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Gondwana Research 79 (2020) 263e282

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Early to mid-Paleozoic magmatic and sedimentary records in theBainaimiao Arc: An advancing subduction-induced terrane accretionalong the northern margin of the North China Craton

Yan Chen a, b, c, Zhicheng Zhang b, *, 1, Xiaoyan Qian b, Jianfeng Li d, Zejia Ji b, Tairan Wu b

a Institute for Peat and Mire Research, State Environmental Protection Key Laboratory of Wetland Ecology and Vegetation Restoration, Northeast NormalUniversity, Changchun, 130024, Chinab The Key Laboratory of Orogenic Belts and Crustal Evolution, Ministry of Education, School of Earth and Space Sciences, Peking University, Beijing, 100871,Chinac Key Laboratory of Geographical Processes and Ecological Security in Changbai Mountains, Ministry of Education, School of Geographical Sciences,Northeast Normal University, Changchun, 130024, Chinad Key Laboratory of Paleomagnetism and Tectonic Reconstruction of Ministry of Land and Resources, Institute of Geomechanics, Chinese Academy ofGeological Sciences, Beijing, 100081, China

a r t i c l e i n f o

Article history:Received 4 September 2018Received in revised form7 August 2019Accepted 14 August 2019Available online 4 November 2019

Keywords:Central Asian Orogenic BeltBainaimiao ArcNorth China CratonGeochemistryDetrital zircon dating

* Corresponding author.E-mail address: zczhang@pku.edu.cn (Z. Zhang).

1 Postal address: School of Earth and Space ScienYiheyuan Road, Haidian District, Beijing 100871, PR C

https://doi.org/10.1016/j.gr.2019.08.0121342-937X/© 2019 International Association for Gond

a b s t r a c t

The early to mid-Paleozoic subduction-induced terrane accretion along the northern margin of the NorthChina Craton is not well understood. To address this issue, we investigate the magmatic and sedimentaryrecords, including both new and previously published geochemical, SreNd isotopic, and zircon UePbeHfisotopic data from the Bainaimiao Arc. The collected gabbroediorites and granitoids have been dated to431e453 Ma. The gabbroediorites have high Mg/(Mg þ Fe) molar ratios (44.41e73.39); depleted Nb, Taand Ti; and negative εNd(t) values (-9.43e-6.80). They were derived from a mantle wedge metasomatizedby subduction-derived fluids with crustal contamination. The granitoids are characterized by high silica,low to high K, low Fe and Mg contents, strong fractionation of rare earth elements, and positive εHf(t)values (þ1.42eþ8.19). They were derived from crustal melts with juvenile additions. The clastic rocksfrom the Baoerhantu Group and Xibiehe Formation are dominated by early Paleozoic zircons, whereasthose from the Bainaimiao Group are dominated by early Paleozoic and Precambrian zircons. Detritalzircon geochronology and field geology confirm their deposition in early to mid-Paleozoic. The UePbages and petrographic and geochemical analyses indicate that the clastic rocks were deposited in arc-related basins with felsic sources from the Bainaimiao Arc. The xenocrystic and detrital zircons in themagmatic and clastic rocks, respectively, imply a Precambrian basement for the Bainaimiao Arc. The earlyPaleozoic magmatic rocks of the Bainaimiao Arc show secular changes with decreasing age: increasingK2O contents and Sr/Y ratios and decreasing Fe2O3

T þ MgO contents and εHf(t) and εNd(t) values. This islikely in response to advancing subduction and related crustal thickening. Accordingly, the followingtectono-paleogeographic model was proposed for the Bainaimiao Arc: (a) ~500e455Ma initial sub-duction and juvenile arc development, (b) ~455e415Ma continuous subduction with mature arcdevelopment, and (c) ~415e400Ma accretion to the North China Craton.© 2019 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved.

1. Introduction

Accretionary orogens have occurred worldwide throughout theEarth’s history and formed by oceanic subduction and/or terrane

ces, Peking University, No. 5hina.

wana Research. Published by Else

accretion before or in the absence of continentecontinent collision(Condie, 2007; Cawood et al., 2009). They are usually characterizedby a long lifespan (50 to more than 300 M.y.) in intra-oceanic oractive continental margin settings (Condie, 2007; Cawood et al.,2009). The advancing (e.g., the Andes) and retreating (e.g., thewestern Pacific) types of accretionary orogens are characterized bycrustal thickening and back-arc extension, respectively (Schellartet al., 2007; Cawood et al., 2009; Sharples et al., 2014). Althoughthe crustal structure, driving mechanisms, and crustal evolutionof accretionary orogens have been generally reviewed by

vier B.V. All rights reserved.

Y. Chen et al. / Gondwana Research 79 (2020) 263e282264

Cawood et al. (2009), our understanding of accretionary orogenswould be improved by investigating specific orogens.

The Central Asian Orogenic Belt (CAOB) has been defined as anaccretionary orogen (e.g., Seng€or et al., 1993; Xiao et al., 2015). It issurrounded by the Siberia Craton to the north, the North China andTarim cratons to the south, the East European Craton to the west,and the Pacific Ocean to the east (Fig. 1a; Jahn et al., 2000; Windleyet al., 2007; Han et al., 2010, 2011; Wilhem et al., 2012; Xiao et al.,2015; Zhou et al., 2018). The CAOB was constructed via a series ofPaleo-Asian oceanic subduction, arc migration, and terrane accre-tion events along the margins of the surrounding cratons (Kr€oneret al., 2014, 2017; Windley et al., 2007; Xiao et al., 2015; Zhouet al., 2018). The Paleozoic orogen to the north of the North ChinaCraton is invaluable for understanding the tectonic history ofancient accretionary orogens (Xiao et al., 2003, 2015).

The Bainaimiao Arc, located to the north of the North ChinaCraton (Fig. 1b; Zhang et al., 2014), documents the complexorogenic processes involving its own magmatic arc development,the Paleo-Asian Ocean, Hunshandake block, and northern conti-nental margin of the North China Craton (Xiao et al., 2003; Jianet al., 2008; Xu et al., 2013; Zhang et al., 2014). However, despitethe large amount of geochronological work that has been per-formed, its subduction-accretion processes are not clearly under-stood, particularly those that occurred during the early to mid-Paleozoic. The magmatic, paleogeographic, and crustal evolutionsinvolved in the subduction-accretion processes remain poorlydefined. Based on the early Paleozoic passive northern margin(~0.52e0.42 Ga) of the North China Craton, Zhang et al., 2014 pro-posed a northward subduction beneath the Bainaimiao Arc.

Fig. 1. (a) Tectonic position of the Central Asian Orogenic Belt (CAOB; after Han et al., 2011)

However, reconstruction of the early Paleozoic trenchearcebackarc suggests a southward subduction (Jian et al., 2008; Xu et al.,2013). Furthermore, the Precambrian basement of the Bainaimiaoarc proposed by Zhang et al., 2014 should be confirmed by moredating work. Further, the termination of the Bainaimiao Arc and itsaccretion to the North China Craton should be revealed in a syn-thesized analysis incorporating different lines of magmatic, meta-morphic, and sedimentary evidence.

The Zhurihe and Damaoqi regions (Fig. 1b) are pivotal for un-derstanding the nature and development of the Bainaimiao Arc(Shao, 1989; Xiao et al., 2003; Zhang et al., 2014). The subduction-accretion evolution is well documented in the ophiolitic m�elanges,plutons, volcanic rocks, sediments, andmetamorphic rocks in theseregions (Jian et al., 2008; Gu, 2012; Zhang et al., 2013, Zhang et al.,2014; Chen et al., 2015). The magmatic rocks have received consid-erable attention, and their geochemical and geochronological datahave been abundantly collected (Zhang and Jian, 2008; Tong et al.,2010; Qin et al., 2013; Liu et al., 2014; Bai et al., 2015). However,few studies have systematically incorporated as many of thesepublished data as possible. Although the clastic rocks in these re-gions are records of their source characteristics and are importantfor tectono-paleogeographic reconstruction (Gehrels et al., 2011;Gehrels, 2014), they have been investigated in less detail. In thisstudy,we report newzirconUePb ages,Hf isotopic data, whole-rockgeochemistry, and SreNd isotopic data of the early tomid-Paleozoicmagmatic and clastic rocks in the Zhurihe and Damaoqi regions.After incorporating a body of published geochemical and geochro-nological data, we examine the subduction-induced terrane accre-tion along the northern margin of the North China Craton.

