Zircon U–Pb–Hf isotopes and geochemistry of two contrasting Neoarchean charnockitic rock series in Eastern Hebei, North China Craton: Implications for petrogenesis and tectonic setting

Precambrian Research 267 (2015) 72–93

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Zircon U–Pb–Hf isotopes and geochemistry of two contrasting Neoarchean charnockitic rock series in Eastern Hebei, North China Craton: Implications for petrogenesis and tectonic setting Xiang Bai a , Shuwen Liu a,∗ , Rongrong Guo a , Wei Wang b a

Key Laboratory of Orogenic Belts and Crustal Evolution, MOE, School of Earth and Space Sciences, Peking University, Beijing 100871, China The State Key Laboratory of Geological Processes and Mineral Resources, School of Earth Science and Resources, China University of Geosciences, Beijing 100083, China b

a r t i c l e

i n f o

Article history: Received 22 October 2014 Received in revised form 2 June 2015 Accepted 3 June 2015 Available online 14 June 2015 Keywords: Charnockitic rock series Orthopyroxene origins Zircon U–Pb–Hf isotopic systematics Petrogenesis and tectonic setting Eastern Hebei North China Craton

a b s t r a c t The Zunhua–Qinglong microblock in Eastern Hebei, North China Craton, exposes two contrasting charnockitic rock series in the Yuhuzhai–Taipingzhai and Cuizhangzi areas: charnockitic plagioclase gneiss series with chemical compositions of dioritic and tonalitic rocks, and charnockite series showing tonalitic and granodioritic components. Petrographic observation reveals that two types of orthopyroxenes are preserved in these rocks. Medium-fine grained anhedral orthopyroxenes are preserved in both series, and coarse grained subhedral orthopyroxenes are preserved only in the charnockite series. In situ trace element analyses reveal that the coarse grained subhedral orthopyroxenes exhibit typical chemical features of magmatic orthopyroxene, whereas the medium-fine grained anhedral orthopyroxenes display lower medium to heavy rare earth elements (REE) and no obvious Eu anomalies, resulting in left-inclined REE patterns. Combined with petrographic features, these new data indicate that the medium-fine grained anhedral orthopyroxenes were formed by granulite–facies metamorphism caused by dehydration reactions, whereas the coarse grained subhedral orthopyroxenes were directly crystallized from their magmatic precursors. Laser ablation inductively coupled plasma mass spectrometry (LA–ICP–MS) zircon U–Pb isotopic dating reveals that the magmatic precursors of the charnockitic plagioclase gneiss series were emplaced between 2530 ± 17 and 2523 ± 12 Ma, and those of the charnockite series were emplaced between 2527 ± 28 and 2515 ± 22 Ma. We conclude that these magmatic precursors are contemporaneous emplacements of two Neoarchean granitoid magmatic events, which were followed by regional granulite–facies metamorphism at ∼2.45 Ga. The charnockitic plagioclase gneiss series and charnockite series exhibit similar ␧Hf (t) values of −2.4 to +5.3 and −3.2 to +5.5, indicating either depleted mantle or juvenile crust affinity, and involvement of ancient crustal components. The charnockitic plagioclase gneiss series exhibit relatively low SiO2 contents (54.1–65.2%), with various MgO contents (1.93–5.36%), Mg# (0.36–0.55), and (La/Yb)N values (3.49–56.00); the charnockite series exhibit higher SiO2 contents (60.1–69.2%), with similar (La/Yb)N values (14.61–42.69), MgO contents (2.10–3.99%), and Mg# (0.29–0.52). These geochemical and zircon Lu–Hf isotopic features, together with chemical modeling, suggest that the magmatic precursors of the charnockitic plagioclase gneiss series may have been primarily derived from the partial melting of a depleted mantle that was metasomatized by slab-derived melts/fluids, and experienced fractional crystallization processes with hornblende and plagioclase as primary fractionated phases. The aforementioned features further suggest that the charnockite series originated from a partial melting of the subducted slab, and it was strongly contaminated by mantle peridotites. These new data, combined with the previous studies of Neoarchean basement rocks in the Zunhua–Qinglong microblock in Eastern Hebei, suggest that the magmatic precursors of these two contrasting charnockitic rock series were formed in a back-arc-related tectonic environment at a convergent plate margin. © 2015 Elsevier B.V. All rights reserved.

∗ Corresponding author. Tel.: +86 010 62754163; fax: +86 010 62754163. E-mail address: [email protected] (S. Liu). http://dx.doi.org/10.1016/j.precamres.2015.06.004 0301-9268/© 2015 Elsevier B.V. All rights reserved.

X. Bai et al. / Precambrian Research 267 (2015) 72–93

1. Introduction As one of the oldest cratonic blocks in the world, the North China Craton (NCC) preserves ∼3.8 Ga crustal rocks (Liu et al., 1992; Song et al., 1996; Zhao, 2014; Zheng et al., 2004). The Precambrian metamorphic basement of the NCC (see the inset of Fig. 1; Zhao et al., 2005) can be divided into the Western and Eastern Blocks that are welded along the Trans-North China Orogen (Condie et al., 1992; Geng et al., 2012; Kröner et al., 2006; Liu et al., 2000, 2002, 2004, 2005, 2006, 2007a, 2007b, 2010, 2011a, 2011b, 2012; Wan et al., 2012; Wang et al., 2011, 2012a, 2012b, 2013a, 2013b; Wilde, 2014; Zhai and Santosh, 2011, 2013; Xia et al., 2006a, 2006b; Yin et al., 2009; Zhang et al., 2012a; Zhao, 2009; Zhao and Cawood, 2012; Zhao et al., 2001, 2005, 2006, 2007, 2008a, 2008b, 2010, 2011, 2012). The Zunhua–Qinglong microblock in Eastern Hebei in the northwestern part of the Eastern Block extensively exposes Neoarchean basement rocks, which are composed of supracrustal rock sequences and varietal granitoid orthogneisses (Fig. 1). These basement rocks were subjected to amphibolite– to granulite–facies metamorphism, recording anticlockwise P–T–t trajectories (Bai et al., 2014a; Guo et al., 2013, 2015; He et al., 1992; Lin et al., 1992; Liu et al., 1991; Nutman et al., 2011; Qian et al., 1985; Sun et al., 1984; Zhang et al., 1986). Recent geochronological data indicate that these metamorphic volcanic sequences erupted between ∼2.61 and 2.51 Ga, granitoid orthogneisses were emplaced between ∼2.55 and 2.50 Ga, and that subsequent regional metamorphism occurred between ∼2.50 Ga and ∼2.37 Ga in the Zunhua–Qinglong microblock of Eastern Hebei (Bai et al., 2014a; Geng et al., 2006; Guo et al., 2013, 2014; Nutman et al., 2011; Yang et al., 2008; Zhang et al., 2012b). Geodynamic models employed to interpret the formation and evolution of the Neoarchean basement in Zunhua–Qinglong microblock are strongly debated. Some researchers suggest the existence of a mantle plume on the basis of the broad distribution and similar emplacement ages of the varietal orthogneisses (Geng et al., 2006, 2010; Yang et al., 2008). Other researchers dispute the mantle plume model and provide evidence for subduction-related settings. Nutman et al. (2011) suggested that the tonalitic-dominated gneisses in the western part of

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Zunhua–Qinglong–Qinhuangdao area in Eastern Hebei display older magmatic crystallization ages of ∼2550–2530 Ma. However, the granitic-dominated gneisses in the east have emplacement ages of ∼2530–2510 Ma, thus indicating a subduction-related lateral accretion. Additionally, whole-rock geochemical and zircon U–Pb–Hf isotopic data of the dioritic–trondhjemitic gneisses reveal that their arc-related geochemical characteristics are similar to those of the Phanerozoic adakites, supporting the arc-related geodynamic models (Bai et al., 2014a). Similarly, recent investigations reveal that the metavolcanic rocks associated with banded iron formations (BIFs) have protoliths consisting chiefly of back-arc MORB basalts, arc-related andesites and basalts, and Nb-enriched basalts, which were likely formed in a subduction-related setting at a convergent plate margin (Guo et al., 2013; Zhang et al., 2012b). Charnockite, termed by Holland (1900) to describe specific orthopyroxenes-bearing granitoid rocks, is one of the major components of worldwide Precambrian high-grade terrains (Frost et al., 2000; Janardhan et al., 1982; Kar et al., 2003; Rajesh, 2012; Santosh and Yoshida, 1992; Yang et al., 2014; Zhang et al., 2010). These rocks can be classified into the following: (1) magmatic type, containing orthopyroxenes crystallized directly from anhydrous magmas, and (2) metamorphic type, the orthopyroxenes of which originate from CO2 -rich fluids related granulite–facies metamorphic dehydration reactions, involving the breakdown of hornblende or biotite (Frost and Frost, 2008; Rajesh and Santosh, 2012). Metamorphic charnockitic rocks are usually distributed as strongly foliated units inside amphibolite–facies gneiss regions, and have been called “incipient charnockite” (Pichamuthu, 1960). In contrast, typical magmatic charnockitic rocks always form massive plutons without obvious foliation. However, it is generally difficult to determine, solely from the degree of deformation, whether orthopyroxenes are crystallized directly from liquid or formed as a result of high-grade metamorphism. It is especially difficult to ascertain the formation process in the presence of multiple phases of later deformation and metamorphism (Frost and Frost, 2008). The Yuhuzhai–Taipingzhai and Cuizhangzi areas of the Zunhua–Qinglong microblock expose widely distributed charnockitic rocks (Fig. 1), of which the genetic mechanism is still controversial. Some investigators consider these rocks to be metamorphic (Geng et al., 1990; Jahn and Zhang, 1984), and

Fig. 1. Geological sketch map of the Neoarchean metamorphic basement region in the Yuhuzhai–Taipingzhai and Cuizhangzi areas of Eastern Hebei, showing the distribution of the charnockitic plagioclase gneiss series and the charnockite series, and the locations of the samples for which U–Pb and Lu–Hf zircon analyses were conducted (modified after Bai et al., 2014a and Guo et al., 2013). The inset shows the tectonic units of the NCC and the location of study area (modified after Zhao et al., 2005).

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others consider them magmatic (Qian et al., 1985; Sun et al., 1984; Zhang et al., 1986). Detailed lithologic, petrographic, and other essentially geochemical characteristics for the charnockitic rocks in the Taipingzhai area were reported by Zhao (1992), who proposed that both metamorphic and magmatic types of charnockitic rocks are exposed in this region. Zhao (1992) also named the two contrasting rock series the “charnockitic plagioclase gneiss series” and the “charnockite series”, and this suggestion has been generally accepted by subsequent researchers (Bai et al., 2014a; Geng et al., 2006; Guo et al., 2013; He et al., 1992; Lin et al., 1992; Yang et al., 2008; Zhao et al., 1998). In this study, we report new zircon U–Pb chronological and Lu–Hf isotopic data, whole–rock geochemical analyses, and in situ trace elements of orthopyroxenes from the two contrasting charnockitic rock series in the Yuhuzhai–Taipingzhai and Cuizhangzi areas of the Zunhua–Qinglong microblock. Integrating these data with previous studies, we propose further constraints on the timing of the formation and petrogenesis of these charnockitic rocks, as well as on the Neoarchean tectonic evolution of the Zunhua–Qinglong microblock in Eastern Hebei.

