were previously considered as the eastern extension of the Bainaimiao arc belt in NE China (e.g., Jia, 1988, 1994; Jia and Lu, 1999). However, recent zircon sensitive high-resolution ion microprobe (SHRIMP) and LA-ICP-MS U-Pb results indicate that most of the “early Paleozoic” plutons are late Paleozoic or Mesozoic in age (e.g., Zhang et al., 2004; Wu et al., 2011), and some “early Paleozoic” meta-morphic sedimentary and volcanic rocks are Carboniferous-Permian in age (e.g., Wang et al., 2013). Therefore, it is still uncertain whether the Bainaimiao arc belt can cross the Songliao Basin and extend to NE China (Fig. 1). The Late Ordovician–Early Silurian tonalite-diorite plu-tons with zircon U-Pb ages of 438 ± 4 Ma and 446 ± 3 Ma recognized from the Gongzhuling area in NE China in this paper provide solid evi-dence for extension of the Bainaimiao arc belt across the Songliao Basin to central Jilin Prov-ince in NE China. Existence of the Bainaimiao arc belt beneath the Songliao Basin is indicated

by a Late Ordovician biotite plagioclase quartz schist with zircon U-Pb age of 446 ± 4 Ma (95% confi dence, mean square of weighted deviates [MSWD] = 1.4, N = 9) from a drill hole in the central Songliao Basin (Wang et al., 2007). Eastward extension of the Bainaimiao arc belt in the Siping area in NE China is very important to understanding the tectonic subdivision and framework of NE China and provides important evidence to connect the Changchun-Yanji suture zone in the NE with the Solonker suture zone in central Inner Mongolia.

Origin of the Magmatic Rocks in the Bainaimiao Arc Belt

The early Paleozoic magmatic rocks in the Bainaimiao arc belt have rock associations of tonalite, diorite, quartz diorite, granodiorite, minor gabbro, andesite, and dacite, with minor granite at late stage. Most of them are calc-alkaline in compositions and belong to I-type granitoids. Their lithological association, min-eral assemblage, and geochemical compositions indicate they are subduction-related magmatic rocks formed in arc environment (Maniar and Piccoli, 1989; Barbarin, 1999), which is further supported by depletion in high fi eld strength trace elements (e.g., P, Nb, Ta, and Ti) on primi-tive mantle–normalized diagrams (Fig. 12).

Most of the early Paleozoic magmatic rocks in the Bainaimiao arc belt are characterized by LREE enrichment, basically no Eu anomaly, high contents of Al2O3, Sr, and Ba, low con-tents of Y and Yb, and high Sr/Y ratios, and they display adakitic geochemical signatures (e.g., Atherton and Petford, 1993; Stern and Kilian, 1996). These rocks were likely produced by par-tial melting of a subducted slab or interactions between a subducted slab, mantle wedge, and continental crust (e.g., Xu et al., 2003; Liu et al., 2003; Tao et al., 2005; Jian et al., 2008).

Whole-rock Sr-Nd and zircon Hf isotopic compositions provide important constraints on the source characteristics of the early Paleo-zoic magmatic rocks in the Bainaimiao arc belt. Both εNd(t) and εHf(t) values increase from the Early Cambrian to Middle–Late Ordovician and decrease from the Middle–Late Ordovi-cian to Middle–Late Silurian (Figs. 13 and 14), indicating changing of source areas during different stages of arc magmatism. The εHf(t) values of detrital zircons from the early Paleo-zoic metasedimentary rocks exhibit a similar trend of increasing from the Early Cambrian to Middle–Late Ordovician and decreasing from the Middle–Late Ordovician to Middle Silurian (Fig. 15B).

The Early Cambrian dacite, which is the ear-liest magmatism recognized in the Bainaimiao

Figure 10. (A) Total alkali (K2O + Na2O) vs. silica (SiO2) and (B) K2O vs. SiO2 classifi cation diagrams for the magmatic rocks in the Bainaimiao arc belt.


Zhang et al.


arc belt, is characterized by high initial 87Sr/86Sr ratios of 0.71064, low initial 176Hf/177Hf ratios from 0.282045 to 0.282157, low negative εNd(t) values of –8.1 to –8.2, low negative εHf(t) val-ues from –14.2 to –10.3, old Nd isotopic TDM model ages of 1.89–3.07 Ga and old Hf isotopic TDM model ages from 1.53 Ga to 1.71 Ga, and TC

DM from 2.14 Ga to 2.38 Ga. These geochemi-cal characteristics indicate that the Early Cam-brian dacite was mainly derived from Protero-zoic ancient lower-crustal material. This partial melting of the ancient lower-crustal material was probably related to underplating of mafi c magma produced by partial melting of mantle wedge and subducted oceanic crust beneath the lower crust during an early stage of subduction.

The Ordovician–Silurian magmatic rocks are calc-alkaline in compositions and belong to I-type granitoids. Some granite with S-type signature was coeval with I-type granites in the Middle–Late Silurian. Early Cambrian–Early Silurian magmatic rocks are characterized by low initial 87Sr/86Sr ratios (0.70492–0.70746), positive to high negative εNd(t) and εHf(t) val-ues, and young Nd and Hf isotopic model ages. Combined with their adakitic geochemical signatures, we proposed that these rocks were mainly produced by partial melting of the sub-ducted slab of the paleo–Asian Ocean. How-ever, the high Mg# values (>50) of some Early Cambrian–Early Silurian magmatic rocks indi-cate involvement of the overlying mantle wedge during their petrogenesis. Variable Sr-Nd-Hf isotopic compositions and the existence of some Precambrian inherited zircons (Table DR1 [see footnote 1]) indicate that some ancient crustal materials could also have been involved.

