New evidence for ~ 4.45 Ga terrestrial crust from zircon xenocrysts in Ordovician ignimbrite in the North Qinling Orogenic Belt, China

Gondwana Research 23 (2013) 1484–1490

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GR Letter

New evidence for ~ 4.45 Ga terrestrial crust from zircon xenocrysts in Ordovician ignimbrite in the North Qinling Orogenic Belt, China Chunrong Diwu a,⁎, Yong Sun a, Simon A. Wilde b, Hongliang Wang a, Zengchan Dong a, Hong Zhang a, Qian Wang b a b

State Key Laboratory of Continental Dynamics, Department of Geology, Northwest University, Xi'an, 710069, China Department of Applied Geology, Curtin University, Kent St, Bentley, WA 6102, Australia

a r t i c l e

i n f o

Article history: Received 19 September 2012 Received in revised form 22 December 2012 Accepted 4 January 2013 Available online 11 January 2013 Handling Editor: M. Santosh Keywords: Hadean Zircon Hf model ages Early Earth Ancient crustal recycling

a b s t r a c t Evidence for the earliest known terrestrial crust comes predominantly from Jack Hills in Western Australia, where hafnium isotopic results from > 3.8 Ga detrital zircons indicate crustal precursors as old as ~4.4– 4.5 Ga. We present evidence from magmatic cores in > 3.9 Ga xenocrystic zircons from a felsic volcanic rock in the North Qinling Orogenic Belt, China, of similar Hf crustal model ages up to 4.45 Ga. These lie on the same Lu/Hf trajectory as the least disturbed Jack Hills and Apollo 14 zircons, therefore providing only the second example of the earliest known generation of continental crust on Earth. In addition, the rims of two zircon grains record later growth at 3.7 Ga and, when combined with the fact that the grains are incorporated in Paleozoic volcanic rocks, imply long-lived crustal residence within the basement of the North China Craton. These results therefore establish the wider distribution and survival of the most ancient crustal material on the Earth and highlight the possibility for the further discovery of ancient crustal remnants. © 2013 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved.

1. Introduction Fragmentary evidence about the nature of the Earth's earliest crust has come from a small suite of > 4.0 Ga detrital or xenocrystic zircons, mostly preserved in the Mt Narryer and Jack Hills belts and adjacent granitoids in the Narryer Terrane of Western Australia [see historical summary in Wilde and Spaggiari (2007) and summary of more recent work in Harrison et al. (2008), Harrison (2009) and Kemp et al. (2010)]. These Hadean zircons serve as a bridge to explore an important chapter of early Earth history, representing the time between the formation of the Earth and the preservation of the oldest known rocks. Hadean zircon occurrences are extremely rare elsewhere, although they have been reported from three other areas; the Southern Cross belt in Western Australia (Wyche et al., 2004), the Acasta gneiss complex in the Northwest Territories of Canada (Iizuka et al., 2006, 2009) and from China, both in Tibet (Duo et al., 2007) and at Qinling along the southern margin of the North China Craton (Wang et al., 2007; Diwu et al., 2010). The large number of > 4.0 Ga zircons found in the Narryer Terrane, including one with a concordant 207Pb/ 206Pb age of 4404 ± 8 Ma (Wilde et al., 2001), has meant that most attention has focused on this region. Although the other occurrences tend to contain just isolated grains, they nonetheless provide an important indication of the widespread preservation of zircon from the period of Earth ⁎ Corresponding author. Tel./fax: +86 29 88302092. E-mail address: [email protected] (C. Diwu).

history before the extant rock record, limited by the 4.03 Ga components of the Acasta gneiss complex (Bowring and Williams, 1999). The two Hadean zircons reported from China include a 4.1 Ga grain from the Burang area of Tibet (Duo et al., 2007), located in a greenschist-facies quartz schist of the Neoproterozoic Qiumgongba Group, and another ~ 4.1 Ga grain identified in the North Qinling Orogenic Belt (Wang et al., 2007; Diwu et al., 2010). The latter discovery occurs in the major Phanerozoic collisional belt (Fig. 1) developed between the North and South China cratons, with the zircon extracted from Ordovician arc volcanic rocks of the Caotangou Group. These rocks were dated at 456 ± 2 Ma (Wang et al., 2007) and ascended though the North China Craton basement, where the zircon xenocryst was incorporated either from the source region or during magma ascent. As such, it is the first report of Hadean crustal material in a Phanerozoic igneous rock.