. (b) Tectonic framework of the southeastern CAOB (modified after Chen et al., 2016a).

Y. Chen et al. / Gondwana Research 79 (2020) 263e282 265

2. Tectonic background

The North China Craton (Fig. 1b) is characterized by an Archeanto Paleoproterozoic crystalline basement that mainly formedaround 2.5 Ga and 1.85 Ga and a Paleoproterozoic to Phanerozoicsedimentary cover (Zhao et al., 2002, 2012; Kusky and Li, 2003). TheNorth China Craton entered the Paleo-Asian Ocean regime after itsfinal breakup during ~1.32e1.35 Ga from the Columbia supercon-tinent (Zhang et al., 2009, Zhang et al., 2017). The detrital zirconreference proposed by Darby and Gehrels (2006) indicates nomagmatic activity in the North China Craton associated with theGrenville event during ~0.85e1.0 Ga; therefore, this craton isdistinguishable from the Tarim Craton and the microcontinents inthe CAOB (Rojas-Agramonte et al., 2011). No early Paleozoicmagmatic activity is evidenced in the northern margin of the NorthChina Craton, which is considered as a passive margin (Zhang et al.,2014). A belt of Devonian alkaline rocks has been identified (Shiet al., 2010; Zhang et al., 2010b, 2010c). In addition,PermoeCarboniferous magmatic rocks exhibit arc-like or post-collisional geochemical features (Zhang et al., 2010c).

The Bainaimiao Arc is located to the north of the North ChinaCraton; they are separated by the ChifengeBayan Obo fault (Fig.1b).To the north of the Bainaimiao Arc, the Ondor Sum accretionarycomplex records the early Paleozoic consumption of the Paleo-Asian Ocean (Xiao et al., 2003; Jian et al., 2008; Xu et al., 2013). Inthe Bainaimiao Arc, abundant early to mid-Paleozoic magmaticrocks are exposed and have been dated at ~518e400Ma (Liu et al.,2013; Chen et al., 2016a, and references therein). Based on theXuniwusu Formation flysch deposits, Hu et al. (1987) proposed aback-arc basin between the Bainaimiao Arc and the North ChinaCraton. However, based on the ophiolites preserved in the Wudeand HarihadeeChegendalai areas, Zhang et al., 2014 proposed awide ocean, denoted as the South Bainaimiao Ocean, on thenorthern passive margin of the North China Craton.

The Hunshandake block, located north of the Bainaimiao Arc(Fig. 1b), is the western component of the SongliaoeHunshandakemicrocontinent (Xu et al., 2013, 2014, 2015, 2016). Researchers havereported ~1.81e1.87 Ga-old magmatic rocks in the easterncomponent (Wang et al., 2006; Pei et al., 2007), thus indicating theexistence of a Precambrian basement. However, the westerncomponent is almost completely buried by the Hunshandakedesert. The Paleo-Asian Ocean existed in the Solonker Zone, lastinguntil the late Permian (Xiao et al., 2003) or forming a new oceanduring the late Paleozoic (Song et al., 2015; Luo et al., 2016a).

The Sonidzuoqi Arc, Xilinhot block, Uliastai continental margin,Hutag Uul block, Enshoo terrane and Nuhetdavaa terrane arelocated to the north of the Solonker Zone (Fig. 1b) and wereinvolved in northward subduction-accretion of the Paleo-AsianOcean (Badarch et al., 2002; Xiao et al., 2003; Xu et al., 2013;Chen et al., 2016). The Hegenshan ophiolite belt was involved in thelate Paleozoic tectonic evolution of the Paleo-Asian Ocean (Zhanget al., 2015; Yang et al., 2017).

3. Regional geology

3.1. Zhurihe region

The Zhurihe region is divisible into three domains: the OndorSum accretionary complex, Bainaimiao Arc, and Xuniwusu Forma-tion flysch deposits in the north, middle, and south, respectively(Fig. 2a; Luo et al., 2016). The early Paleozoic Ondor Sum Groupcomprises metabasalts, gabbros, ferrosilicate rocks, and limestonelenses in its lower section, and quartz schist, ferruginous quartzite,quartzite and phyllite interbedded with greenschist and marblelenses in its upper section (IMBG, 1975; Li et al., 2012; Xu et al.,2016). This group is widely considered to represent an

accretionary complex (Xiao et al., 2003; Jian et al., 2008) with amixture of oceanic relics (e.g., Tulinkai ophiolites; Liu et al., 2003),trenchearc sediments and arc volcanic rocks (Li et al., 2012). Recentdating has confirmed that this group was formed in the earlyPaleozoic (~500e415Ma; Xu et al., 2016), and abundant Precam-brian zircons have been found in the meta-sediments (Li et al.,2012; Xu et al., 2016). The 40Ar/39Ar plateau ages of the quartzitemylonites are ~449e453Ma (de Jong et al., 2006).

The early Paleozoic plutons of the Bainaimiao Arc comprisehornblende gabbro, diorite, quartz diorite, granodiorite, tonalite,and granite (e.g. Fig. 3a and b; Xiao et al., 2003; Zhang and Jian,2008; Gu, 2012). Meanwhile, the early Paleozoic arc-related vol-cano-clastic rocks of the Bainaimiao Group comprise volcanic rocksand breccia alongside variably deformed tuff, tuffaceous sandstone,and mudstone (IMBG, 1975; Hu et al., 1987). Most of these plutonicand volcanic rocks were formed during ~500e410Ma (Nie et al.,1995; Jian et al., 2008; Tong et al., 2010; Gu, 2012; Zhang et al.,2013; Liu et al., 2014; Bai et al., 2015; Qian et al., 2017).

The Silurian Xuniwusu Formation unconformably overlies theearly Paleozoic volcano-clastic rocks of the Bainaimiao Group (Huet al., 1987; Zhang et al., 2017). This formation comprises gravel-bearing coarse-grained sandstone and medium-to fine-grainedsandstone interbedded with siltstone, mudstone, and limestone inthe lower section, and fine sandstone, siltstone, and sandymudstone in the upper section. Thus, the formationwas consideredto be flysch deposits formed in a submarine delta to neritic envi-ronment (Hu et al., 1987). The early Paleozoic plutons, volcano-clastic rocks, and Xuniwusu Formation are unconformably over-lain by the upper Silurian to Devonian Xibiehe Formation molassedeposits (Hu et al., 1987; Zhang et al., 2017).

The widespread PermoeCarboniferous plutons comprise quartzdiorite, granodiorite and granite (Lu et al., 2009; Hao, 2011; Jianget al., 2013; Wang, 2014). The late Carboniferous Amushan For-mation consists mainly of sandstone and siltstone with variabledegrees of deformation and are intruded by the Gutaole pluton(Fig. 2b and c; Fig. 3c and d; IMBG,1975). Based on our dating work,which reassigned the Amushan Formation to the early PaleozoicBainaimiao Group, we refer to the Amushan Formation as “theformer Amushan Formation”. The Permian strata are mainlycomposed of sandstone, slate, limestone lenses and andesite por-phyrite (IMBG, 1975), and were deposited in a coastal to shallow-marine environment (Luo et al., 2016).

3.2. Damaoqi region

In the Damaoqi region, the Bainaimiao Arc contacts with theNorth China Craton along the Wulanbulage fault (Fig. 4a). The earlyPaleozoic tectonic m�elanges with blocks of mafic/ultramafic rocks,granitoids and clastic rocks are exposed along this fault (Jia et al.,2003; Shang et al., 2003; Zhang and Jian, 2008; Zhang et al.,2014). The ultramafic rocks from the Wude tectonic m�elange yiel-ded a SmeNd isochron age of 409Ma and are overlain by upperDevonian deposits (Zhang and Wu, 1999). TheHarihadeeChegendalai ophiolites with similar compositions arethe eastern extension of the Wude ophiolites, both of which areintruded by late Paleozoic plutons (Zhang et al., 2014).

