Zircon U-Pb ages and Hf isotopes of Paleozoic metasedimentary rocks from the Habahe Group in the Qinghe area, Chinese Altai and their tectonic implications

Accepted Manuscript Zircon U-Pb ages and Hf isotopes of Paleozoic metasedimentary rocks from the Habahe Group in the Qinghe area, Chinese Altai and their tectonic implications

Zengchan Dong, Yigui Han, Guochun Zhao, Feng Pan, Kai Wang, Botao Huang, Juanlu Chen PII: DOI: Reference:

S1342-937X(18)30139-4 doi:10.1016/j.gr.2018.05.006 GR 1978

To appear in:

Gondwana Research

Received date: Revised date: Accepted date:

21 April 2018 28 May 2018 28 May 2018

Please cite this article as: Zengchan Dong, Yigui Han, Guochun Zhao, Feng Pan, Kai Wang, Botao Huang, Juanlu Chen , Zircon U-Pb ages and Hf isotopes of Paleozoic metasedimentary rocks from the Habahe Group in the Qinghe area, Chinese Altai and their tectonic implications. Gr (2018), doi:10.1016/j.gr.2018.05.006

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Zircon U-Pb ages and Hf isotopes of Paleozoic metasedimentary rocks from the Habahe Group in the Qinghe area, Chinese Altai

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and their tectonic implications

a

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Juanlu Chenb

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Zengchan Donga,b, Yigui Hanc, Guochun Zhaoc,a*, Feng Panb, Kai Wangb, Botao Huangb,

State Key Laboratory of Continental Dynamics, Department of Geology, Northwest University, Northern Taibai Street 229, Xi’an 710069, China

Key Laboratory for the Study of Focused Magmatism and Giant Ore Deposits， ML Ｒ，, Xi’an Center of Geological Survey, Xi’an, 710054, China

c

Department of Earth Sciences, The University of Hong Kong, Pokfulam Road, Hong Kong, China

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b

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*Corresponding author:

Department of Earth Sciences James Lee Science Building, The University of Hong Kong, Pokfulam Road, Hong Kong

Tel:

852-28578203

Fax:

852-25176912

E-mail:

[email protected]

ACCEPTED MANUSCRIPT Abstract As one of the largest accretionary orogens in Earth’s history, the Central Asian Orogenic Belt has attracted much attention in the past two decades, but there are still many unresolved issues regarding its tectonic nature and evolution. This is the case with the Chinese Altai whose tectonic provenance remains controversial, especially for the Paleozoic era. As the most widely exposed meta-sedimentary rocks in the Chinese

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Altai, the Habahe Group was previously considered as representing a component of Precambrian micro-continent, forming at a passive continental margin. Our new U-Pb

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data for detrital zircons from the Habahe Group in the Qinghe region of the eastern

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Chinese Altai indicate that the strata were deposited from the late Silurian to early Devonian (427~405Ma) and experienced regional metamorphism in the middle

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Devonian (383±9Ma), as constrained by the age of youngest detrital zircons (427±13Ma), the age 405±3Ma of the tonalite that intrudes the Habahe Group, and the

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metamorphic age of 383±9Ma. Additionally, the majority of zircons from the Habahe Group have positive εHf(t) values (+0.22~+15.02). These data, combined with geochemical features and maturity of the sedimentary rocks, indicate an immature

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provenance for the Habahe Group, with the source materials mainly derived from a

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juvenile crust with some old continental components. The juvenile crust was most likely formed in a subduction zone, developing in the Chinese Altai during early-middle Paleozoic time. New geochemical data presented in this study suggest that the Chinese

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Altai was a transitional arc during early Paleozoic time, similar to the Japan-type arc with involvements of old continental components. Therefore, it is proposed that the

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Chinese Altai was likely rifted from the south margin of the Siberia Craton, probably related to the break-up of the Rodinia Supercontinent, and then experienced accretion and reworking with the subduction of the Paleo-Asian Ocean during early Paleozoic time. Key words: Central Asian Orogenic Belt; Chinese Altai; Habahe Group; PaleoAsian Ocean; Subduction

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1. Introduction The Central Asian Orogenic Belt (CAOB) is one of the largest fossil tectonic collages on Earth and has undergone complicated tectonic processes lasting from the Neoproterozoic to Mesozoic, in association with the opening, subduction and closure of the Paleo-Asian Ocean (e.g., Zonenshain et al., 1990; Mossakovsky et al., 1993;

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Sengör et al., 1993; Sengör and Natal'in, 1996; Xiao et al., 2003, 2004, 2009a,b, 2010,

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2013, 2015; Jahn, 2004; Jahn et al., 2000a,b; Windley et al., 2007; Kröner et al., 2008, 2011, 2017; Eizenhöfer et al., 2014, 2015a,b; Eizenhöfer and Zhao, 2018; Han and Zhao,

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2018). The Chinese Altai orogenic belt is a western-central component of the CAOB and southerly adjacent to the Junggar and Tianshan orogenic regions, all of which in

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northwestern China represent important sites in deciphering the histories of accretion, amalgamation and crustal growth of the CAOB (Laurent-Charvet et al., 2003; Xiao et