2. Geological setting and sample selection The WNW-ESE trending Qinling Orogen (Li and Sun, 1999; Zhu et al., 2011) is located in Central China and extends along strike for nearly 2500 km (Fig. 1A). It is subdivided into the North and South Qinling Orogenic Belts (Fig. 1B), separated by the Shangnan–Danfeng suture zone. The North Qinling Orogenic Belt marked the southern margin of the North China Craton in the early Paleozoic, whereas the South Qinling Orogenic Belt was the passive continental margin of the Yangtze Craton prior to collision of the terranes in the early

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Fig. 1. (A) Location of the broad study area within China; (B) More precise location of the study area within the North Qinling Orogenic Belt; (C) Simplified geological map of the Sangyuan area in the western North Qinling Orogenic Belt, Central China. modified from Wang et al., 2007

Mesozoic (Zhang et al., 1996; Dong et al., 2011; Zhai and Santosh, 2011). The original Hadean zircon was extracted from an ignimbrite (welded tuff) belonging to the Ordovician Caotangou Group, which is a low grade greenschist-facies terrigeneous clastic-volcanic association (Diwu et al., 2010). The ignimbrite is exposed along the east bank of a small stream near Sangyuan, in Liangdang County, Gansu Province [GPS: Lat. 34°09′06.7″; Long. 106°31′12.0″] (Fig. 1C). The host is gray–green in color, with a massive structure and eutaxitic texture (Fig. 2A). Lithic fragments and fiamme are flattened and weakly elongated parallel to bedding. Crystal fragments mainly consist of feldspar and quartz, set in a felsophyric matrix with a microcrystalline texture that shows extensive alteration to sericite and chlorite (Diwu et al., 2010) (Fig. 2B). In re-examining this occurrence, a suite of more than 3000 zircon grains was extracted from a new sample of ignimbrite, collected from the same outcrop where the >4 Ga grain was obtained (Diwu et al., 2010), and these were initially analyzed for U–Pb age using LA-ICPMS techniques. From this large suite of zircons, two new grains were identified (denoted here as B and C) with ages ≥3.9 Ga. In addition, the original grain of Wang et al. (2007) (denoted here as Grain A) was re-analyzed for U–Pb age by SHRIMP, and this confirmed the original result. These new U–Pb data were originally published in Chinese (Diwu et al., 2010), but are included here (Table 1) so they are more widely

available and in order that the new Lu–Hf and O data presented here can be placed in a precise temporal context. 3. Analytical methods Both LA-ICP-MS U–Pb and trace element analysis of zircon were carried out using an Agilent 7500a ICP-MS connected to a Geolas-193 UV laser ablation system at the State Key Laboratory of Continental Dynamics, Northwest University in Xi'an. A 20 μm spot diameter was utilized in this study with a laser repetition rate of 6 Hz. The analytical procedures are described in Liu et al. (2007). The U–Pb SHRIMP II analyses were undertaken at the Beijing SHRIMP Center, Chinese Academy of Geological Sciences, following standard operating procedures (Williams, 1998), with a mass resolution of ~5000 (1% definition). The intensity of the primary O −2 ion beam was 6 nA. Spot sizes ranged from 25 to 30 μm, and each site was rastered for 150 s prior to analysis to remove contamination from the gold coating. Five scans through the mass stations were made for each age determination. Standards used were SL13, with an age of 572 Ma and U content of 238 ppm (Williams, 1998), and TEM, with an age of 417 Ma (Black et al., 2004). The ratio of TEM standard analyses to unknown sample analyses was 1:3. Decay constants used for age calculation were those recommended by the Subcommission on Geochronology of the IUGS (Steiger and Jager, 1977). The measured value

Fig. 2. Photographs showing the pyroclastic host rock to the ancient zircon xenocrysts. (A) Field photograph of sample site on the southern side of the stream in the Sangyuan area of the western North Qinling Orogenic Belt, Central China. The lens cap is 60 mm in diameter; (B) Thin section showing altered glass fiamme in recrystallized matrix dominated by sericite alteration. A feldspar phenocryst is present in lower left of image. The scale is 0.2 mm in length. Pl= plagioclase, Mag = magnetite.