2. Geological setting and previous studies The Zunhua–Qinglong microblock in Eastern Hebei has east–west-trending exposures of the rocks of the Neoarchean metamorphic basement region, which are overlain at their periphery by Mesoproterozoic to Mesozoic cover sequences, and are intruded by some Mesozoic granitic plutons (Fig. 1). These basement rocks were once considered an aggregation of several stratigraphic sequences, which were named the Qianxi, Badaohe, and Shuangshanzi groups in early investigations (Qian et al., 1985; Sun et al., 1984). However, subsequent studies identified varietal granitoid orthogneisses in the Zunhua–Qinglong microblock and the residual bodies of supracrustal rocks with variable dimensions in the orthogneisses (Geng et al., 2006; Guo et al., 2013; He et al., 1992; Lin et al., 1992; Zhang et al., 1986). These supracrustal rocks are composed of minor metasedimentary rocks, BIFs, and abundant tholeiitic to calc-alkaline (CA) volcanic rocks with formation ages of 2614–2518 Ma. Moreover, these rocks exhibit arc-related geochemical features indicating that their magmatic precursors formed in an Andean-type continental margin tectonic setting (Guo et al., 2013, 2015; Zhang et al., 2012b). The orthogneisses of Eastern Hebei were classified by He et al. (1992) and Lin et al. (1992) into five lithological series: charnockitic rocks, tonalitic–trondhjemitic gneisses, gneisses, monzogranitic–alkali granodioritic–monzogranitic granitic gneisses, and leucogranites. Both groups of researchers inferred that the magmatic precursors of these rocks have ages between >3.0 and 2.5 Ga (He et al., 1992; Lin et al., 1992). However, recent investigations based on progressive zircon U–Pb dating methods reveal that most of these orthogneiss series have Neoarchean magmatic precursors that were emplaced between 2.55 and 2.50 Ga (Bai et al., 2014a; Geng et al., 2006; Nutman et al., 2011; Yang et al., 2008). Yang et al. (2008) reported plutonic rocks in the Anziling area with emplacement ages between 2526 and 2515 Ma. Of these units, hornblendites were suggested to be derived from an enriched mantle source, monzogranites and alkali granites from a partial melting of a juvenile crust, and diorites and granodiorites from the mixing of mantle-derived mafic and crust-derived felsic magmas, coupled with fractional crystallization. Some dioritic gneisses were identified by Bai et al. (2014a), who summarized previous investigations with respect to granitoid rocks in the Zunhua–Qinglong microblock and updated the classification of the orthogneiss series as dioritic–trondhjemitic gneisses, taxitic dioritic–tonalitic gneisses, and charnockitic rocks

(Fig. 1). The magmatic precursors of the dioritic–trondhjemitic gneisses have formation ages of 2535–2513 Ma and Phanerozoic adakite-like geochemical features, indicating that their magmatic precursors originated from the partial melting of a depleted mantle wedge in a subduction-related environment, with further involvement by fractional crystallization during magmatic evolutionary processes (Bai et al., 2014a). Previous studies have revealed that the Yuhuzhai–Taipingzhai area widely exposes Neoarchean charnockitic rocks, which were also identified in the Cuizhangzi area in this study (Fig. 1). These rocks were subjected to multiple episodes of metamorphism and deformation due to late regional tectonothermal events and were also involved in the evolution of the Central Asian Orogenic Belt (CAOB) from the Paleozoic to the early Mesozoic (Jahn et al., 2000; Windley et al., 2007, 1990; Xiao et al., 2003), and in the Mesozoic lithosphere thinning of the NCC (Gao et al., 2004; Menzies et al., 2007; Yang et al., 2003). It is therefore very difficult to identify the petrogenesis of these charnockitic rocks, a fact that has led to numerous controversies. These charnockitic rocks were initially believed to have formed from remelting (Qian et al., 1985; Sun et al., 1984; Zhang et al., 1986) or metamorphism (Geng et al., 1990; Jahn and Zhang, 1984) of the country rocks with tonalitic or granodioritic lithologies. An alternate model by Wang (1988) proposed that these rocks were magmatic charnockites and were derived directly from a separate source other than the country rocks. Later, as noted above, Zhao (1992) revealed two contrasting charnockitic rock series namely the “charnockitic plagioclase gneiss series” and the “charnockite series”, suggesting that the former was derived from granulite–facies metamorphism caused by dehydration reactions of the dioritic and tonalitic gneisses, whereas the latter was derived from the remelting of the surrounding orthogneisses (He et al., 1992; Lin et al., 1992; Zhao, 1992). The charnockitic plagioclase gneiss series display Rb–Sr and Sm–Nd whole-rock isochrons with ages of ∼2.5 Ga (Jahn and Zhang, 1984), whereas the charnockite series in the Yuhuzhai area have formation age of 2550 ± 2 Ma, as analyzed by zircon stepwise evaporation–deposition technique (Geng et al., 2006). The Yuhuzhai–Taipingzhai and Cuizhangzi areas expose widely charnockitic plagioclase gneiss series, with chemical components of diorite and tonalite. The charnockite series are composed of tonalites and granodiorites, with a distribution of irregular islandlike intrusions inside the charnockitic plagioclase gneiss series in the Yuhuzhai–Taipingzhai area (Fig. 1; He et al., 1992; Lin et al., 1992; Zhao, 1992). The charnockitic plagioclase gneiss series show intrusive contacts with their surrounding supracrustal sequences, as is proved by many lenticular xenoliths of the supracrustal rocks (Fig. 2A and B). The charnockitic plagioclase gneiss series principally display a gradual transition with the regionally dioritic–trondhjemitic gneisses, but are rarely intruded by trondhjemitic veins without orthopyroxene (Fig. 2C). The charnockite series primarily have a gradational contact with the surrounding charnockitic plagioclase gneiss series, whereas the charnockite series locally intruded into gneissic charnockitic plagioclase gneiss series (Fig. 2D) (He et al., 1992; Lin et al., 1992; Zhao, 1992). These geological features indicate that the precursors of the charnockitic plagioclase gneiss series, charnockite series, and trondhjemitic gneisses have similar emplacement ages. However, the magmatic precursors of the charnockitic plagioclase gneiss series were emplaced first, followed by those of the trondhjemitic magmas, and finally those of the charnockite series.

3. Sample petrology Nineteen representative samples were collected from the Yuhuzhai–Taipingzhai and Cuizhangzi areas in this study, nine of

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Fig. 2. Field photographs showing geological relationships and macroscopic lithological features for two contrasting charnockitic rock series in the Yuhuzhai–Taipingzhai and Cuizhangzi area. (A) and (B) Charnockitic plagioclase gneiss series, in which different seizes of lenticular xenoliths of supracrustal amphibolite are preserved. (C) Charnockitic plagioclase gneiss series, with an intruded trondhjemitic (without Opx) vein. (D) An undeformed coarse grained charnockitic vein from the charnockite series intruding into the charnockitic plagioclase gneiss series.

which were from the charnockite series and ten of which were from the charnockitic plagioclase gneiss series. The ten samples of the charnockitic plagioclase gneiss series have lithologies of twopyroxene-hornblende-plagioclase gneisses (four samples, Fig. 3A),

two-pyroxene-plagioclase gneisses (three samples, Fig. 3B), and biotite plagioclase gneisses (three samples). The samples are characterized by a medium-grained granoblastic texture, gneissic structure, and a major mineral assemblage of plagioclase

Fig. 3. Photomicrographs showing petrographic features of representative charnockitic rock samples (viewed under crossed polars). (A) Biotite two-pyroxene-plagioclase gneiss sample 11JD56–1 from the charnockitic plagioclase gneiss series. (B) Biotite plagioclase gneiss sample 11JD25–1 from the charnockitic plagioclase gneiss series. (C) Tonalitic gneiss sample 11JD57–2 from the charnockite series. (D) Granodiorite sample 11JD57–1 from the charnockite series. Mineral abbreviations are as follows: Pl, plagioclase; Hb, hornblende; Qtz, quartz; Bi, biotite; Opx, orthopyroxene; and Cpx, clinopyroxene.

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Table 1 Petrological features of the samples from two contrasting charnockitic rock series. Latitude (N)

Longitude (E)

Charnockitic plagioclase gneiss series 118◦ 39 17 11JD25–3 40◦ 21 48 11JD54–2

40◦ 17 25

118◦ 27 33

11JD56–1

40◦ 11 57

118◦ 25 43

◦





◦





Lithology Biotite two pyroxene plagioclase gneiss Two pyroxene hornblende plagioclase gneiss Biotite two pyroxene plagioclase gneiss Two pyroxene hornblende plagioclase gneiss Two pyroxene hornblende plagioclase gneiss

11JD71–1

40 10 40

118 18 54

11JD72–1

40◦ 13 21

118◦ 19 21

11JD25–1 11JD25–2

40◦ 21 48 40◦ 21 48

118◦ 39 17 118◦ 39 17

11JD50–5 11JD56–3 11JD72–2

40◦ 12 43 40◦ 11 57 40◦ 13 21

118◦ 30 05 118◦ 25 43 118◦ 19 21

Charnockite series 40◦ 07 14 11JD45–1 40◦ 17 25 11JD54–1 40◦ 13 21 11JD57–2 40◦ 16 21 11JD68–2

118◦ 38 40 118◦ 27 33 118◦ 28 26 118◦ 23 19

Tonalite Tonalite Tonalitic gneiss Tonalite

40◦ 11 25 40◦ 13 21 40◦ 16 21 40◦ 14 32 40◦ 10 47

118◦ 33 10 118◦ 28 26 118◦ 23 19 118◦ 23 31 118◦ 21 59

Granodiorite Granodiorite Granodioritic gneiss Granodiorite Granodiorite

11JD47–2 11JD57–1 11JD68–1 11JD70–1 11JD70–3

Biotite plagioclase gneiss Two pyroxene plagioclase gneiss Biotite plagioclase gneiss Biotite plagioclase gneiss Two pyroxene hornblende plagioclase gneiss

Composition

Mineral assemblage Pl(45%) + Qtz(21%) + Opx(10%) + Cpx(9%) + Bi(7%) + Hb(5%) + Kfs(5%)

Dioritic gneiss

Pl(47%) + Qtz(19%) + Opx(13%) + Cpx(10%) + Hb(8%) + Kfs(3%) Pl(51%) + Qtz(20%) + Bi(8%) + Opx(8%) + Cpx(6%) + Kfs(4%) + Hb(3%) Pl(54%) + Qtz(15%) + Opx(11%) + Cpx(8%) + Hb(6%) + Kfs(4%) + Grt(2%) Pl(46%) + Qtz(17%) + Opx(14%) + Cpx(11%) + Hb(9%) + Kfs(3%) Pl(45%) + Qtz(22%) + Bi(9%) + Opx(4%) + Cpx(4%) + Hb(4%) + Kfs(3%) Pl(45%) + Qtz(22%) + Opx(7%) + Cpx(6%) + Kfs(4%) + Bi(3%) + Hb(3%)

Tonalitic gneiss Pl(51%) + Qtz(27%) + Bi(9%) + Opx(4%) + Cpx(3%) + Hb(3%) + Kfsc(3%) Pl(51%) + Qtz(28%) + Bi(8%) + Opx(4%) + Cpx(3%) + Hb(3%) + Kfs(3%) Pl(43%) + Qtz(27%) + Opx(8%) + Cpx(7%) + Hb(7%) + Kfs(5%) + Grt(3%)

Tonalite

Pl(44%) + Qtz(20%) + Bi(14%) + Opx(10%) + Hb(8%) + Kfs(4%) Pl(46%) + Qtz(29%) + Opx(13%) + Bi(8%) + Kfs(4%) Pl(47%) + Qtz(26%) + Opx(16%) + Bi(6%) + Kfs(5%) Pl(50%) + Qtz(21%) + Opx(13%) + Hb(7%) + Cpx(6%) + Kfs(3%)

Granodiorite

Pl(53%) + Qtz(25%) + Opx(12%) + Kfs(8%) + Cpx(4%) Pl(46%) + Qtz(23%) + Kfs(11%) + Opx(12%) + Bi(8%) Pl(54%) + Qtz(24%) + Kfs(12%) + Opx(12%) + Bi(4%) Pl(57%) + Qtz(21%) + Kfs(12%) + Opx(8%) + Bi(2%) Pl(49%) + Qtz(25%) + Kfs(10%) + Opx(8%) + Bi(5%) + Hb(3%)

Note: Pl, plagioclase; Kfs, K-feldspar; Hb, hornblende; Qtz, quartz; Bi, biotite; Opx, orthopyroxene; Cpx, clinopyroxene; Grt, garnet.