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al., 2004, 2009a, 2013; Long et al., 2007, 2008a,b; Sun et al., 2008, 2009; Jiang et al., 2010, 2011; Wong et al., 2010; Cai et al., 2011a,b; Han et al., 2011, 2015, 2016a,b,c;

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Zhang et al., 2016a, b; Li et al., 2017; Tang et al., 2017; Safonova, 2017, Safonova et

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al., 2017a,b). Despite numerous studies in the Chinese Altai, controversy still exists on the tectonic nature and evolution of the region. One school of thought suggests that the Chinese Altai has Precambrian continental basement and represents a micro-continent

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(He et al., 1990; Zonenshain, 1990; Li et al., 2006), whereas others propose that the region represents a paleozoic arc terrane (Sengör et al., 1993; Sengör and Natal’in, 1996;

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Hu et al., 2006; Yuan et al., 2007a,b; Jiang et al, 2011). Even for the arc terrane models, there is a hot debate on its tectonic nature: was it an Andean-type continental margin arc, or a Mariana-type intra-ocean arc, or a Japan-type arc (a type between Andean-type and Mariana-type)? Controversy has also surrounded when and how the arc had developed (Safonova, 2017b), with some arguing it formed in the Early Paleozoic, whereas others proposed it developed in the Later Paleozoic. All of these controversies have stemmed from the lack of reliable geological data for diverse lithotectonic units from the Chinese Altai.

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As the most widely exposed lithotectonic units in the Chinese Altai, the Habahe Group is considered as a crucial target to provide important insights into resolving the above controversial issues because it consists of continental-derived quartzofeldspathic clastic turbidites that contains a great deal of information regarding the tectonic provenance the Chinese Altai. The group occurs extensively in its southern part

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and extends west-east from Habahe, through Fuyun-Qinghe, to the region of Mongolia. The group consists predominantly of clastic sedimentary rocks that have been strongly

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deformed and largely metamorphosed in greenschist facies and locally up to lower

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amphibolite facies. Previous studies speculated that the Habahe Group was deposited in a passive continental marginal basin that developed from the end-Neoproterozoic to

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the Middle Ordovician (Liu et al., 2013). Later, based on the results of U-Pb dating of detrital zircons from clastic rocks and magmatic zircons from migmatites of the group,

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Yuan et al. (2007) concluded that the Habahe Group was deposited in a period between ~470 Ma and ~380 Ma. Thus, consensus has not been reached regarding the depositional and metamorphic timing of the Habahe Group was loosely constrained,

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which has significantly hindered the further understanding of tectonic setting and

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processes of the Chinese Altai. To address these issues, this study carried out detailed U-Pb age and Hf isotopic analyses on detrital and magmatic zircons from the Habahe Group and associated intrusions. The results provide important insights into the tectonic

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the CAOB.

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nature of the Chinese Altai and relevant issues with respect to the tectonic evolution of

2.Geological background and sample descriptions The Chinese Altai is traditionally divided into four NW–SE trending domains (He et al., 1990; Windley et al., 2002; Xiao et al., 2004; Sun et al., 2008; Long et al., 2007, 2008a,b; Cai et al., 2011a; Jiang et al., 2010, 2011), namely the North Altai (N), Central Altai (C), Qiongkuer and Erqis (Q), South Altai (S) domains (Fig. 1). This study focuses on the Qinghe area that lies in the Central Altai Domain (or Terranes 2 and 3 of Windley

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et al., 2002), which is the largest domain in the Chinese Altai. The Central domain of Chinese Altai consists of metamorphosed sedimentary and magmatic rocks. Sedimentary successions include the most widely distributed Habahe Group and those mainly occurred in the western part of Chinese Altai, i.e. Paleoproterozoic Kemuqi Group, Neoproterozoic Fuyun Group, Sinian-Cambrian

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Hanasi Group and Silurian Kulumuqi Group (BGMRX, 1993; Windley et al., 2002).

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Magmatic rocks consist of Late Ordovician Dongxileke volcanic lava and pyroclastic rocks, early Devonian Kangbutiebao Formation volcano-sedimentary suites and late

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Paleozoic granitoid intrusions.

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The Habahe Group consists mainly of continental-derived quartzo-feldspathic clastic turbidites that are metamorphosed in greenschist facies and locally lower

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amphibolite facies (GCRSX, 1981; BGMRX, 1993; Windley et al., 2002; Long et al., 2007, 2008a,b; Cai et al., 2011). The group shows a fault contact with the Kangbutiebao Formation and is intruded by Devonian and Permian plutons, but has no overlying strata

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and unclear relationship with underlying suites. Owing to late-stage deformation and

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metamorphism, original sedimentary textures and structures in the clastic turbidites are mostly absent, and the main metamorphic rocks include migmatite, coarse- and fine-

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grained biotite plagioclase gneiss and mica schist associated with minor amphibolite. We collected eight metasedimentary samples from the Habahe Group and two