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4384 4076 0.7 0.4

4008

3751 3909

A211-3-305

A211-3-305(2) A211-6-386 A211-6-386-Rim C

3709 CTG09-1.1.2

B

4080 4079 4027 CTG09-1.1.1 C1tW24 CTG09-1.1.3 A

The U–Pb isotope data of Hadean–Eoarchean zircon xenocrysts is cited from Diwu et al. (2010).

0.06 0.14 50 85

876 629

0.01816 0.01553

0.00054 0.00071

0.280208 0.280321

0.000019 0.000012

−7.0 0.2

4144 4013

4428 0.7 0.04 31

797

0.0175

0.0007

0.280125

0.000020

−4.5

4270

4357 4115 1.4 −7.1 0.280258 0.00089 0.25 148 37

0.27 0.49 0.18 119 167 61

446 337 330

0.01616

0.01618

0.000038

0.7 −4.6 0.00068

Lu/177Hf 176

Yb/177Hf 176

Th/U U (ppm) Th (ppm) Pb/206Pb(Ma) 207

Age of Sample spot Grain no.

Table. 1 The Hf–O compositions of Hadean–Eoarchean zircon xenocrysts in Ordovician ignimbrite from the North Qinling Orogenic Belt, China.

176

0.280108

Hf/177Hfi

2s

0.000020

εHf(t)

2s

4290

TDM1/Ma

4449

TDMC2/Ma

6 6.3 5 5.1 0.1 −0.2 3.8 5.4 6.6

δ18O (‰)

0.2 0.2 0.2 0.2 0.3 0.3 0.1 0.2 0.2

±2σ

1486

of 204Pb was applied for the common lead correction and data processing was carried out using the Isoplot program (Ludwig, 2003). Zircon Hf isotopic analyses were conducted using a Nu Plasma HR MC-ICP-MS (Nu Instruments Ltd., UK) equipped with a 193 nm laser, hosted at the State Key Laboratory of Continental Dynamics, Northwest University in Xi'an. A spot size of 32 μm diameter was adopted in this study. The Monastery and GJ-1 zircon standards were analyzed as unknowns and their 176Hf/177Hf ratios were 0.282732±0.000012 (n= 12, 2σ) and 0.282013±0.000014 (n =11, 2σ), respectively, during the analytical sessions; in good agreement with the recommended values (see Wu et al., 2006). Data processing involved using a decay constant for 176Lu of 1.867 ×10−11 yr−1 (Albarède et al., 2006) and the presentday chondritic ratios of 176Hf/177Hf=0.282772 and 176Lu/177Hf= 0.0332 (Bouvier et al., 2008) were adopted to calculate εHf values. Single-stage model ages (TDM1) were calculated by reference to the depleted mantle with a present-day 176Hf/177Hf ratio of 0.28325 and 176 Lu/177Hf ratio of 0.0384 (Griffin et al., 2000). The two-stage model age (TDM2) was calculated by projecting the initial 176Hf/177Hf of zircon back to the depleted mantle growth curve using 176Lu/177Hf=0.015 for the average continental crust (Griffin et al., 2000). However, the appropriate value for Hadean crust is not known and some comment is required. In order to calculate the Hf model age of zircon grown in a crustal rock, it is necessary to define the 176Lu/ 177Hf ratio of the protolith that was initially extracted from the mantle. For postHadean zircons, this ratio varies from ~ 0.022 for mafic magmas (Amelin et al., 1999) to around 0.01 for felsic magmas, with an upper continental crustal value of 0.008 (Rudnick and Gao, 2003) and 0.015 for total continental crust (Griffin et al., 2000). For the Precambrian, a 176Lu/ 177Hf value of 0.0093 was proposed for Precambrian granitic crust by Vervoort and Patchett (1996) and 0.005 for Precambrian TTG by Blichert-Toft and Albarede (2008), although the latter leads to Hf isotopic ratios that are too unradiogenic (see Kemp et al., 2010). In view of this uncertainty, and following the proposal that continental crust and oceans were present very early in Earth history (Mojzsis et al., 2001; Wilde et al., 2001; Harrison et al., 2005), we chose the value of 0.015 for the initial calculations. Zircon oxygen isotopes were measured using the Cameca IMS 1280 ion microprobe at the Institute of Geology and Geophysics, Chinese Academy of Sciences in Beijing. The spot size was ~20 μm in diameter. The TEMORA 2 zircon standard during the course of this study yielded a weighted mean of δ18O = 8.14 ± 0.20‰ (n= 13, 2σ), which is consistent, within error, of the reported value (Black et al., 2004). Analytical procedures were similar to those described in Li et al. (2010). 4. Results 4.1. Grain A Cathodoluminescence (CL) imaging of Grain A reveals oscillatory zoning and a core–rim structure (Fig. 3A). A concordant LA-ICPMS 207Pb/ 206Pb age of 4079 ± 5 Ma was reported from this grain by Wang et al. (2007) and to verify this, the mount was re-polished and the grain re-analyzed using the SHRIMP II ion microprobe at the Beijing SHRIMP Center. One site was chosen as close as possible to the initial analytical site and recorded an age of 4080 ± 9 Ma (Diwu et al., 2010), thus confirming the original result. However, because the initial site overlapped a change in CL pattern (Fig. 3A), two new sites were also selected. One site was selected in oscillatory zoned zircon near the center of the grain and revealed an age of 4027 ± 12 Ma. The other was placed in the rim domain and recorded an age of 3709 ± 15 Ma (Diwu et al., 2010). Following SHRIMP analysis, the mount was lightly re-polished and oxygen isotope analyses were determined by Cameca IMS 1280 at the Institute of Geology and Geophysics, Chinese Academy of Science, in Beijing. The site that gave a SHRIMP age of 4027 ± 12 Ma recorded a δ 18O value of 6.0‰ (Table 1), whereas an adjacent site in the core

