Petrogenesis of Late Triassic mafic enclaves and host granodiorite in the Eastern Kunlun Orogenic Belt, China: Implications for the reworking of juvenile crust by delamination-induced asthenosphere upwelling

Journal Pre-proof Petrogenesis of Late Triassic mafic enclaves and host granodiorite in the Eastern Kunlun Orogenic Belt, China: Implications for the reworking of juvenile crust by delamination-induced asthenosphere upwelling

Hongzhi Zhou, Daohan Zhang, Junhao Wei, Dazhao Wang, M. Santosh, Wenjie Shi, Jiajie Chen, Xu Zhao PII:

S1342-937X(20)30098-8

DOI:

https://doi.org/10.1016/j.gr.2020.02.012

Reference:

GR 2321

To appear in:

Gondwana Research

Received date:

3 October 2019

Revised date:

9 February 2020

Accepted date:

16 February 2020

Please cite this article as: H. Zhou, D. Zhang, J. Wei, et al., Petrogenesis of Late Triassic mafic enclaves and host granodiorite in the Eastern Kunlun Orogenic Belt, China: Implications for the reworking of juvenile crust by delamination-induced asthenosphere upwelling, Gondwana Research (2020), https://doi.org/10.1016/j.gr.2020.02.012

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Petrogenesis of Late Triassic mafic enclaves and host granodiorite in the Eastern Kunlun Orogenic Belt, China: Implications for the reworking of juvenile crust by delamination-induced asthenosphere upwelling

Hongzhi Zhoua,b,c, Daohan Zhanga, Junhao Weia, Dazhao Wangd, M. Santoshe,f, Wenjie Shia, Jiajie Chend, Xu Zhaoa School of Earth Resources, China University of Geosciences, Wuhan 430074, China

b

Wuhan Centre of China Geological Survey, Whhan 430205, China

c

Institute of Granitic Diagenesis and Metallogeny, China Geological Survey, Wuhan

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a

School of Earth Sciences, East China University of Technology, Nanchang 330011, China．

School of Earth Sciences and Resources, China University of Geosciences Beijing,

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e

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d

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430205, China

f

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Beijing 100083, P.R. China

Department of Earth Sciences, University of Adelaide, Adelaide SA 5005, Australia

Abstract: Late Triassic granitoids containing abundant mafic microgranular enclaves (MMEs) occur widely in the Eastern Kunlun Orogenic Belt (EKOB). In this work, we present mineral chemistry, zircon U-Pb ages and Lu-Hf isotopes, whole-rock chemistry and Sr-Nd isotope compositions of the MMEs and host granodiorite from the Huda pluton in the Elashan area within the easternmost domain of the EKOB. These rocks contain inherited (Meso- to Neoproterozoic) 

Corresponding author. E-mail address: [email protected] 1

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and xenocrystic (ca. 240 Ma) zircon grains that yield apparent older ages, whereas the magmatic zircons from MMEs and granodiorite yield similar weighted mean ages around 224 Ma, which are interpreted as their crystallization ages. The MMEs have low SiO2 but high TiO2, TFe2O3, CaO, MgO and MnO concentrations

with

relatively

high

Mg#

values

(48-54)

and

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100MnO/(MnO+MgO+TFe2O3) ratios (1.2-1.6). They display identical Sr-Nd-Hf isotope compositions to the host granite. Combined with petrological evidence,

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we suggest that the MMEs are cognate cumulates that formed by pressure

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quenching during the late stage of magma evolution from the same parental

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magma of the host granodiorite, rather than a magma mixing origin. The

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granodiorite is calc-alkaline to high-K calc-alkaline, metaluminous I-type granite.

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They show relatively low SiO2 and MnO, but high MgO, Al2O3, CaO and TFe2O3 contents with Mg# values of 45-50. They are enriched in light rare earth elements

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(LREEs) and large ion lithophile elements (LILEs), such as Rb, Th, K and Pb, and are depleted in P and high field strength elements (HFSE) including Nb, Ta and Ti. These rocks display slightly negative Eu anomalies and low Sr/Y and La/Yb ratios. Together with the rim-ward chemically evolved nature of some phenocrysts, the comparatively high initial Sr isotope (0.70888-0.70912), low whole-rock εNd(t) (-5.6 to -6.0) and zircon εHf(t) (-3.3 to -0.1) values, and low Nb/Th (0.11-0.26) and Ta/U (0.53-0.68) ratios, we suggest that the granodiorite magma was sourced from the lower crust. Considering their comparatively young two-stage Nd and Hf model ages (1.42-1.49 Ga and 1.13-1.42 Ga, respectively) 2

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and same trace element character with the juvenile crust beneath the EKOB, we interpret the juvenile lower crust as the dominant source rocks for the granodiorite. Based on our data and regional geological evidence, we suggest that the partial melting of juvenile crust resulted from delamination-related asthenosphere mantle upwelling. The latter process resulted in extensive melting

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of the lower crust, producing a major Late Triassic magmatic flare-up event in the

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EKOB.

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Keywords: Mafic enclave; I-type granite; Eastern Kunlun; Delamination;

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1. Introduction

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Magmatic flare-up

Mafic microgranular enclaves (MMEs) are common in intermediate to felsic

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granitic intrusions and volcanic rocks. They generally preserve important information on the nature of source rocks, emplacement mechanism, and magma evolution (Barbarin and Didier, 1991; Clemens et al., 2017; Donaire et al., 2005), and have also been used for granite classification in some cases (Barbarin, 1999). Several mechanisms have been proposed for their formation such as magma mixing origin (Barbarin, 2005; Clemens et al., 2017; Plail et al., 2018), restitic origin (Chappell, 1996; Chappell and Wyborn, 2012; White et al., 1999) and cognate origin (Dahlquist, 2002; Donaire et al., 2005; Flood and Shaw, 2014). The most common model considers that MMEs formed as mafic globules via magma mixing. But in many cases 3

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the hybridized mafic blobs may interact with the surrounding host magma during cooling, resulting in chemical equilibrium between them. Consequently, these enclaves might be interpreted as cognate origin and vice versa, which makes the genesis of MMEs ambiguous in these situations. The Late Permian to Triassic I-type granitoids from the Eastern Kunlun Orogenic

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Belt (EKOB), west of China (Fig. 1a, b), contains numerous MMEs. Previous studies interpreted these MMEs to have formed via magma mixing during the injection of

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mantle-derived magma, which, given their ubiquitous occurrence, might play an

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important role in crustal growth in the EKOB (Liu et al., 2004; Mo et al., 2007; Qin et

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al., 2018; Xiong et al., 2012). Recent studies on the Early to Middle Triassic MMEs

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and host granitoids, however, argued against magma mixing origin, favoring a

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cognate origin as early mafic cumulates since they have similar Sr-Nd-Pb-Hf isotope compositions (Huang et al., 2014; Shao et al., 2017).

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The Late Triassic I-type granites in the EOKB are genetically associated with some of the major porphyry-skarn-type Cu-polymetallic deposits (Ma et al., 2015; Wang et al., 2018a). Although several studies were carried out on these rocks, there has been no consensus on their petrogenesis leading to diverse models such as magma mixing (Chen et al., 2005; Liu et al., 2004; Luo et al., 2014; Qin et al., 2018), or sourced from either enriched mantle (Yin et al., 2017), or (thickened) ancient lower crust (Ren et al., 2016; Xia et al., 2014; Xiong et al., 2014), or juvenile crust (Xiong et al., 2016a). Recently, Ma et al. (2015) noted that compared to the Early to Middle Triassic granitoids, the Late Triassic I-type magmatism in the EKOB was 4

Journal Pre-proof characterized by remarkably high magma addition rate (up to 99 km3/m.y.), suggestive of a magmatic flare-up event during this period. Compared to magmatic lulls, at least two orders of magnitude magmatism involving 50% mantle-derived components were added to the arc crust during flare-ups, and thus contribute significantly to the net crustal growth (Ducea et al., 2015; Paterson and Ducea, 2015).

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Therefore, the Late Triassic magmatic flare-up might play an important role in continental growth in the EKOB, although the triggers for this event remain unclear

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(see Ma et al., 2015).

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With a view to understand the petrogenesis of the MMEs and the host Late

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Triassic I-type granites from the EKOB, in this study we present zircon U-Pb ages and

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Lu-Hf isotopes, mineral chemistry, whole-rock chemistry and Sr-Nd isotopic data on

area.

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both enclaves and host granodiorite of the ca. 224 Ma Huda pluton from the Elashan Our results lead to a delamination model to account for the Late Triassic

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magmatic flare-up in the EKOB.