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intrusive rock samples from a batholith that intrude the group in the Qinghe area (Figs. 2 and 3). The samples from the Habahe Group include coarse-grained banded biotiteplagioclase pelitic gneisses (samples D2080-1, D2126-1, D2085-1, D2031-1 and D4014-2) with minor Al-rich metamorphic minerals (garnet, sillimanite, andalusite and/or cordierite), medium-grained biotite-plagioclase gneisses (D2085-2 and D21052) without Al-rich metamorphic minerals, and fine-grained biotite-quartz schists (Pm005-63) with minor garnet (Fig. 4). Of these samples, sample D2085-2 exists as a lens in sample D2085-1 (Fig. 4). The granite (PM004-10) and tonalite (D4785-1)

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samples from the pluton intruding the Habahe Group show a weak gneissic structure and a fine- to medium-grained texture, and thus their ages can be used to constrain the minimum depositional age of the group. 3. Analytical methods

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3.1. Major and trace element analysis

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We selected seven fresh samples for whole-rock major and trace elemental

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analysis. Whole-rock major oxides were analyzed on fused glass disks by X-ray fluorescence spectrometry, and trace elements, including rare earth elements (REEs),

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were determined by an Agilent 7500a ICP-MS. All the geochemical analyses were carried out at the Experimental Center of Xi'an Institute of Geology and Mineral

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Resources (Xi'an Center of Geological Survey), China. Standards GSR-1 (granite) and GSR-3 (basalt) were used as reference materials; analytical uncertainties for the majority of major elements were estimated to be better than ca. 5%. Errors for major

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element analysis are within 1%, except for P2O5 (5%), and for most trace elements

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(including REE) are within 10%.

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3.2. Zircon U-Pb dating

A certain amount of zircons were separated from crushed samples by a series

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conventional process of heavy liquid, magnetic techniques and random handpicking under a binocular microscope. The zircons from each sample were mounted in an epoxy resin and polished down to expose their interior and then photographed under reflected and transmitted light for the Cathodolumnescence (CL) imaging. The CL imaging was conducted by using a FEI Quanta 400 FEG environmental scanning electron microscope equipped with an Oxford energy dispersive spectroscopy and a Gatan CL3+ detector at the Northwest University in Xi’an, China. Target dating sites were marked on zircon images to avoid impurities and inclusions.

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The U-Pb isotopic composition and trace elements of zircon were analyzed on a laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) instrument equipped with a 193nm ArF-excimer laser and a homogenizing, imaging optical system at the State Key Laboratory of Continental Dynamics, Northwest University, China. Most analyses were carried out with a beam diameter of 30 μm. Typical ablation time

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was 30~60 s, resulting in pits 20–30 μm deep. All the data were calculated using the GLITTER 4.0 program, and then the ages were corrected via the Harvard zircon 91500

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as external standard for both instrumental mass bias and depth-dependent elemental and

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isotopic fractionation. Concordia diagrams and weighted mean calculations were made using Isoplot 3.0. In order to figure out the concentrations of the elements, we use 29Si

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as an internal standard and NIST SRM 610 as an external standard.

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3.3. Zircon Hf isotope analysis

Zircons in-situ Hf isotopes were determined using a Nu Plasma HR MCICP-MS (Nu Instruments Ltd., UK) equipped with a Geo Las 2005 193 nm ArF-excimer laser-

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ablation system. The laser repetition rate is 10 Hz with a beam diameter of 44 μm during

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laser ablation. Laser beam energy is 80 mJ/cm2 during a typical ablation time of 50s. Detailed analytical procedures and isobaric interference corrections are documented in

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Wu et al. (2006). To calculate εHf (t) values, we adopted a present-day 176Lu/177Hf value of 0.0332 and a

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Hf/177Hf value of 0.282772 for the chondritic uniform reservoir

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(CHUR) (Bouvier et al., 2008). Single-stage mantle model ages (T1DM) were calculated by reference to depleted mantle with a present-day 176

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Hf/177Hf ratio of 0.28325 and

Lu/177Hf=0.0384 (Griffin et al., 2000). A two-stage continental model age (T2DM) was

calculated by using

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Lu/177Hf =0.022 for the lower continental crust (Rudnick and

Gao, 2003). 4. Result 4.1. Whole-rock geochemistry

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The meta-sedimentary samples from the Habahe Group show large variations in SiO2 (60.77–83.62 wt.%) and Al2O3 (5.60–17.77 wt.%), and have MgO (1.58–4.27 wt.%), CaO (0.43–2.60 wt.%), Na2O (1.04–3.37 wt.%) and K2O (1.30–2.82 wt.%) contents. The samples display much higher SiO2 contents and ICV values (1.25~1.97, Index of Compositional Variability=(Fe2O3+K2O+Na2O+CaO+MgO+TiO2)/Al2O3,

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molar ratio) than PAAS (post-Archean Australian Average Shale, ICV=0.80, Taylor and McLennan, 1985), and lower CIA values (51.6~69.4, Chemical Index of Alteration

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=Al2O3/( Al2O3+CaO*+Na2O+K2O) × 100, molar ratio) than Phanerozoic shales

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(CIA=70~75, Nesbitt and Young, 1982, 1984), and slightly lower TiO2, Al2O3 and K2O (Supplementary Tables 1 and 2).