The Middle–Late Silurian magmatic rocks are calc-alkaline in compositions and exhibit variable SiO2 contents from 59.8 wt% to 78.0 wt%. They are characterized by high initial 87Sr/86Sr ratios of 0.70612–0.71064, negative to low positive εNd(t) and εHf(t) values, and relatively old Nd and Hf isotopic model ages, indicating involvement of more ancient crustal materials in their origin. Although some garnet-bearing granites belong to S-type and are coeval with I-type granites, they are only observed in limited distribution in the Bainaimiao arc. Muscovite in most granite is a secondary min-eral produced by alteration of the biotite. The Middle–Late Silurian magmatic rocks were likely produced by interactions and differential mixing of melts from subducted slab and conti-nental crust. The previously described evolution trend of early Paleozoic magmatic rocks in the Bainaimiao arc belt is consistent with increas-ing of crustal materials and alkaline components with maturation and crustal thickening of the arc (e.g., Wilson, 1989).

A

C D

E F

G H

I J

B

Figure 11. Chondrite-normalized rare earth element (REE) patterns for the magmatic rocks in the Bainaimiao arc belt. The chondrite values are from Taylor and McLennan (1985).




Nature of the Bainaimiao Arc Belt: Island Arc or Continental Arc?

Our new research results indicate that the Bainaimiao arc belt is an ensialic island arc char-acterized by very different tectonic history and basement compositions than the northern North China craton. Different to the Japan-style island arc as previously suggested (e.g., Hu et al., 1990; Tang, 1990; Tang and Yan, 1993; Jia and Lu, 1999; Gao et al., 2001; Shang et al., 2003; Jia et al., 2003), we propose that the Bainaimiao arc belt was separated by a wide ocean (named as the South Bainaimiao Ocean) from the north-ern North China craton prior to its accretion to the northern margin of the North China craton during the Late Silurian–earliest Devonian (Fig. 17). Early Paleozoic subduction of the oceanic plate beneath the Bainaimiao arc was most likely to the north, not to south as previously suggested (e.g., Wang and Liu, 1986; Hu et al., 1990; Tang, 1990; Tang and Yan, 1993; Jia and Lu, 1999; Gao et al., 2001; Shang et al., 2003; Jia et al., 2003; Xiao et al., 2003). This infer-ence is also supported by recent progress on the tectonic evolution of the northern margin of the North China craton.

Most researchers agree that the northern margin of the North China craton was a passive continental margin during the Meso protero-zoic–Neoproterozoic to Cambrian (e.g., Zhang et al., 1986; Wang and Liu, 1986; Hu et al., 1990; Wang et al., 1991; Tang, 1992; Xu and Chen, 1997; Li, 1997; Xiao et al., 2003). How-ever, whether it was transformed to an active continental margin during the Ordovician to Silurian is still highly controversial. Some researchers considered the northern margin of the North China craton as an active continen-tal margin during the Ordovician to Silurian (e.g., Zhang et al., 1986; Wang and Liu, 1986; Hu et al., 1990; Wang et al., 1991; Tang, 1992; Chao et al., 1997; Xiao et al., 2003); others favor it as a passive continental margin during the early Paleozoic (e.g., Hsu et al., 1991; Li, 1997; Xu and Chen, 1997; Li et al., 2009). Our new results support that the northern margin of the North China craton remained as a passive conti-nental margin during the early Paleozoic and transformed to an Andean-style active continen-tal margin from the late Carboniferous (Zhang et al., 2007a, 2009a). Evidence to support the northern margin of the North China craton as a passive continental margin during the early Paleozoic includes the following: (1) Although the early Paleozoic magmatic rocks are widely distributed in the Bainaimiao arc belt, there are no early Paleozoic magmatic rocks in the northern North China craton, and the previously regarded “early Paleozoic” plutons in northern

A B

C D

E F

G H

I J

Figure 12. Primitive mantle–normalized spidergrams for the magmatic rocks in the Bainaimiao arc belt. The primitive mantle values are from Sun and McDonough (1989).


Zhang et al.


North China craton, such as Hejiao granitoid pluton (Zhao and Li, 1987; Wang et al., 1991; Chao et al., 1997), are Archean–Paleo protero-zoic or Carboniferous–Permian in age, as revealed by recent zircon U-Pb dating (IMIGS, 2003; author’s personal observation, 2008, S.H. Zhang); (2) Cambrian–Ordovician epicontinen-tal sea sedimentation is widely distributed in the

North China craton, even near its northern mar-gins, as indicated by the Early Ordovician mar-bles north to Jining with brachiopod fossils such as Allopiloceras sp. and Eoorthis sp. and Late Cambrian limestone in Kalinqinqi near Chifeng with brachiopod fossils such as Billingsella ex. gr. fl ustuosa Nikitin, Eoorthis aff. linnarsoni (Kayser), etc. (BGMRIM, 1991), indicating that

the northern North China craton remained stable during the Cambrian–Ordovician; (3) structural and sedimentary analysis results indicate that the Mesoproterozoic–Neoproterozoic and early Paleozoic sedimentary rocks are widely distrib-uted in the Inner Mongolia paleo-uplift, and most of them have been eroded due to strong exhumation and erosion of the Inner Mongo-lia paleo-uplift during the late Paleozoic–early Mesozoic (Zhao, 1990; Zhang and Zhu, 2000; Zhang et al., 2006). Therefore, the northern North China craton has not been affected by early Paleozoic subduction of the paleo–Asian Ocean to form the Bainaimiao island arc, which cannot be explained by the Japan-style island-arc model as previously proposed (e.g., Hu et al., 1990; Tang, 1990; Tang and Yan, 1993; Jia and Lu, 1999; Gao et al., 2001; Shang et al., 2003; Jia et al., 2003).

