Reassessment of continental growth during the accretionary history of the Central Asian Orogenic Belt

Gondwana Research 25 (2014) 103–125

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Reassessment of continental growth during the accretionary history of the CentralAsian Orogenic Belt

A. Kröner a,b,⁎, V. Kovach c, E. Belousova d, E. Hegner e, R. Armstrong f, A. Dolgopolova f, R. Seltmann f,D.V. Alexeiev g, J.E. Hoffmann h, J. Wong i, M. Sun i, K. Cai i, T. Wang j, Y. Tong j, S.A. Wilde k,K.E. Degtyarev g, E. Rytsk c

a Beijing SHRIMP Centre, Chinese Academy of Geological Sciences, Beijing, Chinab Department of Geosciences, University of Mainz, Germanyc Institute of Precambrian Geology and Geochronology, Russian Academy of Sciences, St. Petersburg, Russiad Australian Research Council Centre of Excellence for Core to Crust Fluid Systems/GEMOC, Department of Earth and Planetary Sciences, Macquarie University, Sydney, NSW 2109, Australiae Department of Geo- and Environmental Sciences, University of Munich, Germanyf CERCAMS, Department of Earth Sciences, Natural History Museum, London, UKg Geological Institute, Russian Academy of Sciences, Moscow, Russiah Department of Geology and Mineralogy, University of Cologne, Germanyi Department of Earth Sciences, The University of Hong Kong, Pokfulam Road, Hong Kong, Chinaj Institute of Geology, Chinese Academy of Geological Sciences, Beijing, Chinak Institute for Geoscience Research, Department of Applied Geology, Curtin University of Technology, Perth, WA 6845, Australia

⁎ Corresponding author at: Department of GeoscienceE-mail address: [email protected] (A. Kröner).

1342-937X/$ – see front matter © 2013 International Ahttp://dx.doi.org/10.1016/j.gr.2012.12.023

a b s t r a c t
a r t i c l e i n f o
Article history:Received 29 June 2012Received in revised form 22 November 2012Accepted 31 December 2012Available online 23 January 2013

Handling Editor: M. Santosh

Keywords:Central Asian Orogenic BeltNd–Hf isotopesCrustal growthCrustal reworkingJuvenile crust

We argue that the production of mantle-derived or juvenile continental crust during the accretionary history ofthe Central Asian Orogenic Belt (CAOB) has been grossly overestimated. This is because previous assessmentsonly considered the Palaeozoic evolution of the belt, whereas its accretionary history already began in the latestMesoproterozoic. Furthermore, much of the juvenile growth in Central Asia occurred in late Permian andMesozoic times, after completion of CAOB evolution, and perhaps related to major plume activity. We demon-strate from zircon ages and Nd–Hf isotopic systematics from selected terranes within the CAOB that manyNeoproterozoic to Palaeozoic granitoids in the accreted terranes of the belt are derived from melting of hetero-geneous Precambrian crust or throughmixing of old continental crustwith juvenile or short-livedmaterial, mostlikely in continental arc settings. At the same time, juvenile growth in the CAOB occurred during the latestNeoproterozoic to Palaeozoic in oceanic island arc settings and during accretion of oceanic, island arc, andPrecambrian terranes. However, taking together, our data do not support unusually high crust-productionrates during evolution of the CAOB. Significant variations in zircon εHf values at a given magmatic age suggestthat granitoid magmas were assembled from small batches of melt that seem to mirror the isotopic characteris-tics of compositionally and chronologically heterogeneous crustal sources. We reiterate that the chemical char-acteristics of crustally-derived granitoids are inherited from their source(s) and cannot be used to reconstructtectonic settings, and thus many tectonic models solely based on chemical data may need re-evaluation. Crustalevolution in the CAOB involved both juvenile material and abundant reworking of older crust with varyingproportions throughout its accretionary history, and we see many similarities with the evolution of the SWPacific and the Tasmanides of eastern Australia.

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Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1042. Terrane evolution within the CAOB, based on isotopic signatures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1053. Post-orogenic plume-related (?) igneous rocks and crustal growth . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1074. Nd isotopes of whole-rock granitoid samples . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1075. Hf isotopes in zircons of granitoid rocks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 108

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104 A. Kröner et al. / Gondwana Research 25 (2014) 103–125

6. Variations in Nd–Hf isotopic composition of felsic igneous rocks in the CAOB . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1096.1. Terranes composed predominantly of rocks produced from juvenile sources . . . . . . . . . . . . . . . . . . . . . . . . . . . . 109

6.1.1. Northeastern and central Kazakhstan oceanic arcs and accretionary complexes . . . . . . . . . . . . . . . . . . . . . . . 1096.1.2. Lake Zone of southern and western Mongolia . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 110

6.2. Terranes predominantly composed of ancient rocks and/or produced by melting of old crust . . . . . . . . . . . . . . . . . . . . . 1106.2.1. Anamakit-Muya Zone of the Baikal-Muya Belt, Siberia . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1106.2.2. Northern and northwestern Mongolia . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1126.2.3. North Tianshan of Kyrgyzstan, Central Tianshan of NW China, and Tarim margin . . . . . . . . . . . . . . . . . . . . . . 112

6.3. Terranes showing mixed isotopic signatures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1136.3.1. Northern Mongolia . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1146.3.2. Chinese Altai . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1156.3.3. Chinese Inner Mongolia . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 116

6.4. Possible decoupling of Nd and Hf isotopes with ambiguous petrogenetic information . . . . . . . . . . . . . . . . . . . . . . . . 1177. Significance of Nd-Hf isotopes for crustal growth during CAÓB accretion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1188. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 118Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 119References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 119

1. Introduction

It is now well established that the Central Asian Orogenic Belt(CAOB) is one of the largest accretionary orogens on Earth (Sengör etal., 1993; Windley et al., 2007, see Fig. 1) and evolved over a period ofsome 720 million years from about 1000 Ma to about 280 Ma(Coleman, 1989; Khain et al., 2003; Kovalenko et al., 2004; Kröner etal., 2007; Windley et al., 2007; Rytsk et al., 2007). The identification ofnumerous island arc assemblages and ophiolites, coupled with a largenumber of whole-rock Sm–Nd isotopic data, has led to the widely-cited conclusion that more than 50% of the crust within the CAOB is

UralMountains

Kazakhstan

T i a n s h a n M o u n t

Tarim Craton

Altai Mts.

Fig. 2

UzbekistanKyrgyzstan

Fig. 3

Russia

Fig. 1. Geological map of Central Asia and environs showing major tectonic entBase map from “Atlas of Geological Maps of Central Asia and Adjacent Areas (200

juvenile, and the CAOB thus represents the largest area of Phanerozoiccrustal growth on this planet (Sengör et al., 1993; Kovalenko et al.,1996, 2004; Jahn et al., 2000a,b; Hong et al., 2004). Most modelscompared the evolution of the CAOB with that of the Eurasian marginin southeast Asia (Kröner et al., 2007; Windley et al., 2007; Xiao et al.,2010), based on the assumption that in Indonesia there was an east-facing Andeanmargin with subduction of Pacific oceanic crust through-out theMesozoic (e.g., Metcalfe, 2009, 2010). However, the arc terranesof Indonesia are predominantly built on, or contain fragments of, conti-nental basement derived from the Australian continent (Smyth et al.,2007; Hall, 2010; Flores and Harris, 2011; Hall and Sevastjanova,

a i n sNorth China

Craton

Mongolia

Pacific terranes

Fig. 11

Russia

China

China

ities, location of areas described in this paper and location of Figs. 2 and 3.8)” and reproduced with permission of Chinese Academy of Geological Sciences, Beijing.

image of Fig.�1

105A. Kröner et al. / Gondwana Research 25 (2014) 103–125

2012), and there is relatively little juvenile crust from accretion ofophiolites and island arcs (Hall, 2009). Hall (2010) pointed out thatthe entire area north of the Java-Sunda trench and west of thePhilippine trench is a compositemosaic of Australia-derived continentalfragments with varying lithospheric thickness.

The unusually high crust-production rate postulated for the CAOBwas originally based on the assumption that most, if not all, of thisnew crust was generated in the Palaeozoic to Mesozoic (Sengör et al.,1993; Han et al., 2006; Jahn et al., 2000a,b, 2004), but this inference isno longer tenable. Khain et al. (2002) demonstrated that ophiolite andarc formation in the northern CAOB had begun at least 1020 Ma ago,confirmed by additional data of Rytsk et al. (2007). There are alsoophiolites at 917±14 Ma in northern Transbaikalia (Gordienko et al.,2009) and at 800±3 Ma in northern Mongolia (Kuzmichev et al.,2005), demonstrating the existence of the Palaeo-Asian Ocean sincethe earliest Neoproterozoic. Furthermore, voluminous Neoproterozoicterranes adjacent to the Siberian craton that were originally consideredto have formed during the Neoproterozoic so-called Baikalian orogeny(e.g., Zonenshayn, 1967) are now shown by most authors to havebeen generated during an early phase of CAOB evolution through con-tinuous subduction–accretion within the long-lived Palaeo-AsianOcean (e.g., Dobretsov et al., 2004a,b; Kuzmichev et al., 2005,2007; Yarmolyuk et al., 2006; Zhmodik et al., 2006; Rytsk et al., 2007,2011). It is therefore difficult to understand, that even recent re-views of the CAOB, such as by Wilhem et al. (2012), only considerthe period ca. 620–250 Ma.

In our view, there is a misconception in the literature about crustalgrowth in the CAOB. Sengör et al. (1993) specifically related theirassumption of anomalous growth to the evolutionary model of a singlelong-lived island arc complex, a model that we consider erroneous anddiscuss further below. Jahn et al. (2000a,b) and Jahn et al. (2004), in

Fig. 2. Simplified geological map of the northeastern part of the CAOB, reproduced from Kovachnote predominant areas of crustal reworking, and green lines denote areas of mixed crust. TheKovach et al. (2005, 2011), Kovalenko et al. (2004), Kozakov et al. (1997, 2003, 2005, 2007a,b, 2Rudnev et al. (2009, 2013), Rytsk et al. (2007, 2011), Turkina et al. (2007), Yarmolyuk et al. (2

contrast, referred to anomalous Phanerozoic crustal growth in CentralAsia, not specifically implying during the evolution of the CAOB butreferring to the region in Asia now occupied by the CAOB. We do notdispute the interpretation of these authors but note that most of theircrustal growth is post-tectonic and due to intraplate magmatism asdiscussed below, and is therefore, in our view, not related to the orogen-ic evolution of the CAOB.

2. Terrane evolution within the CAOB, based on isotopic signatures

The terranes constituting the northernmost parts of the CAOB andbordering the Siberian craton (Fig. 2) contain early Neoproterozoic tolate Palaeozoic magmatic and sedimentary rocks whose age and Ndisotopic characteristics have been summarized by Rytsk et al. (2011).These authors showed that the spatial distribution of whole-rock Ndisotope data suggests that early and late Neoproterozoic (early andlate Baikalian in Russian terminology) rocks with juvenile signaturesmainly occur in relatively narrow and isolated (arc?) terranes, whereasmany other rocks are clearly derived from melting or erosion of mucholder continental material. Most late Palaeozoic granitoids also exhibitPalaeo- to Mesoproterozoic Nd model ages. Importantly, Rytsk et al.(2011) concluded that the popular model of simple arc accretion,from S to N, onto the Siberian craton is inconsistent with their isotopicdata, but they also show that the CAOB terranes are unrelated to theSiberian craton and evolved far away from it, presumably in the widen-ing Palaeo-Asian oceanic archipelago.

Crustal reworking (melting) was an important element of the earlyorogenic evolution of the CAOB in southern Siberia (Baikalian andCaledonian in Russian terminology, see Rytsk et al., 2007, 2011). NewNd andHf isotopic data for metasediments of the Slyudyansky Complexin southeastern Siberia, another terrane bordering the craton, show that

et al. (2013). Red heavy lines encircle fields of predomiantly juvenile crust, blue lines de-fields are based on data summarized in Gordienko et al. (2012), Jian et al. (2010a,b, 2012),008, 2011, 2012), Kruk et al. (2011), Kuzmichev et al. (2005, 2007), Mongush et al. (2011),007, 2008) and unpublished data of the IPGG-IGEM team.

image of Fig.�2


crustal development in the northeastern CAOB was characterized byreworking of early Precambrian continental crust in the early and lateNeoproterozoic, whereas juvenile crust formation predominated inthe latest Neoproterozoic (Ediacaran) to early Palaeozoic (Kovach etal., 2013). Zhou and Wilde (2013) have shown that several terranes inthe easternmost part of the CAOB in northeastern China and borderingRussia such as the Songliao, Khanka, Jiamusi and Bureya terranes are notPrecambrian but early Palaeozoic in age (see also Zhou et al., 2009) andoriginally formed a single block that probably disintegrated during theaccretionary evolution of the CAOB. Limited Nd whole-rock isotopicdata for early Palaeozoic magmatic rocks of the Jiamusi terrane(Fig. 2) suggest derivation from melting of Mesoproterozoic toPalaeoproterozoic sources (Wilde and Jahn, unpubl. data), thusmainly demonstrating crustal reworking rather than juvenilemagmatism.