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The metasedimentary samples display subparallel REE patterns on the chondritenormalized diagrams (Fig. 5a). The REE concentrations (∑REEs) are characterized by

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a narrow range of 112~208ppm, except for sample Pm005-63 with ∑REEs of 75ppm, lower than those of the PAAS and the Upper Continental Crust (UCC; Rudnick and

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Gao, 2003). Moreover, the samples are moderately enriched in light rare earth elements

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(LREEs) and show relatively flat heavy rare earth element (HREE) patterns (LaN/YbN: 7.35~23.01, average 10.22; GdN/YbN: 163~3.08, average 1.91 for the samples; 10.34, and1.69 for UCC and 9.72 and 1.37 for PASS), which are all characterized by moderate

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negative Eu anomalies with the δEu values of 0.53~0.83 (average 0.70) (Supplementary Tables 1 and 2), similar to the mean values of UCC (δEu =0.71; Rudnick and Gao,

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2003), and are slightly higher than PASS (δEu =0.65). The UCC-normalized spider diagram (Fig. 5b) shows that all the trace elements, including large ion lithosphere elements (LILEs, e.g. Rb, Ba, K, Pb and Sr) and high field strength elements (HFSEs, e.g. Nb, Ta, Zr and Hf), are mostly lower than those of PASS and depleted in LILE (e.g. Rb, Ba, Th, U, Sr, Pb) relative to PAAS, but have uniform patterns, except for sample Pm005-63 (Fig. 5b; Supplementary Tables 1 and 2).

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4.2. Zircon U–Pb and Hf-isotope compositions In this study, 306 U–Pb dating and 148 Hf isotopic spots were analyzed on the detrital zircons from the collected eight samples, with results presented in Supplementary Tables 1 and 2, respectively. Zircon U and Th contents, Th/U ratios, morphological features (Fig. 6), U–Pb ages and Hf isotopic compositions for each

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population of the analyzed samples are summarized in Table 3. Detrital zircons from

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the eight metasedimentary samples show grain sizes varying from 48 to135 μm in length and length/width ratios mostly of ca. 1.1 to 2.5, with subeuhedral-euhedral

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shapes and colorless, light pink or rose colors (Fig. 6). According to CL images, 99% of zircons (298 spots) are characterized by clear oscillatory and sector/irregular zoning

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and have high Th/U ratios (mostly 0.2~1.4), indicating magmatic in origin (Hanchar and Rudnick, 1995), although a few of them are probably metamorphic in origin in

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terms of their core-rim structures and low Th/U ratios (＜0.1) (Fig.6; Supplementary Tables 3 and 4). In addition, a few dark color zircons show complicated interior

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structure without zoning and have low Th/U ratios(＜0.1) (~1%), likely reflecting a

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metamorphic origin.

Four samples (D2080-1, D2126-1, D2085-1 and D2031-1) collected from coarse-

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grained banded paragneisses yielded detrital zircon age patterns almost identical to the other three medium fine-grained paragneisses (D2085-2, D4104-2 and D2105-2) and a

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biotite quartz schist (Pm005-63). These ages vary from ~403 to ~3330Ma and cluster around two prominent ages populations on a normalized probability plot: 460~510Ma, 660~870Ma, and a small amount of ages clustering at 361~401Ma, 420~450Ma 940~1800Ma, and minor 2.2~3.0 Ga (Supplementary table 3, Fig. 7, 8 and 12). However, it is noteworthy that the ages of 361-416Ma with low Th/U ratios (＜0.1) were merely recorded in the coarse-grained banded paragneisses, but not in other samples (Fig. 12a, c). Hence, it most likely that banded paragneisses and another samples have undergone different types of metamorphism or under different

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temperature-pressure conditions after deposition. The maximum depositional age of each sample for the sedimentary rocks was calculated by weighted mean of the youngest age population for magmatic origin zircons. The results gave maximum depositional ages of 427± 13 Ma (n=5, MSWD=9.3) for the sample D2080-1, 431 ± 9 Ma (n=15, MSWD=2.6) for the sample D2126-1, 501 ± 9 Ma (n=11, MSWD=0.77) for

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the sample D2031-1, 461 ± 8 Ma (n=13, MSWD=0.12) for the sample D2085-1, 513±8Ma (n=17, MSWD=0.33) for the sample D2085-2, 462 ±13Ma (n=4,

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MSWD=0.42) for the sample D2105-2, 434 ±8Ma (n=9, MSWD=0.72) for the sample

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D4014-2, and 435 ± 9Ma (n=9, MSWD=0.84) for the sample PM005-63 (Figs. 7, 8). In addition, zircons from the gneissic granite (Pm004-10) and tonalite (D4785-1) have

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typical texture of magmatic zircons, although a few zircons surrounded by narrow dark color rims. Consequently, their weighted mean age are yielded at 386 ± 2 Ma (n=27,

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MSWD=1.5) and 405 ± 3 Ma (n=28, MSWD=0.13), with Th/U ratios ranging from 0.28 to 0.94 and 0.42 to 0.75, respectively. (Supplementary Table 3; Fig. 9)

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Hf isotopic analysis of all detrital zircons yielded εHf(t) values varying from

