Petrogenesis of Neoarchean metavolcanic rocks in Changyukou, Northwestern Hebei: Implications for the transition stage from a compressional to an extensional regime for the North China Craton

    Petrogenesis of Neoarchean metavolcanic rocks in Changyukou, Northwestern Hebei: Implications for the transition stage from a compressional to an extensional regime for the North China Craton Peng Liou, Houxiang Shan, Fu Liu, Jinghui Guo PII: DOI: Reference:

S0024-4937(16)30452-2 doi:10.1016/j.lithos.2016.12.019 LITHOS 4180

To appear in:

LITHOS

Received date: Accepted date:

21 October 2016 20 December 2016

Please cite this article as: Liou, Peng, Shan, Houxiang, Liu, Fu, Guo, Jinghui, Petrogenesis of Neoarchean metavolcanic rocks in Changyukou, Northwestern Hebei: Implications for the transition stage from a compressional to an extensional regime for the North China Craton, LITHOS (2016), doi:10.1016/j.lithos.2016.12.019

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ACCEPTED MANUSCRIPT Petrogenesis of Neoarchean metavolcanic rocks in Changyukou, Northwestern Hebei: implications for the transition stage from a compressional to an

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extensional regime for the North China Craton

Peng Lioua,b, Houxiang Shana,b, Fu Liu a, Jinghui Guo a, ∗

State Key Laboratory of Lithospheric Evolution, Institute of Geology and Geophysics, Chinese

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Academy of Sciences, Beijing, 100029, China

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University of Chinese Academy of Sciences, Beijing, 100049, China E-mail address: [email protected]

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∗ Corresponding author

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Abstract

The 2.5Ga metavolcanic rocks in Changyukou, Northwestern Hebei, can be classified into

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three groups based on major and trace elements: high-Mg basalts, tholeiitic basalts, and the calc-alkaline series (basaltic andesites-andesites and dacites-rhyolites). Both high-Mg basalts and tholeiitic basalts have negative anomalies of Nb, Zr, Ti and Heavy Rare Earth Elements (HREE) as well as enrichments of Sr, K, Pb, Ba and Light Rare Earth Elements (LREE) and show typical subduction zone affinities. The petrogenesis of high-Mg basalts can be ascribed to high-degree partial melting of an enriched mantle source in the spinel stability field that was previously enriched in Large Ion Lithophile Elements (LILE) and LREE by slab-derived hydrous fluids/melts/supercritical fluids, as well as the subsequent magma mixing processes of different sources at different source depths, with little or no influence of polybaric fractional 1

ACCEPTED MANUSCRIPT crystallization. The flat HREE of tholeiitic basalts indicates they may also originate from the spinel stability field, but from obviously shallower depths than the source of high-Mg basalts. They may

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form at a later stage of the subduction process when rapid slab rollback leads to extension and

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seafloor spreading in the upper plate.

We obtain the compositions of the Archean lower crust of the North China Craton based on the Archean Wutai-Jining section by compiling the average tonalite–trondhjemite–granodiorite

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(TTG) components, average mafic granulite components, and average sedimentary rock

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components. The modeling results show that the generation of high-Al basalts, basaltic andesites and andesites can be attributed to assimilation by high-Mg basalts (primary basalts) of relatively

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high-Al2O3 thickened lower crust and the subsequent crystallization of prevailing mafic mineral

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phases, while Al2O3-rich plagioclase crystallization is suppressed under high-pressure and nearly

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water-saturated conditions. Dacites and rhyolites may be the result of further fractional crystallization of basaltic andesites (high-Al basalts) and andesites. Mixing of magmas at various

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stages along the fractionation course of basaltic andesites (high-Al basalts) toward rhyolites promotes the trend of the calc-alkaline series. To reconcile the 2.55 to 2.5Ga TTGs derived from overthickened crust, the 2.51 to 2.50Ga calc-alkaline volcanic rocks derived from thickened crust, tholeiitic basalts representing low pressure and an extensional tectonic setting, 2493Ma leucosyenogranites derived from overthickened crust, 2437Ma biotite-monzogranites derived from slightly thinner crust than leucosyenogranites but still thickened, as well as the clockwise hybrid ITD and IBC P–T paths of the HP granulites and widespread extension and rifting setting within the NCC from 2300 Ma, we propose a model of an evolving subduction process. Among them, the composition of the 2.5Ga 2

ACCEPTED MANUSCRIPT Changyukou volcanic rocks and potassic granites as well as the clockwise hybrid ITD and IBC P–T paths of the HP granulites may reveal that the tectonic setting in Northwest Hebei was in a

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transition stage from a subduction-related compressional regime to an extensional regime

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related to plate rollback.

Keywords:

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Volcanic rocks; Northwestern Hebei; North China Craton; Archean Lower Crust; Archean

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subduction zone; Plate rollback

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ACCEPTED MANUSCRIPT 1. Introduction

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The Archaean and Proterozoic boundary marks what is arguably the most fundamental

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change in the evolution of the Earth (Campbell and Griffiths, 2014; Kamber, 2015). The significant

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increases in large-ion lithophile and high-field strength elements and a decrease in Sr in continental crust at the end of the Archean appear to reflect chiefly a decrease in

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tonalite-trondhjemite-granodiorites (TTG) magma production and a corresponding increase in calc-alkaline magma production (Condie, 2008). However, both the tectonic regime during which

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this crust formed and the cause of rapid crustal growth remain controversial (Condie and O’Neill, 2010). In the North China Craton (NCC), three different tectonic models have been proposed to

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explain the petrogenesis of the Neoarchean TTG, granitic rocks nd calc-alkaline volcanic rocks. Zhang et al. (2011); Zhang et al. (2012) concluded the Neoarchean biotite–monzogranites and

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high-Al TTG in the northern NCC derived from a thickened juvenile crust of garnet amphibolite composition. Liu et al. (2012) believed such TTG and dioritic gneisses formed through partial

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melting of subducted oceanic crustal material. Yang et al. (2008) linked the petrogenesis of the granitoids in the northern NCC with mantle plume activity. However, most of the studies were focused on the TTGs and granitoids, the widespread metavolcanic rocks in the northern margin of NCC have been neglected. In addition, the widespread 2.6-2.5Ga magmatic and metamorphic rocks and zircons indicate the late Neoarchean period is the most important tectonic-thermal event in the NCC (Wan et al., 2014), but the NCC became tectonically inactive in the period between 2.5Ga and 2.35Ga Ma (Zhai et al., 2010). It is still unclear that what kind of mechanisms was able to dominate the transition process. In this study, we present combined zircon LA-ICPMS U–Pb age, whole rock geochemistryand 4

ACCEPTED MANUSCRIPT zircon Hf isotope data for the metavolcanic rocks of the Changyukou area in Northern Hebei (Fig. 2), with the hope that the new data here, in combination with previous studies, could provide

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Archean-Proterozoic transition in the north margin of the NCC.

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comprehensive insights into the petrogenesis and crustal evolution of Northern Hebei during the

2．Geological setting

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The North China Craton formed in Paleoproterozoic time by the assembly of two Archean blocks, the Eastern and Western blocks, along the Trans-North China orogen (e.g.,(Zhao and Zhai,

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2013)). The study area is located in the center of the northern segment of the NCC (Fig. 1a), with the supracrustal rock series named Hongqiyingzi Group lying to the north and the Hengshan

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Complex lying to the south (Fig. 1b). The study area is a part of the Huai’an terrane. The Huai’an TTG and dioritic gneisses make up 60% of the terrane. They formed at approximately 2.55-2.45

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Ga and recorded Paleoproterozoic granulite-facies metamorphism at 1.85-1.80 Ga (Guo et al., 2001; Guo et al., 2005; Liu et al., 2012; Liu et al., 2009; Zhang et al., 2011; Zhang et al., 2012).

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The high-pressure (HP) mafic granulites, distributed around the outer part of the Huai’an gneiss dome (Guo et al., 1993; Li et al., 1998; Zhai et al., 1993), occur as enclaves or small sheet-like bodies within the Huai’an TTG gneiss terrane (Fig. 1b). They record a sequential metamorphic history in four stages, with the P-T path involving prograde metamorphism, near-isothermal decompression (ITD) and cooling after peak metamorphism, which implies thickening followed by extension in a collisional environment (Guo et al., 2002). The Neoarchean granitic gneisses, reddish in color, are randomly distributed within the Huai’an Complex as small stocks (Fig. 1b). There are also minor Paleoproterozoic granitic gneisses and syenogranites displaying clear intrusive contacts with the TTG gneisses, which were 5

ACCEPTED MANUSCRIPT considered to be emplaced at ~2.0 Ga(Zhang et al., 2011; Zhao et al., 2008). The metavolcanic rocks are mainly exposed in the areas of Changyukou, Xuanhua, Chongli

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and Chicheng (Fig. 1 and Fig. 2) and have been named “Hongqiyingzi Group” by previous

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studies. Most of them have undergone amphibolite to granulite facies, with strong deformation. In addition, obvious migmatization can be observed in local areas. The metavolcanic rocks in the Changyukou area in this study are in close contact with the Huai’an TTG gneisses. The

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basalts, andesites, dacites and rhyolites.

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metavolcanic rocks are composed of felsic rocks (60%) and basic rocks (40%) ranging from

In the recent research Wan et al. (2015) delineated three ancient terranes (the Eastern

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Ancient Terrane, Southern Ancient Terrane, and Central Ancient Terrane) based on the spatial

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distribution of ancient rocks and zircons. The study area is located in the northwestern margin of

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3．Petrology

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the Central Ancient Terrane (fig.1 a).

The samples in the present study were collected from the Changyukou area in Northwestern Hebei (Fig. 2). Due to the effect of the Paleoproterozoic regional metamorphism, many micro-textures and features are not well preserved, but the macro-textures and structures of the volcanic succession are visible. They show a gneissic or banded structure and have obvious compositional layering (Fig. 3). All of them display a fine-grained homeoblastic texture, and triple junction structure among the minerals can be observed (Fig. 4). Four types of metavolcanic rocks were collected: metabasalts, meta-andesites, metadacites, and metarhyolites. The metabasalts, ranging from Gt-granulite to amphibolite with the latter constituting the major 6

ACCEPTED MANUSCRIPT part, are composed dominantly of clinopyroxene (5-10%), amphibole (45-50%), plagioclase (35-40%), magnetite (< 5%), ilmenite (< 5%) and minor garnet grains (< 5%). The meta-andesites

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are composed mainly of plagioclase (50-55%), amphibole (30-35%), magnetite (< 5%), ilmenite (<

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5%) and minor quartz grains (< 5%). The metadacites are composed mainly of plagioclase (45%), quartz (40%), clinopyroxene (10%), and minor magnetite and ilmenite (< 5%). The metarhyolites are composed mainly of plagioclase (50%), quartz (45%), and minor magnetite and ilmenite (<

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5%).

