Scholarly article on topic 'SHRIMP zircon dating and LA-ICPMS Hf analysis of early Precambrian rocks from drill holes into the basement beneath the Central Hebei Basin, North China Craton'

SHRIMP zircon dating and LA-ICPMS Hf analysis of early Precambrian rocks from drill holes into the basement beneath the Central Hebei Basin, North China Craton Academic research paper on "Earth and related environmental sciences"

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Abstract of research paper on Earth and related environmental sciences, author of scientific article — Yusheng Wan, Xianzheng Zhao, Zejiu Wang, Dunyi Liu, Alfred Kröner, et al.

Abstract The Central Hebei Basin (CHB) is one of the largest sedimentary basins in the North China Craton, extending in a northeast–southwest direction with an area of >350 km2. We carried out SHRIMP zircon dating, Hf-in-zircon isotopic analysis and a whole-rock geochemical study on igneous and metasedimentary rocks recovered from drill holes that penetrated into the basement of the CHB. Two samples of gneissic granodiorite (XG1-1) and gneissic quartz diorite (J48-1) have magmatic ages of 2500 and 2496 Ma, respectively. Their zircons also record metamorphic ages of 2.41–2.51 and ∼2.5 Ga, respectively. Compared with the gneissic granodiorite, the gneissic quartz diorite has higher ΣREE contents and lower Eu/Eu* and (La/Yb)n values. Two metasedimentary samples (MG1, H5) mainly contain ∼2.5 Ga detrital zircons as well as late Paleoproterozoic metamorphic grains. The zircons of different origins have ε Hf (2.5 Ga) values and Hf crustal model ages ranging from 0 to 5 and 2.7 to 2.9 Ga, respectively. Therefore, ∼2.5 Ga magmatic and Paleoproterozoic metasedimentary rocks and late Neoarchean to early Paleoproterozoic and late Paleoproterozoic tectono-thermal events have been identified in the basement beneath the CHB. Based on regional comparisons, we conclude that the early Precambrian basement beneath the CHB is part of the North China Craton.

Academic research paper on topic "SHRIMP zircon dating and LA-ICPMS Hf analysis of early Precambrian rocks from drill holes into the basement beneath the Central Hebei Basin, North China Craton"

Geoscience Frontiers xxx (2014) 1 —14

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Research paper

SHRIMP zircon dating and LA-ICPMS Hf analysis of early Precambrian rocks from drill holes into the basement beneath the Central Hebei Basin, North China Craton

Yusheng Wana,b,c'*, Xianzheng Zhaod, Zejiu Wange, Dunyi Liua,b, Alfred Kronerb, Chunyan Donga,b, Hangqian Xiea,b, Yuansheng Genga, Yuhai Zhang a,b, Runlong Fana,b, Huiyi Sun a,b

a Institute of Geology, Chinese Academy of Geological Sciences, Beijing 100037, China b Beijing SHRIMP Center, Institute of Geology, Chinese Academy of Geological Sciences, Beijing 100037, China

c State Key Laboratory for Continental Tectonics and Dynamics, Institute of Geology, Chinese Academy of Geological Sciences, Beijing 100037, China d North China Oilfield Company of PetroChina, Renqiu 062552, China e Chinese Academy of Geological Sciences, Beijing 100037, China

ARTICLE INFO

Article history: Received 9 January 2014 Received in revised form 17 February 2014 Accepted 21 February 2014 Available online xxx

Keywords: Early Precambrian North China Craton Drill hole Zircon dating Hf isotopic analysis

ABSTRACT

The Central Hebei Basin (CHB) is one of the largest sedimentary basins in the North China Craton, extending in a northeast—southwest direction with an area of >350 km2. We carried out SHRIMP zircon dating, Hf-in-zircon isotopic analysis and a whole-rock geochemical study on igneous and metasedi-mentary rocks recovered from drill holes that penetrated into the basement of the CHB. Two samples of gneissic granodiorite (XG1-1) and gneissic quartz diorite (J48-1) have magmatic ages of 2500 and 2496 Ma, respectively. Their zircons also record metamorphic ages of 2.41—2.51 and ~2.5 Ga, respectively. Compared with the gneissic granodiorite, the gneissic quartz diorite has higher SREE contents and lower Eu/Eu* and (La/Yb) n values. Two metasedimentary samples (MG1, H5) mainly contain ~2.5 Ga detrital zircons as well as late Paleoproterozoic metamorphic grains. The zircons of different origins have eHf (2.5 Ga) values and Hf crustal model ages ranging from 0 to 5 and 2.7 to 2.9 Ga, respectively. Therefore, ~2.5 Ga magmatic and Paleoproterozoic metasedimentary rocks and late Neoarchean to early Paleoproterozoic and late Paleoproterozoic tectono-thermal events have been identified in the basement beneath the CHB. Based on regional comparisons, we conclude that the early Precambrian basement beneath the CHB is part of the North China Craton.

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1. Introduction

Geological, geochemical and geochronological studies revealed many common features in the exposed early Precambrian rocks of

* Corresponding author. Institute of Geology, Chinese Academy of Geological

Sciences, Beijing 100037, China.

E-mail address: wanyusheng@bjshrimp.cn (Y. Wan).