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−24.99 to +15.02, and T2DM model ages from 0.53 to 3.26 Ga (Supplementary Table 4; Fig. 6). Ca.420~510Ma zircons are characterized by a large variation of εHf(t) values (−11.88 to +15.02), and ~400Ma grains show mostly positive εHf(t) values . Especially,

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all εHf(t) values of gneissic granite are positive (+7.59~+15.23) with T2DM model ages

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of ~0.42 Ga to ~0.90 Ga (Supplementary Table 4; Fig. 12 ). 5. Discussion

5.1. Depositional age The metamorphosed sedimentary successions in the Qinghe region in southeastern Chinese Altai were traditionally regarded as the Habahe Group (BGMRX, 1993; Windley et al., 2002). This group has previously been assumed to have formed in the Proterozoic (Windley et al., 2002; Li et al., 1996, 2006), representing Precambrian

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basement rocks of Chinese Altai (Windley et al., 2002), and may extend southwards beneath the Junggar Basin (Li et al., 2006). However, recent research has revealed that the youngest detrital zircons ages of metasedimentary rocks of this group range from 470 Ma to 380 Ma (Long et al., 2007; Yuan et al., 2007; Sun et al., 2008), and suggested that the metamorphosed strata belong to the equivalents of the Habahe Group (Sun et

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al., 2008). However, Long et al. (2008a,b) considered that Habahe flysch sequence in the northwestern Chinese Altai was probably deposited during the late Silurian to early

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Devonian (438~411Ma), because the maximum depositional age of the sequence

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yielded at ~438Ma, and the age of the overlain Doxileke Formation is ~411Ma (Long et al., 2008a,b).

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In this study, the samples collected from the Qinghe region located in the eastern Chinese Altai, show that the youngest age components of 427~513Ma (Supplementary

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Table 3; Figs. 7, 8), which suggest that the meta-sedimentary rocks must have been deposited at some time after 427±13Ma and before the 405 ± 3 Ma (age of gneissic

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tonalite intruded these paragneisses), although the youngest age component of 387

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±8Ma (4 spots) in the sample D2126-1, but it is regarded as the metamorphic time of the strata, based on the metamorphic origin zircons. Therefore, the depositional age of the meta-sedimentary rocks in this region can be constrained between the Late Silurian

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and Early-Devonian (427~405Ma).

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5.2. Metamorphism

This study reveals that some zircon grains (13 rims and 6 cores) have very low Th/U ratios (<0.1) and mostly yielded young 206Pb/238U ages between 361 and 416Ma (Supplementary Table 5), which are probably metamorphic in origin. These ages give a weighted mean of 383±9Ma (MSWD=0.94), and thus can represent the metamorphic time of the sedimentary rocks. Previous studies in Chinese Altai have suggested three episodes of late Paleozoic

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metamorphic events at ~384-390Ma (Long et al., 2007; Jiang et al., 2010), ~365Ma (Zhuang, 1994), and 244~292Ma (Laurent-Charvet et al., 2003; Chen et al., 2006; Hu et al., 2006; Zheng et al., 2007; Briggs et al., 2007; Wang, et al., 2009). It is noteworthy that the main metamorphic episode occurred at ca.384~390Ma (Long et al., 2007; Jiang et al., 2010), which are consistent with metamorphic age of 383±9Ma in this study,

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suggesting that the metamorphic thermal event is widespread at the middle Devonian in the whole Chinese Altai. However, geodynamic causation for the metamorphism

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remains controversial, with two different models arguing for heating from extensive

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granite intrusions (Zhang et al., 2004) and regional rifting (He et al., 1990). In the Qinghe region, the metasedimentary rocks of the Habahe Group exhibit a

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trend of gradually increasing temperature from north to south (Fig. 4), as indicated by the spatial change of metamorphic mineral assemblages, i.e. from chlorite + garnet and

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andalusite + staurolite (medium fine-grained gneisses), through sillimanite + cordierite, sillimanite + garnet, to sillimanite + K-feldspar

(coarse-grained banded gneisses).

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This trend is similar to high-T metamorphic rocks in northwestern Chinese Altai (Long

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et al., 2007; Wei et al., 2007; Sun et al., 2008; Jiang et al., 2010), all of which need abnormally elevated heat flux (Zhuang, 1994; Jiang et al., 2010).

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In the south of the study area, the gneissic granite (PM004-10) intruding banded gneisses yielded emplacement age of 386±2 Ma, which is consistent with the

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metamorphism event of 383±9 Ma, supporting a genetic relation between magmatism and metamorphism in the Middle Devonian. In addition, the geochemistry and Hf isotopic composition of the Devonian gneissic granites are comparable to the contemporaneous magmatic rocks in the whole Chinese Altai (Wang et al., 2005, 2010, Yuan et al., 2007, Cai et al., 2011a,b, He, et al., 2015), with positive εHf(t) and εNd(t) values, indicating that their magma sources are mainly derived from juvenile mantle materials (e.g., Xu et al., 2002; Cai et al., 2010; He et al., 2015; Santosh et al., 2017; Yu, et al., 2017). Combining with late Paleozoic tectonic setting in Chinese Altai, the

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middle Devonian to early Carboniferous magmatic rocks were most likely generated by partial melting of Paleozoic subducted oceanic crust and/or arc rocks, or from a juvenile magma underplated in a subduction setting (Bryant et al., 1997). Hence, the metamorphic rocks were mainly controlled by the regional thermal dynamic events. The heat flux was probably related to the upwelling of fluid and thermal energy from

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upper mantle asthenosphere, resulting from subduction of oceanic crust.

