Episodic crustal growth and reworking at the southeastern margin of the North China Craton:evidence from zircon U-Pb and Lu-Hf isotopes of Archean tonalite-trondhjemite-granodiorite gneisses in the Bengbu-Wuhe area

Jianjun Wan·Andong Wang,3·Jiayong Pan·Chengdong Liu·Yan Zhao·Zhengbing Zhou·Xiandong Luo

Abstract The cratonization history of the North China Craton(NCC)and the nature of tectonothermal events are still highly controversial.Tonalite-trondhjemite-granodiorite(TTG)gneisses,as the dominant lithological assemblages in Archean metamorphic terranes,can provide signif icant clues to the magmatic and metamorphic evolution of Precambrian crust.This study presents zircon laser-ablation inductively-coupled-plasma mass spectrometry U–Pb ages,trace-element,and in-situ LA-MC-ICPMS zircon Hf isotope data for the TTG gneisses from the Bengbu-Wuhe area on the southeastern margin of the NCC.Cathodoluminescence images and trace elements indicated that magmatic zircons display the characteristics of euhedral-subhedral crystals with oscillatory growth zoning structures,highΣREE contents,marked Ce positive anomalies,and Pr–Eu negative anomalies.The metamorphic zircons display the spherical-oval crystals with distinct core-rim structures, high and homogeneous luminescent intensity,lowerΣREE,Nb,Ta,Hf contents,relative f lat REE patterns,weak Ce positive anomalies,and Pr-Eu negative anomalies.The Ti–in–zircon geothermometer data indicate that the crystallization temperature of the TTG gneiss ranged from 754 to 868°C.Zircon U–Pb ages indicate that the TTG gneisses formed at 2.79–2.77 Ga and 2.50 Ga and underwent metamorphism at 2.57–2.52 Ga.The Hf isotopic data indicate that the magmatic zircons exhibit high,positiveεHf(t)values close to those of the coeval depleted mantle,whereas the metamorphic zircons exhibit negative or nilεHf(t)values.This implies that the TTG gneisses were derived from the partial melting of the～2.9–2.6 Ga juvenile crustal sources mixed with～3.0–2.8 Ga ancient crustal materials.Combined with the regional tectonic evolution,we propose that the metamorphic basement at the southeastern margin of the NCC underwent episodic crustal growth at～2.7 and～2.5 Ga and subsequently underwent crustal reworking or re-melting of the ancient crust during the Neoarchean.The Neoarchean TTG gneisses might have been derived from the partial melting of lower crustal materials related to plate subduction.

Keywords In-situ zircon Hf isotope·Neoarchean·North China Craton·TTG gneiss·Zircon U–Pb dating

1 Introduction

Episodic growth of juvenile continental crust has been recognized,and the Early Precambrian is regarded as a crucial period of continental crust formation(e.g.,Rapp et al.1991;Condie 2005).Globally,3.6,2.7,and 1.8 Ga tectonothermal events were signif icant for the growth of continental crust(Barker and Arth 1976;Jahn et al.1981;Martin 1993;Martin and Moyen 2002;Iizuka et al.2005;Santosh 2010;Liu et al.2015;Liu and Cai 2017).Compared with other ancient cratons,the most distinct feature of the North China Craton(NCC)is widespread～2.5 Ga Late Neoarchean tectonothermal activity or magmatism(Zhai et al.2005,2010;Zhao et al.2007,2008;Liu et al.2009,2013a,b,2015;Geng et al.2010;Diwu et al.2011;Zhou et al.2014;Liu and Cai 2017).Abundant Sr,Nd,Pb,and Hf isotopic data from the NCC indicate that the records of～2.7 and 2.5 Ga tectonothermal events may represent important periods of crustal growth(Wu et al.2005;Diwu et al.2011;Gong et al.2012;Liu et al.2013b,2014;Liu and Cai 2017).

The NCC is one of the oldest tectonic units in China and has long evolutionary and complex tectonic histories(Huang et al.1977;Zhao et al.1998,2001;Zhai et al.2000,2005;Santosh et al.2006;Geng et al.2010;Santosh 2010;Wan et al.2011;Liu et al.2014).The NCC comprises an assemblage of Precambrian rocks that were incorporated into f ive major crustal blocks(the Khondalite Belt,Jiao-Liao-Ji Belt,Tans-North China Orogen,Eastern and Western Blocks),which formed the coherent NCC basement at～1.85 Ga,coinciding with the f inal assembly of the Columbia supercontinent(Fig.1a;Jahn et al.1981;Rapp et al.1991;Smithies 2000;Zhao et al.2001,2008;Martin and Moyen 2002;Condie 2005;Liu et al.2009,2018,2019a,b;Kusky 2011;Zhai and Santosh 2011;Gong et al.2012).Abundant structural,petrological,geochemical,and geochronological data on the various metamorphic basement rocks from the NCC crustal blocks have been obtained by previous studies,and tectonic models involving different collisional ages/stages and subduction polarities have been presented in recent decades(Zhai et al.2005,2010;Zhao et al.2007,2008;Liu et al.2009,2013a,2013b,2015;Geng et al.2010;Diwu et al.2011;Zhou et al.2014;Liu and Cai 2017).However,the specif ic characteristics of the tectonothermal events and tectonic models remain controversial.Furthermore,most of these studies have been restricted to the Trans-North China Orogen,Khondalite Belt,and Jiao-Liao-Ji Belt in the central,western,and eastern parts of the NCC,respectively.Moreover,a relatively small number of studies have elucidated the ancient crystallization basement along the southeastern margin of the NCC,while limited research has been conducted on the Mesozoic granitic rock containing deep-seated xenoliths and the detrital zircons derived from metasediments(Wang et al.2009,2014;Yang et al.2009,2010;Liu et al.2013a,2018,2019a,b,2020;Liu and Cai 2017;Ding et al.2018).The boundaries of microblocks are not fully understood;therefore,understanding the magmatism and metamorphism along the margins is important for discussing the formation and collision of these microblocks.

As the dominant component of Archean cratons,tonalite–trondhjemite–granodiorite (TTG) rocks, which account for 70%of the NCC Precambrian basement,have been regarded as a useful tool for understanding the continental crustal evolution of different blocks in the NCC,such as the Zhongtiaoshan-Wutai-Fuping(central NCC),Western Shandong(eastern NCC),and Guyang(western NCC)areas(Barker and Arth 1976;Jahn et al.1981;Zhai et al.2000,2005;Zhao et al.2001,2007,2008;Zhou et al.2009;Wan et al.2011;Gong et al.2012;Moyen and Martin 2012).The Bengbu-Wuhe area,located in northern Anhui Province at the southeastern margin of the NCC,is generally considered to be important because of its location at the boundary between the NCC and the Yangtze Craton(Liu et al.2009,2013a,2015;Yang et al.2010;Wang et al.2013;Liu and Cai 2017).Recent geochronology data from magmatic rocks around the Bengbu area have indicated that two magmatic events occurred at 2.7 and 2.5 Ga(Wan et al.2010;Wang et al.2012,2014;Yang et al.2012;Liu et al.2013a,b);2.9 Ga xenocrystic zircons have also been observed(Yang et al.2012;Liu et al.2015,2018;Liu and Cai 2017).In the present study,three representative TTG gneiss samples were collected from the ancient crystallization basement in northern Anhui Province at the southeastern margin of the NCC.Detailed f ield investigations,petrography,zircon cathodoluminescence(CL)imaging,laser-ablation inductively-coupled-plasma mass spectrometry(LA-ICP-MS)U–Pb age dating,and zircon Hf isotopic analyses using LA-MC-ICP-MS were conducted to better understand the ages of magmatism and metamorphism,as well as the crustal growth and reworking of the Precambrian crust in the Bengbu-Wuhe area.

2 Regional geology and sampling

The Bengbu-Wuhe area is located in the Eastern Block at the southeastern margin of the NCC(Fig.1a).It borders the Tan-Lu fault zone and the Southwest Sulu Orogen to the west and the Qinling-Dabie Orogen and Hefei Basin to the south(Fig.1b)(Zhao et al.2008;Zhai et al.2010;Wan et al.2011;Liu et al.2013a,2015).The Early Precambrian basement rocks in the Bengbu-Wuhe area are dominated by the Huoqiu and Wuhe Complexes(Zhai et al.2005,2010;Liu et al.2009;Wan et al.2011;Wang et al.2013;Zhou et al.2014;Liu and Cai 2017).The Wuhe Complex is distributed throughout the Bengbu-Wuhe area and neighboring Huaiyuan and Fengyang areas and is mainly composed of metamorphosed maf ic/felsic and supracrustal rocks,along with Paleoproterozoic K-feldspar granites and voluminous Mesozoic granitoids(Fig.1c).The main rock types of the Wuhe Complex include granitoid gneisses,quartz–muscovite schists,plagioclase–amphibole gneisses,biotite plagiogneisses,garnet amphibolites,garnet-bearing granulites,meta-sandstones and impure marbles(Wang et al.2009,2013;Liu et al.2013a,2015;Liu and Cai 2017).Zircon U–Pb geochronologies data of these rocks indicate that the Wuhe Complex formed at 3.0–2.8 Ga and～2.5 Ga and underwent metamorphism at 1.9–1.8 Ga(Tu 1994;Santosh et al.2006;Xu et al.2006;Santosh 2010;Wang et al.2013;Liu and Cai 2017;Liu et al.2018,2019a,b).Certain parts of the Paleoproterozoic K-feldspar granitic plutons are located in the study area,including the Zhuangzili(～2104 Ma)and Mopanshan(～2196 Ma)plutons(Fig.1c),which are considered the products of partial melting of Neoarchean juvenile crust in an extensional tectonic setting(Xu et al.2006;Guo and Li 2009;Yang et al.2009,2010).Additionally,1.9–1.7 Ga multi-stage metamorphism has been identif ied in the Bengbu-Wuhe area(Santosh et al.2006;Xu et al.2006;Guo and Li 2009;Liu et al.2009;Santosh 2010;Wang et al.2013;Liu and Cai 2017).