Peer-review under responsibility of China University of Geosciences (Beijing)

the North China Craton (NCC) (Wan et al., 2011a; Zhai and Santosh, 2011; Zhao and Zhai, 2013 and references therein), which are summarized as follows: (1) The NCC underwent a long and complex tectono-magmatic history back to 3.8 Ga, with 2.8—3.8 Ga rocks having been identified in several areas; (2) juvenile additions of crust from mantle sources were generated in the late Mesoarchean to early Neoarchean and constitute an important crust formation event; (3) the NCC is different from several other cratons in having experienced extensive late Neoarchean tectono-thermal events that resulted in recycling of more ancient crustal material, besides juvenile crustal additions; (4) all or parts of the NCC experienced an extensional tectonic event during the latest Neoarchean as a mark of cratonic stabilization; (5) Paleoproterozoic geological processes in the NCC were much more complex than thought before, with 2.4—2.49 Ga metamorphism and 2.0—2.35 Ga magmatism having

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Y Wan et al. / Geoscience Frontiers xxx (2014) 1—14

been identified in many areas; (6) late Paleoproterozoic (1.8—1.95 Ga) tectono-thermal events occurred widely in the NCC and led to a unified craton towards the end of the Paleoproterozoic. However, the NCC is extensively covered by Mesoproterozoic and younger sedimentary sequences. Zircon dating of rocks recovered from drill holes into the basement was carried out in only a few basins. One such study revealed late Paleoproterozoic magmatism and metamorphism in basement rocks beneath the Songliao basin in the northeastern NCC (Pei et al., 2007), whereas another study indicated that the Ordos basement in the western NCC was involved in a widespread late Paleoproterozoic tectono-thermal event (Hu et al., 2012; Wan et al., 2013).

The Central Hebei Basin (CHB) is one of the largest basins in the NCC, and we carried out SHRIMP zircon dating, Hf-in-zircon iso-topic analysis and a whole-rock geochemical study of magmatic and metasedimentary rocks recovered from drill holes that penetrated its basement.

2. Geological background

The CHB extends in a northeast—southwest direction with an area of >350 km2 (Fig. 1). Its basement is entirely covered by Mesoproterozoic and younger sedimentary rocks. Based on drill core data and geophysical investigations (NCOCP, 2012), the basement is composed of magmatic (mainly granitoids) and supra-crustal rocks (mainly amphibolite, biotite plagioclase gneiss and schist) with greenschist- to upper amphibolite-facies meta-morphism and local anatexis. The bottom of the basin shows up-and-down elevations with the greatest depth being >5000 m. In some areas the early Precambrian basement constitutes buried hills that are controlled by northeast—southwest faults and extend roughly in the same direction as the basin.

Early Precambrian rocks occur extensively around the CHB. In the northwest and west of the basin there are the early Precam-brian Fuping and Zanhuang complexes that contain 1.8—1.9,

Figure 1. Geological map of the Central Hebei Basin and surrounding areas (modified after NCOCP, 2012), showing sample locations in this study. The identification of early Precambrian rocks within the Central Hebei Basin is based on drill core data and geophysical investigations.

Y. Wan et al. | Geoscience Frontiers xxx (20Í4) Í—Í4

2.0—2.2, 2.5—2.55 and ~2.7 Ga granitoids and 1.83—1.92 Ga metamorphic rocks (Zhao et al., 2000, 2002; Guan et al., 2002; Cheng et al., 2004; Yang et al., 2004, 2011a, 2011b, 2013; Trap et al., 2008, 2009; Xiao et al., 2011; Han et al., 2012). In eastern Hebei, northeast of the basin, Archean supracrustal and magmatic rocks occur widely (Liu et al., 1992, 2013; Geng et al., 2006; Yang et al., 2008; Wilde et al., 2008; Sun et al., 2010; Nutman et al., 2011; Zhang et al., 2011, 2012a, 2012b). In the southeast of the basin is the Huanghua depression beneath which basement rocks are also suspected. Four samples for this study were obtained from drill holes that went up to one or more than 10 m into the basement rocks. The locations are shown in Fig. 1, and general information about the samples is provided in Table 1. The petrographic features are summarized below.

2.1. Gneissic granodiorite (XG1-1)

This sample was taken from a drill hole in the northeastern portion of the basin. It shows a gneissic structure (Fig. 2a) and consists of plagioclase, quartz and biotite. Fine-grained plagioclase occurs as aggregates and is altered to epidote and sericite with some phenocrysts showing polysynthetic twinning. Some fine-grained dirty grains may be K-feldspar, but this is difficult to determine under the microscope. Quartz grains are coarser-grained than feldspar, show undulose extinction and are oriented as aggregates parallel to feldspar aggregates, defining a strong foliation. There are fine aggregates of chlorite as a result of alteration of biotite.

2.2. Anatectic biotite K-feldspar paragneiss (MG1)

This sample comes from a drill hole in the central portion of the basin. The rock is inhomogeneous and shows an anatectic texture with K-feldspar-rich leucosome batches (Fig. 2b). It is composed of feldspar, quartz and biotite. Feldspar is dirty (probably seriticized) and is considered to be mainly K-feldspar because the rock contains high K2O and low Na2O (Table 1 ). Some quartz grains are oriented as aggregates. Biotite occurs as aggregates parallel to quartz aggregates and is partly altered to chlorite. It is difficult to determine the protolith of the rock because of strong deformation, meta-morphism and anatexis. We prefer a sedimentary origin because of high K2O and low Na2O (although this is partly due to the analyzed sample containing leucosome) and that it is interlayered with garnet two-feldspar paragneiss (NCOCP, 2012).

2.3. Muscovite-bearing K-feldspar quartzite (H5)

This sample is from a drill core in the southwestern portion of the basin and shows a layered structure (Fig. 2c). The rock is mainly composed of quartz and K-feldspar with some muscovite. Quartz is

fine-grained and shows a triple junction texture. K-feldspar occurs as small grains or porphyroclasts. Fine-grained muscovite flakes are scattered between quartz grains but show orientation, and some muscovite seems to have formed by alteration of K-feldspar. These features suggest that the rock underwent recrystallization after strong deformation.

2.4. Gneissic quartz diorite (J48-1)

This sample was taken from a drill hole about 30 km southwest of sample H5. The rock is gray in color and shows a weak gneissic structure (Fig. 2d). It is mainly composed of plagioclase, quartz and biotite. Plagioclase is strongly altered to sericite and epidote. Some feldspar grains show weaker alteration and are considered to be K-feldspar. Many biotite flakes are altered to chlorite and a dark mineral. Quartz does not occur as aggregates, and this suggests that the rock did not undergo strong deformation.

3. Analytical techniques

Major oxides were analyzed by XRF and trace elements by ICPMS in the Institute of Geological Analysis, Chinese Academy of Geological Science (CAGS), Beijing. Uncertainties for XRF and ICPMS are estimated at 3—5% and 3—8%, respectively.