Although it is still uncertain how wide the South Bainaimiao Ocean between the Bainaimiao arc belt and North China craton was, existence of the ocean is supported by many lines of geological evidence. As a long-lived compos-ite accretionary orogen, ophiolitic mélanges are very important to understanding the tectonic framework and evolution history of oceans in the Central Asian orogenic belt (e.g., Robinson et al., 1999; Buchan et al., 2001; Miao et al., 2008; Jian et al., 2010). In the southern Central Asian orogenic belt, ophiolitic mélanges are dis-tributed not only near the Solonker suture zone or in areas north to the Solonker suture, but also in areas between the Bainaimiao island arc and the North China craton (Fig. 1). Some ophio-litic mélanges located between the Bainaimiao island arc and the North China craton have been recognized in Wude north to Bayan Obo (Jia et al., 2003; Shang et al., 2003), Harihada-Chegendalai northeast to Damaoqi (Shao, 1986, 1989, 1991; Tang, 1992), and central-southern Jilin and northern Liaoning Provinces in NE China (Jia, 1988; Jia and Lu, 1999), and they represent the accretion-collision belt between the Bainaimiao island arc and the northern North China craton. Although their ages are not well constrained, fi eld occurrences and relations indi-cate formation of these ophiolitic mélanges dur-ing the early Paleozoic period (e.g., Shao, 1986, 1989, 1991; Tang, 1992; Jia et al., 2003; Shang et al., 2003).The NW-SE–trending Wude ophio-litic mélange is several hundred meters to 2 km wide and 20 km long. It is fault contacted with the Devonian sedimentary rocks on its north-ern side, and its southern side is unconformably overlain by Early Cretaceous volcanic and sedi-mentary rocks (Fig. 2). It is composed of differ-ent structurally contacted rock units including oceanic serpentinite, layered gabbro, diopsidite, deep-water chert, fl ysch-like siltstone, andesite ,

A

B

C

Figure 13. (A) εNd(t) vs. crystallization age and (B) εNd(t) vs. (87Sr/86Sr)i plots for rocks from the magmatic rocks in the Bainaimiao arc belt. The ancient crust evolution line in A is con-structed on the basis of an average 147Sm/144Nd value of 0.118 (Jahn and Condie, 1995).




spilite, andesitic basalt, quartzite, two-mica schist, quartz diorite, and diorite (Figs. 4A–4C; Jia et al., 2003; Shang et al., 2003). The Hari-hade-Chegendalai ophiolitic mélange is 15 km long (Fig. 2) and consists of ophiolitic perido-tite, pyroxene peridotite, gabbro, basalt, dia-base, deep-water chert and Ordovician–Silurian musco vite leptynite, quartzite, marble, plagio-clase amphibole schist, and mica schist (Figs. 4D–4F; Shao, 1986, 1989, 1991; Tang, 1992). It was strongly deformed probably during the lat-est Silurian to Early Devonian with development of steeply dipping NE-trending foliation and dip-parallel stretching lineation (Shao, 1986, 1989, 1991; Tang, 1992).

Tectonic Affi nity of the Bainaimiao Arc Belt

Since little is known about the ages of the meta morphic rocks and compositions of the basement rocks in the Bainaimiao arc belt, tectonic affi nity of the Bainaimiao arc belt has long been controversial. Because the Bainaim-iao arc belt is located south to the late Paleozoic Solonker suture zone and in fault contact with the northern margin of the North China craton (Fig. 1), most researchers considered it as an island arc or continental arc with tectonic affi n-ity to the North China craton (e.g., Hu et al., 1990; Tang, 1990; Tang and Yan, 1993; Jia and Lu, 1999; Gao et al., 2001; Shang et al., 2003; Jia et al., 2003; Xiao et al., 2003; de Jong et al., 2006). However, others considered it as exotic terrains accreted to the northern margin of the North China craton by tectonism, since ages and compositions of rock units and fossil associa-tions of the Silurian strata of the Bainaimiao arc belt are very different from those of the North China craton (e.g., Chen et al., 1993; Li, 1997). Our new zircon U-Pb results of the early Paleo-zoic metamorphic sedimentary and volcanic rocks in the Bainaimiao arc belt and Sr-Nd-Hf isotopic compositions of magmatic rocks pro-vide important constraints on the tectonic affi n-ity of the Bainaimiao arc belt in the southern Central Asian orogenic belt.

New zircon U-Pb dating results indicate that the metamorphic rocks that had previously been regarded as Precambrian basement of the Bainaimiao arc belt (e.g., Nie et al., 1991; Chen et al., 1993; Li et al., 2002; IMIGS, 2003) are all early Paleozoic in age (Table 2). These meta-morphic sedimentary and volcanic rocks are arc-related low-grade metamorphic rocks and were deposited during formation of the arc and were probably metamorphosed during arc-continent collision between the Bainaimiao arc terrain and the northern margin of the North China cra-ton in the latest Silurian. There are no exposed Precambrian basement rocks in the Bainaimiao

A

B

C

Figure 14. εHf(t) vs. U-Pb age for zircons from the magmatic rocks in the Bainaimiao arc belt. (A) Bayan Obo–Damaoqi; (B) Bainaimiao; (C) Gongzhuling, NE China.