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Fig. 4. Rare earth element data for zircon xenocrysts from the western North Qinling Orogenic Belt, China.

δ 18O value of 5.1‰, with an adjacent site (Fig. 3A) giving a value of 5.0‰, both close to the mean mantle value (Fig. 4). The 176Hf/ 177Hf value obtained from the 4027 ± 12 Ma core site was 0.280108 and for the rim 0.280258. The calculated initial εHf(t) value and two-stage model age (TDM2) for the core are − 4.6 and 4449 Ma, respectively, and for the rim they are − 7.1 and 4357 Ma, respectively (Table 1). Trace element analyses (Table 2) of the core and rim yield similar patterns (Fig. 4), both showing prominent positive Ce anomalies, negative Eu anomalies, and heavy rare earth element (HREE) enrichment, typical of magmatic zircon (Cavosie et al., 2006). 4.2. Grain B Grains B and C are located in separate mounts, and the analytical protocol for both was as for Grain A, except that neither was analyzed using SHRIMP, and both were analyzed for trace elements simultaneously during LA-ICP-MS analysis. Grain B has a dark structureless core in CL image with a concordant LA-ICP-MS 207Pb/ 206Pb age of 4008 ± 29 Ma (Fig. 3B) and Th/U ratio of 0.04. The rim is much lighter in CL with broad zoning and records a LA-ICP-MS 207Pb/ 206Pb age of 3751 ± 30 Ma with a Th/U ratio of 0.06. Two sites were selected in the core for oxygen isotope analyses and these record extremely low δ 18O values of 0.2‰ (from the U–Pb site) and − 0.1‰ from an adjacent site (Fig. 3B). The U–Pb rim site has a δ18O value of 3.8‰ (Fig. 3). Lu–Hf isotope data were obtained from both dating sites, with the core location having a 176Hf/177Hf value of 0.280125, an εHf(t) value of −4.4 and a TDM2 model age of 4428 Ma. The rim has a 176Hf/177Hf value of 0.280208, a more negative εHf(t) value of −7.1 and a TDM2 model age of 4384 Ma. Trace element analyses of the core and rim are similar, but are in marked contrast to Grain A (Fig. 4; Table 2), since they have much flatter patterns with light rare earth (LREE) enrichment, and negative Eu anomalies. These features are more typical of hydrothermal zircon (Hoskin, 2005) or grains that have undergone radiation damage (Cavosie et al., 2006).