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5.3 Provenance and source nature

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CIA and ICV values have been frequently used to trace the weathering intensity of sedimentary sources and deduce the nature of sedimentary provenance (Nesbitt and

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Young, 1982; Cox et al., 1995; Fedo et al., 1995; Cullers and Podkovyrov, 2000; Bhat and Ghosh, 2001; Sun et al., 2008). In this study, all metasedimentary samples are

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characterized by low CIA and high ICV values (Fig. 10a), and plot in the Greywacke, Litharenite and Subarkose fields on the classification diagram for terrigenous arenites (Fig. 10b; Pettijohn et al., 1972), implying poor psephicity and low mineralogical

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maturity, reflecting an immature provenance (Van de Kamp and Leake, 1985). In

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addition, Zr/Sc and Th/Sc ratios are useful for provenance study (Long et al., 2008a,b; Chen et al., 2016), because the Th/Sc ratio does not vary much during sedimentary

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recycling, while the Zr/Sc ratios increase significantly during sediment recycling due to zircon enrichment (McLennan et al., 1993; Cullers, 1994). Our samples have a

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variable value of Zr/Sc and Th/Sc ratios with a positive correlation and plot in between the granite and andesite on the Th/Sc–Zr/Sc diagram (Fig. 11a). This indicates variable contributions in the proportions of source material to the sediments, implying a predominant supracrustal/volcanic source (Capistrano et al., 2017) and the provenance was mainly controlled by source composition rather than sediment recycling. Moreover, their low Zr/Sc and Th/Sc ratios also indicate that the samples were deposited close to the source area with the immature provenance. On the La/Th–Hf diagram (Floyd and Leveridge, 1987), most samples plot in an

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acidic arc source field (Fig. 11b), suggesting that the sediments were derived from an acid arc with a low La/Th ratio and Hf content (Capistrano et al., 2017). In this study, most of detrital zircons form prominent age peaks at ca. 463, 504, 514Ma on the age spectrum (Fig. 12a, c), which indicate that the main source of clastic material was derived from the Cambrian to Middle Ordovician rocks (463~514Ma). Additionally,

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the Hf isotopic compositions of these early Paleozoic zircons with ages of 427-514 Ma show both negative εHf(t) and positive εHf(t) values (-11.88 to +15.02) (Supplementary

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Table 6; Fig.12b, d). but characterized by a large variation of mostly positive εHf(t)

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values, suggesting magma derivation from mixed massive juvenile and bits of reworked components(e.g., Santosh et al., 2017; Santosh and Li, 2018). According to the euhedral

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to subhedral morphology features, oscillatory concentric zoning and high Th/U ratios of the zircons (Fig. 6), they are probably magmatic in origin and experienced short-

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distance transportation (Hanchar and Miller, 1993; Corfu et al., 2003; Chen et al., 2015). Therefore, the Habahe Group sedimentary rocks were mainly sourced from early Paleozoic magmatic rocks related to reworking of juvenile crust and mantle

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differentiation, consistent with the northwestern Chinese Altai (Long et al., 2007,

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2008a,b; Sun., 2008; Zhang et al., 2017). Previous research has also indicated that massive Paleozoic magmatic rocks were emplaced in the Central Altai (Windley et al., 2002; Wang et al., 2006; Yuan et al., 2007; Briggs et al., 2007; Sun et al., 2008, 2009),

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thus providing the clastic materials for the Hahabe Group.

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Moreover, our new data indicate that abundant Precambrian ages were detected in the Habahe Group: 75 out of 306 spots analyzed show ages between 550 and 3330 Ma, occupying 25% of all data. Additionally, 143 out of 148 isotopic analyses give twostage model ages of 550~3260 Ma, which indicate that some ancient materials preserve in the Chinese Altai. Furthermore, it is also supported by the Nd-Sr isotopic mapping for the granitoid intrusions and mafic dykes (Wang et al., 2009), which shows that the central Altai contains widespread old crustal components in its basement. Considering that the sediments of the Habahe Group were related to the adjacent early-middle

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paleozoic magmatic rocks, we propose that the Precambrian zircons were probably derived from the source region of the magma. Hence, the Precambrian basement of the Altai orogenic belt may be as the source region existing in the deep crust, rather than the Habahe Group.