2. Geological background

2.1. East Kunlun Orogenic Belt (EKOB) The EKOB is bordered by the Qaidam Basin to the north and the Bayanhar terrane to the south (Fig. 1a). It is a composite subduction-accretion orogen constructed through the Cambrian to Devonian Proto-Tethys and the Carboniferous to Early Triassic Paleo-Tethys orogenic processes (Dong et al., 2018; Pei et al., 2018; Yang et al., 1996). The EKOB can be subdivided into, from north to south, the North 5

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Qimantagh

Belt,

Central

Kunlun

Belt

and

South

Kunlun

Belt

by

Qimantagh-Xiangride and Aqikekulehu-Kunzhong ophiolitic mélange zones (QXM and AKM) (Fig. 1b; Dong et al., 2018). The Precambrian basements beneath the North Qimantagh and Central Kunlun Belts are represented by the Jinshuikou Group that are composed of lower Baishahe and upper Xiaomiao Formations (Dong et al.,

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2018; Wang et al., 2007). The Paleoproterozoic Baishahe Formation (2.2-1.8 Ga) mainly comprises amphibolite- to granulite-facies metamorphic paragneiss, migmatite,

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schist, amphibolite and marble, whereas the Mesoproterozoic Xiaomiao Formation

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(1.7-1.2 Ga) is composed dominantly of greenschist-facies metamorphic marbles,

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amphibolite-facies gneisses, greenschists and quartzites (Chen et al., 1999, 2006;

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Chen et al, 2011, 2014; Dong et al., 2018; Wang et al., 2007). As revealed by zircon

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geochronological studies, the Jinshuikou Group was superimposed successively by 1.0-0.9 Ga and ca. 400 Ma tectono-thermal overprints (He et al., 2016a). The

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Precambrian units of the South Kunlun Belt include the Paleoproterozoic Kuhai Group and Meso-Neoproterozoic Wanbaogou Group. The Kuhai Group is dominated by amphibolite-facies metamorphic gneiss, schist, amphibolite and marble, whereas the

Wanbaogou

Group

is

mainly

composed

of

low-grade

metamorphic

basaltic-andesitic and carbonate rocks interbedded with clastic rocks (Dong et al., 2018; Wang et al., 2007). Three episodes of magmatism during the Neoproterozoic, Ordovician-Silurian and Permian-Triassic were identified in the EKOB (Dong et al., 2018; Mo et al., 2007). The Neoproterozoic magmatic rocks are mainly S-type and A-type granites 6

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with emplacement ages ranging from 1007 to 870 Ma (Chen et al., 2015; He at al., 2016b, 2018; Meng et al., 2017), and these rocks are distributed mainly in the Central Kunlun Belt (Dong et al., 2018). The Ordovician-Silurian (ca. 495-390 Ma) rocks are dominantly A-type granites with subordinate I- and S-type granites (Chen et al., 2016; Wang et al., 2018b; Xin et al., 2018; Xiong et al., 2015; Yan et al., 2019; Zhao et al.,

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2017). The Permian-Triassic granitoids including I-, A- and S-types (ca. 265-200 Ma; He et al., 2018; Luo et al., 2014; Ma et al., 2015; Ren et al., 2016; Xiong et al., 2014,

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2016a; Zhang et al., 2018; Zhao et al., 2018) are volumetrically predominant in the

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EKOB (~25,000 km2) and constitute half of the total outcrop area of all granitoids in

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this region (Fig. 1b; Liu et al., 2004; Mo et al., 2007). Their formation was associated

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with to the closure of the Anyemaqen-Kunlun Ocean (a branch of the Paleo-Tethys

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Ocean) and subsequent continental collision (Dong et al., 2018; Ma et al., 2015; Ren

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et al., 2016; Xiong et al., 2014, 2016a).

2.2. Geology of the Elashan area The Elashan area is located at the easternmost segment of the EKOB, and is disconnected from West Qinling Orogen (WQO) by the northwest-southeast trending strike-slip Wenquan fault (Fig. 1b). It was previously considered as an independent tectonic unit formed by lateral collision between EKOB and WQO during the Late Triassic (Sun et al., 2001, 2004). However, subsequent studies have shown that the Elashan area preserves the same Late Permian to Triassic magmatic events as the EKOB and WQO, and that they have same crustal basement as indicated by 7

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whole-rock Sr-Nd-Pb and zircon Hf isotope evidences (Ren et al., 2016; Shao et al., 2017). Therefore, the EKOB and the WQO more likely belonged to a single orogen that was cut and offset by Wenquan strike-slip fault (Shao et al., 2017; Zhang et al., 2006). The paragneiss of Jinshuikou Group is exposed in the north of Elashan area and

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experienced the Neoproterozoic (1.0-0.9 Ga) and Ordovician-Silurian (450-430 Ma) metamorphism (Meng et al., 2013, 2017). The Jinshuikou Group was covered by

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Early Paleozoic basaltic to andesitic volcanic rocks interbedded with clastic rocks

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(Tanjianshan Group), Late Carboniferous-Permian shallow marine and paralic clastic

Triassic

volcanic-sedimentary rocks

(Naocangjiangou

and

Elashan

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Group),

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strata (Haoteluowa Group), Early Triassic limestone and sandstone (Hongshuichuan

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Formations), and Cenozoic sediments (Li et al., 2012; Ren et al., 2016). The Elashan Formation consists mainly of basalt, andesite, dacite, and rhyolite with interbedded

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clastic rocks (Ma et al., 2016). Zircon U-Pb data suggest that these volcanic rocks erupted at 247-243 Ma (our unpublished data). In addition to minor Neoproterozoic S-type granite (Meng et al., 2017), voluminous Late Permian to Triassic high-K calc-alkaline I-type granitoids (252-218 Ma; Ren et al., 2016; Xiong et al., 2016b; Wang et al., 2019; Zhang et al., 2006) also occur in study area.

3. Sample description and petrography The Huda granodiorite pluton is located in the southeastern part of Elashan area with an outcrop area of approximately 11.6 km2 (Fig. 1c). This oval-shaped pluton 8

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intruded into the Middle Triassic Naocangjiangou Formation and was later intruded by the Hudalongwa quartz-diorite pluton as inferred from their intrusive contact relationship in the field. Samples collected from the Huda granodiorite exhibit medium- to coarse-grained texture and are dominantly composed of 45-50 vol.% plagioclase, 15-20 vol.% quartz, 10-15 vol.% K-feldspar, 8 vol.% biotite and 7 vol.%

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hornblende, with apatite, zircon and titanite as accessory minerals (Fig. 2a-c). The plagioclase phenocrysts vary from 0.5 to 3 mm in grain size, and many grains display

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obvious multiple twinning whereas a few show sericitization. Some plagioclase laths

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exhibit resorption texture (Fig. 2c), indicating either temperature increase (via magma

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mixing; Vernon, 1991) or rapid pressure drop (via fast magma ascent; Nelson and

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Montana 1992). Combined with the limited chemical and isotopic variation of

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granodiorite, however, the overall rim-ward decrease of An contents (see below) support the rapid pressure drop over the magma mixing-induced temperature increase.

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Biotite is yellowish brown in color with subhedral to euhedral forms. Hornblende is euhedral to subhedral and in some cases contains plagioclase and/or quartz inclusions (Fig. 2b).

Abundant subangular to subspherical MMEs are present in the Huda pluton and commonly range in size from few centimeters to decimeters (Fig. 2a). Almost all the MMEs have sharp contacts with the host (Fig. 2a). The MMEs show porphyritic texture, and their mineral assemblage resembles that of Huda granodiorite but with apparently much higher ratio of mafic minerals (Fig. 2d-f). The coarse-grained plagioclase and hornblende crystals represent xenocrysts that were entrained from the 9

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host granodiorite, as inferred from the same resorption texture in some of them (Fig. 2f) and the chemical composition (see below) similar to those of the phenocrysts in granodiorite. They are euhedral with diameters up to 2 mm and in some cases are surrounded by a ring of dominantly fine-grained biotite and hornblende (Fig. 2f). The fine-grained matrix of MMEs consist mainly of 40-45 vol.% plagioclase, 10-17 vol.%

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hornblende, 15-20 vol.% biotite, 5-8 vol.% quartz and less than 1 vol.% accessory minerals including apatite, zircon, titanite and Fe-Ti oxides (Fig. 2d-e). The

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plagioclase laths are euhedral to subhedral with grain size in the range of 0.1 to 0.5

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mm and sometimes cut across hornblende or biotite with sharp grain boundaries.

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Hornblende and biotite are normally subhedral to allotriomorphic and tend to cluster

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together to form large aggregates (Fig. 2e-f). Many apatite grains with typical acicular

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crystal shape are entrapped in fine-grained hornblende, biotite and plagioclase (Fig.

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2e), suggestive of rapid quenching (Wyllie et al., 1962).

4. Analytical methods

The EPMA analyses of major elements for silicate minerals were conducted using a JEOL JXA-8230 microprobe at the Center for Global Tectonics, School of Earth Sciences, China University of Geosciences, Wuhan (CUGW). In-situ zircon U-Pb ages for samples BT10 and HD04 were obtained using a GeoLas 2005 Laser Ablation (Coherent, USA) coupled with an Agilent 7500a ICP-MS at the State Key Laboratory of Continental Dynamics, Northwest University, Xi’an, China; U-Pb ages for samples BT04, BT07, HD02, HD09 and HD12 were determined using a 10

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GeolasPro laser ablation system plus Agilent 7700e ICP-MS instrument at Wuhan SampleSolution Analytical Technology Co., Ltd., Wuhan, China. Some zircon grains were selected for further in-situ Lu-Hf isotope analyses using a Neptune Plus MC-ICP-MS (Thermo Fisher Scientific, Germany) in combination with a Geolas HD excimer ArF laser ablation system (Coherent, Göttingen, Germany) at the Wuhan

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SampleSolution Analytical Technology Co., Ltd. Off-line selection and integration of analyte signals, and mass bias calibrations were performed using ICPMSDataCal

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software (Liu et al., 2010). Whole-rock major element concentrations were

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determined using X-ray fluorescence (XRF) and a PANalytical Axios XRF instrument

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at ALS Chemex Co., Ltd., Guangzhou, China. Whole-rock trace and rare earth

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elements were analyzed using an Agilent 7500a ICP-MS at the GPMR, CUGW.

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Whole-rock Sr-Nd isotope compositions were determined on a Finnigan MAT-261 thermal ionization mass spectrometer (TIMS) also at the GPMR, CUGW. Detailed

5. Results

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analytical conditions and procedures are provided in Appendix.

