Journal of Earth Science  2018, Vol. 29 Issue (5): 1254-1275   PDF    
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Tectonothermal Records in Migmatite-Like Rocks of the Guandi Complex in Zhoukoudian, Beijing: Implications for Late Neoarchean to Proterozoic Tectonics of the North China Craton
Yating Zhong1, Chuan He1, Neng-Song Chen1,2,3, Bin Xia1,4, Zhiqiang Zhou1,4, Binghan Chen1, Guoqing Wang1    
1. School of Earth Sciences, China University of Geosciences, Wuhan 430074, China;
2. Center for Global Tectonics, China University of Geosciences, Wuhan 430074, China;
3. State Key Laboratory of Biogeology and Environmental Geology, China University of Geosciences, Wuhan 430074, China;
4. State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences, Wuhan 430074, China
ABSTRACT: Migmatite-like rocks transformed from strongly metamorphosed and deformed enclave-bearing felsic plutons usually make people confuse with the true migmatites and mistake in interpretation of their petrogenesis and tectonic implications. Here we report a suite of rocks that have long been called as migmatites from the Guandi complex in Zhoukoudian region, southwest of Beijing. The rocks are dominated by felsic gneisses with garnet-free amphibolites. Field occurrence, petrography and geochemistry indicate that the felsic gneisses and amphibolites were metamorphosed from protoliths of intermediate-acid and basic igneous rocks, respectively. New LA-ICP-MS zircon U-Pb dating and geothermobarometry study further reveal that precursor magmas of the two types of rocks were emplaced at 2.54-2.56 Ga and the rocks subsequently underwent medium P/T-type metamorphism with upper amphibolite facies conditions of 0.55-0.90 GPa and 670-730℃ at~2.48-2.50 Ga. Geochemically, precursor magmas of the amphibolites were suggested to be derived from an enriched lithospheric mantle source in continental arc setting, and those of the felsic gneisses are characterized by tonalitic to trondhjemitic magmas that are usually considered to be generated by partial melting of hydrated, thickened metamorphosed mafic crust with garnet as residues, suggesting that the rock associations are not of true migmatites but migmatite-like rocks. Our study reveal that protoliths of the migmatite-like rocks from the Guandi complex, were likely formed via magmatism in a continental arc setting, followed by accretion and collision of the continental arc as well as the intro-oceanic arc terranes to the Eastern Block of the North China Craton in the transition from the Late Neoarchean to Early Paleoproterozoic.
KEY WORDS: Zhoukoudian    migmatite-like rocks    magmatism and metamorphism    tectonic evolution    Late Neoarchean to Early Paleoproterozoic transition    North China Craton    

0 INTRODUCTION

Migmatite was coined by Serderholm (1907) to describe a composite rock with hybrid of metamorphic and magmatic features. Partial melting of metamorphic rocks at high-temperature conditions is the fundamental mechanism for migmatization (e.g., Sawyer, 2008a, b; Kriegsman, 2001; Ashworth, 1985; Mehnert, 1968). Crystallization of melt fractions in various degrees as felsic neosomes or leucosome inside paleosomes or mesosomes (non-melted part; i.e., metamorphic protolith) will generate a variety of migmatites (e.g., Sawyer, 2008a, b, 1999). Along with increasing knowledge of this rocks, people have noted some rocks that appear similar outcrop-scale morphology of migmatites turn out not to be the true migmatites but migmatite-like rocks.

The "migmatite-like rock" is referred to such kind of rocks that have light-colored patches, layers, and veins of felsic compositions within a darker host with complex morphologies, produced by other process rather than by partial melting (Sawyer, 2008a). A typical example is the metamorphosed intrusive mingling rocks with their protoliths formed by injection of felsic magma into the preexisting different compositional rocks that finally become enclaves with lobate and cuspate boundaries (e.g., Barbarin, 1988; Mason, 1985). However, their primary structures and intrusive contact boundary can be blurred or eliminated by strong metamorphism and deformation in post-magmatism, generating secondary structures, like deformed structure or typical migmatite-like structures (Sawyer, 2008a; Hopgood, 1999), making people mistake them for diatexite of migmatites (e.g., Koshida et al., 2016; Nehring et al., 2009). The "migmatite-like rock" has magmatic genetic components and thus would have significant implications for magma generation, crustal evolution and shed light on the regional tectonic evolution.

A small portion of basement rocks of the North China Craton, traditionally called Guandi complex in Chinese literatures (Yuan et al., 2016; Liu Y S et al., 2008; Chen and Wang, 2006; Yan et al., 2005; Wang et al., 1990; Liu and Wu, 1987) is sporadically exposed in Zhoukoudian of the Southwest Beijing City. The Guandi complex developed migmatite-like rocks on some outcrops, with felsic gneisses as the host components and minor amphibolites. Up till now, three schools of thought have been brought for the genesis of these rocks: (1) strongly deformed and metamorphosed rocks of Precambrian strata (He, 1936); (2) marginal migmatization of the Mesoproterozoic supracrustal successions during the peak contact metamorphism in Cretaceous (Guo, 1985); (3) regional migmatization of the Precambrian basement in the Neoarchean (Liu B et al., 2008; Zhao, 2003; Wang and Chen, 1996). Furthermore, tectonic settings for the protoliths of these rocks are also poorly understood. In this contribution, a comprehensive study has been carried out on geology, petrography, mineralogy, geochemistry, geochronology and metamorphic P-T conditions of these rocks, attempting to (1) determine the protolith rocks and their formation ages as well as timing of the metamorphism that had undergone; (2) constrain the petrogenesis of protolith rocks; (3) ascertain whether they are true migmatites or migmatite-like rocks; (4) discuss their tectonic implications for evolution of the North China Craton in the transition from Late Neoarchean to Early Paleoproterozoic.

1 GEOLOGICAL SETTING 1.1 Geological Background

The North China Craton (NCC) is bounded by the Late Paleozoic Central Asian orogenic belt on the north, the Early Paleozoic Kunlun-Qilian orogenic belt on the west, the Early Paleozoic to Early Mesozoic Qinling-Dabie orogenic belt on the south and the Su-Lu ultrahigh-pressure metamorphic belt on the east (Fig. 1a). It is tectonically divided into the Eastern and Western blocks separated by the Trans-North China Orogen (Fig. 1a; Zhao et al., 2005, 2001). The Eastern Block consists of Archean-Paleoproterozoic basements and Mesoproterozoic- Paleozoic covers. The basement exposes predominantly the Late Mesoarchean-Neoarchean rocks, with minor but the oldest rocks of the NCC formed at ~3.8 Ga (e.g., Zhai, 2014; Zhao and Zhai, 2013; Zhai and Santosh, 2011; Liu et al., 2007; Zhao et al., 2001).

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Figure 1. (a) Tectonic framework of the North China Craton (modified after Zhao et al., 2005, with > 2.8, > 2.7, and ~2.5-2.6 Ga crystalline basement sketched from Wang W et al., 2015). Abbreviations for the metamorphic complexes: AB. Anshan-Benxi granite-greenstone belt; CD. Chengde; DF. Dengfeng; EH. eastern Hebei; FP. Fuping; HA. Huai'an; HS. Hengshan; JD. Jiaodong terrain; LS. Lushan; NH. northern Hebei; NL. northern Liaoning; QY. Qingyuan greenstone; SJ. southern Jilin; SL. southern Liaoning; TH. Taihua; WL. western Liaoning; WS. western Shandong; WT. Wutai granite-greenstone belt; ZH. Zanhuang; ZT. Zhongtiao. (b) Simplified geological sketch map of the Guandi complex (modified after He et al., 2005) and the sample localities.