Fig.1 a Distribution of metamorphic basement and tectonic subdivisions of the North China Craton(modif ied after Zhao et al.2007);b Geologic sketch map of the southeastern margin of the NCC(modif ied after Xu et al.2006).The rectangle indicates the location of the Bengbu uplift;c Sketch map of the exposed basement and granitic plutons of the Bengbu uplift(modif ied after Liu et al.2015).The sample locations and geographic names are indicated by yellow stars

Three TTG gneiss samples were collected from the Wuhe Complex in this study.These samples included 1 Shizishan granodioritic gneiss (14SZS01) and 2 Xiangmiaocun trondhjemitic gneisses(14XMC01 and 14XMC02),as shown in Figs.1c,2.Field observations revealed that the dark tongue-shaped Paleoproterozoic amphibolites and white quartz veins distinctly intruded into the light-colored TTG gneisses(Fig.2b,c,f).Gneissic banding with the same direction developed in these rocks,indicating that the TTG gneisses of the Wuhe Complex and Paleoproterozoic amphibolites may have undergone later deformational metamorphism(Fig.2b,c).

Fig.2 Outcrop photographs of granitic gneisses in the Bengbu-Wuhe area,southeastern margin of the NCC.a,b Light-colored Archean granodioritic gneisses(sample 14SZS01)at Shizishan;c,d Light-colored Archean trondhjemitic gneisses(sample 14XMC01)intruded by dark Paleoproterozoic amphibolite at Xiangmiaocun;e,f Latter quartz vein intruding into the Archean trondhjemitic gneisses(sample 14XMC02)at Xiangmiaocun

Fig.3 Representative micrographs of TTG gneisses from the Bengbu-Wuhe area.a Granodioritic gneisses(14SZS01)from Shizishan;(b,c)Trondhjemitic gneisses(14XMC01,14XMC02)from Xiangmiaocun;d–f Oriented biotite and other dark-colored minerals constituting typical gneissic banding of the Bengbu-Wuhe TTG gneisses.a,b,c,e,f are under cross-polarized light,and d is under plane-polarized light.Bi=Biotite;Chl=Chlorite;Chl-Bi=Chloritization of biotite;Kfs=K-feldspar;Mc=Microcline;Per=Perthite;Pl=Plagioclase;Q=Quartz

3 Analytical methods

Sample preparation and zircon separation were conducted at the State Key Laboratory of Nuclear Resources and Environment,East China University of Technology of China in Nanchang.Approximately 8–10 kg of rock was crushed to obtain zircons.Zircon selection was carried out using the heavy-liquid f lotation and magnetic techniques.The selected zircons were f ixed using epoxy resin and a curing agent and then polished to approximately one-third of their original thickness.CL images of the zircons were obtained using a f ield emission scanning electron microscope(Nova NanoSEM 450,Thermo Fisher Scientif ic,Oregon,USA)at the State Key Laboratory of Nuclear Resources and Environment,East China University of Technology.

3.1 Zircon LA-ICP-MS U-Pb dating and trace elemental analyses

U–Pb dating and trace elemental analyses of single zircon grains were conducted using LA-ICP-MS(GeoLas Pro 193 nm ArF excimer laser,Coherent,California,USA,and 7500a ICP-MS,Agilent,California,USA)at the School of Resource and Environmental Engineering,Hefei University of Technology of China in Hefei.The diameter of the laser beam spots was 30μm.Each measurement incorporated a background acquisition of approximately 30 s(gas blank)followed by 60 s of data acquisition from the sample.The degree of data concordance exceeded 95%.The data reduction software ICPMS Data Cal performed off-line selection and the integration of the background and analyte signals,along with time-drift corrections and quantitative calibrations for trace element analyses and U–Pb dating(Liu et al.2008,2010).The detailed analytical procedure is described in Tu et al.(2011).U–Th–Pb ratios were determined relative to the 1065 Ma 91,500 zircon reference material,which has Th and U concentrations of～29 and 81μg/g,respectively(Wiedenbeck et al.1995).Concordia diagrams and weighted mean calculations were generated using Isoplot 3.0 (Berkeley Geochronology Center,USA)(Version 3.00,Ludwig 2003).Pb/Pb ages were used to represent the model age for a single zircon.Trace elemental compositions of the zircons were calibrated using the NIST-610 reference material combined withZr internal standardization.The elemental contents of the NIST-610 material were referenced to the GeoReM database(http://georem.mpchmainz.gwdg.de/).

3.2 In-situ Lu-Hf isotope analyses

In-situ

zircon Lu–Hf isotope analyses were conducted using laser ablation (RESOlution M-50-LR Excimer,Australian Scientif ic,Fyshwick,Australia)and isotope analyses(Neptune Plus MC-ICP-MS,Thermo Fisher Scientif ic,Waltham,MA,USA)at the State Key Laboratory of Isotope Geochemistry,Guangzhou Institute of Geochemistry,Chinese Academy of Sciences in Guangzhou.The diameter of the laser beam spots was 45μm,and the laser repetition rate was 8 Hz at 80 mJ for an ablation time of 40 s.During the analyses,the averageHf/Hf ratio obtained for the Penglai zircon standard was 0.282902±0.000009,which is consistent with the recommendedHf/Hf ratio for this standard(Li et al.2010).The experimental process and instrumentation setup are described in detail by Zhang et al.(2014),in which aLu decay constant of 1.867×10a(So¨derlund et al.2004)was used to calculate theε(t)values and Hf model ages.In this study,thePb/Pb age of each analyzed zircon was used to calculate the corresponding ε(t)value and the Hf model age.

4 Analytical results

4.1 Zircon U-Pb ages and trace elemental compositions

Representative CL images and U–Pb dates for the zircons from samples 14SZS01,14XMC01,and 14XMC02 are shown in Tables 1 and 2,and Fig.4,respectively.

4.1.1 Shizishan granodioritic gneiss(Sample 14SZS01)

Zircons from sample 14SZS01 were mostly euhedral–subhedral in shape,with lengths ranging from 150 to 250μm and aspect ratios of 1–3.The CL images show that most of the zircons exhibit relatively high brightness and visible oscillatory zoning structures,both of which are indicative of magmatic origin.A few zircons had narrow metamorphic growth rims that were diff icult to analyze(Fig.4a).Thirty-three typical magmatic zircons were selected for analysis,the results of which are listed in Tables 1 and 2.Most of the analyzed zircons were characterized by relatively low Th and U contents with concentrations ranging from 49 to 437 ppm and from 33 to 232 ppm(except for spot 27),respectively.The Th/U ratios of zircons ranged from 0.87 to 2.38(except for spot 27),which are consistent with the Th/U discrimination criterion for magmatic zircons proposed by Hoskin and Black(2000)and Wu and Zheng(2004).The total rare earth element content(ΣREE)had an average of 431 ppm,and the Lu/Hf ratio had an average of 0.0025.As shown in Fig.5a,the chondrite normalized REE patterns of the zircons indicate the enrichments in heavy REEs(HREEs),and relative depletions in light REEs(LREEs).A relatively strong negative Eu anomaly(meanδEu=0.24),strong positive Ce anomaly(meanδCe＞10),and weak negative Pr anomaly are also consistent with the typical REE patterns of magmatic zircons(Belousova et al.2002;Rubatto 2002;Hoskin and Schaltegger 2003;Li 2015;Liu et al.2015;Li et al.2018;Zhong et al.2018).The Ti–in–zircon temperatures were calculated using the Ti–in–zircon thermometer formula as follows(Watson and Harrison 2005;Watson et al.2006):

Fig.4 Representative cathodoluminescence(CL)images and analysis spots of magmatic and metamorphic zircons from the TTG gneisses in the Bengbu-Wuhe area.The horizontal line in the upper right corner is a 150 microns scale,the black circle is the analysis spot location,and the lower number is the test point number(laser ablation point)

The Ti contents varied from 4.49 to 9.92(average value of 6.55 ppm),with a corresponding temperature range of 675–740°C (average value of 704°C)(Fig.6).ThePb/Pb ages of spots 25,27,and 28(2635,2639,and 2610 Ma,respectively)were signif icantly older than those of the other zircons,indicating that they may have inherited magmatic zircons.ThePb/Pb ages of the other 30 zircons ranged from 2443 to 2587 Ma,with a weighted mean age of 2501±13 Ma(mean square weighted deviation(MSWD)=0.75).This conforms to an upper intercept age of 2510±21 Ma(MSWD=0.95;Fig.5b)and can be considered the crystallization age of the Shizishan granodioritic gneiss.

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Fig.6 Plot of Ti–in–zircon temperatures vs.207Pb/206Pb ages of the zircons from the TTG gneisses in Shizishan and Xiangmiaocun(Bengbu-Wuhe area)

