Building the Wutai arc: Insights into the Archean – Paleoproterozoic crustal evolution of the North China Craton

Precambrian Research 333 (2019) 105429

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Building the Wutai arc: Insights into the Archean – Paleoproterozoic crustal evolution of the North China Craton

T

Pin Gaoa, M. Santosha,b,c,

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a

School of Earth Sciences and Resources, China University of Geosciences Beijing, 29 Xueyuan Road, Beijing 100083, PR China Department of Earth Sciences, University of Adelaide, Adelaide, SA 5005, Australia c Yonsei Frontier Lab, Yonsei University, Seoul, Republic of Korea b

ARTICLE INFO

ABSTRACT

Keywords: Arc magmatism Geochemistry Zircon U-Pb geochronology Lu-Hf isotopes Crustal evolution

The Wutai complex is located within the Trans–North China Orogen (TNCO) which marks the collision zone between the Eastern and Western Blocks of the North China Craton (NCC). Here we investigate the timings and mechanisms of Precambrian arc building in Wutai from a suite of newly identified greenstone – felsic tuff – Banded Iron Formation (BIF) – phyllite sequence. Zircon grains from the felsic tuff layer intercalated within the greenstones provide robust constraints on the timing of BIF deposition. Geochemical features of the greenstones and felsic tuff suggest E - MORB and volcanic arc affinity, whereas the BIFs are of submarine hydrothermal origin. Zircon grains in the greenstones define an upper intercept age of 2951 ± 41 Ma and a lower intercept age of 1845 ± 190 Ma. Those from the felsic tuff show 207Pb/206Pb weighted mean age of 2557 ± 10 Ma, whereas those from the BIF intercalated with felsic tuff shows an age of 2585 ± 14 Ma. Metamorphic zircon grains in the BIF define two age populations of ca. 2471 and 1906 Ma. Zircon grains in the phyllite yield an age of 2594 ± 14 Ma. Kernel density plots define age peaks at 2873 and 2578 Ma marking the major phase of arc building in Wutai. Zircon εHf (t) values are mostly positive, in the range of 3.15–7.72, suggesting new crustal growth from juvenile/depleted mantle components. Our study provides insights into the Mesoarchean to Neoarchean crust building through subduction-related arc magmatism, followed by Paleoproterozoic metamorphism in the North China Craton.

1. Introduction Archean cratons on the globe are composite collages of ancient microblocks that were welded along zones of ocean closure and involved multiple subduction-accretion with or without plume-arc interaction, and collision (e.g., Zhai and Santosh, 2011; Manikyamba et al., 2012; Jayananda et al., 2018; Tang and Santosh, 2018a,b; Wang and Santosh, 2019). By analogy with global Phanerozoic orogens such as the Central Asian Orogenic Belt (e.g., Xiao et al., 2015), the construction of continents and cratons in the early history of the Earth is presumed to have involved plume-arc accretion, and arc-arc, arc-continent and continent-continent collision (e.g., Santosh et al., 2009; Manikyamba et al., 2012). The North China Craton (NCC) is among the most widely studied cratons over the globe not only in terms of crustal evolution and recycling in the Precambrian Earth, but also for its unique feature of extensive craton destruction in the Mesozoic generating some of the world-class mineral deposits (e.g., Wan et al., 2010; Zhai and Santosh, 2011; Zhao and Zhai, 2013; Groves and Santosh, 2016;

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Kusky et al., 2016; Tang and Santosh, 2018a,b; Yang and Santosh, 2019, among others). The cratonized NCC is composed of the Eastern block and the unified Western block comprising the northern Yinshan and southern Ordos Blocks, with the intervening Paleoproterozoic collisional sutures of Inner Mongolia Suture Zone (IMSZ or, the Khondalite Belt) and the Trans – North Craton Orogen (TNCO) (Zhao et al., 2005; Santosh, 2010; Zhao and Zhai, 2013) (Fig. 1). The Archean tectonic framework of the NCC is defined as a collage of at least seven microblocks: JL – Jiaoliao block, QH – Qianhuai block, OR – Ordos block, JN – Jining block, XCH – Xuchang block, XH – Xuhuai block and ALS – Alashan block (Zhai and Santosh, 2011; Santosh et al., 2016; Tang et al., 2016; Tang and Santosh, 2018a,b). The TNCO which marks the collisional orogen between the Western and Eastern Blocks of the NCC is a complex zone of subduction-accretion-collision and rifting, and has been the focus of several studies leading to diverse tectonic models including: (1) zone of subduction and arc magmatism during Neoarchean to Paleoproterozoic (Kröner et al.,

Corresponding author at: School of Earth Sciences and Resources, China University of Geosciences Beijing, 29 Xueyuan Road, Beijing 100083, PR China. E-mail address: [email protected] (M. Santosh).

https://doi.org/10.1016/j.precamres.2019.105429 Received 24 June 2019; Received in revised form 13 August 2019; Accepted 22 August 2019 Available online 24 August 2019 0301-9268/ © 2019 Elsevier B.V. All rights reserved.

Precambrian Research 333 (2019) 105429

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Fig. 1. Geologic and tectonic framework of the North China Craton, showing the basement rocks in the Eastern Block, Western Block and Trans-North China Orogen (modified after Zhao et al., 2005).

Western block, the Eastern block and the TNCO (Zhao et al., 2000, 2005; Zhao and Zhai, 2013; Santosh, 2010). The amalgamation of micro-blocks along several Neoarchean greenstone belts is presumed to have led to the cratonization of NCC (Bai et al., 1996; Zhai and Santosh, 2011). The TNCO is a complex zone of accretion and collision, within which is located the Hengshan, Wutai, and Fuping complexes (Fig. 1). Some of the previous studies suggested that these complexes constitute a single magmatic arc formed during the Neoarchean to Paleoproterozoic, with the Hengshan and Fuping gneisses representing the arc root, and the Wutai Complex exposing the upper crustal domain (Zhao et al., 2007). The Wutai region is located in northeastern Shanxi province, and dominantly exposes granitoids plutons, ultramafic rocks and Neoarchean to Paleoproterozoic meta-volcano-sedimentary sequences (Tang and Santosh, 2018a,b) metamorphosed under greenschist to lower amphibolite facies (Zhao et al., 2007), traditionally named as the Wutai and Hutuo groups. From the rocks belonging to various subgroups of the Wutai group, previous studies reported Neoarchean ages such as the zircon SHRIMP ages of 2529–2513 Ma from meta- andesite, rhyolite and dacite, zircon LA-ICPMS ages of 2536–2503 Ma from volcanic rocks (Liu et al., 2016), and zircon SHRIMP zircon age of 2528 Ma from felsic schist (Wilde et al., 2004). The previous tectonic models proposed for the formation of the Wutai complex include: (1) greenstones representing ocean floor basalts generated through rifting of the continental basement composed of Hengshan and Fuping Complex (e.g. Tian, 1991; Li and Qian, 1995); (2) Archean continent-arc-continent collisional system with the greenstone belt representing an arc-related mélange zone between two separate Archean continental basement terranes (Hengshan and Fuping) (Li et al., 1990; Bai, 1989) (Fig. 2); (3) oceanic/magmatic arc system where the lower grade rocks represent the upper crustal component between two higher grade units (Hengshan and Fuping) (Li et al., 1990; Kröner et al., 2001; Guan et al., 2002; Wang et al., 2004; Wilde et al., 2004). In this study, we collected representative samples of greenstones, felsic tuff, BIF, BIF intercalated with felsic tuff and phyllite from two