The Late Mesoarchean-Neoarchean rocks in the NCC were mainly formed in ~2.9-2.7 and ~2.6-2.5 Ga periods. The ~2.9- 2.7 Ga rocks are featured in komatiite-basalt-dacite in supra-crustal sequences and dominant of TTGs (tonalite-trondhjemite-granodiorite) with some granitic-granodioritic gneisses (Jia et al., 2016; Wan et al., 2015, 2014b, 2012; Wu et al., 2013; Liu et al., 2009; Jahn et al., 2008). The komatiites are mainly exposed in Taishan complex in West Shandong, the eastern part of NCC (Cheng and Kusky, 2007; Polat et al., 2006; Zhang et al., 1998). Zircon Hf isotopes revealed significant crustal growth during ~2.9-2.7 Ga, which is related to the global growth of Earth's crust recognized from other regions (Wan et al., 2014a). The ~2.6-2.5 Ga rocks were formed as a result of significant crustal reworking, with minor crustal growth at that time in the NCC. The rocks comprises widespread and voluminous magmatic rocks (now mainly orthogneisses of TTG gneisses) with minor gabbros and BIF-bearing supracrustal beds or lenses in the high-grade metamorphic terranes (Wan et al., 2018, 2015, 2014b; Zhou et al., 2018; Li and Wei, 2017; Kusky et al., 2016; Yip, 2016; Geng et al., 2012). The orthogneisses have Hf model ages peaked at ~2.7 Ga (Geng et al., 2012).

Intensive high-grade regional metamorphism occurred over the NCC (e.g., Wang et al., 2014; Wan et al., 2012; Zhai and Santosh, 2011; Jahn et al., 2008), with P-T-t paths showing both clockwise style (e.g., Liu S W et al., 2018; Lu et al., 2017; Liu S J et al., 2015; Zhai et al., 2005) and counter-clockwise patterns (Duan et al., 2017; Wu, 2015; Ge et al., 2003, 1994; Zhao et al., 2001, 1998; Chen et al., 1994; Li, 1993), probably suggesting either continental collision (e.g., Liu et al., 2018; Lu et al., 2017; Kusky et al., 2016; Wang J P et al., 2015; Zhai, 2014), oceanic plate subduction (e.g., Zhou et al., 2018; Bai et al., 2016; Yang et al., 2016; Peng et al., 2015; Diwu et al., 2011), or mantle plume activity (e.g., Li and Wei, 2017; Wang et al., 2014; Zhai and Santosh, 2013, 2011; Zhao and Zhai, 2013; Geng et al., 2012; Yang et al., 2008) tectonic regimes. The metamorphism resulted in local or regional migmatization at different scales (Liu J H et al., 2015; Liu S J et al., 2015; Wang W et al., 2015; Wu et al., 2013; Nutman et al., 2011; Yang et al., 2008).

1.2 Guandi Complex

The Guandi complex is located in Zhoukoudian region, about 40 km southwest to Beijing City, in the western margin of the Eastern Block of the NCC (Fig. 1a). The complex dominantly consists of Late-Neoarchean basement rocks spreading sporadically on the south, northeast and east of the Early Cretaceous (~0.13 Ga) Fangshan plutons (Fig. 1b). It is composed of felsic gneiss, leptynite, amphibolite and migmatite (Liu B et al., 2008; Chen and Wang, 2006; Wang et al., 1990; Liu and Wu, 1987; Guo, 1985; He, 1936). Wang et al. (1990) proposed Archaean regional metamorphic origin for these rocks according to petrological and geochemical studies, which are affirmed by the later geochronological researches which yielded 2.37-2.45 Ga for zircons from the orthogneiss, using single-grain zircon Pb-Pb evaporation method (Chen and Wang, 2006). Yan et al. (2005) reported a SHRIMP Ⅱ zircon U-Pb age of 2 521±20 Ma for the biotite-plagioclase gneiss on the southeast of the Fangshan pluton. Liu B et al. (2008) reported two LA-ICP-MS zircon ages of 2 538±35 and 2 548±24 Ma from the amphibolites and associated leucosome, respectively, on the southeast of the Fangshan pluton, which constrained timing of the high-grade metamorphism and anatexis. Wang et al. (2011) applied thermal ionization mass spectrometer (TIMS) to obtain two U-Pb ages of 2 513±14 and 2 534±29 Ma for zircons from orthogneiss on the northeast of the Fangshan pluton. Most recently, Yuan et al. (2016) obtained 2 551±37 Ma for magmatic zircons from amphibolites in the same area as well. All these data suggest that formation of the Guandi complex and the metamorphic-anatectic event occurred at ~2.50-2.55 Ga in the late to the end of Neoarchean.

2 OCCURRENCE AND PETROGRAPHY OF THE MIGMATITE-LIKE ROCKS

The studied migmatite-like rocks are exposed in the Guandi complex, on the south of the Fangshan pluton (Fig. 1b). The rocks are lithologically composed of amphibolites and felsic gneisses, with amphibolites occurring in various sized lenses or blocks within the felsic gneisses. The amphibolites display dark green color, foliation structure, and medium-grained prismatic blastic texture and consist of dominant amphibole and plagioclase, without garnet on the outcrops (Figs. 2a, 2b). The felsic gneisses show grey color and clear gneissic structure and occur around the lenticular amphibolites (Figs. 2a, 2b). The rocks have been considered to be migmatites, with the amphibolites as the paleosomes and the felsic gneisses as the neosomes in the previous studies (Liu B et al., 2008; Chen and Wang, 2006; Wang et al., 1990; Liu and Wu, 1987; Guo, 1985; He, 1936). Proportion of the amphibolite show diversity over the Guandi complex, frequently disappearing elsewhere (Figs. 2c-2f); the felsic gneisses appear homogeneity in color, mineral composition, texture and structure as a whole on all outcrops.

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Figure 2. Field photos illustrating occurrence and relations between the amphibolites and felsic gneisses. (a) The amphibolite occur as lenses within the felsic gneisses, parallel with the strong foliation of the gneisses; (b) some amphibolites occur as larger blocks with weak foliation or massive structure; (c) the dioritic gneisses were intruded by Cretaceous irregular mafic enclave-bearing quartz-diorite of the Fangshan pluton, with clear boundary clearly crosscutting the former gneissic foliation; (d) outcrops of trondhjemitic gneisses with weakly or without gneissic foliation.

The amphibolites consist of dominant amphibole (50%- 80%), plagioclase (20%-25%), with minor quartz (~5%), and accessory minerals of sphene and ilmenite (Figs. 3a, 3b). Three generations of amphiboles were reported by Liu B et al. (2008), with brown-green-colored hornblendic amphibole (Amp1), bluish-green-colored hornblendic amphibole (Amp2) and pale-green colored to colorless actinolite amphibole (Amp3). However, our samples only developed the first and second generation amphiboles in the thin sections (Figs. 3a, 3b). The second-generation amphiboles occur as single grains or reaction rims around the first-generation amphiboles (Figs. 3a, 3b). The plagioclases show anhedral blasts with polysynthetic twins and coexist with the brownish-green first-generation amphiboles (Fig. 3b).

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Figure 3. Photomicrographs showing representative microstructures of typical lithologies of the amphibolites and felsic gneisses of the Guandi complex. (a) Two generations of amphiboles (Amp) showing brownish or brownish-green color (Amp1) and bluish-green color (Amp2), respectively; (b) the second-generation amphibole (Amp2) grew on edges of the relicts of the first generation amphiboles; (c) the dioritic gneisses have relict clinopyroxene (Cpx) grains preserved as light-yellowish short prismatic crystals, with retrograde hornblende (Hbl) rim surrounding the edges and the Hbl having been further retrograded to pale-green actinolite (Act); (d) the plagioclase (Pl) in the dioritic gneisses occurs as subhedral crystals, showing blastoporphyritic texture; (e) biotite (Bt) shows brownish to dark brownish color and usually occurs as a collection of tiny recrystallized flakes because of the contact metamorphism; (f) felsic gneiss (CJ14-3) shows Pl+Qtz (quartz)+Bt assemblage, also with plagioclase occurring as relict of magmatic subhedral crystals. Abbreviations for minerals are after Siivola and Schmid (2007).

The felsic gneisses consist of predominant plagioclase (55%-60%) and quartz (5%-30%), with minor clinopyroxene (0-10%), hornblende (0-5%), biotite (~4%) and potassium feldspar (< 5%), and/or accessory minerals of apatite, titanite and ilmenite. They can be divided into intermediate and acidic gneisses. The intermediate gneisses or dioritic gneisses (Fig. 2c) has majorly mafic minerals of clinopyroxene, hornblende and biotite and salic minerals of plagioclase, potassium feldspar and minor (~5%) quartz (Fig. 3c). Some of the plagioclases occur as relict subhedral phenocryst (Fig. 3d). The hornblende shows dark green color and occurs as separate matrix grains or rims on the clinopyroxenes. The biotite shows brownish to dark brownish color and also occurs as separate matrix minerals and a collection of tiny flakes along fractures of the hornblendes. The acidic gneisses are mainly composed of quartz, biotite and plagioclase, with minor potassium feldspar and ilmenite (Fig. 3e). The plagioclases show original subhedral crystals with polysynthetic twinning (Fig. 3f). Some plagioclases are locally altered to tiny muscovite assemblage and epidote. The potassium feldspars display anhedral crystals and some of them have been weakly altered to clay minerals.