5.1. Zircon U-Pb ages Zircon cathodoluminescence (CL) images and U-Pb age data are shown in Figs. 3-4 and the U-Pb isotope data are summarized in Table S1. 5.1.1 Granodiorite Zircon grains from granodiorite (samples HD02, HD04, HD09 and HD12) are generally colorless to pale yellow, and euhedral or elongated crystals, ranging in size 11

Journal Pre-proof from 50 to 300 μm, with aspect ratios of 1:1-4:1. As revealed by CL images, most grains show oscillatory zoning, indicative of magmatic origin (Wu and Zheng, 2004), and some inherited grains show overgrowth of magmatic rims (Fig. 3a-d). For sample HD02, two U-Pb analyses were carried out on inherited zircon cores and twenty-nine on magmatic rims. The zircon cores have relatively high Th (347-535

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ppm) and U (639-1359 ppm) concentrations with high Th/U ratios (0.39-0.54). These features suggest that they are also magmatic in origin but were associated with an Pb/238U ages of 866 to 1048 Ma. The

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older magmatic event as implied by their

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zircon rims have Th and U contents of 118-921 ppm and 351-1167 ppm, respectively,

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with high Th/U ratios of 0.34 to 0.73, supporting their magmatic origin (Wu and

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Zheng, 2004). Two U-Pb age groups can be identified, with one group ranging from

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239 Ma to 242 Ma and the other varying from 220 Ma to 228 Ma. We correlate the former group of xenocrystic zircons with earlier magmatism, which were captured

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during ascent of the younger magma, whereas the younger group represents the crystallization age of the Huda granodiorite, with a weighted mean age of 224.3±0.9 Ma (n=24, MSWD=0.35; Fig. 4a). Twenty-seven U-Pb data were obtained on magmatic zircon rims from sample HD04. These rims have variable Th (118-921 ppm) and U (351-1167 ppm) contents with high Th/U ratios of 0.34 to 0.73. Excluding one much younger analysis (spot 1) that probably represents Pb loss, two U-Pb age groups can also be identified, with one group in the range of 237-241 Ma and the other in the range of 222-227 Ma. Again, we suggest that the younger group represents the crystallization age of the Huda 12

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granodiorite, which yielded a weighted mean age of 224.0±1.3 Ma (n=16, MSWD=0.20; Fig. 4b). One U-Pb analysis was made on inherited zircon core and twenty-seven analyses on zircon rims from sample HD09. The inherited zircon core(s) are magmatic origin as substantiated by high Th (162 ppm) and U (1376 ppm) concentrations and Th/U 206

Pb/238U ages of 705 Ma. All zircon rims are also igneous

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ratio (0.12), and yields

origin with Th/U ratios varying from 0.54 to 1.23. Two U-Pb age groups including

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238-248 Ma and 218-227 Ma are also recognized in magmatic rims. The younger

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group shows a weighted mean age of 223.0±1.0 Ma (n=18, MSWD=1.08; Fig. 4c)

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that represents the emplacement age of granodiorite.

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Likewise, two U-Pb age groups are identified in magmatic zircon rims (with

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Th/U ratios of 0.58-1.46) from sample HD12. The older group (239-243 Ma) represents the ages of zircon xenocrysts, whereas the younger represents the

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crystallization age of granodiorite. Sixteen U-Pb ages from the younger group are concordant, and yield a weighted mean age of 223.5±1.2 Ma (MSWD=0.35; Fig. 4d).

5.1.2 MMEs Zircon crystals from MMEs (samples BT05, BT07 and BT10) have similar morphology and internal texture to those from host granodiorite. The grains are colorless to faint yellow and euhedral to subhedral in shape. They range in size from 100 to 300 μm with aspect ratios of 1:1-4:1. The grains display clear oscillatory zoning in the CL images and a few inherited cores are also present (Fig. 3e-g). 13

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Zircon rims from sample BT05 have Th and U contents of 215-898 ppm and 242-613 ppm, respectively, with Th/U ratios ranging from 0.62 to 1.58, implying their magmatic origin. Nineteen analyses on magmatic zircon rims also show two age groups (clustering around 240 Ma and 224 Ma, respectively), similar to the data from the host granodiorite (Fig. 4e). Fifteen concordant data from the younger magmatic

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zircon group yield a weighted mean age of 224.4±1.2 Ma (MSWD=0.48) which is regarded as the crystallization age of MMEs.

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Twenty-three analyses were carried on zircon rims and one analysis on an

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inherited zircon core from sample BT07. The zircon rims have high Th (294-685 ppm)

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and U (351-1024 ppm) concentrations, with high Th/U ratios (0.59-1.07), suggestive

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of magmatic origin. In comparison, zircon core(s) have relatively low Th (236 ppm)

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and U (596 ppm) contents but still high Th/U ratio (0.40). These features suggest that the inherited zircon core(s) are also probably of igneous origin but were derived from

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an older magmatic event as indicated by the

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Pb/206Pb age of ca. 1417 Ma (Fig. 4f

and Table S1). Two U-Pb age groups are identified, with one group ranging from 238 Ma to 246 Ma and the other varying between 221 Ma and 226 Ma. Eighteen concordant U-Pb ages from the younger group give a weighted mean age of 223.4±1.1 Ma (MSWD=1.00; Fig. 4f), which is considered as the crystallization age of MMEs. For sample BT10, twenty-three analyses were carried on zircon rims and one analysis on an inherited zircon core. Zircon rims have relatively variable Th (56-428 ppm) and U (170-747 ppm) contents, and high Th/U ratios (0.20-0.67), signifying 14

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their magmatic origin. The zircon core(s) have relatively low Th and moderate U contents, with relatively low Th/U ratio (0.18; Table S1), indicative of igneous origin, and give

206

Pb/238U age of 894±11 Ma (Fig. 3g and Table S1). The magmatic zircon

rims also display two separate age groups (around 240 Ma and 224 Ma, respectively) (Fig. 4g). Fifteen concordant data from the younger magmatic zircon group yield a

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weighted mean age of 224.2±1.5 Ma (MSWD=0.12) which is regarded as the crystallization age of MMEs.

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In summary, both granodiorite and MMEs have three types (inherited,

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xenocrystic and magmatic) of zircon grains and each type has similar U-Pb age

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patterns. As indicated by the ages from magmatic zircons, both granodiorite and

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MMEs crystallized at ca. 224 Ma. In addition, the xenocrystic zircon of ca. 240 Ma

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does not show any dissolution or recrystallization texture as inherited Neoproterozoic zircons, indicating that they were captured at the late stage of evolution of the Huda

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pluton, where the 240 Ma zircon survived most likely owing to either relatively low temperature or short interaction period, or both.

5.2. Mineral chemistry Representative EPMA analyses of plagioclase, biotite and hornblende that are dominant mineral phases in both granodiorite and MMEs are given in Table S2-4. Plagioclase phenocrysts in granodiorite, plagioclase xenocrysts and fine-grained matrix plagioclase in MMEs were analyzed. In general, they show limited chemical variations with An contents clustering between 30 and 40, and thus can be classified 15

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as andesine (Fig. 5a). Only those crystals displaying core-rim textures from either granodiorite (phenocrysts) or MMEs (xenocrysts) show small differences where the rims commonly have lower An contents than core domains by an order of An5, and also FeO contents (Table S2). In addition, the majority of fine-grained matrix plagioclase from MMEs are less calcic (An30-33; Table S2) and therefore exhibit minor

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chemically evolved feature. Similarly, hornblende from granodiorite and MMEs (including matrix

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hornblende and xenocrysts) have constant compositions with high CaB (> 1.50) and

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high Mg# (51-59) values, and can be categorized as magnesio-hornblende according

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to the classification by Leake et al. (1997) (Fig. 5b). For some large crystals in

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granodiorite, there is a subtle rim-ward decrease of Al2O3 concentrations and Mg#

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values, although not uniform (Table S3). Using the Al-in-hornblende barometer (Anderson and Smith, 1995), the crystallization pressure was calculated with an

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overall pressure range of 1.8-3.1 kbar, among which the rims of hornblende phenocrysts in granodiorite and matrix hornblende in MMEs tend to yield lower pressures (Table S3).

Biotite from granodiorite and MMEs have similar compositions, with high MgO (9.32-10.28 wt.% and 9.32-10.56 wt.%), FeOt (20.1-21.8 wt.% and 19.8-21.9 wt.%) and TiO2 (3.69-4.34 wt.% and 3.58-4.65 wt.%) contents, and low Al2O3 (13.3-13.9 wt.% and 13.2-13.7 wt.%) concentrations. Their Mg# values range from 46 to 50 and from 46 to 51 (Table S4), respectively. Based on the classification diagram, the mineral is classified as magnesian biotite (Fig. 5c). 16

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5.3. Whole-rock geochemistry Whole-rock major and trace element compositions of eight granodiorite and six MME samples are listed in Table S5 and plotted in Figs. 6-8. The granodiorite samples have relatively low SiO2 (60.9-65.8 wt.%) and MnO (0.08-0.10 wt.%) concentrations, but high MgO (1.66-2.76 wt.%), Al2O3 (16.1-17.2 wt.%), CaO

of

(3.83-5.39 wt.%) and TFe2O3 (4.62-6.56 wt.%) contents, with Mg# values of 45-50.

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All samples have moderate to high K2O (2.22-2.88 wt.%) and total alkalis

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(Na2O+K2O) of 5.74-6.76 wt.%, showing dioritic to granodioritic composition (Fig.