Spot No. Contents(ppm) Th/U Isotopic ratios Zircon type 232Th 238U 207Pb/206Pb 1σ 207Pb/235U 1σ 206Pb/238U 1σ 14SZS01 1 348 203 1.71 0.1670 0.0064 10.5798 0.3926 0.4591 0.0067 Mag 2 123 79 1.55 0.1690 0.0065 10.9724 0.4055 0.4707 0.0072 Mag 3 136 86 1.58 0.1697 0.0066 10.9441 0.4098 0.4644 0.0072 Mag 4 183 77 2.38 0.1678 0.0078 11.2779 0.4626 0.4623 0.0079 Mag 5 352 193 1.82 0.1698 0.0073 11.0049 0.4468 0.4606 0.0076 Mag 6 124 86 1.43 0.1668 0.0069 10.4863 0.4089 0.4493 0.0081 Mag 7 49 33 1.49 0.1690 0.0078 11.1350 0.4760 0.4759 0.0093 Mag 8 76 52 1.46 0.1645 0.0067 10.3088 0.3667 0.4321 0.0073 Mag 9 72 55 1.30 0.1705 0.0058 11.3399 0.3876 0.4790 0.0084 Mag 10 292 185 1.58 0.1659 0.0049 11.0253 0.3084 0.4815 0.0066 Mag 11 93 90 1.03 0.1621 0.0051 11.2957 0.3620 0.5036 0.0090 Mag 12 89 63 1.43 0.1611 0.0063 10.6323 0.3950 0.4799 0.0079 Mag 13 359 209 1.72 0.1629 0.0049 10.4051 0.2864 0.4654 0.0054 Mag 14 66 41 1.60 0.1709 0.0070 11.1345 0.4120 0.4686 0.0081 Mag 15 49 33 1.47 0.1591 0.0081 10.2701 0.4357 0.4869 0.0108 Mag 16 215 132 1.63 0.1719 0.0067 10.6920 0.3721 0.4552 0.0060 Mag 17 116 134 0.87 0.1659 0.0061 10.2426 0.3375 0.4499 0.0063 Mag 18 316 171 1.86 0.1636 0.0061 10.7002 0.3688 0.4724 0.0066 Mag 19 247 143 1.74 0.1618 0.0061 10.5874 0.3729 0.4731 0.0069 Mag 20 225 141 1.59 0.1601 0.0063 10.5827 0.4033 0.4750 0.0073 Mag 21 92 65 1.41 0.1614 0.0072 10.5839 0.4545 0.4706 0.0093 Mag 22 68 59 1.14 0.1651 0.0067 10.9612 0.4346 0.4773 0.0084 Mag 23 107 74 1.44 0.1689 0.0069 10.6925 0.4310 0.4565 0.0083 Mag 24 339 193 1.75 0.1696 0.0068 10.7257 0.4156 0.4547 0.0065 Mag 25 88 57 1.54 0.1781 0.0078 11.2706 0.4505 0.4547 0.0076 Mag 26 59 39 1.53 0.1636 0.0099 10.9263 0.5365 0.4402 0.0096 Mag 27 26 403 0.06 0.1785 0.0072 11.4134 0.4566 0.4705 0.0093 Mag 28 437 232 1.89 0.1753 0.0073 11.5523 0.4698 0.4799 0.0069 Mag 29 121 76 1.60 0.1729 0.0082 10.8856 0.4933 0.4533 0.0076 Mag 30 60 48 1.25 0.1664 0.0095 10.8956 0.5722 0.4796 0.0092 Mag 31 102 70 1.46 0.1670 0.0082 10.9811 0.5175 0.4722 0.0089 Mag 32 375 215 1.74 0.1624 0.0069 10.7464 0.4301 0.4815 0.0068 Mag 33 183 103 1.77 0.1588 0.0076 10.2963 0.4353 0.4586 0.0079 Mag Spot No. Contents(ppm) Th/U Corrected ages(Ma) Zircon type 232Th 238U 207Pb/206Pb 1σ 207Pb/235U 1σ 206Pb/238U 1σ 14SZS01 1 348 203 1.71 2528 32 2487 35 2436 29 Mag 2 123 79 1.55 2548 32 2521 34 2487 32 Mag 3 136 86 1.58 2554 32 2518 35 2459 32 Mag 4 183 77 2.38 2535 39 2546 38 2450 35 Mag 5 352 193 1.82 2555 36 2524 38 2442 34 Mag 6 124 86 1.43 2526 35 2479 36 2392 36 Mag 7 49 33 1.49 2548 41 2534 40 2509 41 Mag 8 76 52 1.46 2502 34 2463 33 2315 33 Mag 9 72 55 1.30 2562 28 2551 32 2523 37 Mag

Spot No. Contents(ppm) Th/U Corrected ages(Ma) Zircon type 232Th 238U 207Pb/206Pb 1σ 207Pb/235U 1σ 206Pb/238U 1σ 10 292 185 1.58 2516 25 2525 26 2534 29 Mag 11 93 90 1.03 2477 26 2548 30 2629 39 Mag 12 89 63 1.43 2478 33 2492 35 2527 34 Mag 13 359 209 1.72 2487 25 2472 26 2463 24 Mag 14 66 41 1.60 2566 34 2534 35 2477 36 Mag 15 49 33 1.47 2446 43 2459 39 2557 47 Mag 16 215 132 1.63 2576 33 2497 32 2418 27 Mag 17 116 134 0.87 2517 31 2457 31 2395 28 Mag 18 316 171 1.86 2494 31 2497 32 2494 29 Mag 19 247 143 1.74 2476 32 2488 33 2497 30 Mag 20 225 141 1.59 2457 34 2487 35 2505 32 Mag 21 92 65 1.41 2470 37 2487 40 2486 41 Mag 22 68 59 1.14 2509 34 2520 37 2515 37 Mag 23 107 74 1.44 2547 34 2497 38 2424 37 Mag 24 339 193 1.75 2554 33 2500 36 2416 29 Mag 25 88 57 1.54 2635 36 2546 37 2416 34 Mag 26 59 39 1.53 2494 51 2517 46 2352 43 Mag 27 26 403 0.06 2639 34 2558 37 2486 41 Mag 28 437 232 1.89 2610 35 2569 38 2527 30 Mag 29 121 76 1.60 2587 40 2513 42 2410 34 Mag 30 60 48 1.25 2521 48 2514 49 2526 40 Mag 31 102 70 1.46 2528 41 2522 44 2493 39 Mag 32 375 215 1.74 2481 35 2501 37 2534 30 Mag 33 183 103 1.77 2443 41 2462 39 2434 35 Mag Spot No. Contents(ppm) Th/U Isotopic ratios Zircon type 232Th 238U 207Pb/206Pb 1σ 207Pb/235U 1σ 206Pb/238U 1σ 14XMC01 1 73 131 0.56 0.1876 0.0072 14.5670 0.5212 0.5462 0.0064 Mag 2 78 281 0.28 0.1924 0.0062 15.4225 0.5255 0.5769 0.0084 Mag 3 79 885 0.09 0.1629 0.0054 11.6024 0.4059 0.5124 0.0068 Meta 4 81 135 0.60 0.1605 0.0059 10.6292 0.3932 0.4792 0.0064 Meta 5 137 636 0.21 0.1879 0.0073 14.1288 0.5368 0.5473 0.0069 Mag 6 109 206 0.53 0.1733 0.0059 11.4502 0.4178 0.4827 0.0068 Meta 7 97 225 0.43 0.2088 0.0064 15.7342 0.4592 0.5621 0.0071 Mag 8 5 20 0.28 0.1628 0.0089 8.3188 0.4460 0.3913 0.0095 Meta 9 157 620 0.25 0.1714 0.0066 9.8575 0.3631 0.4267 0.0065 Meta 10 74 176 0.42 0.1689 0.0056 10.3980 0.3305 0.4526 0.0061 Meta 11 121 298 0.41 0.1685 0.0052 10.8266 0.3216 0.4708 0.0057 Meta 12 109 158 0.69 0.1641 0.0058 10.7355 0.3503 0.4768 0.0065 Meta 13 171 330 0.52 0.1898 0.0063 13.0507 0.4554 0.4985 0.0073 Mag 14 107 215 0.50 0.1636 0.0054 10.7865 0.3557 0.4754 0.0061 Meta 15 629 527 1.19 0.1889 0.0058 13.1882 0.4000 0.5035 0.0062 Mag 16 82 181 0.45 0.0959 0.0086 5.9469 0.2294 0.3340 0.0059 Meta 17 98 126 0.78 0.1668 0.0059 10.9269 0.3818 0.4729 0.0065 Meta 18 117 210 0.56 0.1739 0.0054 11.4458 0.3714 0.4757 0.0068 Meta 19 76 111 0.68 0.1728 0.0058 10.8411 0.3633 0.4649 0.0066 Meta

Spot No. Contents(ppm) Th/U Isotopic ratios Zircon type 232Th 238U 207Pb/206Pb 1σ 207Pb/235U 1σ 206Pb/238U 1σ 20 80 133 0.60 0.1607 0.0062 10.0686 0.3520 0.4522 0.0068 Meta 21 82 157 0.52 0.2012 0.0087 15.4109 0.5605 0.5407 0.0087 Mag 22 82 217 0.38 0.1679 0.0068 11.2850 0.4536 0.5018 0.0086 Meta 23 142 238 0.60 0.2066 0.0077 16.3506 0.5858 0.5744 0.0095 Mag 24 88 163 0.54 0.1855 0.0071 14.5628 0.4947 0.5428 0.0092 Mag 25 49 230 0.21 0.1963 0.0059 14.9182 0.4336 0.5576 0.0083 Mag 26 378 398 0.95 0.2062 0.0066 15.6897 0.4583 0.5568 0.0079 Mag 27 12 61 0.19 0.1883 0.0081 13.7931 0.5705 0.5292 0.0102 Mag 28 101 133 0.76 0.1597 0.0057 10.4401 0.3476 0.4724 0.0065 Meta 29 50 263 0.19 0.1856 0.0064 12.6104 0.4771 0.4766 0.0085 Mag 30 89 182 0.49 0.1885 0.0060 14.0220 0.3931 0.5228 0.0069 Mag Spot No. Contents(ppm) Th/U Corrected ages(Ma) Zircon type 232Th 238U 207Pb/206Pb 1σ 207Pb/235U 1σ 206Pb/238U 1σ 14XMC01 1 73 131 0.56 2721 32 2787 34 2810 27 Mag 2 78 281 0.28 2765 27 2842 33 2936 34 Mag 3 79 885 0.09 2487 28 2573 33 2667 29 Meta 4 81 135 0.60 2461 31 2491 34 2524 28 Meta 5 137 636 0.21 2724 32 2758 36 2814 29 Mag 6 109 206 0.53 2591 29 2561 34 2539 30 Meta 7 97 225 0.43 2896 25 2861 28 2875 29 Mag 8 5 20 0.28 2487 47 2266 49 2129 44 Meta 9 157 620 0.25 2572 32 2422 34 2291 30 Meta 10 74 176 0.42 2547 25 2471 30 2407 27 Meta 11 121 298 0.41 2543 24 2508 28 2487 25 Meta 12 109 158 0.69 2498 30 2500 30 2514 28 Meta 13 171 330 0.52 2740 27 2683 33 2608 31 Mag 14 107 215 0.50 2494 28 2505 31 2507 27 Meta 15 629 527 1.19 2733 25 2693 29 2629 27 Mag 16 82 181 0.45 1546 85 1968 34 1858 29 Meta 17 98 126 0.78 2526 30 2517 33 2496 29 Meta 18 117 210 0.56 2595 26 2560 30 2509 30 Meta 19 76 111 0.68 2585 28 2510 31 2461 29 Meta 20 80 133 0.60 2463 33 2441 32 2405 30 Meta 21 82 157 0.52 2836 35 2841 35 2786 36 Mag 22 82 217 0.38 2537 32 2547 38 2622 37 Meta 23 142 238 0.60 2879 30 2898 34 2926 39 Mag 24 88 163 0.54 2702 31 2787 32 2795 39 Mag 25 49 230 0.21 2796 27 2810 28 2857 34 Mag 26 378 398 0.95 2876 26 2858 28 2854 33 Mag 27 12 61 0.19 2727 36 2736 39 2738 43 Mag 28 101 133 0.76 2454 33 2475 31 2494 28 Meta 29 50 263 0.19 2706 29 2651 36 2513 37 Mag 30 89 182 0.49 2729 26 2751 27 2711 29 Mag