3 ANALYTICAL METHODS

Samples were collected for zircon U-Pb dating and Lu-Hf isotopes analyses, whole-rock major and trace elemental contents analyses and electron microprobe analysis (EMPA) for minerals. Clean and transparent zircon grains were hand-picked under a binocular microscope, and then mounted in epoxy resin, polished down to expose the grain centers. U-Pb isotope and trace element analyses of these zircons were conducted synchronously by LA-ICP-MS at Wuhan Sample Solution Analytical Technology Co., Ltd with an Agilent 7700e ICP-MS instrument and a Geo-Las 2005 laser sampling. Detailed operating conditions for the laser ablation system and the ICP-MS instrument and data reduction were described by Liu Y S et al.(2010, 2008). Zircon 91500 and GJ-1 were respectively used as external and internal standards, with twice every 5 analyses during the whole determination. Concordia diagrams and weighted mean age calculations were made using Isoplot/Exver3 (Ludwig, 2003). The Hf isotopic analysis, using laser ablation multi-collector inductively coupled plasma mass spectrometry (LA-MC-ICP-MS), was carried out on zircon domains near the spots where U-Pb dating had been done at the State Key Laboratory of Geological Processes and Mineral Resources of China University of Geosciences, Wuhan. Spot laser ablation was under a 20 s background acquisitions and 50 s sample data acquisitions, with a beam size of 44 μm, laser pulse frequency of 6 Hz. Detailed analytical procedures were described by Hu et al. (2012). Off-line selection and integration of analyte signals, and mass bias calibrations were performed using ICPMS-Data-Cal (Liu et al., 2010).

Samples for whole-rock analyses were crushed and powdered to 200-mesh in an agate mill before being analyzed by X-ray fluorescence spectrometry at College of Chemistry and Materials Science of China University of Geosciences in Wuhan. The relative standard derivations (RSD) are within 5%. The accuracies are better than 5%-10% for most trace elements. The detailed sample preparation and analytical procedure follow Liu B et al. (2008). The compositions of amphibole and plagioclase were analyzed using a JEOL-JXA-8230 electron microprobe at the Materials Research and Testing Center of Wuhan University of Technology. The microprobe analyses were carried out with an accelerating voltage of 15 kV, a sample current of 2×10-8 nA and a beam diameter of 2 μm. Analytical errors for major and minor element oxides are estimated to be less than 1% and 10%, respectively. The Fe2+ and Fe3+ values of hornblende and biotite were estimated after Droop (1987).

4 RESULTS 4.1 Zircon U-Pb Data 4.1.1 Amphibolite (sample CJ14-27)

Zircons from this sample are subhedral, rounded or stubby grains with varying grain sizes of 80-150 μm. Cathodeluminescence (CL) images (Fig. 4a) reveal that some zircons show core-rim structure, with blurry magmatic oscillatory zoning in the cores and dark or bright rims; whereas some zircon grains display cores with blurred irregular banded/sector-zoning, or fir-tree zoning that are commonly considered to be metamorphic origin (Grant et al., 2009; Wu and Zheng, 2004; Corfu et al., 2003; Hoskin and Schaltegger, 2003; Vavra et al., 1999).

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Figure 4. The CL images of representative zircon grains from amphibolites and felsic gneisses showing internal structures, analyzed locations, and calculated apparent 207Pb/206Pb ages (Ma). Spot numbers are consistent with those listed in Tables S1, S2.

Thirteen analyses on the cores (e.g., spots 03, 14 and 15 in Fig. 4a) show consistently Th (> 197 ppm) and U (> 164 ppm) contents and Th/U values (> 0.46). The REE patterns show positive Ce anomalies, moderate negative Eu anomalies and steep HREE patterns (Fig. 5b), further supporting their magmatic origin (Hoskin and Schaltegger, 2003; Rubatto, 2002). These 13 analyses show a linear array and yielded an upper intercept age of 2 553±48 Ma (MSWD=0.71), which is consistent with the concordant 207Pb/206Pb age of 2 560±18 Ma (e.g., spot 10 in Table S1) within error. Thus the 2 560 Ma is interpreted as the crystallization age of the magmatic zircons. The remaining 9 analyses on metamorphic zircons (e.g., spots 07, 09, 12, 13, and 21 in Fig. 4a) form a well-correlated discordia linear array, indicative of different degrees of radiogenic Pb-loss (Corfu et al., 2003), and yielded an upper intercept age of 2 493±16 Ma (MSWD=0.05) (Fig. 5a) which is consistent with the concordant 207Pb/206Pb age of 2 493±29 Ma for spot 07 (Table S1) within error. Thus the age of 2 490 Ma is interpreted to be recrystallization age of the metamorphic zircons and so the regional metamorphic age of the hosted amphibolites. The rare earth elements (REEs) of both the magmatic and metamorphic recrystallization zircons show the same chondrite normalized REE patterns (Fig. 5b), this is probably because the system was closed and few elements had been removed by solid-state replacement or dissolution recrystallization during regional metamorphism (Martin et al., 2008).

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Figure 5. U-Pb concordia diagrams (a), (c) and (e) for zircons from the amphibolites and felsic gneisses and chondrite-normalized REE patterns of the zircons (b), (d) and (f).
4.1.2 Felsic gneisses (samples CJ14-12, CJ14-28)

Zircon grains from these samples mainly exhibit prismatic shapes between 100 and 200 μm in length, and show clear core-rim structures (Figs. 4b and 4c). The zircon cores are dark in color, euhedral or subhedral in shape, and have clear and fine oscillatory zoning; the rims are bright and structureless.

Twenty-two out of twenty-nine analyses on the cores for zircons in sample CJ14-12 give Th/U values of 0.42-0.90, suggesting their magmatic origin (Hoskin and Schaltegger, 2003; Rubatto, 2002). Two data which deviate from the dominance of analyses spots are ruled out, the other twenty analyses show discordant but form a well-correlated linear array and yielded an upper intercept age of 2 544±33 Ma (MSWD=0.55) (Fig. 5c), which is consistent with the concordant207Pb/206Pb age of 2 553±34 Ma for spot 25 (Table S1). Therefore, the 2 550 Ma can be considered to be the crystallization age of zircon from the felsic gneiss. The other 7 analyses on the mantle domains or homogeneous bright rims (e.g., spots 16, 19, 21, 25 and 26 in Fig. 4b) define a well-correlated discordia line and yield an upper intercept age of 2 478±45 Ma (MSWD=0.05) (Fig. 5c), in consistence with the weighted mean 207Pb/206Pb age of 2 497±27 Ma (MSWD=0.21). The age of 2 480 Ma is viewed as the timing of the amphibolite-facies metamorphic peak.

Nineteen analyses on domains with oscillatory zoning of 19 zircon cores from sample CJ14-28 (e.g., spots 01 and 03, 06 in Fig. 4c) yielded Th/U values are of 0.52-1.07 and suggest their magmatic origin (Hoskin and Schaltegger, 2003; Rubatto, 2002). When one data with high concordance and evidently younger apparent 207Pb/206Pb age 2 237±28 Ma (spot 02) were ruled out, the rest 18 analyses show different degrees of radiogenic Pb-loss but define a well-correlated discordia line with an upper intercept age of 2 554±18 Ma (MSWD=0.29) (Fig. 5e), consistent with the weighted mean 207Pb/206Pb age of 2 548±26 Ma (MSWD=0.66) of 6 concordant ages (i.e., spots 01, 04, 06, 12, 19 and 26 in Table S1) within error. Therefore, the 2 550 Ma is interpreted as the timing of crystallization age of the zircons. On the other hand, eight analyses were conducted on the homogeneous bright rims (e.g., spot 18 in Fig. 4c) also define a well-correlated discordia line, yielding an upper intercept age of 2 498±47 Ma (MSWD=0.92) (Fig. 5e), which is consistent with the concordant 207Pb/206Pb age of 2 494±29 and 2 494±35 Ma within error (spots 16 and 23 in Table S1). Therefore, the age of 2 490 Ma is interpreted as the timing of recrystallization age of the metamorphic zircons and peak metamorphism of the granitoid.

