Neoproterozoic continental back-arc rift development in the Northwestern Yangtze Block: Evidence from the Hannan intrusive magmatism

Accepted Manuscript Neoproterozoic continental back-arc rift development in the Northwestern Yangtze Block: Evidence from the Hannan intrusive magmatism

Biji Luo, Rong Liu, Hongfei Zhang, Junhong Zhao, He Yang, Wangchun Xu, Liang Guo, Liqi Zhang, Lu Tao, Fabin Pan, Wei Wang, Zhong Gao, Hui Shao PII: DOI: Reference:

S1342-937X(18)30087-X doi:10.1016/j.gr.2018.03.012 GR 1951

To appear in: Received date: Revised date: Accepted date:

20 March 2017 25 February 2018 1 March 2018

Please cite this article as: Biji Luo, Rong Liu, Hongfei Zhang, Junhong Zhao, He Yang, Wangchun Xu, Liang Guo, Liqi Zhang, Lu Tao, Fabin Pan, Wei Wang, Zhong Gao, Hui Shao , Neoproterozoic continental back-arc rift development in the Northwestern Yangtze Block: Evidence from the Hannan intrusive magmatism. The address for the corresponding author was captured as affiliation for all authors. Please check if appropriate. Gr(2018), doi:10.1016/j.gr.2018.03.012

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ACCEPTED MANUSCRIPT Neoproterozoic continental back-arc rift development in the Northwestern Yangtze Block: evidence from the Hannan intrusive magmatism Biji Luoa,*, Rong Liub, Hongfei Zhanga, Junhong Zhaoa, He Yangc, Wangchun Xua, Liang Guoa, Liqi Zhanga, Lu Taoa, Fabin Pana, Wei Wanga, Zhong Gaoa, Hui Shaoa a

School of Earth Sciences, and State Key Laboratory of Geological Processes and Mineral

Resources, China University of Geosciences, Wuhan 430074, China

Xinjiang Research Center for Mineral Resources, Xinjiang Institute of Ecology and

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Zhejiang Institute of Geology and Mineral Resource, Hangzhou 310007, China

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b

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Geography, Chinese Academy of Sciences, Urumqi 830011, China

*[email protected]

ACCEPTED MANUSCRIPT Abstract: Neoproterozoic intrusions are widespread in the northwestern margin of the Yangtze Block, and their petrogenesis was closely related to the assembly and breakup of Rodinia supercontinent. A combined study of zircon U–Pb dating, geochemistry and Sr–Nd–Hf isotopes was carried out for three types of intrusive rocks from the Hannan region,

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including the Beiba gabbros, the Tianpinghe and Taojiaba I-type granites and the

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Tiechuanshan and Huangguan A-type granites. LA–ICP–MS zircon U–Pb dating results show

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that the studied intrusions have crystallization ages ranging from ca. 880 to 770 Ma. The ca. 880 Ma Beiba gabbros have variable contents of MgO, Al2O3, Cr and Ni and extremely low

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high field strength elements (HFSE) concentrations that indicate a cumulate origin. These 87

Sr/86Sr

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gabbros display subduction-related geochemical signatures with whole–rock initial

(ISr) = 0.704–0.705, εNd(t) = –0.4 to +0.6 and zircon εHf(t) = +6.9 to +8.4, suggesting that they

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were derived from the partial melting of an enriched lithospheric mantle that had been

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modified by slab–derived materials. The ca. 860 Ma Tianpinghe I-type granites have high ISr (> 0.704) and negative εNd(t) (–5.6 to –3.5) values and zircon εHf(t) = –1.9 to +1.6, indicating

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their sources were mixtures of Neoproterozoic juvenile mafic crust and minor

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Paleoproterozoic crustal components, whereas the ca. 770 Ma Taojiaba I-type granites have relatively low ISr (< 0.704) and positive εNd(t) (+1.5 to +2.4) with zircon εHf(t) = +6.2 to +9.5, implying that they were dominantly sourced from the Neoproterozoic juvenile crustal materials. The geochemical and Sr–Nd–Hf isotopic data imply that the ca. 780 Ma Tiechuanshan A-type granites were crystallized from a reduced and peralkaline magma, which was generated by partial melting of a heterogeneous source involving Neoproterozoic juvenile and ancient mafic crust under dry and reducing conditions. In contrast, the parental magma of

ACCEPTED MANUSCRIPT the ca. 774 Ma Huangguan A-type granites was oxidized, aluminous and calc–alkalic and resulted from the dehydration melting of tonalitic to granodioritic rocks under water-subsaturated and oxidizing environments. In combination with previous work, the ca. 880–860 Ma Beiba gabbros and Tianpinghe I-type granites are interpreted to be formed

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during the stage of initial back-arc extension that was caused by subduction–induced

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convection in the mantle wedge. Subsequently, the ca. 780–770 Ma Tiechuanshan and

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Huangguan A-type granites and Taojiaba granodiorites were formed in a continental back–arc rift setting related to the rollback of subducted oceanic slab. Our new results support that an

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arc-back-arc system developed in the northwestern Yangtze Block during the Neoproterozoic.

Keywords: A-type granite; I-type granite; Neoproterozoic magmatism; continental back-arc

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rift; South China

ACCEPTED MANUSCRIPT 1. Introduction

Neoproterozoic igneous rocks, widespread in the South China Block (Fig. 1a), can provide important information on the assembly and breakup of the Rodinia supercontinent. In

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the past two decades, numerous geochemical and geochronological studies have been carried

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out on them (Fig. 1a). However, their petrogenesis and tectonic setting are still controversial.

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Three geodynamic mechanisms have been proposed to account for the generation of these rocks, including plume-rift (Li et al., 1999, 2003a, b, 2008a, b), slab-arc (Zhou et al., 2002a, b;

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Wang et al., 2004, 2006a, b; 2013) and plate-rift models (Zheng et al., 2006, 2007, 2008).

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The early Neoproterozoic igneous rocks (> 830 Ma) along the western and northern margins of the Yangtze Block are consistently considered to have been produced in an active

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continental setting (Zhou et al., 2002a, b; Ling et al., 2003; Li et al., 2008a, b; Dong et al.,

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2012; Zhao and Cawood, 2012; Wang et al., 2013; Li et al., 2017). However, the geodynamic setting of the late Neoproterozoic igneous rocks (830–740 Ma) has long been a matter of

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debate. The plume-rift model considered that ca. 830–795 Ma and ca. 780–745 Ma

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magmatism resulted from mantle superplume pulses which eventually led to the breakup of the Rodinia supercontinent (Li et al., 1999; 2003a, b, 2008a, b; Gao et al., 2016), whereas the arc model argued that they were formed in the long lived continental arc system (named as the Panxi-Hannan arc) along the western and northern margins of the Yangtze Block (Zhou et al., 2002a, b, 2006a, b; Zhao and Zhou, 2008, 2009a, b; Zhao et al., 2010; Dong et al., 2011; 2012; Zhao and Cawood, 2012; Dong and Santosh, 2016). The Hannan region, located at the northwestern margin of the Yangtze Block (Fig. 1), is a

ACCEPTED MANUSCRIPT key area for understanding the tectonic evolution of the Yangtze Block. A large amount of geochronological and geochemical data for the Hannan Neoproterozoic igneous rocks has been accumulating (Fig. 1b) (Dong et al., 2011, 2012; and references therein). Previous studies have mainly focused on the mafic-ultramafic rocks and calc-alkaline I-type granitoids,

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but have paid less attention on some rare A-type granitoids in this region, which could

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provide additionally significant constraints on the geodynamic setting of the Yangtze Block

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(Fig. 1b). Recently, petrogenesis and the tectonic affinity of the Neoproterozoic A-type granitic plutons in the western margin of the Yangtze Block are also still hotly debated

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(Huang et al., 2008; Zhao et al., 2008). In addition, the exact ages for some plutons are still

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unclear or controversial. For example, the Tianpinghe pluton have been dated either at 863 ± 10 Ma by Ling et al. (2006) using LA–ICP–MS zircon dating, but at 762 ± 4 Ma by Zhao and

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Zhou (2009a) using SHRIMP zircon dating. The Beiba complex has been considered to be

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formed at 814 ± 9 Ma based on SHRIMP U–Pb dating on zircon from the gabbros (Zhao and Zhou 2009b), while SIMS U–Pb dating on zircon from the gabbroic diorites yielded a

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crystallization age of 869 ± 5 Ma (Wang et al., 2016). Thus, additional studies on precise ages

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are required before making any further clarification. In this contribution, we report new LA–ICP–MS zircon U–Pb ages, geochemical and Sr–Nd–Hf isotopic data for three types of rocks in the Hannan region, including the Beiba gabbros, the Tiechuanshan and Huangguan A-type and the Tianpinghe and Taojiaba I-type granites. These rocks provide an opportunity to make a further understanding to the petrogenesis and tectonic implications of the Neoproterozoic igneous rocks around the Yangtze Block.