2005, 2006; Zhao et al., 2005, 2007); (2) arc- back arc system with multiple stages of subduction (Wang et al., 2010, Trap et al., 2009, 2012); (3) continental – arc – continental system (Kusky et al., 2016; Wang et al., 2016); and 4) rift – subduction – accretion – collision system (Zhai et al., 2005; Zhai and Santosh, 2011). Among the various ‘complexes’ within the TNCO, the Wutai complex assumes the position as its heart, being located in the central part, and preserves the vestiges of Neoarchean ocean closure during the amalgamation between the Qianhuai and Ordos microblocks (Zhao et al., 2005, 2012; Zhai and Santosh, 2011; Tang and Santosh, 2018a). Previous tectonic models on the formation of the Wutai Complex include: (1) formation through the closure of a crust rift system (Tian, 1991; Li and Qian, 1995; Du et al., 2013); and (2) an arc system associated with subduction (Li et al., 1990; Liu and Wang, 1998; Zhao et al., 1999; Wang et al., 2004; Niu and Li, 2005, Niu and Li, 2006; Polat et al., 2005; Chen et al., 2005; Liu et al., 2016; Tang and Santosh, 2018b). In this study, we report two new localities where a suite of greenstones, felsic tuff, sulfidic banded iron formation, and phyllites occur together within an arc-accretionary system. We present petrological, geochemical and zircon U-Pb and Lu-Hf data from the various lithologies in this suite which provide insights into the timing of deposition of the BIFs, the duration of active arc building, and the overall Archean – Paleoproterozoic crustal evolution associated with the cratonization of the NCC. 2. Geological background and sampling The NCC is one of oldest cratons on the globe, covering an area of more than 150,000 sq. km and carrying vestiges of crust as old as. 3.8 Ga (Bai, 1989; Liu et al., 1992; Zhai and Santosh, 2011; Tang and Santosh, 2018b). The craton is tectonically bordered by the Central Asian Orogenic Belt (CAOB) to the north, the Qinling-Dabie orogenic belt to the south, the Qilian Orogen to the west and the Sulu HP–UHP metamorphic belt to the east. The NCC is generally sub-divided into the 2

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Fig. 2. Geological map of the Fuping, Hengshan and Wutai complexes (modified after Zhao et al., 2007).

newly identified areas surrounding the Mahuanggou and Pushang iron mines in northeastern Wutai area. The geological map of the study area and sampling locations are shown in Fig. 3. The location details and GPS coordinates are given in Table 1. The major rock types in Mahuanggou are meta-basalts (greenstone) and BIF which occur in association with felsic tuff and phyllite (Fig. 4a). The greenish gray colored metabasalt layers are 1–2 thick and fine grained (Fig. 4a, b, c). The BIFs

are 0.5–4 m thick, display banded structure of recrystallized quartz and magnetite with sulfide minerals (mainly) pyrite disseminated along the bands (Figs. 4c, d, 5b). The dark greenish phyllite occurs as thick schistose and platy layers, ranging in thickness from 0.5 to 2 m, and is mainly composed of chlorite and quartz (Fig. 5a). The felsic tuff occurs as a thin continuous layer (< 0.5 m) over the BIF extending for few tens of meters and is dominantly composed of fine-grained feldspar and 3

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Fig. 3. Geological map of the study area showing sample locations (base map modified after Tian, 1991).

Table 1 Sample localities, rock types, GPS reading and mineralogy of the rocks from Wutai arc analyzed in this study. Serial No.

Sample No.

Locality

Rock type

Coordinates

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24

WT-1/1A WT-1/1B WT-1/1C WT-1/1D WT-1/1E WT-1/2A WT-1/2B WT-1/3 WT-1/4A WT-1/4B WT-1/5 WT-2/1A WT-2/1B WT-2/1C WT-2/2A WT-2/2B WT-2/2C WT-2/2D WT-2/2E WT-2/2F WT-2/2G WT-2/2H WT-2/2I WT-2/2J

Mahuanggou Mahuanggou Mahuanggou Mahuanggou Mahuanggou Mahuanggou Mahuanggou Mahuanggou Mahuanggou Mahuanggou Mahuanggou Pushang iron Pushang iron Pushang iron Pushang iron Pushang iron Pushang iron Pushang iron Pushang iron Pushang iron Pushang iron Pushang iron Pushang iron Pushang iron

Greenstone Greenstone Greenstone Greenstone Greenstone Felsic tuff BIF intercalated with the felsic tuff BIF BIF BIF Phylite BIF BIF BIF Carbonate Carbonate Carbonate Carbonate Carbonate Carbonate Carbonate Carbonate Carbonate Carbonate

N38°55′12.02″, N38°55′12.02″, N38°55′12.02″, N38°55′12.02″, N38°55′12.02″, N38°55′12.02″, N38°55′12.02″, N38°55′12.02″, N38°55′12.02″, N38°55′12.02″, N38°55′12.02″, N38°56′12.14″, N38°56′12.14″, N38°56′12.14″, N38°56′12.14″, N38°56′12.14″, N38°56′12.14″, N38°56′12.14″, N38°56′12.14″, N38°56′12.14″, N38°56′12.14″, N38°56′12.14″, N38°56′12.14″, N38°56′12.14″,

deposite deposite deposite deposite deposite deposite deposite deposite deposite deposite deposite deposite deposite

Assemblage E113°18′15.34″ E113°18′15.34″ E113°18′15.34″ E113°18′15.34″ E113°18′15.34″ E113°18′15.34″ E113°18′15.34″ E113°18′15.34″ E113°18′15.34″ E113°18′15.34″ E113°18′15.34″ E113°20′55.66″ E113°20′55.67″ E113°20′55.68″ E113°20′55.69″ E113°20′55.70″ E113°20′55.71″ E113°20′55.72″ E113°20′55.73″ E113°20′55.74″ E113°20′55.75″ E113°20′55.76″ E113°20′55.77″ E113°20′55.78″

Mineral abbreviations: Chl-Chlorite; Qtz-Quartz; Pl-Plagioclase; Cal-Calcite; Py-Pyrite; Mag-Magnetite; Hem-Hematite.