Both the magmatic and metamorphic zircons from the felsic gneisses mostly show uniform REE patterns with strong positive Ce anomaly, apparent negative Eu anomaly and an strong enrichment of heavy REEs (Figs. 5d, 5f), consistent with those of the igneous zircons (e.g., Rubatto, 2002). As for a few other analyses that have anomalously higher LREE contents with negative Eu anomalies might reflect mobility of incompatible elements (LREE, Th and U) resulting from local recrystallization of the original magmatic zircon (Whitehouse and Kamber, 2003) or likely due to breakdown of LREE bearing minerals contemporaneously with zircon growth (Wu and Zheng, 2004; Whitehouse and Platt, 2003).

4.2 Zircon Lu-Hf Isotopes

Thirteen analyses carried out on magmatic zircons from the amphibolite sample (sample CJ14-27) show constant Hf compositions of initial 176Hf/177Hf values (0.281 251-0.281 282), positive initial εHf(t) values (2.6-4.0), with calculation using the formation age (2 560 Ma) of the amphibolite protoliths, corresponding to depleted mantle model ages (TDM) of 2 711-2 763 Ma (Fig. 6). Fourteen spots analyzes on magmatic zircons from the grey gneissic rock (sample CJ14-12) yielded initial 176Hf/177Hf ratios ranging from 0.281 250 to 0.281 312, initial εHf(t) values of 2.8-4.4 (Fig. 6), corresponding to TDM2 ages and two stage depleted mantle model ages (TDM2) of 2 787-2 888 Ma, calculated using the formation age (2 550 Ma) of the gneiss protoliths. Twelve analyses on magmatic zircons show a wide range of initial 176Hf/177Hf values of 0.281 243-0.281 401. The initial εHf(t) values and two stage depleted mantle model ages (TDM2) with the formation age (2 550 Ma) in calculation are 2.8 to 7.1 (Fig. 6), corresponding to TDM2 values of 2 611-2 889 Ma.

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Figure 6. εHf(t) values vs. ages (Ma) diagram for zircons from both the amphibolites and felsic gneisses. Comparison data from Yuan et al. (2016).
4.3 Whole-Rock Geochemistry

Whole-rock major and trace elemental data are listed in Table S3, while the values and ratios of major element reported in the text are normalized to 100% on volatile-free basis.

It is generally known that protolith nature of the studied metamorphic rocks should be discriminated before description of their geochemical characteristics. Relict microtextures preserved within and internal structures of zircons from a metamorphosed rocks can provide important clues for its protolith nature (e.g., Sang and Ma, 2012). Although our studied rocks have been strongly metamorphosed and deformed, the large scale homogeneous occurrence and massive structure of the rocks, the euhedral magmatic plagioclase feldspar retained locally within the felsic gneisses (Fig. 3f) all suggest their magmatic protoliths. Furthermore, the zircons from both the amphibolites and felsic gneisses show both oscillatory zoning (Fig. 4) and mono-age value of ~2 550 Ma (Fig. 5) characterized by a single magmatic pulse, rather than multiple-age spectrum for detrital zircons from sedimentary clastic rocks, thus further suggesting magmatic protolith nature of the studied rocks. Geochemical features of a metamorphic rock are also good indicators for protolith nature, and for such study many discrimination diagrams have been constructed and employed to distinguish ortho-metamorphic from para-metamorphic rocks and even distinct lithologies (e.g., Wang et al., 1987). Among the commonly used diagrams, the MgO-CaO-FeO*diagram (Walker et al., 1959) and SiO2-TiO2 diagram (Tarney, 1976) have been frequently taken to distinguish ortho-amphibolite from para-amphibolite and ortho-gneiss from para-gneiss, respectively. As can be seen on the two diagrams, our amphibolites and felsic gneisses are respectively plotted in ortho-amphibolite domain (Fig. 7a) and ortho-gneiss region (Fig. 7b), once again suggesting magmatic protoliths for the studied rocks.

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Figure 7. Discrimination diagrams for protolith natures. (a) MgO-CaO-FeO* (Walker et al., 1959) is for the amphibolites, with domain Ⅰ referring to ortho- amphibolite and domain Ⅱ to para-amphibolite; and (b) SiO2-TiO2 (Tarney, 1976) is for the felsic gneisses. FeO*=FeO+0.899 8×Fe2O3.
4.3.1 Amphibolites

The studied amphibolites contain low to moderate SiO2 (45.7 wt.%-50.2 wt.%), variable Al2O3(11.5 wt.%-16.2 wt.%), moderate CaO (9.45 wt.%-11.1 wt.%) and low TiO2 (0.54 wt.%-0.96 wt.%), but high MgO (9.90 wt.%-13.5 wt.%) and Mg# (Mg#=100×Mg/(Mg+Fe2+)) (63.5-75.3). All samples show subalkaline basaltic affinity (Fig. 8a) and dominantly olivine gabbros on R1-R2 classification diagram (Fig. 8b), respectively. The amphibolites have chondrite-normalized REE patterns characterized by enriched LREEs relative to HREEs, with (La/Yb)N values of 4.59-12.6, and moderate to weakly negatively to non Eu anomalies with δEu=0.57-0.95 (Fig. 9a), characterized by the Archean TH2 type tholeiite (Condie, 1976). The primitive mantle- normalized incompatible trace elements patterns display enrichment of large ion lithophile elements (LILE) and light rare earth elements (LREE), and negative anomaly in some typical high field strength elements (HFSE) of Nb, Ta, and Ti (Fig. 9b).

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Figure 8. (a) Total alkali-silica (TAS) diagram (Middlemost, 1994) and (b) R1-R2 diagram (De la oche et al., 1980) showing rock series and possible rock types for both the amphibolites and felsic gneisses from the Guandi complex. The symbols refer to the same types of rocks as those in Fig. 7.
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Figure 9. Chondrite-normalized REE diagram and primitive mantle-normalized spidergram for the amphibolites (a)-(b) and the felsic gneisses (c)-(d) of the Guandi complex. The normalized data are from Sun and McDonough (1989). The typical curves of the OIB, E-MORB and N-MORB (e.g., Sun and McDonough, 1989) and of the Archean TTG (Martin, 1994) are also shown for comparison. The grey field represents upper and lower ranges of the curves of the amphibolites from Yuan et al. (2016). OIB. Oceanic island basalt; E-MORB. enriched mid-ocean ridge basalt; N-MORB. normal mid-ocean ridge basalt.
4.3.2 Felsic gneisses

The felsic gneiss samples have wide ranges of SiO2 (54.4 wt.%-70.5 wt.%), covering intermediate to acidic rocks in TAS diagram (Fig. 8a). The rocks has variable and high Al2O3 (15.0 wt.%-20.7 wt. %) and Na2O (4.87 wt.%-6.21 wt.%) but low TiO2 (0.33 wt.%-0.72 wt.%) and P2O5(0.12 wt.%-0.45 wt.%), and high variation of MgO (0.55 wt.%-2.78 wt.%), CaO (1.17 wt.%- 6.69 wt.%) and K2O (0.75 wt.%-3.10 wt.%) concentrations. They also show wide range of Mg# (25-53) and wide-range and high Na2O/K2O values (1.67-8.37). The rocks are plotted mostly in sub-alkaline series domain on TAS diagram (Fig. 8a), with lithology of dominant of quartz monzonite and granulite on R1-R2 diagram (Fig. 8b). The felsic gneiss rocks exhibit moderate variation of total REE contents (44.9 ppm-179 ppm) and highly fractionated REE patterns ((La/Yb)N=17.5-40.1) with evidently positive Eu anomalies (δEu=1.26-1.91) (Fig. 9c). They show low content of Cr (19.0 ppm-26.5 ppm), Ni (6.33 ppm-16 ppm), with only one exception for the intermediate rock sample CJ14-12 which contains Cr of 99.8 ppm and Ni of 48.7 ppm (Table S3). They have high content and highly variable Sr (260 ppm-1 120 ppm) but low Y (3.65 ppm-11.1 ppm) and Yb (0.36 ppm-1.09 ppm) concentrations, with high Sr/Y values of 50.4-139, characterizing high Sr and low Y rock type. The samples display distinct enrichment of large ion lithophile elements (LILE; Rb, Ba, Sr and Pb) whereas negative anomaly of high field strength elements of Nb and Ta (Fig. 9d).