Northern and central Mongolia are underlain by large volumes offelsic rocks, probably generated by crustal reworking as found inthe Andes and other continental margin arcs (Kröner et al., 2007, seeSection 6.2.2). Furthermore, as shown by Badarch et al. (2002),Kozakov et al. (2007a,b, 2012), and Rojas-Agramonte et al. (2011)there are many fragments of Archaean to Neoproterozoic crust inMongolia, and the presence of numerous Precambrian zircon xenocrystsin the Palaeozoic arc terranes of Mongolia either suggests the presenceof older crust at depth or input of old crustal material into subductionzones that later contributed to arcmagmatism. Significantly, Os isotopicdata frommantle xenoliths present inQuaternary basalts in east-centralMongolia reveal the presence of Archaean to Neoproterozoic litho-spheric mantle beneath the region (Wang et al., 2012, 2013). Similarly,chromitite in an early Palaeozoic ophiolite inWest Junggar of NWChinarevealed a Re–Osmantle age of ca. 2.45 Ga and inherited zircons just as

Tianshan

Russia

(X

Tarim cratonTajikistan

Kazakhstan

Turkmenistan

Kokchetav-Ishimbasementblock

Kazakhstan arcterrane

Uzbekistan

Fig. 3. Aeromagnetic base map of part of Central Asia centering on Kazakhstan, Mongolia, ainantly reworked pre-Palaeozoic crust (blue lines), and areas with mixed isotopic signaturesin this paper and are as follows: Fields enclosed by red lines are Chinese Altai (Sun et alMongolia and part of Chinese western Inner Mongolia (Kovach et al., 2011); and Kazakh arcdata). Fields enclosed by blue lines are Central Mongolia (Kröner et al., 2007 and Armstron2011); South Mongolian microcontinent (Wang et al., 2001); Kokchetav-Tianshan basemenUzbekistan and NW China (Kröner et al., 2012, 2013; Ma et al., 2012a,b; CERCAMS, unp(Armstrong et al., 2012), the eastern part of the Chinese Altai (Wang T. et al., 2008; Wanhttp://models.geomag.us/wdmam.html.

old, suggesting the presence of Archaean mantle components beneaththis part of the CAOB (Shi et al., 2012).

The northern Tianshan orogenic belt of Kyrgyzstan which, togetherwith the Chinese Tianshan, is part of the southern domain of theCAOB bordering the Tarim craton, is made up of Neoproterozoic toearly Palaeozoic magmatic rocks that predominantly represent meltsderived from older basement fragments that were most likely torn offfrom the Tarim craton (Kröner et al., 2012, 2013, see Fig. 3 andSection 6.2.3).Ma et al. (2012a,b) concluded from age spectra of detritalzircons and their Hf isotopic patterns in metasediments of the ChineseCentral Tianshan that much of this terrane is of Precambrian age andconstitutes a rifted fragment of the Tarim craton.

A broad belt of Precambrian crytalline basement known as theIshim-Middle Tianshan block extends from the Kokchetav region ofnorthern Kazakhstan into the Tianshan of Kyrgyzstan (Windley et al.,2007) (see Fig. 3) and consists of Archaean to Proterozoic gneisseswhose age patterns (Kasymov, 1994) are also similar to those of theTarim craton (Kröner et al., 2013; Zhang et al., 2013; Zhang et al.,2013). Some of the early Palaeozoic arc terranes to the east of thishuge basement block contain Precambrian xenocrystic zircons up to3.9 Ga (Kröner et al., 2008), suggesting that at least some of these arcswere built on older crust. Similarly, the arc terranes of northwesternMongolia and Tuva (a small autonomous Republic in the Russian Feder-ation just north ofMongolia and in the very center of Asia, see Fig. 3) aretectonically wedged between several Precambrian crustal fragmentssuch as the Baydrag, Dzabkhan, Gargan, Tarbagatay and Tuva–Mongoliacomplexes (e.g., Kozakov et al., 2007b, 2012; see Figs. 2 and 3). Some ofthese microcontinents contain early Palaeozoic Andean-type activecontinental margin assemblages (Kozakov et al., 2008, 2011; Kröneret al., 2011) and, like most other continental blocks in the CAOB,

Mongolia

Tuva

NW Chinainjiang Province)

Inner Mongolia

North China craton

Siberian craton

ChineseAltai

Lake Zone of Mongolia

nd NW China and showing fields of predominantly juvenile crust (red lines), predom-(green lines). The identified fields are based on sources cited below and data reported

., 2008; Wang et al., 2009; Cai et al., 2011b; Liu et al., 2012); Lake Zone of southernterrane (Heinhorst et al., 2000; Kröner et al., 2008; Degtyarev, 2012; CERCAMS unpubl.g et al., 2012); Basement terranes of northwestern Mongolia (Kozakov et al., 2007a,b,t terrane (Turkina et al., 2011); Northern and Middle (Central) Tianshan of Kyrgyzstan,ubl. data). Fields enclosed by green lines are CERCAMS traverse in central Mongoliag Z. et al., 2008) and Chinese central Inner Mongolia. Base map downloadable from

image of Fig.�3

http://models.geomag.us/wdmam.html


are speculated to have been derived from northern Gondwana(Mossakovskii et al., 1993; Didenko et al., 1994). Some contain wellpreserved late Neoproterozoic passive margin deposits (Levashova etal., 2011).

There are several terranes in the CAOB where island-arc crust withprimitive isotopic signatures formed and accreted in theNeoproterozoicandwas subsequently reworked in the early Palaeozoic. Good examplesoccur along the southern margin of Siberia (Rytsk et al., 2011) and inthe Chinese Altai (Wang et al., 2009; Liu et al., 2012). Palaeozoicgranitoid rocks that formed during this reworking process oftenhave relatively primitive isotopic signatures because the isotopicsystems did not have time to evolve sufficiently, but these rocksdid not contribute to crustal growth as demonstrated by Liu et al.(2012). Thus, from the published data, the inference of an excep-tionally high crust-production rate during evolution of the CAOB isquestionable.

It has been inferred from field relationships and geochemistry thatplume-generated ocean islands and small oceanic plateaux developedsporadically within the late Neoproterozoic to early Palaeozoic Palaeo-Asian Ocean (Dobretsov et al., 2004a,b; Gordienko et al., 2007;Safonova, 2009), and there are dismemberedmassive carbonate blocksin some accretionary mélanges that appear to be derived from ancientguyots (A. Kröner, unpubl. field observations). However, the volumesof such intra-oceanic mafic volcanic rocks are insignificant consideringthe size of the CAOB and probably have not contributedmuch to crustalgrowth.

3. Post-orogenic plume-related (?) igneous rocks and crustal growth

The majority of granitoid rocks in the CAOB with juvenile whole-rock Nd isotopic signatures as summarized by Kovalenko et al. (1996)and Jahn et al. (2000a,b, 2004) are between ca. 290 to 100 Ma inage, i.e. Permian to Cretaceous and therefore post-accretional andanorogenic. Only a few samples with juvenile signatures analyzed bythese authors represent early Palaeozoic rocks and are not representa-tive of the entire CAOB. Dobretsov (2003) related the voluminousPermian to Triassic granitoids of Central Asia to plume-induced intra-plate magmatism and postulated the existence of a large superplumeat ca. 280 Ma, whereas the Triassic granitoids were linked to a youngerplume producing the Siberian traps. This interpretation was latertaken up by various authors who postulated that the plume arrivedbeneath the Tarim craton in Permian times at about 275 Ma ago(e.g. Polyakov et al., 2008; Zhang et al., 2008, 2010a,b; Pirajno et al.,2009; Li Y.Q. et al., 2012; Li Z. et al., 2012). All these authors agree thatgranitoid magmatism was anorogenic and occurred after completionof the accretionary history in the CAOB. Some authors have relatedthe voluminous Permian granitoid plutons to post-collisional mag-matism during orogenic collapse (e.g., Han et al., 1999; Tong et al.,2006a,b; Nenakhov et al., 2007), but these rocks occur almost every-where throughout the CAOB and in the northern Tarim craton, andthis synchroneity suggests that Permian granitoids in the CAOB andTarim are probably related to a common geodynamic feature, namelya mantle plume (Zhang et al., 2010b; Zhang et al., 2013; Y. Xu, personalcommunication, 2012). Zr saturation temperatures in plutonic rocks ofXinjiang Province in NW China show a pronounced increase in theearly Permian, and ca. 290 Ma high to ultra-high temperature meta-morphism has been recorded in the Altai region (Li et al., 2010),consistent with a major thermal event. 298–282 Ma Cu−Ni de-posits in the Chinese Altai and eastern Tianshan have been relatedto plume-induced mafic magmatism (Mao et al., 2006). Finally, therecent discovery of V−Ti−Fe ore deposits on the western marginof Tarim shows lithologic similarities with those in Emeishan LIP ba-salts (Li Y.Q. et al., 2012; Li Z. et al., 2012), lending further support tothe plume model.

The Nd isotopic characteristics of the post-collisional Permian toTriassic rocks are likely controlled by reworking (melting) of mixed

short-lived juvenile, and long-lived crustal sources or interaction ofplume-derived magmas and continental crust. Therefore, this typeof crustal growth is not related to accretionary orogeny producingthe CAOB and will not be further discussed here.

It is surprising that the myth of very high crustal growth in theCAOB has persisted for so long and is consistently mentioned in al-most all recent publications on the CAOB despite the fact that it hasbeen known for some time that, globally, crustal recycling basicallybalances crustal growth today and may have done so for the past3 Ga (Armstrong, 1981; Clift et al., 2009; Scholl and von Huene,2009; Belousova et al., 2010). Sediment recycling by subduction, tec-tonic erosion and large-scale crustal recycling is difficult to constrainin accretionary orogens where evidence for much of the early tectonichistory was obliterated during subsequent collision and, therefore,these processes were not discussed in tectonic models for the CAOB.However, Kovalenko et al. (2004) assessed crustal growth duringCAOB formation in southern Siberia and Mongolia on the basis of geo-chronology and isotope geochemistry and concluded that crustalgrowth rates were not unusually high but similar to those of today.Surprisingly, few authors paid attention. Significant sediment sub-duction leading to contamination of basalt sources in the mantlewedge below a Silurian−Devonian arc terrane on the NW marginof the Jungar Basin in NW China has now been documented on thebasis of trace element and isotopic patterns (Shen P. et al., 2012)and may have been more widespread than so far recognized, thusadding to the growing body of evidence that crustal growth rateswere overestimated in the past. Similar amounts of sediment subduc-tion were inferred from Nd isotopic data for latest Neoproterozoic toCambrian and Silurian−Devonian arc terranes of Mongolia byYarmolyuk et al. (2007, 2008) and Kovach et al. (2011) as furtherdiscussed in Chapter 6.3.1 below. A comprehensive study of theprocesses involved in crust-formation and reworking in Central Asiarequires combined geological, geochronological, geochemical, andisotopic investigations of magmatic and associated sedimentaryrocks of presumed juvenile island arc complexes and their evolutionduring accretion-collision.

4. Nd isotopes of whole-rock granitoid samples

The continental crust forms by melting of the upper mantle,resulting in the formation of mafic crustal protoliths that may attain in-termediate and silicic compositions by involvement of recycled olderfelsic material. This crust grew via subduction of oceanic crust in islandarc and active continental margin settings, by accretion of oceanic pla-teaux and islands, and by magmatic underplating and/or intraplatingof plume-derived material. The large fractionation of the Sm/Nd ratioduring melting of the depleted mantle (Sm/Nd~0.355) and formationof granitoid crust (Sm/Nd~0.190) bymelting ofmantle-derived basalticprotoliths and recycled older material ensues a strongly diverging iso-tope evolution of felsic crustal material from that of the depleted man-tle, representing the ultimate source of juvenile continental crust. Theisotope relationship between depleted mantle and continental crust,combined with the largely immobile behavior of the Sm−Nd isotopesystem, provide a robust geochemical framework to constrain thetime of crust formation, sources of rocks, and evolution of continen-tal crust (e.g., DePaolo, 1981; Patchett and Arndt, 1986; Bennett andDePaolo, 1987).

The age of continental crust is interpreted as the time at which theNd of a crustal rock has been isolated from its depleted mantle source(DePaolo et al., 1991). This parameter is calculated as a Nd model age,tNd(DM) and corresponds to the time when the Nd isotopic composi-tion of a crustal rock was identical to its mantle source. This is differ-ent to the age of magma crystallization or subsequent transformationof crustal rocks during reworking, usually determined through zirconages or other mineral dating techniques.


The currently employed Nd-model age concept assumes the isotopecomposition of a model-depleted upper mantle as ultimate source forcontinental crust. As typically a few percent of older crustal material isinvolved in the generation of magmatic arcs (White and Patchett,1984), Ndmodel ages are commonly interpreted as average crustal res-idence ages, or simply Nd-model ages for samples comprising materialfrom older recycled crust (Arndt and Goldstein, 1987). These Ndmodelages reflect an average age of the material involved in the formation ofthe crust. Implicit in the interpretation of Ndmodel ages is the assump-tion that mante-derivedmaterial is rapidly converted to granitoid crust(Jagoutz et al., 2009), a process that may not exceed 100 Ma or so inmagmatic arcs today (e.g., Atherton, 1990). On the other hand, conver-sion of accreted mafic material from oceanic plateaux or magmaticunderplating to granitoids (e.g., Jagoutz et al., 2009) probably exceedsthis time span, and in such cases the Nd model age would underesti-mate the time of separation of material from the mantle.

The source of rocks is characterized by the εNd(t) parameter(DePaolo, 1988) so that positive εNd(t) values indicate derivation ofgranitoids from short-lived juvenile sources, whereas negative εNd(t)values are indicative of rock formation due to reworking of long-lived crustal sources. However, since felsic igneous rocks are oftenformed from sources with different isotopic compositions and ages,such an interpretation is oversimplified (Arndt and Goldstein, 1987).Moreover, anatexis of juvenile rocks shortly after their formation mayresult in granitoids with positive εNd(t). Consequently, one of the isoto-pic criteria to recognize juvenile continental crust is an εNd(t) valueclose to that of the depleted mantle and broad correspondence of theNd model age with the age of crystallization.