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4. Analytical methods

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4.1. Zircon LA-ICP-MS U-Pb dating

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The zircon grains were extracted through heavy liquid and magnetic separation techniques, with subsequent hand-picking under a binocular microscope prior to mounting in epoxy resin

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discs. The internal structures of the zircons were clearly revealed by cathodoluminescence (CL) imaging using a Gatan MonoCL3 cathode light emitter on a JEOL JXA-8100 Electron Microprobe (EMPA) at the Institute of Geology and Geophysics, Chinese Academy of Sciences (IGGCAS).

Zircon U-Pb dating was conducted on an Agilent 7500a quadruple (Q)-ICP-MS equipped with a GeoLas 200M ArF excimer 193 nm laser-ablation system (MicroLas, Germany) at the State Key Laboratory of Lithospheric Evolution in the IGGCAS. A 30-μm spot size was used during the analytic process. The detailed analytical procedures were described in Xie et al. (2008). Standard zircons 91500 and NIST610 were analyzed twice and once every 10 analyses, respectively. 7

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Pb/206Pb,

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Pb/238U,

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Pb/235U and

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Pb/232Th ratios were calculated using the GLITTER 4.0

program (Macquarie University) and calibrated using the Harvard external standard zircon 91500 206

Pb/238U age of 1065.4 ± 0.6 Ma for both instrumental mass bias and

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with a recommended

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depth-dependent elemental and isotopic fractionation. U, Th and Pb concentrations were corrected by the internal standard 29Si and the external standard NIST SRM 610. Isoplot 3.0 was employed to make concordia diagrams and weighted mean calculations with 1σ-error and 95%

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confidence levels (Ludwig, 2003).

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4.2. In situ zircon Lu-Hf isotope analyses

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Zircon Lu-Hf isotopic analyses were performed on a Neptune MC-ICPMS equipped with a 193-nm laser ablation system at the State Key Laboratory of Lithospheric Evolution of the

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IGGCAS. A 60-μm spot size with a laser repetition rate of 8 Hz was used throughout the process. The detailed analytical techniques were described by Wu et al. (2006). The zircon domains

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chosen for Hf isotopic analyses were the same as those for U-Pb dating. During analyses, the Hf/177Hf ratios of the standard zircon (GJ) were 0.282021 ± 0.000010 (2σ, n = 30), which is

consistent with the commonly accepted values measured using the solution method and in situ studies within error (Woodhead and Hergt, 2005; Xie et al., 2008). A decay constant for chondritic ratios of

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Lu of 1.867×10–11 (Söderlund et al., 2004) and the present-day

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Lu/177Hf = 0.0336 and 176Hf/177Hf = 0.282785 (Bouvier et al., 2008) were

adopted to calculate εHf(t) values. The depleted mantle line is defined by a present-day 176

Hf/177Hf ratio of 0.28325 and a 176Lu/177Hf ratio of 0.0384 (Griffin et al., 2000). In addition, a

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Lu/177Hf value of 0.015 for the average continental crust (Griffin et al., 2002) was used during 8

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calculations.

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4.3. Major and trace elements

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The major element compositions were measured by 1500 X-Ray Fluorescence instrument at IGGCAS with the GBW07101-07114 Chinese national standard used for calibration.

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Approximately 0.50 g of rock powder was first ignited at 1000 °C for approximately 1 h to obtain the loss on ignition (LOI) and then fused with 3-4 drops of lithium tetraborate. Uncertainties

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depended upon the concentration in the sample, but generally they were estimated at

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±3–5%.Trace element analyses were determined by LA-ICP-MS at the State Key Laboratory of

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Geological Processes and Mineral Resources, China University of Geosciences, Wuhan. The detailed procedures for ICP-MS analyses and analytical precision and accuracy for trace elements

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were described by Liu et al. (2008); Liu et al. (2010).

5. Results

5.1. Zircon U-Pb dating The CL images of representative zircons are shown in Fig. 5. The U-Pb data of analyzed zircons are listed in Error! Reference source not found. and plotted in Fig. 5 and Fig. 6, respectively. For older zircons (>1.2Ga), due to the intercept ages defined by the discordia line may be disturbed, it is more reasonable to use the weighted mean 207Pb/206Pb ages instead.

5.1.1. 10LF28 (tholeiitic basalt) 9

ACCEPTED MANUSCRIPT Zircon grains are spherical to ovoid or even irregularly shaped, range in size from 50 μm to 120 μm, luminesce dimly in CL (Fig. 5a), and have U and Th contents and Th/U ratios of 123-481

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ppm, 35-112 ppm, and 0.23-0.32, respectively. Nearly all of the grains display faint fir-tree-like or

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patch-like sector zoning, implying that they are typical metamorphic zircons. Among them, 19 analyses give a weighted mean 207Pb/206Pb age of 1796.0 ± 8.5 Ma (MSWD = 0.14, Fig. 6a). The age could be considered as the metamorphic age of the sample, supporting a Paleoproterozoic

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metamorphic event in this area.

5.1.2. 10LF53 (basaltic andesite)

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Zircon grains are tabular or irregularly shaped and range in size from 100 μm to 200 μm. Most of them possess weak magmatic oscillatory zoning (Fig. 5b). Some have homogeneous

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metamorphic overgrowths around the magmatic cores. The magmatic zircon domains have Th and U contents and Th/U ratios of 57-281 ppm, 110-512 ppm and 0.3-0.84, respectively. The 207

Pb/206Pb age of 2504 ±

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seven concordant analyses (95% confidence) yield a weighted mean

14 Ma (MSWD = 0.5, Fig. 6b), which could be considered as the crystallization age of the basaltic andesite. 5.1.3. 10LF73 (meta-andesite) These zircons have prismatic or ovoid shapes and range in size from 50 to 200 μm with length/width ratios of 1:1 to 3:1 (Fig. 5c). Many of them show complex core-rim structures. Magmatic cores have a prismatic habit with typical oscillatory magmatic zoning, but cores are darker in CL than rims. The magmatic zircon domains have Th and U contents and Th/U ratios of 15-1325 ppm, 73-1930 ppm, and 0.2-1.55, respectively. The nine concordant analyses (95% 10

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Pb/206Pb age of 2505 ± 16 Ma (MSWD = 0.02, Fig. 6c),

which could be considered as the crystallization age of the andesite.

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5.1.4. 10LF63 (rhyodacite)

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These zircons have ovoid or prismatic shapes and range in size from 30 to 200 μm (Fig. 5d). Many of them show complex core-rim structures, with thin bright alteration rims surrounding or

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eroding the magmatic cores. Some metamorphic domains crosscut the magmatic domains, implying the role of metamorphic recrystallization. The magmatic zircon domains have Th and

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U contents and Th/U ratios of 199-955 ppm, 246-1853 ppm, and 0.36-1.43, respectively. The twenty concordant analyses (95% confidence) yield a weighted mean

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Pb/206Pb age of 2509 ±

rhyodacite.

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5.1.5. 10LF64 (rhyodacite)

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9 Ma (MSWD = 0.3, Fig. 6d), which could be considered as the crystallization age of the

Most of these zircons have ovoid shapes and range in size from 30 to 200 μm (Fig. 5e).

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Some grains show core-rim structures with darker cores than rims in CL. Many of them have blurred oscillatory zoning, implying that these magmatic domains have undergone later alterations. Twenty-six analyses on the magmatic domains yield Th and U contents and Th/U ratios of 69-3316 ppm, 89-2322 ppm, and 0.35-4.46, respectively. Among them, the eighteen most concordant analyses yield a weighted mean 207Pb/206Pb age of 2507 ± 11 Ma (MSWD = 0.03, Fig. 6e), which could be considered as the crystallization age of the rhyodacite.

5.2. Zircon Hf isotopes The zircon Lu-Hf isotopic results for the metavolcanic rocks are listed in Error! Reference 11

ACCEPTED MANUSCRIPT source not found. and plotted in Fig. 7. For the basaltic andesite (10LF53, Fig. 7a), the seven most concordant age plots give Hf/177Hf values of 0.281293 to 0.281362. When corrected to 2504 Ma, the εHf(t) values range

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have similar

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from 1.7 to 3.9, with TDM ages of 2664-2752 Ma and TDMC ages of 2765-2898 Ma. Other plots 176

Hf/177Hf values of 0.281229 to 0.281396 and similar TDM ages of 2622-2786 Ma,

indicating that the Lu-Hf system in these domains was not modified by the later metamorphism.

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For the andesite (10LF73, Fig. 7b), the eight most concordant age plots give

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Hf/177Hf

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values of 0.281228 to 0.281315. When corrected to 2505 Ma, the εHf(t) values range from 0.6 to 3.9, with TDM ages of 2666-2782 Ma and TDMC ages of 2765-2954 Ma. Other plots have similar Hf/177Hf values of 0.281219 to 0.281369 and similar TDM ages of 2681-2794 Ma, indicating that

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the Lu-Hf system in these domains was not modified by the later metamorphism. 176

Hf/177Hf

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For the rhyodacites (Fig. 7 c and d), twenty analyses of sample 10LF63 give

values of 0.281242 to 0.281334. When corrected to 2509 Ma, the εHf(t) values range from 1.2 to

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4.1, with TDM ages of 2660-2772 Ma and TDMC ages of 2747-2928 Ma. Analyses for sample 10LF64 have similar results. The seventeen most concordant analyses give

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Hf/177Hf values of

0.281275 to 0.281330. The εHf(t) values for t=2507 Ma range from 2.3 to 4.4, with TDM ages of 2647-2727 Ma and TDMC ages of 2730-2860 Ma.