The early Paleozoic plutons mainly comprise granite, diorite,and granitic diorite (IMBG, 1968). The Bailiutu pluton dated to447e453Ma by Li et al. (2010), was selected for further whole-rockgeochemical and SreNd isotopic analyses in this study. The earlyPaleozoic volcano-clastic rocks of the Baoerhantu Group compriseandesitic porphyrite, volcanic tuff, tuffaceous siltstone and sand-stone (IMBG, 1968; Zhang and Jian, 2008). Deformed volcanicbreccia with large clasts (Fig. 5a) and mudstone lenses/interlayerswith clear crossbedding in the volcanic rocks (Fig. 5b) have beenfound. These arc-related volcanic rocks and deposits are associated

Fig. 2. (a) Geology of the Zhurihe region (modified after Luo et al., 2016; Qian et al., 2017) with (b) the dated sample locations and (c) a cross-section for the early PaleozoicBainaimiao Group (formerly the Late Carboniferous Amushan Formation).

Y. Chen et al. / Gondwana Research 79 (2020) 263e282266

with the Bainaimiao Group in the Zhurihe region (Shao, 1989).From bottom to top, the upper Silurian Xibiehe Formation

comprise conglomerate, sandy conglomerate, gravel-bearingcoarse-grained sandstone (Fig. 5c), limestone and moderate tofine-grained sandstone (Fig. 4b and c; IMBG, 1968). This

formation has been widely considered to represent shallowmarine molasse deposits (Zhang et al., 2004, Zhang et al., 2010).It unconformably overlies early Paleozoic volcano-clastic rocks,arc plutons and tectonic m�elange (Zhang et al., 2004, Zhanget al., 2010).

Fig. 3. Field photographs and photomicrographs of the magmatic and clastic rocks of the Zhurihe region. The field photographs show (a), (b) arc plutons and (c), (d) the sandstone ofthe Bainaimiao Group (formerly the Amushan Formation); (d) shows intrusion by the arc plutons. The photomicrographs show (e) gabbroic diorite, (f) granite, and (g) granodioritefrom the arc plutons and (h) quartz micaschist and (i) sandstone from the Bainaimiao Group (formerly the Amushan Formation). Mineral abbreviations: Py for pyroxene; Hb forhornblende; Pl for plagioclase; Q for quartz; Ms for muscovite; and Lv for volcanic lithic fragments.

Y. Chen et al. / Gondwana Research 79 (2020) 263e282 267

4. Sampling

4.1. Zhurihe region

Nine magmatic samples were collected from the Hudugetu,Gutaole and Wulanhada plutons in the Zhurihe region (Fig. 2b;Table 1), including two gabbroic diorite to diorite samples andseven granite to granodiorite samples (Fig. 3a and b). The gabbroicdiorite to diorite samples consist of ~50% plagioclase, ~35% horn-blende and ~10% pyroxene (e.g., Fig. 3e). The granite to granodioritesamples consist of 45e50% plagioclase, 25e30% quartz and 10e15%alkali feldspar (e.g., Fig. 3f and g).

Five sandstones and two quartz micaschists (Table 1) weresampled from the former Amushan Formation (Fig. 3c). The quartzmicaschists consist of ~75e80% quartz, ~20% mica and ~5%plagioclase (Fig. 3h). The quartz is clear but small in diameter,ranging from 0.1 to 0.2mm. The mica is long and striped withdirectional alignment. The sandstones consist of ~50% quartz,~10e15% feldspar, ~20e30% lithic fragments (mostly volcanic rockswith minor metamorphic rocks), and ~10% muscovite (Fig. 3i).Monocrystalline quartz dominates the total quartz fraction.

4.2. Damaoqi region

Eleven magmatic samples, including gabbro to diorite, werecollected from the Bailiutu pluton in the Damaoqi region (Table 1).Their whole-rock geochemical and SreNd isotopic analyses wereconducted in this study.

Seven samples of clastic rocks were collected from the sand-stone/siltstone/mudstone interlayers in the early Paleozoic Baoer-hantu Group (Fig. 4b; Table 1). Samples NM15e161,165, and 168 arelithic sandstones (e.g., Fig. 5d) consisting of ~30e40% quartz,~10e20% feldspar, and ~30e50% lithic fragments. The grain shapesare angular to subangular. Samples NM15e156, 160, 170, and 172are fine-grained clastic rocks ranging from siltstone to mudstone(e.g., Fig. 5e). Six samples were collected from the upper Silurian tolower Devonian Xibiehe Formation (Fig. 4b; Table 1). These rocksare mainly arkose and consist of ~50e70% quartz, ~20e30% feld-spar, and ~10e30% lithic fragments (Fig. 5f).

5. Results

Themethods used in this study are described in the Appendix A.

5.1. Magmatic rocks

5.1.1. Zircon records and SreNd isotopic dataFive magmatic rock samples (NM16e15, 16, 18, 24 and 27)

collected from the Hudugetu and Gutaole plutons in the Zhuriheregion were selected for LAeICPeMS zircon UePb isotopic ana-lyses. The numbers of total and concordant analyses, age range, andweighted mean age of each sample are listed in Table 2. TheLAeICPeMS zircon UePb isotopic data of the magmatic rocks arepresented in Tables S1 of the Appendix A. The zircons from samplesNM16e15 and 16 are mostly euhedral to subhedral with clearoscillatory zoning (Fig. 6a and b), and those from samples

Fig. 4. (a) Geology of the Damaoqi region (modified after Li et al., 2010; Zhang et al., 2014) with (b) the dated sample locations and (c) a cross-section for the upper Silurian toDevonian Xibiehe Formation.

Y. Chen et al. / Gondwana Research 79 (2020) 263e282268

NM16e18, 24, and 27 are more or less rounded with little to poorzoning (Fig. 6cee). The Th/U ratios of the zircons from these fivesamples range from 0.31 to 1.06, usually exceeding 0.5, which in-dicates their magmatic origin (Hoskin and Schaltegger, 2003).Sample NM16e15 yielded a weighted mean age of 436± 2Madetermined from 28 concordant analyses from a total of 30 analyses

(Fig. 7a). Sample NM16e16 has a weighted mean age of 440± 2Madetermined from 24 concordant analyses from a total of 30 analyses(Fig. 7b). Sample NM16e18 has a weighted mean age of 431± 2Madetermined from 21 concordant analyses from a total of 27 analyses(Fig. 7c), although two separated youngest zircons with ages of245Ma and 286Mawere probably affected by later thermal events.

Fig. 5. Field photographs and photomicrographs of the volcanic and clastic rocks in the Damaoqi region. The field photographs show (a) volcanic breccia with large clasts, and (b)volcanic rocks interbedded with mudstones from the Baoerhantu Group, and (c) gravel-bearing coarse sandstone from the Xibiehe Formation. The photomicrographs show (d) lithicsandstone and (e) siltstone from the Baoerhantu Group and (f) arkose from the Xibiehe Formation. Mineral abbreviations: Pl for plagioclase; Q for quartz; Ms for muscovite; and Lvfor volcanic lithic fragments.

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Sample NM16e24 yielded a weighted mean age of 441± 4Madetermined from 10 concordant analyses from a total of 22 analyses(Fig. 7d). Sample NM16e27 has a weighted mean age of 444± 2Madetermined from 28 concordant analyses from a total of 30 analyses(Fig. 7e).

Three magmatic rock samples (NM16e15, 16 and 27) collectedin the Zhurihe region were selected for LAeMCeICPeMS zirconLueHf isotopic analyses. The results are listed in Table S2 of theAppendix A. Samples NM16e15, 16, and 27 have εHf(t) valuesof þ1.42 to þ4.18, þ2.93 to þ5.36, and þ3.57 to þ8.19, respectively,and model ages (TDM2) of 1153e1329Ma, 1082e1236Ma, and905e1199Ma, respectively. Four gabbroediorite samples(NM08e14, 16, 17 and 18) collected from the Bailiutu pluton in theDamaoqi region, were selected for whole-rock SreNd isotopic an-alyses. The SreNd isotopic data for the magmatic rocks are listed inTable S3 of the Appendix A. The four samples yielded negativeεNd(t) values of �9.43 to �6.80 and model ages (TDM) of1131e1357Ma.