4

Chl + Qtz + Pl + Py Chl + Qtz + Pl + Py Chl + Qtz + Pl + Py Chl + Qtz + Pl + Py Chl + Qtz + Pl + Py Qtz + Py + Pl Qtz + Mag + Py Qtz + Mag Qtz + Mag + Cal + Pl Qtz + Mag + Cal + Pl Chl + Qtz + Pl Qtz + Mag + Cal + Py + Hem Cal + Qtz + Mag + Py Cal + Qtz + Mag + Py Cal + Qtz + Py + Mag Cal + Qtz + Py + Mag Cal + Qtz + Py + Mag Cal + Qtz + Py + Mag Cal + Qtz + Py + Mag Cal + Qtz + Py + Mag Cal + Qtz + Py + Mag Cal + Qtz + Py + Mag Cal + Qtz + Py + Mag Cal + Qtz + Py + Mag

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Fig. 4. Representative field photographs. (a) BIF, greenstone, felsic tuff and phyllite from Mahuanggou; (b) greenstone from Mahuanggou; (c) layered BIF, felsic tuff and greenstone from Mahuanggou; (d) BIF from Mahuanggou;

quartz with minor pyrite (Fig. 4a, c). The Pushang iron deposit is located 5 km northeast of Mahuanggou (Yan, 2009). Large rocky exposures composed of several tens of meters wide BIF bands in this location are intercalated with carbonate- and sulfide-bearing iron-rich layers (Fig. 5c). The BIF displays alternating thin bands of quartz and magnetite and contains abundant sulfides (Fig. 5d).

mounted onto epoxy resin discs and then polished to expose the grains. Cathodoluminescence (CL), transmitted and reflected light images were used for checking the internal textures to choose the most suitable sites for U-Pb and Hf analysis. The zircon U-Pb and Lu-Hf analyses were carried out by LA-ICP-MS at the Wuhan Sample Solution Analytical Technology Co., Ltd., Wuhan, China. For U-Pb dating and element analyses, the laser sampling was performed using a GeolasPro laser ablation system that consists of a COMPexPro 102 ArF excimer laser (wavelength of 193 nm and maximum energy of 200 mJ) and a MicroLas optical system. An Agilent 7700e ICP-MS instrument was used to acquire ion-signal intensities. Helium was applied as a carrier gas. The laser spot diameter is 32 μm and the frequency is 5 Hz. The external standards for optimizing instrument used the zircon 91,500 and silica glass NIST610. Each analysis incorporated a background acquisition of approximately 20–30 s followed by 50 s of data acquisition from the sample. An Excel-based software ICPMSDataCal was used to perform off-line selection and integration of background and analyses signals, time-drift correction and quantitative calibration for U-Pb dating and trace element analyses (Liu et al., 2010, 2008) In this study, we used ISOPLOT 4.15 software to made Concordia diagrams and calculated the weighted mean age (Ludwig, 2008). The in-situ Lu-Hf isotopic analyses were performed using a Neptune Plus MC-ISP-MS equipped with a 193 nm laser ablation system (GeoLasPro HD) with a spot size of 44 μm and a repetition rate of 8 Hz. The Lu-Hf isotopes were measured either by ablating into the pre-existing crater from the U-Th-Pb analysis or as new ablations within the same cathodoluminescence domains. Laser parameters used were beam size of 50 µm, 10 Hz repetition rate, and fluence of 6–8 J/cm2. He (650 mL/min) and N2 (2 mL/min) were used as carrier gases to enhance Hf isotope intensity (Iizuka and Hirata, 2005). The interference of 176Lu and 176Yb on the 176Hf signal were corrected by using the procedures of

3. Analytical methods Representative samples from the study area in the Wutai complex were selected for petrographic studies, zircon U-Pb and Lu-Hf analyses, and whole-rock major and trace elements analyses. Polished thin sections were prepared at the Peking University, China. Petrographic studies were carried out at the China University of Geoscience Beijing. Whole rock geochemical analyses including major, trace and rare earth elements were carried out in the Testing Center of the First Geological Institute of the China Metallurgical Geology Bureau, Sanhe City, Hebei Province. Representative samples devoid of surface alteration and weathering were chipped and powered to 200 mesh in size. Loss on ignition was obtained through heating sample powder (1 g) at 980 °C for 30 min. The major and trace elements were analyzed by Xray fluorescence (XRF model PW 4400) and PE300D inductive coupled plasma mass spectrometry (ICP-MS), respectively. The analytical uncertainties for major element oxides are below 0.5%. The accuracy of determination (RSD) for trace elements range from 2% to 10%. Trace and rare earth elements were analyzed with analytical uncertainties 10% for elements with abundance < 10 ppm, and approximately 5% for those > 10 ppm (Gao et al., 2008). Zircon grains were separated from crushed rock fragments at the Yu’neng Geological and Mineral Separation Centre, Langfang City, Hebei Province, China, using gravimetric and magnetic techniques with final hand picking under a binocular microscope. The grains were 5

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Fig. 5. Representative field photographs. (a) Phyllite from Mahuanggou; (b) BIF from Mahuanggou; (c) BIF intercalated with carbonate from Pushang; (d) BIF from Pushang carrying abundant sulfides.

Chu et al. (2002) and Vervoort et al. (2004), respectively. The 176 Lu/177Hf and 176Yb/177Hf ratios were calculated after Iizuka and Hirata (2005). Initial εHf values were calculated using a 176Lu decay constant of 1.865 × 10−11 year−1 (Scherer, 2001) and the chondritic values suggested by Blichert-Toft and Albarède (1997). Single-(TDM) and two-stage model ages (T2DM) were calculated with reference to suggested parameters for depleted mantle (Griffin et al., 2000) and average continental crust (Rudnick and Gao, 2003). All ratios are reported with 2σ errors. During the sample analysis, to evaluate the precision and accuracy of 176Hf/177Hf ratios, two reference zircons 91,500 (0.282297; Griffin et al., 2000) and GJ-1 were repeatedly analyzed at the beginning and end of each analytical session, and at regular intervals during the session. Data reduction was carried out using Iolite 2.5 processing software (Paton et al., 2011), and plotted using Isoplot R (Vermeesch, 2018).