ACCEPTED MANUSCRIPT 2. Geological backgrounds

The South China was formed through amalgamation of the Yangtze Block in the northwest and the Cathaysia Block in the southeast along the Jiangnan orogen in the early

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Neoproterozoic (Wang et al., 2007, 2014; Zhao and Cawood, 2012) (Fig. 1a). The Yangtze

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Block is bounded by the Qinling-Dabie-Sulu orogenic belt to the north and is separated from

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the Songpan-Ganze and Bikou Terranes by the Longmenshan fault to the west (Fig. 1a). Widespread Neoproterozoic to Cenozoic cover overlie the sporadic outcrop of Archaean to

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Mesoproterozoic metamorphic basements in the Yangtze Block (Zhao and Cawood, 2012).

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The Kongling Complex, consisting of the Archean to Paleoproterozoic high-grade metamorphic TTG gneisses, metasedimentary rocks and amphibolites, is the oldest exposed

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rock unit in the northern interior of the Yangtze Block (Gao et al., 1999; Guo et al., 2015 and

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references therein).

The Hannan region is located at the northwestern corner of the Yangtze Block (Fig. 1a).

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Three Precambrian tectono-stratigraphic units, such as the Paleoproterozoic Houhe complex,

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the Meso- to Neoproterozoic Huodiya Group, and the Neoproterozoic Xixiang Group (Fig. lb), are recognized. The upper amphibolite-facies Houhe complex consists of trondhjemitic gneisses, amphibolites and migmatites with minor marbles (Ling et al., 2003; Wu et al., 2012). Zircon U–Pb dating for a felsic gneiss yielded a weighted mean

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Pb/206Pb age of 2081 ± 4

Ma (Wu et al., 2012). The greenschist-facies Huodiya Group, unconformably overlying the Houhe Group, comprises the metasedimentary rocks in the lower part and the volcanic rocks (Tiechuanshan Formation) in the upper part, respectively. The Tiechuanshan Formation is a

ACCEPTED MANUSCRIPT bimodal basalt–dacite/rhyolite sequence with a silica gap between 49.8 wt % and 66.3 wt %. The Tiechuanshan basalt consists of tholeiitic and alkaline members, which have been dated at 817 ± 5 Ma using TIMS U–Pb dating on zircon (Ling et al., 2003) (Fig. lb). The Xixiang Group, a meta-volcano-sedimentary succession, is composed of the lower basalt and andesite

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unit and the upper dacite and rhyolite unit (Gao et al., 1990; Ling et al., 2003). Volcanic rocks

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in the Xixiang group showing arc-like geochemical characteristics have been dated at ca.

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950–730 Ma by using U–Pb zircon dating method (Ling et al., 2003; Xia et al., 2009; Cui et al., 2010; Cui et al., 2013; Deng et al., 2013) (Fig. lb).

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Numerous Neoproterozoic intrusions intrude the Houhe, Huodiya and Xixiang groups in

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the Hannan region (Fig. 1). The ultramafic–mafic intrusions include the 898 ± 10 Ma Liushudian, ca. 880–824 Ma Beiba, ca. 860 Ma Zhengyuan, ca. 820–785 Ma Wangjiangshan,

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ca. 800–750 Ma Bijigou and 746 ± 4 Ma Mujiaba plutons (Zhou et al., 2002a, b; Zhao and

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Zhou, 2009b; Dong et al., 2011; 2012; Wang et al., 2016). According to the geochemical characteristics, the granitoid plutons in this region can be mainly classified into four types:

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diorites, I-type granites, TTGs (tonalite-trondhjemite-granodiorite) and alkali-granites. The

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diorites are composed of the 840 ± 6 Ma Nanjiang diorites (Dong et al., 2012), the 774 ± 37 Ma Guangwushan quartz diorite (Li, 2010) and the ca. 760–770 Ma Shahekan diorites (Zhao and Zhou, 2009a). The I-type granites consist of the 863 ± 10 Ma Tianpinghe granites (Ling et al., 2006), the 760 ± 4 Ma Tianpinghe quartz monzonites (Zhao and Zhou, 2009a) and the 770 ± 3 Ma Taojiaba granodiorites (This study). The TTGs dominated this region, including the ca. 887–840 Ma Xishenba tonalite (Ao, 2015), the ca. 764–718 Ma Wudumen and Erliba tonalites (Zhao et al., 2006; Zhao and Zhou, 2008; Dong et al., 2012) and the 728 ± 3 Ma

ACCEPTED MANUSCRIPT Zushidian trondhjemites (Ao, 2015). The alkali-granites include the ca. 780 Ma Huangguan monzogranite and syenogranite (Zhao et al., 2006; Dong et al., 2012) and the ca. 780 Ma Tiechuanshan aegirine-arfvedsonite (Aeg-arf) granites (Zhang et al., 1994). In this contribution, the Beiba gabbros, two I-type plutons (the Tianpinghe granites and Taojiaba

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granodiorites), and two A-type plutons (the Huangguan syenogranites and the Tiechuanshan

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Aeg-arf granites) were chosen for detailed study.

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3. Field relations and petrography

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3.1. The Beiba intrusive complex

The Beiba intrusive complex, with an outcrop area of ~150 km2 (Fig. 1b), consists of

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olivine gabbro, gabbro, hornblende gabbro and minor diorite (Zhao and Zhou, 2009b; Dong et

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al., 2012; Wang et al., 2016). In this study, the hornblende gabbros were sampled. They are coarse-grained and comprise 60–70% plagioclase, 15–25% hornblende, 10% clinopyroxene,

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5% orthopyroxene and minor Fe-Ti oxide (Fig. 2a and b).

3.2. The Tianpinghe and Taojiaba intrusions The Tianpinghe intrusive complex, with an outcrop area of ~17 km2 (Fig. 1b), is composed of ca. 860 Ma granite (Ling et al., 2006) and ca. 760 Ma quartz monzonite (Zhao and Zhou, 2009a). The collected granite in this study is gray, fine to medium grained, massive and consists of 35–40% K-feldspar, 20–25% quartz, 15–20% plagioclase, 5–8% biotite and 2–3% hornblende with minor apatite, Fe-Ti oxide and zircon (Fig. 2c and d).

ACCEPTED MANUSCRIPT The Taojiaba granodiorite intruded into the Meso- to Neoproterozoic Huodiya Group. These rocks are fine- to medium-grained and consist of 60–65% plagioclase, 15–25% quartz, 15–20% K-feldspar, 7–8% hornblende and 5–7% biotite (Fig. 2e). Accessory minerals include

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apatite, Fe-Ti oxide, titanite and zircon (Fig. 2e).

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3.3. The Huangguan and Tiechuanshan intrusions

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The Huangguan pluton, with an outcrop area of ~42 km2, intruded into the Xixiang group (Fig. 1b). The rocks are mainly of monzogranite and syenogranite. The syenogranite in

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this study is reddish pink, medium-grained and consists of 50–60% microcline, 25–30%

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quartz, 5–10% plagioclase and 2–3% biotite and hornblende with minor Fe-Ti oxide, zircon, allanite and titanite (Fig. 2f).