5.3. Geochemistry The major and trace element data of the metavolcanic rocks from the Changyukou area are listed in Error! Reference source not found.. The rocks have suffered multi-phase deformation and high grade regional metamorphism. It is therefore important to assess the mobility of the 12

ACCEPTED MANUSCRIPT elements. The less mobile element Zr is a sensitive indicator for the mobility of the elements (e.g. (Polat and Hofmann, 2003)). As with Zr, MgO is also an effective indicator due to the good

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correlation between MgO and Zr. It is reasonable to choose the elements that have good

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correlations with MgO. To avoid the effects of the alteration and regional metamorphism on element mobility, most of the elements we used have good correlations with MgO (fig.8 and 9). Although Sr is scattered to different extents for the calc-alkaline series (fig.8d), in the diagram

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Sr/Y-Y, they perform as well as the (La/Yb)N-Yb (fig.15d). Moreover, relatively uniform patterns

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of the spidergrams also imply the availability of the element of Sr (fig. 11). On the diagram of SiO2 vs. Nb/Y, all the basalts plot in the field of sub-alkaline basalt (Fig.

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10a). On the AFM diagram (Fig. 10b), one group plots in the tholeiitic basalt zone while the

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high-Mg basalts plot in the transition zone between tholeiitic basalts and calc-alkaline basalts. In

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fact, the constant FeOT contents (9.19-9.69 wt.%) imply they have no features of the tholeiite series and the constant alkali elements (K2O+Na2O: 3.00-3.92 wt.%) imply they cannot be

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classified in the calc-alkaline series. The felsic volcanic rocks plot in the calc-alkaline series zone (Fig. 10b). Based on the SiO2 vs. Nb/Y diagram, the calc-alkaline series can be further classified as a basaltic andesite (high-Al basalt)-andesite group and a dacite-rhyolite group. Thus, the metavolcanic rocks can be subdivided into four groups geochemically: high-Mg basalts, tholeiitic basalts, basaltic andesites (high-Al basalts) and andesites, as well as dacites and rhyolites. High-Mg basalts are characterized by low Fe2O3T (10.22-10.77 wt.%, 10.58 wt.% on average) and TiO2 (0.43-0.67 wt.%) and high MgO (8.27-16.66 wt.%) contents, whereas the tholeiitic basalts have higher Fe2O3T (14.88-17.45 wt.%) and TiO2 (1.47-1.96 wt.%) and lower MgO 13

ACCEPTED MANUSCRIPT (4.46-6.08 wt.%) contents (Fig. 8). Compared with the two groups of metabasalts, the basaltic andesites-andesites show higher Al2O3 (15.13-19.98 wt.%), lower Fe2O3T (8.20-12.29 wt.%, 10.36

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wt.% on average), moderate TiO2 (1.05-1.24 wt.%) and much lower MgO (2.23-4.31 wt.%)

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contents (Fig. 9).

With respect to trace elements, the high-Mg basalts are characterized by moderate LREE enrichment (LaN/YbN=5.61-11.21) and small negative Eu anomalies (Eu/Eu*=0.65-0.91) (Fig.

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11a). In the primitive mantle (PM)-normalized trace element diagrams (Fig. 11b), they are

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typified by enrichment in large ion lithophile elements (LILE, such as Sr, Ba, and Rb) relative to LREE and HFSE (such as Nb, Ta, P, and Ti). By contrast, the tholeiitic basalts have slight LREE

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enrichment (LaN/YbN=1.50-3.23) and weak negative Eu anomalies (Eu/Eu*=0.89-0.95) (Fig. 11c).

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They have slightly enriched LILE with negative Sr anomalies (Fig. 11d). Both of these basalts

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have typical negative Nb, Ta, and Ti anomalies. In most selected oxides and trace elements versus MgO diagrams, the calc-alkaline

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volcanics (basaltic andesites-andesites and dacite-rhyolites) constitute linear trends. From the basaltic andesite (high-Al basalts) endmember to the rhyolite end-member, the contents of FeOT, Al2O3, P2O5, MnO, Sr, Ni, and V decrease concomitantly with decreasing MgO, while the contents of alkali elements increase (Fig. 9). The basaltic andesites-andesites have lower K2O/Na2O ratios (0.3-0.92) and higher CaO/Al2O3 ratios (0.21-0.43) than the dacites-rhyolites (K2O/Na2O = 0.97-1.48; CaO/Al2O3= 0.10-0.18). With respect to trace elements, the basaltic andesites-andesites are characterized by moderate LREE enrichment (LaN/YbN=11.28-29.14) (Fig. 11e). They show enriched LILE and LREE and strong depletion in HFSE (such as Nb, Ta, and Ti) (Fig. 11f). The dacites-rhyolites have 14

ACCEPTED MANUSCRIPT slightly more enriched LREE and depleted HREE than the former (LaN/YbN=12.92-41.98; Fig. 11h). They have similar Nb, Ta, and Ti depletions and weak Sr depletion. The basaltic andesite (high-Al

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basalt) and andesite endmember have Eu/Eu* ratios of 0.85-1.18, with an average of 1.00. The

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dacite and rhyolite endmembers have slightly wider and lower Eu/Eu* ratios of 0.62-1.18, with a mean value of 0.95.

6.1 Petrogenesis of high-Mg basalt

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6. Discussion

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High-Mg basalts are characterized by low SiO2 (46.65-51.77wt.%), high MgO (8.27-16.66

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wt.%), high Mg-numbers (= 100Mg/(Mg+Fe2+) = 64.3-78.6), and high Cr (488-1638ppm) and Ni

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(127- 518ppm) (Fig. 8). Chondrite-normalized REE diagrams for the Changyukou metavolcanic rocks show typical subduction zone affinities, that is, depletions of Nb, Zr, and HREE and

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enrichments of Sr, K, Pb, and LREE (Fig. 11a, b, c, and d). The flat and high HREE patterns suggest a garnet-free source, which reflects shallower

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magma genesis. Two samples (10CYK01 and 10LF12) can be recognized as primary magmas that were not subsequently modified, on the basis of high Mg# (>66) and high Cr (>1000 ppm), and Ni (>400 ppm) concentrations (Winter, 2014). Would the other samples be the derivative magmas formed by the process of fractional crystallization? 6.1.1 Role of fractional crystallization for the genesis of high-Mg basalts If we assume that fractional crystallization did occur and treat the most primitive high-Mg basalt (10CYK01) as the initial sample, on the Ni vs. Cr diagram, the rapidly falling Ni concentrations indicate olivine involvement in the process of fractional crystallization, while the decreasing Cr concentrations indicate clinopyroxene (cpx) involvement (spinel can be neglected 15

ACCEPTED MANUSCRIPT because its partition coefficient for Cr is similar to that for cpx and because of its very low quantities). The constant Fe concentrations indicate no magnetite and ilmenite involvement.

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Their constant K/Rb ratios imply no amphibole involvement because amphibole has partition

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coefficients of approximately 1.3 for K and 0.3 for Rb. The fractional crystallization of amphibole would lower the K/Rb ratio in the evolved magma. The Eu anomaly in the REE diagram implies that plagioclase (pl) is an important phase during ascent to the surface. Because we have

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recognized several important mineral phases, we could calculate the degree of fractional

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crystallization for these high-Mg basalts in the Ni vs. Cr diagram and the REE diagram. As Fig. 12a illustrates, 2%-9% fractional crystallization of 40% ol, 40% cpx and 20% pl can explain the

D

evolution trends of Ni and Cr, as well as the pattern of REEs. However, such low degrees of

TE

fractional crystallization have no obvious influence on the concentrations of incompatible

CE P

elements. In fact, the primary magma represented by sample 10CYK01 would need up to 65% fractional crystallization of 40% Ol, 40% cpx and 20% pl to reach all the REE concentrations of the

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evolved high-Mg basalts (Fig. 12b, Table a). In addition, contrary to MgO, silica usually varies little in the early stages of magmatic evolution. Thus, 2%-9% fractional crystallization is unable to make SiO2 contents evolve from 46.65% to 51.77%. Continuous fractional crystallization enriches the alkali elements, but their Na2O+K2O contents stay constant. Based on all the above evidence, it can be concluded that the high-Mg basalts are not simply the result of fractional crystallization of primary basalts. 6.1.2 Role of magma mixing for the genesis of high-Mg basalts The strong linear correlations between MgO and most major and trace elements (e.g., TiO2, Al2O3, FeO, MnO, Mg#, K2O+Na2O, Ni, Cr, V, Sr, and Y) and between Ni and Cr indicate that magma 16

ACCEPTED MANUSCRIPT mixing is another possible important process for magmatic differentiation. The lack of strong linear correlations between MgO and some major and trace elements (e.g., P2O5 and Rb) does

IP

T

not necessarily rule out mixing, as the scatter may be attributed to variable endmember

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compositions coupled with fractional crystallization subsequent to mixing. More important, binary mixing can reasonably explain the wide range of REE concentrations of the high-Mg basalts (Fig. 8). In spite of the lack of petrographic evidence to support the process of magma

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mixing, different batches of similar mafic melts could easily mingle to result in homogeneous

MA

mixtures. The petrographic evidence for such mixtures, however, is usually obscured. The two endmembers represented by samples 10CYK01 and 10LF37 obviously did not result from various

D

extents of partial melting of one particular source because such a process would generate

TE

different slopes in the REE patterns. In contrast, the most probable genesis is that they derived

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from separate sources of the heterogeneous mantle. In fact, their different degrees of depletion or enrichment of HREE and various La/Yb ratios may point to source depths from the garnet

AC

stability field to the spinel stability field. Although incorporation of crustal material during the ascent process of high-Mg basalts may be able to generate higher concentrations of REE and SiO2, we think the likelihood is low due to the constant FeO and alkali metals (N2O+K2O) and the negative anomalies of Zr and Hf, as well as their higher MgO contents and higher Cr and Ni concentrations. In other words, crustal assimilation is inevitable for any basalt, but for these high-Mg basalt samples, the influence can be neglected so that they can still be used to reveal the mantle source environment. 6.1.3 Tectonic setting of high-Mg basalts The depletions of Nb, Ta, and Ti and enrichments of LILEs are characteristic of arc signatures. 17

ACCEPTED MANUSCRIPT On the Th/Yb vs. Nb/Yb plot, most of the high-Mg basalts have higher Th/Yb ratios than MORB (Fig. 13b), indicating that they were affected by fluid addition since Th is mobile in a fluid system.