%) (Fig. 6c), showing laminated distribution. Medium to fine grained pyrite (0.10–0.25 mm) occurs as pale yellow cubic crystals. Pressure shadow around pyrites grains is filled by fine grained quartz. 4.1.3. BIF The BIF samples from Mahuanggou (WT-1/3 and WT-1/4A, B) and Pushang (WT-2/1 A, B, C) show alternating bands of quartz and magnetite on a fine scale, ranging in width from 0.1 to 1.5 mm in width (Fig. 7a, b, d). The quartz is fine grained (0.05–0.10 mm), colorless or grey, rounded to anhedral. Magnetite is medium to coarse grained (0.30–1.5 mm). The main minerals in Sample WT-1/4 A, B are quartz (35–40 vol%), magnetite (30–35 vol%), calcite (10–15% vol%) and plagioclase (5–10 vol%). Medium to coarse grained magnetite (0.25–0.80 mm) occurs as bands or as inclusions in quartz. Calcite in this sample is fine-grained (0.05–0.10 mm), showing high order colors. Plagioclase is fine-grained (0.05–0.15 vol%), colorless or grey, and subhedral to anhedral. Sample WT-2/1 contains pyrite and hematite in addition to magnetite and quartz (Fig. 7d). The quartz is fine to medium grained (0.10–0.30 mm), magnetite is coarse grained (0.30–1.00 mm), and calcite is very fine-grained (0.05–0.15 mm). Pale yellow cubic crystals of pyrite are fine grained (0.10–0.20 mm) and occurs as thin bands. Hematite is fine-grained (0.05–0.15 mm) and shows bright red or brick red color. Compared with the WT-2/1A, samples WT-2/1B and WT-2/1C contain more calcite than quartz with no hematite. The main minerals in WT-2/1B and WT-2/1C are calcite (30–35 vol%), quartz (25–30%), magnetite (20–25 vol%) and pyrite (10–15 vol%).

4. Results 4.1. Petrology 4.1.1. Greenstones The five greenstone samples in this study from Mahuanggou (WT-1/ 1 A–E) are dominantly composed of chlorite (40 vol%), quartz (20–35 vol%), and plagioclase (10–15 vol%) with pyrite (5–10 vol%) as the major accessory (Fig. 6a, b). The chlorite is fine-grained (< 0.02 mm), greenish and defines the schistosity. Quartz and plagicalse are also fine grained (0.01–0.02 mm). The pyrite (0.10–0.25 mm) grains show typical cubic crystal habit and are euhedral to subhedral.

4.1.4. BIF intercalated with felsic tuff The BIF sample WT-1/2B from Mahuanggou is intercalated with felsic tuff (Fig. 6d). The major minerals are quartz (40–60 vol%), magnetite (30–35 vol%) and minor pyrite (~5% vol%). The quartz is fine grained (0.05–0.20 mm), grey or colorless, subhedral to anhedral,

4.1.2. Felsic tuff The felsic tuff sample WT-1/2A Mahuanggou is composed of fine grained and subhedral quartz (65–70 vol%), and feldspars (35–15 vol 6

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Fig. 6. Representative photomicrographs of thin sections. (a) and (b) greenstone from Mahuangou (WT-1/1A) showing mineral assemblage of chlorite, quartz, plagioclase and pyrite. (c) Felsic tuff from Mahuanggou (WT-1/2A) showing mineral assemblage of plagioclase, quartz and pyrite. (d) BIF intercalated with felsic tuff from Mahuanggou (WT-1/2B); Mineral abbreviations: Chl – Chlorite; Py – Pyrite; Qtz – Quartz; Pl – Plagioclase; Mag – Magnetite; Cal – Calcite.

and occurs as bands with a width of 8 mm, alterating with magnetite bands ranging from 0.1 to 0.2 mm in width. The magnetite is fine to medium grained (0.10–0.25 mm). The pyrite grains in this sample are fine fine-grained (0.01–0.02 mm), pale yellow and cubic, and occurs scattered as euhedral to subhedral crystals.

slopes with (La/Yb)N: 2.19–3.36, (Ce/Yb)N: 1.78–2.68, (La/Sm)N: 1.71–1.96, (Gd/Yb)N: 1.03–1.40. In Fig. 10a the positive Eu anomalies conform to plagioclase crystallization. In Th/Yb vs. Nb/Yb diagram (Fig. 11), all greenstone samples fall near the E-MORB field and show active continental margin/alkaline oceanic arcs characteristics.

4.1.5. Phyllite The phyllite sample WT-1/5 from Mahuanggou (Fig. 7c) is dominantly composed of chlorite (55 vol%) and quartz (45 vol%). The chlorite is fine-grained (0.01–0.25 mm), greenish and defines the schistosity. The quartz is fine grained (0.05–0.15 mm) and subhedral to anhedral and occurs as alternating layers between chlorite-rich bands.

4.2.2. Felsic tuff The felsic tuff s characterized by high SiO2 (63.18 wt%), moderate Al2O3 (15.64 wt%), Fe2O3 (3.06 wt%), FeO (3.67 wt%), Na2O (5.38 wt %) and low TiO2 (0.71 wt%), MnO (3.09 wt%), CaO (0.73 wt%), K2O (1.15 wt%), P2O5 (0.13 wt%) (Supplementary Table 1). In total alkali vs. silica plot (Fig. 8a) the felsic tuff sample falls in the andesite field and in the K2O vs. SiO2 plot, the rock shows sub-alkalic affinity (Fig. 8b). The rock shows low transition trace element contents: Cr (19.65 ppm), Co (7.55 ppm) and Ni (21.95 ppm). For Large Ion Lithophile Elements (LILE) are enriched, with high Rb (32.30 ppm) and Ba (259.16 ppm) and moderate Sr (60.70 ppm). The Zr (163.81 ppm) content of this rock is higher than that in the greenstone and BIF samples. In normalized REE patterns (Fig. 9a), the rock shows LREE enriched nature with (La/Yb)N: 3.21, (Ce/Yb)N: 3.39, (La/Sm)N: 2.23 and (Gd/Yb)N: 1.26 and δEu of 1.06. In primitive-mantle normalized trace elements diagram (Fig. 9b), the rock show obvious Rb, K, Pb, Zr, Hf enrichment and U, Ta, Pr, Sr, Nd, Sm, Y depletion. In Th/Yb vs. Nb/ Yb diagram (Fig. 11), the felsic tuff sample falls in the calc-alkaline field and shows active continental margin setting with oceanic arcs characteristics.