Fig. 3. Zircon cathodoluminescence images. (A) Grain A; (B) Grain B; (C) Grain C. The red circles and numbers show the δ18O results, the blue circles show the location of the U–Pb analytical sites with age in Ma, and they also show the εHf(t) values, where available.

(Fig. 3A) recorded a value of 6.3‰. Owing to damage incurred during previous analytical sessions, it was not possible to obtain precise data from the site with an age of 4080 ± 9 Ma. Importantly, oxygen data obtained from the SHRIMP site near the rim of the grain recorded a

4.3. Grain C Grain C displays a core–rim structure in CL and is similar to Grain B, with a dark core and lighter rim, although the core shows traces of magmatic oscillatory zoning (Fig. 3C). The core records a concordant LA-ICP-MS 207Pb/206Pb age of 3909± 45 Ma with a Th/U ratio of 0.14 (Table 1). Oxygen isotope data were collected from the U–Pb site and from the lighter rim. The δ18O value of the core is 5.4‰, whereas the rim value is 6.6‰ (Figs. 3C). The core site has a 176Hf/177Hf value of 0.280321, an εHf(t) value of 0.2, and a TDM2 model age of 4076 Ma.

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Table 2 Rare earth element data for Hadean–Eoarchean zircon xenocrysts in Ordovician ignimbrite from the North Qinling Orogenic Belt, China. Grain No

Spot number

La

Ce

Pr

Nd

Sm

Eu

Gd

Tb

Dy

Ho

Er

Tm

Yb

Lu

A

Core 1 Rim Core Rim Core

0.01 0.02 30.6 65 0.04

9 15 193 289 11

0.06 0.05 14.1 26.9 0.14

1 1 67 130 3

3 1 32.4 60.5 7.7

0.2 0.1 1.5 2 0.8

12 6 22 90 48

5 3 9 28 19

59 35 123 338 253

22 15 52 153 101

99 75 252 688 451

21 18 66 165 94

270 185 739 1504 870

62 54 195 320 178

B C

Trace element data from the core site define a trend similar to Grain A (Fig. 4), with a prominent positive Ce anomaly, negative Eu anomaly, and HREE enrichment, typical of magmatic zircon (Cavosie et al., 2006).

5. Discussion and conclusions 5.1. Source regions The three ancient zircon grains identified in this and companion studies (Wang et al., 2007; Diwu et al., 2010) from the Ordovician Caotangou Group volcanics in the North Qinling Orogenic Belt of central China, provide important evidence regarding the nature of the basement to the North China Craton, from which this portion of the orogenic belt is considered to be derived (Zhang et al., 1996; Dong et al., 2011). Note that initially the cores of Grains B and C were interpreted to be of metamorphic origin (Diwu et al., 2010), but this is re-evaluated here. Importantly, the new SHRIMP data confirm the antiquity of the core of Grain A, as reported by Wang et al. (2007). Furthermore, the REE data (Fig. 5) support the interpretation from the CL images that the oscillatory zoned cores in Grains A and C record magmatic crystallization of the zircon xenocrysts. The dark oscillatory zoned core of Grain C records a δ 18O value of 5.4‰, consistent with its derivation from either juvenile crustal rocks or direct from the mantle, which has a value of 5.3‰ ± 0.3‰ (Valley et al., 1998). In contrast, the core of Grain A records slightly elevated δ 18O values of 6.0–6.3‰. Such values are close to the maximum that can result from fractionation of mantle-derived magmas (Taylor and Sheppard, 1986) and just overlap the field of ‘supracrustal’ zircon as defined by Cavosie et al. (2005), most likely implying magma derivation