5.1.2. Geochemical dataThe major and trace element concentrations in the magmatic

rocks are listed in Table S4 of the Appendix A.The gabbroediorites yielded low to moderate amounts of SiO2

(48.52e57.77%), K2O (0.32e2.01%), and TiO2 (0.45e0.84%), low tohigh amounts of Na2O (0.33e4.85%), and abundant Fe2O3

T

(7.58e11.59%), MgO (3.38e15.18%), Al2O3 (7.39e18.90%) and CaO(5.61e14.18%). The Na2O þ K2O versus SiO2 diagram (Middlemost,1994) defined these rocks as gabbro, gabbroic diorite, diorite, andmonzoediorite (Fig. 8a). The K2O versus SiO2 diagram (Peccerilloand Taylor, 1976) showed that the gabbroediorites belonged tolow-to high-K series (Fig. 8b). The gabbroediorites are metal-uminous to peraluminous with Al2O3/(CaO þ Na2O þ K2O) ratios of0.51e1.57 and Al2O3/(Na2O þ K2O) ratios of 1.95e12.52. They havehigh Mg/(Mg þ Fe) molar ratios of 44.41e73.39. Thesegabbroediorites have mostly low to moderate rare earth element(REE) contents of 32.20e110.51 ppm and are mostly slightly tomoderately enriched in light REEs (LREEs) relative to heavy REEs

(HREEs) [(La/Sm)N and (La/Yb)N ratios of 0.98e3.26 and 1.42e6.36,respectively; Fig. 8c]. Further, slightly positive to negative Euanomalies with Eu* values of 0.87e1.07 were detected. Thesegabbroediorites are depleted in Nb, Ta and Ti, slightly depleted inZr, and enriched in Hf (Fig. 8d). Their Sr/Y ratios were low tomoderate (11.62e47.72), and their Y contents were11.70e22.40 ppm.

The granitoids are abundant in SiO2 (64.18e76.11%) and Al2O3(13.40e17.37%), moderately abundant in Na2O (typically2.52e4.53%), CaO (0.2e4.53%) and K2O (1.33e5.33%), and low inFe2O3

T (0.48e5.35%) and MgO (0.08e2.20%). The granitoids belongto the granodiorite to granite category in the Na2O þ K2O versusSiO2 diagram (Middlemost, 1994, Fig. 8a) and moderate-to high-Keven shoshonitic series in the K2O versus SiO2 diagram (Peccerilloand Taylor, 1976, Fig. 8b). These granitoids are peraluminous withAl2O3/(CaO þ Na2O þ K2O) ratios of 1.46e2.92 and Al2O3/(Na2O þ K2O) ratios of 1.75e3.25. They yielded low Mg/(Mg þ Fe)molar ratios of 23.27e44.89. Furthermore, these granitoids havemoderate to high REE contents (56.96e116.50 ppm) and highlyfractionated REE patterns [(La/Sm)N and (La/Yb)N ratios of3.93e6.18 and 5.63e17.66, respectively; Fig. 8c]. Slightly negative topositive Eu anomalies with Eu* values of 0.78e1.27 were detected.These granitoids are obviously depleted in Nb, Ta and Ti butenriched in Zr and Hf (Fig. 8d). They have variable Sr/Y ratios of10.94e108.51 and low to moderate Y contents of 6.11e29.50 ppm.

5.2. Clastic rocks

5.2.1. Detrital zircon recordsSamples NM16e20 and 25 collected from the former Amushan

Formation in the Zhurihe region, and samples NM15e165, 174 and176 collected from the Baoerhantu Group and the Xibiehe Forma-tion in the Damaoqi region, were selected for LAeICPeMS zirconUePb isotopic analyses. The numbers of total and concordant an-alyses, age groups, and main peak ages of each sample are shown inTable 2. Their LAeICPeMS zircon UePb isotopic data are presentedin Table S5 of the Appendix A. The detrital zircons from the five

Table 1Summary of the sampling locations, units, rock types and analytical methods of all the samples.

Sample No. Location Units Rock type Methods

Zhurihe regionNM16e15 42�15053.800N, 112�51027.700E Hudugetu pluton Granodiorite GA, UePb, HfNM16e16 42�17016.600N, 112�51020.300E Granite GA, UePb, HfNM16e17 Granodiorite GANM16e18 42�18042.900N, 112�5209.500E Gabbroic diorite GA, UePbNM16e19 42�18036.700N, 112�52020.900E Diorite GANM16e23 42�17022.300N, 112�5906.500E Gutaole pluton Granodiorite GANM16e24 42�17033.100N, 112�59015.400E Granodiorite GA, UePbNM16e27 42�17059.700N, 113�00015.100E Granite GA, UePb, HfNM16e28 42�26010.300N, 113�38040.700E Wulanhada pluton Granodiorite GANM16e22 42�17033.100N, 112�59015.400E Bainaimiao Group (formerly

the Amushan Formation)Meta-sandstone GA

NM16e25 42�17036.000N, 112�59018.600E Meta-sandstone GA, UePb, HfNM16e30 Meta-siltstone GANM16e31 Meta-siltstone GANM16e32 Meta-siltstone GANM16e20 42�18048.300N, 112�51044.700E Quartz micaschist GA, UePb, HfNM16e21 42�18048.300N, 112�51044.700E Quartz micaschist GADamaoqi regionNM08e12 From Li et al., 2010 Bailiutu pluton Diorite GANM08e13 Gabbroic diorite GANM08e14 Gabbroic diorite GA, SreNdNM08e15 Gabbroic diorite GANM08e16 Gabbro GA, SreNdNM08e17 Gabbroic diorite GA, SreNdNM08e18 Gabbroic diorite GA, SreNdNM08e19 Diorite GANM08e20 Gabbro GANM08e21 Gabbro GANM08e22 Gabbro GANM15e156 41�59048.400N, 110�9045.600E Baoerhantu Group Mudstone GANM15e160 41�59057.600N, 110�9054.900E Siltstone GANM15e161 42�000.300N, 110�9048.800E Lithic sandstone GANM15e165 42�0015.200N, 110�1000.000E Lithic sandstone GA, UePbNM15e168 42�0024.100N, 110�10026.400E Lithic sandstone GANM15e170 42�0031.000N, 110�10034.900E Siltstone GANM15e172 42�0033.100N, 110�10041.400E Siltstone GANM15e173 41�58050.500N, 110�605.000E Xibiehe Formation Arkose GANM15e174 41�58050.500N, 110�605.000E Arkose GA, UePbNM15e175 41�58021.700N, 110�602.800E Arkose GANM15e176 41�58022.200N, 110�602.200E Arkose GA, UePb, HfNM15e178 41�58024.100N, 110�5055.200E Arkose GANM15e180 41�58025.400N, 110�5052.400E Quartz sandstone GA

Note: GA: geochemical analysis, UePb: zircon UePb isotopic analysis, Hf: zircon Hf isotopic analysis, SreNd: whole-rock SreNd analysis.

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clastic rocks are mostly euhedral to subhedral with clear zoning(Fig. 6fej), indicating a magmatic origin. The zircons from samplesNM16e20 and 25 showed similar features. Some of the earlyPaleozoic zircons are cracked, and some Precambrian zircons arerounded. The zircons from sample NM15e165 exhibited the clear-est and widest zoning. The zircons from samples NM15e174 and176 are similar, exhibiting both obvious zoning and no zoning.Approximately 66%, 22% and 11% of the valid zircons yielded Th/Uratios of�0.5, 0.4e0.5, and 0.1e0.4, respectively. Only three zirconswith Paleoproterozoic ages yielded Th/U ratios below 0.1. These Th/U ratios support a predominantly magmatic derivation of the zir-cons (Hoskin and Schaltegger, 2003).

The Precambrian and early Paleozoic zircons each comprisedhalf of samples NM16e20 and NM16e25 (Fig. 9a and b), whichwere collected from the Zhurihe region. The zircon ages for sampleNM16e20 (Fig. 9a) were divided into five clusters: (a) 408e493Ma,with a dominant peak at 460Ma and secondary peaks at 414Maand 490Ma; (b) 553e604Ma, with a peak at 590Ma; (c)779e1394Ma with peaks at 1011Ma and 1339Ma; (d)1500e1651Ma, with a peak at 1546Ma; and (e) 1739e2713Ma,with a peak at 2479Ma. The zircon ages of sample NM16e25(Fig. 9b) were grouped as follows: (a) 438e523Ma, with a domi-nant peak at 456Ma and a secondary peak at 512Ma; (b)557e704Ma; (c) 1008e1205Ma, with a peak at 1120Ma; (d)1323e1586Ma, with a peak at 1466Ma; and (e) 1746e2716Ma,

with a peak at 1918Ma. In samples NM15e165, NM15e174 andNM15e176, which were collected in the Damaoqi region, all of thedetected zircons date from the early Paleozoic (Fig. 9cee). The mostsignificant age peaks of samples NM15e165, 174, and 176 are480Ma, 461Ma and 453Ma, respectively.