Zircon dating was carried out on the SHRIMP II ion microprobe at the Beijing SHRIMP Center, Institute of Geology, CAGS. The analytical procedures were similar to those described by Williams (1998). Five scans through the mass stations were made for each age determination. The intensity of the primary ion beam was 3—4 nA. Primary beam size was ~30 mm, and each site was rastered for 120—200 s prior to analysis. Standards M257 (U = 840 ppm, Nasdala et al., 2008) and TEMORA 1 (206Pb/238U age = 417 Ma; Black et al., 2003) were used for calibration of U abundance and 206Pb/238U ratio respectively. The TEMORA 1 to unknown ratio was 1:3—4. Data processing and assessment was carried out using the SQUID and ISOPLOT programs (Ludwig, 2001, 2003). The measured 204Pb was used for common lead correction. 207Pb/206Pb ratios were used to assess the age of all samples. The uncertainties for individual analyses are quoted at the 1s sigma level, whereas uncertainties on weighted mean ages are quoted at the 95% confidence level.

In-situ zircon Hf isotopic analyses were conducted on a Finnigan Neptune MC-ICPMS with a NewWave UP213 laser ablation microprobe at the Institute of Mineral Resources, CAGS. The detailed analytical procedures were described by Hou et al. (2007). Analyses were carried out using a laser beam diameter of 55 mm. The standard GJ-l zircon was analyzed to check for instrument reliability and stability. Calculation of eHf(t) values was based on a decay constant for 176Lu of 1.867 x 10""a"1 (Soderlund et al., 2004) and

Table 1

Nature and location of early Precambrian rock samples from drill cores beneath the Central Hebei Basin, North China Craton.

Sample No.

Rock name

Location

Depth (m)

Weight (kg)

Main minerals

Protolith

Formation (deposition or intrusion) age (Ga)

XG1-1 Gneissic North of basin, -30 km 4795-4798 0.9

granodiorite west of Lanfang

MG1 Anatectic biotite Central basin, -30 km 2481-2487 1.5

K-feldspar gneiss northwest of Xianxian

H5 Muscovite-bearing South of basin, -40 km Uncertain 0.7

K-feldspar quartzite northwest of Hengshui

J48-1 Gneissic quartz South of basin, -55 km west of 1855-1857 0.7

diorite Hengshui

Plagioclase, quartz, K-feldspar Quartz, K-feldspar, biotite

Quartz, K-feldspar, muscovite

Plagioclase, K-feldspar, quartz, biotite

Magmatic

Detrital

sedimentary

Detrital

sedimentary

Magmatic rock

l.8—2.5

l.8—2.5

Figure 2. Photographs of early Precambrian rocks obtained from drill cores into the basement beneath the Central Hebei Basin, North China Craton. (a) Gneissic granodiorite (XG1-1); (b) anatectic biotite K-feldspar paragneiss (MG1); (c) muscovite-bearing K-feldspar quartzite (H5); (d) gneissic quartz diorite (J48-1). The coin is ~1.5 cm in diameter.

the present-day chondritic ratios of 176Hf/177Hf = 0.282772 and 176Lu/177Hf = 0.0332 (Blichert-Toft and Albarede, 1997). Two-stage Hf model ages (crustal model ages tDM2(CC)) were calculated by assuming a mean 176Lu/177Hf value of 0.01 (Kroner et al., 2014), rather than 0.015 (Griffin et al., 2000) for the average continental crust.

4. Zircon dating

4.1. Gneissic granodiorite (XG1-1)

The zircons are stubby, elliptical or round in shape and show complex textures in cathodoluminescence (CL) images (Fig. 3a—c). Magmatic zircons show oscillatory zoning and recrystallization to different degrees. The recrystallized domains can be subdivided into two types, inner dark and outer light domains. Some light domains show weak zoning, but others are homogeneous. It cannot be excluded that some light domains are overgrowth rims. It is evident that the rounded terminations are not the original shapes of the zircons but resulted from "metamorphic corrosion", a typical feature in medium to high-grade metamorphic rocks due to dissolution of the pyramidal terminations (Kroner et al., 2013). This indicates that the relationships between outer shape and inner texture cannot be used as indictors of zircon origins. Twenty-one analyses were taken on 18 zircons (Table 2). Three analyses on magmatic domains yielded U contents and Th/U ratios of 155—267 ppm and 0.58—0.86, two of these have a weighted mean 207Pb/206Pb age of2500 ± 15 Ma (MSWD = 0.004) (Fig. 4a), which is taken as the formation age of the granodiorite. Eleven analyses on recrystallized dark domains have U contents and Th/U ratios of 265—1015 ppm and 0.29—1.64 and show a variation in 207Pb/206Pb

age from 2386 to 2511 Ma, except for spot 12.1RC(d) which has a 207Pb/206Pb age of 2189 Ma because of strong lead loss. Four analyses plot on or near concordia with the youngest ages yielding a weighted mean 207Pb/206Pb age of 2412 ± 17 Ma (MSWD = 1.9), which is considered to record a tectono-thermal event at the early Paleoproterozoic. Seven analyses on recrystallized light domains have much lower U contents (13—21 ppm) and higher Th/U ratios (1.75—4.44) than the dark ones and show a similar age variation to the dark domains but with large errors due to low U contents. The variations in U contents and Th/U ratios resulted from redistribution of U and Th in the recrystallized parts of magmatic zircons. However, some recrystallized grains almost entirely became light domains, suggesting that U and Th were removed from the zircons with U easier to be removed than Th, probably under fluid conditions.