(Pl, 43–54%), quartz (Qtz, 15–28%), orthopyroxene (Opx, 4–14%), clinopyroxene (Cpx, 3–11%), hornblende (Hb, 3–9%), biotite (Bi, 0–9%), K-feldspar (Kfs, 3–5%), and garnet (Grt, 0–3%), and accessory minerals are zircon, apatite, and magnetite (Table 1). The orthopyroxenes and clinopyroxenes from these charnockitic plagioclase gneiss samples are anhedral granular and hold plagioclase, quartz, and hornblende as inclusions, showing equilibrium association to hornblendes or reactive products of hornblendes. The garnets are only preserved in samples 11JD71–1 and 11JD72–2, which display anhedral granular texture and reactive products of pyroxenes. The hornblendes are characterized by two generations: the first generation is medium euhedral–subhedral granular minerals, and the second is fine anhedral mineral aggregates along the edges of pyroxene grains. These petrographic features reveal that the charnockitic plagioclase gneiss series were subjected to granulite–facies metamorphic overprint. In addition, the gneissose fabrics in these samples are characterized by the preferred orientations of mafic minerals (Fig. 3B) and stretched quartz grains (Fig. 3A). The nine samples of the charnockite series are coarse-grained, containing the lithologies of four tonalitic samples and five granodioritic samples (Table 1). The tonalitic samples display a mineral assemblage of Pl (44–50%), Qtz (20–29%), Opx (10–16%), Cpx (0–6%), Hb (0–8%), Bi (0–14%), Kfs (3–5%) (Fig. 3C). The granodioritic samples display a mineral assemblage of Pl (46–57%), Qtz (21–25%), Opx (8–12%), Cpx (0–4%), Hb (0–3%), Bi (2–8%), and Kfs (8–12%) (Fig. 3D). All nine samples contain accessory minerals of zircon, apatite, magnetite, and allanite. In contrast to the charnockitic plagioclase gneiss series, the charnockite series display mainly a massive structure, except for tonalitic gneiss sample 11JD57–2 and granodioritic gneiss sample 11JD68–1, which show some weak foliations (Fig. 3C). The samples from the charnockite series also exhibit lower contents of clinopyroxenes and hornblendes, and the hornblende grains are only preserved at the edges of the pyroxene grains as mineral aggregates. Although few orthopyroxenes in the charnockite series are medium-fine grained,

most of them have a subhedral coarse grained texture, indicating that they were formed either from sufficient granulite–facies metamorphism (i.e., sufficient to transform almost all hornblende grains into orthopyroxenes), or directly by the crystallization of their magmatic precursors, coupled with later metamorphism and deformation (He et al., 1992; Lin et al., 1992; Zhao, 1992). 4. Analytical techniques 4.1. Whole-rock geochemical analyses Whole-rock major oxide and trace element analyses were conducted for all samples of the charnockitic plagioclase gneiss series and the charnockite series. Before the geochemical analyses, each sample was screened to remove any weathered surfaces, and the fresh surfaces were chipped and powdered in an agate mill to a mesh size of 200. Major oxides were analyzed using X-ray fluorescence (XRF) on a Thermo Arl Advant XP+, at the Key Laboratory of Orogenic Belts and Crustal Evolution, Ministry of Education, School of Earth and Space Sciences, Peking University. Sample powders were fused with lithium metaborate in a 1:10 ratio, placed into glass disks and fused again at 1100 ◦ C in a Pt–Au crucible for 20–40 min. The resultant melt was then poured into a 34 mm diameter preheated pellet and prepared for major element analysis. XRF analyses were calibrated against national standards GBW02103 (GSR-1) and GBW02104 (GSR-2). The proportion of volatile components was calculated by measuring the different weights of each sample before and after heating at 1050 ◦ C. The analytical precision of the measurement of major oxides was ≤0.5%. The whole-rock powders for trace element analyses were prepared at Peking University, 25 mg of each sample were decanted into Savillex teflon beakers, placed in a high-pressure bomb with a 1:1 mixture of HF and HNO3 , and heated for 24 h at 80 ◦ C. After evaporation, 1.5 ml HNO3 , 1.5 ml HF, and 0.5 ml HClO4 were added, and the beakers were capped for digestion at 180 ◦ C in a hightemperature oven for a minimum of 48 h until the powders were

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completely digested. Finally, the residue was diluted with 1% HNO3 to a volume of 50 ml. Trace elements, including rare earth elements (REEs), were measured using a VG Axiom multicollector (MC), highresolution (HR) ICP–MS at the Key Laboratory of Orogenic Belts and Crustal Evolution, Ministry of Education, School of Earth and Space Sciences, Peking University. National standards GSR-1 and GSR-2 were used for analytical quality control, and the precision of measurements of trace elements is 0.5‰. The results of the wholerock major oxides and trace element analyses are listed in online supplementary Table S1. Supplementary Table S1 related to this article can be found, in the online version, at http://dx.doi.org/10.1016/j.precamres.2015. 06.004 4.2. In situ trace element analyses for single minerals Orthopyroxenes from sample 11JD56–1 (a sample of two-pyroxene-plagioclase gneiss), and sample 11JD72–2 (a sample of two-pyroxene-hornblende-plagioclase gneiss) from the charnockitic plagioclase gneiss series, and a sample 11JD47–2 (granodiorite) of the charnockite series were selected for in situ mineral trace element analyses. The samples were processed using an LA–ICP–MS instrument (Agilent 7500ce) at the Key Laboratory of Orogenic Belts and Crustal Evolution, Ministry of Education, School of Earth and Space Sciences, Peking University. Ablation spot size was 60 ␮m with a repetition rate of 5 Hz and laser energy of 100 mJ. Helium was used as the cell gas. The acquisition times for the background and the ablation interval were 15–20 and 60–45 s, respectively. Synthetic glasses NIST 610, 612, and 614 were used as external standards. The calculation of trace element concentrations was performed by GLITTER version 4.4.2. Internal standards were Si. Results of the in situ trace element analyses for orthopyroxenes are given in online Supplementary Table S2. Supplementary Table S2 related to this article can be found, in the online version, at http://dx.doi.org/10.1016/j.precamres.2015. 06.004

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2003). All zircon U–Pb isotopic data and analyzed ages are listed in online Supplementary Table S3. Supplementary Table S3 related to this article can be found, in the online version, at http://dx.doi.org/10.1016/j.precamres.2015. 06.004 Lu–Hf isotope analyses were conducted using a Neptune Plus MC–ICP–MS (Thermo Fisher Scientific, Germany) in combination with a Geolas 2005 excimer ArF laser ablation system (Lambda Physik, Göttingen, Germany) that was hosted at the state Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences, Wuhan. The energy density of the laser ablation used in this study was 5.3 J cm−2 . Helium was used as the carrier gas within the ablation cell and was merged with argon (the make-up gas) after the ablation cell. As demonstrated by our previous study, using the 193 nm laser, helium consistently enhanced the signal by twofold, as compared with argon gas (Hu et al., 2008b). We used a simple Y junction downstream from the sample cell to add small amounts of nitrogen (4 ml min−1 ) to the argon makeup gas flow (Hu et al., 2008a). Compared with the standard arrangement, the addition of nitrogen along with the use of the newly designed X skimmer cone and Jet sample cone in Neptune Plus improved the signal intensity of Hf, Yb, and Lu by factors of 5.3, 4.0, and 2.4, respectively. In this study, all data were acquired on zircon in single spot ablation mode at a spot size of 44 ␮m. Each measurement consisted of 20 s of acquisition of the background signal followed by 50 s of acquisition of the ablation signal. Detailed operating conditions for the laser ablation system and the MC–ICP–MS instrument and analytical method are the same as those described by Hu et al. (2012). The zircon Lu–Hf analytical data and calculated results are listed in online Supplementary Table S4, in which the 176 Hf/177 Hf and 176 Lu/177 Hf ratios of chondrite and depleted mantle are 0.282772 and 0.0332, and 0.28325 and 0.0384, respectively (Blichert-Toft and Albarède, 1997; Griffin et al., 2000), and  = 1.867 × 10−11 a−1 (Söderlund et al., 2004). Supplementary Table S4 related to this article can be found, in the online version, at http://dx.doi.org/10.1016/j.precamres.2015. 06.004

4.3. Zircon isotopic analysis 5. Analytical results Six representative samples were selected for zircon U–Th–Pb and Lu–Hf isotopic analyses, charnockitic plagioclase gneiss series and charnockite series, three samples each. The charnockitic plagioclase gneiss series comprises one sample of two-pyroxene-hornblende-plagioclase gneiss (11JD72–1) and two samples of biotite plagioclase gneiss (11JD25–1 and 12JD50–5), and from the charnockite series came one tonalite sample (11JD54–1), one tonalitic gneiss sample (11JD57–2), and one granodiorite sample (11JD47–2). Zircon grains were separated from each sample by standard density and magnetic techniques, and then purified by handpicking under a binocular microscope. More than 1000 zircon grains were mounted in epoxy resin discs and polished to half the thickness of the modal grains. Prior to analysis, cathodoluminescence (CL) images were obtained using a scanning electron microscope (SEM) at the Peking University. All six samples were simultaneously analyzed for zircon U–Pb isotopes and trace elements using an LA–ICP–MS at the Geological Lab Center, China University of Geosciences, Beijing (CUGB). Harvard zircon 91500 was used as an external standard for all U–Th–Pb isotopic analyses, and NIST 610 was used as an external standard to calculate the contents of U, Th, Pb, and other trace elements in the analyzed zircon grains. 207 Pb/206 Pb and 206 Pb/238 U ratios were calculated using the GLITTER program (Van Achterbergh et al., 2001); a common Pb was corrected using the method of Andersen (2002), and all geochronological calculations and concordia plot constructions were conducted using Isoplot (ver. 3.0) (Ludwig,

5.1. Whole-rock chemical compositions 5.1.1. Charnockitic plagioclase gneiss series Ten samples from the charnockitic plagioclase gneiss series were selected for whole-rock chemical composition analyses. Five samples have low SiO2 contents of 54.1–60.3%, and they plot in the fields of gabbro diorite, diorite, and monzonite in the total alkali versus silica (TAS) diagram (Fig. 4A; Middlemost, 1994). The other five samples exhibit higher SiO2 contents of 61.2–65.2%, and plot in the field of tonalite in the TAS diagram (Fig. 4A). Therefore, these samples from the charnockitic plagioclase gneiss series are classified into five dioritic gneisses and five tonalitic gneisses according to their chemical compositions (Table S1). The five dioritic gneiss samples have SiO2 contents of 54.1–60.3%, Na2 O contents of 3.71–4.72%, CaO contents of 4.16–8.35%, K2 O contents of 0.52–2.53%, and Al2 O3 contents of 15.1–16.9%. The K2 O/Na2 O, CaO/Na2 O, and A/CNK ratios are 0.12–0.62, 1.12–1.97, and 0.72–0.97 (metaluminous), respectively. Their MgO contents range from 2.66 to 5.36%, with Mg# (Mg/(Mg + Fetota l )) ranging from 0.46 to 0.55. In the MgO versus SiO2 plot from Martin et al. (2005) (Fig. 4C), four of the dioritic gneiss samples plot in the low-silica adakite (LSA) field, one sample (11JD25–3) plots at the edge of the LSA field, and three samples (11JD54–2, 11JD56–1, and 11JD72–1) exhibit MgO contents higher than the experimental melt range of basalts or amphibolites (PMB).