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6a) and calc-alkaline to high-K calc-alkaline character (Fig. 6b). They are

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metaluminous with low A/CNK (molar Al2O3/(CaO+Na2O+K2O)) values of 0.93-0.99

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(Fig. 6c). The rocks display clear linear trends in the Harker diagrams (Fig. 7). The samples are enriched in LREE compared to HREE (Fig. 8), with relatively flat HREE

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pattern ([Gd/Yb]N=1.49-1.67) in chondrite-normalized REE patterns. They exhibit slightly negative Eu anomalies (Eu/Eu*=0.68-0.84, Fig. 8a). In primitive mantle-normalized trace element diagrams, the granodiorite samples are enriched in large ion lithophile elements (LILE), such as Rb, Th, K and Pb, and depleted in P and high field strength elements (HFSE) including Nb, Ta and Ti. In addition, they have low Sr/Y (12.1-18.3) and La/Yb (11.0-17.1) ratios. Compared to host granodiorite, the MME samples contain lower SiO2 contents (50.2-55.3 wt.%) but higher TiO2 (0.86-1.06 wt.%), TFe2O3 (8.13-11.20 wt.%), CaO (6.64-7.67 wt.%), MgO (4.00-4.81 wt.%) and MnO (0.15-0.25 wt.%) concentrations, 17

Journal Pre-proof with higher Mg# of 49-54 (Fig. 6d) and 100MnO/(MnO+MgO+TFe2O3) ratios (1.2-1.6; Table S5). They have similar Al2O3 (16.9-18.3 wt.%) and total alkalis contents as host granodiorite and show metaluminous high-K calc-alkaline features (Fig. 6b). The MME samples have higher REE contents than granodiorite and exhibit more flat HREE patterns ([Gd/Yb]N=1.38-1.15) (Fig. 8a), consistent with their higher

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modal proportion of hornblende. In the trace element spidergram, the MME samples show notable enrichment in K, Pb, Sm and Nd, and depletion in Th, U, Nb, Ta, P and

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Ti (Fig. 8b).

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5.4. Sr-Nd isotopes

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The Sr-Nd isotopic data for granodiorite and MMEs are listed in Table S6 and

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plotted in Fig. 9a. The granodiorite and MMEs have similar Sr-Nd isotope compositions. The measured 87Sr/86Sr and 143Nd/144Nd ratios of granodiorites fluctuate

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from 0.710806 to 0.711998 and from 0.512227 to 0.512257, respectively, whereas those of MMEs range from 0.710624 to 0.711585 and from 0.512241 to 0.512285, respectively. Their initial

87

Sr/86Sr and εNd(t) values were calculated at 224 Ma. The

granodiorite has initial 87Sr/86Sr ratios of 0.70888-0.70912 and εNd(t) values of -5.6 to -6.0, with T2DM(Nd) in the range of 1450 to 1490 Ma. The MMEs yield initial 87Sr/86Sr ratios of 0.70863-0.70910 and εNd(t) values of -5.2 to -5.8, with T2DM(Nd) varying from 1418 to 1468 Ma.

5.5. Zircon Lu-Hf isotopes 18

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All zircon Lu-Hf isotope compositions of granodiorite and MMEs are listed in Table S7 and plotted in Fig. 9b. 5.5.1 Granodiorite Fourteen Lu-Hf isotopic analyses were conducted on zircon crystals from sample HD02. Ten analyses on ca. 224 Ma magmatic grains reveal that the magmatic grains

of

have 176Hf/177Hf ratios of 0.282571-0.282592, εHf(t) values of -2.0 to -1.3, and T2DM(Hf) ages of 1211-1248 Ma (Table S7). Four analyses on ca. 240 Ma xenocrystic zircons

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give similar results that this zircon group has 176Hf/177Hf ratios of 0.282571-0.282592

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and εHf(t) values of -2.0 to -1.3, corresponding to T2DM(Hf) of 1211-1248 Ma (Table

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S7).

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Seventeen Lu-Hf isotopic analyses were performed on magmatic zircon grains

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from sample HD04, including fourteen on ca. 224 Ma magmatic grains and three on ca. 240 Ma xenocrystic zircon. The results show that the two zircon groups also have 176

Hf/177Hf ratios (Table S7). The ca. 224 Ma zircon grains have

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similar

176

Hf/177Hf

ratios of 0.282588-0.282633 and εHf(t) values of -1.8 to -0.1, corresponding to T2DM(Hf) of 1134-1225 Ma, while ca. 240 Ma zircons have

176

Hf/177Hf ratios of

0.282585-0.282646 and εHf(t) values of -1.6 to +0.6, corresponding to T2DM(Hf) of 1106-1228 Ma. Five Lu-Hf isotope analyses on ca. 240 Ma xenocrystic zircon grains and nine on ca. 224 Ma magmatic grains from sample HD09 reveal that they also have similar 176

Hf/177Hf ratios (0.282552-0.282589 vs. 0.282565-0.282609), εHf(t) values (-2.7 to

-1.4 vs. -2.6 to -1.2) and T2DM(Hf) ages (1216-1290 Ma vs. 1191-1268 Ma) (Table S7). 19

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One analysis on inherited zircon core shows comparatively high

176

Hf/177Hf ratio of

0.282319, εHf(t) value of -1.2 and T2DM(Hf) age of 1576 Ma. Two zircon groups from sample HD12 also have similar Lu-Hf isotope compositions (Table S7). Nine analyses on ca. 224 Ma zircons yield 176Hf/177Hf ratios of 0.282569-0.282610, corresponding to εHf(t) values of -2.4 to -1.0 and T2DM(Hf) ages of 1322-1406 Ma. Three analyses on ca. 240 Ma zircons give

176

Hf/177Hf ratios of

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0.282567-0.282615, and the calculated εHf(t) values range from -2.2 to -0.4, and

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5.5.2 MMEs

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T2DM(Hf) ages from 1295 to 1407 Ma.

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The two zircon groups from MME sample also show similar Lu-Hf isotope

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compositions and resemble those of the host granodiorite (Table S7). Twelve analyses 176

Hf/177Hf ratios of

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on ca. 224 Ma magmatic zircon grains from sample BT05 show

0.282552-0.282601, and their εHf(t) values and T2DM(Hf) ages range from -3.1 to -1.3

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and from 1201 to 1297 Ma, respectively. Four analyses on ca. 240 Ma xenocrystic zircon grains yield

176

Hf/177Hf ratios of 0.282546-0.282583, and their corresponding

εHf(t) values and T2DM(Hf) ages are from -2.8 to -1.5 and 1223-1299 Ma, respectively. Ten Lu-Hf analyses on ca. 224 Ma magmatic zircons and two on ca. 240 Ma xenocrystic ones from sample BT07 also suggest that they have almost identical Lu-Hf isotope compositions (Table S7), with 176Hf/177Hf ratios of 0.282553-0.282595 and 0.282522-0.282582, respectively. Their corresponding εHf(t) values and T2DM(Hf) ages are from -3.0 to -1.5 and 1209-1292 Ma, and from -3.5 to -1.5 and 1219-1335 Ma, respectively. One analysis on inherited zircon grains shows distinct176Hf/177Hf 20

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Hf/177Hf ratios of 0.282556-0.282621, and their εHf(t) values and T2DM(Hf) ages vary

from -2.9 to -0.5 and from 1156 to 1284 Ma (Table S7), respectively. Four analyses on ca. 240 Ma xenocrystic zircon grains yield similar results that 176Hf/177Hf ratios are in

of

the range of 0.282563-0.282621, and their corresponding εHf(t) values and T2DM(Hf) ages are from -2.3 to -0.2 and 1152-1270 Ma, respectively. One analysis on inherited

Hf/177Hf=0.282267, εHf(t)=1.1, and T2DM(Hf)=1597Ma (Table S7).

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176

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Neoproterozoic zircon core yield different Hf isotopic composition with initial

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The indistinguishable Lu-Hf isotope compositions between granodiorite and

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MMEs further confirm their genetic relation. Moreover, both 224 Ma and 240 Ma

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zircon grains have similar Lu-Hf isotope compositions, indicating the same source

6. Discussion

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regions for their parental magmas.

6.1. Mafic enclaves: cognate cumulate via pressure quenching The MMEs in silicic plutons are normally interpreted as mafic blobs formed during magma mixing (Barbarin, 2005; Clemens et al., 2017; Plail et al., 2018; Xu et al., 2012a, b; Yang et al., 2004, 2005a), cognate fragments (of cumulate minerals, chilled margins, or pressure quenching cumulates) (Dahlquist, 2002; Donaire et al., 2005; Flood and Shaw, 2014), refractory restites of source rocks over partial melting (Chappell, 1996; Chappell and Wyborn, 2012; White et al., 1999), or xenoliths of 21

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country rocks incorporated during magma ascent (Vernon, 1983). The latter two mechanisms can be excluded in the case of MMEs from the Huda pluton. The refractory restites are characterized by metamorphic texture and have much older, and a wider range of, zircon U-Pb ages than host granite. This is not consistent with the igneous texture of MMEs and their same zircon age patterns with those of the host

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granodiorite in this study. Much older (and perhaps variable) zircon ages are also expected for wall-rock xenoliths, as well as the contrasting isotopic compositions.

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Nevertheless, the identical isotope compositions between MMEs and Huda

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granodiorite (Fig. 9; Tables S6 and 7) eliminate the xenolith origin.