Due to the fact that the sedimentary cycle does notmodify the Sm/Ndratio in fine-grained material, such as pelites, from that of the source(Taylor andMcLennan, 1985), Ndmodel ages for fine-grained sedimentsprovide large-scale information on crustal evolution (e.g., Allègre andRousseau, 1984). Constraints on crustal growth versus recycling ofolder crust can be derived for samples for which crystallization ageshave been determined. In these cases simple two-component mixing in-volving melting of material from the depleted mantle and older crust,provide a rough estimate of the proportions of juvenile versusrecycled older crustal material (e.g., Patchett and Bridgwater, 1984).The validity of such crust-mantle mixing models relies on the properchoice of the composition of the mixing members. The crustal endmember can often be identified from the geologic context, and for thedepleted mantle a linear isotopic evolution (e.g., Goldstein et al.,1984) or an increasingly incompatible element depletion of the uppermantle have been proposed (DePaolo, 1981). For the predominantlyearly Palaeozoic samples of this study a leucogabbro from an ophioliticassemblage in the Kyrgyz North Tianshan (KG 67, see Figs. 9 and 10)suggests a depleted upper mantle with εNd(t) of 7.6. Reproducibilityfor the εNd(t) values reported in this study is ca. 0.3ε-units, includingall errors (Hegner et al., 2010; Kovach et al., 2011). The Nd modelages mentioned in the text for the Anamakit-Muya Zone of Siberiaand the Lake Zone of Mongolia are based on the depleted mantlemodel of Goldstein and Jacobsen (1988), whereas all others are basedon that of DePaolo (1981).

5. Hf isotopes in zircons of granitoid rocks

Hafnium (Hf) is a geochemically importantmajor element in zircon,because its isotopic composition is a sensitive tracer of crustal andmantle processes, similar to Nd isotopes (Vervoort et al., 1996;Vervoort and Patchett, 1996; Kemp et al., 2006; Belousova et al., 2006and references therein). Zircon retains a reliable memory of its initialisotopic composition because of its high Hf concentration (0.5−2.0wt.%, Hoskin and Schaltegger, 2003), low Lu/Hf ratio (Blichert-Toftand Albarède, 1997) and its general ability to survive sedimentary andmost metamorphic processes (Andersen et al., 2002). During mantlemelting, Hf is partitioned more strongly into the melt than Lu.

Therefore, over time, the 176Hf/177Hf ratio evolves to higher values inthe depleted mantle than in enriched crustal rocks. During the produc-tion of granitoid magmas, high values of 176Hf/177Hf (i.e. εHf(t)≫0) in-dicate ‘juvenile’ mantle input, either directly via mantle-derived maficmelts, such as generated during subduction-induced island arc forma-tion, or by remelting of young mantle-derived mafic lower crust. Lowvalues of 176Hf/177Hf (εHf(t)bb0) provide evidence for melting of oldcontinental crust, whereas intermediate values and those around zerosuggest mixing of old crust and depleted mantle-derived material dur-ing granite production. Significant variations in the Hf isotopic compo-sition of zircons from the same granitoid rock (including zoning)usually indicate a heterogeneous source and magma mixing in theproduction of a pluton, as documented by Griffin et al. (2002) andBelousova et al. (2006). The zircons usually preserve the chemicalevidence of such mixing which is particularly well preserved in zirconsof S-type granitoids (e.g., Phillips et al., 2011; Villaros et al., 2012).Stevens et al. (2007) and Reichhardt et al. (2010) have provided evi-dence that most granitoid plutons sample heterogeneous crust whensmall batches of melt segregate and feed larger magma chambers. Hfisotopic heterogeneity is also observed in magmatic zircons of I-typegranitoids generated in arc environments, thus confirming field andpetrological evidence that most granitoid protoliths are not chemicallyhomogeneous (e.g., Clemens et al., 2011; Miller et al., 2011). Thus, agranitoid generated in a continental margin arc such as Japan or theAndes most likely exhibits variable mixing of a juvenile melt witholder crust as themantle-derivedmelt underplates the crust and causespartial melting and thus becomes contaminated. This commonly gener-ates a Hf-in-zircon isotopic pattern with εHf(t)-values from negative topositive, as shown below by examples from Central Asia.

The advantage of the Hf-in-zircon isotopic data over whole-rock Ndisotopic patterns is that the former, if combined with geochronology,crystal structure, cathodoluminesce imaging (CL), and trace elementchemistry has the potential to provide insights into the sequence ofprocesses generating a pluton (Kinny and Maas, 2005; Kemp et al.,2006; Belousova et al., 2006), whereas the latter generally reflects theend-product of magma formation and mixing and therefore may maskdetails of important petrogenetic processes. Also, the resistance of zir-con to Hf isotopic disturbance presents an important advantage overgeochemical tracing based on whole-rock Nd isotope compositions(Kinny and Maas, 2003) since, under certain circumstances, Sm/Nd ra-tios may be disturbed during metamorphism (e.g., Gruau et al., 1996).Due to the chemical properties, the Lu–Hf and Sm–Nd isotopic systemsare correlated and thus provide the same answer, at least in terms ofmean compositions and model ages (e.g., Vervoort et al., 1999), but inpractice the Hf-in-zircon data often detect evidence of crustal contami-nation which the whole-rock Sm–Nd system does not record becausethe Nd content in zircon is highly variable but generally low (0.04–15 ppm, Hoskin and Ireland, 2000; Yokoyama et al., 2011), whereasHf in zircon is high (up to 2 wt.% or more, see above). A good exampleis the occurrence of old xenocrystic zircons and other crust-derivedminerals in ophiolites, oceanic crust, or mantle-derived rocks(e.g., Bortnikov et al., 2008; Siebel et al., 2009) often suspected to bedue to laboratory contamination but recently confirmed by thin sectionevidence and micro-analysis (e.g., Jian et al., 2012).

As pointed out by Gerdes and Zeh (2009), the combined use of theU–Pb and Lu–Hf isotopic systems to understand zircons provides apowerful tool to unravel crustal growth and evolution. The U–Pb sys-tem, in combination with CL, is used to establish the crystallizationage of a zircon and to understand processes of alteration, recrystalliza-tion and Pb-loss (e.g., Pidgeon, 1992; Pidgeon et al., 1998). CL zircontextures and trace elements also help to understand magmatic andmetamorphic processes (Corfu et al., 2003; Belousova et al., 2006;Chen et al., 2010) and mixing in granitoid magmas (e.g., Gagnevin etal., 2010). The zircon age, if verified to reflect magma crystallization, issubsequently used to calculate εHf(t) values used for understandingcrustal evolution.


There are currently five different techniques to obtain U–Pb zirconages and Lu–Hf isotopic characteristics on individual zircon grains.One is to date the sectioned zircon exposing its interior on a high-resolution ion microprobe such as SHRIMP II or Cameca 1280, andthen determine the Lu–Hf on the same but generally somewhat largerspot using a laser-ablation ICP-MS. The advantage is that the ion-microprobe pit is extremely shallow (>0.4 μm), thus making surethat the same material is analyzed for U–Pb and Lu–Hf (e.g., Wu et al.,2008). Alternatively, both U–Pb and Lu–Hf isotopes are determined si-multaneously by LA-ICP-MS by splitting the gas stream of sputteredsample material and analyzing the ionized elements in two separatemass spectrometers (Xie et al., 2008; Yuan et al., 2008). This createsan ablation pit in the zircon about 30 μm deep and also guaranteesthat the same material is analyzed for both isotopic systems. A thirdandmost commonly usedmethod is to separately analyze two adjacentspots on the same zircon by LA-ICP-MS, each generating a pit about20–30 μm deep (e.g., Knudsen et al., 2001; Griffin et al., 2004). Thedisadvantage of this method is that domains of potentially differentage and isotopic composition may be sampled. Alternatively, the Hfisotope information is obtained by ablating directly over a smaller di-ameter (20–30 μm) laser pit fromwhere the U–Pb age was determined(e.g., Gerdes and Zeh, 2006). Woodhead et al. (2004) and Kemp et al.(2009) proposed a method to monitor the 207Pb/206Pb ratio in a zirconduring Hf isotope analysis, thereby constraining the age of the sampledzircon volume as ablation proceeds. However, this method does notallow recognition of discordance and non-zero Pb-loss and is thusonly able to estimate minimum 207Pb/206Pb zircon ages. Another meth-od is to analyze U–Pb and Lu–Hf sequentially (quasi simultaneously) onthe same spot by excimer laser-ablation multiple-collector ICP-MS(Kempet al., 2009;Xia et al., 2011). The instrument isfittedwith amod-ified collector block (U–Pb block) that contains 12 Faraday collectorsand 4 ion-counting detectors, allowing simultaneous acquisition ofion signals ranging from mass 204Pb to 238U, an important factor inobtaining highly precise and accurate U–Pb age determinations(Kemp et al., 2009; Xia et al., 2011). The disadvantage of this methodis that ablation time is usually 60 s, resulting in relatively deep pits, andthis requires relative large and homogeneous zircons.

If the crystallization age of the zircon is known, it is possible toestimate its so-called Hf crustal model age by calculating a growthcurve with a Lu/Hf ratio corresponding to the whole-rock through thezircon initial ratio (Griffin et al., 2000; Belousova et al., 2006). We aretalking here NOT about zircon host rock, but we use the Lu/Hf ratio ofthe source or protolith for the host rock— and that is usually unknown.That is why we are using the Lu/Hf of the average continental crust.

Table 1Lu–Hf isotopic composition of three felsic igneous rocks of the CAOB.a

Sample Lu (ppm) Hf (ppm) 176Lu/177Hf 176Hf/177Hf

KG-128 0.446 7.296 0.008668±2 0.282933±10T4-270 0.163 2.071 0.011170±2 0.282700±8T1-171 0.941 16.53 0.008076±2 0.282518±7

KG 128: Ordovician metadacite of Ulakhol Formation, northern slope of Terskey Ridge,Kyrgyz North Tianshan.T4-270: Quartzmonzodiorite (granodiorite), Central Kazakhstanmagmatic belt, collectednorth of Karagandy from the eastern part of the Pushkinskiy pluton.T1-171: Alkalifeldspar granite of unnamed igneous complex, collected 35 km fromJargalant on SE bank of River Toin Gol, central Mongolia.

a Lu andHf concentrations and isotopic compositionswere obtained by isotope dilutionemploying mixed 175Lu–180Hf tracers (Münker et al., 2001) and using the NeptuneMC-ICP-MS at the University of Bonn, Germany. Lu and Hf separation was afterMünker et al. (2001). Samples were digested in Parr bombs (Hoffmann et al., 2011).Measured 176Hf/177Hf was corrected using an exponential law to a 179Hf/177Hf=0,07235.The Münster AMES standard (isotopically identical to JMC 475) yielded a mean of0.282160. External reproducibility of the standard is ca. 40 ppm (2σ). All data are givenrelative to the standard value of 0.282160. External reproducibility for 176Lu/177Hf was0.2% and includes imparted uncertainties by Yb interference. Total procedural blankswere b20 pg for Lu and b70 pg for Hf.

There is some uncertainty in the calculation of this model age becausethe whole-rock 176Lu/177Hf ratio of the protolith is not known, andtherefore most authors use an assumed value for the continentalcrust, varying between 0.009 and 0.015 (Vervoort and Patchett, 1996;Belousova et al., 2006).We analyzed thewhole-rock Lu–Hf compositionof three representative granitoid samples of the CAOB, yielding acombined mean value of 176Lu/177Hf=0.01 (Table 1), and we assumethat the protoliths of the CAOB felsic magmas had approximatelythis value. Griffin et al. (2000) also recommended this value for thecontinental crust that we use in our Hf evolution diagrams below.Within-run errors for individual Hf zircon analyses in the laboratoriesinvolved in this study are generally about 0.5 ε-units, and externalreproducibility of the Hf standards is at ca. 1ε-unit (e.g., Griffin et al.,2000, 2004; Xia et al., 2011).

6. Variations in Nd–Hf isotopic composition of felsic igneous rocksin the CAOB

We now discuss, as examples, three regions of the CAOB wherethe available Nd and/or Hf isotopic data, combined with field geologyand chemical data, either suggest formation in predominantly oceanicdomains or by processes strongly dominated by mantle-sourcedmagmatism, either above subduction zones or above hypothesizedmantle plumes. This is followed by three regions where crustal melt-ing involves variable but relatively minor input from mantle-derived,juvenile source material. Our data are based on an extensive surveyalong several traverses across parts of the CAOB undertaken by theCERCAMS team(R. Seltmann, E. Belousova, A. Dolgopolova, R. Armstrong,R. Pankhurst, D.V. Alexeiev and co-workers), on work undertaken inSiberia and Mongolia by the IPGG team in St. Petersburg (V.P. Kovach,E. Yu. Rytsk, I.K. Kozakov and co-workers) in cooperation with the IGEMteam in Moscow (V.V. Yarmoljuk, A.M. Kozlovskii and co-workers), inthe Chinese Altai by the Hong Kong and CAGS teams (M. Sun, K. Cai,J. Wong and co-workers, T. Wang and Y. Tong), and in the KyrgyzTianshan by the Mainz-Munich-Moscow-Bishkek team (A. Kröner,Y. Rojas-Agramonte, E. Hegner, D.V. Alexeiev, A. V. Mikolaichuk,V.V. Kisilev). Based on our Nd whole-rock and Hf-in-zircon isotopicdata for granitoid and felsic volcanic rocks we have delineated severalareas of the CAOB in Figs. 2 and 3 which are characterized by either apredominance of juvenile rocks (indicated by positive εNd(t)- andεHf(t)-values), by rocks predominantly derived from melting of mucholder crust (indicated by negative εNd(t)- and εHf(t)-values), and byareas with mixed signatures where rocks from juvenile and reworkedsources occur together.