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Hydrothermally altered MORBs and sediments in the subducting lithosphere are widely accepted

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as the primary source of subduction zone fluid phases (Tatsumi and Kogiso, 2003). For the high-Mg basalts in Changyukou, the enrichments in the LILEs (e.g., K, Rb, Ba, Pb, and Sr) and depletions in the HFSEs (e.g., Nb, Ta, Zr, and Ti) are broadly consistent with selective transport of

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elements by aqueous fluids from both subducting sediments and oceanic crust.

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All the volcanic rocks have Zr/Yb ratios within the enriched MORB range (Fig. 13a). These geochemical characteristics, together with their high LREE/HREE ratios (LaN/YbN = 5.6 – 11.2),

D

represent mantle sources that were variably enriched relative to average MORB by both

TE

slab-derived aqueous fluids and partial melts or supercritical fluids because fluids can bring

CE P

mobile elements and melts may enrich the mantle with REEs (Bau and Knittel, 1993). The supercritical fluids play a major role at depths greater than 100 km (Mibe et al., 2011).

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The Ti/V ratios (18.2-25.7) for the high-Mg basalts in Changyukou, however, are moderately lower than those of average MORB (25.4). According to Macdonald et al. (2000), three interpretations exist: (1) The mantle sources are slightly depleted in Ti relative to V compared with those of N-MORB. (2) The Changyukou basalts represent higher melt fractions than N-MORB. (3) Small differences in partition coefficients for early crystallized phases such as spinel and clinopyroxene have caused small decreases in Ti/V during high-pressure fractionation of the parental basalts. Given the evidence from Zr/Nb ratios and the Zr/Yb vs. Nb/Yb plot (Fig. 13), it appears the most likely explanation is that these primary magmas in Changyukou represent higher degree melts than MORB because Ti, an incompatible element, is increasingly depleted in 18

ACCEPTED MANUSCRIPT the melt as the degree of partial melting increases (Pearce, 2014). If carbonates are the dominant subduction components (indeed, carbonates are often the

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major marine sediments), the efficiency of metasomatizing the depleted mantle with LILEs and

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LREEs will be more enhanced.

6.2.1 Petrogenesis of tholeiitic basalts

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6.2 Petrogenesis of tholeiitic basalts

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The tholeiitic basalts are characterized by uniform SiO2 (49.62-49.85 wt.%), moderate MgO (4.46-6.08 wt.%, Mg-number=37.3-48.8), and high FeO (13.39-15.70 wt.%) and TiO2 (1.47-1.96

TE

D

wt.%). They display weakly LREE-enriched patterns (LaN/YbN=1.5-3.2, GdN/YbN=1.16-1.25). On the Zr/Yb vs. Nb/Yb diagram, most of the samples plot in the enriched mantle field, implying their

CE P

sources are slightly enriched compared with N-MORB (Fig. 13). The increasing Th/Yb trend implies the involvement of hydrous fluids, which is impossible in the MOR setting. In addition, the

AC

obvious negative anomalies of Nb and Ta and the enrichments of Rb, Ba, and Pb suggest that the magmatic precursors of these rocks were hybridized by minor liquids and sediments derived from the subducted slab. Though all the samples indicate mid-ocean ridge tectonic affinity in the Ti-Sm-V discrimination diagram designed by Vermeesch (2006), the original subarc mantle has been generally accepted as identical to the MORB source mantle because of the uniform chemistry of MORBs and the similarity in high field strength element ratios between MORBs and arc lavas (Tatsumi, 2001). The hybrid mixture of MORB-like and arc-like element signatures reflects repeated melting of a MORB mantle source in a subarc-forearc wedge that has been modified by fluids released from a subducting oceanic slab and sediment-derived melts (Dilek 19

ACCEPTED MANUSCRIPT and Polat, 2008). The flat HREE patterns of tholeiitic basalts indicate they may originate from the spinel stability field, shallower than the source of the high-Mg basalts. Their diverse slopes of REE

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patterns imply that different degrees of melting occurred in their source. In fact, the higher Ti/V

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ratios (24.15-29.01) than the average MORB (25.4) imply a slightly lower degree of mantle melting than for MORB. This process, as well as the contributions from enriched slab-derived fluids and sediments, can result in the slightly enriched geochemical character of the tholeiitic

MA

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basalts.

D

6.2.2 Comparison between high-Mg basalts and tholeiitic basalts

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We can infer that geochemical variation in high-Mg basalts is dominated by magma mixing

CE P

and different source characteristics at different source depths, with little or no influence of polybaric fractional crystallization. Their geochemical features can be ascribed to the high-degree

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partial melting of an enriched mantle source that was previously enriched in LILE and LREE by slab-derived hydrous fluids prior to magma generation. For the tholeiitic basalts, however, melting occurred at relatively low pressures within the stability field of spinel. Their geochemical variation can be ascribed to different degrees of mantle melting and different degrees of metasomatism from enriched slab fluids. This conclusion can also be supported by quantitative calculation. In the Nb vs. Y diagram (Fig. 14) for distinguishing source enrichment/depletion and variations in degrees of partial melting proposed by Portnyagin et al. (2007), the mantle sources of high-Mg basalts are generally more enriched than primitive mantle, and the degree of melting ranges from 10% to 40% in the spinel facies. By contrast, the mantle sources of tholeiitic basalts 20

ACCEPTED MANUSCRIPT are more enriched than enriched MORB mantle (EMM) but slightly lower than the primitive mantle. The degree of melting is usually below 10% in the spinel facies.

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The shallower and slightly enriched tholeiitic basalts, together with the deeper and

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moderately enriched high-Mg basalts, indicate the complex magmatic evolution, which includes melt aggregation, mixing, and differentiation processes that occur in different pressure ranges at

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multiple levels within the subarc mantle wedge (e.g., Dilek et al. (2008); Dilek and Thy (2009)).

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6.3 Calc-alkaline rocks

Calc-alkaline rocks (including high-Al basalts, basaltic andesites, andesites, dacites, and

TE

D

rhyolites) are typically widespread in subduction zones. They usually have multiple origins and multiple components. For example, they could be derived from the tholeiitic, alkalic, or

CE P

high-alumina series by fractionation under high oxygen pressure (Kuno, 1966; Sheth et al., 2002). Crustal assimilation and magma mixing of crustal and deeper subduction zone melts may also be

AC

effective ways to generate calc-alkaline rocks, e.g., Roman et al. (2006) and Myers et al. (1985). For the Changyukou calc-alkaline rocks, despite the linear trends on variation diagrams (Fig. 9) that indicate magma mixing is an essential process, we still need to understand the origin of the two endmembers, namely, basaltic andesites and andesites, as well as dacites and rhyolites.

6.3.1 Basaltic andesites and andesites 6.3.1.1 Possible genesis of basaltic andesites and andesites Geochemical data reveal that basaltic andesites and andesites have moderate SiO2 contents of 50.58 to 63.02 wt.% with an average of 54.70 wt.%, lower MgO of 1.46 to 4.31 wt.% with an 21

ACCEPTED MANUSCRIPT average of 2.88 wt.%, and higher Al2O3 of 16.06 to 19.98 wt.% with an average of 17.2 wt.%. In fact, based on their geochemical characteristics, most of them can be classified as high-Al basalts

IP

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as defined by Crawford et al. (1987) (SiO2 < 54 wt.%, Al2O3 > 16.5 wt.%). High-Al basalts have

SC R

been proposed as derived from partial melting of subducted oceanic crust (Marsh, 1982) or as differentiation products of Mg-rich mantle-derived basalts (Crawford et al., 1987; Gust and Perfit, 1987). Some high-Mg (> 8 wt.% MgO) high-Al basalts may be primary melts of peridotite (Winter,

NU

2014), but due to their low MgO and low Cr and Ni concentrations, samples from Changyukou are

MA

too evolved to be primary. In addition, their lower Sr/Y and La/Yb (Fig. 15) ratios imply they are less likely to have originated from slab melting since garnets are stable in the source at the

D

pressure and temperature of melting. Their relatively steeper slopes of REE patterns (LaN/YbN =

TE

11.28-29.14, Fig. 11e and f) than high-Mg basalts and tholeiitic basalts (LaN/YbN = 1.5-11.21, Fig.

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11a, b, c and d) indicate that they cannot be evolved from basalts via closed-system fractionation because the fractionating phases such as olivines, pyroxenes, and feldspars do not selectively

AC

incorporate LREEs or HREEs. Even hornblendes preferentially incorporate MREEs over LREEs and HREEs to form a characteristic listric REE profile but with no influence on their slopes, e.g., La/Yb ratios.

It should be noted that both the high-Mg basalts and the tholeiitic basalts have negative anomalies of Zr and Hf (Fig. 11a and c), while the lower and upper crust lack these features. However, for the basaltic andesites and andesites, Zr and Hf show either negative anomalies or positive anomalies (Fig. 11f and h). Thus, it can be deduced that assimilation by mantle-derived melts of crustal material or mixing processes between mafic and felsic magmas play a major role in the evolution of the basaltic andesites and andesites. 22

ACCEPTED MANUSCRIPT The simple mixing process, however, is less likely since these rocks do not show the characteristic linear trends in the Harker diagrams or other bivariate diagrams (diagrams not

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shown). Finally, it should be noted that unlike mafic melts discussed above, perfect mixing of

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mafic and felsic melts might be improbable in nature. Assimilation of crustal material is an energy-consuming process and would likely cause massive crystallization (DePaolo, 1981; Portnyagin et al., 2013).