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The magma mixing model is the most common and prevailing mechanism to

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explain the origin of MMEs in granitoids (Barbarin, 2005; Clemens et al., 2017; Xu et

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al., 2012a, b; Yang et al., 2004, 2005a) and was advocated to interpret the formation of ubiquitous MMEs within Triassic granitic plutons in the EKOB (Chen et al., 2013b,

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2018; Liu et al., 2004; Qin et al., 2018; Wang et al., 2019; Xia et al., 2014). Most MMEs of magma mixing origin generally have contrasting chemical and isotopic compositions with host granites (Clemens et al., 2017). However, in some cases, variable degree of interaction between MMEs and the host during the time-scales of thermal equilibrium and final pluton consolidation may result in their chemical and isotopic equilibration owing to efficient diffusion of highly mobile elements (e.g. K, Na, Rb, Sr and Pb) (Blundy and Sparks, 1992; Elburg, 1996; Holden et al., 1987, 1991; Pankhurst et al., 2011). In contrast, Sm, Nd and HFSE are considerably less mobile and thus their concentrations as well as relevant Sm-Nd isotopes are 22

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representative of initial melt composition (Holden et al., 1987, 1991; Pankhurst et al., 2011). Accordingly, chemical character of these elements and especially Sm-Nd isotopes for the MMEs of magma mixing origin are expected to be distinct from those of the host magma. The MMEs and the host Huda granodiorite have consistent bulk-rock Sr isotopes (Table S6), with comparable Sm-Nd isotopes and εNd(t) ranging

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from -6.0 to -5.2, precluding magma mixing and subsequent extensive equilibrium model. Moreover, the uniform Hf isotope compositions of magmatic zircon grains

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from the MMEs and host granite suggest that the zircon grains crystallized from the

-p

same parental magma (see above) rather than through late isotope equilibrium, which

re

in turn excludes the magma mixing process. In fact, some MMEs having identical

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Sr-Nd isotopic compositions from host rocks were also documented in Late Permian

na

to Early Triassic plutons from the EKOB (Xiong et al., 2012; Li et al., 2018), and they were considered to be originated from the enriched mantle sources which accounts for

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the similar isotopic compositions as their crustal-derived host granites (Xiong et al., 2012; Li et al., 2018). In this scenario, the hot mafic melt intruding comparatively cool felsic crystal-rich mushes would become quenched because of a combined effect of rapid cooling and water loss, and produce the peculiar mafic rind composed principally of fine-grained biotite and hornblende (Blundy and Sparks, 1992; Farner et al., 2014) and/or comb layering textures at the contacts with their silicic host rocks (Pistone et al., 2016). However, such textures are not present in the outer margins of Huda MMEs. In addition, the constant low bulk-rock Mg# values (< 55) (Fig. 6d) and the rim-ward decrease in An contents and Mg# values for plagioclase and hornblende 23

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xenocrysts in MMEs (Tables S2 and 3) are also not consistent with magma mixing process. The cognate origin MMEs occur through three intrinsically different magma emplacement/evolution possesses: early cumulate segregation (Dahlquist, 2002), chilled margin by rapid cooling (Donaire et al., 2005), and late cumulate by pressure

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quenching (Flood and Shaw, 2014). All MMEs formed by these processes exhibit same isotope composition as their hosts. In this situation, only mineral and textural

ro

evidence can be used to determine the process whereby the MMEs from the Huda

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pluton formed. Huang et al. (2014) and Shao et al. (2017) reported some analogous

re

MMEs in Triassic granitoids from the EKOB, and interpreted the MMEs as

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cumulates formed at the early evolution stage of host magma. Early cumulates are

na

characterized by cumulus texture with coarse-grained early crystallizing phases (e.g. hornblende, biotite and plagioclase) (Dahlquist, 2002). This feature differs with the

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fine- to medium-grained texture and subhedral to allotriomorphic morphology of hornblende and biotite as observed in MMEs from the Huda pluton (Fig. 2c, d), implying that they are not early cumulates. Likewise, rapid cooling model is also inappropriate for the Huda enclaves. In this model MMEs were suggested to form along cold magma conduits during magma ascent, thus experience rapid cooling and be eventually incorporated into later host magma (Donaire et al., 2005). Because the rapid heat loss to the surroundings resulted in melt temperature drop to/across its liquidus, thus abundant near-liquidus mafic minerals (especially biotite) crystallized in consequence of their fast nucleation compared to feldspar and quartz (Naney and 24

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Swanson, 1980). The MMEs of this origin are therefore distinguished by high volume of biotite that in some cases may be up to 50 vol.% as in the case of the Los Pedroches batholith, Spain (Donaire et al., 2005). Obviously, this is much higher than biotite concentration (less than 20 vol.%) in our MME samples. In addition, wall-rock xenoliths are also expected to be common in the pluton if the quenched

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margins were incorporated into the magma (Flood and Shaw, 2014). However, such xenoliths have not been found in the Huda pluton yet.

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The MMEs in the Huda pluton are best explained as late cumulates formed by

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pressure quenching. In contrast to thermal quenching for enclaves of chilled margin

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origin, this model emphasizes pressure quenching without changing magma

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temperature. As explicated by Flood and Shaw (2014), the exsolved magmatic fluids

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will concentrate near the roof of magma chamber when the magma (or residual melt) become water saturated. Accumulating fluids will generate overpressure to the magma

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chamber roof, resulting in the formation of roof fractures, consequent fluid escape, and final pressure reduction. The sudden pressure drop induces the increase of solidus and decrease of liquidus temperatures of water-saturated magma (or residual melt) and leads to cotectic shift from Q–Ab–Or system to K-feldspar and quartz. Near-liquidus biotite and hornblende and to a less extent plagioclase, tend to nucleate on pre-exiting mineral phenocrysts (e.g. quartz and plagioclase) to form crystal cumulates as enclaves. The mafic ring consisting of fine-grained biotite and hornblende around plagioclase phenocryst in the Huda enclaves (Fig. 2d) is consistent with

this

process.

Moreover,

the

high

MnO

contents

and 25

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100MnO/(MnO+MgO+TFe2O3) ratios of Huda enclaves are also the distinct character for enclaves formed via pressure quenching process (Flood and Shaw, 2014). Additionally, this process generally operates at the shallow emplacement level of magma, which is also substantiated by the low crystallization pressure for MMEs estimated with Al-in-hornblende geobarometer (as low as 1.6 kbar; Table S3), and can

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explain why the texture and composition of zircon grains (especially ca. 240 Ma xenocrystic zircon group) are indistinguishable between MME and Huda pluton.

ro

Therefore, we suggest that the MMEs in the Huda pluton are of cognate origin

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formed by pressure quenching. This mechanism may be also applicable for the MMEs

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from the other Triassic I-type granitoid plutons/batholiths in the EKOB, because they

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also display identical zircon crystallization ages (as well as young xenocrystic zircon

na

ages in some cases) and Sr-Nd-Hf(-Pb) isotopes to their host granites. They also commonly show fine- to medium-grained texture and have moderate biotite volume

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(no more than 20 vol. %), and no country-rock xenoliths are present in their host (Huang et al., 2014; Shao et al., 2017; Xia et al., 2015; Xiong et al., 2012), all which suggest that they cannot be interpreted as either early cumulates or chilled margin as discussed above. Recently, Chen et al. (2018) presented a detailed mineralogical work on the MMEs from ca. 255 Ma Hulagatu batholith in the EKOB. They found that hornblende in the matrix of enclaves crystallized at lower temperature and pressure but under more oxidized conditions than those from host granites. This phenomenon cannot be interpreted as the consequence of magma mixing as proposed by these authors, because the extremely rapid thermal equilibration between enclave and host 26

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at the level where mafic magma intruded into volumetrically dominant felsic magma chambers, would result in similar crystallization temperature and pressure between these newly crystallized minerals. On the contrary, this phenomenon can be best explained by our favored model of pressure quenching process. In this mechanism, MMEs formed during the late magma evolution stage, and the degassing of exsolved

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fluids can cause the pressure reduction and oxygen fugacity increase of the remaining melt, all of which can explain the relatively low crystallization temperature and

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pressure for near-liquidus mafic phase (e.g. hornblende) at more oxidized conditions.

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6.2. Host granodiorite: Partial melting of juvenile crust

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Zircon U-Pb ages reveals that the Huda granodiorite intruded in Late Triassic,

na

during which time synchronous granitoids of either I-, A- or S-type were generated in the EKOB (Hu et al., 2016; Mo et al., 2007; Xiong et al., 2014). The following lines

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of evidence preclude the possibility of the Huda granodiorite being either S-type granite or A-type granite (Chappell and White, 1992; Chappell et al., 2012; Whalen et al., 1987): (1) the absence of characteristic Al-rich minerals (e.g. cordierite and muscovite); (2) the low A/CNK values (﹤1.0; Fig. 6c); (3) the negative relationship between P2O5 and SiO2 (Fig. 7h); (4) low FeOt/MgO and Ga/Al ratios; and (5) low Zr, Nb, Ce and Y concentrations (Table S5). In contrast, all the above features of Huda intrusion, together with high volume of hornblende and biotite that resembles regional contemporaneous I-type granites (Xiong et al., 2014; Ren et al., 2016), suggest that the Huda granodiorite belongs to I-type granite. 27

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The I-type granites could be produced via three principal mechanisms: (1) mixing of crustal and mantle-derived melts (Collins, 1998; Gray and Kemp, 2009; Keay et al., 1997), (2) fractionation of mantle-derived mafic magma, accompanied with crustal assimilation (Clemens et al., 2009; Nandedkar et al., 2014; Sisson et al., 2005), and (3) partial melting of metaigneous (or metasedimentary) rocks from the

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lower crust (Altherr et al., 2000; Chappell and White, 1992; Gao et al., 2016; Kemp et al., 2007; Roberts and Clemens, 1993; Wu et al., 2003; Zhang et al., 2015). The

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relatively higher Mg# (45-50) than pure crustal partial melt (< 45; Fig. 6d; Altherr et

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al., 2000; Rapp and Watson, 1995), and elevated Cr (9-17 ppm), Co (73-204 ppm) and