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Fig. 4. Geochemical classification diagrams for analysis of two contrasting charnockitic rock samples and trondhjemitic gneisses (without Opx) in the Taipingzhai area as reported by Bai et al. (2014a). (A) Total alkali versus silica (TAS) diagram (Middlemost, 1994). (B) An–Ab–Or diagram (Barker, 1979). (C) MgO versus SiO2 plot (PMB, experimental partial melts from basalts or amphibolites; LSA, low-silica adakite; HSA, high-silica adakite; after Martin et al., 2005). (D) K2 O–Na2 O–CaO diagram (Barker, 1979), with the black arrows showing the tonalite–trondhjemite (TT) trend and calc-alkaline (CA) trend highlighted by these samples. The symbols are samples with compositions of dioritic gneiss (solid diamonds) and tonalitic gneiss (open diamonds) for the charnockitic plagioclase gneiss series, and those of tonalite (open circles) and granodiorite (solid circles) for the charnockite series, and the trondhjemitic gneiss samples (inverted open triangles).

The five tonalitic gneiss samples are characterized by higher SiO2 (61.2–65.2%), but lower CaO (3.92–5.76%), K2 O (0.91–1.62%), MgO (1.93–3.55%) contents. Their Na2 O contents (3.79–4.65%) are similar to that of the dioritic gneiss samples, and results in ratios of 0.23–0.47 for K2 O/Na2 O, 0.95–1.28 for CaO/Na2 O, and 0.36–0.48 for Mg#. They have Al2 O3 contents of 14.0–16.7%, and A/CNK ratios ranging from 0.84 to 1.00 (metaluminous). In the An–Ab–Or diagram, these samples plot in the tonalite field (Fig. 4B; Barker, 1979). All five tonalitic gneiss samples plot in the high-silica adakite (HSA) field in the MgO versus SiO2 plot (Martin et al., 2005), and two of them (11JD50–5 and 11JD72–2) have MgO contents higher than the PMB (Fig. 4C). Combined with the published data for the trondhjemitic gneisses (without orthopyroxene) in the Taipingzhai area (Bai et al., 2014a), data for these dioritic and tonalitic gneiss samples from the charnockitic plagioclase gneiss series form a tonalite–trondhjemite (TT) trend in the K2 O–Na2 O–CaO diagram (Barker, 1979) (Fig. 4D). The five analyzed dioritic gneiss samples have total rare earth element (REE) contents of 78–155 ppm. They have nearly

consistent and moderately fractionated chondrite-normalized REE patterns with (La/Yb)N ratios of 3.54–15.06, and weakly positive to negative Eu anomalies (EuN /EuN * = 0.65–1.16) (Fig. 5A). In contrast, the five tonalitic gneiss samples exhibit higher total REE (TREE) contents of 93–330 ppm, and more strongly fractionated chondritenormalized REE patterns, with higher (La/Yb)N ratios of 3.49–56.00 and weakly negative Eu anomalies (EuN /EuN * = 0.73–1.00) (Fig. 5C). In the primitive mantle-normalized multielement patterns, the five dioritic gneiss samples are characterized by Ba, Rb, K, and slight light rare earth element (LREE) enrichments, and by depletions of U, Th, Nb, Ta, and Ti (Fig. 5B). Similarly, the five tonalitic gneiss samples display Ba, Rb, K, Zr, Hf, and LREE enrichments; U, Nb, Ta, and Ti depletions; and positive to negative Th anomalies (Fig. 5D). 5.1.2. Charnockite series Nine samples from the charnockite series were included in the whole-rock composition analyses, which were composed of four tonalitic samples and five granodioritic samples. In the An–Ab–Or

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Fig. 5. Chondrite-normalized REE patterns and primitive mantle-normalized spider diagrams for the charnockitic plagioclase gneiss series with dioritic gneiss (A and B) and tonalitic gneiss (C and D) compositions, and for the charnockite series with tonalite (E and F) and granodiorite (G and H) compositions. Chondrite-normalized REE patterns for model magmas generated by partial melting (1–25%) of a mantle source (Martin, 1986, 1987, 1994) are shown in (A), and average component for low-Ti sanukitoids (SiO2 < 62%) (Martin et al., 2010) are shown in (E) and (G). Symbols are the same as those given in Fig. 4, and the chondrite and primitive mantle values are after Sun and McDonough (1989).

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Fig. 6. Photomicrographs showing petrographic features of analyzed orthopyroxenes and locations of in situ trace element analyses (viewed under crossed polars). (A) Sample 11JD56–1, a dioritic gneiss from the charnockitic plagioclase gneiss series. (B) Sample 11JD72–2, a tonalitic gneiss from the charnockitic plagioclase gneiss series. (C) and (D) Sample 11JD47–2, a granodiorite from the charnockite series.

diagram (Barker, 1979), the four tonalitic samples plot in the tonalite field, whereas the five granodioritic samples plot in the granodiorite field (Fig. 4B). Considering that most of the representative samples show a massive structure, we divided the samples from the charnockite series into four tonalites and five granodiorites. The four tonalite samples have SiO2 contents of 60.1–69.2%, CaO contents of 3.36–4.95%, Na2 O contents of 3.47–4.34%, K2 O contents of 0.92–2.03%, and Al2 O3 contents of 13.2–17.5%. The samples have K2 O/Na2 O, CaO/Na2 O, and A/CNK ratios of 0.27–0.47, 0.85–1.28, and 0.95–0.99 (metaluminous), respectively. In the TAS diagram (Middlemost, 1994), two samples (11JD54–1 and 11JD57–2) plot in the granodiorite field, with the other two samples (11JD68–2 and 11JD45–1) in the diorite field (Fig. 4A). These tonalite samples exhibit MgO contents of 2.10–2.93%, with Mg# ranging from 0.29 to 0.52. All samples plot in the HSA field in the MgO versus SiO2 plot (Martin et al., 2005) (Fig. 4C). The analytical results suggest that two of these samples (11JD54–1 and 11JD57–2) have MgO contents higher than the PMB (Fig. 4C). The five analyzed granodiorite samples have SiO2 contents of 62.3–66.6%, Na2 O contents of 3.57–4.49%, and Al2 O3 contents of 14.5–16.7%. Their A/CNK ratios are 0.90–0.99 (metaluminous). In contrast to the tonalite samples, these granodiorite samples are characterized by higher K2 O (2.17–3.54%) and MgO (2.39–3.99%), but lower CaO (3.19–4.19%). They have K2 O/Na2 O, CaO/Na2 O, and Mg# ratios of 0.56–0.98, 0.82–1.12, and 0.44–0.52, respectively. In the TAS diagram (Middlemost, 1994), they plot in the fields of tonalite (11JD70–1 and 11JD70–3), monzonite (11JD68–1), quartz monzonite (11JD57–1), and granodiorite (11JD47–2) (Fig. 4A). They plot in the HSA field in the MgO versus SiO2 plot (Martin et al., 2005), and most of them display MgO contents higher than the PMB, except for one sample 11JD68–1 (Fig. 4C). The nine samples

of the charnockite series constitute a calc-alkaline (CA) trend from tonalite samples to granodiorite samples in the K2 O–Na2 O–CaO diagram (Fig. 4D; Barker, 1979). The four tonalite samples have TREE contents of 71–119 ppm and strongly fractionated chondrite-normalized REE patterns. They have (La/Yb)N ratios of 15.50–25.18 and positive to negative Eu anomalies (EuN /EuN * = 0.89–3.61) (Fig. 5E). The five granodiorite samples exhibit similar TREE contents of 58–147 ppm and strongly fractionated chondrite-normalized REE patterns. They have higher (La/Yb)N ratios of 14.61–42.69, and positive Eu anomalies (EuN /EuN * = 1.26–2.03) (Fig. 5G). In the primitive mantlenormalized multielement patterns, the four tonalite samples are characterized by Ba, Rb, K, and LREE enrichments, and by depletions of U, Th, Nb, and Ta (Fig. 5F). The five granodiorite samples display Ba, Rb, K, Zr, Hf, and LREE enrichments, and U, Th, Nb, and Ta depletions (Fig. 5H). 5.2. In situ trace elements of orthopyroxenes Thirty-four in situ trace element analyses were conducted on the samples of the charnockitic rock series from the Yuhuzhai–Taipingzhai and Cuizhangzi areas. Five analyses were done on the dioritic gneiss sample 11JD56–1 (of the charnockitic plagioclase gneiss series), seven analyses were done on the tonalitic gneiss sample 11JD72–2 (of the charnockitic plagioclase gneiss series), and twenty-two analyses were done on the granodiorite sample 11JD47–2 (of the charnockite series) (Table S2). The orthopyroxenes of the dioritic gneiss sample 11JD56–1 (Fig. 6A) and the tonalitic gneiss sample 11JD72–2 (Fig. 6B) are characterized by medium-fine grained anhedral crystalloblastic texture, and they hold plagioclase, quartz and hornblende as inclusions, indicative of their metamorphic origins. The twenty-two analyses of the

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following: higher TREE contents of 59–182 ppm; strongly negative Eu anomalies (EuN /EuN * = 0.27–0.56); less fractionated chondritenormalized REE patterns; depleted LREE and flat HREE patterns; and (La/Yb)N , (La/Sm)N , and (Gd/Yb)N ratios of 0.03–0.35, 0.01–0.24, and 0.82–1.88, respectively. In the whole-rock normalized REE patterns (Fig. 7B), the analyzed orthopyroxenes in the dioritic (11JD56–1) and tonalitic (11JD72–2) gneiss samples, and the anhedral orthopyroxenes of the granodiorite sample 11JD47–2, have similar fractionated REE patterns, with depleted LREE and enriched HREE and weak depletions in Eu. However, the coarse grained subhedral orthopyroxenes in sample 11JD47–2 show less fractionated REE patterns and stronger LREE and Eu depletions, with flat HREE patterns that are consistent with the pattern of orthopyroxene REE coefficients in dioritic to granitic melts, although the analyzed orthopyroxenes show slightly higher HREE contents (Rollinson, 1993, and references therein). 5.3. Zircon U–Pb and Lu–Hf isotopes

Fig. 7. Chondrite (A) and whole-rock (B) normalized REE patterns for analyzed orthopyroxenes, showing the partition coefficients for orthopyroxene in dioritic to granitic melts in (B) as white solid triangles (La, 0.015; Ce, 0.15; Nd, 1.25; Sm, 1.6; Eu, 0.825; Tb, 1.850; Dy, 1.800; Yb, 2.35; Lu, 2.70; after Rollinson, 1993, and references therein). Other symbols are the same as those for their hosted rocks given in Fig. 4, and the chondrite values are after Sun and McDonough (1989).

granodiorite sample 11JD47–2 included analyses of eighteen spots on three coarse grained subhedral orthopyroxenes (Fig. 6C), and four analyses of spots on one medium grained anhedral orthopyroxenes (Fig. 6D). The orthopyroxenes of the dioritic gneiss sample 11JD56–1 are characterized by low TREE contents of 9–18 ppm, negative Eu anomalies (EuN /EuN * = 0.31–0.36), and fractionated chondritenormalized REE patterns. They have enriched to depleted LREE and highly enriched heavy REE (HREE) levels, and (La/Yb)N , (La/Sm)N , and (Gd/Yb)N ratios of 0.01–0.20, 0.27–2.71, and 0.07–0.14, respectively (Fig. 7A). The orthopyroxenes of tonalitic gneiss sample 11JD72–2 exhibit similar REE features of low TREE contents from 10 to 17 ppm, negative Eu anomalies (EuN /EuN * = 0.35–0.88), and strongly fractionated chondrite-normalized REE patterns. They have enriched to depleted LREE and highly enriched HREE, and (La/Yb)N , (La/Sm)N , and (Gd/Yb)N ratios of 0.01–0.17, 0.04–6.09, and 0.03–0.07, respectively (Fig. 7A). The granodiorite sample 11JD47–2 from the charnockite series contains two kinds of orthopyroxenes, one of which displays anhedral orthopyroxenes, showing REE features similar to those of the above charnockitic plagioclase gneiss samples: low TREE contents of 5–10 ppm; moderately negative Eu anomalies (EuN /EuN * = 0.72–0.89); strongly fractionated chondritenormalized REE patterns; enriched to depleted LREE and highly enriched HREE; and (La/Yb)N , (La/Sm)N , and (Gd/Yb)N ratios of 0.04–0.58, 0.52–2.77, and 0.10–0.21, respectively (Fig. 7A). However, the coarse grained subhedral orthopyroxenes in the same sample have distinctly different chemical features, including the