Samples NM16e20, NM16e25 and NM15e176 were selected forLAeMCeICPeMS zircon LueHf isotopic analyses. The results arelisted in Table S6 of the Appendix A. Samples NM16e20 and 25,collected in the Zhurihe region, have similar εHf(t) distributions.Their εHf(t) values are all negative at >1700Ma, all positive at~1250e1650Ma, both positive and negative at ~900e1150Ma,nearly all negative at ~550e750Ma, and mostly positive at~410e520Ma. Their model ages (TDM2) are 698e3429Ma. SampleNM15e176, collected in the Damaoqi region, yielded all positiveεHf(t) values between þ5.19 and þ 8.95 with young model ages(TDM2) of 861e1100Ma.

5.2.2. Geochemical dataThe major and trace element concentrations of the clastic rocks

are listed in Table S7 of the Appendix A.The mica-quartzose schists (samples NM16e20 and 21) are

quite high in SiO2, moderate to low in Al2O3, K2O and Na2O, and lowin other major oxides. Their parent rocks are classified as clasticrocks on a P2O5/TiO2 vs. MgO/CaO diagram (not shown; Werner,1987), which is consistent with their petrographic features and

Table 2Summary of the zircon UePb and Hf isotopic analyses of the dated samples with interpretations.

Sample No. Region Dated units Rock type Zircon UePb isotopic analyses εHf(t) Interpretation

Totalanalyses

Concordantanalyses

Age rangs(groups)/Ma

WeightedMean ages(main peakages)/Ma

Magmatic rocksNM16e15 Zhurihe Hudugetu

plutonGranodiorite 30 28 429e449 436± 2

(MSWD¼ 1.01)1.42e4.18 arc setting

NM16e16 Granite 30 24 438e450 440± 2(MSWD¼ 0.73)

2.93e5.36

NM16e18 Gabbroicdiorite

27 21 424e439 431± 2(MSWD¼ 0.55)

NM16e24 Gutaole pluton Granodiorite 22 10 432e466 441± 4(MSWD¼ 0.76)

NM16e27 Granite 30 28 437e448 444± 2(MSWD¼ 0.36)

3.57e8.19

Clastic rocksNM16e20 Zhurihe Bainaimiao

Group(formerly theAmushanFormation)

Quartzmicaschist

75 74 408e493, 553e604, 779e1394, 1500e1651, 1739e2713

414, 460, 490,590, 1011,1339, 1546,2479

�26.55e23.13 arc andPrecambrianbasementderived

NM16e25 Meta-sandstone

72 70 438e523, 557e704, 1008e1205, 1323e1586, 1746e2716

456, 512, 1120,1466,1918

�10.50e11.90

NM15e165 Damaoqi BaoerhantuGroup

Lithicsandstone

50 49 431e506 433, 445, 480 arc derived

NM15e174 XibieheFormation

Arkose 77 77 412e506 461, 505NM15e176 Arkose 75 74 409e500 453 5.19e8.95

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zircon age distribution. The sandstones are abundant in SiO2(71.22% on average) and Al2O3 (13.56% on average), moderatelyabundant in Fe2O3

T (4.96% on average), and moderate to low in K2O(2.41% on average), CaO (1.58% on average), MgO (1.28% on

Fig. 6. CL images for zircons from the magmatic and clastic rocks. The 206Pb/238U

average), and Na2O (1.38% on average). The Xibiehe Formationclastic rocks have the highest SiO2 (74.64% on average) and CaO(3.13% on average). Meanwhile, the Bainaimiao Group clastic rockshave the highest Fe2O3

T þ MgO (8.39% on average) and K2O (3.96%

age for each zircon is shown. The black bar in each figure is 100 mm long.

Fig. 7. Zircon UePb isotopic concordia plots and weighted mean ages for the collected magmatic rocks. The numbers (represented by n) of concordant analyses used to calculate theweighted mean age and total analyses for each sample are shown in the figure.

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on average). The Baoerhantu Group clastic rocks have the highestAl2O3 (15.08% on average) and Na2O (2.23% on average). These rockshave been identified as litharenite, wacke, shale, Fe-shale, and Fe-sandstone with low maturity based on log (Fe2O3/K2O) versus log(SiO2/Al2O3) plots (not shown; Pettijohn et al., 1972).

The clastic rocks are mostly enriched in LREEs with flat HREEpatterns (Fig. 10a) and their Eu anomalies are slightly negative orabsent. These rocks are depleted in Nb, Ta and Ti (Fig. 10b). The REEand trace element patterns of most of the samples are similar to

those of the upper crust (Fig. 10c and d), but sample NM15e156 isdepleted in LREEs relative to HREEs and highly depleted in Ce.

6. Discussion

6.1. Petrogenesis of the magmatic rocks

The sampled 431e453Ma gabbroediorites, including samplesNM08e12 to 22 from the Bailiutu pluton in the Damaoqi region and

Fig. 8. (a) Na2O þ K2O versus SiO2 diagram (after Middlemost, 1994), (b) K2O versus SiO2 diagram (after Peccerillo and Taylor, 1976), (c) REE patterns, and (d) trace-element patternsfor the collected magmatic rocks. The parameters of C1 Chrondrite and Primitive Mantle for REE and trace-element normalization are after Sun and McDonough (1989).

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samples NM16e18 and 19 from the Hudugetu pluton in the Zhuriheregion, yielded moderate to high Fe2O3

T þ MgO contents(11.58e26.10%) and high Mg/(Mg þ Fe) molar ratios (44.41e73.39).This indicates that mantle-derived magmas, likely from a mantlewedge, were involved in their source (Rapp andWatson, 1995). Thelow to moderate REE fractionation and slight Eu anomalies (Fig. 8c)suggest that the middle to lower crust re-melted. The Nb, Ta and Tidepletions indicate the metasomatism of melted mantle by fluidsfrom the subducted slab. However, the variable K2O indices (Fig. 8b)indicate some crustal contamination. The La/Nb ratios of thegabbroediorites (0.81e3.41) overlap that of the primitive mantle(0.96; Sun and McDonough, 1989) and those of the lower, middle,and upper continental crust (1.6, 2.4 and 2.8, respectively; Rudnickand Gao, 2003), further indicating some degree of crustal assimi-lation. Furthermore, the gabbroediorites collected in the Damaoqiregion yielded negative εNd(t) values of �9.43 to �6.80 and modelages (TDM) of 1131e1357Ma, indicating crustal recycling or a highdegree of crustal contamination in the basic melts. Therefore, thesegabbroediorites were derived from the mantle wedge metasom-atized by subduction-related fluids and were partially contami-nated by the crust when the magma ascended.

The collected 436e441Ma granitoids, including samplesNM16e15 to 17 from the Hudugetu pluton, NM16e23, 24 and 27from the Gutaole pluton, and NM16e28 from the Wulanhadapluton in the Zhurihe region, have high silica, low to high K and lowFe andMg contents. These samples were likely derived from crustalmelts. Their similar REE and trace element patterns indicate similarparent magmas. These rocks are enriched in LREEs and relativelydepleted in HREEs (Fig. 8c), with depletion in Nb, Ta and Ti (Fig. 8d);

thus, they are similar to arc-related magmatic rocks. The relativedepletion in MREEs (Fig. 8c) indicates the crystallization of horn-blende. This finding and the slight Eu anomalies of the granitoidsindicate their likely derivation from the middle crust. Furthermore,all the zircons from the granitoids yielded positive εHf(t) values of1.42e8.19 and moderate to young model ages of 905e1329Ma.These results indicate abundant juvenile additions mixed with theMesoeNeoproterozoic crustal materials. Therefore, these granit-oids were sourced from mixed crustemantle, likely in an arcsetting.