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Ni (28-100 ppm) contents of Huda granodiorite suggest the involvement of mantle

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component. The widespread MMEs in Triassic I-type granites from the EKOB were

na

considered as a robust petrological evidence for the formation of the host magmas via extensive magma mixing (Chen et al., 2005; Liu et al., 2004; Luo et al., 2014; Qin et

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al., 2018). The prerequisite for such argument is, however, that the MMEs themselves were the (hybridized) mafic globules of mantle origin. Although MMEs are also commonly present in the Huda pluton and display linear relationship among major elements in Harker diagrams (Fig. 7), implying magma mixing process, these enclaves are proved to be cognate origin as the host granodiorite, as discussed above. In addition, mixing of two chemically contrasting end members would result in conspicuous chemical and isotopic variation in the resultant melt. Nonetheless, the consistent whole-rock major and trace element (Figs. 6-8; Table S5) and Sr-Nd isotopic compositions (initial 87Sr/86Sr = 0.70888-0.70912 and εNd(t) = -5.6 to -6.0) of 28

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the Huda granite contradict with this mixing process. Note that the comparatively homogeneous whole-rock composition could probably result from complete hybridization via magma convection due to the relatively long evolution time after mixing in the magma chamber. In this case, several lines of evidence (e.g. chemical and isotopic heterogeneity) for the magma mixing can still be preserved in the mineral

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scale, i.e., εHf(t) values of zircon grains crystallizing from mixed melts commonly display a wide variation range (up to 10 units; Davidson et al., 2007; Shaw and Flood,

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2009; Sun et al., 2010). However, the Lu-Hf isotope compositions of zircon grains

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from both granodiorite and MME are rather homogeneous with a total variation of

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only about 3 εHf(t) units, ranging from -3.3 to -0.1 and from -3.1 to -0.5, respectively

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(Fig. 9b; Table S7). Furthermore, some plagioclase phenocrysts with clear core-rim

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texture from the Huda granodiorite normally display rim-ward decrease in An contents (Fig. 2c; Table S2), suggesting typical fractional crystallization rather than

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magma mixing process. Consequently, magma mixing process cannot adequately explain the formation of the Huda granite (and MMEs). The process involving extensive fractionation of mantle-derived magma is also not consistent with the enriched isotope character of Huda pluton exhibiting high initial Sr isotope and low εNd(t) and εHf(t) values (Fig. 9; Tables S6 and 7), unless the mantle was previously metasomatized by subduction fluids/melts. Given the relative incompatibility of Nb, Th, Ta, and U (DNb≈DTh < DTa≈DU; Niu and Batiza, 1997; Niu and O'Hara, 2009), the Nb/Th and Ta/U ratios of basaltic melt are normally inherited from the source region and remain constant during the subsequent 29

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differentiation (Huang et al., 2014; Shao et al., 2017). The Huda granodiorite has much lower Nb/Th (Nb*=0.11-0.26) and Ta/U (Ta*=0.53-0.68 with two exceptions) ratios than the primitive mantle and the differentiates of mantle-derived mafic melts (Fig. 10), which probably resulted from the contamination of crustal components. It has to be noted that MMEs have much higher Nb*and Ta* values than the host

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granodiorite (Fig. 10), which is more likely related to their higher proportion of hornblende that preferentially incorporates Nb and Ta over Th and U elements (Shao

ro

et al., 2017). Hence, the derivation from enriched mantle with different degree of

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crustal contamination and fractional crystallization as suggested for the generation of

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Triassic I-type granitoids from the EKOB, including Halagatu (248-244 Ma; Li et al.,

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2018), Nan’getan (243 Ma; Xia et al., 2015) and Yemaquan (228-221 Ma; Yin et al.,

na

2017) plutons, seems to be feasible for the Huda pluton as well. The available Sr-Nd-Hf isotope data of the Triassic mafic dykes from the EKOB, however, suggest

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that the underlying enriched mantle was isotopically more enriched than the Huda granite (Xiong et al., 2011; Zhao et al., 2018; Fig. 9). Thus, partial melting of the enriched mantle cannot produce the Huda granodiorite via fractionation and assimilation of even more enriched crustal components. On the other hand, the volume of granitic differentiate from basaltic magma is quite limited, generally less than 25% (Sisson et al., 2005), which means that at least three times the volume of contemporaneous mafic rocks should be present. This contradicts the fact that the intermediate to felsic granitoids of Late Permian to Triassic age volumetrically dominate the EKOB with the exposed region of ca. 23,000 km2, up to 50% of all 30

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exposed granitoids of Proterozoic to the late Mesozoic ages in the EKOB (Liu et al., 2004; Mo et al., 2007). The coeval mafic magmas are rather rare and were commonly intruded as dykes or small intrusions (Hu et al., 2016; Liu et al., 2017; Xiong et al., 2011; Zhao et al., 2018; Zhang et al., 2018). Furthermore, extensive bulk assimilation is also not achievable since this process in general is severely energy-limited to

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dissolve crustal xenoliths during basalt magma ascent (Glazner, 2007). Consequently, differentiation of mantle-derived magma augmented by crustal assimilation is not

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suitable for the generation of Huda granodiorite either.

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Therefore, we propose that the Huda granodiorite formed through the partial

re

melting of lower crust, as proposed for many other coeval I-type granites in the

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EKOB (Luo et al., 2014; Xia et al., 2014; Xiong et al., 2014, 2016a; Zhang et al.,

na

2012). Both ancient basement and juvenile crust were present in the lower crust beneath the EKOB during Late Triassic, and either of these (or both) can be potential

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source for the Huda granite. The ancient basement is represented by Paleo- to Mesoproterozoic metasedimentary rocks of Jinshuikou Group (including Baishahe and Xiaomiao Formations) (Dong et al., 2018; Wang et al., 2007a), whereas the juvenile crust was produced by partial melting of the subducted Paleo-Tethyan oceanic crust with terrigenous sediments (represented by the Mesoproterozoic Shaliuhe gneiss) under amphibolite-facies conditions over the closure of Paleo-Tethyan ocean (Huang et al., 2014; Hu et al., 2016). The relatively old two-stage Nd and Hf model ages of Huda pluton (varying from 1.45 to 1.49 Ga and from 1.13 to1.42 Ga, respectively) may suggest the ancient crustal sources. However, 31

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the basement rocks beneath the EKOB are generally much older than isotope model ages of Huda pluton (see Fig. 9b), which is also evidenced by their more enriched whole-rock Nd-Hf isotope compositions (εNd(t) = -17.1 and εHf(t) = -15.7; t=224 Ma; Shao et al., 2017). In addition, the partial melts from these ancient metasedimentary basement rocks are commonly more enriched in LILE (Rb, Th, U, K and P; Fig. 8b)

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than Huda granodiorite. Accordingly, the underlying ancient metasedimentary basement rocks were not the major sources for the Huda granite. Instead, the juvenile

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crust seems to be the dominant crustal sources, because Huda granodiorite displays

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same REE and trace element patterns as the average juvenile crust (Fig. 8). The

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Sr-Nd-Hf isotope compositions of Huda granodiorite also corroborate with the hybrid

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source for the juvenile crust that was composed of depleted mantle and the

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Mesoproterozoic supracrustal crust (Fig. 9) (Huang et al., 2014). Thus, we infer that the Huda granodiorite originated from the partial melting of crustal sources composed

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principally of juvenile crust beneath the EKOB. The Huda granodiorite has relatively low SiO2 (60.9-65.8 wt. %), and high FeOt (4.16-5.90 wt. %), MgO (1.66-2.76 wt. %), Al2O3 (16.1-17.2 wt. %), CaO (3.83-5.39 wt. %) contents, consistent with experimentally produced melts from basaltic to andesitic rocks at fluid-absent, low pressure (5-10 kbar) and temperature (≤ 1000 °C) conditions (Gao et al., 2016 and references therein). The mafic to intermediate rock sources for Huda granodiorite is also substantiated by its high CaO/(MgO+FeOt) and low Al2O3/(MgO+FeOt) molar ratios (Fig. 11a), and all the Huda samples are plotted within the amphibolite source region (Fig. 11b, c). Because the K2O concentrations in 32

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the melt is primarily controlled by the source rocks, the comparatively high K2O contents but low K2O/Na2O ratios (0.63-0.75) resemble those from dehydration melting of medium-K basaltic rocks at relatively high temperatures above the biotite exhaustion (≥ 850 °C; Sisson et al., 2005). The low-pressure melting condition is also validated from the trace element evidence. The Huda granite has elevated HREE and

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Y contents and exhibits flat HREE patterns with low (Dy/Yb)N ratios (1.17-1.26), indicative of partial melting below the garnet stability field (≤ 10 kbar). Under these

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normal crustal conditions, dehydration melting of basaltic to andesitic source rocks is

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promoted by the incongruent breakdown of hydrous minerals (principally biotite and

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amphibole) and will produce I-type melt and residual pyroxene (Chappell et al., 2012).

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Given the fact that most pyroxenes formed during the dehydration melting

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experiments have considerably low ASI values (0.1-0.3; Sisson et al., 2005; Wolf and Wyllie, 1994), almost all resultant I-type melts are therefore peraluminous unless

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higher temperature has been reached, i.e., up to 1000 °C, at which condition residual pyroxene can be further consumed during the melting process and then the melts could eventually become metaluminous in composition (Beard and Lofgren, 1991; Chappell et al., 2012). In the case of metaluminous Huda granodiorite, such high temperature at the lower crustal level was probably associated with the upwelling of asthenospheric mantle (see below). Moderate crust-mantle interaction probably resulted in elevated Mg# value and Cr, Co and Ni concentrations of Huda granodiorite.