Spot No. Contents(ppm) Th/U Isotopic ratios Zircon type 232Th 238U 207Pb/206Pb 1σ 207Pb/235U 1σ 206Pb/238U 1σ 14XMC02 1 156 321 0.49 0.1933 0.0079 13.9515 0.5054 0.5086 0.0074 Mag 2 68 1886 0.04 0.1703 0.0063 10.8097 0.3510 0.4480 0.0053 Meta 3 267 435 0.61 0.1998 0.0072 14.7336 0.4859 0.5211 0.0063 Mag 4 50 118 0.43 0.2003 0.0074 14.2454 0.4873 0.5057 0.0070 Mag 5 42 114 0.37 0.1975 0.0077 13.7737 0.4928 0.4987 0.0073 Mag 6 102 234 0.44 0.1948 0.0078 13.0641 0.4892 0.4805 0.0068 Mag 7 71 205 0.35 0.2003 0.0072 14.6366 0.4934 0.5246 0.0070 Mag 8 84 206 0.41 0.1986 0.0064 15.0878 0.4686 0.5464 0.0074 Mag 9 86 196 0.44 0.1648 0.0054 10.7098 0.3670 0.4676 0.0078 Meta 10 119 242 0.49 0.1662 0.0054 11.1072 0.3551 0.4783 0.0065 Meta 11 159 235 0.68 0.1764 0.0051 11.5104 0.3487 0.4670 0.0069 Meta 12 124 183 0.68 0.1726 0.0050 11.3921 0.3225 0.4748 0.0060 Meta 13 169 267 0.63 0.1778 0.0054 11.5811 0.3117 0.4692 0.0055 Meta 14 76 159 0.47 0.1641 0.0056 10.5173 0.3308 0.4607 0.0064 Meta 15 193 591 0.33 0.2039 0.0084 15.4438 0.6060 0.5403 0.0080 Mag 16 9 2075 0.00 0.1696 0.0063 11.5105 0.3980 0.4860 0.0066 Meta 17 104 258 0.40 0.1994 0.0079 15.2658 0.5132 0.5278 0.0070 Mag 18 134 260 0.52 0.1675 0.0061 10.1910 0.3543 0.4382 0.0059 Meta 19 149 820 0.18 0.1674 0.0060 10.6355 0.3663 0.4575 0.0052 Meta 20 108 199 0.54 0.1912 0.0077 14.2921 0.5134 0.5357 0.0069 Mag 21 114 236 0.48 0.2032 0.0071 15.9312 0.5407 0.5684 0.0077 Mag 22 315 393 0.80 0.1625 0.0056 10.0089 0.3405 0.4387 0.0063 Meta 23 177 377 0.47 0.1960 0.0068 14.3372 0.5045 0.5299 0.0075 Mag 24 144 339 0.42 0.1746 0.0071 11.3688 0.3947 0.4403 0.0065 Meta 25 90 464 0.19 0.1716 0.0068 10.9779 0.3910 0.4403 0.0078 Meta 26 65 164 0.40 0.2000 0.0072 15.7632 0.5232 0.5541 0.0081 Mag 27 94 327 0.29 0.1953 0.0066 14.3297 0.4756 0.5198 0.0075 Mag 28 49 125 0.39 0.2071 0.0077 16.3516 0.5870 0.5617 0.0090 Mag 29 33 98 0.34 0.1946 0.0079 13.8000 0.5565 0.5033 0.0091 Mag 30 82 168 0.49 0.1764 0.0064 12.4442 0.4208 0.4923 0.0074 Meta Spot No. Contents(ppm) Th/U Corrected ages(Ma) Zircon type 232Th 238U 207Pb/206Pb 1σ 207Pb/235U 1σ 206Pb/238U 1σ 14XMC02 1 156 321 0.49 2770 34 2746 34 2651 32 Mag 2 68 1886 0.04 2561 31 2507 30 2387 24 Meta 3 267 435 0.61 2824 30 2798 31 2704 27 Mag 4 50 118 0.43 2829 30 2766 33 2638 30 Mag 5 42 114 0.37 2805 32 2734 34 2608 32 Mag 6 102 234 0.44 2783 33 2684 35 2529 30 Mag 7 71 205 0.35 2829 30 2792 32 2719 29 Mag 8 84 206 0.41 2815 26 2821 30 2810 31 Mag 9 86 196 0.44 2505 27 2498 32 2473 34 Meta 10 119 242 0.49 2520 27 2532 30 2520 28 Meta 11 159 235 0.68 2620 24 2565 28 2470 30 Meta

Spot No. Contents(ppm) Th/U Corrected ages(Ma) Zircon type 232Th 238U 207Pb/206Pb 1σ 207Pb/235U 1σ 206Pb/238U 1σ 12 124 183 0.68 2583 8 2556 27 2505 26 Meta 13 169 267 0.63 2633 25 2571 25 2480 24 Meta 14 76 159 0.47 2498 29 2481 29 2443 28 Meta 15 193 591 0.33 2858 34 2843 38 2785 34 Mag 16 9 2075 0.00 2554 31 2565 32 2553 29 Meta 17 104 258 0.40 2821 33 2832 32 2732 30 Mag 18 134 260 0.52 2533 30 2452 32 2343 26 Meta 19 149 820 0.18 2531 30 2492 32 2429 23 Meta 20 108 199 0.54 2753 33 2769 34 2765 29 Mag 21 114 236 0.48 2854 29 2873 33 2901 32 Mag 22 315 393 0.80 2483 26 2436 31 2345 28 Meta 23 177 377 0.47 2794 29 2772 34 2741 32 Mag 24 144 339 0.42 2602 34 2554 33 2352 29 Meta 25 90 464 0.19 2574 33 2521 33 2352 35 Meta 26 65 164 0.40 2828 29 2863 32 2842 34 Mag 27 94 327 0.29 2787 28 2772 32 2698 32 Mag 28 49 125 0.39 2883 30 2898 34 2873 37 Mag 29 33 98 0.34 2783 34 2736 38 2628 39 Mag 30 82 168 0.49 2620 31 2639 32 2580 32 Meta

4.1.2 Xiangmiaocun trondhjemitic gneisses(Samples 14XMC01 and 14XMC02)

Zircons from the Xiangmiaocun trondhjemitic gneisses(samples 14XMC01 and 14XMC02)are mostly subhedral and oval.Their lengths vary from 100 to 200μm,with aspect ratios of 1–2.Based on their interior structures shown by the images,these zircons can be classif ied into three types:subhedral grains with low brightness,relatively visible oscillatory zoning,and narrow edges,indicating a magmatic origin with potential inf luences from later metamorphism or recrystallization to some degree(type 1;e.g.,zircons in Fig.5b,d)(Hoskin and Black 2000;Wu and Zheng 2004);irregular grains with inner cores and relatively bright wide rims(core–rim texture),cores with weak or no oscillatory zoning,and rims with uniform brightness(type 2;e.g.,spots 11,14,and 17 in Fig.5c and spots 12,14,16,and 18 in Fig.5e);spherical or oval grains with relatively high uniform brightness and no oscillatory growth zoning(type 3;e.g.,spots 3,6,9,and 22 in Fig.5c and spots 2,9,and 10 in Fig.5e).The CL images indicate that type 1 zircons are magmatic in origin,whereas type 3 zircons and the rims of type 2 zircons are metamorphic in origin(Hoskin and Black 2000;Rubatto 2002;Wu and Zheng 2004;Li et al.2018;Zhong et al.2018).The distinction between magmatic and metamorphic zircons is further demonstrated by their trace elemental characteristics.

Fig.5 Chondrite-normalized rare earth element(REE)patterns,U–Pb Concordia diagram,and weighted mean ages of zircons from the Shizishan granodioritic gneisses(14SZS01)