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The Tiechuanshan Aeg-arf granite, with an outcrop area of ~9 km2, is located at the west

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side of the Beiba complex and intruded into the Tiechuanshan Formation (Fig. 1b). The pluton is light pink, fine- to medium-grained and massive, and is composed of 40–45%

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microcline, 20–25% quartz, 10–20% albite plagioclase and 3–5% dark minerals including

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aegirine, arfvedsonite and augite (Fig. 2g and h). Accessory minerals include zircon, monazite, niobite, allanite and apatite.

4. Analytical methods

Fresh rock samples were crushed in a steel crusher and then powdered in an agate mill to a grain size <200 mesh. Major elements were analyzed at the Hubei Geological Analytical

ACCEPTED MANUSCRIPT Center, Wuhan. For analytical methods see Zhang et al. (2002a). The analytical uncertainty is generally <5%. Trace elements were measured using Agilent 7500a ICP–MS at the State Key Laboratory of Geological Processes and Mineral Resources (GPMR), China University of Geosciences, Wuhan. Sample-digesting procedure for ICP–MS analyses and analytical

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precision and accuracy are described by Liu et al. (2008). Whole–rock Sr and Nd isotopic

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analytical procedures are described by Gao et al. (2004).

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ratios were measured by a Triton thermal ionization mass spectrometer at GPMR. For details

Zircons were separated using heavy-liquid and magnetic separation techniques before

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examination under a binocular microscope, and then were mounted in epoxy resin and

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polished. Cathodoluminescence (CL) images, taken at Northwest University, Xi’an, were used to check the internal textures of individual zircon and to guide U–Pb dating and Hf isotope

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analysis. U–Pb zircon dating was carried out using LA–ICP–MS on an Agilent 7500 equipped

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with a 193 nm ArF excimer laser at GPMR. Operating conditions are the same as described by Liu et al. (2010). A beam diameter of 32 µm was used. Zircon 91500 and the NIST610

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glass were used as an external standard for Pb/U ratio and concentration, respectively.

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Common Pb correction is made by using the program of ComPbCorr#3–17 (Andersen, 2002). Off-line selection and correction and quantitative calibration were conducted by ICPMSDataCal (Liu et al., 2010). In situ Zircon Hf isotope analysis was carried out using a Geolas 2005 193nm ArF excimer laser, attached to a Nu plasma multi–collector ICP–MS, at the Key Laboratory of Continental Dynamics, Northwest University. The detailed analytical technique is similar to that described by Yuan et al. (2008). The decay constant for

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Lu of 1.865×10-11yr-1 was

ACCEPTED MANUSCRIPT adopted (Scherer et al., 2001). Initial 176Hf/177Hf ratio, denoted as εHf(t), is calculated relative to the chondritic reservoir with a

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Hf/177Hf ratio of 0.282772 and

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Lu/177Hf of 0.0332

(Blichert-Toft and Albarède, 1997). Single-stage Hf model ages (TDM(Hf)) are calculated relative to the depleted mantle which is assumed to have a linear isotopic growth from Hf/177Hf = 0.279718 at 4.55 Ga to 0.283250 at present, with

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Lu/177Hf ratio of 0.0384

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(Vervoort and Blichert-Toft, 1999).

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5. Results

5.1. Zircon U–Pb geochronology

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Five samples were collected for LA–ICP–MS U–Pb zircon dating. The results are listed in the supplementary Table S1. Detailed information on grain sizes, internal structures, Th and U

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contents, Th/U ratios and weighted mean summarized in Table 1. The weighted mean

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Pb/238U ages of the analyzed zircons are

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Pb/238U ages are interpreted as the magma

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crystallization ages. Representative zircon CL images and their U–Pb concordia plots are shown in Fig. 3. Zircons from all the samples are subhedral to euhedral, short to long

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prismatic and 50–350 μm in length, with aspect ratios of 1:1–4:1. Most zircon grains exhibit clear oscillatory zoning and zircons from the Beiba gabbro show broad oscillatory zoning and sector zoning (Fig. 3), indicative of magmatic origin. Some zircons have inherited cores, which display fragmented and ovoid shapes with no zoning and bright luminescence (Fig. 3). Seventeen analyses on zircons from the Beiba gabbro (sample L52) yield

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Pb/238U ages

from 853 to 890 Ma, giving a weighted mean of 879 ± 6 Ma (2σ; MSWD = 0.81) (Fig. 3a). Three analyses give younger 206Pb/238U ages of 826–830 Ma, constituting a Pb loss trend.

ACCEPTED MANUSCRIPT For the Tianpinghe granite (sample L53), five analyses on inherited cores give an upper intercept age of 2023 ± 94 Ma (Fig. 3b), while three analyses yield younger

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Pb/238U ages

(738–808 Ma), probably due to Pb loss (Fig. 3c). The remaining nine spots yield

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Pb/238U

ages from 843 to 867 Ma, with a weighted mean of 860 ± 6 Ma (2σ; MSWD = 1.4) (Fig. 3c).

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Twenty analyses on zircons from the Taojiaba granodiorites (sample L38) have 206Pb/238U

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Pb/238U age of ca. 820 Ma. Three spots plot

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MSWD=0.43). One inherited core gives a

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ages ranging from 760 to 781 Ma, with a weighted mean age of 770 ± 3 Ma (Fig. 3d) (2σ，

below the concordia line, constituting a Pb loss trend.

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For the Huangguan Syenogranite (sample L64), nineteen analyses yield

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Pb/238U ages

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ranging from 762 to 788 Ma with a weighted mean of 774 ± 3 Ma (2σ; MSWD = 1.4) (Fig. 3e). Three analyses give younger 206Pb/238U ages of 694 and 730 Ma, probably due to Pb loss.

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Fifteen analyses on zircons from the Tiechuanshan Aeg-arf granite (sample L42)

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yield 206Pb/238U ages between 766 and 787 Ma, with a weighted mean of 782 ± 4 Ma (2σ; MSWD=1.3) (Fig. 3f). Two inherited cores give 206Pb/238U ages of ca. 910 Ma. Five analyses

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plot slightly away from the concordia line, constituting a Pb loss trend.

5.2. Major and trace elements Whole–rock major and trace element compositions of twenty-four samples are listed in the supplementary Table S2. Most samples have low loss on ignition (LOI) values (<2.8 wt %) and both mobile and immobile elements of these samples do not exhibit any correlation with the LOI (not shown), indicating insignificant alteration.

ACCEPTED MANUSCRIPT 5.2.1. The Baiba gabbros The Beiba gabbros are low-K to medium-K subalkaline (Fig. 4a and Table S2). They have low SiO2 (44.76–50.75 wt %), high CaO (9.01–12.05 wt %) and MgO (6.62–11.35 wt %) and variable Al2O3 (12.02–19.47 wt %) and TiO2 (0.23–2.18 wt %) contents. They also have high

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Cr (82–337 ppm) and Ni concentrations (85–427 ppm) (Fig. 5 and Table S2). In trace element

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spider diagram (Fig. 6a), the gabbros show distinct negative Th, U, Nb, Ta, Zr and Hf

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anomalies and obvious positive Sr, Ba and K anomalies. They have low total rare earth (REE) contents (11–55 ppm) (Table S2) and show flat chondrite-normalized REE patterns with

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(La/Yb)N ratios of 2.03–3.28 and Eu/Eu* values of 0.89–1.68 (Figs. 5g, h and 6b).