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5.4. Tectonic implications

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5.4.1. Tectonic nature of the Chinese Altai

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The tectonic nature of the Chinese Altai is controversial, with competing models advocating a Precambrian passive continental margin (He et al., 1990; Li et al., 2006,

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Liu et al., 2013), a Precambrian micro-continent (Li et al., 2006), an active continental margin in the early Paleozoic (Long et al., 2007; Sun et al., 2008), and a Paleozoic

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island arc (Sengör and Natal’in et al., 1996) or fold belt (Ren, 1997). It thus remains unknown or controversial regarding whether or not Precambrian basement exists in the Altai. Some researchers regard some mesometamorphic rocks in the Altai as

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Precambrian units (Li et al., 1996; Windley et al., 2002; Wang et al., 2009). However,

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other studies argue that these rocks resulted from high-temperature metamorphism of Paleozoic strata, reflecting the tectonic nature of magmatic arc or active continental margin on the basis of their protolith, depositional age and geochemistry (Sengör et al.,

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1993; Zhuang, 1994; Sun et al., 2008; Long et al., 2008a,b; Jiang et al., 2011). As the most widely exposed and old sedimentary strata in the Chinese Altai, the Habahe Group

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can potentially provide important clues for the tectonic nature of the Chinese Altai. The metasedimentary rocks of the Habahe Group in the Qinghe region show Ti/Zr and La/Sc ratios quite similar to those of sediments from typical continental arc settings (Fig. 13a, 13b; Bhatia and Crook, 1986), implying mixtures of the Paleozoic andesites and felsic volcanic rocks as dominant sediment sources (Fig. 13b; Chen et al., 2016). Furthermore, almost all samples plot in the continental arc field on the La–Th–Sc and Th–Sc–Zr/10 diagrams (Fig. 13a, b) (Bhatia and Crook, 1986). In summary, together

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with above-mentioned maximum depositional ages, metamorphism, and provenance of the Habahe Group, this study infers that the strata were sourced from adjacent magmatic rocks (514~427Ma). This is also supported by the dominantly positive εHf(t) values for detrital zircons from the meta-sedimentary rocks, which imply a juvenile and immature continental arc on the active margin of the Chinese Altai in the early-middle Paleozoic.

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Therefore, we conclude that the Habahe Group formed in a active continental margin

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5.4.2. Continental crustal reworking and arc type

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arc tectonic setting due to oceanic subduction in the early Paleozoic.

A <300 Ma difference between zircon crystallization age and TCDM Hf model age

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suggests that the source materials had been extracted from the mantle shortly before the formation of the juvenile rocks (e.g., Dhuime et al., 2011; Arndt and Davaille et al.,

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2013; Zou et al., 2017). Our data show that the model ages of the majorities of zircons are >300 Ma larger than the U-Pb ages (Fig. 13), suggesting that these zircons crystallized from magmatic rocks that contained abundant old crustal materials (Zou et

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al., 2017). Therefore, an ancient continental crust was most likely involved in the

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formation of precursor of the sedimentary rocks. In addition, it is worth to note that zircons with ages of ca. 403-534Ma (TCDM Hf model ages of 503-793Ma) and 698-951

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Ma (TCDM Hf model ages of 847-1122Ma) have the age differences less than 300Ma, implying that their source materials came from the juvenile crust (Supplementary Table

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6; Fig. 14). This suggests that the ancient crust have intensely reworked by mantlederived juvenile materials in the early Neoproterozoic, which produced juvenile source for the Habahe Group (Zou et al., 2017). Previous studies suggest that the tectonic history of the CAOB is similar to the present evolution of southwest Pacific（Xiao et al., 2004; Windley et al., 2007; Safonova et al., 2017a,b). As a crucial part of the CAOB, the Chinese Altai orogen might have experienced coincident geodynamic processes. The Pacific-type continental margin may have been classified into three subclasses, i.e. Cordilleran, Andeans and Oceanic-

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Island types. The two formers are characterized by low angle oceanic crust subduction and trench-mountain arc compound system, also called Peru-Chile subduction zone (Xiao et al., 2009c). The latter presents high angle subduction with the complicated system of trench-island arc-back arc basin, also named Japan-type subduction zone (Xiao et al., 2009c). Although our data and previous studies consistently suggest that

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the Chinese Altai probably represents an active continental margin arc in the earlymiddle Paleozoic (Sun et al., 2008, 2009; Long et al., 2008a,b; Cai et al., 2011a,b), the

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specific type of the continental arc and its evolution are still ambiguous.

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This study shows that the sources are complicated for the meta-sedimentary rocks of the Habahe Group, not only involving immature materials originated from mantle-

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derived juvenile crust, but also the remnants of ancient crust (Kröner et al., 2017). It belongs to the transitional arc with reworked continental crust, which was probably

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separated from continental margin and subsequently experienced magmatic activities, similarity to the Japan arc in the west Pacific (Hu, et al., 2017), such as the classical

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Japan Island and the Ryukyu Islands (Li et al., 1996). This type of arc is distinct from

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intra-oceanic island arc that develops large amounts of island type basalt and volcanic complexes, such as the typical Mariana arc and Tonga Kermadec island, and also different from the Andes arc characterized by the continental magmatic arc (Li et al.,

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1996; Xiao et al., 2009c). Hence, we propose that the Altai orogen, as a fragment with Precambrian basement, was probably rifted from the southern margin of the Siberia

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Craton, resulting from the break-up of Rodinia in the early Neoproterozoic (Cocks and Torsvik, 2007; Xiao et al., 2010; Wilhem et al., 2012; Yakubchuk, 2017; Likhanov and Santosh, 2017; Song et al., 2017; Zong et al., 2017). Subsequently, the northward subduction of the Paleo-Asian Ocean in the late Cambrian significantly reworked the continental sliver, accompanied by deformation caused by the erosion of tectonism and magmatism (Wang et al., 2006, 2010, 2017; Yuan et al., 2007), finally forming the transitional continental arc.