6.1. Terranes composed predominantly of rocks produced from juvenilesources

6.1.1. Northeastern and central Kazakhstan oceanic arcs and accretionarycomplexes

The region of northern, northeastern and central Kazakhstan (Figs. 1and 3) contains some Precambrian microcontinental blocks in the NWwithin the Kokchetav area and several early Palaeozoic arc terranes,including the Boshekul, Shyngyz, and Baidaulet-Aqbastau arcs that areassociated with the Maikain–Balkybek, Junggar–Balkash and Irtysh–Zaisan accretionary complexes, respectively (Windley et al., 2007;Degtyarev, 2011). These heterogeneous domainswere welded togetherbefore the middle Silurian and were followed by middle and latePalaeozoic continental arcs known as the Central Kazakhstan Devonianand late Palaeozoic Balkhash-Yili magmatic belts (Degtyarev, 2011,2012). The latter two in particular are well-expressed in the regionalmagnetic field and appear as concave structures with light colours inthe left central part of Fig. 3.

We show, as an example, the Hf isotopic composition of a 420 Magranite, sample T4-270, which was collected north of Karagandyfrom the eastern part of Pushkinskiy Pluton (Chakabaev, 1978).


This pluton belongs to the Central Kazakhstan Devonian magmaticbelt (Fig. 3), that is underlain by the Ordovician Baydaulet-Akbastau oceanic arc terrane (Windley et al., 2007; Degtyarev,2011, 2012). The granite is characterized by variable but positiveεHf(t) values, yielding crustal Hf model ages between 0.56 and0.81 Ga (Fig. 4). The whole-rock εNd(t) value is +4.8, and the corre-sponding model age is 0.67 Ma (Table 2). The variation in εHf(t) valuesreflects a somewhat heterogeneous but juvenile source, supported bythe Nd isotopic data, and this is best explained by melting of asubduction-related (underplated?) gabbroic reservoir at the base ofan Ordovician oceanic arc from which the granite formed by partialmelting. No evidence of contamination with much older crust isdetected in this sample, and this is in agreement with regional geologi-cal interpretations (Degtyarev, 2011).

The northeastern Kazakhstan region delineated in Fig. 3 predomi-nantly consists of granitoid rocks with strongly positive Nd and Hfisotopic signatures similar to those shown in Fig. 4 and can thus beconsidered as a major area of juvenile crust formation in the earlyPalaeozoic. This terrane consists of early Palaeozoic arcs and ophiolitesthat were welded together in the Late Ordovician, and there is nofield evidence for a Precambrian basement (Windley et al., 2007;Degtyarev, 2011, 2012). Similarly, most of central Kazakhstan consistsof early to middle Palaeozoic arc terranes hosting major sulfide ore de-posits and shown by Heinhorst et al. (2000) and Degtyarev (2012) toconsist predominantly of granitoid rockswith positive εNd(t) signatures.Kröner et al. (2008) extended this survey in central Kazakhstan and alsofound mostly positive εNd(t) values in whole-rock samples of four earlyPalaeozoic granitoid complexes located between Aksu in the north andKounrad at Lake Balkash. Significantly, one Ordovician granite fromStepnyak, close to the Kokchetav basement, contains a 3.9 Ga zirconand has the only negative εNd(t) value of −1, suggesting an Andean-type continental margin magmatic arc (Kröner et al., 2008). The centraland northeastern predominantly juvenile magmatic arc terrane ofKazakhstan is approximately delineated by a red line in Fig. 3.

6.1.2. Lake Zone of southern and western MongoliaThe Lake Zone of southwestern andwesternMongolia (Figs. 2 and3)

is a major area of arc complexes and ophiolites in the central CAOB, andthere is much information on the rock types and structures (Dergunov,2001 and references therein; Lehmann et al., 2010) as well as petrology,geochemistry and isotopes (Kovalenko et al., 1996; Rudnev et al., 2009;Kröner et al., 2010; Yarmolyuk et al., 2011; Kovach et al., 2011).

-10

0

10

20

0 200 400 600 800 1000

t (Ma)

T4-270 - Granite of Central Kazakhstan

0.56Ga

0.81Ga

εNd(t) = 4.8Nd model age: 0.67 Ga

ε Hf(

t)

Fig. 4. Hf isotope evolution diagram for zircons from granite sample T4-270, centralKazakhstan. Note spread in εHf(t) values suggesting a heterogeneous protolith withshort crustal residence time.

The fold-and-thrust system of the Lake Zone is built up of arc-relatedmagmatic rocks, associated metasediments and supra-subductionophiolites (Khain et al., 1995; Gibsher et al., 2001; Dijkstra et al., 2006;Yarmolyuk et al., 2011). High-Ti sub-alkaline pillow basalts associatedwith beds and lenses of siliceous siltstone and siliceous–carbonaterocks are similar to those of modern oceanic plateaux and have εNd(t)values from +4.8 to +7.5 (Fig. 5) and may have formed above aplume source (Kovach et al., 2011). Basalt–andesite sequenceswith rare dacites formed at the Ediacaran–Cambrian boundary atca. 545 Ma and have low TiO2 contents and geochemical features typicalof oceanic island arcs (Kovalenko et al., 1996, 2004). The geochemicaldata and εNd(t) values from +7.3 to +9.9 suggest that these rockswere mainly derived from a subduction-modified depleted mantlewith varying contributions of (subducted) sedimentary material.Sediments associated with these island arc volcanic rocks have slightlylower εNd(t) values of +6.8 to +7.4 and Nd model ages from 0.86 to0.76 Ga (Fig. 5).

Clastic sediments associatedwith the arc assemblageswere deposit-ed in an accretionary wedge environment (Yarmolyuk et al., 2011) andhave positive εNd(t) values from +4.9 to +7.8, confirming juvenilearc sources for their provenance. Arc-related igneous assemblages,including layered gabbros and quartz diorite-tonalite-trondhjemite-granodiorite plutons, were emplaced between about 530 and 480 Ma(Rudnev et al., 2009; Yarmolyuk et al., 2011). They are characterizedby positive εNd(t) values of +6.5 to +9.0 and NdDM model ages of0.73–0.50 Ga. In the εNd vs. age diagram (Fig. 5), granitoids of the islandarc and accretion phases plot on the Nd isotope evolution trend of thearc volcanic rocks. This suggests isotopically similar sources for the arcvolcanics and granitoids and the formation of granitoids by melting ofjuvenile arc material during accretion within the Lake Zone. Thus, thegeochemical and Nd isotopic data suggest that most rocks of the LakeZone were formed in a large ocean basin far from continental domains.

However, plume sources also seem to have contributed to theformation of at least some accretion-related magmatic rocks. This isexemplified by the peralkaline granites of the Bomin-Kharin massif(511±2 Ma, εNd(t)=+6.0 to +7.4 and tNd(DM)=0.80 Ga), whosecompositional characteristics suggest formation of the parental meltfrom short-lived mafic crust, followed by magma mixing (Kovach etal., 2011).

Similar patterns of crust formation and evolution were described byPfänder et al. (2002), Mongush et al. (2011), Gordienko et al. (2006),Gordienko et al. (2012), Kruk et al. (2011), and Rudnev et al. (2013)for the latest Neoproterozoic to Ordovician eastern Tannuola andDzhida zones, and the Altai-Sayan folded region of the eastern CAOB(see Fig. 2 for location). In summary, the Ediacaran to Ordovician evolu-tion of the central CAOB was characterized by extensive formation ofjuvenile crust, probably in intra-oceanic arc complexes of the evolvingPalaeo-Asian Ocean.

6.2. Terranes predominantly composed of ancient rocks and/or producedby melting of old crust

6.2.1. Anamakit-Muya Zone of the Baikal-Muya Belt, SiberiaThe Anamakit–Muya zone of the Baikal-Muya Belt (BMB), close to

the Siberian craton (Fig. 2), is a good example of crustal melting andmixing of juvenile with old crustal material. The BMB is the type areaof Neoproterozoic assemblages known in the Russian literature asBaikalides (Zonenshayn, 1967). It is bounded by the NeoproterozoicBaikal-Patom passive margin of the Siberian craton in the north andthe Barguzin-Vitim Superterrane in the south (Fig. 2). The structureof the BMB is characterized by a combination of linear and mosaic-type fold-and-thrust zones affecting many different volcanic, sedi-mentary and intrusive assemblages (Fig. 2). The geology, geochronol-ogy and Nd isotope geochemistry of rocks in the BMB and adjacentareas of the CAOB were summarized by Rytsk et al. (2007, 2011).

image of Fig.�4

Table 2Sm–Nd isotopic data for magmatic rocks of the Kyrgyz North Tianshan (a) and granitoids from the CERCAMS data base (b).

Sample Age [Ma] Rock type Sm [μg/g] Nd [μg/g] 147Sm/144Nd 143Nd/144Nd (m) εNd(t) tDM Ref.

KG 61a 490 Foliated granite 2.394 12.86 0.1125 0.512300±13 −1.3 1.12 1KG 63ca 451 Porphyritic andesite 4.858 26.38 0.1113 0.512136±11 −4.9 1.34 1KG 67a 491 Leucogabbro 1.017 2.807 0.2191 0.513101±7 +7.6 – 1KG 83a 456 Porphyritic granite 9.958 61.94 0.09721 0.512057±9 −5.5 1.28 1KG 95a 467 Felsic volcanic 4.091 22.45 0.1102 0.512244±10 −2.5 1.17 1KG 102a 420 Foliated diorite 6.190 30.21 0.1239 0.512378±9 −1.2 1.13 1KG 116a 466 Undeformed K-granite 2.227 13.34 0.1009 0.511993±9 −6.9 1.41 1MAV185/6a 445 Metarhyolite 6.500 33.40 0.1177 0.512276±8 −2.6 1.21 1KG 119a 746 Grey granite gneiss 11.20 58.21 0.1164 0.512300±9 +1.1 1.16 1KG 22a 764 Rhyolite 12.84 59.89 0.1296 0.512195±10 −2.1 1.53 1KG 128a 483 Porphyritic dacite 4.106 21.71 0.1143 0.512255±9 −2.4 1.20 1T5-029a 440 Granite 6.31 30.61 0.1246 0.512282±5 −2.9 1.29 1T4-270b 420 Granodiorite 4.45 21.57 0.1249 0.512685±5 +4.8 0.67 1T1-126b 428 Quartz monzonite monzonite 6.02 31.41 0.1157 0.511739±5 −13.1 1.97 1T1-153b 417 Granite 2.74 20.17 0.08220 0.511549±5 −15.2 1.70 1T9-135a 807 Granite 4.48 25.48 0.1063 0.511446±5 −14.0 2.26 1KG1a 844 Granite gneiss 5.875 34.76 0.1022 0.511543±10 −11.2 2.04 2KG2a 810 Granite gneiss 4.829 29.87 0.09773 0.511668±10 −11.8 1.80 2KG3a 562 Granite gneiss 10.25 50.77 0.1220 0.512115±8 −4.8 1.53 2KG5a 834 Granite gneiss 4.791 28.77 0.1007 0.511872±11 −4.7 1.57 2KG8a 600(?) Eclogitic dyke 3.533 12.87 0.1659 0.512708±10 +3.7 2KG11a 810 Tonalitic gneiss 5.100 29.64 0.1040 0.511514±8 −12.3 2.11 2KG13a 461 Diorite 5.715 29.56 0.1169 0.512177±10 −4.3 1.36 3KG14a 1153 Granite gneiss 10.93 70.58 0.09358 0.511435±11 −8.3 2.03 3KG15a 451 Diorite 6.636 32.14 0.1248 0.512224±11 −3.9 1.40 3KG16a 441 Granite 4.649 27.07 0.1038 0.512199±8 −3.3 1.19 3KG37a 472 Granodiorite 2.231 10.63 0.1269 0.512297±10 −2.5 1.30 2KG38a 472 Granodiorite 2.814 12.43 0.1369 0.512357±9 −1.9 1.35 2T5-140b 1131 Granite 5.92 44.46 0.0805 0.511456±9 −6.3 1.78 3

References are: (1) this paper; (2) Kröner et al. (2012); (3) Kröner et al. (2013).All KG samples as well as T9-135 are shown in Fig. 9. Sample T4-270 is from Kazakhstan, T1-153 is fromMongolia, and T9-135 is from the Chinese Tianshan. These cannot be shownin Fig. 9.

a Munich data: 143Nd/144Nd normalized to 146Nd/144Nd=0.7219. External precision for 143Nd/144Nd is ~1.1×10−5 (2 s.d.). Error for 147Sm/144Nd ~0.15% (2 s.d.). The La Jolla Ndstandard solution yielded 143Nd/144Nd=0.511847±8 (2 s.d., N=10). Sm and Nd concentrations determined by isotope dilution, m. = measured ratio, t=initial ratio. Model agecalculation according to DePaolo (1981). For analytical procedure see Kröner et al. (2012).

b CERCAMS data: 143Nd/144Nd b1×10−5, tDM as above. Model age calculation according to DePaolo (1981). For analytical details see Kröner et al. (2013). No tDM can be calculatedfor KG8.