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6.3.1.2 Composition of Archean lower crust in the North China Craton

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Simultaneous assimilation and fractional crystallization (AFC, DePaolo (1981)) is an extremely useful quantitative model for testing whether various trace element concentrations

D

(and isotopic ratios) in two magmas are related, solving the relationship between the assimilation

TE

rate and the fractional crystallization rate, and finding the fraction of magmatic liquid remaining

CE P

and the bulk partition coefficient for any trace element, e.g., Hunter (1998). For geochemical modeling, we must determine the chemical composition of the Archean lower crust of the North

AC

China Craton. However, knowledge of the lower crustal composition is limited by the ambiguity in deriving chemical compositions from seismic velocities, the lack of high-quality data for a number of trace elements and the still-fragmentary knowledge of the seismic structure of the continental crust (Rudnick and Gao, 2014). The nature of the lower crust is usually examined by evidence from granulites and xenoliths of lower crustal origin, which has revealed a great diversity in lithologies within Archean cratons. Weaver and Tarney (1984) determined the Archean lower crustal composition from the average of Archean Scourian granulites in the Lewisian complex, Scotland. However, their estimates are too high, with SiO2 contents of 68.1 wt.%, and may represent the evolved lower crust in Archean cratons (Rudnick and Gao, 2014). 23

ACCEPTED MANUSCRIPT The Wutai–Jining Zone is suggested to be an exposed cross-section through the Archean North

China

Craton

and

has

lower

crust

lithologies

composed

of

54%

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tonalitic-trondhjemitic-granodioritic-granitic gneiss, 32% mafic granulite, 6% metapelite and 8%

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metasandstone (Kern et al., 1996; Rudnick and Gao, 2014). For the TTG endmember, we compile 2.5-Ga TTGs in Huai’an, Lushan, Wutai-Fuping and Eastern Hebei from Liu et al. (2012), Zhou et al. (2014), Liu et al. (2004) and Bai et al. (2014), respectively, and screen out 108 set of data based

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on the definition of TTGs (SiO2 > 56 wt.%, Na2O > 3.0 wt.%, K2O/Na2O < 0.5) by Moyen and

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Martin (2012) to obtain the average TTG compositions of the Archean North China Craton (table4). Their average major and trace elements are similar to the average global components of

D

1439 TTGs (s.l.) given by Moyen and Martin (2012). For the mafic granulite endmember, we use

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the average components from two subgroups of mafic granulites compiled by Zhai et al. (2001).

CE P

For the metasedimentary (metapelite and metasandstone) endmember, since Taylor and McLennan (1995) deduced the average compositions of Archean Upper Crust (AUC) based mainly

AC

on the Archean sedimentary record, it is reasonable to substitute the AUC for the metasedimentary endmember. Thus, we can deduce the composition of the Archean lower crust of the North China Craton based on the proportions of the three endmembers proposed by Kern et al. (1996). 6.3.1.3 Combined assimilation and fractional crystallization for the genesis of basaltic andesites (high-Al basalts) and andesites We choose high-Mg basalts as parental magmas rather than tholeiitic basalts to interact with lower crust via underplating or intraplating processes based on the following reasons: first, we have deduced that tholeiitic basalts in Changyukou may represent the products of an earlier 24

ACCEPTED MANUSCRIPT stage of the subduction process composed of proto-arc basalts or a later stage of the subduction process with rapid slab rollback leading to extension and seafloor spreading in the upper plate,

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which means they had little chance to assimilate lower crust; second, the basaltic andesites and

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andesites have much higher Sr contents (with an average of 885.20 ppm) than tholeiitic basalts (with an average of 210.83 ppm) and Archean lower crust of the NCC (with an average of 301.36 ppm), which means the tholeiitic basalts cannot attain such high concentrations via assimilation

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or these concentrations could form only at unrealistically high degrees of fractional crystallization

MA

(>85%) since Sr and Rb are highly incompatible elements.

Due to the influence of widespread Archean metamorphism and deformation, we cannot

D

determine the crystallization behavior of high-Mg basalts by means of phenocryst mineralogy

TE

and composition (e.g., Hunter and Blake (1995)); nevertheless, fractional crystallization

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experiments at high fO2 and 10 kbar of picrobasalt (SiO2 = 47.56 wt.%, MgO = 17.09 wt.%), representative of hydrous primary arc magmas, revealed a crystallization mineral assemblage of

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ol + cpx + spl + opx + hbl + ilm when cooling (Müntener and Ulmer, 2006). Phase equilibrium experiments of primitive basaltic andesite (SiO2 = 51.68 wt.%, MgO = 10.79 wt.%) at 8 kbar under H2O-saturated conditions revealed a crystallization sequence of ol + opx + cpx + amp (Grove et al., 2003). In some cases (e.g., thickened crust), garnet occurs as a crystallized phase and dramatically influences compositions at the base of the crust, e.g., Chung et al. (2003). We thus test this crystallization assemblage: ol + cpx + opx + amp ± Gt. We then use the quantitative model proposed by DePaolo (1981) to simulate the assimilation and fractional crystallization process during the ascent of magma to the base or the interior of the Archean lower crust. Important parameters are the ratio (r) of assimilated material 25

ACCEPTED MANUSCRIPT to crystallized material, the fraction (F) of magmatic liquid remaining, and the bulk partition coefficient (D) for any trace element. From this definition, models with r < 0.5 are dominated by

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fractional crystallization while models with r > 0.5 are dominated by assimilation. To accurately

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determine the value of r is nearly impossible. Given that the quantity of the acid volcanics (60%) is greater than that of mafic and intermediate volcanics (40%), r > 0.5 is an educated guess. We thus take 0.6 as an estimated value of r.

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Modeling results for the REEs favor the involvement of garnet (ol:opx:cpx:amp:gt =

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20:20:25:20:15) in the AFC process to generate the slightly depleted HREEs than that of the high-Mg basalts (Fig. 16a, Table b). The existence of minor garnets in the crystallized mineral

D

residue, as well as the adakitic character shown by some samples, may be the result of slightly

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thickened lower crust. Modeling results for other trace elements (Fig. 16b, c, and d; Table c)

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further confirm the effectiveness of combined assimilation and fractional crystallization between high-Mg basalts and Archean lower crust to evolve the high-Al basalts, basaltic andesites or even

AC

andesites when the ratio of assimilated material to crystallized material is approximately 0.6 and the fraction of magmatic liquid remaining is 75% to 93% (namely, the degree of fractional crystallization is 7% to 25%). In brief, the generation of high-Al basalts, basaltic andesites and andesites can be attributed to the assimilation by high-Mg basalts of relatively high-Al2O3 lower crust and the subsequent crystallization of prevailing mafic mineral phases, while Al2O3-rich plagioclase crystallization is suppressed under high-pressure and nearly water-saturated conditions. It should be noted that the degree of AFC progress is not exact because it is the modeling result when average high-Mg basalts are taken as the initial material and the putative average 26

ACCEPTED MANUSCRIPT Archean lower crust of the NCC as the assimilated material; therefore, the ratio may change to some degree due to the heterogeneity of high-Mg basalts noted above and the complexity of

6.3.2 Petrogenesis of dacites and rhyolites

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AFC progress, which may be attributed to these reasons.

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Archean lower crustal composition. In fact, not all the data plot strictly on the evolution line of

Like basaltic andesites and andesites, this endmember has large variations of εHf(t) (sample

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10LF63 with εHf(t) from 1.2 to 4.1; sample 10LF64 with εHf(t) from 2.3 to 4.4), which may indicate

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they were derivative liquids that have undergone at least some degree of fractional crystallization from basaltic andesites and andesites. In fact, the dominant process of differentiation in the mid-

D

to upper crust is crystal-melt fractionation; though assimilation can occur, this process is

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insignificant because silicic melts are cooler and upper crustal temperatures are lower, making

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significant crustal assimilation more difficult (Ducea et al., 2015). The water content in the magma increases due to crystallization (Kay and Kay, 1985). Higher

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oxidation states of arc lavas are also a consequence of shallow-level differentiation processes (Lee et al., 2010). Elevated parental magma water contents and oxygen fugacity can drive the differentiating magma towards the calc-alkaline trend by suppressing plagioclase fractionation and bringing oxide and amphibole fractionation closer to the liquidus (Cai et al., 2015). Amphibole is commonly observed in arc andesites, dacites, and rhyolites since it requires certain conditions to saturate: generally >3 wt.% Na2O, >3 wt.% H2O in the melt and <1100℃ magmatic temperatures (Grove et al., 2003). Amphibole removes Fe from these silicic liquids, driving further Fe depletion and producing calc-alkaline trends (Zimmer et al., 2010). Amphibole fractionation should also yield increasing LREE/MREE but steady or decreasing MREE/HREE ratios 27

ACCEPTED MANUSCRIPT ≈ or <1 (Richards and Kerrich, 2007), as is observed in our samples (Fig. 11g and h). In natural systems, amphibole fractionation is seldom isolated and is typically accompanied by plagioclase

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removal (Moyen, 2009), as is reflected by the decreasing contents of Sr, Eu, and Na in our

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samples.

Quartz becomes stable only after large degrees of crystallization, with temperatures < 840℃

considered as one of the fractionating phases.

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and when melt SiO2 contents exceed 72 wt.% (Macdonald et al., 2000); therefore, it cannot be

MA

Thus, it can be deduced that plagioclase and amphibole should be the crystallizing phases. Modeling results further confirm that the basaltic andesites-andesites end-member can evolve to

D

the dacites-rhyolites endmember by fractional crystallization of 80% pl +20% amp (Fig. 17a, b, c,

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and d; Table d). The general absence of pronounced Eu anomalies is interpreted as a reflection of

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high oxygen fugacities during differentiation involving significant fractionation of plagioclase (Gertisser and Keller, 2000). The obvious decrease of V, Ti, and P contents may indicate the

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involvement of magnet, ilmenite, and apatite during the intra-crustal fractional process. 6.3.3 Magma mixing process Magma mixing is a common process in arc magmatism and is important to the development of the calc-alkaline trend. The occurrence of magma mixing can be ascertained by examining macroscopic to microscopic textural relationships in rock samples, as well as by examining the zonation patterns and compositional variations in phenocrysts (Hunter, 1998). Such measures, however, are not suitable to Archean metamorphic volcanic rocks because their original mineralogical assemblages have been transformed. Nevertheless, the somewhat linear trends on variation diagrams still suggest the role of mixing of two endmember components in the whole 28

ACCEPTED MANUSCRIPT calc-alkaline series (Fig. 9).

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7. Tectonic implications

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Geochemical characteristics of volcanic rocks provide information concerning their tectonic setting and geodynamic evolution. Three types of metabasalts were identified in this area,

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namely, tholeiitic basalts, high-Mg basalts, and high-Al basalts. All of them have marked negative Nb, Ta, and Ti anomalies, LILE enrichment, and enrichment of the LREEs relative to the HREEs.

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Such geochemical features, especially those of high-Mg basalts that show typical features of primary magma, are usually seen as arc-related mafic rocks. The clear signature of enrichment in

TE

D

fluid-mobilized elements of the primary basalts may imply an enriched mantle source derived from the slab via hydrous fluids and/or melts or supercritical fluids.