5.2.2. The I-type granites

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The Tianpinghe granites have relatively high SiO2 (70.69–72.88 wt %) contents, while the

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Taojiaba granodiorites have relatively low SiO2 (66.46–67.39 wt %) contents. Rocks from the two intrusions have similar geochemical compositions. They are slightly peraluminous with

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A/CNK values [molar Al2O3/(CaO+Na2O+K2O)] ranging from 1.00 to 1.05 and belong to the

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calc-alkaline and magnesian rocks (Frost et al., 2001) (Fig. 4). They also have low MgO (<2 wt %), Mg# (<50), Cr (<10 ppm), Ni (<10 ppm) values and low K2O/Na2O ratios of 0.6 to 1.0 (Fig. 5 and Table S2). In Fig. 6c, all samples show negative Nb, Ta, P and Ti anomalies and are relatively enriched in Rb, Ba, Th, U and K. The Taojiaba granodiorites display higher Th and U and lower Nb and Ta than the Tianpinghe granite. They show moderately fractionated chondrite-normalized REE patterns with (La/Yb)N = 9.0–14.3 and weak to negligible negative Eu anomalies (Eu/Eu* = 0.71–0.95) (Fig. 6d).

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5.2.3. The A-type granites The Huangguan syenogranites and the Tiechuanshan Aeg-arf granites are enriched in SiO2 (73–77 wt %) and K2O+Na2O (7–9 wt %) contents (Fig. 4a). They are characterized by low

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MgO, CaO, Al2O3, Cr, Sr and P, and high Zr and Nb contents (Figs. 5 and 6). The

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Tiechuanshan Aeg-arf granites are mainly peralkaline and alkalic-calcic and are ferroan

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(Fe-index = ~99) (Fig. 4b, c and d) (Frost et al., 2001). The Huangguan syenogranite are metaluminous to weakly peraluminous and calc-alkalic, and straddle the boundary between

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ferroan and magnesian series (Fe-index = 77–95), with the majority being ferroan (Frost et al.,

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2001) (Fig. 4b, c and d). The ununiformity of the Huangguan syenogranites indicates that the studied Huangguan granites samples do not represent a same unit or a same granitic series.

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The ferroan samples have more geochemical affinity with A-type granites, while two samples

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with magnesian character have more geochemical affinity with calc-alkalic magnesian granites. This research mainly focuses on the ferroan samples of the Huangguan

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syenogranites.

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In the spider diagram (Fig. 6e), they show relatively weak negative anomalies of Ba, Nb and Ta, and obvious negative anomalies of Sr, P, Eu and Ti, which also agree with those of other Neoproterozoic (820–780 Ma) A-type magmatic rocks in the South China Block (Huang et al., 2008; Zhao et al., 2008b; Wang et al., 2010). The analyzed Tiechuanshan Aeg-arf granitic samples are more enriched in HFSE (e.g. Th, U, Nb, Ta, Zr and Hf), and have high total REE contents than those of the Huangguan syenogranites (Fig. 6e). They also exhibit strong negative Eu anomalies (Eu/Eu* = 0.10–0.44) (Figs. 5h and 6f). The Huangguan

ACCEPTED MANUSCRIPT syenogranites display more strongly fractionated REE patterns ((La/Yb)N = 7.4–34.1) than the Tiechuanshan Aeg–arf granites ((La/Yb)N = 5.28–6.22) (Figs. 5g and 6f).

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5.3. Whole–rock Sr–Nd isotopes and zircon Hf isotopes

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Whole–rock Sr and Nd isotopic data are presented in Table 2. For comparison, the initial Sr/86Sr isotopic ratio (ISr) and εNd(t) values of all samples are calculated at 780 Ma and

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plotted in Fig. 7a. Zircon Lu–Hf isotopic data are listed in Supplementary Table S3 and the

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initial zircon εHf(t) values are calculated at their magma crystallization ages, respectively. The

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results are briefly summarized in Table 1 and plotted in Fig. 7b. The Beiba gabbros have ISr = 0.7042–0.7045, εNd(t) = –0.39 to +0.58 and zircon εHf(t)

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values of +6.9 to +8.4 (Fig. 7) with mantle depletion model ages of TDM(Nd) = 1.80–2.52 Ga

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and TDM(Hf) = 1.10–1.16 Ga. The Tianpinghe granites have ISr = 0.7044–0.7048, εNd(t) = –5.6 to –3.5, and zircon εHf(t) values of –1.9 to +1.6 (Fig. 7) with TDM(Nd) = 1.59–1.74 Ga and

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TDM(Hf) = 1.35–1.49 Ga. The Taojiaba granodiorites have ISr = 0.7032–0.7035, εNd(t) = +1.5

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to +2.36 and zircon εHf(t) values of +6.2 to +9.5 (Fig. 7) with TDM(Nd) = 1.21–1.32 Ga and TDM(Hf) = 0.98–1.09. The Tiechuanshan Aeg-arf granites show large variation in ISr (0.7079 to 0.7123), and have εNd(t) = –2.7 to –0.7 and zircon εHf(t) values of +0.5 to +5.3 (Fig. 7) with TDM(Nd) = 1.54–1.78 Ga and TDM(Hf) = 1.14–1.32 Ga. Due to the low Sr contents and high Rb/Sr raitos, their Sr isotopes have no petrogenetic significance. The Huangguan syenogranites show a restricted ISr (0.7024 to 0.7032), εNd(t) (+0.89 to +3.07) and zircon εHf(t) values (+1.9 to +6.9) (Fig. 7) with TDM(Nd) = 1.21–1.25 Ga and TDM(Hf) = 1.07–1.27 Ga.

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6. Discussion

6.1. Gabbros derived from the subduction modified lithospheric mantle

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The Beiba gabbros are characterized by extreme depletions of Th, U, Nb and Ta, and two

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samples have total REE contents even lower than those of the N–MORB (Fig. 6a, b). The

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petrographic and geochemical data indicate that the Beiba gabbros are either cumulates or mixtures of cumulates and the evolved melts (Fig. 2a). Sample L51 has relatively high MgO

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(11.35 wt %), Cr (337 ppm) and Ni (427 ppm), extremely low Al2O3 (12.02 wt %) and total

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REE (55 ppm) abundances (Figs. 4 and 5a, b), suggesting preferential accumulation of hornblende and pyroxenes (Fig. 2a). Compared with sample L51, Sample L50 has higher

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Al2O3 (19.47 wt %) and shows more positive Eu anomaly (Eu/Eu* = 1.69) than sample L51,

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indicating preferential accumulation of hornblende, pyroxenes and plagioclase. Two samples (L49 and L52) show relatively low MgO (6.62–9.02 wt %), Cr (76–82 ppm) and Ni (85–90

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ppm) contents, relatively high Al2O3 (15.63–16.16 wt %) contents, and positive Eu anomaly

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(Eu/Eu* = 1.12) (Figs. 4 and 5a, b), indicating preferential accumulation of plagioclase from a magma that had undergone fractional crystallization of olivine and pyroxene. In addition, extremely enriched Fe2O3t (15.16 wt %) and TiO2 (2.18 wt %) in sample L49 is indicative of accumulation of Fe-Ti oxides. These features suggest that these samples can not be equivalent to the direct solidification of a homogenous magma. One of the most important issues for cumulate gabbros is to identify their parental magmas. During the magma emplacement, the chemical composition of primitive basaltic

ACCEPTED MANUSCRIPT melt is commonly changed by fractional crystallization and assimilation. The Beiba gabbros have Th, U, Nb and Ta concentrations much lower than the lower continental crust (Rudnick and Gao, 2003) (Fig. 6a). They display homogenous whole–rock ISr (~0.704) and εNd(t) (~1 unit) values, and zircon εHf(t) (~1 unit) values (Fig. 7), indicating insignificant crustal

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assimilation during the magma evolution.

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The parental magma of the Beiba gabbros is hydrous as evidenced by the existence of

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hornblende and biotite in the studied samples. The hydrous fluids were most likely derived by dehydration of subducted slab, because the Beiba gabbros show enrichment of LILE (Rb, Ba,

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Sr and K) and LREE and depletion of HFSE (e.g., Nb and Ta) (Fig. 6a), clearly resembling to

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the subduction-related arc magmas. As shown in Fig. 7a, the Beiba gabbros are plotted in the field of the ca. 950–820 Ma Xixiang and Tiechuanshan basaltic rocks, implying similar

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sources for all these mafic magmas (Ling et al., 2003). The Beiba gabbros have lower

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whole–rock εNd(t) (–0.4 to +0.6) and zircon εHf(t) (+7 to +8) values than those of the depleted mantle (Fig. 7), suggesting that their primary magmas were derived from the partial melting

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materials.