4.2. Anatectic biotite K-feldspar paragneiss (MG1)

The detrital zircons are stubby with rounded terminations and show complex textures in CL images (Fig. 3d—f). The cores show oscillatory zoning, but most show recrystallization. It is difficult to determine when the inner light domains underwent recrystalliza-tion (before or after deposition), but we treat them as cores. Sixteen analyses on the cores yielded U contents and Th/U ratios of 152—1574 ppm and 0.22—1.26, excepting 6.1D and 20.1D which have Th/U ratios of 3.14 and 6.41 (Table 2) probably due to analysis spots onTh-high mineral inclusions. They show strong lead loss but define a discordia line with an upper intercept age of w2.5 Ga (Fig. 4b). Analysis 2.2D is on an inherited core and has a 207Pb/206Pb age of 2634 Ma (Fig. 3e, grain 2). This suggests that the magmatic rocks in the source region at least partly contained older

Y. Wan et al. / Geoscience Frontiers xxx (2014) 1—14

50 u m RC(d)

f О"Л(2.480а) 8.1 RC(d) U Y ^2.40Ga)

' s !Ki

14.1 MA (2.50Ga)

8.2RC(I) (2.30Ga)

50 u m

18.1 RC(d) (2.51 Ga)

11.1 RC(I) ~(2.48Ga)

/ /9.1 MA

V ,/(2.49Ga)

9 9П (2 63Ga)

7 2D > /О

i.gecaTT^^ O) /

9.1 D > ' -MG1 Vïf^^eGa)

50 n m /

/ -10 "

о ? /

1 / 17.1 RC

/ > (1.61 Ga) 14.1D 11.1D

¿-17.2D (2.33Ga)

/ > (1.61 Ga) I V

14.1 D , 11.1 D (БШ!

(2.45Ga) /(2.39Ga) ; ^ ,

I.IRCJV^y

(1.59Ga)

50 um (2.47Ga) 16.1D

19.1 RC 50pm (1.64Ga)

4.1 RC V**^"

3.1 RC f

(1.66Ga)

1 96Ga) J\ ГГ\

; 10.1D

, v, (2.36Ga)

4 V7 f% 1

4.2DV MG1 (2.40Ga)

y 3.2D (2.43Ga)

Figure 3. Cathodoluminescence images of zircons from early Precambrian rocks obtained from drill cores into the basement beneath the Central Hebei Basin, North China Craton. (a)-(c) Gneissic granodiorite (XG1-1); (d)-(f) anatectic biotite K-feldspar paragneiss (MG1); (g)-(i) muscovite-bearing K-feldspar quartzite (H5); (j)-(l) gneissic quartz diorite (J48-1). MA, D, C, RC(d) and RC(l) represent magmatic, detrital and core zircons and recrystallized dark and light domains, respectively.

continental material. The outer dark domains look like overgrowth rims, but some show blurred zoning (Fig. 3d, grain 17; Fig. 3f, grain 19), indicating that they formed by recrystallization, probably under fluid conditions. Seven analyses on the dark domains have U

contents and Th/U ratios of 841—2268 ppm and 0.09—0.44 (Table 2). They show stronger lead loss than the cores, and no precise ages could be obtained. We speculate that recrystallization was related to a tectono-thermal event at the end of the

Y. Wan et al. / Geoscience Frontiers xxx (2014) 1—14

50 u m

1.1D (2.48Ga)

ШШ *

(2.61 Ga)

50 u m

9.1D (2.446 a)/ /

5.1D 4 (2.53Ga)

4.1D (2.51 Ga)

50 y m

8.1D (2.456a)

12.1D > (2.50Ga)

$À Ш

' 10.1D (2.52Ga)

50 y- m

20.2RC(d) к

(2.29Ga)13 1R+RC(|)

, (1.736a)

7.1 RC(I) (2.50Ga)

л омд ' 20.1 MA ; ■

4.2MA (2 46Ga) r—^(2.5C

(2.496a) gM^

\ ^^ a) 1ШЗ

J48-1 (2.226a)

50 y m

1.2MA (2.406a)

3.1 RC(I) (2.386a)

x 1 MA

11.1 RC(d)+R (1.656a) В % ,-jN.

II 2.1С —F p/(2.696a) i

X11.2C (2.646a)

—— _„лд 26.1 MA 50 У m 32.1 MA (2 476a)

/л К

26.2RC(I) -Л^ЙВ (2.266a)

^ 9 1 MA ;/fS "

X9.2RC(d) 16.1RC(dT^ J48-1 (2.276a) (2.306a)

Figure 3. (continued).

Paleoproterozoic because no tectono-thermal event <1.8 Ga has so far been reported in the central NCC. The outermost light "rims" are also considered to have formed as a result of recrystallization during which U and Th were redistributed between the light and dark domains.

4.3. Muscovite-bearing K-feldspar quartzite (H5)

Besides their shapes, the zircons show large variations in internal texture, such as oscillatory zoning (Fig. 3g, grain 1), banded zoning (Fig. 3h, grain 9), sector zoning (Fig. 3g, grain 3) or strong

Table 2

SHRIMP U-Pb data for zircons from early Precambrian rocks from drill cores beneath the Central Hebei Basin, North China Craton.

Spot 206Pbc U Th Th/U 206Pb* 207Pb*/206Pb* ±% 207Pb*/235U ±% 206Pb*/238U ±% Errcorr 206Pb/238U age 207Pb/206Pb age Discordant

(%) (ppm) (ppm) (ppm) (%)

Gneissic granodiorite (XG1-1)

1.1RC(d) 0.23 684 218 0.33 263 0.1632 0.62 10.05 1.8 0.447 1.7 0.94 2380 ±34 2489 ±11 4

1.2RC(l) — 20 67 3.43 9 0.1559 3.6 10.51 4.5 0.488 2.7 0.59 2563 ±56 2414 ±61 -6

2.1RC(l) 1.68 13 25 1.96 5 0.1469 6.5 8.60 7.1 0.422 3.0 0.43 2271 ±57 2320 ±110 2

3.1MA 0.01 155 87 0.58 65 0.1642 0.58 11.09 1.9 0.490 1.8 0.95 2569 ±38 2500 ±10 -3

3.2RC(d) 0.05 347 100 0.30 135 0.1649 0.39 10.25 1.8 0.451 1.7 0.98 2399 ±35 2507 ±7 4