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The abundant calc-alkaline felsic rocks are generally the dominant series in convergent boundaries due to the crucial role of water in the genesis of the calc-alkaline series. Primary

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magmas are ponded at the base of the crust where they melt the less refractory lower crustal rocks and undergo extensive assimilation and fractional crystallization (AFC processes) to generate high-Al basalts or basaltic andesites and andesites. The subsequent shallow intracrustal differentiation further promotes the generation of dacites and rhyolites. Mixing of magmas at various stages along the fractionation course of high-Al basalt toward rhyolite may contribute to the calc-alkaline trends as well. Modeling results show that minor garnets (ca. 15 wt.%) may be present during the melting of the lower crust. Garnet is a common metamorphic mineral in crustal rocks and increases in abundance with depth from mid-crustal levels in amphibolites to eclogite facies rocks in the lower crust (Richards and Kerrich, 2007). Garnets may form in basaltic 29

ACCEPTED MANUSCRIPT bulk compositions by the breakdown of amphibole + plagioclase under fluid-absent conditions by the following reaction: amphibole + plagioclase ± quartz = garnet + clinopyroxene + melt + new

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plagioclase at 12 and 18 kbar (Haschke et al., 2002), which implies that the crust above the

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Changyukou subduction zone must have been thickened to at least 40 km, a typical crustal thickness in a mature continental arc setting. This inference can also be supported by the voluminous felsic calc-alkaline volcanics in the Changyukou area and the widespread

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TTG-like/adakitic intrusions in Northwest Hebei. The continuously thickening crust produced

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coeval TTGs with high positive εNd(t) generated by partial melting of newly formed lower crust that are common in this area (Liu et al., 2012; Liu et al., 2009; Yang et al., 2008; Zhang et al.,

D

2012). However, it is still an open question as to which was the dominant factor in this crustal

TE

thickening event – lateral crustal shortening caused by subduction or vertical magmatic addition

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caused by magma underplating.

The Changyukou metavolcanic rocks in the Huai’an Complex developed by subduction at a

AC

thickened continental margin. In addition, tholeiitic basalts in Changyukou may represent the production of an earlier stage of the subduction process composed of proto-arc basalts (Guo et al., 2013) or a later stage of the subduction process with rapid slab rollback leading to extension and seafloor spreading in the upper plate (Dilek and Furnes, 2011). At the same time, geochemical

compositions

reveal

that

both

the

leucocratic

syenogranites

and

biotite–monzogranites from the Huai’an Complex were derived from partial melting of thickened juvenile lower crust (Zhang et al., 2011), similar to the tectonic setting of the Changyukou calc-alkaline volcanics. Not only in Northwest Hebei but also in Eastern Hebei, 2550- to 2530-Ma tonalites and quartz diorites and 2530- to 2510-Ma granites formed in a complex arc with local 30

ACCEPTED MANUSCRIPT incorporation of older continental crust (Nutman et al., 2011). For the 2.55 to 2.5Ga TTGs in the Huai’an Complex, different models exist. According to

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Zhang et al. (2012), low-Al trondhjemites were formed by intraplating of mantle-derived magmas,

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and the coeval high-Al TTGs were produced by partial melting of a thickened lower crust triggered by underplating. Apparently, both the leucosyenogranite (2493 ± 6 Ma) and the biotite–monzogranite (2437 ± 10 Ma) in the Huai'an Complex and the granites (2530-2510 Ma) in

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Eastern Hebei slightly postdate the era of TTGs (2550-2500 Ma) in this area, implying their

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post-and/or syn-tectonic emplacement (Nutman et al., 2011; Zhang et al., 2011). By modeling calculations, Zhang et al. (2011) determined that the compositions of the 2493-Ma

D

leucosyenogranites reflect their derivation from partial melting of overthickened juvenile lower

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crust of eclogitic composition while the 2437-Ma biotite–monzogranites formed from a thickened

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juvenile crust of garnet amphibolite composition. Zhang et al. (2011) linked the 2500-Ma and 2440-Ma granitic magmatism to amalgamation of microblocks and concomitant overthickening of

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the lower crust of the NCC, with subsequent crustal extension that triggered underplating to induce newly accreted lower crust and reworking to generate the charnockites and the 2.44-Ga granitic magmas.

In addition, high-pressure basic granulites are widely distributed as enclaves and sheet-like blocks in the Huai’an TTG gneiss terrane. According to Guo et al. (2002), the clockwise hybrid ITD and IBC P–T paths of the HP granulites in this area imply a model of thickening followed by extension in a collisional environment. Furthermore, the relatively high pressures (6–14.5 kbar) of the four metamorphic stages and the geometry of the P–T paths suggest that the HP granulites, together with their host Huaian TTG gneisses, represent the lower plate in a crust thickened 31

ACCEPTED MANUSCRIPT during collision. To reconcile the widespread 2.55 to 2.5Ga TTGs derived from overthickened crust, the 2.51-

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to 2.50-Ga calc-alkaline volcanic rocks derived from thickened crust, the tholeiitic basalts that

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represent low pressure and an extensional tectonic setting, the 2493Ma leucosyenogranites derived from overthickened crust, the 2437Ma biotite-monzogranites derived from slightly thinner crust than the leucosyenogranites but still thickened, as well as the clockwise hybrid ITD

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and IBC P–T paths of the HP granulites and the widespread extension and rifting setting within

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the NCC from 2300 to 1950 Ma, we propose a model of an evolving subduction process. First, continuous subduction and collision caused the continental crust above the arc to

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continuously thicken (Fig. 18a). Then, as the subducted plate was becoming older and denser, the

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subducting plate began to roll back, and the trench migrated away from the continent,

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accompanied by voluminous TTG magmas and charnockites due to the extension of the thickened continental arc and subsequent mantle upwelling (Fig. 18b). Next, with the

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development of plate rollback, the overthickened continent began to thin. The potassic granites and calc-alkaline volcanics were widespread in this stage. Extension and seafloor spreading above the subducted plate led to the occurrence of tholeiitic basalts (Fig. 18c, e). Finally, continuous crustal thinning caused by plate rollback and magmatic underplating created a post-2.4-Ga extensional regime in Northwest Hebei (Fig. 18d). Under extension, the upper plate developed basins that could occur anywhere from the backarc to the forearc region, with crustal thickness remaining relatively small (Ducea et al., 2015).

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ACCEPTED MANUSCRIPT 8. Conclusions

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1. We report the results of major and trace element analysis as well as comprehensive LA-ICP-MS

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zircon U–Pb dating and Hf analyses. The felsic volcanic rocks record crystallization ages of

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2504-2509 Ma. Based on their geochemical characteristics, the Archean Changyukou volcanic rocks can be classified into three groups: high-Mg basalts, tholeiitic basalts, and the calc-alkaline

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series (basaltic andesites-andesites and dacites-rhyolites).

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2. The petrogenesis of high-Mg basalts can be ascribed to the high-degree partial melting of an enriched mantle source and the subsequent magma mixing process of different sources at

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different source depths. For the tholeiitic basalts, however, melting occurred at relatively low pressures within the stability field of spinel. Their geochemical variation can be ascribed to

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different degrees of mantle melting and different degrees of metasomatism from enriched slab fluids. the generation of Changyukou high-Mg basalts and tholeiitic basalts indicate a complex

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magmatic evolution, which includes melt aggregation, mixing, and differentiation processes that occur in different pressure ranges at multiple levels within the subarc mantle wedge.

3. The generation of basaltic andesites and andesites can be attributed to the assimilation by high-Mg basalts (primary basalts) of overthickened lower crust and the subsequent crystallization of prevailing mafic mineral phases, while Al2O3-rich plagioclase crystallization was suppressed under high-pressure and nearly water-saturated conditions. Dacites and rhyolites may be the result of further fractional crystallization of basaltic andesites and andesites. Mixing of magmas at various stages along the fractionation course of basaltic andesites (high-Al basalts) toward 33

ACCEPTED MANUSCRIPT rhyolites promotes the trends of the calc-alkaline series.

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4. We obtain the compositions of the Archean lower crust of the North China Craton by compiling

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the average TTG components, average mafic granulite components and average sedimentary rock components based on the Archean Wutai-Jining section, which is composed of 54% tonalitic-trondhjemitic-granodioritic-granitic gneiss, 32% mafic granulite, 6% metapelite and 8%

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metasandstone.

5. The 2.5-Ga tholeiitic basalts, high-Mg basalts, and high-Al basalts, as well as other abundant

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calc-alkaline volcanic rocks in Changyukou, require a subduction tectonic regime of Archean

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lithospheric plates similar to the Phanerozoic Earth.

6. A model of evolving subduction process was proposed. Among them, the composition of the

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2.5-Ga Changyukou volcanic rocks and potassic granites and the clockwise hybrid ITD and IBC P–T paths of the HP granulites may reveal that the tectonic setting in Northwest Hebei was in a transition stage from a subduction-related compressional regime to an extensional regime related to plate rollback.

Acknowledgments We express our sincere thanks to the anonymous reviewers for the helpful and instructive comments. We thank Prof. Mingguo Zhai and Prof. Huafeng Zhang for their suggestions. This work was financed by National Natural Science Foundation of China (projects 41672190), and 34

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Chinese Academy of Science priority strategic program (No. XDB18030205).

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ACCEPTED MANUSCRIPT Fig. 1 (a) Distribution of ancient (>2.6 Ga) terranes in the North China Craton (Wan et al., 2015). EH:

Eastern Hebei; ES: Eastern Shandong; WS: Western Shandong; EAT: Eastern Ancient Terrane; SAT:

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Southern Ancient Terrane; CAT: Central Ancient Terrane. YB: Yinshan Block; KB: Khondalite Belt; OB;

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Ordos Block; WB: Western Block; TNCO: Trans-North China Orogen; EB: Eastern Block; JLJB: Jiao-Liao-Ji Belt. (b) Simplified geological map of the Huai’an Complex, showing major lithological units.

Modified from Guo et al. (2002) and Liu et al. (2012). (c) Tectonic subdivisions of the North China Craton

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Craton; the red star indicates the study area.

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modified after Zhao and Zhai (2013). EB: Eastern Block; WB: Western Block; TNCO: Trans-North China

Fig. 2 Geological sketch map of the Changyukou metavolcanic rocks and the sample locations. Modified

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from the Chicheng (1:200000) and Changyukou (1:50000) geological maps and Wang et al. (2012).