Zhang et al.


arc belt in the southern Central Asian orogenic belt. However, high initial 87Sr/86Sr ratios, low εNd(t) and εHf(t) values, and Paleoproterozoic–Mesoproterozoic Nd and Hf isotopic model ages (Tables DR4 and DR5 [see footnote 1]) of the Early Cambrian dacite (sample 08505–1) and the Silurian granite, tonalite, and quartz dio-rite (samples 08480–1, 08487–1, 08515–2 and 070813–7 and 08417–1) indicate existence of some Precambrian basement rocks beneath the Bainaimiao arc belt.

Different to the Cambrian to Permian strata in the North China craton (e.g., Cope et al., 2005; Darby and Gehrels, 2006; Yang et al., 2006), there are abundant late Mesoproterozoic–Neo-proterozoic (1.25–0.60 Ga) detrital zircons from the metasedimentary rocks in the Bainaimiao arc belt (Fig. 8; Table 2). These detrital zircon compositions are very similar to those from the Neoproterozoic to Paleozoic arc terranes (e.g., Shi et al., 2003; Xue et al., 2009; Li et al., 2011; Rojas-Agramonte et al., 2011; Yang et al., 2011; Jiang et al., 2012) and microcontinents (e.g., Wang et al., 2001; Zhao et al., 2006; Demoux et al., 2009a; Levashova et al., 2010, 2011; Kozakov et al., 2012; Kröner et al., 2011, 2014) in central-southern Mongolia and Inner Mongo-lia in the Central Asian orogenic belt (Fig. DR4 [see footnote 1]), indicating that the Bainaimiao arc belt has similar tectonic affi nity to most of the arc terranes and microcontinents in the southern Central Asian orogenic belt. Recent geological, geochronological, and paleomag-netic results suggested that the microcontinents (e.g., Wang et al., 2001; Zhao et al., 2006; Demoux et al., 2009a; Levashova et al., 2010, 2011; Han et al., 2011; Kröner et al., 2011, 2014) and Neoproterozoic to Paleozoic arc terranes

(e.g., Rojas-Agramonte et al., 2011) in central-southern Mongolia and NE China share similar tectonic affi nity to the Tarim or South China (Yangtze) cratons. Therefore, we propose that the Bainaimiao arc belt has a tectonic affi nity to the Tarim or Yangtze (South China) cratons and was developed upon some crustal fragment with affi nity to the Tarim or Yangtze (South China) cratons during the early Paleozoic.

Evolution of the Bainaimiao Arc Belt

Evolution of the Bainaimiao arc belt during the Mesoproterozoic to late Paleozoic period is shown by schematic cartoons in Figure 17. Prior to formation of the continental margins, the northern margin of the North China craton was connected with Laurentian, Siberian, or Indian cratons within the Columbia supercon-tinent (e.g., Wang et al., 1991; Condie, 2002; Wilde et al., 2002; Zhao et al., 2003a, 2003b; S.H. Zhang et al., 2009c, 2012; S. Zhang et al., 2012). During the latest Paleoproterozoic to mid-Mesoproterozoic, the northern North China craton was characterized by continental rifting with deposition of the thick clastic and carbonate sequence of the Bayan Obo Group (Fig. 17A). Formation of the passive continental margin of the northern North China craton and initiation of the paleo–Asian Ocean occurred during fi nal breakup of the North China craton from the Columbia supercontinent in the mid-Mesoproterozoic at 1.33–1.32 Ga (S.H. Zhang et al., 2009c, 2012; Zhao et al., 2010), which is supported by paleomagnetic results from the North China craton (e.g., Wu et al., 2005; Pei et al., 2006; S. Zhang et al., 2012; Chen et al., 2013). Initiation of the paleo–Asian Ocean dur-

ing the mid-Mesoproterozoic at ca. 1.32 Ga is supported by the 1.3–1.1 Ga Baikal-Muya (Sklyarov et al., 1994) ophiolite and the 1.02 Ga (Khain et al., 2002, 2003) Dunzhugur ophiolite in southern Siberia.

Initiation of the Bainaimiao island arc in the southern Central Asian orogenic belt occurred during the Early Cambrian period at ca. 0.52 Ga (Figs. 17C–17D). The arc was formed by north-ward subduction of the paleo–Asian oceanic plate beneath a Proterozoic microcontinent with tectonic affi nity to the Tarim or Yangtze (South China) cratons (Fig. 17D). The North China craton remained stable during this period with deposition of Cambrian–Ordovician passive-margin deposits. Successive northward sub-duction during the Early Cambrian to Middle Silurian (0.52–0.42 Ga) resulted in contraction of the South Bainaimiao ocean located between the Bainaimiao island arc and the northern North China craton and fi nal accretion of the Bainaimiao island arc to the North China cra-ton by arc-continent collision during the latest Silurian (Fig. 17E). The arc-continent collision was accompanied by metamorphism and defor-mation of the early Paleozoic sedimentary and magmatic rocks and deposition of the latest Silurian–earliest Devonian continental molasse or quasi-molasse of the Xibiehe Formation (e.g., Zhang and Tang, 1989; Tang, 1990; BGMRIM, 1991; Su, 1996; Xu et al., 2003; Wang, 2005; Chen and Boucot, 2007; Y.P. Zhang et al., 2010) and then followed by emplacement of the Early–Middle Devonian alkaline rocks in the northern North China craton and southern Central Asian orogenic belt (e.g., Luo et al., 2001; S.H. Zhang et al., 2007b, 2009d; X.H. Zhang et al., 2010; Shi et al., 2010; Wang et al., 2012). During the