from material that has undergone prior interaction with surface waters. Similar, to slightly higher, values (average 6.5‰) are reported from detrital zircons in the Caozhuang quartzite in Eastern Hebei Province, North China (Wilde et al., 2008) with U–Pb ages up to 3.86 Ga. These were interpreted as resulting from rapid recycling of altered juvenile crust, since zircon Hf data from the same location (Wu et al., 2005) indicated chondritic εHf(t) values and TDM2 model ages close to the formation ages. Unfortunately, the core of Grain B is dark, structureless and metamict, implying that the δ 18O values of 0.2 to − 0.1‰ are not primary. This is substantiated by the REE data (Fig. 4), where the flatter pattern of Grain B, with LREE enrichment, is considered to reflect radiation damage and subsequent alteration (see Cavosie et al., 2006). Unlike the detrital zircons entrained in the Caozhuang quartzite (Wilde et al., 2008) – or the magmatic grains in the nearby Anshan gneisses, where the oldest zircon has a 207Pb/206Pb age of 3887 ± 5 Ma, εHf(t) value of 1.07 and a TDM2 model age of 4008 Ma (Wu et al., 2008) – the εHf(t) values of the North Qinling zircons range from chondritic to −4.6, with TDM2 model ages ranging from 4076 to 4449 Ma. The latter are the oldest Hf model ages recorded from the North China Craton, and fall within the range of values reported for the ancient detrital zircon suite from Jack Hills, Western Australia (Harrison et al., 2005, 2008; Blichert-Toft and Albarede, 2008; Harrison, 2009; Kemp et al., 2010). To the best of our knowledge, this makes North Qinling only the second locality on Earth where Hf model age data indicate the presence of source rocks as old as ~4.45 Ga. Importantly, the North Qinling data fall on the 176Lu/ 177Hf= 0.020 trend as defined by the most pristine oscillatory zoned zircons from Jack Hills and zircons from the Apollo 14 breccias (Taylor et al., 2009; Kemp et al., 2010). This lends credence to the idea that the earliest crust on Earth may have evolved from a KREEP-like reservoir, similar to that on the Moon (see Kemp et al., 2010). 5.2. Crustal reworking

Fig. 5. Plot of εHf(t) versus age. The yellow circles show the Hf isotope composition of xenocrystic zircons from the western North Qinling Orogenic Belt, China; the shaded diamonds are the Hf isotopic data from the Jack Hills detrital zircon suite (after Kemp et al., 2010). The isotope trajectories of putative upper continental crust (176Lu/177 Hf = 0.008; Rudnick and Gao, 2003), mafic crust (176Lu/177 Hf = 0.022) and 177 TTG reservoirs (formed at 4.3 Ga with 176Lu/ Hf = 0.005, Blichert-Toft and Albarede, 2008) are shown for reference.

All three zircon xenocrysts have rims (Fig. 3), with those of Grains A and B dated at 3709 ± 15 Ma and 3751 ± 30 Ma, respectively. The rim of Grain A is oscillatory zoned, has a Th/U ratio of 0.25, a REE pattern consistent with a magmatic origin (Fig. 4), and records mantle δ 18O values of ~ 5.0‰; most likely indicating incorporation of the original grain within a mantle-derived magma. However, the εHf(t) value of − 7.1 and TDM2 model age of 4357 Ma suggest that a component from the ancient lower crust was also involved. Grain B has a rim showing weak banding (Fig. 3B) and a fairly low Th/U ratio of 0.06. The oxygen data (Table 1) and the REE pattern (Fig. 5) indicate that the core of this grain has undergone substantial alteration, as discussed above. However, its rim has a εHf(t) value of − 7.0 and TDM2 model age of 4384 Ma, indicating that the Lu–Hf system was not disturbed by this alteration and thus it records values comparable to the rim of Grain A, implying a similar genesis. The rim to Grain C was not dated, but the elevated δ 18O value of 6.6‰ suggests growth in a melt that incorporated a supracrustal component (Cavosie et al., 2005). Although the data are too few to determine a precise origin for the ~ 3.7 Ga rims, it is clear that they record a tectonothermal event involving crust as old as 4.3 Ga and thus attest to Archean reworking of Hadean crustal material beneath the North China