The locations of the six samples that were conducted for zircon U–Pb–Hf isotopic analyses are shown in Fig. 1. Cathodoluminescence (CL) images show that the analyzed zircon grains from the charnockitic gneiss series (samples 11JD72–1, 11JD25–1, and 11JD50–5) and charnockite series (samples 11JD47–2, 11JD54–1, and 11JD57–2) show short or long prismatic shapes, with lengths of 70–310 ␮m and length/width ratios of 4:3–4:1 (Fig. 8). Most of these zircon grains are characterized by core–rim structure, with blurry oscillatory zoning at the euhedral or subhedral cores, which were surrounded by homogeneous dark or bright rims (Fig. 8). Oscillatory zoning structures are usually considered to be common characteristics of magmatic zircons, whereas homogeneous rims generally indicate metamorphic growth around preexisting magmatic zircon grains during later metamorphism (Hoskin and Schaltegger, 2003). Therefore, we indicate that the analyzed zircon grains from the two contrasting charnockitic rock series were of magmatic origin, and experienced significant alteration or recrystallization during later tectonothermal events. Twenty-five U–Pb isotopic analyses were conducted on twentythree zircon grains of the sample 11JD72–1 (a dioritic gneiss) of the charnockitic plagioclase gneiss series. Thirteen analyses from inner cores showing blurry oscillatory zoning (e.g., spots #17, #19, #21, and #22) (Fig. 8A) show Th/U values of 0.14–0.87 that are suggestive of a magmatic genesis (Rubatto, 2002). They display a weighted mean apparent 207 Pb/206 Pb age of 2523 ± 12 Ma (MSWD = 0.046), interpreted as the magmatic crystallization age of the sample magmatic precursor. Other six analyses from blurry oscillatory-zoned inner cores (e.g. spot #2; Fig. 8A) show an older weighted mean apparent 207 Pb/206 Pb age of 2621 ± 16 Ma (MSWD = 0.073), with Th/U values of 0.31–0.63. Although these six analyses are plotted on or near the concordia curve (Fig. 9A), they are less significant than above thirteen analyses in the histogram of apparent 207 Pb/206 Pb ages (Fig. 9B), but consistent with the magmatic crystallization ages of captured zircon grains (∼2.61 Ga) of the dioritic–trondhjemitic gneisses in Eastern Hebei (Bai et al., 2014a) and eruption ages of early metavolcanic rocks (∼2614 Ma) (Guo et al., 2013, 2014). Based on geological relationships on field outcrops, the charnockitic plagioclase gneiss series show significant intrusive contacts with metavolcanic rocks (Fig. 2A and B), and the emplacement of these granitoid gneisses should be later than the metavolcanic rocks with eruption ages of ∼2.61 Ga. Therefore, we suggest that these old zircon grains were captured from the surrounding metavolcanic rocks. The remaining six analyses were conducted on four homogeneous domains (e.g., spot #3) (Fig. 8A) and two blurry oscillatory-zoned inner cores. They are scattered under the concordia curve (Fig. 9A),

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Fig. 8. Cathodoluminescence images of representative zircon grains showing internal structures, analyzed locations, and calculated apparent 207 Pb/206 Pb ages. (A) Dioritic gneiss sample 11JD72-1 (charnockitic plagioclase gneiss series). (B) Tonalitic gneiss sample 11JD25–1 (charnockitic plagioclase gneiss series). (C) Tonalitic gneiss sample 11JD50–5 (charnockitic plagioclase gneiss series). (D) Granodiorite sample 11JD47–2 (charnockite series). (E) Tonalitic gneiss sample 11JD54–1 (charnockite series); (F) Tonalite sample 11JD57–2 (charnockite series).

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Fig. 9. Concordia plots showing zircon U–Pb isotope data (A, C, D, E, F, and H) and histograms of apparent contrasting charnockitic rock series.

207

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Pb/206 Pb ages (B and G) for the analyzed samples from two

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Fig. 10. ␧Hf (t) versus age plots for dated zircon grains. (A) Dioritic gneiss sample 11JD72–1 (charnockitic plagioclase gneiss series). (B) Tonalitic gneiss sample 11JD25–1 (charnockitic plagioclase gneiss series). (C) Tonalitic gneiss sample 11JD50–5 (charnockitic plagioclase gneiss series). (D) Granodiorite sample 11JD47–2 (charnockite series). (E) Tonalitic gneiss sample 11JD54–1 (charnockite series). (F) Tonalite sample 11JD57–2 (charnockite series).

suggesting multiple episodes of Pb-loss (Corfu et al., 2003). The obtained apparent 207 Pb/206 Pb ages range from 2443 ± 18 Ma to 2274 ± 21 Ma, which are concordant with the age of regional metamorphism in Eastern Hebei (Bai et al., 2014a; Guo et al., 2013, 2014; Nutman et al., 2011; Yang et al., 2008), and are interpreted as multiple episodes of Pb-loss that were caused by later multistage tectonothermal events. Nineteen Lu–Hf isotopic analyses were performed on laser ablation spots of dated zircon grains, of which five analyses of the captured zircon grains yield their ␧Hf (t) values (for t = 2621 Ma) range from +2.9 to +5.8, and their TDM

ages of 2805–2694 Ma (Fig. 10A). The remaining fourteen analyses were conducted from zircon domains with magmatic crystallization ages, with their ␧Hf (t) values (for t = 2523 Ma) range from −2.4 to +4.9, and TDM ages of 2927–2644 Ma (Fig. 10A). Sample 11JD25–1 (a tonalitic gneiss) of the charnockitic plagioclase gneiss series was collected for thirty five U–Pb isotopic analyses on thirty-one zircon grains. Nine analyses of oscillatory zoning domains (e.g., spot #21) (Fig. 8B) display Th/U values of 0.26–0.58 and an old weighted mean apparent 207 Pb/206 Pb age of 2615 ± 28 Ma (MSWD = 0.0065) (Fig. 9C), suggesting these

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zircon domains were captured from the country metavolcanic rocks. Other twenty-five analyses were conducted on twentytwo inner cores that show oscillatory zoning structures (e.g., spots #9, #12, #18, #23, and #31) and three homogeneous rims (e.g., spot #22) (Fig. 8B). These analyses plot on or below the concordia curve, with a linear alignment that indicates Pbloss (Corfu et al., 2003) and gives an upper intercept age of 2531 ± 25 Ma (MSWD = 0.013) (Fig. 9C). They show a weighted mean 207 Pb/206 Pb age of 2530 ± 17 Ma (MSWD = 0.017), with Th/U values of 0.18–1.16, and this weighted mean age is interpreted as the crystallization age of the magmatic precursor. The last one analysis (spot #32) from a homogeneous rim (Fig. 8B) display a Th/U value of 0.35, but the youngest apparent 207 Pb/206 Pb age of 2471 ± 47 Ma, which is considered as recording a late metamorphic or alteration event. Twenty-five Lu–Hf isotopic analyses were performed on dated zircon grains from this sample. Six analyses from captured zircon grains exhibit ␧Hf (t) values (for t = 2615 Ma) ranging from +3.5 to +7.0, with TDM ages of 2777–2644 Ma (Fig. 10B). The other nineteen analyses of zircon domains with magmatic crystallization ages show ␧Hf (t) values (for t = 2530 Ma) of +1.1 to +5.2 and TDM ages of 2794–2640 Ma (Fig. 10B). Thirty U–Pb isotopic analyses were performed on twentyfive zircon grains in sample 11JD50–5 (a tonalitic gneiss) of the charnockitic plagioclase gneiss series. Spot #13 from a blurry oscillatory-zoned inner core (Fig. 8C) indicates a Th/U value of 1.06, and apparent 207 Pb/206 Pb age of 2620 ± 20 Ma, which is much older than the ages revealed by other analyses of this sample (Fig. 9D), suggesting that this inner core was captured from ancient metavolcanic rocks. Other twenty-four analyses were conducted from inner cores that show oscillatory zoning structures (e.g., spots #7, #20, and #26) or homogeneous domains (e.g., spot #20) (Fig. 8C). They are gathered on the concordia curve (Fig. 9D), with Th/U values of 0.25–1.73 (except for spot #28 showing a low Th/U value of <0.01), and the weighted mean apparent 207 Pb/206 Pb age of 2524 ± 8 Ma (MSWD = 0.079), indicating the crystallization age of the magmatic precursor of this sample. The remaining five analyses were performed on homogeneous domains of three rims (e.g., spot #19) and two cores (e.g., spot #11) (Fig. 8C). They were plotted on or below the concordia curve, with Th/U values of 0.16–0.45 (Fig. 9D). These analyses display younger apparent 207 Pb/206 Pb ages from 2446 ± 20 Ma to 1750 ± 65 Ma, considered to record late-stage tectonothermal events. Nineteen Lu–Hf isotopic analyses were conducted on dated zircon domains of this sample, of which one was located on the captured zircon grain and exhibit the ␧Hf (t) value (for t = 2620 Ma) of +6.7, and the TDM age of 2660 Ma (Fig. 10C). The remaining eighteen analyses were performed on zircon domains recording magmatic crystallization ages and yield ␧Hf (t) values (for t = 2524 Ma) from +3.5 to +5.3 and TDM ages of 2696–2629 Ma (Fig. 10C). Sample 11JD47–2 (a granodiorite) was performed for thirty-five U–Pb isotopic analyses on twenty-nine zircon grains. Spot #8, at a bright structureless zircon domain (Fig. 8D), has the oldest apparent 207 Pb/206 Pb age in this sample at 2644 ± 34 Ma (Fig. 9E). Its Th/U value is 0.38, indicating that this zircon grain was captured from the ancient metavolcanic wall rocks in Eastern Hebei. Ten analyses were conducted on subhedral blurry oscillatory zoning inner cores (e.g., spots #7, #20, #22, and #24) (Fig. 8D). These cores show high Th/U values of 0.56–2.15, suggesting their magmatic genesis (Hoskin and Schaltegger, 2003; Rubatto, 2002). These analyses display the weighted mean apparent 207 Pb/206 Pb age of 2515 ± 22 Ma (MSWD = 0.0040) (Fig. 9E), which is considered as the crystallization age of the sample magmatic precursor. Other seventeen analyses—five of blurry oscillatory-zoned inner cores (e.g., spot #32) and twelve of structureless zircon domains, consisting of six inner cores and six rims (e.g., spots #6 and #23)