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6. Conclusions (1) The sedimentary rocks of the Habahe Group in the southeastern Chinese Altai were deposited during a period between the late Silurian and early Devonian (427~405Ma), and subsequently experienced regional metamorphism in the middle

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Devonian (~383±9Ma). (2) The source materials of the Habahe Group were mainly derived from an

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immature provenance, primarily adjacent early-middle Paleozoic magmatic rocks. The

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sources contain not only immature materials derived from mantle-derived juvenile crust,

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but also remnants of ancient crust.

(3) The Chinese Altai was a transitional arc during early Paleozoic time, similar to

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the Japan-type arc with involvements of old continental components, rather than the

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Acknowledgments

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Andean-type continental margin arc or the Mariana-type island arc.

This study was financially supported by the National Key R & D Program of China

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(2017YFC0601205), NSFC Key Project (41730213), Program of Geological Survey Bureau China (121201011000150003) and Hong Kong RGC GRF Project (17301915).

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We thank Drs. Yan Zhao, Bing Xu, Wenhao Ao and Hai Zhou for their laboratory assistance.

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Figure captions Fig. 1. Simplified geologic map of the Chinese Altai (modified from He et al., 1990; Li et al., 1996; Windley et al., 2002). The inset figure shows the extension of the CAOB. Abbreviations: CAOB = Central Asian Orogenic Belt; SC = Siberia Craton; TC = Tarim Craton; NC = North China Craton; N = North Domain, C =

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Central Domain, Q = Qiongkur Domain, S = South Domain. Fig. 2. Geological map showing major geological units in the Qinghe area of the eastern

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Chinese Altai Orogen (modified after 1:50000 Qinghe geologic map, 2016).

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Fig. 3. Simplified geological map showing the location of gneissic tonalite in the Qinghe area of the eastern Chinese Altai Orogen (modified after 1:50000 Qinghe

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geologic map, 2016).

Fig. 4. Petrologic features of studied samples from the metasedimentary strata and

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intrusive rocks in the eastern Chinese Altai. All micro-images are under crosspolarized light. Abbreviations for minerals: Q = quartz; Pl = plagioclase; Bi = biotite; Ms = muscovite; St = staurolite; Cord = cordierite; Am = amphibole.

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Fig. 5. Chondrite-normalized REE patterns and Upper Crust-normalized spider diagram

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of the sedimentary rocks. The geochemical data of the Chondrite and Upper Crust are from Sun and McDonough (1989) and Rudnick and Gao (2003), respectively.

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Fig. 6. Representative cathodoluminescence (CL) images for detrital zircons from eight samples in this study. Note explanations at the top right for labeled numbers.

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Fig. 7. U–Pb concordia diagrams and age histogram and relative probability plots for zircon samples of the banded gneisses in the eastern Chinese Altai. Fig. 8. U–Pb concordia diagrams and age histogram of relative probability plots for zircon samples of the sedimentary rocks in the eastern Chinese Altai. Fig. 9. U–Pb concordia diagrams and cathodoluminescence (CL) images for zircon samples of the gneissic granite and gneissic tonalite in the eastern Chinese Altai, Symbols as in Fig. 5. Fig. 10. CIA-ICV and rock classification diagrams for metasedimentary rocks (after

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Herron, 1988). Data of the PAAS are from Taylor and McLennan (1985), Symbols as in Fig. 4. Fig. 11. Geochemical diagrams showing source composition for the metasedimentary rocks from the Chinese Altai. Zr/Sc–Th/Sc diagrams after McLennan et al. (1993); La/Th–Hf diagram after Floyd and Leveridge (1987). Symbols are the same as

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those in Fig. 4. Fig. 12. Zircon age histogram and age -εHf discrimination pattern for the

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metasedimentary and granitic rocks. Symbols are the same as those in Fig. 4.

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Fig. 13. Tectonic discrimination diagrams for the metasedimentary rocks from the Chinese Altai, Th–Sc–Zr/10, La/Sc–Ti/Zr and La–Th–Sc diagrams (after Bhatia

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and Crook, 1986). Abbreviations for tectonic settings: ACM, active continental margin; PM, passive continental margin; CIA, continental island arc; OIA, oceanic

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island arc. Symbols are the same as those in Fig. 3. Fig. 14. Plots of Hf model age (TCMD) vs. crystallization age of the detrital zircons from

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growth in Chinese Altai.

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the Habahe Group. The yellow area in (a) represents periods of juvenile crust

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Highlights: The Habahe Group was formed in the late Silurian to early Devonian

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Regional thermal dynamic metamorphism occurred in the middle Devonian

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The Chinese Altai represents an active continental margin in the early Paleozoic.

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