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of an enriched lithospheric mantle source that had previously been modified by slab-derived

6.2. I-type granitoids produced by partial melting of mafic crust The Tianpinghe granites and Taojiaba granodiorites are slightly peraluminous and contain biotite and hornblende, similar to typical I-ype granitoids (Chappell and White, 1992). They fall in the fields of I-, S- and M-type granitoids (Fig. 7a–c) and are also plotted in the field of typical arc magmatic rocks (Fig. 9). All these features indicate that the Tianpinghe granites

ACCEPTED MANUSCRIPT and Taojiaba granodiorites are I-type granitoids. Calc-alkaline I-type granitic rocks are commonly generated by partial melting of mafic to intermediate rocks, differentiation of mantle-derived magmas or mixing between crustal- and mantle-derived melts (Clemens et al., 2009). The studied Tianpinghe and Taojiaba I-type

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granitoids have low MgO (Mg#), Cr and Ni contents (Fig. 5 and Table S2) and display a large

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compositional gap with the contemporaneous mafic-ultramafic rocks in the Hannan region,

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indicating that they were predominantly of crustal origin and mantle-derived melt was not involved in their petrogenesis.

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The 730 Ma adakitic rocks in the Hannan region have been suggested to be produced by

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partial melting of a thickened Neoproterozoic juvenile mafic crust with garnet-bearing and plagioclase-free residuals (Zhao and Zhou, 2008; Liu et al., 2009). By contrast, the

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Tianpinghe and Taojiaba I-type granitoids show lower Sr, Sr/Y and (La/Yb)N values (Figs. 4

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and 8), suggesting that they were derived from a relatively shallower source (<0.8–1.0 Gpa). They also have high K2O/Na2O ratio than that of the Hannan adakitic rocks (Fig. 5c),

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indicating a medium- to high-K basaltic protolith.

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The Sr–Nd–Hf isotopic compositions of the studied I-type granitoids are different from those of the basement rocks (Fig. 7), such as the Kongling complex (Gao et al., 1999; Ma et al., 2000; Zhang et al., 2006) and the Houhe complex (Lin et al., 1997; Zhang et al., 2002b), indicating that they were not derived from the partial melting of the ancient crustal rocks. The Taojiaba granodiorites have low ISr (0.7032–0.7035) and positive εNd(t) (+1.5 to +2.4) and zircon εHf(t) (+6.2 to +9.5) values, similar to those of the Neoproterozoic mafic-ultramafic rocks in the western margin of the Yangtze Block (Fig. 7) (Zhou et al., 2002a; Zhao et al.,

ACCEPTED MANUSCRIPT 2008a; Zhao and Zhou, 2009b), suggesting that they were more likely derived from the partial melting of the juvenile mafic lower crust. The Tianpinghe granites have slightly high ISr (0.7044–0.7048), low zircon εHf(t) (–1.9 to +1.6), and negative εNd(t) (–5.6 to –3.5) values

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(Fig. 7), implying that ancient crustal components were involved in their formation.

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6.3. Petrogenesis of A-type granites

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The Tiechuanshan and Huangguan A-type granites have higher zircon saturation temperatures (TZr) (836–997℃ and 768–821℃, respectively) (Boehnke et al., 2013) than

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those of the Neoproterozoic I-type granitoids (this study) and adakitic rocks (Zhao and Zhou,

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2008; Liu et al., 2009) in the northwestern Yangtze Block (Fig. 9e). In the plot of FeOT/(FeOT + MgO) vs. Al2O3 (Dall'Agnol and de Oliveira, 2007) (Fig. 9d), the parental magmas of the

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Tiechuanshan Aeg-arf granites are reduced, while the parental magmas of the Huangguan

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ferroan syenogranites are mainly oxidized. The Tiechuanshan and Huangguan A-type granites also have chemical compositions and TZr values similar to other Neoproterozoic A-type felsic

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rocks in the South China Block (Ling et al., 2003; Huang et al., 2008; Zhao et al., 2008b;

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Wang et al., 2010) (Figs. 4, 5, 6 and 9). Several mechanisms have been proposed for the generation of A-type granites, including fractionation of basaltic magmas with or without crustal assimilation, partial melting of residual crust, partial melting of mafic or felsic rocks, and mixing between crustal and mantle-derived melts (Collins et al., 1982; Whalen et al., 1987; King et al., 1997; Dall'Agnol and de Oliveira, 2007; Frost and Frost, 2011). King et al. (1997) and Frost and Frost (2011) suggested that partial melting of tonalitic to granodioritic rocks can form metaluminous to peraluminous calc-alkalic A-type granites.

ACCEPTED MANUSCRIPT Dall'Agnol and de Oliveira (2007) suggested that oxidized A-type granites could be derived from dehydration melting of quartz-dioritic, tonalitic and granodioritic rocks under oxidizing conditions, which is also consistent with experimental results (Skjerlie and Johnston, 1993; Patiño Douce, 1997; Dall'Agnol et al., 1999; Klimm et al., 2003). The Huangguan ferroan

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A-type granites are oxidized, aluminous and calc-alkalic, indicating that they were probably

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produced by melting of tonalitic or granodioritic rocks. The Huangguan A-type granites have

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similar whole–rock Sr–Nd and zircon Hf isotopic compositions to those of the widespread early Neoproterozoic I-type igneous rocks in the western margin of the Yangtze Block (Fig. 7)

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(Zhou et al., 2002a; Ling et al., 2006; Sun and Zhou, 2008; Dong et al., 2012; Du et al., 2014;

Neoproterozoic I-type igneous rocks.

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Ao, 2015), indicating that they were probably derived from the reworking of these early

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In contrast, the reduced, peralkaline and alkalic-calcic A-type granites are suggested to be

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produced by extreme differentiation of basaltic melts (King et al., 1997; Dall'Agnol and de Oliveira, 2007; Frost and Frost, 2011). Nevertheless, this scenario can not be applied to the

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Tiechuanshan A-type granites. Firstly, there are no contemporary mafic-ultramafic rocks

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associated with the Tiechuanshan granites. The adjacent Beiba gabbros were emplaced earlier than the Tiechuanshan granites. Secondly, the Zr, as well as Nb, Ta and Hf (not shown) contents of the Tiechuanshan A-type granites exhibit a larger variation (Fig. 5f), but Eu/Eu* values keep constant at a restricted range of SiO2 (Fig. 5h), which is inconsistent with crystal fractionation. On the other hand, the rocks have homogenous whole–rock εNd(t) and zircon εHf(t) values and extremely low MgO and Cr contents, ruling out the influence from crustal assimilation and magma mixing during the magma evolution. Thus, the Tiechuanshan A-type

ACCEPTED MANUSCRIPT granites were most likely generated by partial melting processes. In the Rb/Nb–Y/Nb and Y-Nb-Ce diagrams (Fig. 9f), the Tiechuanshan A-type granites fall into the A2 group, indicating that the primary magmas of the Tiechuanshan A-type granites were produced by melting of continental mafic crust (Eby, 1992).