6.2. Depositional age and provenance of the clastic rocks

In the Zhurihe region, sandstone and quartz micaschist werecollected from the former Amushan Formation. The youngestgrouping of five zircons from sample NM16e25 has a weightedmean age of 440± 5Ma, indicating their deposition after 440Ma(Dickinson and Gehrels, 2009). These sediments were intruded bythe Gutaole pluton (Fig. 3d), fromwhich the granitic diorite sampleNM16e24 was dated at 441± 4Ma. Therefore, the sandstone musthave been deposited at ~440Ma (within the age error). The quartzmicaschist, sample NM16e20, has an age pattern very similar tothat of sample NM16e25, indicating that they have a similarprovenance and depositional age. However, the youngest peak ageof the micaschist (414Ma) is consistent with a 414Ma meta-morphic event (Liu et al., 2003), which probably affected themicaschist after deposition of the parent rocks. Therefore, thesandstone and quartz micaschist were deposited during the earlySilurian and have been ascribed as early Paleozoic volcano-clastic

Fig. 9. Zircon age distributions for the (aee) clastic rocks, (f) Bainaimiao Arc (data for magmatic rocks are from Liu et al., 2003; Jian et al., 2008; Li et al., 2010; Gu, 2012; Hao andHou, 2012; Li et al., 2012b; Zhang et al., 2013; Wang, 2014; Zhang et al., 2014; Bai et al., 2015; Qian et al., 2017 and this study), and (g) North China Craton (after Rojas-Agramonteet al., 2011).

Y. Chen et al. / Gondwana Research 79 (2020) 263e282274

rocks of the Bainaimiao Group in the Zhurihe region. In the Dam-aoqi region, sample NM15e165 from the lower Paleozoic volcano-clastic rocks of the Baoerhantu Group yielded a weighted mean ageof 432± 6Ma for the youngest three zircons, indicating depositionafter the early Silurian. The dacite and andesite from the Baoer-hantu Group have been dated to 518± 3Ma and 474± 5Ma,respectively (Zhang et al., 2014). The clastic rocks are interbeddedwith the volcanic rocks and were likely deposited simultaneouslywith the volcanic rocks during the early Paleozoic. SamplesNM15e174 and 176 from the Xibiehe Formation with the youngestzircons dated at ~410e420Ma, were deposited after the earlyDevonian. The Xibiehe Formation unconformably overlies earlyPaleozoic volcano-clastic rocks, arc plutons, and tectonic m�elange,and is intruded by late Paleozoic plutons (Zhang et al., 2004, Zhanget al., 2010). Accordingly, the Xibiehe Formation was most likelydeposited during the Devonian.

The lithic sandstones from the Baoerhantu Group and arkosesfrom the Xibiehe Formation in the Damaoqi region are texturallyimmature with weak sorting and rounding (e.g., Fig. 5d, f), indi-cating the fast exhumation of the source rocks and short trans-portation of sediments. The lithic sandstones comprise ~30e40%

quartz, ~10e20% feldspar and ~30e50% lithic fragments, indicatingtheir derivation from both volcanic and plutonic rocks. The arkosescomprise ~50e70% quartz, ~20e30% feldspar and ~10e30% lithicfragments, indicating their derivation dominantly from plutonicrocks and subordinately from volcanic rocks. The sandstones fromthe Bainaimiao Group in the Zhurihe region are variable in sorting,indicating the transportation of sediments over various distances.They comprise ~50% quartz, ~10e15% feldspar, ~20e30% lithicfragments and ~10% muscovite, indicating their derivation fromvolcanic and plutonic rocks with later deformation. Themica quartzschist from the Bainaimiao Group has a zircon age spectrumwith awide range and multiple peaks, indicating clastic rocks as parentrocks. The composition of dominant quartz indicates the derivationof felsic rocks. The geochemical characteristics of sediments canreveal their source rocks and tectonic settings (Bhatia, 1983; Bhatiaand Crook, 1986). High silica and aluminum contents, moderate tolow Fe and Mg contents, and mostly upper crust-like REE patternsindicate predominantly felsic sources. Furthermore, the presentclastic rocks have low Hf contents and La/Th ratios, which is similarto the upper crust and felsic sources (Fig. 11). In the tectonicdiscrimination triangular diagrams of LaeTheSc and TheSceZr/10

Fig. 10. Trace element characteristics for the clastic rocks in the Bainaimiao Arc with parameters after Sun and McDonough (1989) and Taylor and McLennan (1985).

Y. Chen et al. / Gondwana Research 79 (2020) 263e282 275

(Bhatia and Crook, 1986, Fig. 12a and b), the clastic rocks from theBaoerhantu Group fell within the domains of oceanic island arcsand continental arcs; those from the Bainaimiao Group fell withinthe field of continental arcs; and those from the Xibiehe Formationfell within the field of active continental margins. This indicates atransition from arcs to continental margins.

Detrital zircon UePb ages and Hf isotopes are robust tracers ofsource blocks (Gehrels, 2014). The clastic rocks from the Baoer-hantu Group and Xibiehe Formation are dominated by earlyPaleozoic zircons, and those from the Bainaimiao Group containboth early Paleozoic and Precambrian zircons (Fig. 9). The

Fig. 11. La/Th versus Hf source discrimination for the clastic rocks (after Floyd andLeveridge, 1987).

Sonidzuoqi arc, Xilinhot block, and other terranes to the north ofthe Paleo-Asian Ocean were not amalgamated with the BainaimiaoArc when the clastic rocks were deposited (Xiao et al., 2003; Jianet al., 2008; Zhang et al., 2014); therefore, they are excluded asthe provenance of the clastic rocks. Further, the North China Cratonis excluded as the provenance of the clastic rocks because Meso-Neoproterozoic to early Paleozoic magmatic rocks are absent inthe northern part of the North China Craton (Fig. 9; Darby andGehrels, 2006; Rojas-Agramonte et al., 2011); therefore, no corre-sponding materials are available for the clastic rocks. The Bainai-miao Arc, which contains widespread early Paleozoic magmaticrocks, is the most likely source of the early Paleozoic zircons in theclastic rocks. Moreover, nearly all of the εHf(t) values of these earlyPaleozoic zircons fell within the εHf(t) ranges of the Bainaimiao Arc-related magmatic rocks (Fig. 13c). Currently, the Bainaimiao Arc isconsidered to have evolved on a Precambrian basement, whichlikely formed during ~0.60e1.25 Ga based on the detrital zirconages (Zhang et al., 2014). However, the UePb dating of the zirconscaptured from a quartz monzodiorite dike suggested a moreancient Precambrian basement (1.26e1.9 Ga) for the BainaimiaoArc (Zhang et al., 2009). Accordingly, a Precambrian basement,which can provide Precambrian materials for the clastic rocks, hasbeen confirmed for the Bainaimiao arc. Moreover, the abundantmagmatic rocks ranging from basic to intermediateeacid types inthe Bainaimiao Arc overlap the source rock compositions of thecollected clastic rocks. Therefore, the clastic rocks were derivedfrom the Bainaimiao Arc itself.

6.3. Tectonic implications

6.3.1. Identification of a Precambrian basement for the BainaimiaoArc

The limited understanding of the nature of the Bainaimiao Arc is

Fig. 12. (a) LaeTheSc and (b) TheSceZr/10 tectonic discriminations for the clastic rocks (after Bhatia and Crook, 1986). The arrows roughly show the tectonic shift with decreasingage.

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due to a lack of exposed Precambrian magmatic rocks. Fortunately,the crustal indicators of Hf isotopes in the early Paleozoic magmaticrocks and the Bainaimiao Arc-derived sediments can help to resolvethis problem (Zhang et al., 2014; Qian et al., 2017). The collectedgranodiorite to granite samples (NM16e15, 16 and 27) from theHudugetu and Gutaole plutons in the Zhurihe region yielded low tohigh positive εHf(t) values (þ1.42 to þ8.19) and model ages of905e1329 Ma, indicating the crustal reworking of aMesoeNeoproterozoic basement. The Bainaimiao Arc-derivedclastic rocks contain abundant Proterozoic zircons, indicating aPrecambrian basement for the Bainaimiao Arc. This proposition issupported by the Proterozoic zircons captured from the magmaticrocks in the Zhurihe and Damaoqi regions (Zhang and Jian, 2008;Zhang et al., 2009). Based on the UePb dating of detrital zircons inclastic rocks and xenocrytic zircons in magmatic rocks, Zhang et al.,2014 further concluded that the Bainaimiao Arc is an ensialic islandarc evolved on a Precambrian basement. Thus, we advocate that thePrecambrian basement of the Bainaimiao Arc extends fromPaleo-toMeso-to Neoproterozoic. The Precambrian basement of the Bai-naimiao Arc is considered to have been involved with the Grenvilleevent (Zhang et al., 2009) and is thus similar to the Tarim Cratonand microcontinents in the CAOB, but has no tectonic affinity withthe North China Craton (Zhang et al., 2014). Therefore, the Bainai-miao Arc probably evolved on an isolated Precambrian terranerather than the continental marginal arc of the North China Craton.The Bainaimiao Arc is separated from the North China Craton by theSouth Bainaimiao Ocean, which is represented by the Wude andHarihadeeChegendalai ophiolites (Zhang et al., 2014).