4.1RC(l) — 21 89 4.44 9 0.1567 1.6 10.21 3.7 0.472 3.3 0.90 2493 ±68 2422 ±28 -3

5.RC(d) 0.06 1015 635 0.65 400 0.1620 0.23 10.25 1.7 0.459 1.7 0.99 2434 ±35 2477 ±4 2

6.1RC(d) — 690 1092 1.64 248 0.1536 0.27 8.85 1.7 0.418 1.7 0.99 2250 ±33 2386 ±5 6

7.1RC(l) 0.42 15 28 1.89 7 0.1712 2.2 12.16 4.5 0.515 3.9 0.87 2678 ±85 2570 ±37 -4

8.1RC(d) 0.13 409 246 0.62 160 0.1552 0.35 9.77 1.8 0.457 1.7 0.98 2424 ±35 2404 ±6 -1

8.2RC(l) — 21 78 3.76 9 0.1457 3.8 9.18 5.4 0.456 3.8 0.71 2424 ±77 2298 ±65 -5

9.1MA 0.16 180 127 0.73 74 0.1631 0.73 10.74 2.0 0.477 1.9 0.93 2516 ±39 2488 ±12 -1

10.1RC(d) 0.13 292 177 0.63 112 0.1578 0.72 9.67 1.9 0.444 1.8 0.93 2370 ±35 2432 ±12 3

11.1RC(l) — 19 65 3.52 9 0.1625 3.3 11.73 4.3 0.523 2.8 0.65 2711 ±62 2484 ±55 -9

12.1RC(d) — 934 560 0.62 195 0.1368 0.50 4.56 1.8 0.242 1.7 0.96 1395 ±21 2189 ±8 36

13.1RC(l) 1.07 16 27 1.75 7 0.1498 6.2 9.54 6.9 0.461 3.3 0.48 2443 ±65 2348 ±100 -4

14.1MA 0.24 267 221 0.86 110 0.1643 0.69 10.83 1.9 0.478 1.8 0.93 2519 ±37 2501 ±12 -1

15.1RC(d) — 336 215 0.66 128 0.1560 0.39 9.52 1.8 0.443 1.7 0.98 2363 ±34 2413 ±7 2

16.1RC(d) 0.01 265 92 0.36 120 0.1637 0.46 11.84 1.8 0.525 1.8 0.97 2720 ±39 2494 ±8 -9

17.1RC(d) — 334 93 0.29 131 0.1575 0.92 9.92 2.0 0.457 1.8 0.89 2425 ±36 2429 ±16 0

18.1RC(d) 0.07 324 125 0.40 136 0.1654 0.51 11.11 1.8 0.487 1.8 0.96 2560 ±38 2511 ±8 -2

Anatectic biotite K-feldspar gneiss (MG1)

1.1RC 0.40 2268 201 0.09 388 0.0983 0.84 2.69 1.7 0.198 1.5 0.87 1166 ±16 1592 ±16 27

2.1D 0.37 276 166 0.62 84 0.1611 0.85 7.82 1.8 0.352 1.6 0.88 1944 ±26 2467 ±14 21

2.2D 0.34 257 136 0.55 82 0.1779 0.78 9.04 1.8 0.369 1.6 0.90 2022 ±28 2634 ±13 23

3.1RC 0.56 1389 199 0.15 226 0.1020 0.59 2.65 1.6 0.189 1.5 0.93 1113 ±16 1660 ±11 33

3.2D 0.34 390 218 0.58 94 0.1577 0.71 6.04 1.8 0.278 1.7 0.92 1582 ±23 2430 ±12 35

4.1RC 0.47 841 236 0.29 162 0.1205 0.52 3.70 1.6 0.223 1.5 0.95 1297 ±18 1963 ±9 34

4.2D 0.22 547 412 0.78 148 0.1550 0.42 6.70 1.6 0.314 1.5 0.96 1759 ±24 2402 ±7 27

5.1D 0.43 217 142 0.68 67 0.1628 0.71 8.06 1.7 0.359 1.6 0.91 1979 ±27 2485 ±12 20

6.1D 0.41 194 590 3.14 65 0.1651 1.3 8.86 2.0 0.389 1.6 0.79 2119 ±29 2508 ±21 16

7.1RC 0.56 1314 301 0.24 240 0.1083 0.72 3.16 1.7 0.212 1.5 0.91 1237 ±17 1771 ±13 30

7.2D 0.68 1053 954 0.94 205 0.1200 0.75 3.72 1.7 0.225 1.5 0.90 1306 ±18 1956 ±13 33

8.1RC 0.69 1857 654 0.36 283 0.1016 0.77 2.47 1.7 0.176 1.5 0.89 1045 ±15 1654 ±14 37

9.1D 0.32 530 401 0.78 80 0.1605 0.75 3.88 1.8 0.175 1.6 0.91 1040 ±15 2461 ±13 58

10.1D 0.11 1574 341 0.22 537 0.1510 0.19 8.26 1.6 0.397 1.5 0.99 2155 ±28 2357 ±3 9

11.1D 0.35 440 383 0.90 81 0.1535 0.57 4.54 1.6 0.215 1.5 0.94 1253 ±17 2386 ±10 47

12.1D 0.34 262 220 0.87 73 0.1630 1.1 7.26 1.9 0.323 1.5 0.82 1805 ±24 2486 ±18 27

13.1D + RC 0.42 548 224 0.42 119 0.13945 0.55 4.82 1.6 0.251 1.5 0.94 1441 ±20 2220 ±10 35

14.1D 0.16 864 308 0.37 175 0.15910 0.51 5.15 1.6 0.235 1.5 0.95 1360 ±19 2446 ±9 44

15.1D + RC 0.43 383 329 0.89 109 0.1527 0.66 6.93 1.7 0.329 1.6 0.92 1834 ±25 2376 ±11 23

16.1D 0.61 152 186 1.26 42 0.1626 1.3 7.06 2.1 0.315 1.7 0.79 1766 ±26 2482 ±22 29

17.1RC 1.01 1596 244 0.16 267 0.09924 0.90 2.63 1.8 0.193 1.5 0.86 1135 ±16 1609 ±16 29

17.2D 0.26 509 306 0.62 116 0.1484 0.76 5.43 1.7 0.266 1.6 0.90 1518 ±21 2328 ±13 35

18.1D 0.80 736 342 0.48 121 0.1485 0.81 3.87 1.8 0.189 1.6 0.89 1117 ±16 2329 ±14 52

19.1RC 0.96 1558 663 0.44 275 0.1006 1.1 2.82 1.9 0.203 1.5 0.82 1193 ±16 1636 ±20 27

20.1D 0.62 637 3955 6.41 87 0.1410 1.1 3.08 1.9 0.158 1.5 0.81 948 ±14 2239 ±19 58

Muscovite-bearing K-feldspar quartzite (H5)