Test spots La Ce Pr Nd Sm Eu Gd Tb Dy Zircon type 14SZS01 1 0.19 44.7 0.10 1.84 3.42 1.03 12.2 4.31 50.3 Mag 2 0.01 26.5 0.03 1.25 2.09 0.27 7.69 2.21 29.0 Mag 3 0.01 28.6 0.05 1.21 1.69 0.28 8.28 2.66 27.5 Mag 4 0.01 40.0 0.06 1.49 3.31 0.47 12.2 3.83 43.8 Mag 5 0.07 44.5 0.17 1.90 3.27 0.57 13.5 4.39 52.5 Mag 6 0.17 28.9 0.18 1.39 2.64 0.90 9.14 2.68 30.7 Mag 7 0.01 30.1 0.06 1.35 2.80 0.29 11.5 3.92 44.0 Mag 8 0.04 33.7 0.08 0.98 2.45 0.34 12.1 3.58 41.7 Mag 9 0.01 37.8 0.09 1.40 2.41 0.34 12.1 3.65 41.6 Mag 10 0.60 41.9 0.33 2.87 3.95 1.37 17.0 4.93 55.7 Mag 11 0.04 29.6 0.06 0.65 2.05 0.37 7.94 2.32 26.6 Mag 12 0.01 27.9 0.07 0.96 2.38 0.40 11.8 3.31 41.3 Mag 13 0.03 44.7 0.06 1.32 2.73 0.66 14.4 4.50 52.9 Mag 14 0.01 23.8 0.09 0.89 1.56 0.30 7.55 2.11 23.9 Mag 15 0.01 26.1 0.04 1.00 1.86 0.38 8.80 2.98 32.4 Mag 16 0.01 35.0 0.08 0.93 2.81 0.37 11.3 3.49 42.1 Mag 17 90.1 260 30.9 135 29.6 2.05 32.5 6.19 56.3 Mag 18 0.02 42.2 0.09 1.65 2.57 0.48 13.6 4.21 50.8 Mag 19 0.03 37.1 0.12 1.77 2.75 0.50 12.3 4.02 44.1 Mag 20 0.01 34.9 0.05 1.06 2.09 0.30 11.1 3.56 40.3 Mag 21 0.01 26.7 0.10 1.26 2.24 0.35 10.2 2.96 34.6 Mag 22 0.01 28.8 0.07 1.08 1.93 0.43 9.18 2.55 31.4 Mag 23 0.01 28.7 0.01 1.43 2.53 0.33 11.3 3.15 37.8 Mag 24 0.02 43.4 0.07 1.60 2.44 0.50 14.1 4.32 48.5 Mag 25 0.01 26.5 0.07 0.81 2.20 0.21 8.64 2.53 29.7 Mag 26 0.01 26.2 0.06 0.74 1.20 0.40 9.34 2.53 31.6 Mag 27 56.5 119 11.0 36.2 4.94 0.44 7.37 1.48 16.2 Mag 28 0.04 49.8 0.09 1.55 2.78 0.58 16.6 5.18 60.9 Mag 29 0.10 29.7 0.15 1.62 2.67 0.73 12.9 3.68 42.5 Mag 30 0.02 23.8 0.06 0.82 1.49 0.16 7.80 2.36 27.6 Mag 31 0.01 28.5 0.04 1.48 1.88 0.49 8.59 3.02 32.9 Mag 32 0.04 47.1 0.10 0.94 2.60 0.32 12.6 3.91 44.9 Mag 33 0.05 32.3 0.09 0.85 2.27 0.43 9.93 3.38 38.4 Mag Test spots Ho Er Tm Yb Lu Eu/Eu* P Ti TZr(°C) Zircon type 14SZS01 1 18.6 92.5 22.0 230 49.7 0.43 218 6.93 710 Mag 2 11.3 51.7 13.0 128 28.7 0.18 175 6.06 699 Mag 3 11.0 53.7 12.9 134 29.4 0.19 217 6.30 702 Mag 4 16.2 79.4 19.4 204 44.8 0.20 261 6.83 709 Mag 5 20.1 98.3 23.2 241 52.7 0.22 213 7.05 711 Mag 6 11.7 55.3 13.0 139 29.8 0.50 206 4.92 682 Mag 7 16.8 74.9 16.5 161 33.2 0.13 225 9.11 733 Mag 8 15.4 71.0 15.2 150 29.9 0.16 250 5.06 684 Mag 9 14.9 68.7 14.6 141 28.6 0.16 262 9.92 740 Mag 10 20.8 95.2 21.7 233 49.6 0.44 335 5.13 686 Mag 11 9.97 47.3 11.3 128 27.6 0.24 168 5.18 686 Mag 12 15.1 70.5 16.2 161 34.1 0.19 208 7.64 718 Mag

Test spots Ho Er Tm Yb Lu Eu/Eu* P Ti TZr(°C) Zircon type 13 20.3 99.6 24.0 246 54.0 0.26 266 6.59 706 Mag 14 9.21 43.8 10.4 106 22.8 0.22 131 8.25 724 Mag 15 12.0 57.2 12.6 133 27.3 0.24 203 7.18 713 Mag 16 15.9 78.2 18.6 198 42.1 0.17 246 6.07 699 Mag 17 19.3 87.0 19.5 208 43.7 0.20 7247 5.71 694 Mag 18 19.8 95.3 22.5 240 51.9 0.20 223 5.88 696 Mag 19 17.5 85.3 19.9 211 45.8 0.22 241 6.27 702 Mag 20 15.4 74.2 17.9 189 41.2 0.15 212 6.01 698 Mag 21 12.5 61.5 13.8 147 30.9 0.19 234 7.22 713 Mag 22 11.8 57.4 13.1 137 31.0 0.26 212 6.30 702 Mag 23 14.0 66.0 15.5 159 33.7 0.16 254 6.28 702 Mag 24 19.5 92.9 22.0 229 50.1 0.20 211 5.07 685 Mag 25 10.7 51.7 11.7 120 25.8 0.13 166 7.46 716 Mag 26 11.4 56.1 12.8 140 30.3 0.26 166 5.46 690 Mag 27 6.58 33.6 8.60 100 23.0 0.22 173 6.60 706 Mag 28 23.6 113 26.1 272 59.5 0.20 750 4.49 675 Mag 29 16.7 76.4 18.2 190 39.4 0.31 279 6.19 700 Mag 30 9.76 47.1 11.0 116 24.1 0.12 210 7.89 721 Mag 31 12.2 56.2 13.5 137 28.9 0.31 192 8.14 723 Mag 32 17.1 80.8 19.2 205 44.2 0.14 198 7.68 718 Mag 33 15.2 72.4 17.1 174 38.3 0.23 254 5.19 686 Mag Test spots La Ce Pr Nd Sm Eu Gd Tb Dy Zircon type 14XMC01 1 0.03 4.61 0.19 2.77 3.69 1.46 18.5 6.39 77.0 Mag 2 0.17 10.3 0.08 0.40 1.25 0.27 9.95 3.93 51.6 Mag 3 0.89 15.6 0.50 3.08 2.92 0.24 16.1 6.96 99.2 Meta 4 0.01 20.3 0.03 0.97 2.42 0.07 13.3 6.04 82.9 Meta 5 0.01 12.4 0.02 0.28 1.90 0.11 11.5 5.16 78.3 Mag 6 24.6 88.7 11.0 55.6 17.9 0.51 31.8 8.47 97.7 Meta 7 0.03 6.25 0.09 1.10 2.22 0.74 11.7 4.40 58.4 Mag 8 0.23 2.20 0.09 0.69 0.49 0.08 1.27 0.40 5.41 Meta 9 13.0 80.9 7.79 38.6 17.6 7.26 30.9 9.84 108 Meta 10 0.01 19.0 0.02 0.53 1.66 0.07 11.1 4.39 61.7 Meta 11 0.01 15.4 0.03 0.82 1.81 0.12 14.0 5.86 84.6 Meta 12 0.01 17.4 0.02 0.54 1.82 0.07 14.0 6.27 79.5 Meta 13 0.01 19.9 0.12 2.06 4.80 0.44 27.2 9.48 122 Mag 14 0.01 20.3 0.04 1.05 2.31 0.11 15.5 6.08 84.8 Meta 15 5.07 34.5 1.44 11.1 14.3 5.85 67.4 21.2 246 Mag 16 0.15 17.8 0.09 1.15 2.08 0.11 13.6 5.46 69.0 Meta 17 0.02 19.9 0.02 0.59 2.37 0.10 13.9 5.80 81.6 Meta 18 0.03 14.7 0.05 1.66 3.65 0.47 21.0 7.32 92.6 Meta 19 0.01 21.1 0.07 0.56 1.94 0.07 12.5 5.45 73.7 Meta 20 0.01 21.6 0.06 0.77 1.78 0.08 13.3 5.36 74.6 Meta 21 0.01 8.23 0.14 3.07 5.01 1.12 27.5 8.89 108 Mag 22 0.01 16.5 0.04 0.88 1.90 0.09 15.3 6.13 87.1 Meta 23 0.05 10.3 0.26 5.16 9.49 2.21 49.9 15.8 181 Mag 24 0.01 12.2 0.01 0.81 1.91 0.28 12.7 5.20 64.2 Mag

Test spots La Ce Pr Nd Sm Eu Gd Tb Dy Zircon type 25 0.01 4.05 0.01 0.19 0.48 0.01 3.86 1.73 26.1 Mag 26 0.02 41.9 0.51 10.3 13.5 5.05 56.2 16.1 184 Mag 27 0.03 1.55 0.02 0.19 0.35 0.26 2.38 0.98 13.5 Mag 28 0.06 17.8 0.11 0.80 2.44 0.18 16.4 7.35 98.0 Meta 29 0.01 8.94 0.03 0.64 1.79 0.28 12.8 4.16 56.7 Mag 30 0.03 8.56 0.05 1.31 2.91 0.62 15.4 5.52 72.6 Mag Test spots Ho Er Tm Yb Lu Eu/Eu* P Ti TZr(°C) Zircon type 14XMC01 1 31.1 151 35.7 367 82.0 0.44 151 3.79 662 Mag 2 19.9 93.1 20.2 199 39.7 0.16 323 7.27 714 Mag 3 41.1 196 44.8 432 86.6 0.09 676 6.65 706 Meta 4 34.5 171 38.9 383 76.4 0.03 526 4.47 675 Meta 5 35.3 189 46.3 505 108 0.05 453 8.93 731 Mag 6 37.7 182 41.5 424 86.3 0.07 4921 5.02 684 Meta 7 24.9 129 30.7 320 71.6 0.36 178 4.62 677 Mag 8 2.10 11.7 3.67 48.2 15.8 0.31 522 6.33 702 Meta 9 39.5 198 49.7 528 110 0.94 538 9.50 736 Meta 10 26.5 128 28.8 289 55.5 0.04 449 4.72 679 Meta 11 36.7 190 44.4 440 87.1 0.05 530 4.92 682 Meta 12 32.3 154 34.4 332 65.6 0.03 481 6.03 698 Meta 13 48.6 231 51.6 491 97.4 0.09 522 6.52 705 Mag 14 35.8 171 38.3 363 73.2 0.04 550 6.30 702 Meta 15 94.7 448 102 1019 212 0.48 642 32.3 856 Mag 16 30.4 148 33.8 336 68.4 0.05 488 5.01 684 Meta 17 34.0 170 39.1 392 78.8 0.04 558 4.28 671 Meta 18 37.3 175 38.1 373 75.0 0.13 499 3.89 664 Meta 19 31.3 150 33.9 325 63.4 0.03 531 6.40 703 Meta 20 30.9 151 33.8 333 66.0 0.04 500 3.89 664 Meta 21 42.1 195 41.5 398 81.1 0.23 259 5.19 686 Mag 22 36.0 179 39.5 384 77.5 0.04 515 4.59 677 Meta 23 69.2 304 62.6 584 117 0.25 210 6.69 707 Mag 24 27.0 132 29.3 284 59.9 0.13 320 4.48 675 Mag 25 12.7 69.2 17.8 190 43.1 0.01 119 5.47 691 Mag 26 69.1 321 74.1 774 165 0.48 396 8.16 723 Mag 27 5.96 32.1 7.97 97.3 23.3 0.66 77.3 6.37 703 Mag 28 43.2 210 46.6 436 83.6 0.07 651 5.03 684 Meta 29 24.0 116 26.8 264 56.4 0.13 236 5.45 690 Mag 30 29.5 140 31.2 303 62.1 0.23 253 4.50 675 Mag Test spots La Ce Pr Nd Sm Eu Gd Tb Dy Zircon type 14XMC02 1 0.70 17.5 0.35 3.06 4.15 0.71 21.5 7.08 91.0 Mag 2 0.25 5.83 0.13 0.99 1.81 0.34 12.4 4.34 45.5 Meta 3 4.13 50.6 3.40 19.5 11.9 2.80 36.8 11.8 137 Mag 4 0.02 7.70 0.03 0.72 1.91 0.46 9.29 3.22 43.4 Mag 5 0.03 8.24 0.01 0.64 1.82 0.33 11.5 4.02 51.4 Mag 6 0.43 17.0 0.31 1.91 3.25 0.55 18.1 6.68 86.4 Mag 7 0.02 10.1 0.01 1.06 2.33 0.25 12.1 4.93 66.5 Mag