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(Fig. 8D)—have Th/U values of 0.01–0.88 and a younger weighted mean apparent 207 Pb/206 Pb age of 2455 ± 18 Ma (MSWD = 0.025), which is corresponded to the upper intercept age of 2456 ± 26 Ma (MSWD = 0.016) (Fig. 9E). The internal structural features suggest that these zircon domains are due to either metamorphism or alteration. The remaining seven analyses are from homogeneous rims (e.g., spot #21) (Fig. 8D), with Th/U values of 0.01–0.65. They were plotted on or below the concordia curve (Fig. 9E), exhibiting the youngest apparent 207 Pb/206 Pb ages from 2393 ± 33 Ma to 2259 ± 34 Ma, which are considered as recording later metamorphic or alteration events. Ten Lu–Hf isotopic analyses were conducted at zircon dating spots, showing ␧Hf (t) values (for t = 2515 Ma) from +1.7 to +4.1, and TDM ages of 2759–2670 Ma (Fig. 10D). Thirty-two U–Pb isotopic analyses were conducted on twentynine zircon grains for sample 11JD54–1 (a tonalite) from the charnockite series, with the exclusion of spot #8 that showing anomalous isotopic spectral lines. Spot #7 from an oscillatoryzoned inner core (Fig. 8E) are plotted on the concordia curve (Fig. 9F), with a Th/U value of 0.64. This analysis shows the oldest apparent 207 Pb/206 Pb age of 2847 ± 56 Ma, indicating that it was captured from ancient Mesoarchean crustal rocks. Seven analyses were performed from oscillatory-zoned inner cores (e.g., spot #1) (Fig. 8E), with Th/U values of 0.24–0.57. They are all plotted on the concordia curve, displaying a weighted mean apparent 207 Pb/206 Pb age of 2641 ± 44 Ma (MSWD = 0.39) (Fig. 9F), which is suggested to be captured from the Mesoarchean country rocks during magmatic emplacement. The largest group is made up of fifteen analyses, which were also conducted from inner cores showing oscillatory zoning structures (e.g., spots #5, #20, and #29) (Fig. 8E), with Th/U values of 0.07–1.30. These analyses exhibit a weighted mean apparent 207 Pb/206 Pb age of 2517 ± 31 Ma (MSWD = 0.020). As these analyses outnumber the analyses showing the ages of 2641 ± 44 Ma, and are more significant in the histogram of apparent 207 Pb/206 Pb ages (Fig. 9G), we suggest the weighted mean age of 2517 ± 31 Ma as the emplacement age of this sample magmatic precursor. The remaining eight analyses, comprising two at blurry oscillatory zoning inner cores and six at homogeneous rims, exhibit younger apparent 207 Pb/206 Pb ages from 2470 ± 63 Ma to 2330 ± 59 Ma, and Th/U values from 0.03 to 1.27 (e.g., spots #21 and #30) (Fig. 8E). We consider these obtained zircon ages as recording either metamorphism or alteration during later tectonothermal events. Twenty Lu–Hf isotopic analyses were conducted from the dating spots of the zircon grains from this sample, of which six analyses from captured zircon domains yield the ␧Hf (t) values (for t = 2641 Ma) range from +4.9 to +7.0, with TDM ages of 2746–2666 Ma (Fig. 10E). The remaining fourteen analyses at zircon domains with magmatic emplacement ages have ␧Hf (t) values (for t = 2517 Ma) ranging from −3.2 to +4.9, with TDM ages of 2943–2641 Ma (Fig. 10E). Twenty-nine U–Pb isotopic analyses were conducted on twenty-seven zircon grains for sample 11JD57–2 (a tonalite) from the charnockite series. All twenty-nine analyses were conducted from oscillatory-zoned zircon domains (e.g., spots #6, #7, #13, #19, #23, and #24), with high Th/U values from 0.56 to 1.13 (Fig. 8F). These analyses plot on or near the concordia curve, showing a weighted mean apparent 207 Pb/206 Pb age of 2527 ± 28 Ma (MSWD = 0.27) (Fig. 9H). This age is consistent with the magmatic emplacement ages of the abovementioned samples (11JD47–2 and 11JD54–1) from the charnockite series within the error range. Therefore, although the analyses for this sample have large errors, we still suggest the weighted mean age of 2527 ± 28 Ma as the crystallization age of the sample magmatic precursor. Twenty-four Lu–Hf isotopic analyses were conducted from the dated zircon domains, showing ␧Hf (t) values (for t = 2527 Ma) from +2.1 to +5.5, with TDM ages of 2753–2623 Ma (Fig. 10F).

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6. Discussion

6.2. Zircon U–Pb chronology

6.1. Orthopyroxene origins and implications for two contrasting charnockitic rock series

The LA–ICP–MS zircon U–Pb isotopic ages for the dated charnockitic plagioclase gneiss series indicate that the magmatic precursor of the dioritic gneiss (11JD72–1) was emplaced at 2523 ± 12 Ma, and those of the tonalitic gneisses (11JD25–1 and 11JD50–5) were emplaced between 2530 ± 17 and 2524 ± 8 Ma. These data indicate the existence of an important Neoarchean episode of dioritic–tonalitic plutonic magmatism during 2530 ± 17 and 2523 ± 12 Ma. In contrast, the magmatic precursors of the charnockite series, comprising two tonalites (11JD54–1 and 11JD57–2) and one granodiorite (11JD47–2), were emplaced between 2527 ± 28 and 2515 ± 22 Ma. The fact that they are slightly younger than the charnockitic plagioclase gneiss series indicates another episode of charnockitic magmatism, which is almost contemporaneous to but slightly younger than the dioritic–tonalitic magmatism. These data are consistent with the geological features that the charnockite series primarily has a gradational contact with the surrounding charnockitic plagioclase gneiss series (He et al., 1992; Lin et al., 1992; Zhao, 1992), whereas some veins or apophyses of charnockite series clearly intruded into charnockitic plagioclase gneiss series and truncate the gneissosity of charnockitic plagioclase gneiss series, but no obvious finegrained chilling margin is preserved (Fig. 2D). These indicate that the charnockite series emplaced at a deep level with a relatively high temperature, and slightly later than the emplacement of charnockitic plagioclase gneiss series. Zircon U–Pb isotopic data reveal that ∼2.45 Ga records are pervasively preserved in all analyzed samples from the two contrasting charnockitic rock series (Table S3), indicating this age is an important episode of regional metamorphism or an alteration event. Previous investigations on the dioritic–trondhjemitic gneisses and metavolcanic rocks in the Zunhua–Qinglong microblock suggested these rocks were subjected to amphibolite–to granulite–facies metamorphism, with a peak metamorphic timing record at ∼2.45 Ga (Bai et al., 2014a; Guo et al., 2013, 2014), which is consistent with the charnockitic rock series analyzed in this paper. Therefore, we suggest this age of ∼2.45 Ga as a signature of the most important episode of regional tectonothermal event, which led to granulite–facies metamorphism in the Yuhuzhai–Taipingzhai and Cuizhangzi areas, and amphibolite–facies dominated metamorphism is also recorded in other terrenes of the Zunhua–Qinglong microblock from Eastern Hebei.

Two types of orthopyroxenes are identified in the charnockitic rocks from the Yuhuzhai–Taipingzhai and Cuizhangzi areas of Eastern Hebei (see Section 3). The medium-fine grained anhedral orthopyroxenes are preserved in samples of both the charnockitic plagioclase gneiss series and the charnockite series, with distinct crystalloblastic texture indicating metamorphic origin. However, the coarse grained subhedral orthopyroxenes are only preserved in the charnockite series, indicating that they were probably crystallized directly from the magmatic precursors (He et al., 1992; Lin et al., 1992; Zhao, 1992). In situ trace element analyses reveal that the coarse grained subhedral orthopyroxenes in the charnockite series of granodiorite (11JD47–2) (Fig. 6C) display REE features that are totally different from those of the medium-fine grained anhedral orthopyroxenes in the charnockitic plagioclase gneiss series, e.g. the dioritic gneiss (11JD56–1, Fig. 6A) and the tonalitic gneiss (11JD72–2, Fig. 6B), as well as the granodiorite from charnockite series (11JD47–2, Fig. 6D), because of the higher medium to heavy REE contents in the coarse grained subhedral orthopyroxenes (Fig. 7A and B) that correspond to the pattern of orthopyroxene REE coefficients in dioritic to granitic melts, further indicating their magmatic genesis. The medium-fine grained anhedral orthopyroxenes of samples from the charnockitic plagioclase gneiss series clearly exhibit metamorphic reaction textures. For example, symplectic two pyroxenes around the hornblende and the reaction remains of plagioclase, hornblende, and quartz within some large anhedral orthopyroxenes, thus indicating that they were derived from dehydration reactions during granulite–facies metamorphism, involving the breakdown of hornblende and/or biotite minerals (Liu et al., 1991; Lin et al., 1992). These orthopyroxenes exhibit left-inclined REE patterns showing essentially no Eu anomalies in the whole-rock normalized REE spectrums, which are totally different to partition coefficient curve of magmatic orthopyroxenes (Fig. 7B), further supporting that the medium-fine grained anhedral orthopyroxenes have metamorphic origins. The petrographic and chemical features of medium-fine grained anhedral orthopyroxenes from the charnockitic plagioclase gneiss series reveal that these orthopyroxenes were all derived from granulite–facies metamorphism, and no magma genetic orthopyroxene is preserved in the samples of the charnockitic plagioclase gneiss series. Similarly, in the samples of the charnockite series, medium-fine grained anhedral orthopyroxenes have concordant chemical features to those metamorphic genetic orthopyroxenes of samples from the charnockitic plagioclase gneiss series. In addition, some remains of early minerals are preserved in some bigger anhedral orthopyroxenes, revealing that these samples from the charnockite series also experienced the regional granulite–facies metamorphism in Eastern Hebei. Combining petrographic features with in situ trace element analyses of the coarse grained subhedral orthopyroxenes from samples of the charnockite series, we suggest that these coarse grained subhedral orthopyroxenes were directly crystallized from magmatic precursors. These petrographic and chemical characteristics for the orthopyroxenes suggest that the charnockitic plagioclase gneiss series belong to the metamorphic type, whereas the charnockite series belong to the magmatic type according to the previous classification by Frost and Frost (2008) and Rajesh and Santosh (2012). This classification is only based on orthopyroxene origins, and in fact, the two contrasting charnockitic rock series both originated from plutonic igneous rock associations, and then they were metamorphosed in a granulite–facies condition.

6.3. Petrogenesis 6.3.1. Element mobility during granulite–facies metamorphism Petrological data presented in this study show that these two contrasting charnockitic rock series experienced granulite–facies metamorphism (Fig. 3), as proved by abundant metamorphogenetic pyroxenes in samples from the two lithological series. Liu (1995, 1996) and Liu and Lin (1992) reveal that the peak metamorphism for the granulites in the Zunhua–Qinglong microblock occurred at ∼850 ◦ C and ∼10 kbar, and the fluid inclusions display their compositions of CO2 dominated, with almost pure CO2 in the peak stage fluid inclusions, indicative of an anhydrous metamorphic condition. Rutter and Wyllie (1988) reported water vapor-absent melting experimental data for a tonalite sample at 10 kbar, indicating that the melting begins at ∼825 ◦ C, while the melt percentage is less than 10% till ∼875 ◦ C. Similarly, Skjerlie and Johnston (1992) did water vapor-absent melting experiment at 10 kbar for another Archean tonalitic gneiss sample, revealing that the melting begins at ∼875 ◦ C, and the melt abundance is still less than 10% up to 950 ◦ C. When the melt percentage is less than 10%, the produced melts are preserved at the boundaries of minerals instead of gathering as independent magmas