6.3.2. Early Paleozoic magmatic and crustal evolution in theBainaimiao Arc

The early Paleozoic magmatism involved in the Bainaimiao Arcdevelopment lasted for approximately 100 M.y., from 500Ma to400Ma (Liu et al., 2003, 2013; Tao et al., 2005; Jian et al., 2008; Liet al., 2010, 2012b; Gu, 2012; Hao and Hou, 2012; Zhang et al.,2013; Wang, 2014; Bai et al., 2015; Qian et al., 2017). The implica-tions of the 518± 3Ma dacite reported by Zhang et al., 2014 will bediscussed subsequently. The magmatic rocks are composed mainlyof gabbro, tonalite, diorite, granodiorite, granite, andesite, daciteand rhyolite. They are dominated by intermediate to basic rocks inthe early stage (>455Ma) and by intermediate to acid rocks in thelater stage (<455Ma; Liu et al., 2003, 2013; Jian et al., 2008; Li et al.,2010, 2012b; Gu, 2012; Hao and Hou, 2012; Zhang et al., 2013;Wang, 2014; Bai et al., 2015; Qian et al., 2017). Most of themagmatic

rocks yielded Rb contents below 200 ppm and Y þ Nb contentsbelow 50 ppm, placing in the arc field (not shown; Pearce, 1982).Therefore, they were confirmed to have been formed in an arcsetting (Jian et al., 2008; Zhang et al., 2013, Zhang et al., 2014;Zhang, 2013; Wang, 2014; Bai et al., 2015; Qian et al., 2017).

To decipher the magmatic and crustal evolution, the publishedgeochemical and isotopic data of the early Paleozoic magmaticrocks have been incorporated in the present study, as shown inFig. 13. The K2O contents of the magmatic rocks increased withdecreasing age (Fig. 13a), indicating greater involvement of conti-nental materials in younger times. Moreover, the Fe2O3

T þ MgOcontents showed an opposite trend (Fig. 13b), indicating lessinvolvement of oceanic materials in younger times. Both findingssupport that the intermediateebasic rocks dominated the earlystage (>455Ma) and that the intermediateeacid rocks dominatedthe later stage (<455Ma). A large amount of zircon Hf isotopic dataincluding our own results and published data has been incorpo-rated in this study (Fig. 13c). The magmatic rocks of ~500e400Mashowed a decreasing trend in εHf(t) values (Fig. 13c). Nearly all therocks aged ~500e455Ma have positive εHf(t) values, indicatingsignificant juvenile addition. The ~455e415Ma magmatic rocksyielded both positive and negative εHf(t) values, indicating bothsignificant juvenile addition and crustal reworking. The 518± 3Madacite yielded only negative εHf(t) values (�14.2 to �10.3), indi-cating the partial melting of the Precambrian basement of theBainaimiao Arc (Zhang et al., 2014). The εHf(t) values of the412± 1Ma rhyolite were strongly negative, ranging from �15.0to �7.1 (Qian et al., 2017). Further, strong negative εHf(t) values(between �22.0 and �16.4) were reported for the 404± 1Ma vol-canic rocks in the northern Chifeng area (Liu et al., 2013). Theserocks were interpreted to have originated from the partial meltingof the continental basement of the North China Craton (Liu et al.,2013; Qian et al., 2017). The εNd(t) values in Fig. 13d trend simi-larly to the εHf(t) values in Fig. 13c, supporting the aforementionedinterpretation.

To explain the decreasing trend of εHf(t) values in the rocksbetween ~500Ma and ~400Ma, Han et al. (2017) proposed anadvancing accretion model, in which the orogens are usuallycharacterized by crustal thickening (Cawood et al., 2009). The Sr/Yratios of the intermediate magmatic rocks under the filter condi-tions of SiO2¼ 55e70% and MgO¼ 1e6% are positively correlatedwith the arc crustal thickness; therefore, they are useful tracers ofcrustal thickness (Chapman et al., 2015). Generally, the Sr/Y ratiosof intermediate magmatic rocks increase with decreasing age

Fig. 13. Age-dependent variations of (a) K2O contents, (b) Fe2O3T þ MgO contents, (c) εHf(t) values, (d) εNd(t) values and (e) Sr/Y ratios for the magmatic rocks in the Bainaimiao Arc.

The εHf(t) values for the clastic rocks (this study) and the Precambrian basement of the North China Craton (Yang et al., 2006) are also shown in (c). The published data are from Nieet al. (1995), Jian et al. (2008), Gu (2012), Hao and Hou (2012), Liu et al. (2013, 2014), Zhang, 2013; Zhang et al. (2013, Zhang et al., 2014, Wang et al. (2012), Bai et al. (2015) and Qianet al. (2017).

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(Fig. 13e). This indicates crustal thickening with arc development,which is consistent with the advancing accretionmodel. The crustalgrowth was likely compensated by crustal reworking in the laterstage, which minimized the net crustal growth in the accretionaryorogens (Cawood et al., 2009).

6.3.3. A tectono-paleogeographic reconstructionBefore proposing an early Paleozoic tectono-paleogeographic

evolution model for the Bainaimiao Arc, we should clarify thesubduction polarity. From north to south, the Bainaimiao Arc andadjacent domains are structurally composed of the early PaleozoicOndor Sum accretionary complex (Xiao et al., 2003; Xu et al., 2013),Bainiamiao Arc-related magmatic rocks (Jian et al., 2008), andSouth Bainaimiao Ocean (Zhang et al., 2014). The early PaleozoicOndor Sum accretionary complex is widely considered to recordthe southward subduction-accretion of the Paleo-Asian Ocean (Liet al., 2012; Xu et al., 2016). The Bainaimiao Arc-related magmaticrocks are widely considered to have been formed by the southwardsubduction of the Paleo-Asian Ocean (Jian et al., 2008; Xu et al.,2013; Bai et al., 2015; Qian et al., 2017). Based on the northernpassive margin of the North China Craton, Zhang et al., 2014 insteadproposed an early Paleozoic northward subduction of the SouthBainaimiao Ocean beneath the Bainaimiao Arc. However, thisnorthward subduction is inconsistent with the south-dippingcrustal structure revealed through both seismic and geological in-vestigations (Xu et al., 2013; Zhang et al., 2014b). Moreover, it isdifficult to imagine that the narrow Bainaimiao Arc received boththe southward subduction of the Paleo-Asian Ocean and thenorthward subduction of the South Bainaimiao Ocean. Therefore,we prefer a southward subduction of the Paleo-Asian Oceanbeneath the Bainaimiao Arc.

Based on the above discussion and previous studies, thefollowing updated tectono-paleogeographic reconstruction of theBainaimiao Arc (Fig. 14) is proposed:

(a) Stage I: Initial subduction and juvenile arc development(~500e455Ma).

The initial subduction of the Paleo-Asian Ocean probablyoccurred during the Cambrian (Jian et al., 2008; Xu et al., 2013). TheTulinkai ophiolites representing the Paleo-Asian oceanic crust wereformed during 497e477Ma, indicating that the subduction of thePaleo-Asian Ocean began at ~500Ma (Jian et al., 2008). The earlyPaleozoic Ondor Sum Group was formed during ~500e415Ma andrecorded the initial consumption of the Paleo-Asian Ocean at~500Ma (Xu et al., 2016). The 499± 2Ma biotite leptite from theBainaimiao Group was formed by the initial subduction (Zhang,2013). Zhang et al., 2014 reported a dacite with an age of518± 3Ma in the Damaoqi region and considered the dacite to beevidence of the initial arc magmatism caused by subduction.However, considering the conspicuous age gap and abrupt changein the εHf(t) values between the 518± 3Ma dacite and later mag-matism at ~500e455Ma, the 518± 3Ma dacite might represent adifferentmagmatic event that might be unrelated to the BainaimiaoArc development. More research is needed in the future to revealthe tectonic implications of the 518± 3Ma dacite. Nevertheless, weadvocate that the initial subduction of the Paleo-Asian Oceanoccurred at ~500Ma based on the above discussion. The magmaticrocks formed during ~500e455Ma yielded dominantly positiveεHf(t) and εNd(t) values (Fig. 13c and d) with geochemical affinitymostly to basiceintermediate rocks. This indicates that they weremainly derived from depleted mantle and formed within a juvenilearc setting (de Araujo et al., 2014). The early Paleozoic volcano-clastic rocks were formed from the eruption of volcanic rocks andthe exhumation of plutonic rocks. As previously discussed, a Pre-cambrian basement has been identified for the Bainaimiao Arc.