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The Tiechuanshan A-type granites have distinct whole–rock εNd(t) values (–2.7 to –0.7)

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and TDM(Nd) ages (1.54–1.78 Ga), indicating a distinct magma source from that of the

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Huangguan A-type granites. The Tiechuanshan A-type granites have higher TZr (836–997℃) values (Fig. 9e) and HFSE and HREE contents (Fig. 6e, f) than those of the Huangguan

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A-type granites, indicating a drier and hotter source. Although partial melting of residual

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granulite of previous I-type granitoids can result in some geochemical characteristics of A-type granites (Collins et al., 1982; Whalen et al., 1987), this residual model cannot account

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for high Fe-index of some A-type granites (Creaser et al., 1991; Wang et al., 2010). The

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Tiechuanshan A-type granites have geochemical features (Fig. 6e, f) and εNd(t) (Fig. 7a) values similar to those of the 820 Ma Tiechuanshan dacites and rhyolites, implying that they

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may share similar magma source. Moreover, the Tiechuanshan A-type granites have εNd(t)

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values between that of the Neoproterozoic Tiechuanshan basalts and the Paleoproterozoic Houhe meta-mafic rocks (Ling et al., 2003) (Fig. 7a). Thus, the most plausible magma source for the Tiechuanshan A-type granites is a mixture of the newly underplated juvenile mafic crust represented by the Neoproterozoic Tiechuanshan basalts and the Paleoproterozoic Houhe meta-mafic rocks. Vervoort et al. (1999) pointed out that Hf–Nd isotopic data of igneous rocks generally show a coupled relationship. Base on the ‘terrestrial array’ (εHf = 1.36*εNd + 2.95) evolution equation, εHf(780 Ma) values calculated for the Tiechuanshan

ACCEPTED MANUSCRIPT basalts and the Houhe meta-mafic rocks are +2.8 to +9.8 and –5.4 to +2.0 (Fig. 7b), respectively. The Tiechuanshan A-type granites display an intermediate zircon εHf(t) value (+0.5 to +5.3) between those of the Tiechuanshan basalts and the Houhe meta-mafic rocks (Fig. 7b), supporting a mixing source as we proposed above.

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In summary, we favor that the oxidized, aluminous and calc-alkalic Huangguan A-type

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granites were derived from dehydration melting of tonalitic to granodioritic rocks under

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water-subsaturated and oxidizing conditions (Skjerlie and Johnston, 1993; Patiño Douce, 1997; Dall'Agnol et al., 1999; Klimm et al., 2003). In contrast, the reduced, peralkaline and

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alkalic-calcic Tiechuanshan A-type granites were generated by partial melting of a

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heterogeneous source involving underplated juvenile mafic crust and ancient mafic crust

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under dry and reducing environments (Frost and Frost, 2011).

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6.4. Neoproterozoic arc-back-arc development in the Northwestern Yangtze Block The inconsistent geochronological results between earlier studies for the same pluton

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could lead to the conflicting tectonic interpretations. The Beiba gabbro has a LA–ICP–MS

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zircon U–Pb age of 879 ± 6 Ma (Fig. 3a), which is in agreement with the SIMS zircon U–Pb age of 869 ± 5 Ma for the Beiba gabbroic diorite within errors (Wang et al., 2016). The Tianpinghe granite has a LA–ICP–MS zircon U–Pb age of 860 ± 6 Ma (Fig. 3c), consistent with the previously reported LA–ICP–MS zircon age of 863 ± 10 Ma within errors (Ling et al., 2006). The Huangguan syenogranite has a LA–ICP–MS zircon U–Pb age of 774 ± 3 Ma (Fig. 3e), which is comparable to the TIMS U–Pb zircon age of 778 ± 5 Ma (Zhao et al., 2006). These repeated, but independent, results suggest that our zircon U–Pb dating results

ACCEPTED MANUSCRIPT are reliable. The possible cause of the inconsistency of the previous geochronological data could be that the studied plutons are intrusive complex with multistage emplacement. This possibility is supported by the geochemical observations. The 762 ± 4 Ma Tianpinghe quartz monzonites (Zhao and Zhou, 2009a) show a higher whole–rock εNd(t) and zircon εHf(t) values

PT

than those of the Tianpinghe granites in this study (Fig. 7). The 814 ± 9 Ma Beiba gabbros

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reported by Zhao and Zhou (2009b) also have a larger variation of zircon εHf(880 Ma) values

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(+0.8 to +12.8) than those of the Beiba gabbros (+6.9 to +8.4) in this study. These observations indicate that the Beiba and Tianpinghe plutons were constructed from multiple

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injections of magma over a long period of time.

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The new zircon dating results, together with previous geochronological studies clearly show that the Neoproterozoic magmatism in the Northwestern Yangtze Block was active from

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ca. 950 Ma to 700 Ma (Fig. 1). This long duration (over 200 m.yr) of the Neoproterozoic

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magmatism in the Northwestern Yangtze Block is similar to the arc-related magmatism in Andean-type active continental margin (Hervé et al., 2007), but in strong contrast to the short

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pulses (~1–5 m.yr) of magmatism related to mantle plume (Bryan and Ernst, 2008). In

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addition, the Neoproterozoic lithological assemblages in the Northwestern Yangtze Block are composed of abundant intermediate and felsic granitoids and minor mafic-ultramafic magmas with typical subduction-related geochemical fingerprint (Zhou et al., 2002a, b; Zhao and Zhou, 2008, 2009b; Zhao et al., 2010; Dong et al., 2011, 2012), which is inconsistent with massive continental flood basaltic volcanism that is generally associated with plume-related magmatism (Condie, 2011; Bryan and Ferrari, 2013). In view of these, we incline to the view that the Northwestern Yangtze Block was in a continental arc setting for a long time span

ACCEPTED MANUSCRIPT during the Neoproterozoic. The ca. 880–870 Ma Beiba gabbros and gabbroic diorites have arc-related geochemical signatures and were derived from the partial melting of the enriched mantle wedge that had been modified by subduction-related materials (Wang et al., 2016). The ca. 860 Tianpinghe

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granites resulted from reworking of juvenile basaltic crust with contamination by some

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Paleoproterozoic basement materials. It has been reported that the approximately coeval

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subduction-related gabbros, gabbroic diorites and granitoids are widely distributed in the northern and western margins of the Yangtze Block, such as the ca. 870–855 Ma Wangcang

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granodiorites and Zhengyuan gabbronorites and the ca. 900 Ma Liushudian gabbros (Dong et

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al., 2012) and Xishenba tonalites (Ao, 2015) in the Hannan region, the ~880 Ma Guankouya and Pingtoushan diorites and Liujiapu gabbros (Xiao et al., 2007) and the ca. 880 Ma

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Tongchang diorites (Wang et al., 2012) in the Bikou terrane, the ca. 860 Ma Guandaoshan

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gabbros, diorites and granodiorites in the western margin and the ca. 870–860 Ma Sanligang mafic dikes and granitoids in the northern margin of the Yangtze Block (Xu et al., 2016) (Fig.

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1). The presence of these subduction-related magmatic rocks indicates that there was an

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oceanic slab subduction beneath the northern and western margins of the Yangtze Block during the early Neoproterozoic. The presence of the ca. 950–895 Ma N-MORBs, boninites and the low-Ti tholeiitic basalts in the lower Xixiang group (Ling et al., 2003) and the ca. 970 Ma mylonitize granites (Geng, 2010) and the ca. 900 Ma gabbros (Dong et al., 2011) intruded in Xixiang group suggests that these magmatic rocks formed in a continental arc setting and the onset of oceanic slab subduction beneath the northwestern margin of the Yangtze Block occurred at least since ca.

ACCEPTED MANUSCRIPT 960 Ma (Ling et al., 2003; Geng, 2010; Dong et al., 2011) (Fig. 10a). Recently, the ca. 970 Ma volcanic rocks of the Tongmuliang group with the spilite-keratophyre-quartz-keratophyre associations have been reported to occur to the southwest of the Hannan batholith, also suggesting that there was an ocean subduction along the northwestern margin of the Yangtze

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Block during early Neoproterozoic (Li et al., 2017). During the ca. 890–830 Ma, with

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sequential subduction, arc volcanic rocks, mafic rocks and calc-alkaline granitoids continued

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to be generated in the Hannan arc (Fig. 10b) (Ling et al., 2006; Xia et al., 2009; Ao, 2015). Consequently, subduction-induced convection in the mantle wedge beneath the Hannan region

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resulted in an initiation of back-arc extension, in which the Beiba gabbros and the Tianpinghe

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granites were generated (Fig. 10b).