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Fig. 3 Field photographs of the Changyukou metavolcanic rocks. (a) Thick interbedded metabasalts,

andesites, and dacites; (b) and (c) Thick layered metadacites; (d-f) Thin interbedded metabasalts,

andesites, dacites, and rhyolites; (g) Metabasalts (garnet granulites); (h) Massive metabasalts

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(amphibolites); (i) High-Mg basalts; (j) Metadacites; (k) Metarhyolites; (l) Metadacites-metarhyolites.

Fig. 4 Photomicrographs of the Changyukou metavolcanic rocks. (a) High-Mg basalts; (b) Metabasalts

(amphibolites); (c) Metabasalts (garnet granulites); (d-f) Meta-andesites; (g-i) Metadacites-metarhyolites.

The length of the scale bar is 1 mm.

Fig. 5 Representative cathodoluminescence (CL) images and corresponding

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Pb/

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Pb ages of zircons

from the Changyukou metavolcanic rocks. (a) Tholeiitic basalt; (b) and (c) Basaltic andesite; (d) Andesite;

(e) and (f) Rhyodacite.

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ACCEPTED MANUSCRIPT Fig. 6 Concordia diagrams for zircon U-Pb dating of the Changyukou metavolcanic rocks.

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Fig. 7 ɛ Hf(t) values of zircons from the Changyukou metavolcanic rocks.

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Fig. 8 Diagrams of MgO versus selected major and trace elements for high-Mg basalts (black solid boxes)

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and tholeiitic basalts (black solid triangles). Trends in these figures indicate binary mixing of two

endmembers of high-Mg basalts.

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Fig. 9 Diagrams of MgO versus selected major and trace elements for basaltic andesites-andesites (gray

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solid circles) and dacites-rhyolites (white circles). Trends in these figures indicate binary mixing of

basaltic andesites and rhyolites.

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Fig. 10 (a) SiO2 versus Nb/Y diagram for the Changyukou volcanic rocks, according to Winchester and

Floyd (1977). (b) AFM diagram proposed by Irvine and Baragar (1971) for the Changyukou volcanic

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rocks. A = Na2O+K2O (wt.%); M = MgO (wt.%); F is calculated as (FeO) + (Fe2O3 expressed as FeO) in

wt.%. Black solid boxes: high-Mg basalts; black solid triangles: tholeiitic basalts; gray solid circles:

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basaltic andesites-andesites; white circles: dacites-rhyolites.

Fig. 11 (a), (c), (e), and (g) Chondrite-normalized REE patterns; and (b), (d), (f), and (h) Primitive mantle-normalized trace element spidergrams for the Changyukou volcanic rocks. Normalization values are from Sun and McDonough (1989).

Fig. 12 Modeling results of fractional crystallization (FC) based on the Rayleigh fractionation model

proposed by Rayleigh (1896). While 2%-9% fractional crystallization of 40% ol, 40% cpx and 20% pl can

explain the evolution trend of Ni and Cr (Fig. 12a), up to 65% fractional crystallization is required to cover

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ACCEPTED MANUSCRIPT all the REE concentrations of evolved high-Mg basalts (Fig. 12b). The inconsistent results demonstrate

that fractional crystallization is not a feasible way to evolve the high-Mg basalts. Sample 10CYK01 was

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used as the primary basalt. The shaded area in Fig. 12b represents the REE range of all the high-Mg

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basalts.

Fig. 13 (a) Zr/Yb vs. Nb/Yb diagram proposed by Pearce and Peate (1995); (b) Th/Yb vs. Nb/Yb diagram

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according to Pearce (2014); (c) Sm-Ti-V discrimination diagram proposed by Vermeesch (2006). These

diagrams demonstrate that the mantle sources of high-Mg basalts and tholeiitic basalts have been

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enriched by slab-derived fluids/melts/ supercritical fluids.

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Fig. 14 Diagram for distinguishing source enrichment/depletion and variations in degrees of partial

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melting based on the systematics of Nb and Y concentrations in the Changyukou volcanic rocks as

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proposed by Portnyagin et al. (2007). Different mantle sources of oceanic magmas are depleted MORB

mantle (DMM), enriched MORB mantle (EMM), average MORB mantle (AMM), depleted mantle (DM),

primitive mantle (PM), and primitive mantle plus 5% of average ocean island basalts (PM+5%OIB).

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Curves labeled as “SpP” were calculated for DMM, PM and PM+5%OIB and refer to melting in the spinel facies (ol:opx:cpx:spl=57:28:13:2). Shaded field labeled as “GaP” encloses melt compositions produced

by mixing of melts from an Nb-enriched PM+5%OIB source in the garnet facies (ol:opx:cpx:ga

=53:8:34:5).

Fig. 15 Plots of (a) Sr/Y vs. Y and (b) (La/Yb)N vs. YbN (Defant and Drummond, 1990), showing fields for

adakites and normal calc-alkaline andesite-dacite-rhyolites. Most Changyukou felsic volcanic rocks are

typical calc-alkaline magmas, and a few belong to adakites.

Fig. 16 Reconstruction of the AFC trends (vs. bulk mixing) for the high-Mg basalts, basaltic

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ACCEPTED MANUSCRIPT andesites-andesites, and dacites-rhyolites. The initial point is average high-Mg basalt (Average HM). The

assimilant is represented by the Archean Lower Crust of the NCC (ALC) with trends for r = 0.6. Dashed

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lines with vertical crosses refer to the magma mixing process of Average HM and ALC. In (a), the line

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with vertical cross labeled with 15%Gt refers to an AFC process with a fractionating assemblage of

20%Ol+20%Opx+25%Cpx+20%Amp+15%Gt and the line with diagonal cross labeled with No Gt refers

to an AFC process with a fractionating assemblage of 20%Ol+20%Opx+30%Cpx+30%Amp; in (b), (c)

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and (d), the line with vertical cross refers to an AFC process with a fractionating assemblage of

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20%Ol+20%Opx+25%Cpx+20%Amp+15%Gt. All the AFC lines increase with steps of 5% crystallization.

(a) La/Yb vs. Yb (ppm), (b) Ti/Eu vs. Ni (ppm), (c) K (ppm) vs. Ni (ppm), (d) Ti/Ni vs. Ni (ppm).

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Fig. 17 Selected major and trace element variation and inter-element ratio diagrams to model the

fractional crystallization process. Average basaltic andesites-andesites (Average BA) were assumed as

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the initial material, and 80% plagioclase and 20% amphibole were assumed as fractionating phases in

the evolution process from basaltic andesites to rhyolites. All the FC lines increase with steps of 3%

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crystallization. (a) Ba (ppm) vs. Sr (ppm), (b) Eu (ppm) vs. Sr (ppm), (c) Sr/Y vs. Sr (ppm), (d) La/Yb vs.

Sr (ppm).

Fig. 18 (a) Continuous subduction and collision caused the continental crust above the arc to continuously thicken. (b) As the subducted plate grew colder and denser, the subducting plate began to roll back, and the trench migrated away from the continent, accompanied by voluminous TTG magmas and charnockites due to the extension of thickened continental arc and subsequent mantle upwelling. (c) With the development of plate rollback, the overthickened continent began to thin. The potassic granites and calc-alkaline volcanic rocks were widespread in 44

ACCEPTED MANUSCRIPT this stage. The extension and seafloor spreading above the subducted plate led to the occurrence of tholeiitic basalts. (d) Continuous crustal thinning caused by plate rollback and magmatic

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underplating created a post-2.4-Ga extensional regime in Northwest Hebei. (e) The detailed

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mantle process of fig. c. Table 1

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(a) The average TTG (s.l.) compositions are from Moyen and Martin (2012). “TTG (s.l.)” is used in a broad

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sense to refer to both low- and high-Al2O3 sodic (meta-) plutonic rocks.

(b) Average TTG compositions in the NCC are based on 2.5-Ga TTGs in Huai’an, Lushan,

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Wutai-Fuping and Eastern Hebei from Liu et al. (2012), Zhou et al. (2014), Liu et al. (2004) and Bai

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et al. (2014), respectively.

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(c) Average mafic granulites in the NCC are based on two subgroups of mafic granulites compiled by Zhai et al. (2001).

(d) The metasedimentary (metapelite and metasandstone) compositions are based on the

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average compositions of Archean Upper Crust (AUC) compiled by Taylor and McLennan (1995). (e) The average Archean Lower Crust compositions in the NCC are based on the proportions of the three endmembers (54% tonalitic-trondhjemitic-granodioritic-granitic gneiss of NCC, 32% mafic granulite, 14% metasediments composed of 6% metapelite and 8% metasandstone) in the Wutai–Jining cross-section proposed by Kern et al. (1996).

Table 2 Partition coefficients (D) and modeling results of AFC and FC processes. Data sources for the AFC process: DREE and Ti for Ol, Cpx, Pl, Opx, Amp and Gt from McKENZIE and O'NIONS (1991); DNi, Cr for

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ACCEPTED MANUSCRIPT Ol, Cpx, Opx, Amp and Gt and DK for Gt from Rollinson (1993); DK for Ol from Zanetti et al. (2004); DK for Cpx, Pl, and Opx from Schnetzler and Philpotts (1970); DK for Amp from Dalpé and Baker (1994).

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The AFC modeling process is based on DePaolo (1981). For the FC process of the calc-alkaline series,

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DSr, Y, Ba for Amp and Pl from Rollinson (1993); DLa, Eu, Yb for Pl from Fujimaki et al. (1984); DLa, Eu, Yb for Amp from Klein et al. (1997). The FC modeling process is based on Rayleigh (1896). (a) Modeling results of a fractionating assemblage of 40%Ol+40%Cpx+20%Pl for high-Mg basalts; (b) Modeling of

an

AFC

process

with

a

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results

fractionating

assemblage

of

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20%Ol+20%Opx+25%Cpx+20%Amp+15%Gt; (c) Modeling results of an AFC process with a fractionating assemblage of 20%Ol+20%Opx+30%Cpx+30%Amp; (d) Modeling results of an FC process

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with a fractionating assemblage of 20%Amp+80%Pl for the calc-alkaline series.