4.4 Mineral Chemistry and Estimation of the Peak Metamorphic Conditions 4.4.1 Compositions of amphibole and plagioclase

In order to estimate peak conditions of the ~2.48-2.50 Ga metamorphism, electronic probe analyses were carried out only on both the core and rim domains of the first-generation amphiboles (Amp1) and co-existed plagioclases from five amphibolite samples, and the resultant data for both the amphiboles and plagioclases are listed in Tables S4 and S5, respectively.

The first-generation amphiboles display the same or minor variation compositions for both the core and rim of a given amphibole grain. Analyses on the core show Si=6.43-6.62, CaB= 1.92-2.00, NaB=0.00-0.08, (Ca+Na)B=1.97-2.00, (Na+K)A= 0.56-0.72, Fetot/(Fetot+Mg)=0.37-0.46, Fe3+/(Fe2++Fe3+)=0.00- 0.12; those on the rim show Si=6.51-6.68, CaB=1.90-2.00, NaB=0.00-0.05, (Ca+Na)B=1.96-2.00, (Na+K)A=0.56-0.76, Fetot/(Fetot+Mg)=0.38-0.46, Fe3+/(Fe2++Fe3+)=0.00-0.07. They are pargasite and edenite in Si vs. Mg/(Mg+Fe2+) diagram (Fig. 10) proposed by Leake et al. (1997). It is noteworthy that with one exception (spot 1.2 in sample CJ14-27), the core analyses show Al2O3 content and AlTvalues higher than the rim data, indicating that the cores may record higher pressure and temperature approaching the peak metamorphic conditions, based on a fundamental principle that the AlT content of calcic amphiboles is proportional to metamorphic pressure and temperature increasing (Plyusnina, 1982; Graham, 1974; Hietanen, 1974). The co-existed plagioclase crystals have anorthite contents ranging from 22.87% to 36.58%. Six out of nine selected plagioclase grains show positively zoned anorthite components, with anorthite contents of 24.41%-27.25% in the core increasing to 25.12%-36.58% in the rim, suggesting the prograde process; the other three grains, however, show negatively zoned variations, probably recording the retrograde process. Therefore, the positively zoned anorthite compositions in the rim probably record the compositions representing nearer peak metamorphic conditions.

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Figure 10. Sivs. Mg/(Mg+Fe2+) diagram (Leake et al., 1997) for amphiboles from the amphibolites of the Guandi complex.
4.4.2 Estimation of P-T conditions

The P-T conditions of metamorphic equilibration for amphibolites can be estimated by application of the amphibole-plagioclase thermometry and related barometry, among which combination of the amphibole-plagioclase thermometry calibrated by Holland and Blundy (1994) and the Al-in-hornblende barometer calibrated by Anderson and Smith (1995) have been frequently used for metamorphic rocks and proven to be well performed (e.g., Li et al., 2017; Xia et al., 2014). However, the plagioclase/amphibole Al-Si partitioning barometer of Molina et al. (2015), based on calibration of the plagioclase/amphibole the Al-Si partitioning barometer by Fershtater (1990) are proven to be more accurate than amphibole-only thermobarometers and can give more reliable estimates.

The rim composition of positively zoned plagioclases and core composition of the co-existed amphibole (e.g., Tables S4 and S5) were chosen for P-T estimations since they are approximating the peak compositions. The P-T estimation for each co-existed Pl-Ampl pair was performed by iterative calculation with the pressure values obtained by the barometer of Anderson and Smith (1995) at various temperature values by thermometers (thermometer Ta and Tb) of Holland and Blundy (1994). Iterative calculation with the pressure values obtained by the barometer of Molina et al. (2015) at various temperature values by Holland and Blundy (1994) thermometers was also performed for comparison. Ta(PAS), Tb(PAS), Ta(PM) and Tb(PM) are temperature values in centigrade; PAS(Ta), PAS(Tb), PM(Ta) and PM(Tb) are pressure values in kbar, but have been changed to GPa in final list in Table S6.

By comparison of calculation results from the thermometer of Anderson and Smith (1995) to those from the hornblende-plagioclase thermometer B (edenite-richterite reaction) of Holland and Blundy (1994), the latter has been considered to give more reliable data because they reproduce more precise temperature results than the thermometer of Anderson and Smith (1995) did. Therefore, the temperature results (Tb) from the calibration of reaction B of Holland and Blundy (1994) and pressure results using temperature parameter by Tb are preferred here. Comparison also shows that the pressure results PAS(Ta) and PAS(Tb) by barometer of Anderson and Smith (1995) are a little lower than those PM(Ta) and PM(Tb) by barometer of Molina et al. (2015), but consistent with each other within their error ranges. Thus we consider the estimation of 0.55-0.9 GPa and 674- 727 ℃ as the best fitting pressure and temperature conditions for the peak metamorphism of the amphibolites from the Guandi complex, Zhoukoudian, Southwest Beijing.

5 DISCUSSION 5.1 Timing of Emplacement of Magmatism and Metamorphism

The previously studies suggested that the magmatic protoliths of the felsic gneisses from Guandi complex were formed at 2.53-2.55 Ga (Yuan et al., 2016; Liu B et al., 2008; Yan et al., 2005). Liu B et al. (2008) obtained a metamorphic age of ~2.54 Ga for the metamorphic zircons, the same as those for the magmatic zircons, suggesting that the metamorphism occurred right following the magma emplacement. Our new data reveal that protoliths of the amphibolites and felsic gneisses of the Guandi complex are of magmatic rocks and that emplacement of their precursor magmas occurred at 2.54-2.56 Ga, which is consistent with previous data (Yuan et al., 2016; Liu B et al., 2008; Yan et al., 2005). However, dating of the metamorphic zircons from the three samples all points to that the timing of the peak of the following regional metamorphism occurred during ~2.48-2.50 Ga in this region, rather than ~2.54 Ga estimated by Liu B et al. (2008).

5.2 Petrogenesis of the Amphibolites and Felsic Gneisses

The studied amphibolites have moderate LOI values varying from 0.89 wt.% to 1.80 wt.% for 14 samples, with only one sample having a high LOI value of 2.30 wt.% (Table S3), suggesting most of the samples have undergone little influence of later retrograde processes. Since LILE (e.g., K, Rb, Cs and Ba) may become remobilization during the upper amphibolite-facies metamorphic overprint (Humphris and Thompson, 1978), relatively immobile elements such as the high field strength elements (HFSE: Ti, Zr, Y, Nb, Ta, Hf), Th and the rare earth elements (REE) are employed for following discussions of petrogenesis and tectonic implications.

5.2.1 Petrogenesis of the amphibolites

It has been known that basaltic magmas are mantle derived and usually chemically interact to some extent with the continental crust during ascent through the crust and/or residence in crustal magma chambers (e.g., Dharma Rao et al., 2004; Ashwal et al., 1986). However, the precursor magmas of the amphibolites of the Guandi complex show negligible crustal assimilation, as evidenced by that they have (1) remarkably lower Th/La values (0.02-0.13) in comparison with 0.36 of the average Archean upper crust (Taylor and McLennan, 1985); (2) low La/Sm values (2.35-4.31, except for one sample with 5.30) in comparison with high La/Sm values (> 4.5), a criteria indicative of the existence of crustal contamination (Shan et al., 2015; Lassiter and Depaolo, 1997); (3) no field evidence about emplacement of the ~2.55 Ga amphibolites into the Guandi complex has been reported and no crustal xenolith has been found within the amphibolites.