A B

Figure 15. εHf(t) vs. U-Pb age for zircons from the metasedimentray rocks in the Bainaimiao arc belt.




late Carboniferous to Middle Permian period, the northern North China block (including the northern North China craton and the accreted Bainaimiao arc belt) evolved as an Andean-style active continental margin due to southward sub-duction of the paleo–Asian oceanic plate (Fig. 17F). The latest Silurian arc-continent colli-sion between the Bainaimiao island arc and the North China craton was very likely responsible for Paleozoic reversal of arc polarity and transi-tions of the northern North China craton from passive to active continental margin as sug-

gested in other places such as Cenozoic Taiwan, northern New Guinea, Northwest Pacifi c, and the Irish Caledonides (e.g., McKenzie, 1969; Johnson and Jaques, 1980; Konstantinovskaia, 2001; Clift et al., 2003).

Implications for Crustal Accretion in the Central Asian Orogenic Belt

As the world’s largest site of juvenile crust formation in the Phanerozoic, the mechanism of continental crustal growth in the Central Asian

orogenic belt has long been highly controver-sial (e.g., Sengör et al., 1993; Xiao et al., 2003; Windley et al., 2007; Li et al., 2009; Safonova et al., 2011; Wilhem et al., 2012; Kovach et al., 2013; Kröner et al., 2014). As suggested by many previous studies (e.g., Badarch et al., 2002; Xiao et al., 2003; Windley et al., 2007; Safonova et al., 2011; Wilhem et al., 2012; Kröner et al., 2014), formation of the Central Asian orogenic belt during the Neoproterozoic to late Paleozoic can be explained in terms of southwest Pacifi c–style accretion of arcs and

TABLE 2. SUMMARY OF ZIRCON U-Pb AGES OF THE MAGMATIC AND METASEDIMENTRAY ROCKS FROM THE BAINAIMIAO ARC BELT

.feRdohteM)aM(egAepytkcoRnoitacoL)N°(edutitaL)E°(edutignoL.onelpmaS)3,1(PMIRHSetitiblAiakniluT––2-1TM)3,1(PMIRHSetiroidztrauQiakniluT––1-1TM)3,1(PMIRHSeticaDiakniluT––11-1TM)3,1(PMIRHSsetimejhdnorTiakniluT––4-1TM)3,1(PMIRHSorbbagateMiakniluT––6-1TM)3,1(PMIRHSetilanoTiakniluT––8-1TM

DRAB01 110°34′11.4″ 41°50′16.1″ )3,2(PMIRHSetilanoTiqoamaDBYH01 110°07′15.3″ 41°55′06.2″ )3,2(PMIRHSetiroidonarGiqoamaDBYH03 110°06′58.7″ 41°59′14.5″ )3,2(PMIRHSetiroidztrauQiqoamaDBYH02 110°07′20.7″ 41°55′54.0″ )3,2(PMIRHSetiroiDiqoamaDNM08-13 110°25′48.0″ 42°07′00.0″ )4(PMIRHSorbbagednelbnroHiqoamaDNM08-17 110°25′58.0″ 42°07′03.0″ )4(PMIRHSorbbagednelbnroHiqoamaD

)5(SMITetiroiDiqoamaD–––)5(SMITetilanoTiqoamaD–––)5(SMITetiroidonarGiqoamaD–––

BD09-007 113°37′05.2″ 42°24′18.8″ )6(SM-PCI-ALetiroidonarGoaimadaH)7(PMIRHSyryhpropetiroidonarGiqoamaD––3-MNB)8(PMIRHSetiloyhRoaimianiaB––40MNB)8(PMIRHSeticaDoaimianiaB––20MNB)8(PMIRHSeticaDoaimianiaB––10MNB)8(PMIRHSssiengetinamilliSoaimianiaB––70MNB)8(PMIRHSssiengednelbnroh-esalcoigalPoaimianiaB––60MNB)8(PMIRHSetiroidateMoaimianiaB––80MNB)8(PMIRHSekidetitamgePoaimianiaB––90MNB)8(PMIRHSetiroiDoaimianiaB––50MNB)9(SMITetiroidztrauQoaimianiaB––11-99)9(SMITetiroidztrauQoaimianiaB––41-99)01(SM-PCI-ALetisednacihpromateMoaimianiaB–––)11(SM-PCI-ALetiloyhrcihpromateMoaimianiaB––1-45b61P)11(SM-PCI-ALetiloyhrcihpromateMoaimianiaB––1-9b61P