Test spots La Ce Pr Nd Sm Eu Gd Tb Dy Zircon type 8 0.11 12.3 0.10 1.61 3.03 1.03 18.2 7.06 92.8 Mag 9 0.01 11.2 0.02 1.16 4.25 0.84 22.0 7.46 95.6 Meta 10 41.4 108 13.3 64.5 17.9 1.45 37.8 10.1 112 Meta 11 0.16 21.2 0.16 2.07 4.07 0.56 23.8 8.82 115 Meta 12 0.01 21.1 0.03 0.91 2.53 0.12 16.6 6.40 86.3 Meta 13 0.01 22.0 0.04 0.81 2.67 0.05 19.9 7.55 104 Meta 14 0.01 16.1 0.01 0.65 1.44 0.10 11.9 4.97 65.5 Meta 15 0.55 17.6 0.58 6.24 7.50 1.90 40.1 13.2 158 Mag 16 0.84 6.62 0.55 2.91 1.13 0.57 4.71 2.95 50.8 Meta 17 0.01 13.2 0.03 1.13 2.52 0.40 16.7 5.61 78.0 Mag 18 0.01 17.8 0.05 0.49 2.56 0.18 14.9 6.10 79.7 Meta 19 0.01 11.0 0.02 0.55 1.64 0.06 12.7 5.09 69.4 Meta 20 27.1 88.1 11.0 56.2 19.0 1.75 43.7 12.2 131 Mag 21 0.01 11.8 0.11 2.79 5.18 1.46 38.0 14.8 197 Mag 22 21.4 138 10.8 49.2 21.4 3.46 28.9 9.22 100.0 Meta 23 0.02 23.4 0.07 1.61 4.67 0.80 27.2 9.68 121 Mag 24 0.82 18.0 0.49 2.85 4.54 0.63 24.0 8.95 116 Meta 25 0.01 5.31 0.01 0.62 1.15 0.24 8.38 3.16 43.3 Meta 26 0.01 11.3 0.08 1.84 3.08 0.65 20.1 6.85 83.9 Mag 27 0.01 8.60 0.08 1.98 4.33 0.83 27.2 9.14 119 Mag 28 0.46 8.72 0.27 1.85 2.71 0.88 17.5 6.33 76.6 Mag 29 0.01 5.73 0.03 0.67 0.79 0.32 5.19 1.98 27.4 Mag 30 10.4 98.1 4.83 25.8 11.4 1.49 32.2 9.76 116 Meta Test spots Ho Er Tm Yb Lu Eu/Eu* P Ti TZr(°C) Zircon type 14XMC02 1 35.1 158 35.2 340 68.2 0.19 171 254 765 Mag 2 13.7 50.1 9.03 72.6 13.5 0.19 171 13.1 742 Meta 3 52.2 240 51.8 500 97.8 0.37 425 10.2 682 Mag 4 17.9 81.9 18.9 194 40.8 0.28 3125 4.89 690 Mag 5 20.3 101 22.3 228 46.9 0.17 661 5.40 676 Mag 6 37.1 180 40.3 403 83.9 0.17 572 4.55 687 Mag 7 27.2 135 31.0 314 65.1 0.12 653 5.21 674 Mag 8 39.6 196 43.3 442 91.1 0.33 440 4.42 663 Mag 9 37.7 176 38.3 361 72.2 0.37 425 3.81 681 Meta 10 43.3 200 44.5 439 91.7 0.28 3125 4.84 815 Meta 11 46.4 220 49.7 480 95.9 0.17 661 22.0 699 Meta 12 35.5 171 39.8 390 77.2 0.17 572 6.07 694 Meta 13 42.6 198 43.9 413 78.5 0.12 653 5.68 669 Meta 14 27.7 136 30.1 300 58.6 0.33 440 4.14 834 Meta 15 62.7 285 64.4 647 134 0.27 502 26.3 724 Mag 16 24.7 126 28.9 277 52.3 0.27 168 8.17 988 Meta 17 31.7 154 34.8 349 69.0 0.14 266 95.6 696 Mag 18 34.1 170 40.1 409 83.4 0.14 540 5.86 765 Meta 19 26.3 122 26.7 259 50.2 0.18 387 7.39 715 Meta 20 48.3 207 43.2 419 82.0 0.18 3827 4.63 678 Mag 21 79.8 370 79.0 751 148 0.23 310 7.29 714 Mag 22 36.2 171 39.4 401 80.9 0.23 1777 9.35 735 Meta

Test spots Ho Er Tm Yb Lu Eu/Eu* P Ti TZr(°C) Zircon type 23 50.1 232 51.4 505 102 0.17 361 7.67 718 Mag 24 48.0 227 50.5 487 97.6 0.17 925 5.06 684 Meta 25 17.5 90.0 22.0 240 60.1 0.19 199 7.88 720 Meta 26 34.3 166 36.7 376 76.7 0.19 230 3.59 658 Mag 27 47.3 223 48.0 473 96.7 0.18 373 5.70 694 Mag 28 30.9 139 31.2 303 61.6 0.29 352 3.43 655 Mag 29 11.6 59.2 14.6 156 35.1 0.36 160 4.44 674 Mag 30 42.8 195 41.3 402 81.1 0.18 1587 9.47 736 Meta

Among the 30 zircons from sample 14XMC01,14 are type 1 and characterized by Th contents of 12–629 ppm,U contents of 61–636 ppm,and Th/U ratios of 0.19–1.19.The average value of higherΣREE is 937 ppm,and the chondrite-normalized REE diagrams show strong enrichment in HREEs with negative Pr–Eu anomalies(meanδEu=0.41)and a positive Ce anomaly(meanδCe＞10),which are characteristic of magmatic zircons(Fig.7a)(Belousova et al.2002;Hoskin and Schaltegger 2003;Li 2005;Liu et al.2008,2010;Zhong et al.2018).The average Ti content is 7.84 ppm(ranging from 3.79 to 32.31 ppm)and yields a mean Ti–in–zircon temperature of 707°C(ranging from 662 to 855°C;Fig.6).The 207Pb/206Pb ages of these 14 zircons range from 2702 to 2896 Ma and yield an upper int ercept age of 2748±77 Ma(MSWD=0.95),which is in accordance with the weighted mean age of 2777±41 Ma(MSWD=1.50;Fig.7b).The weighted mean age could represent the crystallization age of the trondhjemitic gneiss.The other 16 analyzed zircons are type 3 and the rims of type 2,with lower Th contents(5–157 ppm),lower U contents(20–885 ppm),and lower Th/U ratios(0.09–0.78).Compared to the type 1 magmatic zircons,theΣREE of the metamorphic zircons are lower(meanΣREE=817 ppm),and exhibit moderately fractionated REE patterns with slightly negative Pr–Eu anomaly and positive Ce anomalies(Fig.7a).The Ti contents vary from 3.89 to 9.50 ppm(average value of 5.44 ppm).ThePb/Pb ages of these zircons range from 2591 to 2454 Ma(except spot 16 with an age of 1546 Ma,Fig.7b).Moreover,the weighted mean age of the metamorphic zircons is 2528±30 Ma(MSWD=0.67),which can be considered the age of metamorphism.

Among the 30 zircons from sample 14XMC02,16 are type 1 magmatic zircons for which the Th,U,and Th/U ranges are 33–267 ppm,98–722 ppm,and 0.29–0.98,respectively.Their averageΣREE is 926.35 ppm,and the average Lu/Hf ratio is 0.0082.As shown in Fig.7c,the chondrite normalized REE patterns are similar to those of the magmatic zircons in sample 14XMC01(meanδEu=0.28,meanδCe＞10)(Hoskin and Schaltegger 2003;Liu et al.2008,2010).Their Ti contents vary widely,ranging from 3.43 to 95.55 ppm(average value of 12.88 ppm,except for the spot 1 outlier),with a corresponding Ti–in–zircon temperature range of 655–987°C(average value of 717°C;Fig.6).ThePb/Pb ages of these zircons range from 2883 to 2753 Ma and yield an upper intercept age of 2878±43 Ma(MSWD=0.95),which approaches the weighted mean age of 2790±17 Ma(MSWD=1.20;Fig.7d)that represents the crystallization age of the trondhjemitic gneiss.The other 14 analyzed zircons are of metamorphic origin,with lower Th contents(9–315 ppm),higher U contents(159–2075 ppm),and lower Th/U ratios(0.004–0.680).These metamorphic zircons have lower ΣREE contents(meanΣREE=826.33 ppm)and exhibit relatively f lat REE patterns,with slightly negative Pr–Eu anomalies and slightly positive Ce anomalies(Fig.7c).The Ti contents vary from 3.81 to 22.02 ppm(average value of 8.06 ppm).ThePb/Pb ages of the 13 zircons range from 2483 to 2633 Ma,with a weighted mean age of 2570±23 Ma(MSWD=3.60;Fig.7d),which represents the age of metamorphism.