1.1D 0.75 296 103 0.36 63 0.1626 0.91 5.48 2.3 0.244 2.2 0.92 1409 ±27 2483 ±15 43

2.1D 1.47 518 320 0.64 47 0.1476 1.3 2.14 2.5 0.105 2.2 0.85 643 ±13 2319 ±23 72

3.1D 1.19 30 47 1.63 13 0.1751 2.8 11.70 3.8 0.485 2.7 0.69 2547 ±56 2607 ±46 2

4.1D 0.64 60 46 0.79 24 0.1652 1.5 10.64 2.8 0.467 2.4 0.84 2471 ±49 2509 ±26 2

5.1D 1.10 52 44 0.87 17 0.1668 1.5 8.52 2.9 0.370 2.4 0.84 2031 ±42 2526 ±26 20

6.1D 1.04 47 36 0.78 20 0.1641 1.8 11.07 3.1 0.489 2.5 0.80 2568 ±52 2498 ±31 -3

7.1D 0.75 304 439 1.49 58 0.1604 0.93 4.86 2.3 0.220 2.2 0.92 1280 ±25 2460 ±16 48

8.1D 2.48 508 377 0.77 79 0.1597 1.4 3.89 2.6 0.177 2.1 0.84 1050 ±21 2452 ±24 57

9.1D 1.44 274 200 0.75 35 0.1584 1.8 3.22 2.9 0.147 2.2 0.78 886 ±19 2438 ±30 64

10.1D 0.30 181 139 0.80 46 0.1658 1.5 6.68 2.7 0.292 2.3 0.83 1652 ±33 2516 ±26 34

11.1D 3.18 80 150 1.94 20 0.1541 2.9 5.96 3.8 0.280 2.4 0.63 1594 ±34 2392 ±50 33

12.1D 0.84 407 320 0.81 54 0.1603 10 3.38 2.4 0.153 2.1 0.91 917 ±18 2459 ±17 63

13.1D 1.18 58 65 1.16 16 0.1681 2.5 7.17 3.5 0.309 2.5 0.70 1736 ±37 2539 ±42 32

14.1D 0.36 176 138 0.81 42 0.1626 0.97 6.17 2.4 0.275 2.2 0.92 1567 ±31 2483 ±16 37

15.1D 0.90 149 234 1.62 47 0.1606 1.1 8.10 2.5 0.366 2.2 0.89 2010 ±38 2461 ±19 18

16.1D 2.99 767 590 0.80 105 0.1406 1.7 3.00 2.7 0.155 2.2 0.79 928 ±19 2235 ±29 58

17.1D 1.23 171 206 1.25 51 0.1554 1.6 7.36 2.8 0.344 2.2 0.81 1903 ±37 2406 ±28 21

18.1D 0.77 98 143 1.51 31 0.1612 1.5 8.15 2.9 0.367 2.4 0.85 2015 ±42 2468 ±25 18

(continued on next page)

Y. Wan et al. / Geoscience Frontiers xxx (2014) 1—14

Table 2 (continued )

Spot 206Pbc (%) U (ppm) Th (ppm) Th/U 206Pb* (ppm) 207Pb*/206Pb* ±% 207Pb*/235U ±% 206Pb*/238U ±% Errcorr 206Pb/238U age 207Pb/206Pb age Discordant (%)

Gneissic quartz diorite (J48-1)

1.1MA 0.11 495 179 0.36 110 0.1519 0.48 5.42 2.9 0.259 2.9 0.99 1484 ±38 2368 ±8 37

1.2MA 0.44 442 162 0.37 117 0.1546 1.1 6.51 2.1 0.305 1.8 0.86 1715 ±27 2400 ±18 29

2.1C 0.21 76 39 0.53 32 0.184 0.85 12.51 2.2 0.493 2.0 0.92 2583 ±42 2690 ±14 4

3.1RC(l) 3.62 26 1 0.03 5 0.1516 6.2 4.27 6.5 0.203 2.7 0.41 1190 ±29 2376 ±100 50

4.1Rc(d) - 314 85 0.28 49 0.1540 0.82 3.83 1.9 0.180 1.8 0.91 1067 ±17 2392 ±14 55

4.2MA 0.19 285 132 0.48 101 0.1630 0.57 9.27 1.8 0.412 1.8 0.95 2226 ±33 2487 ±10 11

5.1RC(d) 1.51 687 170 0.26 64 0.1208 2.3 1.79 2.9 0.107 1.7 0.61 655 ±11 1973 ±41 67

6.1Rc(l) + R 0.98 418 19 0.05 35 0.1177 2.6 1.55 3.1 0.095 1.8 0.58 585 ±10 1927 ±45 70

7.1Rc(l) 0.35 46 1 0.02 16 0.1637 1.3 8.91 2.5 0.394 2.1 0.84 2143 ±38 2495 ±22 14

7.2Rc(d) 0.48 646 336 0.54 80 0.1392 1.2 2.75 2.1 0.143 1.7 0.83 861 ±14 2220 ±20 61