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(Arzi, 1978). Hence the whole-rock chemical analyzed data can still represent the compositions for the protoliths of the two contrasting charnockitic rock samples. This is also evidenced by the geological features that the Zunhua–Qinglong microblock preserves hardly any anatectic potassic granite veins (He et al., 1992; Lin et al., 1992). Furthermore, Bai et al. (2014a) reported wholerock chemical compositions for eight Neoarchean dioritic gneiss samples in Gualanyu–Shangying area, with five of them subjected to granulite–facies metamorphism, and the other three were amphibolite-facies metamorphosed. We evaluated the major and trace element analyzed results for these eight samples (see the online Supplementary Fig. S1), which indicate that contents of major oxide (e.g. CaO, Al2 O3 , MgO, Fe2 O3T , and SiO2 ), REEs, and other trace elements (e.g. Sr, Nd, Sm, Zr, and Hf) exhibit no obvious difference between the granulite–facies and amphibolite-facies metamorphosed samples, further suggesting that there was no obvious composition loss during the granulite–facies metamorphism. Supplementary Fig. S1 related to this article can be found, in the online version, at http://dx.doi.org/10.1016/j.precamres.2015. 06.004 Granulite–facies metamorphism commonly results in the mobilization of large-ion lithophile elements (LILEs), including Rb, Ba, U, Th, and K, which is also exhibited in the studied samples by intensively negative U and Th anomalies, and Rb in partial samples (Fig. 5). In addition, Bai et al. (2014a) and Guo et al. (2013) reported that Neoarchean dioritic–trondhjemitic gneisses and metavolcanic rocks in Eastern Hebei also exhibit mobilizations of LILEs, further indicating that these elements should be excluded in petrogenesis discussion. However, previous investigations suggest that REEs (e.g., Yb), high field strength elements (HFSEs) (e.g., Zr and Hf), and some major compositions (e.g., Ca, Al, Fe, and Mg) are less subjected to modification during metamorphism (Kerrich et al., 1998; Pearce et al., 1992; Wang et al., 2012a, 2013a, 2015). This is also verified in the samples from regional Neoarchean metamorphic basement rocks (Bai et al., 2014a; Guo et al., 2013). In addition, the analyzed samples display no obvious Ce anomaly (Fig. 5A, C, E, and G), further indicating that the REEs remained stable during the metamorphism (Polat and Hofmann, 2003; Polat et al., 2002). Therefore, these essentially inactive elements can be chosen to identify petrogenesis and tectonic settings of the charnockitic plagioclase gneiss series and the charnockite series from the Zunhua–Qinglong microblock in Eastern Hebei. 6.3.2. Charnockitic plagioclase gneiss series The charnockitic plagioclase gneiss series in the Yuhuzhai– Taipingzhai and Cuizhangzi areas have chemical compositions of dioritic and tonalitic gneisses. These gneisses display similar chondrite-normalized REE patterns (Fig. 5A and C) and depletions in Nb, Ta, and Ti (Fig. 5B and D). These factors indicate that their magmatic precursors might both be parts of a cogenetic magma evolution series. In addition, LA–ICP–MS zircon U–Pb dating reveals that their precursors have contemporaneous emplacement ages from 2530 ± 17 to 2523 ± 12 Ma, further suggestive of their close time correlations. Considering Hf as an incompatible and Yb as a compatible element in dioritic to granitic magmas (see Rollinson, 1993, and references therein), the dioritic and tonalitic gneiss samples construct an upward curve that shows a mixing or fractional crystallization trend in the Hf versus Hf/Yb plot (Fig. 11A) (Schiano et al., 2010). The samples also display a curved trend of partial melting or fractional crystallization in the 1/Yb versus Hf/Yb plot (Fig. 11B) (Schiano et al., 2010). This indicates the magmatic precursors of these dioritic and tonalitic gneisses experienced a primary fractional crystallization process during the evolution process. These dioritic and tonalitic gneisses display their mineral assemblages as a

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sequential sequence, with increasing quartz contents but decreasing two pyroxenes and hornblende contents from dioritic to tonalitic gneisses (Table 1). In addition, some hornblendes and pyroxenes are concentrated as a possible accumulation phase in the field outcrops (Fig. 2D). Petrographic and in situ trace elementary research has revealed that the pyroxene grains are granulite–facies metamorphic products of the hornblende or biotite minerals in dioritic and tonalitic gneisses, thus suggesting hornblende or biotite are probably primary crystallization fractionation phases during the magmatic evolution process. Hornblendes are characterized by LREE and week Eu depletions, and HREE enrichments that are particularly distinct for Dy, Ho, Er, Tm, and Yb in a dioritic to granitic magmatic system (see Rollinson, 1993, and references therein). Although the dioritic and tonalitic gneiss samples exhibit similar chondrite-normalized REE patterns, tonalitic gneisses are more enriched in LREE, and slightly more depleted in HREE from Dy to Yb (Fig. 5A and C; Table S1). This supports that hornblende is the primary fractionated phase during the magmatic evolution process. However, the Eu anomalies of the tonalitic gneisses (EuN /EuN * = 0.73–1.00) are similar to those of the dioritic gneisses (EuN /EuN * = 0.65–1.16), suggesting that the best interpretation is involvement of both hornblende and plagioclase during fractionation crystallization processes. In this situation, positive Eu anomalies of fractionated plagioclases may make up for the negative Eu anomalies of separated hornblendes, eliminating or reducing Eu anomalies of the rock samples. It is confirmed by petrological investigations that plagioclase contents for dioritic gneisses (45%–54%) are a little higher than those for tonalitic gneisses (43%–51%). Lu–Hf isotopic analyses reveal that the tonalitic gneiss samples 11JD25–1 and 11JD50–5 have positive ␧Hf (t) values for magmatic zircons, from +1.1 to +5.2, and from +3.5 to +5.3, respectively (Fig. 10B and C). These data indicate their depleted mantle affinity. The dioritic gneiss sample 11JD72–1 exhibits various ␧Hf (t) values from −2.4 to +4.9, with TDM ages of 2927–2644 Ma (Fig. 10A), suggesting minor ancient crustal materials were involved in the magmatic evolution process of these dioritic–tonalitic magmas. Given that no significant mixing process is revealed and the lack of gabbroic member, we consider that these dioritic–tonalitic magmas were contaminated by ancient continental crustal materials. Previous experimental investigations revealed that crust-derived melts with a source composition of garnet-amphibolite or eclogite display Mg# values no higher than ∼0.4 (Kelemen, 1995; Stern and Kilian, 1996). The samples from the charnockitic plagioclase gneiss series exhibit low SiO2 contents of 54.1–65.2%, but various MgO contents and Mg# of 1.93–5.36% and 0.36–0.55, respectively. Some of these obtained MgO contents are higher than those of the experimental crust-derived melts (Fig. 4C), indicating they were derived mainly from partial melting of depleted mantle rather than juvenile crustal basalts (Kelemen, 1995; Martin et al., 2005; Rapp et al., 1999; Rapp and Watson, 1995; Stern and Kilian, 1996). In addition, the dioritic gneisses have low (La/Yb)N values of 3.54–15.06, and chondrite–normalized REE patterns accordant with those of the mantle derived magmas with low melting degrees of 1–25% (Martin, 1986, 1987, 1994) (Fig. 5A). These findings further support a supposition of their sources being based on depleted mantle. Although pure mantle sources produce basaltic magmas rather than dioritic magmas, previous studies have revealed that a mantle source that is strongly modified by slabderived felsic melts can provide dioritic magmas, such as low-silica adakitic and sanukitoid magmas (Martin et al., 2005, 2010). Therefore, we suggest the magmatic precursors of these Neoarchean charnockitic plagioclase gneiss series originated from a depleted mantle source that had been modified by reaction with silicarich melts/fluids, which were probably derived from a subducted slab.

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Fig. 11. Petrogenetic discrimination diagrams for two types of charnockitic samples. (A) Hf (ppm) versus Hf/Yb plot, showing distinct curves that reflect compositional evolution due to partial melting and mixing or fractional crystallization (Schiano et al., 2010). The inset is a schematic CI versus CI /CC plot. (B) 1/Yb versus Hf/Yb plot, showing different curves between partial melting or fractional crystallization and mixing evolutionary process (Schiano et al., 2010). The inset is a schematic 1/CI versus CI /CC plot, with CI and CC in (A) and (B) being an incompatible and a compatible element, respectively. (C) (La/Yb)N versus YbN plot summarizing different models for basalt melting (Martin, 1986, 1987), with the source of an average Archean tholeiite (AT) transformed into garnet-free amphibolite (G0 ), 10% (G10 ) and 25% (G25 ) garnet-bearing amphibolite, and eclogite (E). The gray field represents the component of Archean TTG gneisses. (D) TiO2 versus MgO plot showing different fields of high- and low-Ti sanukitoids (Martin et al., 2010). Symbols are the same as those given in Fig. 4.

Furthermore, the charnockitic plagioclase gneiss series in the Yuhuzhai–Taipingzhai and Cuizhangzi areas display mainly a gradual transition to the dioritic–trondhjemitic gneisses, but were intruded by trondhjemitic veins locally (Fig. 2C). This suggests they are closely related in petrogenesis. Bai et al. (2014a) reported detailed geochronological and whole-rock geochemical data for the dioritic–trondhjemitic gneisses, revealing that they have magmatic emplacement ages of 2535 ± 23–2513 ± 8 Ma, and a primary genetic mechanism of fractional crystallization, and the dioritic member had a modified depleted mantle source. In addition, the charnockitic plagioclase gneiss series, together with the samples of trondhjemitic gneisses in the Taipingzhai area reported by Bai et al. (2014a), constitute a TT trend in the K2 O–Na2 O–CaO diagram (Fig. 4D) (Barker, 1979). These two gneiss series have uniform magmatic emplacement ages, genetic mechanisms, and magmatic sources, and we therefore suggest that the precursors of the charnockitic plagioclase gneiss series and dioritic–trondhjemitic gneisses formed from the evolution of cognate magmatism, indicating a widespread Neoarchean dioritic–tonalitic–trondhjemitic (DTT) magmatism in the Zunhua–Qinglong microblock of Eastern Hebei. 6.3.3. Charnockite series In contrast to the charnockitic plagioclase gneiss series, the charnockite series is characterized by massive to week gneissic structure, coarse grained texture, and magma genetic orthopyroxenes. The samples of this series also have higher contents of K-feldspar (3–12%), and construct a CA trend in the K2 O–Na2 O–CaO

diagram (Fig. 4D) (Barker, 1979), suggesting that their magmatic precursors might be derived from more potassic sources. U–Pb zircon analyses reveal that the charnockite series and the charnockitic plagioclase gneiss series have similar magmatic emplacement ages (2527 ± 28–2515 ± 22 Ma and 2530 ± 17–2523 ± 12 Ma, respectively), thus excluding the possibility that the charnockite series originated from remelting of the charnockitic plagioclase gneiss series, which was proposed by previous investigations (He et al., 1992; Lin et al., 1992; Zhao, 1992). In addition, the interpretations for element mobility (see section 6.3.1.) have excluded the possibility of high degree partial melting during the granulite–facies metamorphism, further proving that the charnockite series were not derived from remelting of the charnockitic plagioclase gneiss series. The samples from the charnockite series have chemical compositions of tonalite and granodiorite, which exhibit similar magmatic emplacement ages and trace element features, including chondrite-normalized REE patterns (Fig. 5E and G) and primitive mantle-normalized multielement patterns (Fig. 5F and H), suggesting that they have closely genetic correlations. Geochemical modeling reveals that the tonalite and granodiorite samples from the charnockite series construct a partial melting trend in the Hf versus Hf/Yb plot (Fig. 11A), suggesting their magmatic precursors were directly derived from common sources (Schiano et al., 2010). These rocks have SiO2 contents (60.1–69.2%) higher than those of the charnockitic plagioclase gneiss series, indicating their sources were more related to crustal materials than directly depleted mantle. Previous investigations have revealed that