However, some Stage I basic rocks exhibit geochemical character-istics of an oceanic island arc (Jian et al., 2008). The source rocks ofsome Baoerhantu Group clastic rocks were formed in an oceanicisland arc setting (Fig. 12). Therefore, we consider that the Bainai-miao Arc is a long arc belt that evolved partly on an ensialic base-ment and partly on an oceanic crust. The South Bainaimiao Oceandeveloped between the Bainaimiao Arc and the North China Craton(Jia et al., 2003; Shang et al., 2003; Zhang et al., 2014). The Hun-shandake block moved along with the Paleo-Asian Ocean (Xu et al.,2016).

(b) Stage II: Continuous subduction with mature arc develop-ment (~455e415Ma).

As discussed in the petrogenesis section, the collectedgabrroediorites and granitoids had a mixed crustemantle source.During Stage II, a portion of the magmatic rocks is characterized bySiO2�56%, Al2O3�15%, MgO�3%, low HREE and Y contents, andhigh Sr/Y ratios; therefore, they show adakitic affinity (Jian et al.,2008; Zhang and Jian, 2008; Zhang et al., 2014a; Bai et al., 2015)as defined by Defant and Drummond (1990). These rocks wereprobably formed by the partial melting of a previously subductedslab in a high-pressure environment caused by continuous sub-duction of the Paleo-Asian Ocean and a thickened arc crust (Jianet al., 2008; Zhang and Jian, 2008; Zhang et al., 2014a; Bai et al.,2015). The ~455e415Ma magmatic rocks yielded both positiveand negative εHf(t) and εNd(t) values (Fig. 13c and d) with a maingeochemical affinity to calc-alkali intermediateeacid rocks.Therefore, they were probably sourced from interactions/mixturesof a modified metasomatic mantle wedge, partial melting of asubducted slab, and partial melting of continent crust, and formedwithin a more mature arc (de Araujo et al., 2014; Zhang et al., 2014).The above discussion on magmatic and crustal evolution suggestsan advancing subduction. The phengite and glaucophane fromblueschists with Ar/Ar ages of 445e453Ma mixed within theOndor Sum Group were formed likely by the advancingsubduction-induced metamorphic event (Tang, 1992; de Jong et al.,2006). The Hunshandake block shifted towards the Bainaimiao Arcon the Paleo-Asian Ocean. Xu et al. (2013) considered that theHunshandake block collided with the Bainaimiao Arc during~450e440Ma, which is incompatible with attributing the latermagmatism to continuous subduction (Jian et al., 2008; Zhanget al., 2014; Qian et al., 2017). The arc basins of the Bainaimiaoand Baoerhantu groups received detritus from both the earlyPaleozoic arc-related magmatic rocks and Precambrian basementof the Bainaimiao Arc. In contrast, the Silurian Xuniwusu Formationflysch deposits that formed in the South Bainaimiao Ocean basinreceived detritus only from the Bainaimiao Arc in the early stageand from both the Bainaimiao Arc and North China Craton in thelater stage (Zhang et al., 2017). This indicates that the South Bai-naimiao Ocean shortened with the advancing subduction of thePaleo-Asian Ocean and was narrow at least during the Silurian.

(c) Stage III: Accretion to the North China Craton(~415e400Ma).

The Bainaimiao Arc is considered to have been accreted to theNorth China Craton during ~415e400Ma based on the followingevidences. The A-type rhyolite of 412± 1Ma exhibited similar Hfisotopic characteristics as the North China Craton; therefore, it wasformed by the anatexis of the continental crust of the North ChinaCraton, which was caused by the accretion of the Bainaimiao Arc tothis craton (Qian et al., 2017). The tectonic m�elange with ~409Maultramafic rocks in the Wude and HarihadeeChegendalai areasdocuments this collisional event and the closure of the South Bai-naimiao Ocean (Zhang and Wu, 1999; Jia et al., 2003; Shang et al.,

Fig. 14. Sketches showing the tectono-paleogeographic reconstruction for the Bainaimiao Arc and adjacent terranes.

Y. Chen et al. / Gondwana Research 79 (2020) 263e282 279

2003; Zhang et al., 2014). The early Paleozoic magmatism ceased at~415e400Ma (Zhang et al., 2014; Chen et al., 2016a; Qian et al.,2017), likely by the accretion of the Hunshandake block(Eizenh€ofer et al., 2014, 2015). The 399± 6Ma garnet amphibolitein centralewest Inner Mongolia is considered to have been formedby a collisional event following the closure of the Paleo-AsianOcean (Chen et al., 2015) between the Bainaimiao Arc and theHunshandake block (Xu et al., 2013, 2016). The molasse deposits ofthe Devonian Xibiehe Formation unconformably overlie the earlyPaleozoic ophiolitic m�elange, arc-related magmatic rocks andforearc to back-arc sediments, thus marking the termination of thisorogenic event (Zhang et al., 2004, Zhang et al., 2010) although theywere mostly derived from early Paleozoic magmatic rocks. Paleo-magnetic data show that the southeastern CAOB was amalgamatedbefore the late Devonian (Zhao et al., 2013). Furthermore, in a post-

collisional extensional setting after the amalgamation, a Devonianalkaline rock belt formed along the northern margin of the NorthChina Craton (Shi et al., 2010; Zhang et al., 2010b, 2010c). Theserobust evidences support a final collision at ~415e400Ma, closingthe oceans between the Hunshandake block, Bainaimiao Arc andNorth China Craton during this time.

7. Conclusions

The following conclusions were made based on the newly andpreviously reported magmatic and sedimentary records of theBainaimiao Arc.

(a) The gabbroediorites and granitoids aged 431e453Ma wereformed in an arc setting with mantleecrust interactions.

Y. Chen et al. / Gondwana Research 79 (2020) 263e282280

(b) The Baoerhantu Group and Xibiehe Formation formed duringthe early Paleozoic and the Devonian, respectively. Theformer Late Carboniferous Amushan Formation has beenascribed to the Bainaimiao Group of early Paleozoic age. Theclastic rocks from these three units yielded early Paleozoicand/or Precambrian zircons. Based on the UePb ages andgeochemical data, we suggest that these rocks formed fromfelsic sources related to the Bainaimiao Arc.

(c) Precambrian basement was identified for the Bainaimiao Arcbased on the UePb ages of xenocrystic zircons in magmaticrocks and detrital zircons in clastic rocks. The ~500e400 Mamagmatic rocks of the Bainaimiao Arc show the secularchanges of increasing K2O contents and Sr/Y ratios anddecreasing Fe2O3

T þMgO contents and εHf(t) and εNd(t) valueswith decreasing age. The changes are the likely responses toadvancing subduction and related crustal thickening. Theproposed tectono-paleogeographic model for the BainaimiaoArc involves a three-stage progression of (a) ~500e455Mainitial subduction and juvenile arc development, (b)~455e415Ma continuous subduction with mature arcdevelopment, and (c) ~415e400Ma accretion to the NorthChina Craton.

Acknowledgements

We thank the Associate Editor Inna Safonova and two anony-mous reviewers for their careful reviews and invaluable sugges-tions, which help to improve our manuscript. The National KeyResearch and Development Project of China (2017YFC0601302),Fundamental Research Funds for the Central Universities(2412019QD002), China Postdoctoral Science Foundation(2018M640275), National Key Basic Research Program of China(2013CB429801), National Natural Science Foundation of China(41372225) and China Scholarship Council (201606010074) areacknowledged.

Appendix A. Supplementary data

Supplementary data to this article can be found online athttps://doi.org/10.1016/j.gr.2019.08.012.

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