Fig.7 Chondrite-normalized rare earth element(REE)patterns,U–Pb Concordia diagram,and weighted mean ages of zircons from the Xiangmiaocun granodioritic gneisses(14XMC01 and 14XMC02)

In summary,two genetic types of zircons from samples 14SZS01,14XMC01,and 14XMC02 indicate that the TTG gneisses formed at 2.7 and 2.5 Ga and underwent metamorphism at 2.5 Ga.

4.2 In-situ zircon Lu-Hf isotopic compositions

As shown in Table 3,all the zirconLu/Hf ratios were less than 0.002,which indicates that the quantity of radioactive Hf was extremely limited.The values of

f

below the continental crustal value(-0.55)indicate that the two-stage Hf model ages(T)may represent accurate times when the source materials were extracted from the depleted mantle or the average crustal residence age of the source materials(Griff in et al.2002).

For the magmatic zircons from sample 14SZS01,theLu/Hf andHf/Hf ratios ranged from 0.000340 to 0.001132 (average 0.000571)and from 0.281329 to 0.281470(average 0.281378),respectively.The calculated ε(t)values were all positive ranging from+2.6 to+9.1(average+5.83),which are in agreement with those of the Huai’an and Songshan TTG gneisses(Liu et al.2009;Zhou et al.2009).The Tranged from 2474 to 2755 Ma(average 2644 Ma),which are close to the T(2496–2648 Ma, average 2590 Ma). Moreover, the youngest two-stage Hf model age of 2474 Ma(spot 4 from sample 14SZS01)is similar to the crystallization age obtained from sample 14SZS01(2501±13 Ma),thereby indicating a juvenile crust.In thePb/Pb ages vs.ε(t)(pink circles in Fig.8)diagram,the analyzed spots fall within the area between the 2.5 and 2.7 Ga crustal evolution lines.

For ten magmatic zircons from sample 14XMC02,theLu/Hf andHf/Hf ratios ranged from 0.000737 to 0.002537(average 0.0014085)and from 0.281132 to 0.281293(average 0.2812048),respectively.The calculatedε(t)values ranged from+3.6 to+7.1(average+4.80;light-blue circles in Fig.8),which are in agreement with those of the Huoqiu and Jiaobei TTG gneisses(Liu et al.2013b,2014).The Tage ranged from 2778 to 3066 Ma(average 2937 Ma),indicating the presence of juvenile components.The other ten metamorphic zircons from sample 14XMC02 hadLu/Hf andHf/Hf ratios that ranged from 0.046375 to 0.090974(average 0.001072)and from 0.001038 to 0.001956(average 0.2812182),respectively.These metamorphic zircons yieldedε(t)values of-1.0 and+2.2(average 0.51;dark-blue circles in Fig.8),with Tages of 2888 and 3130 Ma,which is older than the formation age and in agreement with the age of the Huoqiu TTG gneiss(Liu et al.2014),suggesting mixing with older crustal materials.

Fig.8 Diagrams ofεHf(t)-ages of zircons in the TTG gneisses from Shizishan and Xiangmiaocun(Bengbu-Wuhe area).Data source:TTG gneiss of Huai’an used by Liu et al.(2009);TTG gneiss of Songshan used by Zhou et al.(2009);granitoid gneiss of Jiaobei terranes used by Liu et al.(2013b);and TTG gneiss of Huoqiu used by Liu et al.(2014)

Spot No 176Yb/177Hf 176Lu/177Hf 176Hf/177Hf 2σm Age(Ma) εHf(t) TDM 1(Ma) f Lu/Hf TDM 2(Ma) Zircon type 14SZS01 1 0.021375 0.000495 0.281379 0.000012 2528 6.6 2584 -0.99 2618 Mag 2 0.014536 0.000340 0.281361 0.000012 2548 6.7 2597 -0.99 2628 Mag 3 0.021300 0.000497 0.281331 0.000011 2554 5.5 2648 -0.99 2706 Mag 4 0.050051 0.001042 0.281470 0.000013 2535 9.1 2496 -0.97 2474 Mag 5 0.024614 0.000568 0.281378 0.000012 2555 7.1 2589 -0.98 2610 Mag 6 0.016671 0.000390 0.281329 0.000013 2526 4.9 2644 -0.99 2717 Mag 7 0.022593 0.000554 0.281377 0.000013 2548 6.9 2590 -0.98 2616 Mag 8 0.015815 0.000348 0.281330 0.000013 2502 4.5 2640 -0.99 2725 Mag 9 0.016330 0.000378 0.281365 0.000012 2562 7.1 2594 -0.99 2614 Mag 10 0.049740 0.001132 0.281431 0.000013 2516 7.1 2555 -0.97 2577 Mag 11 0.038531 0.000913 0.281409 0.000013 2477 5.8 2571 -0.97 2626 Mag 12 0.019365 0.000448 0.281399 0.000013 2478 6.3 2554 -0.99 2600 Mag 13 0.021659 0.000507 0.281355 0.000013 2439 3.7 2617 -0.98 2726 Mag 14 0.025188 0.000576 0.281374 0.000011 2421 3.9 2596 -0.98 2703 Mag 15 0.016733 0.000392 0.281350 0.000013 2390 2.6 2616 -0.99 2755 Mag 16 0.021932 0.000549 0.281396 0.000012 2428 4.9 2564 -0.98 2647 Mag 17 0.025218 0.000597 0.281398 0.000012 2487 6.2 2564 -0.98 2611 Mag 18 0.025125 0.000589 0.281362 0.000013 2566 6.7 2613 -0.98 2641 Mag 19 0.022965 0.000531 0.281391 0.000012 2446 5.1 2569 -0.98 2644 Mag 14XMC02 1 0.054915 0.001166 0.281208 0.000015 2770 4.8 2863 -0.96 2918 Mag 2 0.046375 0.001038 0.281171 0.000010 2561 -1.0 2904 -0.97 3106 Meta 3 0.063336 0.001339 0.281143 0.000016 2824 3.3 2965 -0.96 3047 Mag 4 0.056549 0.001334 0.281132 0.000015 2829 3.1 2979 -0.96 3066 Mag 5 0.032778 0.000737 0.281146 0.000012 2805 4.2 2914 -0.98 2980 Mag 6 0.062011 0.001341 0.281185 0.000013 2783 3.9 2908 -0.96 2980 Mag 7 0.041969 0.000927 0.281204 0.000012 2829 6.4 2850 -0.97 2863 Mag 8 0.075375 0.001641 0.281237 0.000015 2815 5.9 2859 -0.95 2883 Mag 9 0.055262 0.001257 0.281186 0.000012 2505 -2.1 2900 -0.96 3130 Meta 10 0.059515 0.001308 0.281208 0.000012 2520 -1.0 2874 -0.96 3078 Meta 11 0.040559 0.000860 0.281192 0.000012 2620 1.4 2862 -0.97 3007 Meta 12 0.049708 0.001070 0.281228 0.000011 2583 1.5 2828 -0.97 2972 Meta 13 0.038287 0.000849 0.281196 0.000012 2633 1.9 2855 -0.97 2988 Meta 14 0.043361 0.000956 0.281223 0.000013 2498 -0.4 2827 -0.97 3022 Meta 19 0.046933 0.001038 0.281280 0.000010 2531 2.2 2755 -0.97 2888 Meta 20 0.116650 0.002537 0.281291 0.000016 2753 4.8 2852 -0.92 2904 Mag 22 0.021052 0.000388 0.281229 0.000012 2483 0.4 2778 -0.99 2960 Meta 29 0.061553 0.001497 0.281209 0.000015 2783 4.5 2887 -0.95 2946 Mag 30 0.090974 0.001956 0.281269 0.000012 2620 2.2 2837 -0.94 2957 Meta 31 0.063474 0.001566 0.281293 0.000014 2773 7.1 2776 -0.95 2778 Mag

5 Discussion

5.1 Protolith and metamorphism ages of the TTG gneisses and multi-stage tectonothermal events

Previously obtained zircon ages from metamorphic rocks in the NCC have been diff icult to interpret due to the lack of visible internal structures and

in-situ

analysis,especially from Archean–Paleoproterozoic rocks that underwent multi-stage tectonothermal events.Tu(1994)acquired mixed U–Pb ages of 2458±10–2408±13 Ma for the TTG gneiss in the study area using the single-grain zircon evaporation method.Based on the detailed CL images and trace elemental compositions obtained in this study,we were able to acquire more precise ages for the different geologic events.The U–Pb ages of the magmatic zircons from samples 14SZS01,14XMC01,and 14XMC02 are 2501±13,2777±41,and 2790±17 Ma,respectively,which represent the～2.7 and～2.5 Ga magmatic events.The metamorphic zircons from samples 14XMC01 and 14XMC02 yielded U–Pb ages of 2528±30 and 2570±23 Ma,which represent the metamorphism that occurred at～2.5 Ga.

The geochronology data collected from previous studies indicated an important tectonic event of～2.5 Ga in late Neoarchean,with magmatic and metamorphic peak ages of 2530 and 2490 Ma,respectively(Fig.9;e.g.,TTG gneisses,syntectonic granites,and other metamorphic supracrustal rocks of greenschist to granulite facies grade)(Tu 1994;Zhao et al.1998,2001;Xu et al.2006;Zhai et al.2010;Diwu et al.2011;Zhai and Santosh 2011;Liu and Cai 2017;Ding et al.2018;Liu et al.2019a,b).As for the TTG magmatism activity of the NCC,cooling and crystallization of magmas are known to be slow processes,and the magma emplacement requires a particular time period instead of being instantaneous.The value of 2510 Ma represents the relatively late age of this magmatic evolution.Furthermore,the LA-ICP-MS test process leads to analytical errors,and the weighted average ages of ancient U–Pb zircons can sometimes add up to±50 Ma;these factors should be taken into account.Consequently,although the magmatism at 2510 Ma is younger than the metamorphism studied herein(2528–2570 Ma),the late Neoarchean TTG magmatism and high-grade metamorphism are believed to be the contemporary products of the～2.5 Ga late tectonothermal event for a certain error range(Zhai et al.2005;Zhao et al.2007;Guo and Li 2009;Liu et al.2009,2013a,b,2014;Zhou et al.2009,2014;Geng et al.2010).In addition,abundant～2.8–2.7 Ga magmatic zircons have been observed in the TTG and metasedimentary rocks of the NCC(e.g.,in Huai’an,Fuping,Wutai,Huoqiu,Lushan in Henan Province,the western Shandong Province,and the Jiaodong Peninsula)(Zhao et al.2007,2008;Liu et al.2009,2014;Wan et al.2011;Zhou et al.2014;Xie et al.2015).In this study,the～2.7 Ga magmatic ages of the southeastern margin of the NCC also imply that the～2.8–2.7 Ga tectonothermal events may have been more extensive than previously thought and that this period may also have been an important stage of episodic tectonothermal activity for the NCC (Fig.9, Wan et al. 2011; Liu et al.2013a,b,2015,2018,2019a,b;Wang et al.2013;Zhou et al.2014;Xie et al.2015).