The occurrence of the ca. 780 Ma Tiechuanshan and Huangguan A-type granites manifests

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an extensional environment in the Hannan region at that time (Eby, 1992). The ca. 820 Ma

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Tiechuanshan bimodal volcanic rocks in the Hannan region consist of tholeiitic and alkaline basalts and A-type dacites and rhyolites, which have previously been interpreted to be formed

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in an intra-continent rift setting (Ling et al., 2003). However, the bimodal volcanic rocks and

AC

the A-type magmas are not inevitably indicative of an intra-continent rift setting and can also occur in a back-arc setting (e.g. Espinoza et al., 2008; Wang et al., 2017a). Both modern and ancient back-arc magmatism show a wide range of compositions, including calc-alkaline basalts, andesites and rhyolites, tholeiitic basalts, alkaline basalts and perakaline rhyolites (Luhr, 1997; Rivers and Corrigan, 2000; Espinoza et al., 2008; Viruete et al., 2008). Furthermore, some ca. 820–770 Ma mafic and felsic intrusions with typical arc signature in the Hannan region have been suggested to be formed in a subduction related environment

ACCEPTED MANUSCRIPT (Zhou et al., 2002a, b; Zhao and Zhou, 2009a; Dong et al., 2011, 2012; Zhao and Cawood, 2012). Thus, the ca. 820 Ma Tiechuanshan bimodal volcanic rocks and the ca. 780 Ma Tiechuanshan and Huangguan A-type granites were most probably formed in a back-arc rift environment (Fig. 10c). Similarly, the ca. 820 Ma Daxiangling A-type granites in the western

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margin and the Shuangqiaoshan A-type rhyolitic magma in the eastern margin of the Yangtze

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Block were also considered to be formed in a back-arc basin setting (Zhao et al., 2008b; Li et

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al., 2016).

The Huangguan A-type granites located at the continental fore-arc region and the

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northward magmatic migration in the Hannan region (Fig. 1b) were probably caused by the

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rollback or steepening of the southward subducted oceanic slab, which resulted in the upwelling of the asthenosphere and back-arc rift (Fig. 10b). The ca. 770–760 Ma Taojiaba

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I-type granodiorites and Tianpinghe monzogranites (Zhao and Zhou, 2009a) were generated

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by remelting of juvenile mafic crust, which are the products of the increasing degree of the rifting in the active continental margin. Dong et al. (2011, 2012) also suggested that the ca.

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824–750 Ma magmatism in the inner part of the Hannan regions were formed in a back-arc

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basin setting related to the roll back of the oceanic slab. The occurrences of ca. 760–710 Ma adakitic (TTG) rocks (Zhao et al., 2006; Zhao and Zhou, 2008; Dong et al., 2012) in the Hannan region indicate that there was a crustal thickening process (Fig. 10d). This crustal thickening process also signified that the final termination of subduction, which may be related to the collision between the Hannan arc and some microterranes around the northwestern margin of the Yangtze Block (Fig. 10d) ( e.g. Xiao et al., 2007; Dong et al., 2012; Hu et al., 2016; Zhang et al., 2016).

ACCEPTED MANUSCRIPT

6

Conclusions

A series of igneous rocks with variably chemical compositions from the Hannan region have been studied in details for their petrogenesis and tectonic affinity. The ca. 880 Ma Beiba

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gabbros are of cumulate origin and their parental magmas were generated by the partial

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melting of an enriched lithospheric mantle source that had been modified by slab-derived

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materials. The ca. 860 Ma Tianpinghe I-type granites were dominantly derived from

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Neoproterozoic juvenile mafic crust, with minor Paleoproterozoic components. The ca. 770 Ma Taojiaba I-type granodiorites were produced by the dehydration melting of the newly

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formed Neoproterozoic mafic crust. The ca. 780 Ma Tiechuanshan Aeg-arf granites are reduced, peralkaline and alkalic-calcic A-type granites, which were generated by the partial

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melting of a heterogeneous source involving juvenile and ancient crustal materials under dry

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and reducing conditions. The ca. 775 Ma Huangguan A-type granites are oxidized, aluminous and calc-alkalic, which were derived from the dehydration melting of tonalitic to granodioritic

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rocks under water-subsaturated and oxidizing conditions. All of these igneous rocks were

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formed at an extensional active continental margin, probably a subduction-related back-arc rift. Our new results support a model that the Yangtze Block was surrounded by ocean and arc magmatism in its northern and western margins during the Neoproterozoic.

Acknowledgments

We are grateful to Prof. Roberto Dall'Agnol and two anonymous reviewers for their thoughtful and insightful reviews, which greatly helped us to improve the paper. We also

ACCEPTED MANUSCRIPT acknowledge Prof. Dong Yunpeng for editorial handling. This research was supported by Natural Science Foundation of China (No. 41403026 and 41573021), China Geological Survey (No. 12120113100900) and MOST Special Fund from State Key Laboratory of Geological Processes and Mineral Resources (MSFGPMR201601-2).

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ACCEPTED MANUSCRIPT Figure Captions Fig.1. (a) Simplified geological map of China, showing major tectonic units of China; and (b) Geological map of the Hannan region, Northwestern Yangtze Block (modified after Dong et al., 2012). Data sources are mainly from Table 3 in Dong et al., (2012). Abbreviations in Fig.1a: WQ, West Qinling; EQ, East Qinling; SPGZ, Songpan-Ganzi terrane; DB, Dabie belt; SL, Sulu belt; QD, Qaidam; QL, Qilian Shan belt; KL (EKL), Kunlun Shan belt; NQL, North

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Qinling; SQL, South Qinling; SCS, South China Sea.

Fig. 2. Field photographs and photomicrographs of representative samples of the intrusive

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rocks from the Hannan region. (a) Coarse-grained layered Beiba gabbro; (b) Beiba gabbro (L51); (c) The outcrop of the Tianpinghe granite; (d) Tianpinghe granite (L53); (e) Taojiaba

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granodiorite (TJB) (L37); (f) Huangguan syenogranite (L63); (g) and (h) Tiechuanshan aegirine-arfvedsonite granite (L45). Hbl, hornblende; Bt, biotite; Opx, orthopyroxene; Cpx,

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clinopyroxene; Pl, plagioclase; Kfs, K-feldspar; Qz, quartz; Aeg, aegirine; Arf, arfvedsonite;

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Ttn, titanite.

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Fig. 3. Representative the CL images of zircon samples and Zircon U–Pb Concordia diagrams. (a) L-52, Beiba gabbrs; (b) (c) L-53, Tianpinghe granite; (d) L-38, Taojiaba granodiorite; (e) L-64, Huangguan syenogranite; (f) L-42, Tiechuanshan aegirine-arfvedsonite granite. The

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smaller solid line circles show LA–ICP–MS dating spots, and the bigger broken line circles

calculation.

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show Lu–Hf isotope analysis. Gray shaded and dashed ellipses are not included in the age

Fig. 4. Plots of (a) K2O+Na2O versus SiO2 (Le Bas et al., 1986), (b) MALI (K2O+Na2O-CaO) versus SiO2 (Frost, 2001), (c) A/NK [molar ratio Al2O3/(Na2O+K2O)] versus A/CNK molar ratio [Al2O3/(CaO + Na2O + K2O)] (Maniar and Piccoli, 1989) and (d) FeOt/(FeOt+MgO) versus SiO2 (Frost, 2001). Hannan adakitic rocks from Zhao and Zhou, (2008b) and Liu et al. (2009); ca. 820 Ma Tiechuanshan A-type dacites and thyolites from Ling et al. (2003); ca. 820 Ma Daxiangling plution from Zhao et al. (2008b); ca. 780 Ma Mianning pluton from Huang et al. (2008); 830–740 Ma calc-alkaline granitoids and volcanic rocks in the South China Block

ACCEPTED MANUSCRIPT (SCB) from Wang et al. (2010).