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Craton. Furthermore, the occurrence of these three ancient grains in an Ordovician volcanic rock indicates that some of these ancient components were still present in the basement of the North China Craton during the early Paleozoic. 5.3. Significance The oldest known rocks on Earth are the 4.03 Ga components of the Acasta gneiss complex in the Slave Province of northwestern Canada (Bowring and Williams, 1999) from which a 4.2 Ga zircon xenocryst has been reported (Iizuka et al., 2006, 2009). Hafnium data for zircons from the Acasta gneisses, some as old as 3.94 Ga, reveal negative εHf(t) values from −6.7 to −1.2 and TDM2 model ages (using a 176Lu/177Hf value of 0.093) extending back to 4.16 Ga (Iizuka et al., 2009). However, the oldest known crystals on Earth are the >4.0 Ga detrital zircons described from Mt Narryer and Jack Hills (see summary in Wilde and Spaggiari, 2007) in the Narryer Terrane of Western Australia, with 207 Pb/206Pb ages extending back to 4.4 Ga (Wilde et al., 2001). Their hafnium data (Harrison et al., 2005, 2008; Blichert-Toft and Albarede, 2008; Harrison, 2009; Kemp et al., 2010) reveal extraction of the zircon protoliths from a much older source at ~4.4–4.5 Ga. Although there is some debate as to the veracity and significance of the reported positive εHf(t) values (Harrison et al., 2005; Valley et al., 2006; Kemp et al., 2010), the Hf TDM2 model ages of the zircons are not in dispute and, although the errors are large, their extraction age cannot be substantially younger. The ancient protoliths of these zircons have been variously attributed to the products of subduction (Harrison et al., 2008; Harrison, 2009; Kemp et al., 2010) or melting of differentiated components of a terrestrial magma ocean (Blichert-Toft and Albarede, 2008; Kemp et al., 2010), with perhaps the intermediate generation of TTGs (Blichert-Toft and Albarede, 2008). China is one of the few places in the world where ~ 3.8 Ga crustal material has been identified, with rocks of that age present in the North China Craton near Anshan (Liu et al., 1992, 2008; Song et al., 1996) and abundant detrital zircons up to 3860 ± 3 Ma in the Caozhuang quartzite in Eastern Hebei (Wilde et al., 2008) (Fig. 1). However, initial Lu–Hf investigations at these two localities failed to identify a significantly older source, with maximum Hf model ages of ~4 Ga (Wilde et al., 2008; Wu et al., 2008). The study of Liu et al. (2008) did, however, identify several zircons from a ~3.8 Ga trondhjemitic gneiss of the Shengoushi Complex at Anshan with TDM2 model ages >4 Ga; the oldest recording a model age of 4264 Ma. But this is still significantly younger than the TDM2 model ages reported here from zircon obtained from the Ordovician ignimbrite in the North Qinling Orogenic Belt. Based on the available evidence from the global Hadean zircon record, early magmatic events on Earth are inferred to have occurred throughout the period 4.4 Ga to 3.9 Ga (Nemchin et al., 2006; Cavosie et al., 2005; Harrison et al., 2008; Kemp et al., 2010). Within this time-span, Cavosie et al. (2005) proposed that a significant change occurred by 4.2 Ga, with a marked increase in the proportion of zircons showing elevated δ 18O values (6.3–7.5‰), thus implying extensive crustal reworking from that time onward. The zircon Lu– Hf data from North Qinling provide important new information, being only the second locality on Earth where the hafnium model ages extend back significantly beyond ~ 4.2 Ga. This indicates that preservation of the earliest crustal components (4.2–4.5 Ga) has occurred at more than one locality on Earth. Since the zircons from North Qinling occur in Ordovician volcanic rocks, it may also indicate that the possibility of obtaining additional Hadean material entrained in younger magmatic rocks in this and other cratonic areas may be greater than previously thought. Acknowledgments This study was financially supported by the National Basic Research Program of China (973 Program; grant no. 2012CB416606),

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the National Natural Science Foundation of China (NSFC; grant no. 41272004) and the MOST Special Funds from the State Key Laboratory of Continental Dynamics. We are grateful to Y.S. Wan and W. Wang for assistance with zircon SHRIMP dating and X.H. Li and Q.L. Li for help with the zircon O isotope analyses. We thank Profs. Mingguo Zhai and Alfred Kröner for their constructive reviews that helped improved the quality of the paper and Prof M. Santosh, for his speedy handling of the paper. References Albarède, F., Scherer, E.E., Blichert-Toft, J., Rosing, M., Simionovici, A., Bizzarro, M., 2006. γ-Ray irradiation in the early solar system and the conundrum of the 176Lu decay constant. Geochimica et Cosmochimica Acta 70, 1261–1270. Amelin, Y., Lee, D.-C., Halliday, A.N., Pidgeon, R.T., 1999. Nature of the Earth's earliest crust from hafnium isotopes in single detrital zircons. Nature 399, 252–255. 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