The studied amphibolites are characterized by high MgO (9.90 wt.%-13.47 wt.%), Mg#=63-75, with 6 out of 7 samples > 67, and Ni (105 ppm-424 ppm) contents similar to the primitive magma (MgO=8 wt.%-15wt.%, Mg#≥68 and Ni≥250 ppm; see Gill, 2010), with Cr (822 ppm-2 145 ppm) concentrations also approaching the primitive magma (2 520 ppm-2 625 ppm) (McDonough, 2003; Palme and O'Nell, 2003), these make their precursor magma appear to be derived from the asthenosphere mantle, similar to the komatiites or ocean island basalts (OIB). However, the komatiites mostly have depleted LREE patterns, despite that there are some Al-depleted komatiites displaying enriched LREE (or depleted HREE) and low Al2O3 (5.47 wt.%) and Al2O3/TiO2 values of ~5 (e.g., Xie and Kerrich, 1994). Our amphibolites show obvious enriched LREE patterns (Fig. 9a) and their TiO2 contents (0.61 wt.%-1.65 wt.%) are obviously lower than the average TiO2 value of 2.86 wt.% of the OIB that are derived from the asthenospheric mantle, thus also precluding an OIB affinity. Therefore, precursor magma of the amphibolites was not derived from asthenosphere mantle.

The amphibolites show positive zircon εHf(t) values of 2.6-4.0 (Table S2), corresponding to depleted mantle model ages TDM of 2 711-2 763 Ma (Fig. 6), seemingly suggesting a depleted mantle source for their precursor magmas. However, the REE patterns and primitive mantle-normalized trace element patterns show enrichment in LREEs and LILEs and depletion in HFSEs, with negative Nb, Ta, Ti and P anomalies (Fig. 9a), a characteristic feature shared by many arc-related magmas (Sajona et al., 1996; Drummond and Defant, 1990), suggesting the magmas from an enriched lithospheric mantle source due to subduction induced metasomatism (e.g., Pearce, 2008; Pearce and Peate, 1995; Hawkesworth et al., 1993; McCulloch and Gamble, 1991; Saunders et al., 1991) or melts from a depleted mantle source contaminated by crustal materials during magma ascent and emplacement (Yang et al., 2008; Dharma Rao et al., 2004; Ashwal et al., 1986). However, the former explanation is preferred according to statement of negligible crustal assimilation for these amphibolites. Additional supports to this explanation are (1) they have relatively homogeneous Hf isotopic compositions (εHf(t)=2.6-4.0); (2) they plot nearby the enriched mantle source trend in Th/Yb-Nb/Yb diagram (Fig. 11a); (3) they have higher La/Sm and lower Lu/Hf ratios than the primitive mantle (those of which respectively are 1.59 and 0.24 in McDonough and Sun, 1995), as well as the higher values of La/Ta (24.6-75.0) and La/Nb (2.43-6.60) respectively compared to those values of 16.76 and 0.96 for the primitive mantle (Sun and McDonough, 1989); and (4) they have the (Hf/Sm)N values higher than 1 and wide range of (Ta/La)N values between 0.1-1.0 and plotted within or around the melt-related subduction metasomatism domain (Fig. 11b). Thus the precursor magmas of the amphibolites are considered to be sourced from an enriched lithospheric mantle metasomatized by subduction-derived melts or fluids.

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Figure 11. (a) Th/Yb vs. Nb/Yb diagram (after Pearce, 2008; Pearce and Peate, 1995); (b) (Ta/La)N vs. (Hf/Sm)N diagram (after La Flèche et al., 1998). The symbols refer to the same types of rocks as those in Fig. 7.
5.2.2 Petrogenesis of the felsic gneisses

The studied felsic gneisses display high Na2O concentrations (4.87 wt.%-6.21 wt.%) and low to moderate K2O concentrations (0.75 wt.%-3.10 wt.%), high Na2O/K2O (1.67-8.37) and Sr/Y (50.4-139) values and highly fractionated REE patterns (Fig. 9c) with significantly positive Eu anomalies (1.26-1.91), mostly plot into trondhjemite and tonalitic domains (Fig. 12a) (Moyen and Martin, 2012; Martin et al., 2009, 2005). The TTG rocks have been generally considered to be generated by partial melting of hydrated metamorphosed mafic rocks in high pressure conditions, such as eclogite-facies or high-pressure amphibolite- and granulite-facies conditions to stabilize garnet±amphibole±rutile in the residue (Xiong et al., 2009; Martin et al., 2005; Rapp et al., 2003; Foley et al., 2002; Smithies, 2000). It has been proposed that most of Archean TTGs are formed by (1) differentiation of basaltic magmas (Arth, 1979; Barker and Arth, 1976); (2) partial melting of a subducted oceanic slab (Martin et al., 2005; Foley et al., 2002; Martin, 1999; Stern and Kilian, 1996; Drummond and Defant, 1990); (3) partial melting of a tectonically or magmatism- induced thickened lower crust (Condie, 2005; Smithies, 2000; Atherton and Petford, 1993); or (4) partial melting of delaminated mafic crust in an apparently non-subduction related setting (Wang et al., 2007; Bédard, 2006; Xu et al., 2002; Zegers and van Keken, 2001; Kay and Kay, 1993). However, there are several lines of observation to suggest that the TTG type melts did not form by fractional crystallization: (1) The rocks show significantly positive Eu anomalies (1.26-1.91), indicative of no obviously fractional crystallization of plagioclase; (2) the rocks show two partial melting subseries on La/Sm-La variation plotting (not shown); (3) no coeval cumulated ultramafic rocks has been observed in the Guandi complex. These observations rule out the possibility of differentiation for the protolith of the felsic gneisses.

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Figure 12. (a) Normative An-Ab-Or diagram (Barker, 1979) for the TTG gneisses; (b) MgO vs. SiO2 diagram. PMB. Experimental partial melts of basalts from amphibolites; LSA. low silica adakite; HAS. high silica adakite (after Martin et al., 2005). The symbols refer to the same types of rocks as those in Fig. 7.

Partial melting of a young and hot subducted oceanic slab will result in TTG melts with higher contents of MgO, Cr (> 36 ppm), Ni (> 24 ppm) and high values of Mg#(> 50) as suggested by Lu et al. (2015) and Martin et al. (2005) because such slab-derived melts will traverse the mantle wedge and interact with the peridotite, leading to significant increase in MgO, Cr and Ni concentrations but decrease in SiO2. Additionally, Mg# is proven to be very sensitive to mantle contribution (Lu et al., 2015), and only small contamination (~10% peridotite) will cause significant (~20%) increase in Mg# (Rapp et al., 1999). On the other hand, partial melting of a delaminated mafic lower crust will also cause increasing in MgO (> 3 wt.%), TiO2 (> 0.9 wt.%) and compatible element contents in TTG rocks due to metasomatism of mantle peridotite (Wang et al., 2007; Xu et al., 2002) and the resultant melts would show similar geochemical characteristics to those from partial melting of the subducting oceanic slab. On contrast to these two processes, partial melting of TTG melts from thickened mafic lower crust of a continental arc is geochemically characterized by low MgO, Ni and Cr contents and Mg#, due to no interaction with the mantle peridotite. Majority of the TTG samples from the Guandi complex, Zhoukoudian area display low MgO (0.55 wt.%-2.78 wt.%), Cr (19.0 ppm-26.5 ppm) and Ni (6.33 ppm-16.0 ppm) and low Mg# values (25.0-53.2) as a whole (Table S3). They mostly plot in the field comparable with those experimentally-derived partial melts from metamorphosed basaltic rocks, and display high-Si and low-Mg geological features (Fig. 12b) (Martin et al., 2005). Furthermore, the TTG gneisses have magmatic zircons showing TDM2 of 2.61-2.89 Ga, suggesting the generation of the basaltic protolith is about 0.1-0.4 Ga older than the formation or emplacement age (~2.54-2.56 Ga) for their precursor melts (e.g., Yuan et al., 2016; Liu B et al., 2008; and this study). Therefore partial melting of thickened lower mafic crust is preferred here than partial melting of delaminated lower crust or subducted hot and young oceanic lithosphere for the precursor magmas of the TTG rocks of the Guandi complex.

5.3 Migmatites or Migmatite-Like Rocks?

Migmatites are metamorphic rocks underwent melt-producing reactions during or close to peak metamorphism, usually of upper amphibolite and lower granulite facies (e.g., Vernon and Clarke, 2008; Braun and Kriegsman, 2001). Migmatization provides a possible link between high-grade metamorphism and crustal anatexis according to the "ultra-metamorphism" theory (e.g., Yakymchuk, 2014; Brown, 2007; Hinchey and Carr, 2006; Sawyer, 1998). However, migmatites are not always easily to be distinguished from the high-grade metamorphosed and strongly deformed mafic enclave-rich intrusive rocks.