4.2. Geochemistry 4.2.1. Greenstones The greenstone samples in this study show low to moderate SiO2 (47.82–51.05 wt%), Al2O3 14.35–15.53 wt%, and Fe2O3 1.97–3.47 wt %, and relatively high FeO (7.91–9.10 wt%), MgO (5.89–6.84 wt%), and CaO (4.72–6.66 wt%) (Supplementary Table 1). Based on the total alkali contents vs. silica (Fig. 8a) and K2O vs. silica (Fig. 8b) variations, the greenstones correspond to low K subalkaline basalts. Compared with the transitional trace elements of primitive mantle values (Ni: > 400 ppm, Cr: > 800 ppm), the samples show extremely depleted concentrations (Ni: 81.05–103.30 ppm, Cr: 133.80–186.40 ppm). The rocks are characterized by high Sr (128.90–179.10 ppm) with low Ba (14.47–47.84 ppm) and Rb (1.96–6.15 ppm). Primitive mantle normalized trace element abundance patterns (Fig. 9b) reflect negative anomalies at Ba, Nb, Ce, Pr, P, Hf, Sm and Yb, K and Y in especially. Positive anomalies of Rb, Ta, La, Sr, Eu and Ho, Pb are also displayed. The chondrite normalized REE patterns (Fig. 9a) show LREE to HREE

4.2.3. BIF The BIF samples in this study from Mahuanggou and Pushang show low TiO2 (0.00–0.12 wt%), Al2O3 (0.23–3.81 wt%), MnO (0.01–0.28 wt %), P2O5 (0.16–0.21 wt%), and Na2O of WT-1/4A and WT-1/4B are 7

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Fig. 7. Representative photomicrographs of thin sections. (a) BIF from Mahuanggou (WT-1/3) showing magnetite and quartz bands. (b) BIF from Mahuanggou (WT1/4B) showing magnetite, quartz and calcite bands. (c) Phyllite from Mahuanggou (WT-1/5) showing mineral assemblage of chlorite and quartz. (d) BIF from Pushang (WT-2/1) showing banded magnetite, quartz and calcite. Mineral abbreviations: Chl – Chlorite; Py – Pyrite; Qtz – Quartz; Pl – Plagioclase; Mag – Magnetite; Cal – Calcite.

Fig. 8. (a) Na2O + K2O vs. SiO2 TAS diagrams. (b) K2O vs. SiO2 diagram. Fields in (a) modified after Le Maitre (2002), (b) modified after Middlemost (1977).

0.11 and 0.04 (Supplementary Table 1). the Na2O and K2O in other BIF samples are below detection limit. The six typical BIF samples (WT-1/3, WT-1/4 A, B, WT-2/1 A, B, C) show high Fe2O3 (29.45–42.16 wt%), FeO (11.53–18.09 wt%), moderate SiO2 (25.96–47.60 wt%) and low to moderate CaO (0.40–7.76 wt%). In chondrite normalized REE pattern (Fig. 9a) they display LREE enrichment, and positive Nd and Eu anomalies. In Post-Archean Australian Shale (PAAS) normalized REE patterns (Fig. 10a) these rocks show positive Eu, Dy, Tm anomalies with negative Pr, Y, Yb, Lu anomalies. The BIF samples fall in the hydrothermal area in SiO2 vs Al2O3 diagram (Fig. 10b) and show trend from

depleted mantle source to upper crust (Fig. 10c). 4.2.4. BIF intercalated with felsic ruff The BIF intercalated with felsic tuff from Mahuanggou (WT-1/2B) is characterized by high Fe2O3 (32.40 wt%) and FeO (12.87 wt%), moderate SiO2 (46.02 wt%), and low TiO2 (0.12 wt%), Al2O3 (3.81 wt%), MnO (0.05 wt%), MgO (1.88 wt%), CaO (0.28 wt%), Na2O (0.26 wt%), P2O5 (0.15 wt%) with low alkali contents (0.65 wt%) (Supplementary Table 1). In chondrite normalized REE pattern (Fig. 9a), the rock displays REE content intermediate between BIF and felsic tuff. The ΣREE 8

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Fig. 9. Chondrite-normalized REE (a), and Primitive mantle normalized trace element abundance (b) patterns for the samples analyzed in this study (normalization values are from Sun and McDonough, 1989).

Fig. 10. (a) PAAS normalized multi-element diagram for BIF, BIF intercalated with felsic tuff, and felsic tuff (normalization values from Taylor and Mclennan, 1985). (b) SiO2 vs. Al2O3 diagram showing submarine hydrothermal affinity for the BIF (after Zhu et al., 2015). (c) Th/U vs. Th plot showing that the BIF has weathering trend (after McLennan et al., 1993).

and Y of WT-1/2B is 38.39 ppm and 8.83 ppm are higher than the other BIF samples, and exhibit a wide ΣREE variation from 14.05 to 21.82 ppm and 4.93 to 6.68 ppm. The Y/Ho values of the rock (24.43) lower than that of other BIF samples which range from 27.77 to 29.58. In Post-Archean Australian Shale normalized diagram (Fig. 10a), the BIF intercalated with the felsic tuff shows higher ΣREE than BIF, LREE enrichment and positive Eu anomaly.

4.3. Zircon morphology and U-Pb geochronology The U-Pb age data are presented in Supplementary Table 2 and plotted in concordia diagrams together with age data histograms and bar charts (Figs. 13 and 14). Representative cathodoluminescence (CL) images of zircon grains from the different rocks are also shown in Fig. 12 together with the analytical spots. We give below a brief summary of the zircon characteristics and age results in individual samples.

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Fig. 11. Th/Yb vs. Nb/Yb plots (after Pearce, 2008) showing active continental margin affinity for the greenstone and felsic tuff from Wutai arc corroborating oceancontinent convergence. Abbreviations: CA - calc alkaline; TH - tholeiitic; SHO - shoshonite.