Evolution of the BMB occurred predominantly during the earlyBaikalian (1000±100 to 720±20 Ma) and late Baikalian (700±10 to590±5 Ma) tectonic cycles which are separated by deformation, meta-morphism, and granite formation at 0.80–0.78 Ga and 0.61–0.59 Garespectively. Early Baikalian juvenile rocks formed in several relatively

Fig. 5. Whole-rock Nd isotope evolution diagram for rocks from the Lake Zone, south-western Mongolia. The light brown field shows data from the Precambrian Dzabkhanmicrocontinent, the purple field encompasses data from island arc terranes.Modified from Kovach et al. (2011)

narrowand spatially separated zones,whereas formation and reworkingof late Baikalian juvenile crust occurred in the intervening zones (Fig. 2).

The Anamakit–Muya Zone occupies the central part of the BMB(Fig. 2). The basement is composed of Precambrian metamorphicrocks (Nd model ages of 1.5–1.7 Ga), overlain by a Neoproterozoiccarbonate-clastic sequence with rare mafic and felsic metavolcanicrocks (828±3 to 814±5 Ma; for details of all ages see Rytsk et al.,2011). The Muya gabbro–diorite–tonalite complex (812±19 Ma),high-Ti gabbro bodies and rare dunite–troctolite–gabbro intrusions(835±12 Ma) are associated with the volcanic rocks. Metamorphismanddeformation of these complexeswere accompanied by the emplace-ment of partly allochthonous syntectonic granite–gneiss massifs at784±6 and 786±9 Ma. Younger basins with subaerial trachyrhyolites(723±4 Ma), coeval hypabyssal potassic granitoids, and layeredgabbro–anorthosite and harzburgite–pyroxenite–gabbro intrusionsformed during the orogenic phase of Anamakit-Muya zone.

Early Neoproterozoic metasediments of the Anamakit–Muya Zonehave εNd(0.83Ga) of −3.9 to +0.4 (Fig. 7) and tNd(DM) of 2.1–1.5 Ga,suggesting Palaeoproterozoic to Neoproterozoic sources. Metapelitesand quartzites of the carbonate-clastic formations are characterized bypositive εNd(0.83Ga) values of+1.1 to+4.2 (Fig. 6) andMesoproterozoicNdmodel ages of 1.5–1.2 Ga. In contrast, a greenschist (metatuff) has anegative εNd(0.83Ga) value of −16.6. These data indicate that the sedi-mentary material was supplied from both ancient crustal and juvenileNeoproterozoic sources.

The felsic volcanic sequences exhibit a range of Nd isotopic compo-sitions with εNd(t) values ranging from−13.6 to−2.6 and tNd(DM) from2.4 to 1.6 Ga (Fig. 6). Tonalites of the Muya gabbro–diorite–tonalitecomplex are characterized by negative εNd(t) values of −2.4 to −10.7and Palaeoproterozoic Nd model ages of 2.0-1.8 Ga.

image of Fig.�5


The above volcanic and intrusive complexes were previously inter-preted as having formed in an oceanic island arc setting (Tsygankov,2005 and references therein), but the Nd isotopic data clearly indicatea predominance of long-lived crustal sources of Palaeoproterozoicand/or Neoarchaean age with the addition of juvenile Neoproterozoicmaterial (Rytsk et al., 2011).

A similarly wide range of Nd isotopic compositions was found inthe syntectonic gneissic granites with εNd(t)=−10.6 to −0.4 andtNd(DM)=2.6–1.6 Ga, the subaerial trachyrhyolites and rhyolites of theyounger basins with εNd(t)=−17.6 to −1.4, tNd(DM)=2.9–1.5 Ga, andcoeval granites with εNd(t)=−4.1 to −3.9, tNd(DM)=1.9–1.6 Ga(Rytsk et al., 2011). Both ancient crustal and Neoproterozoic juvenilesources have contributed to the genesis of these rocks. In summary, theNeoproterozoic Anamakit-Muya zone is characterized by reworking ofearly Precambrian crust with variable additions of early Neoproterozoicjuvenile (underplated?) material.

6.2.2. Northern and northwestern MongoliaLate Neoproterozoic to early Palaeozoic stable continental margin

(carbonates) and arc formations dominate much of northern and cen-tral Mongolia, overlain and intruded by voluminous Permian felsicvolcanic rocks and granites (Badarch et al., 2002; Kröner et al.,2007). Most of the arc-related rocks of northern and central Mongoliaconsist of Ordovician to Silurian volcanic and volcaniclastic sequences,intruded by a variety of granitoids, and these rocks were generated inthe relatively short time period from 460 to 417 Ma (Kröner et al.,2007). However, there are also Carboniferous arc sequences, particular-ly around Ulaanbaatar (Popeko, 2002). One striking aspect of theserocks is a predominance of felsic compositions, mainly dacites andrhyolites and their sedimentary derivatives. Although these rocksare chemically arc-related, andesites and basalts are relatively rare(Kröner et al., 2007).

We consider it unlikely that these large volumes of predominantlyfelsic rocks were generated entirely from mafic juvenile sources sincesome samples contain Precambrian xenocrystic zircons, and Nd meancrustal residence ages for some of these felsic rocks are between 600and 1300 Ma, suggesting that at least some older material was involvedin their generation. A comparison of Nd isotope data for southern Siberia

Fig. 6. Whole-rock Nd evolution diagram for rocks of the Anamakit-Muya zone of theBaikal-Muya Belt, Siberia (Fig. 2). Blue and violet fields characterize juvenile crustformed in other than Anamakit-Muya zones of the Baikal-Muya Belt during the earlyNeoproterozoic (1000±100 to 720±20 Ma) and late Neoproterozoic (700±10 to590±5 Ma) tectonic cycles (Rytsk et al., 2007). Pink field characterizes samples ofthe Anamakit-Muya zone derived from reworking of Precambrian crust with variableadditions of early Neoproterozoic juvenile material.Modified from Rytsk et al. (2007, 2011).

and Mongolia shows that Transbaikalia and northern Mongolia reflectthe input of older crust,whereas southernMongolia appears to be largely,but not exclusively, juvenile, supporting the data of Jahn et al. (2004),Helo et al. (2006) and Yarmolyuk et al. (2007, 2008).

We present data for a 417 Ma quartz monzonite sample, T1–126,that was collected from the extensive Telmen Complex in the HövsgölAimag (Figs. 3 and 11) and intrudes Neoproterozoic schists (MongolianGeological Survey Sheet M47-XXVIII, scale 1:200000, see Gurlhaajav etal., 1974–75). The rock has an inequigranular, hypidiomorphic textureand consists of plagioclase, quartz, K-feldspar, biotite, hornblende, andsmall relicts of clinopyroxene. The magmatic zircons all have stronglynegative εHf(t) values and Hf crustal model ages between 1.7 and1.9 Ga, and this is compatible with the whole-rock initial Nd isotopiccomposition and corresponding model age of 2.0 Ga (Fig. 7a, Table 2).

Sample T1–153 is a leucocratic granite and was collected fromanother pluton of the TelmenComplex (Fig. 11). As in the previous sam-ple, the zircons have strongly negative εHf(t) values andHf crustalmodelages between 1.7 and 1.9 Ga, similar to the whole-rock initial Nd isoto-pic composition and corresponding model age of 1.7 Ga (Fig. 7b,Table 2). There is no doubt that the above two samples have entirelycrustal sources, suggesting thatmuch of northernMongolia is undertainby old crust.

By the end of the Ordovician, the northern part of the CAOB hadprobably amalgamated into a new continental margin (the MainMongolian Lineament of Windley et al., 2007). Evidence for stabiliza-tion by this time of the northern region is provided by Eocambrian toCambrian shelf carbonates, Ordovician clastic basins, and relativelylittle deformed, extensive Ordovician, ash-fall tuffs, dacites and rhyo-lites (Kröner et al., 2007). The Main Mongolian Lineament appears tobe a major tectonic boundary separating crustal provinces with differ-ent isotopic characteristics, as documented by Kovalenko et al.(2004), Yarmolyuk et al. (2007, 2008), and Kovach et al. (2011).

The scenario of a stabilized accreted margin in Ordovician times innorthern and central Mongolia provides a suitable setting to generateextensive volumes of felsic volcanic rocks through large-scale crustalmelting processes (Bryan et al., 2002), and the zircon ages and Ndwhole-rock systematics summarized in Kröner et al. (2007) supportthis interpretation. Input of ancient material is not only recorded inthe above silicic volcanic rocks but also in volcanic-derived arc sedi-ments and in mafic igneous rocks of the arc sequences (Kröner etal., 2007).

Whole-rock Nd and Hf-in-zircon isotopic data for granitoids intwo N–S traverses across Mongolia obtained by the CERCAMS team(Fig. 3) also confirm that much of northern and central Mongolia con-sists of reworked and mixed crust with relatively little input of juve-nile material, whereas southern Mongolia predominantly exposesgranitoids derived from young (underplated?) crust (Figs. 2 and 3).The juvenile nature of southern Mongolia is also evident in thegiant Oyu Tolgoi porphyry district where Dolgopolova et al. (inpress) reported Sr–Nd–Pb–Hf isotopic data for Devonian to earlyCarboniferous plutonic and volcanic rocks that show derivationfrom juvenile precursors generated within a subduction setting.

6.2.3. North Tianshan of Kyrgyzstan, Central Tianshan of NW China, andTarim margin

The Tianshan thrust-and-fold belt is part of the southern CAOBand extends for about 2000 km from Uzbekistan to eastern XinjiangProvince of China (Fig. 1). It evolved during several tectonic episodesfrom the Neoproterozoic to early Mesozoic and was reactivated due touplift and deformation in the late Tertiary and Quaternary. Precambrianand Palaeozoic rock assemblages in the western part of the belt withinUzbekistan and Kyrgyzstan have traditionally been grouped into threemajor fault-bounded tectonic zones named North, Middle, and SouthTianshan, which have distinctly different geological histories and struc-tural patterns (Bakirov and Maksumova, 2001, and references therein).In Xinjiang Province of NW China the Middle Tianshan is not present,

image of Fig.�6

-20

-10

0

10

20

0 500 1000 1500 2000

t (Ma)

εNd(t) = -13.4Nd model age: 1.97 Ga

Quartz monzonite, TelmenComplex, northern Mongolia

1.68Ga

1.92Ga

-20

-10

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10

20

0 500 1000 1500

t (Ma)

1.68Ga

1.91Ga

2000


Leucogranite, TelmenComplex, northern Mongolia

a bT1-126 T1-153

ε Hf(

t)

Fig. 7. Hf isotopic evolution diagram for zircons from two granitoid samples from the Telmen Complex, Hövsgöl Aimag of northern Mongolia. (a) Quartz monzonite sample T1-126,(b) Leucogranite sample T1−153. The spread in negative εHf(t) values and similar Nd isotopic data for both samples suggests derivation of the protoliths from a Palaeoproterozoiccrustal source.


andwhat is known there as Central Tianshan corresponds largely to theKyrgyz North Tianshan, whereas the South Tianshan is continuous fromKyrgyzstan into China.

The North Tianshan of Kyrgyzstan and its continuation into NWChina (Fig. 3) represents one of the oldest orogenic domains of theCAOB and contains large volumes of early Palaeozoic granitoids. Itrepresents an amalgamation of Precambrian continental fragments,generally interpreted as microcontinents, early Palaeozoic ophiolite-decorated sutures, and high-grade metamorphic domains includingHP to UHP eclogite-facies rocks, all welded together prior to theMiddle Ordovician (see Kröner et al., 2012, 2013, and referencestherein). In the Middle to early Late Ordovician the North Tianshanwas dominated by continental arc volcanism, followed by emplacementof granitoid batholiths in the latest Ordovician and early Silurian(Bakirov and Maksumova, 2001; Konopelko et al., 2008; Glorie et al.,2010; Kröner et al., 2012).

Details on the rock types and tectonics of the Middle and SouthTianshan can be found in Bakirov and Maksumova (2001) and Biskeand Seltmann (2010), and are not discussed here because our samplesfor isotopic analysis all come from the Kyrgyz North Tianshan. TheSouth Tianshan is a late Palaeozoic accretionary and collisionalthrust-and-fold belt that formed during convergence of the Kyrgyz–Kazakh continent with the Tarim craton (Biske, 1995). No Precambrianrocks are known within the South Tianshan, but underthrust continen-tal crust of the Tarim craton can be inferred at depth from seismic evi-dence (Makarov et al., 2010).

Nd and Hf isotopic data together with SHRIMP zircon ages for se-lected areas of the Kyrgyz North Tianshan were published by Kröneret al. (2012, 2013) and mainly relate to granitoid rocks and felsic vol-canic assemblages ranging in age from late Mesoproterozoic to earlyPalaeozoic. All these rocks show derivation from older crust andthus have negative εNd(t) and Hf-in-zircon εHf(t) values, irrespectiveof their zircon ages (Fig. 8). Some of the Palaeozoic granitoids havemodel ages around 1100 Ma, and it is thus possible that they werederived from melting of Grenville-age basement terranes that weredelineated by Kröner et al. (2013). Whole-rock Nd isotopic data forPalaeozoic granitoids and felsic volcanics and Neoproterozoic samplesare listed in Table 2, including those published in Kröner et al. (2012,2013), and their locations are shown in Fig. 8. The εNd(t) values and iso-topic evolution of samples not previously published are graphicallyshown in Fig. 9 which demonstrates that all samples except three

show an old crustal signature and have Ndmodel ages ranging between1.1 and 1.5 Ga (Table 2). The most positive sample (KG67, see Fig. 9)straddles the depleted mantle evolution curve of DePaolo (1981) andis a leucogabbro from an ophiolitic assemblage, thus convincinglyrevealing its juvenile origin. Sample KG 119 (see Fig. 8) is an homoge-neous grey, porphyritic, Neoproterozoic granite-gneiss from the Chon-Ashu Complex in the eastern part of the North Tianshan, and its Ndmodel age suggests derivation from Grenville-age basement which isexposed nearby (Kröner et al., 2013). Sample KG 8 (for location seeFig. 8) is a mantle-derived gabbroic dyke with eclogite mineralogy ex-posed in the Aktyuz area and demonstrating HPmetamorphism duringearly Palaeozoic subduction (Kröner et al., 2012).