33

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In summary, we interpret that the Huda granodiorite was produced through the partial melting of juvenile crust at normal crustal pressure (≤ 10 kbar) but at unusually high temperature (up to 1000 °C) that was induced by asthenosphere upwelling. In addition, its parental magma underwent assimilation and fractionation crystallization (AFC) during ascent, as implied by the presence of inherited and xenocrystic zircon

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grains and the rim-ward chemically evolved character for some plagioclase phenocrysts (Table S2) that, together with negative Eu anomalies, most likely resulted

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from the fractionation of plagioclase, K-feldspar, hornblende, biotite and Fe-Ti oxides.

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The parental magma finally stalled and accumulated at upper crustal level (6-10 km as

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indicated by hornblende geobarometer). When the magma reached water saturation

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along with fractionation and cooling, the exsolved fluids were focused near the roof of

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magma chamber, creating overpressure and resulting in fracture formation with consequent pressure quenching that ultimately led to the formation of ubiquitous

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MMEs in Huda pluton.

6.3. Geodynamic implications Although

the

precise

timing

of

initial

subduction

and

closure

of

Anyemaqen-Kunlun ocean remains controversial (Ding et al., 2014; Wang et al., 2011; Yin and Zhang et al., 1997; Yuan et al., 2009), the subduction initiation of the Anyemaqen-Kunlun ocean beneath the EKOB is generally considered as Early Permian, with closure in Early-Middle Triassic, resulting in continental collision between the EKOB and Bayanhar terrane (Hu et al., 2016; Li et al., 2018; Liu et al., 34

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2017; Mo et al., 2007; Xiong et al., 2014). The EKOB evolved into post-collisional setting during the Late Triassic (Dong et al., 2018). This is evidenced by the fact that voluminous extension-related I-type and A-type granitoids (Hu et al., 2016; Xia et al., 2014; Yin et al., 2017; Zhang et al., 2017) and mafic dyke swarms (Liu et al., 2017; Zhang et al., 2018) were produced at this time. Compared to the Early- to Middle Triassic magmatism in the EKOB, the Late Triassic is characterized by the highest

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magma addition rate, approximate to 100 km3/m.y. (Ma et al., 2015), which is

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comparable to those of Late Mesozoic magmatic flare-up events from the central

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Sierra Nevada arc, California (Paterson and Ducea, 2015). The magmatic flare-up

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events are commonly associated with a series of successive processes involving

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(Ducea et al., 2015).

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crustal thickening, delamination, and subsequent lithospheric mantle upwelling

Partial melts derived from thickened lower crust generally show chemical

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affinity to adakites, characterized by high Sr/Y and La/Yb ratios (e.g. Chiaradia, 2015; Profeta et al., 2015). Two episodes of Late Triassic adakitic rocks were reported from the EKOB. The early episode is represented by the Xiao-Nuomuhong (ca. 222 Ma; Xia et al., 2014), Xiangride (223 Ma; Xiong et al., 2014) and Helegang-Xilikete (225 Ma; Chen et al., 2013b) plutons (or batholiths), and these rocks generally have low Mg# values and Nb/Ta ratios. Their high Sr/Y characters suggest they were derived from thickened crust at the depth exceeding 35-40 km (Annen et al. 2006). If we adopt the positive correlation between La/Yb ratio and inferred depth for the source rocks as computed by Chung et al. (2009), the highest La/Yb ratios varying from 32 35

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to 52 of these three plutons further suggest that the crust was probably thickened to 45-50 km at this period. In contrast, the late episode of adakitic rocks represented by 218 Ma Kekealong pluton (Chen et al., 2013a) shows high Mg# values, Nb/Ta ratios (16.2-21.9), and Cr and Ni contents, which are chemically similar to the partial melts from eclogitic lower crust with subsequent interaction with surrounding mantle

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peridotites during ascent (Gao et al., 2009; Wang et al., 2007; Xu et al., 2008). This indicates that the thickened lower crust already delaminated prior to 218 Ma, and was

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foundered into deeper level (exceeding 1.5 GPa, corresponding to more than 45-50

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km), in the rutile stability field, as implied by the superchondritic Nb/Ta ratios of the

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partial melts (Xiong et al., 2005). This delamination and resultant asthenospheric

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upwelling process was proposed to explain the planar distribution character of

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voluminous Late Triassic granitoids (Luo et al., 2014) and the generation of 228-213 Ma OIB-type mafic dykes and high-Nb-Ta rhyolites in the EKOB (Ding et al., 2011;

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Hu et al., 2016; Liu et al., 2017). Such spatial and temporal associations between lowand high-Mg adakitic rocks have also been reported from the southern and eastern margins of the North China Craton, where extensive lithospheric delamination and thinning processes following earlier crustal thickening occurred during the Late Mesozoic (Gao et al., 2004, 2009; Huang et al., 2008; Wang et al., 2007a; Xu et al., 2008). Compared to coeval low-Mg adakitic rocks (222-225 Ma), the Huda pluton in this study has lower Sr/Y and La/Yb ratios, implying that the partial melting occurred at normal crust thickness. Many synchronous I-type granites with low Sr/Y ratios are 36

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also present in the EKOB (Luo et al., 2014; Xiong et al., 2016a; Yin et al., 2017). This could not be explained as non-uniform thickness of crust, because both coeval highand low Sr/Y granites can be found in the same batholith, i.e., ca. 218 Ma granites from Xiangride batholith display both chemical features (Luo et al., 2014; Xiong et al., 2014). Considering that the crustal thickening and delamination processes took place

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during the Late Triassic, one possible explanation is that part of thickened lower crust (plus lithospheric mantle) delaminated and was foundered into asthenospheric mantle

ro

at this time, which resulted in the upwelling of hot asthenosphere, ultimately leading

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to the whole-scale melting of lower crust (Nelson and Montana, 1992). Consequently,

re

both high- and low Sr/Y rocks could be generated at the same time, via partial melting

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of remnant thickened lower crust in deep and normal lower crust at shallower level,

na

respectively (Fig. 12). The resultant high-Sr/Y rocks have comparatively low Mg# and subchondritic Nb/Ta ratios because of no (or restricted) interaction with surrounding

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peridotite and lower melting pressure without rutile formation. In summary, we suggest that the Huda pluton was produced from the partial melting of juvenile crust formed from delamination-related asthenosphere mantle upwelling. The latter process gave rise to the whole scale melting of lower crust and produced the Late Triassic magmatic flare-up event in the EKOB. Such high magma fluxes are necessary to create magma chamber in upper crust level, and keep it molten thus leading to a long-time evolution rather than fast solidification (Richards, 2005; Zhang and Audétat, 2017). Such a setting would facilitate the production of mafic enclaves via pressure quenching as we interpret in the case of the Huda MMEs in this 37

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study during the late evolution stage of magma system.

7. Conclusions Based on integrated mineralogical, geochronological, geochemical and isotopic studies on the MMEs and host granodiorite in the Huda pluton from the EKOB, the

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following conclusions are drawn. (1) The MMEs and host granodiorite have same crystallization ages (ca. 224 Ma)

ro

and identical Sr-Nd-Hf isotope compositions. Combined with mineralogical evidence,

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we interpret that the MMEs are of cognate origin and formed by pressure quenching

re

rather than magma mixing or early cumulates as proposed in previous studies. Our

na

granitoids in the EKOB.

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model may also be applicable for the formation of MMEs in the other Triassic

(2) The Huda granodiorite is metaluminous I-type granite and, as inferred from

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the major and trace elements and isotope compositions, was produced from the partial melting of crustal sources constituted mainly by juvenile crust at normal lower crustal depth level with heat input from asthenosphere upwelling. (3) Considering the coexistence of high- and low Sr/Y rocks in the EKOB, and contemporaneous mafic dykes displaying asthenospheric mantle source feature, we suggest that the Late Triassic asthenospheric upwelling was probably associated with earlier crustal thickening and delamination, and induced the whole-scale melting of lower crust, leading to the Late Triassic magmatic flare-up event in the EKOB.

38

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Acknowledgements We want to thank Associate Editor Toshiaki Tsunogae and an anonymous reviewer for their constructive comments that really improve our manuscript, and Mr. Zhongxian Ma and Pengyu Li for their field assistance. This study was jointly funded

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by the National Natural Science Foundation of China (41772071 and 41802086), the Fundamental Research Funds for the Central Universities, China University of

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Geosciences (Wuhan) (CUG180609) and the China Postdoctoral Science Foundation

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na

lP

re

-p

(BX20190302).

39

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Zhang, D.H., Wei, J.H., Fu, L.B., Chen, H.Y., Tan, J., Li, Y.J., Shi, W.J., Tian, N., 2015. Formation of the Jurassic Changboshan-Xieniqishan highly fractionated I-type granites, northeastern China: implication for the partial melting of juvenile crust induced by asthenospheric mantle upwelling. Geological Journal 50, 122-138. Zhang, D., Audétat, A., 2017. What caused the formation of the giant Bingham Canyon porphyry Cu-Mo-Au deposit? Insights from melt inclusions and magmatic sulfides. Economic Geology 112, 221-244. Zhang, H.F., Chen, Y.L., Xu, W.C., Liu, R., Yuan, H.L., Liu, X.M., 2006. Granitoids 57

Journal Pre-proof around Gonghe basin in Qinghai province: petrogenesis and tectonic implications. Acta Petrologica Sinica 22, 2910-2922 (in Chinese with English abstract). Zhang, J.Y., Ma, C.Q., Xiong, F.H., Liu, B., 2012. Petrogenesis and tectonic significance of the Late Permian-Middle Triassic calc-alkaline granites in the Balong region, eastern Kunlun Orogen, China. Geological Magazine 149, 892-908. Zhang, J.Y., Yang, Z.B., Zhang, H., Ma, C.Q., Li, J.W., Pan, Y.M., 2017. Controls on

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the formation of Cu-rich magmas: Insights from the Late Triassic post-collisional

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Saishitang complex in the eastern Kunlun Orogen, western China. Lithos 278, 400-418.