Fig. 5. Harker plots of selected major and trace elements of the intrusive rocks from the Hannan region. The 780 Ma Daolinshan A-type granites from Wang et al. (2010). Other data are from the same references as those in Fig. 4.

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Fig. 6. Primitive-mantle normalized trace element spider diagrams and chondrite–normalized

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REE patterns. Chondrite, Primitive-mantle, N-MORB and E-MORB values from Sun and

from the same references as those in Figs. 4 and 5.

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McDonough (1989); Lower continental crust from Rudnick and Gao (2003). Other data are

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Fig. 7. (a) whole rock εNd(t) versus ISr and (b) zircon εHf(t) versus Age (Ma) diagrams for intrusive rocks from the Hannan region. Reference lines representing Chondrite Hf evolution

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and the Depleted mantle are from Blichert-Toft et al. (1997) and Vervoort and Blichert-Toft, (1999), respectively. The dashed lines represent average continental crust evolution lines 176

Lu/

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Hf value of 0.015 (Griffin et al., 2002). Hannan adakites from

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assuming a mean

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Zhao and Zhou, (2008b) and Liu et al. (2009); Hannan-panxi mafic-ultramafic rocks from Zhou et al. (2002a), Zhao et al., (2008) and Zhao and Zhou, (2009b); Xixiang group from Zhang et al. (2002b) and Ling et al. (2003); Houhe complex from Ling et al. (1997), Zhang et

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al. (2002b) and Wu et al. (2012); Kongling group from Gao et al. (1999), Ma et al. (2000) and Hu et al., (2016a); ca. 820 Ma Tiechuanshan (TCS) basalts and dacites and rhyolites from

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Ling et al. (2003); ca. 760 Tianpinghe (TPH) I-type granite from Zhao and Zhou, (2009a); Other data are from the same references as those in Figs. 4 and 5.

Fig. 8. Plots of (a) Sr/Y versus Y and (b) (La/Yb)N versus YbN (Martin, 1999). Hannan adakitic rocks from Zhao and Zhou, (2008b) and Liu et al. (2009).

Fig. 9. Plots of (a) FeOT/MgO versus Zr + Nb + Ce + Y (Whalen et al., 1987), (b) (K2O+Na2O)/CaO versus Zr + Nb + Ce + Y (Whalen et al., 1987), (c) 10000×Ga/Al versus Zr + Nb + Ce + Y (Eby, 1990), (d) FeOT/(FeOT + MgO) versus Al2O3 (wt %) (Dall’Agnol and de

ACCEPTED MANUSCRIPT Oliveira, 2007), (e) Zircon saturated temperature (◦C) (Boehnke et al., 2013) versus SiO2 (wt %) and (f) Rb/Nb versus Y/Nb, and insert shows Y-Nb-Ce (Eby, 1992). FG-field for fractionated I-type granitoids; OGT-field for I-, S- and M-type granitoids. A1, anorogenic A-type granites; A2, post-collisional A-type granites. Plume- or hotspot-related A-type granites from Wang et al. (2010). Other data sources are from the same references as those in

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Figs. 4 and 5.

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Fig. 10. A sketch of tectonic evolution model for magma generation of the Hannan region in

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the Northwestern margin of the Yangtze Block during the Neoproterozoic. See text for details.

ACCEPTED MANUSCRIPT Table 1 Magmatic zircons sizes, internal structures, U and Th contents and Th/U ratio and weighted mean 206

Pb/238U ages for the magmatic rocks in this study.

Sample

length

aspect

internal

µm

ratios

structures

U

Th

Age (2σ)

Th/U

ppm

Ma

εHf(t)

TDM1 (Ga)

Beiba gabbro L52

100-300

1-1.5

sector or broad zoning

34-78

24-68

0.64-1.04

115-697

40-334

0.21-1.01

52-208

38-154

0.56-0.94

116-678

79-321

879 ± 6

+6.9 to +8.4

1.10-1.16

-1.9 to +1.6

1.35-1.49

770 ± 3

+6.2 to +9.5

0.98-1.09

0.47-0.77

774 ± 4

+1.9 to +6.9

1.07-1.27

0.07-1.43

782 ± 4

+0.5 to +5.3

1.14-1.32

80-300

2-3.5

oscillatory zoning

L38

150-350

1:2-1:4

oscillatory zoning

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Tiaojiaba granodiorite

L64

100-300

2-3.5

oscillatory zoning

Tiechuanshan A-type granite oscillatory zoning

444-952

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51-1337

MA

1-1.5

D

50-200

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L42

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Huanggun syenogranite

860 ± 6

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L53

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Tianpinghe granite

ACCEPTED MANUSCRIPT Table 2 Data for whole-rock Sr and Nd isotopes

87

87

Sr/86Sr

2σ

Isr

147

Sm/144Nd

L-49

0.124

0.705783

3

0.704402

0.1794

L-50

0.065

0.704914

4

0.704190

L-51

0.032

0.704825

2

L-52

0.055

0.7051

143

Nd/144Nd

2σ

εNd(t)

TDM(Ga)

0.512579

4

0.6

2.52

0.1720

0.51251

3

0.0

2.33

0.704469

0.1500

0.512399

1

0.0

1.79

2

0.704487

0.1633

0.512447

2

-0.4

2.12

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Rb/86Sr

Sample Beiba gabbro

Tianpinghe granite 0.5

0.710084

2

0.70394

0.1090

0.512002

3

-2.8

1.67

L-54

0.501

0.710027

3

0.70388

0.1015

0.511948

2

-3

1.63

L-55

0.493

0.710302

3

0.70425

0.1202

0.511958

1

-4.9

1.94

L-56

0.548

0.710683

2

0.70395

0.1120

0.512026

4

-2.6

1.68

L-57

0.436

0.709437

3

0.70407

0.1065

0.511913

2

-4.2

1.76

0.1158

0.512301

1

1.5

1.32

0.1082

0.512291

2

2.1

1.24

0.1052

0.512291

2

2.4

1.21

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L-53

0.428

0.708065

3

0.703298

L-38

0.467

0.70863

3

0.703429

L-39

0.454

0.708266

4

0.703210

L-40

0.437

0.708382

3

0.703515

0.1119

0.512295

2

1.8

1.28

L-41

0.478

0.708812

2

0.703488

0.1068

0.512278

2

2.0

1.24

MA

L-37

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Taojiaba granodiorite

Huanguan syenogranite 2.077

0.726328

9

0.703195

0.0910

0.512143

0.7

1.0

1.25

L-67

2.799

0.733862

3

0.702688

0.1161

0.512383

1

3.1

1.20

L-68

2.715

0.73262

4

0.702382

0.1176

0.512368

2

2.63

1.24

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L-63

Tiechuanshan aegirine-arfvedsonite granite 10.183

0.820678

9

0.707264

0.1280

0.512147

1

-2.7

1.78

L-43

41.252

1.11117

8

0.651723

0.1228

0.512202

0.7

-1.1

1.59

L-44

8.77

0.832964

5

0.735288

0.1317

0.512268

0.7

-0.7

1.64

L-45

47.596

1.195191

6

0.665087

0.1250

0.512216

0.8

-1.1

1.60

L-46

43.758

1.156381

6

0.669023

0.1154

0.512153

1

-1.4

1.54

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L-42

Notes: 87Rb/86Sr and 147Sm/144Nd ratios are calculated using Rb, Sr, Sm and Nd contents (Table S2), measured by ICP-MS.

ACCEPTED MANUSCRIPT Highlights • The ca. 870 Ma gabbros and I-type granites formed during the initiation of extension. • The ca. 780 Ma Tiechuanshan A-type granites are reduced, peralkaline and alkalic. • The ca. 774 Ma Huangguan A-type granites are oxidized, aluminous and calc-alkalic.

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• The A-type granites formed in a continental rift setting due to slab rollback.

Graphics Abstract

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Figure 7

Figure 8

Figure 9

Figure 10