4.3.1. Greenstone (WT-1/1E) Zircon grains from the greenstone are transparent and colorless, prismatic, showing euhedral to subhedral morphology. The grains size is variable, 60–150 μm in length and 30–80 μm in width. In CL (Fig. 12), most grains show oscillatory zoning and banded zoning, with a few grains showing core-rim texture. Some of the magmatic cores display oscillatory zoning and are surrounded by thin light-colored metamorphic rim. Thirty-three spots were analyzed from 33 grains from the greenstone sample (WT-1/1E). Among these, six spots show low concordance. The remaining data define an upper intercept age of 2591 ± 41 Ma and a lower intercept age of 1845 ± 190 Ma (MSWD = 2.0, n = 27) (Fig. 13a). Their Th, U contents show a range of 32.98–413.04 ppm, 80.44–466.91 ppm with Th/U values in the range of 0.20–0.88 ppm. In age data histogram, the data show clear age peaks at 1987 Ma and 2881 Ma (Fig. 13b).

with the felsic tuff, among which one spot shows low concordance. The Th, U and Th/U are in the range of 17.61–62.59 ppm, 36.52–90.33 ppm and 0.40–0.81 ppm. The data yield weighted mean age of 2585 ± 14 Ma (MSWD = 0.98, n = 29) (Fig. 14a). 4.3.4. BIF The BIF sample WT-1/3 contains only very few zircon grains. BIF is a marine metasedimentary rock and the few zircon grains in this sample were sourced from the intercalated volcanics during deposition, and also generated during subsequent metamorphism. The zircon grains in the BIF are transparent or brownish, fusiform or irregular, ranging in length from 100 to 110 μm, and width of 50–80 μm. In CL (Fig. 12), most of the grains are unzoned and homogeneous. Some grains show high luminescence domains. The zircon grains in this rock are mostly of metamorphic origin. Four spots were analyzed from the BIF sample (WT-1/3), which yielded two groups of ages. The older group includes three grains, with 207 Pb/206Pb weighted mean age of 2471 ± 46 Ma (MSWD = 1.02, n = 3) (Fig. 14b). The younger group includes one grain, with 207 Pb/206Pb age spot age of 1906 ± 115.74 Ma. The Th, U contents and Th/U ratios are in the range of 35.12–79.84 ppm, 31.53–178.75 ppm and 0.42–1.11.

4.3.2. Felsic tuff Zircon grains from the felsic tuff sample WT-1/2A are transparent or light-brownish, stumpy or rounded, showing subhedral to anhedral morphology. The size of the grains shows a length of 40–100 μm and a width of 30–100 μm. In CL (Fig. 12), most grains show oscillatory zoning, and a few grains are homogenous without any zoning. Fifty-two spots were analyzed from 52 grains from the felsic tuff sample (WT-1/2A), among which three spots show low concordance. The Th, U contents and Th/U ratios in the range of 15.85–129.63 ppm, 30.11–147.72 ppm with Th/U values in the range of 0.44–0.96. The data define weighted mean 207Pb/206Pb age of 2557 ± 10 Ma (MSWD = 0.89, n = 48) (Fig. 13c).

4.3.5. Phyllite The zircon grains in phyllite sample WT-1/5 are dark or brownish. Most of the grains are stumpy, subhedral to anhedral, and a few grains are elongated. They show length of 30–130 μm and width of 50–80 μm. In CL (Fig. 12), the grains display sector zoning or patchy zoning, with some grains possessing weak oscillatory zoning or banded zoning. Thirty-two spots were analyzed from 31 grains in the phyllite sample (WT-1/5) and the results show Th, U contents and Th/U ratios in the range of 13.38–55.83 ppm, 29.59–76.83 ppm and 0.45–0.83. The 207 Pb/206Pb weighted mean age of this sample is 2594 ± 14 Ma (MSWD = 0.88, n = 26) (Fig. 14c).

4.3.3. BIF intercalated in felsic tuff The zircon grains in BIF sample WT-1/2B which is intercalated with felsic tuff are transparent or light-brownish, and subhedral to anhedral. They display variable size range, with length of 30–100 μm and width of 20–80 μm. In CL (Fig. 12), the grains display weak oscillatory zoning, and some grains show sector or patchy zoning. Thirty spots were analyzed from 30 grains from the BIF intercalated 10

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Fig. 12. Representative Cathodoluminescence (CL) images of zircon grains Greenstone (WT-1/1E), Felsic tuff (WT-1/2A), BIF intercalated with felsic tuff (WT-1/2B), BIF (WT-1/3) and Phyllite (WT-1/5). Zircon U-Pb ages (Ma) and εHf(t) values are also shown. The smaller yellow circles indicate spots of LA-ICP-MS U-Pb dating, whereas the larger red circles represent locations of Hf isotopic analyses. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Ce* and Eu/EU* values range of 2.16–5.17 and 0.13–0.16.

4.4. Zircon rare earth elements The rare earth element (REE) data on zircon grains from five samples are given in Supplementary Table 3. Their chondrite normalized REE patterns (Sun and McDonough, 1989) are shown in Fig. 15. The zircon grains are characterized by depleted LREE and enriched HREE. They display positive Ce anomalies and moderately negative Eu anomalies. The Ce/Ce* values show a range of 6.35–205.94 (WT-1/1E), 3.82–19.42 (WT-1/2A), 3.52–31.55 (WT-1/2B), 19.43–41.33 (WT-1/3) and 3.79–65.21 (WT-1/5). The Eu/EU* values of these zircon grains show a range of 0.21–0.7 (WT-1/1E), 0.10–0.24 (WT-1/2A), 0.12–0.27 (WT-1/2B), 0.02–0.74 (WT-1/3) and 0.11–0.43 (WT-1/5). One grain from WT-1/1E shows Ce/Ce* and Eu/EU* values of 2.4 and 0.31. Twenty zircon grains from WT-1/2A display Ce/Ce* and Eu/EU* values range of 0.10–0.36 and 1.23–6.60. Three zircon grains from WT-1/2B show Ce/Ce* and Eu/EU* values rang of 0.99–2.50 and 0.13–0.18. Two zircon grains from WT-1/3 show Ce/Ce* and Eu/EU* values range of 2.59–2.85 and 0.16–0.66. Seven zircon grains from WT-1/5 show Ce/

4.5. Zircon Lu-Hf results In situ Lu-Hf isotopes were analyzed on the same or immediately adjacent domains from where the U-Pb data were gathered. A total of twenty-four zircon grains were analyzed for Lu-Hf isotopes. The results are presents in Supplementary Table 4 and plotted in Fig. 16. The salient features are briefly evaluated below. 4.5.1. Felsic tuff (sample WT-1/2A) Fourteen zircon grains from this sample show initial 176Hf/177Hf values between 0.281253 and 0.281349. They show positive ɛHf(t) values from 3.15 to 6.95 when calculated by 207Pb/206Pb age from 2505 Ma to 2607 Ma. The Hf depleted model ages (TDM) are between 2593 Ma and 2727 Ma. 11

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Fig. 13. U-Pb concordia plots and weighted average plots include the Kernel density distribution of ages for WT-1/1E. All data point uncertainties are 2σ.