The presence of old crust in Palaeozoic to Mesoproterozoic felsicmagmatic rocks in the Kyrgyz North Tianshan is even more evident inthe Hf-in-zircon evolution diagram of Fig. 10 where most analyseshave significantly negative εHf(t) values with only a few data in the pos-itive εHf(t) field. The corresponding model ages vary between ca. 1.2 Gaand 2.6 Ga. As in the case of the Nd data, Mesoproterozoic model agesagain suggest derivation of the felsic magmas from Grenville-agecrust, whereas some Neoproterozoic gneisses reflect input fromPalaeoproterozoic to Archaean crust in their genesis. Kröner et al.(2013) argued that the Precambrian crustal components in the KyrgyzTianshan are fragments of the Tarim craton, where Mesoproterozoicto Archaean crust is abundant, andMa et al. (2012a,b) suggested a sim-ilar scenario for the Chinese Central Tianshan. This is supported by de-trital zircon ages reported by Lin et al. (2011) for metasedimentsranging from 530 to 3324 Ma, and Guo et al. (2007) found xenocrysticzircons between 600 and 1200 Ma in age in a Devonian leucogranitein the eastern Chinese Central Tianshan. Furthermore, Shi et al. (2010)reported zircon ages of 1453–1458 Ma from granitoid gneisses inBeishan, the eastern extension of the Chinese Central Tianshan. Thus,the Tianshan is shown in Fig. 3 as consisting predominantly of reworkedPrecambrian crust.

6.3. Terranes showing mixed isotopic signatures

There are several areaswithin the CAOBwhere the available isotopicdata showmixed compositions, that is some rocks were apparently de-rived frommelting of old crust with variable input of juvenile material,whereas others show derivation from short-lived juvenile crust. Suchvariation often occurs when the entire Palaeozoic history of a terrane

image of Fig.�7

Fig. 8. Geological map of part of the Kyrgyz Tianshan and adjacent areas (based on Osmonbetov, 1980) showing major tectonic subdivision and location of dated samples analyzedfor Nd isotopic composition as shown in Table 2. Blue symbols denote samples with negative εNd(t) values and origin from ancient crust, and red symbols are for samples withpositive εNd(t) values, either of juvenile origin or containing substantial amounts of juvenile material.


is considered. For example, in the Kurama Ridge of Uzbekistan, in thewesternmost CAOB, late Silurian granites show a strong continentalinput, whereas late Carboniferous granites in the same area displayjuvenile characteristics (CERCAMS, unpublished data). In the easternChinese Tianshan of China, Sr and Nd isotopics show an increase injuvenile, mantle-derived compositions from syn-orogenic to post-orogenic granites, accompanied by mafic and ultramafic intrusionssuggesting vertical continental growth throughmagmatic underplating(Wang T. et al., 2008). Therefore, these mixed sugnatures may eithersignify changes in the tectonic setting or in the depth of magmageneration, or both. Particularly good examples are parts of northernMongolia and the Chinese Altai, as summarized below.

ε Nd(

t) 0

8

4

-4

-8

-120 0.5 1.0 1.5 2.0

Ga

Fig. 9. Nd isotope evolution diagram for whole-rock samples of felsic igneous rocks andone gabbro from the Kyrgyz North Tianshan. For location see figure, data are summarizedin Table 2. All numbers have the prefix KG.

6.3.1. Northern MongoliaOur data for northern Mongolia come from part of a CERCAMS

traverse (Figs. 3 and 11) that contains granitoid rocks ranging in agefrom Silurian to Permian. Nd and Hf isotopic data of several samplessuggest mixtures of juvenile material with old continental crust.Samples from near the large E–W Bolnai Fault (Fig. 11) are strikinglyjuvenile and are probably related to a deep magma source associatedwith a major deep structure (Zorin et al., 1993) and as also visible onthe teleseismic profile MOBAL 2003 (Mordvinova et al., 2007; Tiberiet al., 2008).

Fig. 10. Hf-in-zircon evolution diagram for felsic magmatic rocks of the Kyrgyz NorthTianshan ranging in age between ca. 460 and 1300 Ma.Data from Kröner et al. (2013) and unpublished data of A. Kröner, D.V. Alexeiev and J.Wong.

image of Fig.�8

image of Fig.�9

image of Fig.�10


6.3.2. Chinese AltaiThe Chinese Altai is a general term used for the Chinese segment

of the Altai-Mongolian terrane that is situated in the central part ofthe CAOB (Figs. 1 and 3). This region is mainly composed of variablydeformed and metamorphosed sedimentary and volcanic rocks aswell as granitoid intrusions (Zou et al., 1988; He et al., 1990, 1994;Qu and Zhang, 1991; Windley et al., 2002; Chen and Jahn, 2002;Xiao et al., 2009; Wang et al., 2009; Liu et al., 2012).

The terrane is characterized by a thick and strongly deformedmetasedimentary sequence known as the Habahe Group (e.g., BGMRX,1993), and these rocks contain flysch-type sedimentary features,suggesting deposition and evolution in an accretionary wedge alongan active continental margin (Windley et al., 2002; Li et al., 2006;Long et al., 2008, 2010). Sediments of this group, nowvariably deformedand metamorphosed, were predominantly deposited in the northwest-ern and central region and probably extend into Mongolia and Russia.

Fig. 11. Simplified geological map of part of the Hövsgöl Aimag in northern Mongolia (for losamples of the CERCAMS data collection with their respective mean εHf(t)-in-zircon isotopi

Long et al. (2007, 2010) and Yuan et al. (2007b) reported numerous de-trital zircon ages for Habahe metasediments. Those from northwesternareas are predominantly 500 to 438 Ma old, and those from the easternChinese Altai are mostly 540 to 460 Ma. An overlying rhyolite yielded aU–Pb zircon age of 411±5 Ma, thus the depositional age of the HabaheGroup is well constrained between 438 and 411 Ma (Long et al., 2010).Chen and Jahn (2002) determined whole-rock Nd–Sr isotopic composi-tions of Habahe Group metasediments from the central and easternparts of the Chinese Altai, which yielded negative εNd(t) values of −3.4to −5.0, equivalent to Proterozoic depleted mantle model ages of 1.5to 1.8 Ga, and high initial Sr values of 0.710 to 0.712. These authorssuggested that the provenance of the early Palaeozoic sedimentsconsisted of evolved continental crust with minor Palaeozoic juvenileinput. Long et al. (2007, 2010) carried out detrital zircon Hf isotopicstudies of Habahe Group metasediments in the northwestern ChineseAltai, which yielded a wide range of εHf(t) values of −20 to +17,

cation see Fig. 1), showing plutonic rocks and their ages as well as location of granitoidc signatures. Note that the juvenile rocks are concentrated along the Bolnai Fault.

image of Fig.�11


although most were positive. Geochemical features demonstrate thatthese sediments were mainly derived from immature sources with sig-nificant input of juvenilematerial and onlyminormaterial from evolvedcontinental crust (Long et al., 2007, 2010). Sun et al. (2008) and Liu et al.(2012) speculated that the Habahe detrital zircons were derived fromthe Tuva–Mongolian microcontinent in southern Siberia, but this is un-likely because the Chinese Altai is separated from the Tuva–Mongolianblock by a wide terrane of early Palaeozoic arc assemblages in Tuva(Tannuola arc, Kuzmichev et al., 2001; Mongush et al., 2011; seeFig. 2) and the Mongolian Altai.

Some high-grade metamorphic rocks, including schist, gneiss,migmatite, and amphibolite, are widespread in the central andsouthern Chinese Altai and were originally considered to be part ofthe Habahe Group. Precambrian feldspar Pb model ages, Nd modelages, and multigrain zircon ages between 900 and 2600 Ma (Quand Zhang, 1991; Hu et al., 2000, 2002) led to interpretation ofthese rocks as a Precambrian basement (Li et al., 1996; Li andPoliyangsiji, 2001). However, the above Precambrian ages are notsupported by modern single grain zircon dating, and SHRIMP analy-ses of zircons from gneisses and granulites yielded ages between 279and 451 Ma (Chen et al., 2006; Hu et al., 2006; Briggs et al., 2007).Zircons from banded paragneisses in the western Chinese Altaishow a predominant age population at 466 to 528 Ma, with minorgrains of Precambrian age. Gneissic granitoids intrusive into thesebanded gneisses yielded emplacement ages of 380 to 450 Ma.Hf-in-zircon isotopic data suggest that these rocks were derived froma predominantly juvenile provenance, similar to that of the Habaheflysch sequence (Sun et al., 2008).

Granitoid rocks, partly transformed into gneisses, are extensivelydeveloped in the Chinese Altai and occupy at least 40% of the entirearea (Zou et al., 1988; Tong et al., 2005; Wang et al., 2009). Singlezircon ages suggest that the granitic plutons were emplaced duringthe early to middle Palaeozoic (e.g., Windley et al., 2002; Wang et al.,2006; Briggs et al., 2007; Yuan et al., 2007a; Sun et al., 2008; Cai et al.,2011b), and the oldest granitic intrusion was dated at 479 Ma (Cai etal., 2011b). Granitoid plutonism was most dominant and continuousfrom the early to middle Palaeozoic, whereas late Palaeozoic andMesozoic plutonism was relatively weak with restricted distribution(Wang et al., 2006, 2010; Cai et al., 2011b; Liu et al., 2012).

The predominant early to middle Palaeozoic granitoids consist oftonalite, granodiorite andmonzogranite, and their geochemical featuresare consistent with parental magmas generated via dehydration melt-ing of a hornblende-bearing mafic to intermediate middle crust abovethe garnet stability field (Wang et al., 2006; Yuan et al., 2007a; Sun etal., 2008; Cai et al., 2011a,b). Small bodies of A-type granites are widelydistributed in the central and southeastern Chinese Altai with crystalli-zation ages younger than 300 Ma and are interpreted as post-tectonicgranitoids (e.g., Tong et al., 2006a,b, 2012; Shen et al., 2011).

Whole-rock initial Nd–Sr isotopic data for granitic rocks yieldedεNd(t) values of +2.1 to −4.4, corresponding to model ages of 0.7 to1.6 Ga, and Sr ratios of 0.705 to 0.714. These data were interpretedto mirror mixtures of juvenile arc material and older continentalcrust (Zhao et al., 1993; Chen and Jahn, 2002; Yuan et al., 2007a).Regionally, whole-rock Nd and Sr isotopic mapping of granitoid intru-sions yielded εNd(t) values from−4 to +2 in the central Altai to +1.4to +6 in the southern region and corresponding tDM Nd model agesdecreasing from 1.6–1.1 Ga to 1.0–0.5 Ga (Wang et al., 2009). Thesedata led Wang et al. (2009) to propose that new crust grew south-westwards in the Chinese Altai through processes of horizontal aswell as vertical accretion (Fig. 12).

Hf-in-zircon isotopic data for post-400 Ma granitoid rocks in thewestern part of the Chinese Altai support this model, and almost allmagmatic zircons exhibit positive εHf(t) values of 0 to +9, andinherited zircons show a comparable range of εHf(t) values of +2.5to +12 (Fig. 13). These data document that juvenile material waspredominant, whereas ancient material was relatively minor in the

magma source (Cai et al., 2011b). Sun et al. (2008) and Cai et al.(2011b) related the significant increase in magmatism from juvenilesources at about 400 Ma to ocean ridge subduction, whereas Wanget al. (2009) speculated that this was due to slab breakoff. However,Liu et al. (2012) reviewed the published whole-rock Nd and Hf-in-zircon isotopic data for the Chinese Altai and concluded that thesynorogenic granitoids in the Chinese Altai (460–360 Ma) weremainly derived frommelting of subducted early Palaeozoic sedimentsand felsic volcanic rocks, with some contribution from underplatedmafic crust. This implies cannibalistic reworking of early Palaeozoiccrustal material.

The East Junggar terrane borders the Chinese Altai in the south-west and consists of early Palaeozoic volcanic assemblages thatwere interpreted as oceanic crust and/or island arcs, representing ju-venile additions to the CAOB (e.g., Windley et al., 2007; Zheng et al.,2007). However, new isotopic data for at least part of this terrane sug-gest the presence of older crust beneath the volcanic assemblage andthe existence of an Andean-type continental margin arc (Xu et al., inpress).

In summary, the available data suggest that the southwesternChinese Altai represents a huge Palaeozoic subduction–accretioncomplex, and there is no evidence for a Precambrian basement. Thegranitoid rocks resulted from partial melting of short-lived juvenilecrustal sources, heated by addition of mantle-derived basaltic melts,possibly with minor contamination by old crustal material (Sun etal., 2008; Cai et al., 2011b), and this supports the contention of Liuet al. (2012) that there was massive syn-orogenic granitoid plutonismbut not massive new growth. In contrast, the available Nd isotopicdata for the eastern and central regions of the Chinese Altai favorthe existence of ancient material in the deep crust, and the overallpattern suggests that juvenile material became increasingly addedsouthwards to the Altai crust with time, suggesting southward accre-tionary crustal growth (Wang et al., 2009).