Felsic gneisses of the migmatite-like rocks of the Guandi complex are compositionally comparable with the Archean TTG suite. As aforementioned, precursor TTG melts of the protoliths of the felsic gneisses were formed by partial melting of metabasites at thickened lower crust during ~2.54-2.56 Ga, coevally with the generation of the basaltic melts at ~2.56 Ga for the amphibolites. The TTG magmas generated from the metabasites within garnet-bearing domains under which pressure and temperature conditions will be as high as 1.0-2.5 GPa and 800-1 000 ℃, according to a recently phase-equilibration modeling in Wei et al. (2017), in order to stabilize garnet the residue sources. Though the studied amphibolites underwent metamorphism with variable pressure conditions of 0.55-0.9 GPa at peak temperature conditions of 674-727 ℃, with the highest pressure conditions approaching the lower pressure limit of > 1.0 Ga for generation precursor magmas of the coeval TTG rocks at ~2.50 Ga (Fig. 5), garnet constitute has not been observed on outcrops (Figs. 2a-2b) nor in thin sections (Figs. 3a-3b) of these mafic metamorphic rocks, thus clearly indicating no genetic relationships between the protoliths of the TTG melts and the formation of the amphibolites. Therefore the association of the felsic gneisses and amphibolites of the Guandi complex in Zhoukoudian area is not of truth migmatites but migmatite-like rocks, but is in fact of a suite of the mingled Na-rich acidic intrusive rocks with mafic enclaves that has undergone amphibolite-facies metamorphism and intensive deformation later at 2.49-2.50 Ga.

5.4 Tectonic Setting and Implications 5.4.1 Tectonic setting for the Late Neoarchean magmatism

Our zircon U-Pb ages and whole-rock geochemical data indicate that two constitutes of the migmatite-like rocks recorded a Late Neoarchean (~2.54-2.56 Ga) magmatic intrusion. It is well known that basaltic magmas generate in multiple tectonic settings majorly including subduction-related volcanic arc, within plate and mid-ocean ridge (Pearce, 1996). Subduction-related volcanic arc basalts generally have remarkable enrichment of LILE but depletion of HFSE due to generation from mantle metasomatized by fluids released from the dehydration of a subducting oceanic slab (e.g., Spandler and Pirard, 2013; Pearce, 2008, 1996; Pearce and Peate, 1995; Hawkesworth et al., 1993; McCulloch and Gamble, 1991; Saunders et al., 1991). The normal mid-ocean ridge basalts (N-MORB) generally show depletion in LILE and LREE (e.g., McDonough and Sun, 1995; Sun and McDonough, 1989), and the within plate basalts similar to the oceanic island basalts (OIB) are characterized by high contents of LREE and positive Nb, P, Zr and Hf anomalies (Hollings and Kerrich, 2004). The amphibolites of the Guandi complex have obvious Nb-Ta and Zr-Hf troughs, negative Th anomalies, and positive Rb and Ba anomalies on the primitive mantle normalized spidergram (Fig. 9b), thus showing characteristics of subduction-related arc basalts rather than within plate basalts (WPB). Plots on Ti/100-Zr-3Y diagrams also support this observation (Fig. 14a). Geochemistry-based discrimination on diagrams of MnO×10-P2O5×10-TiO2and Hf/3-Th-Nb/16 (Figs. 14b-14c) further suggest subduction-related arc environments, with minor overlapping with mid-ocean ridge settings, however, plots on the Ti/Sc-Zr diagram preferentially suggests arc environment (Fig. 14d). In addition, these amphibolites have obviously fractionated chondrite-normalized REE patterns, different from those of both MORB and OIB (Figs. 9a-9b) and negative Nb-Ta, Ti, and P anomalies, further suggesting derivation from an enriched mantle source metasomatized at diverse degree by subduction-derived fluids or melts (Fig. 11b) (e.g., Pearce, 2008; Pearce and Peate, 1995; Hawkesworth et al., 1993; McCulloch and Gamble, 1991; Saunders et al., 1991). A previous study by Yuan et al. (2016) on the same age (2 551±37 Ma) amphibolite samples from the Guandi complex, northeast of the Fangshan pluton (Fig. 1b) also showed similar geochemical features and tectonic setting signature to our studied amphibolites (see Figs. 7a, 8a-8b, 9a-9b, 11a-11b, and 14a-14f). Our TTG type gneisses have zircon Hf isotopic TDM ages of 2.61-2.89 Ga, suggesting an ancient continental crust beneath the study area and thus a continental arc setting already existed for generation of precursor magmas of protoliths of the amphibolites and TTG in the end of the Late Neoarchean.

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Figure 13. (a) Sr/Y vs. Y diagram (Martin, 2005); (b) (La/Yb)N vs. YbN diagram (Drummond and Defant, 1990) for TTG and classic arc calc-alkaline magmatic rocks. The symbols refer to the same types of rocks as those in Fig. 7.
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Figure 14. (a) Ti/100-Zr-3Y (Pearce and Cann, 1973); (b) MnO×10-P2O5×10-TiO2 (Mullen, 1983); (c) Hf/3-Th-Nb/16 diagram (McCulloch and Gamble, 1991); (d) Ti/Sc vs. Zr diagram (after Gribble et al., 1996); (e) La vs. Yb discrimination diagram (Ross and Bédard, 2009); (f) Ce vs. Yb diagram (modified after Hawkesworth et al., 1993). BABB. Back arc basin basalt; CAB. calc-alkalic island arc basalt; IAT. island arc tholeiite basalt; OIA. seamount alkalic; OIT. seamount tholeiitic; WPAB. within-plate alkaline basalts; WPB. within-plate basalt; WPT. within-plate tholeiites. The symbols refer to the same types of rocks as those in Fig. 7.
5.4.2 Tectonic evolution for NCC during Neoarchean- Early Paleoproterozoic transition

The fundamental geodynamic mechanism for the North China Craton during the Late Neoarchean has been debated and two main stream models of mantle plume (Li and Wei, 2017; Wang et al., 2014; Zhai and Santosh, 2013, 2011; Zhao and Zhai, 2013; Geng et al., 2012; Yang et al., 2008) and plate tectonics (Deng et al., 2018, 2016; Zhou et al., 2018; Kusky et al., 2016; Tang et al., 2016; Wang W et al., 2015; Kusky and Zhai, 2012; Kusky and Li, 2010) were proposed. The mantle plume model is supported by evidences including (1) the exceptionally large exposure of granitoid intrusions that formed over a relatively short time period (2.55-2.50 Ga), without systematic age progression across a ~800 km wide block; (2) affinities of mafic rocks to continental tholeiitic basalts; (3) dominant diaprism-related domal structures; and (4) bimodal volcanic assemblages; (5) short time interval between initial magmatism and regional high-grade metamorphism (Wang et al., 2014; Zhao and Zhai, 2013; Zhao et al., 2001, 1998). The plate tectonic model is suggested by arc magmatism (Zhou et al., 2018; Bai et al., 2016; Yang et al., 2016; Peng et al., 2015; Wang W et al., 2015; Nutman et al., 2011) and arc-continental or continental-continental collision process (Liu et al., 2018; Lu et al., 2017; Chen et al., 2016; Deng et al., 2016; Kusky et al., 2016; Tang et al., 2016; Liu S J et al., 2015; Wang J P et al., 2015). Accordingly, the arc magmatism was established by: (1) the typical island arc-related lithological assemblages, e.g., sanukitoids (Wan et al., 2010; Wang et al., 2009), back arc basin basalt (BABB; Bai et al., 2016), high Mg group of TTG (Wang et al., 2013, 2012; Zhai et al., 2005), meta-volcano-sedimentary rocks (Guo et al., 2015; Peng et al., 2015; Lü et al., 2012; Zhai et al., 1985); (2) geochemical affinities with calc-alkaline plutons in modern magmatic arcs (Zhai, 2014; Wang et al., 2013, 2012; Zhai and Santosh, 2011; Zhai et al., 2005). The collisional process can be evidenced by (1) several geological evidences related to the suture zone of ca. 2.5 Ga arc-continent collisions are recognized (Deng et al., 2016; Tang et al., 2016); (2) Late Neoarchean to Early Paleoproterozoic granulite facies metamorphism with clock-wise P-T-t paths in the Eastern Block of the NCC, indicating subduction-collision tectonics (Liu S W et al., 2018; Lu et al., 2017; Liu S J et al., 2015); (3) post arc-continent collision magmatism such as 2.5-2.4 Ga mafic dykes and 2.50-2.45 Ga potassic granites occurred in the NCC (Deng et al., 2018).