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magmatic orthopyroxenes can only be crystallized from anhydrous magmas with high temperatures above ca. 850–1000 ◦ C (Frost and Frost, 2008; Frost et al., 2000; Madsen, 1977). In contrast, the partial melting of depleted mantle involving slab melts/fluids always produces wet and cool magmas. Therefore, this finding rules out the existence of depleted mantle-based sources. Charnockitic magmas could be formed by crustal melting or have incorporated a large component of crustal melt, according to previous researches (Bhattacharya and Sen, 2000; Kar et al., 2003; Rajesh and Santosh, 2004; Young et al., 1997; Zhang et al., 2010). In addition, the charnockite series display high (La/Yb)N values of 14.61–42.69, and they follow the model for basalt melting with a source component of Archean tholeiite transformed into 25% garnet bearing amphibolite in the (La/Yb)N versus YbN plot (Martin, 1986, 1987). This further suggests the samples’ crustal origins. Lu–Hf isotopic analyses for these rocks reveal that samples 11JD47–2 and 11JD57–2 display positive ␧Hf (t) values from +1.7 to +4.1, and +2.1 to +5.5, respectively, indicating their magmatic precursors were mainly derived from partial melting of juvenile crustal basalts. Sample 11JD54–1 displays various ␧Hf (t) values from–3.2 to +4.9, with the TDM ages of 2943–2641 Ma, reflecting the minor involvement of ancient crustal constituents. Although the magmatic precursors of these charnockite series were mainly derived from crustal sources, most of the analyzed samples exhibit MgO contents and Mg# ratios higher than those of the experimental crustal basalt derived melts (Martin et al., 2005; Kelemen, 1995; Stern and Kilian, 1996), indicating that their primary magmas were contaminated by mantle materials during the evolutionary processes. These rocks are characterized by high (La/Yb)N values of 14.61–42.69 and Mg# from 0.43 to 0.52 (except for sample 11JD68–2, which has a low Mg# of 0.28). They constitute a CA trend in the K2 O–Na2 O–CaO diagram (Fig. 4D), resembling those of the sanukitoids, which are considered to be formed by reaction between depleted mantle and subducted slab components (Martin et al., 2005, 2010; Shirey and Hanson, 1984). Martin et al. (2010) classified sanukitoids into high-Ti and lowTi types, suggesting that the former is formed by partial melting of slab melt–metasomatized mantle peridotites, whereas the latter originated from slab melts that were strongly contaminated by mantle peridotites. Most of the analyzed samples from the charnockite series plot in the field of low-Ti sanukitoids, except for sample 11JD57–2 (Fig. 11D), and their chondrite-normalized REE patterns are similar to the average values of low-Ti sanukitoids with SiO2 < 62% (Fig. 5E) (Martin et al., 2010), although the analyzed samples exhibit lower REE contents, which might have resulted in their higher SiO2 contents of 60.1–69.2%. These charnockite series are closely associated with the regional DTT gneisses that were emplaced in a Neoarchean arc tectonic setting (Bai et al., 2014a; Nutman et al., 2011). The contemporary metavolcanic rocks associated with these granitoid rocks in the same region are characterized by typical arc associations of N-MORB basalts, Nb-rich basalts, low-Ti tholeiitic basalts, and calc-alkaline basaltic–andesitic–dacitic–rhyolitic rocks, further suggesting that the charnockite series were also emplaced in a subduction-related environment (Guo et al., 2013; 2015). Therefore, the charnockite series have magmatic precursors similar to the low-Ti sanukitoid magmas and originated from subducted slab-derived melts that were strongly contaminated by depleted mantle during the rising process. 6.4. Implications for tectonic setting Many investigations suggested that the interior of the Eastern Block was under a plume tectonic setting (Geng et al., 2012; Polat et al., 2006; Wu et al., 2014a, 2014b; Zhai and Santosh,

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2013; Zhao et al., 2005, 2012), whereas the north margin (including Chengde, Zunhua–Qinglong microblock of Eastern Hebei, and Chaoyang–Fuxin and Fushun–Qingyuan ranges of Liaoning) is considered to be formed in subduction-related environments in Neoarchean (Bai et al., 2014a, 2014b; Guo et al., 2013; Liu et al., 2010, 2011a, 2011b; Nutman et al., 2011; Wang et al., 2011, 2012b, 2013b). Our new whole-rock geochemical, in situ orthopyroxene trace element, and zircon U–Pb and Lu–Hf isotopic data suggest that the charnockitic plagioclase gneiss series was derived from partial melting of depleted mantle components that were metasomatized by slab-derived melts/fluids, and the charnockite series originated from slab-derived melts that were strongly contaminated by depleted mantle peridotites. The charnockite series are akin to low-Ti sanukitoids, which are pervasively linked to arc magmatism (Heilimo et al., 2010; Lobach-Zhuchenko et al., 2008; Ma et al., 2014; Martin et al., 2005, 2010; Stevenson et al., 1999). As such, we consider that the two types of charnockitic rock series may be formed in a continental back-arc-related setting, integrating with major lines of evidence as bellow:

(1) Investigations on Neoarchean metavolcanic rocks reveal that they are characterized by a metamorphic suite consisting mainly of tholeiites, calc-alkaline basalts to basaltic andesites, and dacites, associated with small amounts of Nb-rich basalts and N-MORB basalts, with the eruption ages of ∼2614–2518 Ma in the Zunhua–Saheqiao–Qinglong range located in the north of Zunhua–Qinglong microblock. These metamorphic volcanic rocks are not produced in mantle-plume tectonic regime but in slab subduction related tectonic regimes due to a mass of metamorphic calc-alkaline rock with a consecutive composition with identified Nb, Ta and Ti depletions. Studies on petrogenesis indicate the calc-alkaline magmas were derived from partial melting of mantle wedge rocks that were altered by subducted slab-fluids or melts, and therefore, they were formed in a subduction-related tectonic setting (Guo et al., 2013, 2014; Zhang et al., 2012b); (2) Guo et al. (2013) reveal that the N-MORB-like basalts located in the south of the Zunhua–Qinglong microblock have eruption ages (∼2525 Ma) younger than the island-arc basalts and andesites in the north (∼2614–2518 Ma). This eruptive sequence of magmatic precursors of metamorphic volcanic rock assemblages is consistent with the typical back-arc evolution process; (3) Nutman et al. (2011) indicate the Neoarchean metamorphic volcanic rocks and granitoid gneisses were produced in magmatic arc at a convergent plate boundary based on the lithological assemblages in Eastern Hebei and their zircon U–Pb–Hf isotopic systematics, combining with trace element characteristics. (4) Previous investigations revealed that charnockitic magmatism can be formed in Archean (Pouclet et al., 2007; Rajesh, 2012; Shaji et al., 2014), Proterozoic (Peng et al., 2012; Yang and Santosh, 2015), and Phanerozoic (Ma et al., 2013; Yin et al., 2013; Zhang et al., 2011) subduction-related environments. These magmas were generally derived from partial melting of the subducted slab or arc root materials that were heated by the upwelling hot, dry, and potassium enriched magmatism from deep mantle in specific dynamic mechanisms of slab window (Yin et al., 2013; Zhang et al., 2011), ridge subduction (Peng et al., 2012; Yang and Santosh, 2015; Zhang et al., 2010), or back-arc extension (Chiarenzelli et al., 2010; Fernández et al., 2008; Keppie and Ortega-Gutiérrez, 2010). Therefore, the magmatic precursors of the charnockite series in the Yuhuzhai–Taipingzhai area were likely emplaced in a Neoarchean back-arc-related environment.

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(5) The magmatic precursors of the charnockitic plagioclase gneiss series, and those of the dioritic–trondhjemitic gneisses reported by Bai et al. (2014a), experienced fractional crystallization-dominated evolutionary processes and require a relatively stable tectonic environment. Zircon Lu–Hf systematics reveal that the primary and evolutionary magmas of these charnockitic plagioclase gneiss series only experienced slight contaminations of older crustal materials, with only one captured zircon grains older than ∼2.7 Ga (Table S3). All these Neoarchean orthogneisses are also characterized by clearly negative Nb, Ta and Ti anomalies; therefore, their tectonic environment is likely to be an arc to back-arc tectonic setting; (6) Furthermore, the Zunhua–Qinglong microblock is adjacent to the Caozhuang complex in the south that preserves plenty of old detrital zircon grains up to ∼3.8 Ga in age (Liu et al., 1992, 2013; Wilde et al., 2008). The charnockitic plagioclase gneiss series sample 11JD72-1 and charnockite series sample 11JD54-1 exhibit negative to positive ␧H f(t) values of −2.4 to +4.9 and −2.9 to +5.2, respectively, suggesting that ancient crustal materials were involved in their magmatic evolutionary processes. Slightly negative ␧Hf (t) value signatures are also revealed in the Neoarchean quartz-rich trondhjemitic gneisses (∼2535 Ma) of −3.4 to +4.1 (Bai et al., 2014a), and the metavolcanic rocks (∼2614–2514 Ma) of −2.5 to +9.0 (Guo et al., 2013, 2014), supporting that the weakly crustal assimilation was involved in an active continental marginal back-arc setting. Our unpublished data suggest that the felsic paragneisses in north side of Zunhua town were deposited in Late Neoarchean, with detrital zircon age signatures up to ∼3.4 Ga, which proves the existence of ancient continental crust in the south part of the Zunhua–Qinglong microblock; (7) Zhang et al. (2012b) indicated the Shirengou metavolcanic rocks (∼2553–2541 Ma) associated with BIFs from the north Zunhua area were formed in a Neoarchean arc-related basin; (8) Liu et al. (2014) suggest that the metasedimentary rocks from the Shuangshanzi and Qinglonghe Groups in Eastern Hebei were formed in a continental intra-arc basin, and these tectonic implications are also corresponded to the features of the active continental marginal back-arc related tectonic settings. Based on above essentially geological facts and recent investigations in the Neoarchean metamorphic volcanic rock assemblages, BIF deposits, metamorphic sedimentary sequences, and plutonic dioritic–trondhjemitic gneisses, together with our studies, the two contrasting charnockitic rock series are considered to be produced in an arc to back-arc tectonic setting from north to south, and the Zunhua–Qinglong microblock was likely in a Neoarchean back-arcrelated environment. 7. Conclusions Petrological, geochemical, geochronological, and Lu–Hf isotopic data obtained from two contrasting charnockitic rock series in the Yuhuzhai–Taipingzhai and Cuizhangzi Neoarchean basement range of Eastern Hebei have led us to the following major conclusions. (1) These charnockitic rock series can be classified as the charnockitic plagioclase gneiss series and charnockite series on the basis of petrographic and chemical features. All the orthopyroxenes in the former are metamorphic orthopyroxenes, which were derived from hornblendes or biotites under a granulite–facies condition. Whereas the later contains not only metamorphic orthopyroxenes, but also magmatic orthopyroxenes that crystallized directly from charnockitic magmatism.

Magmatic orthopyroxenes are characterized by high contents of the medium to heavy REE and strongly negative Eu anomalies, which are significantly distinguished from metamorphic orthopyroxenes showing lower contents of the medium to heavy REE and hardly any Eu anomalies. (2) Magmatic precursors of the charnockitic plagioclase gneiss series were emplaced between 2530 ± 17 and 2523 ± 12 Ma, whereas those of the charnockite series were emplaced between 2527 ± 28 and 2515 ± 22 Ma, suggesting two contemporary periods of Neoarchean granitoid magmatism. This was followed by a ∼2.45 Ga regional tectonometamorphic and alteration event leading to granulite–facies metamorphism. (3) Magmatic precursors of the charnockitic plagioclase gneiss series were derived from the partial melting of depleted mantle materials that had been metasomatized by slab-derived melts/fluids and experienced fractional crystallization of hornblende and plagioclase during the evolutionary processes. The magmatic precursors of the charnockite series formed from partial melting of subducted oceanic slabs, and were contaminated by mantle peridotites. (4) Magmatic precursors of these two contrasting charnockitic rock series were formed in a back-arc-related tectonic environment at a convergent plate margin along the north margin of the Eastern Block.

Acknowledgements We wish to thank Bin Yang, Fang Ma, and Libing Gu for their assistance in the whole–rock geochemical analyses at the Key Laboratory of Orogenic Belts and Crustal Evolution, Ministry of Education, School of Earth and Space Sciences. We also wish to thank Li Su for the LA–ICP–MS zircon U–Pb isotope analyses at the Geological Lab Center, China University of Geosciences, Beijing, and Zhaochu Hu for the ICP–MS zircon Lu–Hf isotope analyses at the state Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences, Wuhan. This study was financially supported by the National Natural Science Foundation of China (Grants: 41472165, 41272209, 41121062, and 41072143).

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