8.1C 0.07 197 87 0.46 83 0.1718 0.48 11.55 1.9 0.488 1.8 0.97 2560 ±38 2576 ±8 1

9.1MA 0.75 243 139 0.59 81 0.1632 0.48 8.69 1.8 0.386 1.8 0.97 2104 ±31 2489 ±8 15

9.2RC(d) 2.54 457 40 0.09 39 0.1430 1.5 1.92 2.5 0.097 2.0 0.79 597 ±11 2267 ±26 74

10.1RC(l) 1.32 213 15 0.07 30 0.1353 1.8 3.00 2.5 0.160 1.8 0.71 958 ±16 2172 ±31 56

11.1RC(d) + R 2.50 464 16 0.04 15 0.1006 5.5 0.51 5.7 0.037 1.9 0.33 232 ±4 1646 ±100 86

11.2C 0.04 118 72 0.63 53 0.1787 0.69 12.97 2.0 0.526 1.9 0.94 2725 ±41 2642 ±11 -3

12.1RC(l) 0.84 94 3 0.03 21 0.1540 2.4 5.34 3.1 0.251 1.9 0.64 1444 ±25 2396 ±40 40

12.2MA - 347 314 0.93 86 0.1658 0.68 6.58 1.9 0.288 1.8 0.93 1631 ±25 2516 ±11 35

13.1R + RC(l) - 1106 328 0.31 31 0.1054 1.7 0.47 2.4 0.032 1.7 0.72 206 ±4 1726 ±31 88

14.1MA 0.08 224 106 0.49 85 0.1646 0.46 9.99 1.8 0.440 1.8 0.97 2352 ±35 2503 ±8 6

15.1MA 0.55 153 131 0.89 55 0.1625 0.65 9.38 2.0 0.419 1.9 0.95 2255 ±36 2482 ±11 9

16.1RC(d) 1.37 213 66 0.32 35 0.1454 1.2 3.82 2.1 0.190 1.8 0.84 1122 ±18 2296 ±20 51

17.1MA 0.98 368 169 0.47 76 0.1553 0.58 5.11 1.9 0.238 1.8 0.95 1379 ±22 2405 ±10 43

18.1RC(d) 0.93 296 164 0.57 87 0.1626 0.49 7.62 1.8 0.340 1.7 0.96 1886 ±29 2484 ±8 24

19.1RC(l) 0.66 40 1 0.02 15 0.1669 1.8 9.83 3.4 0.427 2.9 0.86 2290 ±56 2529 ±29 9

20.1MA - 263 167 0.66 75 0.1604 0.61 7.27 2.0 0.329 1.9 0.95 1831 ±31 2460 ±10 26

20.2RC(d) 0.74 386 41 0.11 36 0.1448 1.4 2.14 2.4 0.107 1.9 0.81 654 ±12 2288 ±24 71

21.1RC(d) 1.30 337 46 0.14 46 0.1447 0.90 3.14 2.0 0.157 1.8 0.89 940 ±16 2285 ±15 59

22.1RC(d) 0.87 465 9 0.02 48 0.1314 1.4 2.17 2.3 0.119 1.8 0.79 727 ±12 2120 ±24 66

23.1RC(d) 0.12 311 154 0.51 38 0.1361 1.7 2.64 2.8 0.141 2.2 0.79 848 ±17 2182 ±30 61

24.1RC(d) - 279 20 0.07 40 0.1482 1.2 3.36 2.2 0.164 1.8 0.83 980 ±16 2327 ±21 58

25.1RC(d) 0.37 740 149 0.21 29 0.1108 2.3 0.68 2.9 0.045 1.8 0.61 281 ±5 1819 ±41 85

26.1MA - 198 142 0.74 76 0.1617 0.63 9.88 1.9 0.443 1.8 0.94 2363 ±36 2474 ±11 4

26.2RC(l) - 43 10 0.24 13 0.1423 4.0 6.78 4.6 0.345 2.5 0.53 1909 ±41 2261 ±68 16

27.1RC(l) 0.64 104 33 0.33 23 0.1547 1.5 5.47 3.2 0.256 2.8 0.88 1469 ±37 2400 ±25 39

28.1RC(l) 1.75 193 52 0.28 18 0.1268 3.5 1.91 3.9 0.109 2.0 0.50 665 ±12 2060 ±59 68

29.1MA 1.05 271 121 0.46 69 0.1595 0.98 6.49 2.0 0.295 1.8 0.88 1667 ±26 2451 ±16 32

30.1RC(d) - 392 338 0.89 94 0.1568 0.70 6.00 1.9 0.278 1.8 0.94 1579 ±26 2422 ±12 35

31.1MA 0.06 520 331 0.66 88 0.1402 0.84 3.78 1.9 0.195 1.7 0.90 1150 ±18 2232 ±14 48

32.1MA - 397 307 0.80 77 0.1529 0.81 4.75 1.9 0.225 1.8 0.91 1309 ±21 2381 ±14 45

Note: (1) Common lead corrected using measured 204Pb; (2) 206Pb* is radiogenic lead; (3) Discordance(%) is defined as [(1-(206Pb/23sU)age)/(207Pb/206Pb)agej x 100; (4) age in Ma; (5) MA, D, C, RC(d) and RC(l) represent magmatic, detrital and core zircons and recrystallized dark and light domains, respectively.

recrystallization (Fig. 3h, grain 5; Fig. 3i, grains 8 and 12). Eighteen analyses yielded U contents and Th/U ratios of 30—767 ppm and 0.36—1.94 (Table 2) and exhibit lead loss to different degrees but plot roughly on a discordia line with 3 analyses on concordia (Fig. 4c). It is evident that the source region(s) are mainly composed of ~2.5 Ga rocks. Some zircons have metamorphic rims, but these are too narrow for SHRIMP analysis (Fig. 3h, grain 5; Fig. 3i, grain 12). We consider that metamorphism occurred at the end of the Paleoproterozoic for the same reason as mentioned for sample MG1.