Fig.9 Accumulative curves of zircon U–Pb ages for Precambrian metamorphic rocks and granites on the southeastern margin of the North China Block.Statistical data are from Xu et al.2006;Guo and Li 2009;Liu et al.2009,2013a,2014;Yang et al.2010;Wang et al.2012,2013.The red curve represents the ages of magmatic zircon and the black curve represents the ages of metamorphic zircons

5.2～2.5 and～2.7 Ga episodic crustal growth events

The Hf isotope analyses of the widely exposed TTG rocks in the NCC can provide key evidence for the large-scale crustal growth events and evolution of the NCC basement(Martin 1993;Zhao et al.1998,2001,2007,2010;Condie 2005; Zheng et al. 2006; Liu et al.2009,2013b,2014,2019b;Zhai et al.2010;Diwu et al.2011;Wan et al.2011;Zhai and Santosh 2011;Liu and Cai 2017).Zheng et al.(2006)emphasized that the Hf model age could only be interpreted as the period of juvenile crustal growth when the maximum positiveε(t)value and the corresponding Hf model age are close to the initial Hf isotopic ratio of the depleted mantle and the U–Pb age,respectively.Theε(t)values of the magmatic zircons from the～2.5 Ga sample 14SZS01 and～2.7 Ga sample 14XMC02 are positive,and most are close to the values of the contemporary depleted mantle(Fig.8).The Hf model ages(T)are also approximately equal to the U–Pb ages,which illustrates that they possess juvenile crustal characteristics.Some of the zircon plots near the chondrite line,indicating that the magma source was derived from the crust-mantle interaction region or the enriched mantle region;however,most of the zircons are dominated by depleted mantle signatures(Iizuka et al.2005;Wan et al.2011;Xie et al.2015;Liu and Cai 2017).The～2.5 Ga metamorphic zircons from sample 14XMC02 exhibit nil or negativeε(t)values and the corresponding Tages are older than the U–Pb ages,which indicates that they resulted from the mixing of ancient continental crust,as the residence time of the source material is relatively long(approximately 0.4–0.5 Ga)(Zheng et al.2006;Gong et al.2012;Liu et al.2013b;Xiao et al.2019).

As one of the oldest cratons in the world,the NCC is marked by～2.5 Ga crustal growth events(Fig.8;Zhao et al.2008;Kusky 2011;Wan et al.2011;Zhai and Santosh 2011).Liu and Cai(2017)analyzed detrital zircons from metasedimentary rocks in the Bengbu-Wuhe area,and their results showed that the positiveε(t)values were characterized by Tages of～2.57–2.46 Ga,with a peak at～2.52 Ga(Fig.8).In recent years,mounting evidence has suggested that the～2.8–2.7 Ga magmatism was widely distributed throughout the NCC (Wan et al.2011,2013;Liu et al.2013b,2018,2019a,b;Zhou et al.2014;Xie et al.2015).This period may have also been a crucial crustal growth stage(e.g.,the discovery of～2.8 Ga TTG rocks from the Jiaodong Peninsula,western Shandong,Hengshan,Zhongtiaoshan,and Zuoquan metamorphic complexes in the NCC)(Liu et al.2009;Wan et al.2011;Xie et al.2015;Xiao et al.2019).Zircon isotope studies of TTG gneisses from the Bengbu-Wuhe area also support the evidence that both the～2.7 and～2.5 Ga were important periods for episodic crustal growth and that the reworking of ancient continental material also occurred at～2.5 Ga.

5.3 Implications for Neoarchean crustal evolution in the southeastern NCC

Based on the statistical results of zircon U–Pb and Hf model ages mentioned above,different blocks of the NCC exhibit diverse age peaks(～2.5 and～2.8 Ga)for Late Archean crustal growth.The products of～2.5 Ga tectonothermal events are widely distributed throughout the NCC,while the～2.8–2.7 Ga magmatism and juvenile crust are also distributed more extensively in the NCC than previously thought;however,the crustal evolution history is more complex(Geng et al.2010;Zhai et al.2005,2010;Diwu et al.2011).These magmatic rock types change over time;the TTG rocks observed at～2.8–2.7 Ga were primarily the trondhjemite and tonalite,while the ratio of granodiorite increased at the end of the Late Neoarchean(～2.5 Ga),which was accompanied by the typical crustal source potassium-rich granitoid(e.g.,monzonitic granite and syenogranite).It is possible that these results ref lected the characteristic evolutionary trend of magma,crustal thickness,and the high maturity of continental crust at～2.5 Ga(Zhai et al.2000;Moyen and Martin 2012;Wan et al.2017;Zhang et al.2017).The distinct similarities in the rock assemblages(e.g.,amphibolite,garnet amphibolite,granulite,and granitic gneiss),formation ages,and evolutionary characteristics imply that various rock complexes from the basement may have formed in the same tectonic environment.Therefore,it is likely that the southeastern margin of the NCC formed a unif ied basement during the Late Archean(Zhao et al.2001,2008;Wu et al.2005;Zhai et al.2005;Guo and Li 2009;Geng et al.2010).The emergence of ultramaf ic–alkaline dykes during the same period of granitic magmatism activities also indicates that the lithosphere of the NCC became thick and stable at the end of the Archean(Zhai and Santosh 2011).

Signif icant differences in the whole rock and magmatic zircon Hf–Nd isotopic ratios from the NCC suggest that ancient continental material may have been involved in crustal recycling,and the residence times of the crust are also slightly different(Figs.8,9).The magmatism at～2.5 Ga,metamorphism event,different degrees of magmatism,and partial melting of lower crustal rocks indicate that the continental block formed during the Archean.The characteristic episodic tectonothermal events in the Late Archean might represent the convergence of macro-blocks via subduction and collision(e.g.,the Jiaoliao,Qianhuai,Fuping,and Ji’ning blocks)(Rapp et al.1991;Zhai et al.2005,2010;Wang et al.2009;Liu et al.2014,2020).Subsequently,the NCC underwent granulite-facies metamorphism and the intrusion of granitic magma from a crustal source,and cratonization was complete by the end of the Neoarchean(Zhao et al.1998,2001;Zhai et al.2000;Wu et al.2005;Santosh et al.2006;Kusky 2011).This period could have been an important tectonic transition period for the NCC.Evidence from experimental petrology has shown that the partial melting of the crust could generate granitic magma at low temperatures(≤800°C),but only if a certain amount of f luid is added to the interior crustal sources(Watson et al.2006;Wu et al.2007).The results of Ti–in–zircon geothermometry from the Bengbu-Wuhe TTG gneisses yielded a relatively low crystallization temperature(Fig.6),implying the involvement of f luids from a subducted slab(Martin 1993).For the zircon trace-element features,the average Ti content in the Bengbu-Wuhe area(6.85 ppm)is signif icantly lower than those in the Phanerozoic continental arc and continental hotspot(8 ppm;Carley et al.2014).On the other hand,considering the high U/Yb ratios of 1.04,low Yb/Gd ratios of 18.49,and Nb–Ta-Ti negative anomalies for the plagioclase amphibolite in the 2.5 Ga Jiagou xenoliths(Liu et al.2013a),a late Neoarchean subduction-related setting at the southeastern NCC is more likely to occur.The studies of the Neoarchean TTG gneisses of the Songshan and Huoqiu areas,located adjacent to the study area,have also indicated that f luid from a subducted slab was contributed to deep lithospheric processes and the source region(Fig.8;Zhou et al.2009;Liu et al.2014,2020).The underplating of mantle-derived magma relates to the breakoff and exhumation of a subducted slab that occurred during the Late Archean,and the juvenile crust was formed at the bottom of the ancient crust.Meanwhile,granitic rocks were formed and reworked from the juvenile crust under the inf luence of underplating,and the partial melting of the TTG crust with a metasomatic f luid was observed.Finally,the growth and reconstruction of the Earth’s crust were completed.

6 Conclusions

1. The magmatic zircons sourced from the TTG gneisses in the Bengbu-Wuhe area recorded ages of～2.5 and～2.7 Ga,which were associated with the episodic Neoarchean basement magmatism.

2. The metamorphic zircons from the TTG gneisses recorded an age of～2.5 Ga,thereby indicating that the basement rocks at the southeastern margin of the NCC underwent metamorphism caused by the～2.5 Ga tectonothermal event.

3. The positiveε(t)values of the～2.7 and～2.5 Ga magmatic zircons represent episodic Neoarchean juvenile crustal accretion events in the NCC.Negative or nilε(t)and older Tages of the～2.5 Ga metamorphic zircons suggest the re-melting and reworking of ancient continental crust(～3.0–2.7 Ga).

4. The Neoarchean TTG gneisses may have been derived from the partial melting of lower crustal materials and formed in an arc environment related to plate subduction.

Acknowledgement

s This research was jointly supported by the National Natural Science Foundation of China(Nos.41303041 and 41763005),Open Fund(Nos.Z1909,Z1912,RGET1804,15LCD08)of the State Key Laboratory of Nuclear Resources and Environment,Fundamental Science on Radioactive Geology and Exploration Technology Laboratory,and State Key Laboratory of Continental Dynamics.We are grateful to the State Key Laboratory of Isotope Geochemistry,Guangzhou Institute of Geochemistry,Chinese Academy of Sciences(GIGCAS)for their assistance with the zircon Hf isotope analyses.

Compliance with ethical standards

Conf lict of interest

The authors declare that there is no conf lict of interest.