07130-1 110°12′23.3″ 41°53′47.4″ )21(SM-PCI-ALetiroidztrauQiqoamaD08480-1 110°36′35.4″ 41°48′40.1″ )21(SM-PCI-ALetiroidztrauQiqoamaD08487-1 110°47′22.9″ 41°49′17.5″ )21(SM-PCI-ALetinargrapsdlefilaklAiqoamaD08500-1 110°07′04.6″ 41°59′12.8″ )21(SM-PCI-ALetilanoTiqoamaD08502-1 110°07′12.8″ 41°59′52.5″ )21(SM-PCI-ALetilanoTiqoamaD08504-1 110°09′50.3″ 42°00′00.8″ )21(SM-PCI-ALetisednAiqoamaD08505-1 110°10′55.1″ 41°59′50.6″ )21(SM-PCI-ALeticaDiqoamaD08515-2 110°34′11.5″ 41°50′16.1″ )21(SM-PCI-ALetilanoTiqoamaD08556-1 109°54′05.4″ 41°59′32.3″ )21(SM-PCI-ALeticaDobOnayaB08557-1 109°54′11.8″ 41°59′53.8″ )21(SM-PCI-ALetiroidztrauQobOnayaB070813-7 112°34′40.3″ 42°14′14.8″ )21(SM-PCI-ALetiroidztrauQoaimianiaB08404-1 112°43′15.1″ 42°16′49.6″ )21(SM-PCI-ALetiroidztrauQoaimianiaB08406-1 112°43′19.9″ 42°17′34.4″ )21(SM-PCI-ALetinargetivocsuMoaimianiaB08409-1 112°36′36.0″ 42°12′47.4″ )21(SM-PCI-ALetiroidztrauQoaimianiaB08413-1 112°31′56.2″ 42°13′24.1″ )21(SM-PCI-ALetilanoToaimianiaB08417-1 112°33′31.7″ 42°14′16.3″ )21(SM-PCI-ALetilanoToaimianiaB08429-1 112°37′48.4″ 42°16′13.0″ )21(SM-PCI-ALetilanoToaimianiaB08432-1 112°37′27.6″ 42°17′05.9″ )21(SM-PCI-ALetiroiDoaimianiaB09341-2 124°59′40.1″ 43°27′36.7″ )21(SM-PCI-ALetilanoTgniluhzgnoG09344-1 124°56′15.6″ 43°27′38.4″ )21(SM-PCI-AL

2±5243±4543±6542±2742±0847±0942±7142±0442±6443±2543±3545±744

274724054

3±6446±5447±4747±3549±63411±264

5±7342±8348±114

01±9143±954

41±4544±0542±9942±8742±3544±6344±9242±3743±0745±4743±8156±3344±5443±5442±1243±9345±2343±0345±9343±0343±3344±6343±6444±834etiroidztrauQgniluhzgnoG

09224-3 110°29′06.8″ 41°52′05.1″ Damaoqi Metamorphic tuffaceous sandstone Main peaks: 440, 453, 466 LA-ICP-MS (12)09225-1 110°26′59.2″ 41°52′29.0″ Damaoqi Metamorphic sandstone Main peak: 458;

Minor peaks: 622, 960, 1158, 3109LA-ICP-MS (12)

09226-1 110°27′32.7″ 41°52′26.6″ Damaoqi Metamorphic sandstone Main peaks: 464, 499, 534;Minor peaks: 612, 845, 986, 1106, 1254, 1607

LA-ICP-MS (12)

08411-1 112°33′06.7″ 42°12′58.8″ Bainaimiao Biotite quartz schist Main peaks: 446, 469, 706, 981;Minor peaks: 591, 889, 1134, 2174, 2954

LA-ICP-MS (12)

Note: Methods are as follows: SHRIMP—sensitive high-resolution ion microprobe; LA-ICP-MS—laser-ablation–inductively coupled plasma–mass spectrometry; TIMS—thermal ionization mass spectrometry. References are as follows: (1) Liu et al. (2003); (2) Zhang and Jian (2008); (3) Jian et al. (2008); (4) Li et al. (2010); (5) Xu et al. (2003); (6) Hao and Hou (2012); (7) Li et al. (2012); (8) Zhang et al. (2013); (9) Tong et al. (2010); (10) Gu (2012); (11) Liu et al. (2013); (12) this study.


Zhang et al.


microcontinents. Therefore, island arcs and microcontinents may have played impor-tant roles in the formation of the huge Central Asian orogenic belt. In addition to the exposed Precambrian microcontinents recognized from the Central Asian orogenic belt (e.g., Gordi-enko, 1996; Wang et al., 2001; Badarch et al., 2002; Dobretsov et al., 2003; Zhao et al., 2006; Demoux et al., 2009a; Levashova et al., 2010, 2011; Rojas-Agramonte et al., 2011; Kozakov et al., 2012), some Precambrian microconti-nents could be buried beneath island arcs, as suggested by our geochronological and geo-chemical results of the early Paleozoic meta-morphic and magmatic rocks in the Bainaimiao arc belt in the southern Central Asian orogenic belt. As indicated by our whole-rock Sr-Nd and zircon Hf isotopic data from the Bainaimiao arc belt, formation of juvenile crust through tonalite-trondhjemite-granodiorite (TTG)–type andesitic magmatism was dominant for crustal growth in construction of the Bainaimiao arc. However, during the earliest and latest stages of arc construction, recycling of ancient crustal materials was likely a very important mecha-nism for crustal formation. Paleozoic island arcs with buried Precambrian microcontinents are probably very common within the Central Asian orogenic belt. Considering the different evolution history of the Bainaimiao arc and other island arcs in the Central Asian orogenic

belt (e.g., Demoux et al., 2009b; Rojas-Agra-monte et al., 2011; Wilhem et al., 2012, and references therein), we contest the evolutionary model for the Central Asian orogenic belt pro-posed by previous authors in terms of a single, long island arc (e.g., Sengör et al., 1993; Sengör and Natal’in, 1996) and further support the evolutionary model that involved subduction-collision-accretion processes of multiple island arcs and microcontinents (e.g., Xiao et al., 2003; Windley et al., 2007; Kröner et al., 2007; Wilhem et al., 2012).