6.3.3. Chinese Inner MongoliaMost of Inner Mongolia straddles the northern margin of the

North China Craton (NCC) and consists of two major belts (Sengöret al., 1993), named the Northern and Southern Orogens respectively(Jian et al., 2008), and separated by the ophiolite-decorated SolonkerSuture Zone (Xiao et al., 2003; Jian et al., 2010a,b). The SouthernOrogenincludes an early Palaeozoic subduction–accretion complex (Xiao et al.,2003), an E–W trending Late Ordovician ophiolite belt containingseveral southward-verging thrust sheets (Wang et al., 1991), and anOrdovician island arc chain that is in fault contact with the NCC (Taoet al., 2005). The early Permian Solonker Suture Zone represents amajor palaeo-plate boundary in Asia that stretches northeastwardsover 2500 km in southern Mongolia and Inner Mongolia of China (Li,2006) and contains a spectacular ophiolitic mélange, interpreted byJian et al. (2010b) as part of a fossil arc-trench system. The NorthernOrogen forms a north-dipping fold-and-thrust belt (Xiao et al., 2003)containing a low-P/T metamorphic complex including old continentalfragments, a subduction–accretion complex, and extensive TTG plutons(for details see Xiao et al., 2003; Jian et al., 2008, 2010a,b). TheHegenshan mafic–ultramafic complex, previously interpreted as aCarboniferous ophiolite but now re-interpreted as the result ofCarbonifeerous and Cretaceous intraplate extension (Jian et al.,2012) is part of this belt. Recent summaries of the rock types, geochem-istry, and zircon ages in these zones, as well as tectonic models, wereprovided by Jian et al. (2008, 2010a,b, 2012).

The Southern Orogen was built on the margin of the NCC as docu-mented by numerous inherited zircons up to 2.5 Ga in age found inthe early Palaeozoic arc assemblages (Zhang and Su, 2002; Jian etal., 2008; P. Jian, personal communication, 2012). The NorthernOrogen contains continental fragments and xenocrystic zircons invarious rocks up to 2614 Ma in age (Miao et al., 2008; Jian et al.,2012) and the available εNd(t)-values for Palaeozoic rocks are strongly

Habahe

49o

48o

47o

86o 90o88o

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48o

47o

87o

2

4

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5

6

1

N

3

3

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0 40 km

Younger crust (0.8-1.4 Ga)

Younger crust (0.6-1.0 Ga)

Youngest crust (0.4-0.5 Ga)

Old crust (0.8-1.5 Ga)

Granitoid intrusions

2

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6

Terranes based on Nd model ages:

Qinhe

Fujun

Hanasi

China

Kazakhstan

MongoliaCAOB

Siberian Craton

Tarim CratonNorth China

Craton

CAOB

Altai

Erqis Fault

Buerjin

Much younger crust (0.6-0.7 Ga)

1

Russia

ChinaCAOB

Younger crust (0.6-1.0 Ga)(Lake Zone of Mongolia?)

Mongolia

Russia

Fig. 12. Simplified geological map of the Chinese Altai, modified from Wang et al. (2009), showing Nd whole-rock model age provinces decreasing from old in the NE (unit 2) toyoung in the SW (unit 6). Unit 1 may be part of an arc terrane in the Lake Zone of Mongolia.


negative (Wang Z. et al., 2008). Thus there is evidence for the involve-ment of variable amounts of older crust in the Palaeozoic generationof the arc terranes in Inner Mongolia. In view of limited Nd isotopicdata and no published Hf-in-zircon isotopes it is difficult to assessthe proportion of juvenile versus old crust in Inner Mongolia, andwe provisionally classify it as a terrane with mixed signatures(Fig. 3). New trace element data for peridotite xenolith of the sub-crustal lithospheric mantle from beneath Inner Mongolia and otherparts of the eastern CAOB reveal moderately refractory compositionsmixed with fertile material, interpreted as reflecting mixtures be-tween relatively old and young mantle (Pan et al., 2013).

6.4. Possible decoupling of Nd and Hf isotopes with ambiguous petrogeneticinformation

Although the systematics of the Lu–Hf decay system closely parallelthose of the Sm–Nd system (Vervoort et al., 1999), the latter is moreprone to disturbance by metamorphism and/or alteration (e.g., Blackand McCulloch, 1987; Poitrasson et al., 1995; Vervoot and Blichert-Toft, 1999), becauseNd is usuallymoremobile thanHf in aqueous fluids(e.g., Thompson et al., 2008; Martin et al., 2010). Thus, the whole-rockNd-isotope composition for samples affected by hydrothermal orseawater alteration has to be treated with caution. In contrast, zirconis stable up to high metamorphic grades, and because of its very low

Lu/Hf, can preserve the initial 176Hf/177Hf of the source magma at thetimeof crystallization. This stabilitymeans that the linkbetween the zir-con crystallization age and the isotopic composition of the host magmais more likely to be preserved than in whole-rock isotopic systems.

However, there are also other processes that may potentiallycause decoupling of the Hf–Nd isotopic systems. These include frac-tionation between muds and sands in passive margin sediments dueto concentration of low Lu/Hf, low 176Hf/177Hf, Hf-rich zircons in ma-ture sands. This is the so-called ‘zircon effect’, where the concentrationof zircon preferentially in the sand fraction will produce a higher Lu/Hfand thus higher 176Hf/177Hf in the silt–clay fraction but little change inSm/Nd (e.g., Vervoort et al., 1999; Veevers et al., 2008). Lu–Hf systemat-ics can be also decoupled from Sm–Nd systematics in the lower crustthrough high-grademetamorphism and partial-melting processes. Sev-eral previous studies (e.g., Schmitz et al., 2004; Halpin et al., 2005;Martin et al., 2010; Valley et al., 2010; Hoffmann et al., 2011) demon-strated that the breakdown of garnet with a high Lu/Hf ratio duringgranulite-facies metamorphism liberates Lu. This results in the growthof zircons enriched in 176Hf that show anomalously juvenile Hf-isotope compositions with εHf values up to +40.

An example from the CAOB revealing somewhat diverse isotopiccompositions is shown by our sample T5-29, obtained from a 440±6 Ma pluton exposed east of Kochkorka in the Kyrgyz North Tianshan(Fig. 9). Similar SHRIMP II zircon ages were recently reported for

image of Fig.�12

Fig. 13. Hf isotope evolution diagram for zircons from Palaeozoic rocks of the ChineseAltai, based on data summarized in Cai et al. (2011b). The vast majority of analyses sug-gest a predominantly juvenile source, but results near εHf(t)=0 may reflect minor imputof detrital crustal material. Analyses with clearly negative εHf(t) values are crustally-derived xenocrysts or old detrital zircons. Data are mainly from zone 4 in Fig. 12.


granitoid plutons from the same area (Kröner et al., 2013). The plutonbelongs to a major syncollisonal granitoid belt that extends for about2000 km from the Kokchetav area of northern Kazakhstan throughthe Kyrgyz North Tianshan into the Chinese Central Tianshan(Windley et al., 2007; Degtyarev, 2011). The Hf-in-zircon isotopicdata are remarkably uniform in their slightly positive εHf(t) values,suggesting that the granite was derived from an isotopically homoge-neous protolith (Fig. 14), possibly derived froma source ofmixedorigin,i.e. partly juvenile and partly including old crust. However, the Hf isoto-pic compositions also permits the interpretation of a homogeneous,possibly magmatic protolith underplated in the Mesoproterozoic, assuggested by the Hf crustal model age of 1.21 Ga. This interpretationis supported by the fact that extensive Mesoproterozoic crust in theKyrgyz North Tianshan was documented by Kröner et al. (2013).However, the whole-rock Nd isotopic composition with an εNd(t) valueof−2.9 and amodel age of 1.29 Ga (Table 2), if interpreted in isolation,

-10

0

10

20

0 500 1000 1500

t (Ma)

1.21Ga

Granite in the Kyrgyz North Tianshan


T5-29

ε Hf(

t)

Fig. 14. Hf-in-zircon isotope evolution diagram for granite sample T5–29 in the KyrgyzNorth Tianshan with contrasting isotopic signatures (Nd-value see Table 2, Hf datafrom unpublished CERCAMS data base). Note that all εHf(t) values are virtually identical,suggesting a very homogeneous protolith.

would suggest derivation of the granite from an early Mesoproterozoiccrustal source, and input of juvenile material would not be suspected.

7. Significance of Nd-Hf isotopes for crustal growth duringCAÓB accretion

The Nd-Hf isotopic systematics of magmatic rocks in the CAOB,coupled with single-grain zircon dating, provide a record of heteroge-neous crustal growth during the accretionary evolution of this hugeorogenic domain which, in many ways, mirrors the evolution of thepresent SW Pacific (e.g., Hall, 2008, 2010). Large areas of CentralAsia are not covered by modern isotopic data, in particular Kazakh-stan, Uzbekistan, the Russian Altai and eastern Mongolia, and our re-cord is therefore somewhat fragmentary. In addition, there is noreliable estimate as to how much crust has been recycled back intothe mantle along subduction zones, but if present-day estimates(e.g., Clift and Vannuchi, 2004; Von Huene et al., 2004; Clift et al.,2009) also apply to the Palaeozoic and Neoproterozoic of CentralAsia, the total volume must be considerable, perhaps equalling thevolume of juvenile additions (e.g., Scholl and von Huene, 2009).

The available record implies that early crustal growth in the CAOB inthe Neoproterozoic mainly occurred in terranes now bordering theSiberian craton, and some of this crust was reworked shortly after itsgeneration. As already noted in previous data compilations, growth inthe eastern CAOBwas generally fromnorth to south, and a large terraneextending from the Siberian margin to the Central Mongolian Linea-ment was already consolidated in the early Palaeozoic (Kovalenko etal., 2004; Kröner et al., 2007;Windley et al., 2007). Likewise, in Kazakh-stan, crustal growth apparently proceeded in a west–east direction byaddition of arc terranes to the Kokchetav–Tianshan continental domainthroughout the early Palaeozoic (Windley et al., 2007; Degtyarev,2011). Some terranes seem to record significant changes in their tec-tonic setting during CAOB evolution, i.e. they contain magmatic as-semblages recording both crustal reworking and juvenile additions,probably as a result of changing palaeogeography and plate geome-try, similar to the evolution of the SW Pacific as shown in thecomputer-based reconstructions of Hall (2002). The isotopic dataare therefore compatible with evolution of the CAOB in an archipelago-type Palaeo-Asian Ocean in which magmatic rocks were generated inintra-oceanic as well as continental margin tectonic settings, andwhere “soft” collisions occurred between arc and microcontinentalterranes, similar to scenarios postulated for the evolution of theNeoproterozoic to earliest Palaeozoic Arabian-Nubian Shield (Johnsonet al., 2011).

One inference from our documentation of numerous crustally-derived magmatic rocks is that these assemblages evolved on ancientcontinentalmargins, perhaps themargins of continental fragments large-ly derived from the Tarim craton and northern Gondwana (Dobretsov etal., 2003; Kröner et al., 2013; Zhang et al., 2013; Zhang et al., 2013). Jahn(2010) also concluded that the subduction–accretion complexes inJapan are mainly composed of recycled continental crust, probably ofProterozoic age, thus supporting the idea that proto-Japan was initiallydeveloped along the southeastern margin of the South China Block.Therefore, continental arcs may have been more common in the CAOBthan so far envisaged, in line with a suggestion of Condie and Kröner(2013) that such arcs played a major role in continental evolutionthrough geologic time. A corollary from this is thatmanyof the chemicalcharacteristics of crustally-derived granitoids in the CAOB are inheritedfrom their source(s) and cannot be used to reconstruct tectonic settings(Kröner et al., 2012, 2013).

8. Conclusions

Our survey of Nd-Hf isotopic data for felsic magmatic rocks in theCAOB does not support unusually high crust-production rates duringthe accretionary history of this extensive orogenic belt of Central Asia.

image of Fig.�14

image of Fig.�13


Data for many areas document reworking, either of Precambrian crustmost likely representing fragments of the Tarim craton or the northernGondwana margin, or of crust generated during the Neoproterozopicevolution. Consequently, continental growth in the CAOB cannot beassumed to be proportional to the apparent increase in area by terraneaccretion. There is no doubt that considerable andmassive new growthoccurred in selected areas of the CAOB such as the arc terranes of north-eastern and central Kazakhstan, the southwestern Chinese Altai, theAltai-Sayan region of Siberia, the Dzida Zone of Transbaikalia, and theLake Zone of southern and western Mongolia. In contrast, the KyrgyzNorth Tianshan, most of northern and central Mongolia, andmost likelyalso the Chinese Central Tianshan, show evidence of extensive crustalreworking and little new growth.

Newdata fromother accretionary orogens such as theNeoproterozoicto earliest Palaeozoic Arabian Nubian Shield (ANS), the Tasmanidesof Eastern Australia, and SW Japan also show evidence of involvementof much old crust, and juvenile additions are less voluminous thanpreviously assumed. For instance, numerical modelling in English(Reymer and Schubert, 1986) and Nd isotopic data (Stoeser and Frost,2006) suggested anomalously high crust-production rates for the ANS,whereas new SHRIMP zircon ages (Kennedy et al., 2004, 2005) andthe discovery of numerous xenocrystic zircons in assumed juvenile arcvolcanic rocks as well as Hf-in-zircon isotopic data (Ali et al., in press)imply the involvement of some old continental crust in the generationof ANS igneous rocks. The generation of extensive granitoids in SWJapan was probably dominated by remelting of Precambrian crustalsources underlying the “thin” roof-pendants of accretionary complexes(Jahn, 2010). Similarly, data from the southern Tasmanides show thatgrowth of this classical series of accretionary orogens, that added~30% of the area of Australia from 830 until 340 Ma, proceeded largelywithout the addition of new juvenile material (Glen, in press).