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Figure 1

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Figure 10

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Figure 12

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Figure 13

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Figure 14

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Figure 15

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Figure 16

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Figure 17

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Figure 18

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T

61.59 0.49 15.01 6.61

4.70 6.20 3.30 1.80

3.81 5.82 3.22 1.65

50.00 265.00

43.18 494.51

301.36 94.24 12.08 2.47 74.50 156.51 150.48 0.64 3.13 15.19 31.74 14.65 3.22 0.98

1.67

240.00 125.00 18.00 3.00 105.00 180.00 195.00 1.50 5.70 20.00 42.00 20.00 4.00 1.20 3.40 3.40 2.10 2.00

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60.00 0.20 15.30 8.00

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49.60 0.84 14.78 12.01 0.24 7.28 10.47 1.88 0.54 0.17 8.77 253.72 6.32 0.41 168.67 104.89 16.39 1.99 154.11 352.17 310.56 0.25 0.63 5.91 16.00 11.92 4.23 1.26

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69.10 0.36 15.07 3.05 0.06 1.53 2.97 4.00 2.27 0.14 61.81 696.70 5.08 0.59 395.90 79.96 8.00 2.61 19.42 34.48 44.07 0.66 3.95 19.44 38.41 14.88 2.43 0.76 2.04 1.35 0.74 0.77

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69.15 0.36 15.53 2.73 0.05 1.16 3.14 4.84 1.70 0.12 55.55 530.64 5.20 0.68 492.91 139.51 9.18 4.00 36.35 40.36 32.86 1.42 5.72 24.73 47.15 18.16 3.03 0.84 2.33 1.70 0.85 0.71

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SiO2 TiO2 Al2O3 FeOt MnO MgO CaO Na2O K2O P2O5 Rb Ba Nb Ta Sr Zr Y Hf Ni Cr V U Th La Ce Nd Sm Eu Gd Dy Er Yb

Archean Upper average Lower Crust Crustd in NCCe

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average TTG average TTG in average mafic (s.l.)a NCCb granulites in NCCc

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Table 2

Fractionating assemblages of 40%Ol+40%Cpx+20%Pl

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CE P AC

Cr Ni La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu

Ol Cpx Pl 0.7000 34.0000 0.0100 29.0000 14.0000 0.0100 0.0006 0.0540 0.2700 0.0005 0.0980 0.2000 0.0008 0.1500 0.1700 0.0010 0.2100 0.1400 0.0013 0.2600 0.1100 0.0016 0.3100 0.7300 0.0015 0.3000 0.0660 0.0015 0.3100 0.0600 0.0017 0.3300 0.0550 0.0016 0.3100 0.0480 0.0015 0.3000 0.0410 0.0015 0.2900 0.0360 0.0015 0.2800 0.0310 0.0015 0.2800 0.0250

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Partition Coefficient

(b) AFC process Fractionating assemblages of 20%Ol+20%Opx+25%Cpx+20%Amp+15%Gt 66

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(a) FC process

Degree of crystallization 0% Cr(ppm) Ni(ppm) La(ppm) Ce(ppm) Pr(ppm) Nd(ppm) Sm(ppm) Eu(ppm) Gd(ppm) Tb(ppm) Dy(ppm) Ho(ppm) Er(ppm) Tm(ppm) Yb(ppm) Lu(ppm)

3%

6%

9%

65%

1638.00 1106.39 738.15 486.05 0.00 518.00 316.17 190.01 112.32 0.00 14.50

14.91

15.35 15.82 36.34

35.07

36.10 37.19 85.17

4.90

5.04

5.18

21.10

21.68

22.29 22.94 51.00

4.02

4.13

4.24

4.37

9.58

1.08

1.10

1.13

1.16

2.23

3.38

3.47

3.57

3.67

8.00

0.48

0.49

0.51

0.52

1.13

2.54

2.61

2.68

2.75

5.95

0.47

0.48

0.50

0.51

1.11

1.36

1.40

1.44

1.48

3.23

0.18

0.18

0.19

0.20

0.43

1.18

1.21

1.25

1.28

2.83

0.17

0.17

0.18

0.18

0.41

5.34 12.06

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Partition coefficient Degree of crystallization Ol Cpx Pl Opx Amp Gt 0% 5% 10% 15% 20% 25% 30% 35% 40% 45% 0.0006 0.0540 0.2700 0.0020 0.1700 0.0100 La(ppm) 23.26 25.47 27.90 30.59 33.58 36.94 40.74 45.07 50.05 55.86 0.0015 0.2800 0.0310 0.0490 0.5900 4.3000 Yb(ppm) 1.95 2.08 2.22 2.36 2.52 2.70 2.89 3.10 3.33 3.58

Yb Ni Ti Eu K

Cpx

Pl Opx Amp Gt 0.270 0.002 0.170 0.010 0.0006 0.0540 0 0 0 0 La(ppm) 0.031 0.049 0.590 4.300 0.0015 0.2800 0 0 0 0 Yb(ppm) 29.000 14.000 0.010 5.000 6.800 5.100 0 0 0 0 0 0 Ni(ppm) 0.040 0.024 0.690 0.100 0.0060 0.1000 0 0 0 0 Ti(ppm) 0.730 0.013 0.880 0.320 0.0016 0.3100 0 0 0 0 Eu(ppm) 0.156 0.009 0.015 0.0130 0.0072 0 1 1.36 0 K(ppm)

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La

Degree crystallization

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Ol

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Partition Coefficien t

of

of

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( c) AFC process Fractionating assemblages 20%Ol+20%Opx+30%Cpx+30%Amp

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CR

La Yb

0%

5%

10%

15%

20%

25%

30%

35%

40%

45%

23.26

25.52

28.02

30.80

33.90

37.38

41.33

45.85

51.07

57.18

1.95

1.93

1.92

1.90

1.89

1.87

1.86

1.84

1.83

1.81

15.78

5.88

4.07

3.77

3.72

3.72

3.72

3.72

290.20 65.01

3527.73 3854.69 4209.28 4595.57 5018.53 5484.27 6000.45 6576.73 7225.63 7963.57 1.35 1.44 1.54 1.64 1.76 1.88 2.01 2.16 2.32 2.50 11787.8 13033.4 14367.2 15800.9 17348.6 19027.3 20858.0 22866.9 25087.3 27562.0 5 7 9 9 5 2 2 5 3 4

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3% 6% 9% 12% 15% 18% 21% 24% 27% 819.70 757.21 697.69 641.09 587.35 536.42 488.26 442.81 400.01 21.80 21.61 21.42 21.22 21.01 20.80 20.59 20.36 20.14 45.40 46.35 47.36 48.42 49.54 50.74 52.00 53.35 54.79 1.84 1.88 1.92 1.97 2.01 2.06 2.11 2.17 2.23 2.24 2.18 2.12 2.06 1.99 1.93 1.86 1.80 1.74 1253.37 1281.27 1310.73 1341.89 1374.90 1409.97 1447.28 1487.08 1529.65

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Sr(ppm) Y(ppm) La(ppm) Yb(ppm) Eu(ppm) Ba(ppm)

0% 885.20 21.99 44.49 1.81 2.31 1226.90

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Sr Y La Yb Eu Ba

Pl Amp 4.4000 0.0200 0.1000 6.0000 0.3926 0.1200 0.1323 1.1500 2.1100 1.0800 0.3600 0.0440

Degree of crystallization

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Partition Coefficient

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(d) FC process Fractionating assemblages of 80%Pl+20%Amp

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The generation of basaltic andesites (high-Al basalts) and andesites can be attributed to the assimilation by high-Mg basalts (primary basalts) of relatively

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2.

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Highlights 1.The petrogenesis of high-Mg basalts can be ascribed to high-degree partial melting of an enriched mantle source that was previously enriched in LILE and LREE by slab-derived hydrous fluids/melts/ supercritical fluids, as well as the subsequent magma mixing process of different sources at different source depths, with little or no influence of polybaric fractional crystallization. For the tholeiitic basalts, however, melting occurred at relatively low pressures within the stability field of spinel. Their geochemical variation can be ascribed to different degrees of mantle melting and different degrees of metasomatism from enriched slab fluids. The shallower and slightly enriched tholeiitic basalts, together with the deeper and moderately enriched high-Mg basalts, indicate a complex magmatic evolution, which includes melt aggregation, mixing, and differentiation processes that occur in different pressure ranges at multiple levels within the subarc mantle wedge.

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high-Al2O3 overthickened lower crust and the subsequent crystallization of prevailing mafic mineral phases, while Al2O3-rich plagioclase crystallization was

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suppressed under high-pressure and nearly water-saturated conditions. Dacites and rhyolites may be the result of further fractional crystallization of basaltic

andesites and andesites. Mixing of magmas at various stages along the fractionation course of basaltic andesites (high-Al basalts) toward rhyolites promotes the

trends of the calc-alkaline series.

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and

average

sedimentary

rock

components

based

on

the

Archean

Wutai-Jining

section,

which

is

composed

of

54%

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components

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We obtain the compositions of the Archean lower crust of the North China Craton by compiling the average TTG components, average mafic granulite

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3.

The 2.5-Ga tholeiitic basalts, high-Mg basalts, and high-Al basalts, as well as other abundant calc-alkaline volcanic rocks in Changyukou, require a subduction

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4.

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tonalitictrondhjemitic-granodioritic-granitic gneiss, 32% mafic granulite, 6% metapelite and 8% metasandstone.

5.

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tectonic regime of Archean lithospheric plates similar to the Phanerozoic Earth.

To reconcile the widespread 2.55 to 2.5Ga TTGs derived from overthickened crust, the 2.51 to 2.50Ga calc-alkaline volcanic rocks derived from thickened crust,

tholeiitic basalts that represent low pressure and an extensional tectonic setting, 2493Ma leucosyenogranites derived from overthickened crust, 2437Ma

biotite-monzogranites derived from slightly thinner crust than the leucosyenogranites but still thickened, as well as the clockwise hybrid ITD and IBC P–T paths of

the HP granulites and widespread extension and rifting setting within the NCC from 2300 to 1950 Ma, we propose a model of an evolving subduction process.

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Among them, the composition of the 2.5Ga Changyukou volcanic rocks and potassic granites and the clockwise hybrid ITD and IBC P–T paths of the HP granulites

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may reveal that the tectonic setting in Northwest Hebei was in a transition stage from a subduction-related compressional regime to an extensional regime related

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to plate rollback.

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