Arc-continent collision is generally thought to have been the most important process involved in the growth of the continental crust over geological time (e.g., Rudnick and Fountain, 1995; Clift et al., 2010). Collisions between oceanic island-arc terrains and pas-sive continental margins are thought to have been important in the formation of continental crust throughout much of Earth’s history (e.g., Draut et al., 2009). Paleozoic arc-continent col-lision has been reported in the southern Urals, Kazakhstan, and southern Mongolia in the Cen-tral Asian orogenic belt (e.g., Brown et al., 1998, 2006; Alvarez-Marron et al., 2000; Johnson et al., 2008; Degtyarev and Ryazantsev, 2007). As an actualistic example for the Paleozoic Cen-tral Asian orogenic belt, Cenozoic arc-continent collision is very common in the western Pacifi c region, such as Taiwan, Philippines, Malaysia,

Indonesia, northwest Australia, and NE Russia (e.g., Charlton, 1989; Rangin et al., 1990; Kon-stantinovskaia, 1999, 2001; Huang et al., 2000, 2006; Elburg et al., 2002; Clift et al., 2003; Dimalanta et al., 2009; Whattam, 2009; Houri-gan et al., 2009; Harris et al., 2009; Brown and Huang, 2009; Herrington et al., 2011). The lat-est Silurian arc-continent collision as reported in this contribution led to the accretion of the Bainaimiao island arc to the North China cra-ton and formation of the composite North China block south to the Solonker suture (Fig. 1). Therefore, arc-continent collision could have been an important mechanism for continental crustal growth and formation of the huge Cen-tral Asian orogenic belt.

CONCLUSIONS

(1) New zircon U-Pb geochronological results indicate that the area east to Gong-zhuling, Siping, in NE China, is the eastward extension of the Bainaimiao arc belt in the southern Central Asian orogenic belt.

(2) Zircon U-Pb geochronological and geo-chemical results from the arc-related magmatic rocks in the Bainaimiao arc belt indicate that the arc was active from the Early Cambrian to Middle Silurian (0.52–0.42 Ga). These mag-matic rocks were produced by partial melting of subducted slab or interactions among subducted slab, mantle wedge, and continental crust. Involvement of ancient crustal material was very signifi cant during the early and late stages of arc magmatism.

(3) New zircon U-Pb geochronological results of the metamorphic sedimentary and volcanic rocks in the Bainaimiao arc belt indicate that they are all early Paleozoic in age. They are arc-related low-grade metamorphic rocks deposited during formation of the arc, not Precambrian basement rocks as previously regarded.

(4) Detrital zircon analyses of the early Paleozoic metasedimentary rocks and Sr-Nd-Hf geochemical results of the magmatic rocks in the Bainaimiao arc belt indicate existence of some Proterozoic basement rocks beneath the arc and that the Bainaimiao arc was built upon a Precambrian microcontinent. Similar to the microcontinents in central-southern Mongolia, which share tectonic affi nity to the Tarim or South China (Yangtze) cratons, the Bainaimiao arc belt has a tectonic affi nity to the Tarim or Yangtze cratons and was developed upon some crustal fragment with affi nity to the Tarim or Yangtze (South China) cratons during the early Paleozoic.

(5) The Bainaimiao arc belt is an ensialic island arc characterized by a different evolution history and basement compositions from the

Figure 16. Zircon U-Pb age probability density plot of magmatic rocks in the Bainaimiao arc belt.




northern North China craton. It was separated by a wide ocean (South Bainaimiao Ocean) from the northern North China craton during the Cambrian–Ordovician period. Successive northward subduction during the Early Cam-brian to Middle Silurian (0.52–0.42 Ga) resulted in contraction of the ocean and fi nal accretion of the Bainaimiao island arc to the northern mar-gin of the North China craton during the Late Silurian–earliest Devonian by arc-continent collision.

APPENDIX. METHODS AND ANALYTICAL PROCEDURES

Sample Preparation and Imaging

Zircons were separated using conventional crushing and separation techniques and were then handpicked under a binocular microscope. They were mounted in epoxy resin and polished to expose the cores of the grains in readiness for photomicrograph, cath-odoluminescence (CL), laser-ablation–inductively coupled plasma–mass spectrometry (LA-ICP-MS) U-Pb, and in situ Lu-Hf isotopic analyses. Zircons were imaged using the HITACHI S3000-N scanning electron microscope attached with a GATAN Chroma CL detector at the Beijing Sensitive High-Resolution Ion Microprobe (SHRIMP) Center. CL images of analyzed zircon grains are shown in Figures DR2 and DR3 (see footnote 1).