4.5.2. BIF intercalated with the felsic tuff Ten zircons show initial 176Hf/177Hf values between 0.281280 and 0.281316. They display positive ɛHf(t) values from 4.35 to 7.72 when calculated by 207Pb/206Pb age from 2513 Ma to 2651 Ma. The Hf depleted model ages (TDM) are between 2635 Ma and 2686 Ma.

close to the age defined by zircon in the felsic tuff, suggesting contribution from an active arc. Zircon grains in the BIF sample define two groups, the older one showing 207Pb/206Pb weighted mean age of 2471 Ma and the younger group defining an age of 1906 Ma (Fig. 14b). The older group of zircon might represent inheritance from arc sources, whereas the younger group represents the timing of metamorphism. Zircon grains in the phyllite sample define a 207Pb/206Pb weighted mean age of 2594 Ma which is identical to the age from the felsic tuff, indicating formation along an active convergent margin. The NCC records a long history of Archean-Paleoproterozoic crustal evolution (Zhai and Santosh, 2011; Zhao and Zhai, 2013). In a recent study, Tang and Santosh (2018a) summarized the Neoarchean to Paleoproterozoic evolution of the Trans-North China Craton (TNCO) from various previous studies and noted four major Precambrian events as follows: (1) initial cratonization of the NCC (2.58–2.45 Ga); (2) postcollisional magmatism resulting in the opening of oceanic basin (2.5–2.45 Ga); (3) two periods of subduction-rift system which generated various arc and rift related rocks; and (4) the terrane assembly and collision between the Eastern and Western blocks (1.96–1.90 Ga). The Wutai complex is located at the heart of the TNCO and is a therefore a key region to address the tectonic evolution and geological events of the NCC. Several previous studies were carried out on the Wutai region. Chen et al. (2005) reported zircon U-Pb age of 2509 Ma from a gneissose potassic granite, which was considered to constrain the time of

5. Discussion 5.1. Age constrains from zircon U-Pb data In this study, we obtained zircon U-Pb data from a suite of greenstone, felsic tuff, BIF, BIF intercalated with felsic tuff, and phyllite from two new localities in the Wutai Complex. The lithological association represents a sequence of oceanic and trench material in an active continental margin which were subsequently metamorphosed, as also confirmed by petrographic studies. The U-Pb data from these samples are compiled in Figs. 13 and 14. Zircon grains in the greenstone sample (WT-1/1E) define an upper intercept age of 2951 ± 41 Ma and lower intercept age of 1845 ± 190 Ma (Fig. 13a). These ages indicate the timing of formation of the protolith during Mesoarchean and subsequent metamorphism during Late Paleoproterozoic (Santosh et al., 2007, 2013). Zircon grains from the felsic tuff sample (WT-1/2A) yield a 207Pb/206Pb weighted mean age of 2557 Ma (Fig. 13c), marking the timing of active arc magmatism. Zircon grains from the BIF intercalated with the felsic tuff yielded a weighted mean age of 2585 Ma which is 12

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Fig. 14. U-Pb concordia plots and weighted average plots. All data point uncertainties are 2σ.

subduction in the Wutai arc. Du et al. (2010) reported two groups of zircon grains with 207Pb/206Pb ages of 2433–2558 Ma and 2140 Ma from basaltic andesite in the Hutuo group. In a subsequent study, Du et al. (2013) obtained 207Pb/206Pb weighted mean age of 2137 from zircon grains in a porphyritic granite and considered the age to represent the timing of rifting event in NCC. Du et al. (2017) presented zircon age data from the Doucun and Dongye subgroups (2.5 Ga, 2.2–2.1 Ga with a minor group at 2.7 Ga), Guojiazhai sub-group of the Hutuo group (2.4 Ga, 2.2–2.1 Ga and 2.0–1.9 Ga) and inferred a period of major crustal growth in the TNCO during 2.6–2.9 Ga. Han et al. (2017) investigated the Algoma-type BIF and reported zircon SIMS ages of 2693 Ma, 2528 Ma and 2543 Ma from associated plagioclase gneiss and chlorite sericite schist, and correlated the Wutai complex with Marina-type arc system and intra – oceanic subduction. Liu et al. (2016) reported detrital zircon ages in the range of 2769–2650 Ma, 2565–2465 Ma and 2372–2331 Ma from the quartz schist, meta-conglomerate, quartzite and felsic dyke in the Wutai group, and interpreted an active continental margin and continental island arc setting. Wilde et al. (2004) investigated metamorphosed intermediate to felsic rocks from three subgroups of Wutai area and reported zircon SHRIMP 207 Pb/206Pb weighted mean age of 2523 and suggested that that Wutai complex containing both island arcs and forearc components. Wang et al. (2014) obtained zircon SIMS 207Pb/206Pb weighted mean age of 2542 Ma from amphibolite intercalated with the BIF. In Fig. 17 we compile the zircon U-Pb age data from various studies

in the Wutai Complex, where most of the age peaks cluster in the range of 2499–2545 Ma and 2115–2135 Ma. The age data from our present study show two sharp peaks at 2578 Ma and 2873 Ma. The 2578 Ma age peak in this study correlates well with the 2499–2545 Ma age peaks in previous studies and mark the initial cratonization of the NCC at 2.58–2.45 Ga (Tang and Santosh, 2018a,b), possibly through extensive arc magmatism. The ca. 2873 Ma age peak in our study suggests that active subduction and arc building were possibly initiated in the Mesoarchean and continued into Neoarchean, thus suggesting episodic crust building during a protracted subduction realm. The Banded Iron Formations in the NCC are considered to have formed during Eoarchean to Paleoproterozoic (3.8–2.3 Ga), and mainly in the late Neoarchean (2.5–2.55 Ga) (Wan et al., 2012). The BIFs in Wutai area are considered as Algoma – type. As chemical sedimentary rocks, they generally lack syngenetic zircon grains, and the ages from volcanic intercalations, or the overlying or underlying rocks were used to constrain the formation age of BIFs (Wang et al., 2014). In this study, the BIF from Wutai is intercalated with felsic tuff and the weighted mean of 2557 Ma from magmatic zircon grains in the tuff marks the minimum time of deposition of the BIF. The weighted mean ages of 2558 and 2471 Ma from zircon grains in the felsic tuff intercalated with the BIF and the discrete felsic tuff layer obtained in this study provide a robust estimate of the minimum time interval of deposition of the BIFs in Wutai. The weighted mean age of 1906 Ma from the BIF corresponds to the timing of metamorphism.