The amphibolites and TTG gneisses of the Guandi complex geochemically suggest a subduction-related continental arc setting for generation of their precursor magma. The metamorphism of the amphibolites at ~2.50 Ga, with pressure conditions of 0.55- 0.90 GPa at peak temperature conditions of ~674-727 ℃ is characterized by medium P/T-type amphibolite-facies metamorphism (Fig. 15) in a collisional tectonics (Chen et al., 2013; Miyashiro, 1973). This metamorphic event occurred coevally with and is comparable to the metamorphism recently reported for the high pressure mafic granulites (2 473±6 Ma; Liu S J et al., 2015) in the Jiaodong Group, eastern Shandong Province and the pelitic granulites (2 497±17 to 2 499±11 Ma; Lu et al., 2017) in the Jidong region, Hebei Province, which are characterized by clockwise P-T-t path pattern which has been usually related to arc-continent collision or continent-continent collision process (Groppo and Rolfo, 2008; Brown, 2007, 1995; Bohlen, 1987). In the meantime, ~2.49 Ga monzogranites and potassium-rich granites, which are considered to be products representing final amalgamation of the continental arc and the ancient continent (e.g., Deng et al., 2018; Wan et al., 2012) have also been observed in the Guandi complex (our unpublished data). Therefore, magmatism and metamorphism of both the amphibolites and felsic gneisses of the Guandi complex might suggest a subduction-collision process, with accretion of continental arc terrane to main continental block during the Late Neoarchean.

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Figure 15. P-T diagram showing plots of pressure-temperature estimations for peak metamorphism of the amphibolites from the Guandi complex. The results are expressed with error bars. The cross red and blue error bars are referred to P-T results by thermometer of Holland and Blundy (1994) and barometer of Molina et al. (2015) and those purple and green error bars are the P-T results by the same thermometer of Holland and Blundy (1994) and barometer of Anderson and Smith (1995). Three major types of metamorphic facies series proposed by Miyashiro(1994, 1973) are also shown. P-T fields of different metamorphic facies are also shown. Ky. Kyanite; And. andalusite; Sil. sillimanite; AEH. albite-epidote hornfels facies; HH. hornblende hornfels facies; PH. pyroxene hornfels facies; S. sandinine facies. The total figure framwork was redrawn from Winter (2010) with minor modification.

Based on a comprehensive study of the basement terranes in the Eastern Block of the NCC, a Late Neoarchean (~2.5-2.6 Ga) intra-oceanic arc system with relict MORB-like basaltic rocks has been recognized bordering the northwestern margin of the continental nucleus (ca. 2.7-3.8 Ga) along the proposed "Wutai-Zunhua-Majuanzi" boundary, extending from northern Liaoning Province, through western Liaoning Province, the Zunhua-Qinglong Block of eastern Hebei Province, northern Hebei Province, and the Huai'an-Xuanhua complexes of northwestern Hebei Province, up to the Wutai complex. The intra-oceanic arc system is characterized by the general lack of ≥2.7 Ga crystalline basement, but it has Late Neoarchean metamorphosed basaltic rocks showing geochemical affinities to MORBs-IATs-CABs. Southeast to the proposed "Wutai-Zunhua-Majuanzi" boundary, an array of coeval high grade metamorphic terranes have been understood parallel to the intra-oceanic arc along the inner margin of the Eastern Block of North China Craton (Fig. 1a; Wang W et al., 2016, 2015 and references therein), from northeast to southwest, including the granite-greenstone belts in southern Jilin regions (SJ in Fig. 1a; Guo B R et al., 2017, 2016), the Qingyuan greenstone in northern Liaoning (QY in Fig. 1a; Wang et al., 2016; Peng et al., 2015), the southern Anshan-Benxi granite-greenstone belt (AB in Fig. 1a; Guo R R et al., 2017), eastern Hebei (EH in Fig. 1a; Bai et al., 2016), Hengshan and Fuping (HS and FP in Fig. 1a; Tang et al., 2016; Liu et al., 2004, 2002). These terranes have zircon Hf isotopic TDM ages of > 2.7 Ga, suggesting their ancient basements (> 2.7 Ga) older than those of the intra-oceanic arc system to the north, and show a giant continental arc system. They have evolved a tectonic setting of active convergent margin, documenting an arc-continental accretion between the arc terranes and the interior Eastern Block of North China Craton (Fig. 1; Wang W et al., 2016, 2015 and references therein). The amphibolites and TTG type of rocks from the Guandi complex formed at ~2.54-2.56 Ga. The amphibolites have affinities dominantly to CABs rather than MORBs and IATs; the TTG-type of rocks have affinities to the low Mg TTGs, which are different from that of the 2.5-2.6 intra-oceanic arc system; both of their zircon Hf isotopes show that the source region are predominantly composed of 2.7-2.9 Ga crusts (Table S2), thus they should be part of the continental arc system rather than of the 2.5-2.6 intra-oceanic arc system. In addition, this TTG-type of rocks formed at ~2.54-2.55 Ga, about 50 Ma earlier than the accretion of the continental arc terrane to the interior Eastern Block of North China Craton. This allow a tectonic model to be proposed with more complex than that proposed by Wang W et al. (2015) in which there was a continental marginal arc developed locally while the parallel pared intra-oceanic and continental arc systems were prevailing regionally (Fig. 16).

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Figure 16. Late Neoarchean tectonic evolution model for the Guandi complex on northwestern margin of the eastern block of North China Craton (modified after the model proposed by Wang W et al., 2016; Wang J P et al., 2015). (a) Subduction-accretion and arc magmatism at 2.54–2.56 Ga; (b) arc-continent collision and metamorphism at 2.48–2.50 Ga.
6 CONCLUSIONS

(1) This study suggests that the "migmatite-like" rocks from the Guandi complex in Zhoukoudian area, southwestern Beijing turn out to be a suite of metamorphosed mingled intrusive rocks composed of TTGs (felsic gneisses) and basaltic components (amphibolites). The two kinds of rocks have their magmatic protoliths formed coevally during ~2.56-2.54 Ga. The precursor magmas of protoliths of the amphibolites were sourced from an enriched lithospheric mantle source metasomatized by subduction-derived melts and/or fluids or in arc-related setting; the TTG magmas were generated by partially melting of hydrated metabasites from thickened lower crust in an arc terrane. Both the lithospheric mantle and the lower crust were formed in 2.7-2.9 Ga, thus probably representing a segment of the giant Neoarchean continental arc system along the inner margin of the interior Eastern Block of the North China Craton.

(2) The TTGs and basic rocks underwent ~2.50-2.49 Ga medium-P/T type upper amphibolite-facies metamorphism, with peak temperature conditions of 674-727 ℃ at pressure conditions of 0.55-0.90 GPa, coevally with metamorphic events occurred in other Archean basement complexes in the Eastern Block of the North China Craton, documenting an accretion or collision tectonic event.

(3) The Guandi complex in Zhoukoudian area, southwestern Beijing probably experienced a complex plate tectonic history with thickening of a continental marginal arc system conducted locally but coevally with the regional development of the pared continental margin arc and intra-oceanic arc along the northwestern border of the interior Eastern Block of North China Craton.

ACKNOWLEDGMENTS

This paper is dedicated to the celebration of Prof. Zhendong You's 90th birthday. The corresponding author Neng-Song Chen has been benefited, throughout his life from Prof. Zhendong You who is the supervisor of both the Master's and Doctoral degrees for him, and led him into the world of Metamorphic Geology. We would like to express our great thanks to two anonymous reviewers, whose constructive suggestions and comments have led to great improvements in the quality of this manuscript. We express our gratitude to Prof. Walter Mooney for his helpful suggestion and discussion. Professor Jingbo Liu is specially thanked for his kindly helps with English correction. This study was supported by the National Natural Science Foundation of China (No. 41672060) and the Undergraduate Teaching Projects of China University of Geosciences (Nos. ZL201610 and 2018G36). The final publication is available at Springer via https://doi.org/10.1007/s12583-018-0856-7.

Electronic Supplementary Materials: Supplementary materials (Tables S1-S6) are available in the online version of this article at https://doi.org/10.1007/s12583-018-0856-7.


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