LA-ICP-MS U-Pb and Trace-Element Analyses

LA-ICP-MS U-Pb and trace-element analyses were performed on an excimer (193 nm wavelength) LA-ICP-MS at the State Key Laboratory of Geo-logical Processes and Mineral Resources, China University of Geosciences, Wuhan, and the State Key Laboratory of Continental Dynamics, Northwest Uni-versity, Xi’an. Detailed operating conditions for the laser ablation system and the ICP-MS instrument and data reduction are the same as the description by Liu et al. (2008, 2010a, 2010b) and Yuan et al. (2004), re-spectively. The ICP-MS used is an Agilent 7500a. The GeoLas 2005 laser-ablation system was used for the laser-ablation experiments. Nitrogen was added into the central gas fl ow (Ar + He) of the Ar plasma to de-crease the detection limit and improve precision (Hu et al., 2008). Sites for analyses were selected on the basis of CL and photomicrograph images. The spots used were 30–32 μm in diameter. U, Th, and Pb con-centrations were calibrated by using 29Si as an internal standard and NIST SRM 610 as the reference stan-dard. Isotopic ratios were calculated using GLITTER 4.0 (Macquarie University) and ICPMSDataCal 5.0 (Liu et al., 2008, 2010a), which were then corrected for both instrumental mass bias and depth-dependent elemental and isotopic fractionation using Harvard zircon 91500 as external standard. GEMOC GJ-1 zircon standard (TIMS U-Pb age = 608.5 ± 0.4 Ma; Jackson et al., 2004) was used as a monitor of data quality during analyses. Concordia diagrams and weighted mean ages were produced using the program ISOPLOT/Ex 3.23 (Ludwig, 2003).

Major- and Trace-Element Geochemistry

Major elements were analyzed on fused glass discs by X-ray fl uorescence spectrometry at the Institute of Geology and Geophysics, Chinese Academy of

A

B

C

D

E

F

Figure 17. Schematic cartoons showing evolution of the Bainaimiao arc belt during the Meso proterozoic to late Paleozoic period. See text for discussion. Not to scale. NCC—North China craton.


Zhang et al.


Sciences , Beijing. Trace elements were determined by ICP-MS (VG Plasma Quad PQ2 Turbo ICP-MS) at the same institute. About 100 mg whole-rock pow-ders were weighed and then dissolved in distilled HF-HNO3 in Tefl on screw-cap beakers and high-pressure Tefl on bombs at 200 °C for 4 d, dried, and then digested with HNO3 at 150 °C for 1 d. Dissolved samples were diluted to 50 mL with 1% HNO3 before analysis. A blank solution was prepared, and the total procedural blank was <50 ng for all trace elements. Indium was used as an internal standard to correct for matrix effects and instrument drift. The Chinese na-tional standards GSR-1 (granite) and GSR-3 (basalt ) were used to monitor analyses. Errors for major-element analysis are within 1%, except for P2O5 (5%), and analyses for most trace elements (including REE) are within 10%.

Rb-Sr and Sm-Nd Isotopic Analyses

Samples for Rb-Sr and Sm-Nd isotopic analyses were dissolved in Tefl on bombs after being spiked with 84Sr, 87Rb, 150Nd, and 149Sm tracers prior to HF + HClO4 dissolution. Rb, Sr, Sm, and Nd were sepa-rated using conventional ion exchange procedures and measured using a Finnigan MAT 262 multi collector mass spectrometer at the Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing, and the Analytical Laboratory of the Beijing Research Institute of Uranium Geology. Procedural blanks were <100 pg for Sm and Nd and <500 pg for Rb and Sr. 143Nd/144Nd was corrected for mass fraction-ation by normalization to 146Nd/144Nd = 0.7219, and 87Sr/86Sr ratios were normalized to 86Sr/88Sr = 0.1194. Typical within-run precision (2σ) for Sr and Nd was esti mated to be ±0.000015. The measured values for the JNDi-1 Nd standard and the NBS-987 Sr stan-dard were 143Nd/144Nd = 0.512113 ± 5 (N = 5) and 87Sr/86Sr = 0.710269 ± 19 (N = 3) during the period of data acquisition, respectively. The U.S. Geologi-cal Survey standard BCR-2, prepared using the same procedures as the samples, yielded Rb = 45.92, Sr = 327.2, 87Rb/86Sr = 0.4061, 87Sr/86Sr = 0.705036 ± 11 (2σ), Sm = 6.63, Nd = 29.07, 147Sm/144Nd = 0.1380, and 143Nd/144Nd = 0.512620 ± 13 (2σ).

In Situ Lu-Hf Isotope Analyses

In situ Lu-Hf isotope analyses were performed using a NewWave UP213 laser-ablation microprobe attached to a Finnigan Neptune multicollector ICP-MS at the Institute of Mineral Resources, Chinese Academy of Geological Sciences, Beijing, using techniques and analytical procedures described by Wu et al. (2006) and Hou et al. (2007). The spots were 50–55 μm in diameter. The measured 176Hf/177Hf ratio on the standard zircon (GJ-1) was 0.282009 ± 0.000004 (N = 96), similar to the commonly accepted 176Hf/177Hf ratio of 0.282000 ± 0.000005 measured using the solu-tion method (Morel et al., 2008). For the calculation of initial Hf isotope ratio, the decay constant for 176Lu proposed by Söderlund et al. (2004) was used. For the calculation of εHf(t) values, we adopted the chondritic values of Blichert-Toft and Albarede (1997). Hf model ages (TDM and TDM

C) were calculated on the basis of the depleted mantle model described by Griffi n et al. (2000) and Yang et al. (2006).

ACKNOWLEDGMENTS

This research was fi nancially supported by the National Natural Science Foundation of China (40772144 and 41372230), the International Sci-ence & Technology Cooperation Program of China

(2014DFR21270), and the China Geological Survey (1212011085476). We thank Y.S. Liu, X.M. Liu, C.Y. Diwu, K.J. Hou, C.F. Li, H. Li, and X.D. Jin for their analytical assistance. We are grateful to Inna Safonova , Wenjiao Xiao, and Richard Ernst (GSA Bulletin associate editor) for their thoughtful and thorough reviews that signifi cantly improved the qual-ity of the manuscript. This paper is a contribution to IGCP 592 “Continental Construction in Central Asia.”

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