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Zhang, M.D., Ma, C.Q., Wang, L.X., Hao, F.H., Zheng, S.J., Zhang, L., 2018.

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Subduction-Type Magmatic Rocks in Post-Collision Stage:Evidence from Late

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Triassic Diorite-Porphyrite of Naomuhungou Area,East Kunlun Orogen. Earth Science 43, 1183-1206 (in Chinese with English abstract).

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Zhao, F.F., Sun, F.Y., Liu, J.L., 2017. Zircon U-Pb Geochronology and Geochemistry of the Gneissic Granodiorite in Manite Area from East Kunlun,with Implications

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for Geodynamic Setting. Earth Science 42, 927-940,1044 (in Chinese with English abstract).

Zhao, X., Fu, L.B., Wei, J.H., Zhao, Y.J., Tang, Y., Yang, B.R., Guan, B., Wang, X.Y., 2018. Geochemical Characteristics of An'nage Hornblende Gabbro from East Kunlun Orogenic Belt and Its Constraints on Evolution of Paleo-Tethys Ocean. Earth Science 43, 354-370 (in Chinese with English abstract).

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Journal Pre-proof Figure captions:

Fig. 1 (a) Geological sketch map of the Qinghai-Tibetan Plateau showing the principal blocks/terrains (modified after Chen et al., 2016). TRMB = Tarim block; ALSB = Alashan block; INDB = India block; YZB = Yangtze block. (b) Tectonic map of the East Kunlun Orogenic Belt displaying the ophiolitic mélange zones and tectonic

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divisions (modified after Dong et al., 2018; Xia et al., 2015). (c) Simplified geological

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map of the Elshan area located at the easternmost part of the EKOB, in which the

=

Aqikekulehu-Kunzhong

ophiolitic

mélange

zones;

BAM

=

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AKM

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sample locations are marked. QXM = Qimantagh-Xiangride ophiolitic mélange zones;

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Buqingshan-Anemaqen ophiolitic mélange zones. Zicon U-Pb ages of the magmatic rocks shown in Fig. 1b are collected from the following references: 1-Yang et al.

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(2013); 2-Xiong et al. (2014); 3-Xiong (2014); 4-He (2016a); 5-Zhang et al. (2006);

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6-Xiong et al. (2013); 7-Dai et al. (2013); 8-Xiong et al. (2011); 9-Zhang et al. (2012); 10-Chen et al. (2012); 11-Wang et al. (2009); 12-Song et al. (2013); 13-Feng et al. (2011); 14-Gao et al. (2012); 15-Xia et al. (2014); 16-Xiao et al. (2013); 17-Liu et al. (2012a, b); 18-Chen et al. (2013a); 19-Yin et al. (2013); 20-Ding et al. (2014); 21-Ren et al. (2016); 22-Chen et al. (2017).

Fig. 2 (a) Field photographs of MMEs in host granodiorite. (b-c) Photomicrographs of host granodiorite. The red circles and digits in (c) show the locations of EPMA analysis and the corresponding An contents. (d) Photograph of polished thin section of

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MME, showing the presence of plagioclase and hornblende xenocrysts. (e-f) Microphotographs of MMEs. (b-c) and (f) are in transmitted light, while (e) is in reflected light. Abbreviation: Ap = apatite; Bt = biotite; Hb = hornblende; Kfs = potassic feldspar; Pl = plagioclase; Q = quartz.

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Fig. 3 Cathodoluminescence (CL) images of representative zircon grains from host granodiorite (a-d) and MMEs (e-g) (b). The red solid and blue dashed circles show the

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Pb/238U ages (red digits) and εHf(t) values (blue digits) are also shown.

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206

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locations for U-Pb and Lu-Hf isotope analysis, respectively. Their corresponding

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Fig. 4 Zircon U-Pb concordia plots of the Huda granodiorite (a-d) and the MMEs (e-g)

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in pluton. MSWD = mean square of the weighted deviation.

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Fig. 5 Composition of plagioclase (a; Deer et al., 1992), hornblende (b; Leake et al., 1997) and biotite (c; Foster, 1960).

Fig. 6 Plots of (a) Na2O+K2O vs. SiO2 (Middlemost, 1994), (b) K2O vs. SiO2 (Middlemost, 1985; Peccerillo and Taylor, 1976), (c) A/NK [Al2O3/(Na2O+K2O)] vs. A/CNK [Al2O3/(CaO+Na2O+K2O)] (Maniar and Piccoli, 1989) and (d) Mg# vs. SiO2 for

the

Huda

granodiorite

and

MME

samples.

The

composition

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experimental-derived crustal melts from dehydration melting of basaltic to pelitic rocks shown in (d) are from Patino Douce and Johnston (1991), Rapp and Watson 60

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(1995) and Sisson et al. (2005).

Fig. 7 Harker diagrams for Huda granodiorite and MME samples. Symbols are the same as in Fig. 6.

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Fig. 8 Chondrite-normalized REE patterns (a) and primitive mantle-normalized trace element spiderdiagrams (b) for the Huda granodiorite and MME samples. Chondrite

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and primitive mantle-normalized values are from Sun and Mcdonough (1989). The

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average ancient crust (Ave. Ancient crust) composition is from Xiong et al. (2012,

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2014), and the average juvenile crust (Ave. Juvenile crust) composition is from Huang

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et al. (2014).

Fig. 9 Plots of εNd(t) vs. ISr (a) and εHf(t) vs. U-Pb age (b) for the Huda granodiorite

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and MME samples. In the εNd(t) vs. ISr diagram (a), the Sr-Nd isotopic compositions of magmas from different source regions in the EKOB are compiled from Xiong et al. (2014), and also for the isotopic fields of A’nyemaqen midocean ridge basalt (MORB) and ocean island basalt (OIB). And for the isotope modeling, the composition of Paleo-Tethyan MORB (87Sr/86Sr: 0.7055,

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Nd/144Nd: 0.51313, Sr: 69.07 ppm, Nd:

6.5 ppm) and Shaliuhe gneiss (87Sr/86Sr: 0.7180,

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Nd/144Nd: 0.5121, Sr: 300 ppm,

Nd: 33 ppm) are from Huang et al. (2014) and references therein. In εHf(t) vs. U-Pb age diagram (b), the Hf isotope data of magmas generated from ancient crust are from Xiong et al. (2012 and 2014), of juvenile crust-derived magmas from Huang et al. 61

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(2014), and of enriched mantle (EM) from Xiong et al. (2011) and Zhao et al. (2018).

Fig. 10 Ta* vs. Nb* diagram for the Huda granodiorite and MME samples (after Niu and Batiza, 1997). The compositions of the primitive mantle (PM) and the average oceanic basalts (OIB, E-MORB, N-MORB) are collected from Sun and McDonough

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(1989), and the crustal compositions (BCC, LCC, UCC) are from Rudnick and Gao

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(2003).

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Fig. 11 Source discrimination diagrams for the Huda granodiorite and the MME

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samples. (a) is after Altherr et al. (2000), (b-c) are after Patiño Douce (1999). Symbols

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are the same as in Fig. 6.

Fig. 12 A proposed model for the geodynamic evolution of the EKOB and the

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mechanism for the magmatic flare-up during Late Triassic (from ~230 Ma to 218 Ma). (a) The successive subduction of Anyemaqen-Kunlun ocean and subsequent continental collision resulted in the crustal thickening, and part of lower crust were transformed into garnet-bearing facies. The following slab break-off (Chen et al., 2015; Liu et al., 2017; Xia et al., 2014; Xiong et al., 2014) triggered asthenosphere upwelling which softened the overlying lithospheric mantle. (b) Due to the gravitational instability, part of the thickened lower crust and lithospheric mantle delaminated and foundered into deeper level into the rutile stability field. The delamination resulted in large-scale upwelling of asthenospheric mantle and led to the 62

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whole-scale melting of lower crust and the metasomatized subcontinental lithospheric mantle. The melting of the remaining thickened lower crust generated the 225-222 Ma low-Mg adakitic rocks (1), whereas the melting of the delaminated lower crust produced the high-Mg adakitic rocks (2; e.g. 218 Ma Kekealong quartz diorites; Chen et al. 2013a). The production of 225~222 Ma magmas with normal Sr/Y ratios (3; e.g.

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Huda pluton in this study) was related to the melting of juvenile crust at shallower depth. The melting of the subcontinental lithospheric mantle that once was

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metasomatized during early subduction generated the 228-218 Ma OIB-type mafic

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dykes (4) and the associated high Nb-Ta rhyolites in the EKOB (Ding et al., 2011; Hu

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et al., 2016; Liu et al., 2017).

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Journal Pre-proof Credit Author Statement

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Hongzhi Zhou: Data curation, Writing- Original draft preparation. Daohan Zhang: Conceptualization, Methodology, Writing- Reviewing and Editing. Junhao Wei: Supervision. Dazhao Wang: Formal analysis. M. Santosh: Writing- Reviewing and Editing, Wenjie Shi: Data curation. Jiajie Chen: Data curation. Xu Zhao: Data curation.

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Declaration of competing interests The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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Graphical abstract

Highlights

 Mafic microgranular enclaves are cognate cumulates.

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 The 224 Ma granodiorite was derived from the partial melting of juvenile crust.

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 The Late Triassic magmatic flare-up event was associated with delamination

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process.

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