Lastly, it has become fashionable to use trace element characteris-tics of magmatic rocks to reconstruct tectonic settings in the CAOBand to develop tectonic models. We caution against such practicebecause the chemical characteristics of crustally-derived granitoidsare inherited from their source(s), and many tectonic models solelybased on chemical data may therefore be erroneous. Crustal growthin the CAOB was heterogeneous and varied throughout its accretion-ary history, and we see many similarities with the evolution in theSW Pacific (Hall, 2009) and the Tasmanides of eastern Australia (Glen,in press).

Acknowledgments

This review is based on discussion with many colleagues, in partic-ular Y. Rojas-Agramonte, Yu.S. Biske, D. Konopelko, V. Yarmolyuk,A. Kotov, I. Kozakov, D. Gladkochub, K. Schulmann, A. Mikolaichuk,P. Jian, Y. Xu, V.V. Kiselev, L. Shu, D. Glen, R. Herrington, andR. Pankhurst.We also acknowledge the support of scientists from Russian, Kyrgyz,Mongolian, and Chinese field teams as well as laboratory teams of theBeijing and VSEGEI SHRIMP Centres, the Institute of Precambrian Geol-ogy and Geochronology in St. Petersburg and the Department of EarthSciences, University of Hong Kong. L. Iaccheri helped with isotope anal-yses at Munich University. We are grateful for constructive reviews byB.-M. Jahn and W. Xiao that led to clarification and improvement ofthemanuscript. Fig. 2 was drawn by G.P. Pleskach. A.K. acknowledgesfinancial support of the Deutsche Forschungsgemeinschaft (DFG grantKR590/90-1), the Beijing SHRIMPCentre, and a PPP-Grant (Mainz-HongKong) of the Deutscher Akademischer Austauschdienst (DAAD). V.K.acknowledges financial support of the Russian Foundation for Basic Re-search (RFBR grant 11-05-92003) and the National Science Council ofTaiwan (NSC grant 100-2923-M-002-010). This is a contribution toIGCP Project 592. This is contribution 280 from the ARC Centre of Excel-lence for Core to Crust Fluid Systems (http://www.ccfs.mq.edu.au) and871 in the GEMOC Key Centre (http://www.gemoc.mq.edu.au).

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Alfred Kröner is an emeritus Professor of Geology at MainzUniversity, Germany, and an Honorary Professor and SeniorResearcher at the Beijing SHRIMP Centre, Institute of Geology,

Germany and Austria he joined the University of Cape Town(UCT), South Africa, where he obtained his Ph.D. degree in

Chinese Academy of Geological Sciences, Beijing, China. He isalso anHonorary Professor at Northwest University, Xi'an andthe University of Hong Kong, China. After studying geology in

1968. After a short spell as exploration geologist in Namibiahe joined thePrecambrianReseachUnit of UCT as a Senior Re-search Fellow in 1969, working in South Africa and Namibia.In 1977 he returned to Mainz, Germany, and has undertakenmany research projects in cooperation with the Max-Planck-Institut fürChemie inMainz. Hewas co-editor of Precambri-

an Research (1984–2007) and Terra Nova (1996–2012) and is an Honorary Fellow ofthe Geological Societies of America and South Africa and a recipient of the Chinese
Friendship Award. His research interest is in crustal evolution from the Archaean tothe Palaeozoic with field-based emphasis on southern Africa and Central Asia.
Victor Kovach is a Senior Research Scientist at the Instituteof Precambrian Geology and Geochronology of the Russian
Academy of Sciences (IPGG RAS) in St.-Petersburg, Russia.He was educated in the Geological Department of Leningrad(now St.-Petersburg) State University from1976 to 1981 andreceived his Ph.D. degree in igneous and metamorphicpetrology and isotopic geology from IPGG RAS in 1994. Hisresearch interests focus on radiometric ages, sources of igne-ous rocks andmechanisms of continental crust formation, aswell as evolution in the Central Asian Orogenic Belt and thesouthern Siberian craton on the basis of geochronological,geochemical and isotopic data.
Elena Belousova is an Australian Research Council (ARC) Fu-ture Fellow at the ARC Centre of Excellence for Core to CrustFluid Systems and GEMOC National Key Centre at MacquarieUniversity, Sydney, Australia. She graduated with B.Sc.(Hons) degree in geology fromKiev StateUniversity, Ukraine,in 1988 and obtained her Ph.D. degree from Macquarie Uni-versity, Sydney, in 2000 by studying the trace element signa-tures of zircon and apatite in a wide range of rock types andmineral deposits. Previously Elena was an Australian Post-doctoral Fellow and then a holder of theMacquarie Universi-ty Vice-Chancellor's Innovation Fellowship. Elena's currentresearch work includes the study of processes relating tothe generation of continental crust through time, using in-
situ U–Pb age dating and Hf-isotopic composition of zircons.
Ernst Hegner is a Professor of Geochemistry at theUniversity of Munich, Germany, and in charge of anisotope laboratory, concentrating on Sm-Nd isotopicsystematics. He studied geology and palaeontology atthe Universities of Mainz (Germany) and Natal at Pie-termaritzburg (South Africa). Research for his doctoralthesis was carried out at the Max-Planck-Institut fürChemie and Mainz University. He was a PostdoctoralFellow at the United States Geological Survey in Den-ver and the Colorado School of Mines in Golden, USA,and then established an isotope laboratory at the Uni-versity of Saskatchewan, Saskatoon, Canada, and alsoundertook research at the Geological Survey of Canada

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in Ottawa. In 1990 he returned to Germany and was aSenior Lecturer at the University of Tübingen, teaching and operating an isotopelaboratory.

Robin Armstrong is the leader of the Mining ConsultancySector in the Department of Earth Sciences at the NaturalHistory Museum, London. He received his B.Sc. (Hons)degree in Geology and Applied Geology from the Universityof Glasgow (1994), a DEA from the Universite Blaise Pascalin Volcanlogy and Magmatic Studies (1995) and a Ph.D. inEconomic Geology and Petrology from the University ofSouthampton (1999). He has worked in economic geologyresearch and exploration consultancy for the Natural HistoryMuseum since 1999. He is an associate of the CERCAMSgroup.

Alla Dolgopolova is a Researcher at the Centre for Russianand Central EurAsian Mineral Studies (CERCAMS), NaturalHistory Museum, London. She received her B.Sc. degree inGeology from the Kazakh National Technical University inAlmaty, Kazakhstan, in 1996, an M.Sc. degree in Environ-mental Science from the University of Strathclyde (Glasgow,Scotland, 1998) and a Ph.D. degree in EnvironmentalGeochemistry from the Imperial College London in 2005.Her research interests include isotope and environmentalgeochemistry, isotope mapping and studies of mineraldeposits, mainly in Central Asia.

Reimar Seltmann is a Researcher in Petrology and MineralDeposits in the Department of Earth Sciences at the NaturalHistory Museum, London, where he serves, since 2002, asHead of the Centre for Russian and Central EurAsian MineralStudies (CERCAMS). He graduated from BergakademieFreiberg (Germany) with an M.Sc. in exploration geology(1984) and a Ph.D. in 1987. He has led several internationalprojects (e.g., IGCP 373, 473, 592; research network“Metallogeny of the Altaids”). His research focusses ongeodynamics andmetallogeny of ore provinces in the Altaidsandmineral deposits of Russia, Central Asia, China andMon-golia.

Dmitriy V. Alexeiev is a senior scientist in the Geologi-cal Institute (GIN) of the Russian Academy of Sciencesin Moscow, where he has worked since 2006. He re-ceived his M.Sc. and Ph.D. degrees from Moscow StateUniversity in 1985 and 1993 respectively. Before joiningGIN he worked as a mapping geologist in Kazakhstan,and later as a senior scientist in the Institute of Oceanol-ogy of the Russian Academy of Sciences in Moscow. Hewas a Postdoctoral Fellow at the GeoForschungsZentrumin Potsdam, Germany, and at the University of Munich,Germany. His main research interests are in Palaeozoictectonics, stratigraphy, structural geology of Kazakhstan
and Tianshan as well as in general questions on the tec-tonics of accretionary orogens.
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Elis Hoffmann is a post-doc at the University of Cologne,Germany. He received his Ph.D. degree from the Universityof Bonn in 2011 and B.Sc. and M.Sc. degrees from theUniversity of Münster in 2007. His main research interestis in Archaean geology, and his speciality is in combiningfield geology and advanced analytical techniques in thefield of isotope and trace element geochemistry, petrologyand geochemical modeling to place constraints on theevolution of the early continental crust and the Archaeanmantle. He has worked in Archaean terranes of southernWest Greenland and recently began to investigate ancientrocks in the eastern Kaapvaal Craton of southern Africa to

A. Kröner et al. / Gondwana

understand the tectonic setting and evolution of Archaeangreenstone belts and the early continental crust.

JeanWong is in theDepartment of Earth Sciences, Universityof Hong Kong and in charge of two ICP-MS laboratories.She was trained in geochemistry and geochronology andobtained her B.Sc. (2005), M. Phil. (2007) and PhD degrees(2011) from the University of Hong Kong. Her primaryresearch interests are in the tectonic evolution of SouthChina, with especial emphasis to the Mesozoic and in tech-niques and applications in isotope geochemistry and geo-chronology.

Min Sun is a Professor and Head at the Department ofEarth Sciences, University of Hong Kong (HKU), China. Hegraduated with a B.Sc. degree from Peking University in1982, then went to the University of British Columbia inVancouver, Canada, where he obtained his M.Sc. degreein 1985 and his Ph.D. in 1991. From 1991 to 1994 he wasa Postdoctoral Research Fellow at the University ofSaskatchewan, Saskatoon, Canada, and joined the Universityof Hong Kong in 1994 as Lecturer, later Associate Professor.He received a Collaborative Research Award for OutstandingYoung Researchers in Hong Kong from the National NaturalScience Foundation of China in 1999 and an OutstandingYoung Researcher Award of HKU in 2001. He is a member
of the Editorial Boards of Precambrian Research, Geochimica
et Cosmochimica Acta, and Acta Geoscientia Sinica. His research focusses on the applica-tion of zircon U–Pb and Hf isotopic compositions to the evolution of the North ChinaCraton and the South China Block as well as the Chinese and Mongolian Altai.

Keda Cai is a Research Assistant at the Department ofEarth Sciences, University of Hong Kong. He received hisM.Sc. degree in 2007 at the Guangzhou Institute ofGeochemistry, Chinese Academy of Sciences and his Ph.Ddegree in geochemistry in 2011 at the University of HongKong. His research involves continental crustal growthand the tectonic evolution of the Central Asian OrogenicBelt (CAOB). He is currently undertaking research usingan integrated approach of mineralogy, petrology and geo-chemistry to reveal the geological evolution and magmaticactivity in the Altai-Mongolian terrane of the CAOB.

Tao Wang is a Professor (Senior Researcher) and Directorof the Office of Sciences and Technology in the Instituteof Geology, Chinese Academy of Geological Sciences,Beijing, China. He is a member of the Editorial Board of theChinese journal Acta Petrologica et Mineralogica, DeputySecretary of the Structure Committee of the GeologicalSociety of China, and a Key Member of the Petrological andGeochemical Society of China. His research interests are inMesozoic crustal extension in NE Asia, granitoidmagmatismand continental growth in the Central Asian Orogenic Belt, aswell as comparative studies of Palaeozoic magmatism andtectonics along the border areas of China,Mongolia and Russia.

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Ying Tong is an Associate Professor at the Institute ofGeology, Chinese Academy of Geological Sciences, Beijing,China. He studied geology at the Xi'an College of Engineering(nowChang'anUniversity), graduatingwith a B.Sc. degree in1998. After two years as exploration geologist in the QinghaiBureau of Geology, he continued his studies in the School ofEarth and Space Sciences, Peking University, and obtainedhis M.Sc. degree in 2003. He then undertook his Ph.D.research in the Chinese Academy of Geological Sciencesand at China University of Geosciences in Beijing where heobtained his Ph.D. degree in 2006. From 2006 to 2009 hewas a Research Associate at the Institute of Geology, ChineseAcademy of Geological Sciences. He has been a Visiting
Scholar at Tübingen University, Germany (2009) and at theUniversity of Hong Kong, China (2010 and 2011).
Kirill Ye. Degtyarev is a Corresponding Member of theRussian Academy of Sciences (RAS) andHead of a Laboratoryin the Geological Institute in Moscow, where he has workedsince 1991. He received his M.Sc. degree fromMoscow StateUniversity in 1986, his Ph.D. degree from the GeologicalInstitute of RAS in 1997, and his D.Sc. degree from MoscowState University in 2010. His main research interests arein Palaeozoic tectonics, stratigraphy, geochronology andmagmatism of Kazakhstan, the Urals and the Tianshan.

Evgeniy Rytsk is a Senior Scientist in the Institute ofPrecambrian Geology and Geochronology, Russian Academyof Sciences (IPGG RAS), St.-Petersburg. He was educated atthe Leningrad (now St.-Petersburg) Mining Institute in1970–-1975 and obtained his Ph.D. degree in structuralgeology and metallogeny from the All-Russian Insrtituteof …(VSEGEI) in 1982. His research interest focusses onthe tectonic evolution of the continental crust and thegeodynamics of the Transbaikalia region in Siberia. He haspublished more than 40 research papers on this topic inRussian and international journals.

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Reassessment of continental growth during the accretionary history of the Central Asian Orogenic Belt

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