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Fig. 15. Zircon Chondrite-normalized REE diagrams. The red lines are from magmatic grains and blue lines are from metamorphic grains, chondrite values are after Sun and McDonough (1989). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

5.2. Building the Wutai arc

Wutai group (Fig. 14). Du et al. (2017) presented the two stage Hf model ages on detrital zircon grains from the Hutuo group, which show a range of 2.9–2.6 Ga. Despite the relatively large range in εHf (t) values, a major juvenile crustal source is indicated (Fig. 16). The available zircon Lu-Hf data indicate that the Wutai arc was constructed during ca. 3.0–2.45 Ga (Fig. 16). Diverse tectonic models were proposed for the origin of the Wutai Complex in various studies, most of which converge on the concept that it represents an arc (Bai, 1989; Li et al., 1990; Zhao et al., 1999; Wang et al., 2004; Polat et al., 2005; Zhao et al., 2007), which formed within subduction-related setting (Wilde et al., 1997; Zhao, 2001). Our geochemical data are also consistent with arc magmatism along a convergent margin. The greenstone samples show low K sub-alkalic characteristics and crustal contamination trend (Fig. 8b). In Th/Yb – Nb/Yb diagram (Fig. 11), the greenstones display active continental margin/ alkaline oceanic arc features, with source components corresponding to

The zircon εHf (t) values of zircon grains in the felsic tuff range from 3.15 to 6.95 and the tuff intercalated with BIF shows values in the range of 4.35 to 7.72. The 176Hf/177Hf and positive εHf (t) values indicate Neoarchean juvenile sources (Fig. 16). The data presented in this study from the various rocks types yield Hf depleted mantle model ages (TDM) of 2592–2727 Ma and Hf crustal residence ages (T) of 2619–2832 Ma (Supplementary Table 3). Most of the data fall in the area between the depleted mantle and CHUR line indicating the addition of juvenile components from Mesoarchean to Neoarchean (Fig. 16). In previous studies, Du et al. (2013) reported εHf (t) values ranging from −1.72 to +1.84 and TDM1 (single stage) ages of 2429–2554 Ma from porphyritic granites, indicating minor recycling from pre-Neoarchean continental together with depleted mantle components (Fig. 16). Liu et al. (2016) also reported juvenile crust source for meta-sedimentary rocks from

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to upper crust, and display Eu positive anomalies (Fig. 10a, c), typical of high-T hydrothermal (Fig. 10b) (Wang et al., 2014) or marine hydrothermal activity. Zhao (2001) proposed the sub-division of NCC into the Eastern and Western Blocks with a paleo-ocean in between. Tang and Santosh (2018a) in a comprehensive overview of various complexes from the TNCO noted that the amalgamation of microblocks along ca. 2.5 Ga granite-greenstone belts occurred through multiple subduction, with or without arc-plume interaction. According to their model on the evolution of the TNCO, the Wutai ocean separated two Archean microblocks – the Ordos and Qianhuai at > 2.58 Ga; at ca. 2.58–2.48 Ga, three microblocks – Ordos, Qianhuai and Xuchang – were amalgamated, coinciding with the initial cratonization of the NCC (Fig. 18a, b). The magmatic zircon grains in the different rock types in our study define an age range of 2873–2450 Ma (Fig. 17), which when combined with the age data from previous studies, suggest that the Wutai arc was built through prolonged subduction system during Meso- to Neoarchean in the central NCC (Fig. 18c).

Fig. 16. Zircon Hf isotopic evolution diagram compared with the previous study. Error bars represent 2σ uncertainties. CHUR-chondritic uniform reservoir. New crust line is after Dhuime et al. (2011). Crustal model ages (TDMC) were calculated using representative bulk crustal value 176Lu/177Hf = 0.015 (Griffin et al., 2002). Previous data include porphyritic granite age from Du et al. (2013), meta-sedimentary samples from Liu et al. (2016), and greywacke, sandstone, quartz arenite and conglomerate from Du et al. (2017).

6. Conclusions

• The greenstone – felsic tuff – BIF – phyllite succession reported in

E–MORB, indicating addition of materials from an enriched mantle source. Melts/fluids derived from the downgoing slab and overlying sediments are considered to be responsible for the mantle enrichment. Our rocks from Wutai display similar REE trend with E–MORB (Fig. 9a), although the ∑REE is slightly lower than that of E–MORB (Fig. 9a). The BIF samples in this study show weathering trend from depleted mantle

•

this study from the Wutai Complex within the Trans-North China Orogen represents an accreted and metamorphosed oceanic and trench succession along a paleo-convergent margin between two microblocks in the North China Craton. Geochemical data show E-MORB affinity for the greenstones, oceanic arc feature for the felsic tuff, and marine hydrothermal setting for the BIF, confirming an active subduction system.

Fig. 17. Zircon 207Pb/206Pb age distribution curve compared with previous those from previous studies The background shows evolution of the Trans – North China Orogen (TNCO) during Neoarchean-Paleoproterozoic (after Tang and Santosh, 2018b).

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Fig. 18. (a) and (b): Schematic illustration showing the Neoarchean evolution of TNCO (after Tang and Santosh, 2018b). (c) Schematic plate tectonic diagram illustrating the formation of the Wutai arc, and the active subduction at ca. 2557 Ma indicated by the arc-related felsic tuff (plate tectonic model modified from Schroeder, 2016).

• Zircon U-Pb data indicate that the convergent margin was active • •

Acknowledgements

during 2951–2471 Ma with peak arc building at around 2.5–2.6 Ga. The timing of metamorphism is constrained between 1906 and 1845 Ma. The 176Hf/177Hf and positive εHf (t) values of zircon from the various rock types indicate contribution from Neoarchean juvenile sources. The Hf depleted mantle model ages of 2592–2727 Ma and Hf crustal residence ages of 2619–2832 Ma suggest addition of juvenile components from Mesoarchean to Neoarchean. In conjunction with published data, we conclude that the Wutai arc was built during ca. 2.87–2.45 Ga, which also marks the timing of the initial cratonization of NCC through the amalgamation of the microblocks and intervening arcs.

We thank Prof. Guochun Zhao, Editor and two anonymous referees for constructive suggestions and detailed reviews which improved our manuscript. This study forms part of the PhD research work of Pin Gao at the China University of Geosciences Beijing. Dr. Chengxue Yang and Mr. Haidong Liu accompanied the field work in which the samples for this study were collected, and we thank them for their valuable participation and help. This study was funded by Foreign Expert grants from CUGB to M. Santosh.

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Appendix A. Supplementary data

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