4.4. Gneissic quartz diorite (J48-1)

The magmatic zircons are stubby in shape and show oscillatory zoning but with most undergoing recrystallization (Fig. 3j—l). Two recrystallized domains can be subdivided. One is homogeneous and light gray in CL images (Fig. 3j, grain 3; Fig. 3k, grain 7), another is inhomogeneous with light and dark intergrowths that commonly occur in the outermost domains of zircons (Fig. 3j, grains 2 and 11 ; Fig. 3l, grain 9). Forty-nine analyses were carried out on 32 zircons (Table 2). Thirteen analyses on magmatic domains have U contents of 153—520 ppm and Th/U ratios of 0.36—0.93. They show strong lead loss but define a discordant line with 7 analyses closest to concordia yielding an upper intercept age of 2496 ± 14 Ma

(MSWD = 1.7) (Fig. 4d) that is interpreted as the intrusive age of the quartz diorite. Thirteen analyses on inhomogeneous, dark domains have U contents of 279—740 ppm and Th/U ratios of 0.02—0.89. Eight analyses on the homogeneous, light domains have U contents of 26—213 ppm and Th/U ratios of 0.02—0.33, being lower than the magmatic domains. This is different from the zircons of sample XG1-1 in which the recrystallized, light domains have much higher Th/U ratios than magmatic domains, probably indicating that their recrystallization occurred under different conditions (mainly fluid composition, redox condition, pH value and temperature). The analyses on recrystallized domains plot on the same discordia line as the analyses on magmatic domains. Therefore, we consider that the rock underwent metamorphism soon after its formation. There are also trapped 2576—2690 Ma zircon cores (Fig. 3j, grains 2 and 11; Fig. 4d).

5. Geochemistry and Hf-in-zircon isotopes

5.1. Whole-rock compositions

The chemical analyses of the dated rock samples are listed in Table 3 and shown in Fig. 5. The gneissic granodiorite (XG1-1) has SiO2, MgO + FeOT, Na2O and K2O contents of 66.54, 2.54, 3.82 and 2.79 wt.%; it has SREE contents of 95.9 ppm and shows a large (La/

Y. Wan et al. / Geoscience Frontiers xxx (2014) 1—14

Figure 4. Concordia diagrams showing U-Pb data for zircons from early Precambrian rocks obtained from drill cores beneath the Central Hebei Basin, North China Craton. (a) Gneissic granodiorite (XG1-1); (b) anatectic biotite K-feldspar paragneiss (MG1); (c) muscovite-bearing K-feldspar quartzite (H5); (d) gneissic quartz diorite (J48-1). D, C and RC(d) represent detrital and core zircons and recrystallized dark domains, respectively.

Yb)n ratio (46.1) and positive Eu anomaly (2.35). Compared with the gneissic granodiorite, the gneissic quartz diorite (J48-1) is lower in SiO2 (61.56 wt.%) and much higher in MgO + FeOT (8.23 wt.%), and has higher SREE (140.4 ppm) and lower Eu/Eu* (0.84) and (La/Yb)n (11.5) values. The two metasedimentary samples show large variations in SiO2 such as 63.06 wt.% (MG1) and 79.77% (H5), respectively, but both are similar in having very high K2O (6.22—8.72 wt.%) and very low Na2O contents (0.14—0.30 wt.%). The anatectic biotite K-feldspar paragneiss (MG1) has high MgO + FeOT contents (7.50 wt.%), consistent with its high biotite content. Both samples have SREE contents of 48.1—107.6 ppm, Eu/Eu* of 0.84—0.89 and (La/Yb) n of 12.0—29.5. All dated samples are enriched in large ion lithophile (LIL) elements (K, Rb, Ba, Th) (except for sample XG1-1 that has a low Th content) and is depleted in Nb, P and Ti.

5.2. Hf-in-zircon isotopes

In-situ Hf-in-zircon isotopic analyses were obtained from all the dated samples. Analyses were taken on the SHRIMP spots, and we used the 207Pb/206Pb ages obtained for the dated domains to calculate eHf(t) values. The results are listed in Table 4 and shown in Fig. 6.

Three analyses on magmatic zircons from gneissic granodiorite sample XG1-1 have eHf(t) values of 1.21—2.89 and crustal Hf model ages tDM2(CC) of 2.8—2.9 Ga. Eleven analyses on magmatic zircons from gneissic quartz diorite sample J48-1 have eHf(t) values and

tDM2(CC) ages ranging from -1.57 to 4.47 and 2.6 to 2.9 Ga, respectively. The xenocrystic zircons have tDM2(CC) ages of 2.9—3.0 Ga (spots 2-1C, 8-1C). For the two magmatic rocks, the recrystallized zircon domains show similar Hf isotopic compositions to the magmatic zircons. This suggests that the Lu-Hf isotopic system was not disturbed during metamorphism. Detrital zircons from the two metasedimentary samples have eHf(t) values and tDM2(CC) ages ranging from -3.34 to 5.74 and 2.7 to 3.0 Ga, respectively, but those from sample H5 show stronger depletion in Hf isotopes. The somewhat large eHf(t) variations in these zircons are mainly due to 207Pb/206Pb age variations because of lead loss. Given an age of 2.5 Ga to calculate eHf(t), the values are mainly between 0 and 5, with most zircons having tDM2(CC) ages of 2.7—2.9 Ga.

6. Discussion

We identified ~2.5 Ga old granodiorite and quartz diorite although their spatial distribution is uncertain. The granodiorite (XG1-1) has a relatively high K2O content (2.79 wt.%) and low MgO + FeOT (2.54 wt.%) and Cr (19 ppm) contents and shows a positive Eu*/Eu anomaly (2.35). Combined with the magmatic zircons having low eHf(t) values (1.2—2.9), the rock is considered to be derived from, or at least partly influenced by, continental material. Its strongly fractionated REE pattern dose not mean that it formed by partial melting of a basaltic source under high pressure conditions but reflects the compositional feature of a continental source

Y. Wan et al. / Geoscience Frontiers xxx (2014) 1 —14

Table 3

Chemical compositions of early Precambrian rocks from drill cores beneath the Central Hebei Basin, North China Craton.

Sample No. XG1-1 MG1 H5 J48-1

Rock type Gneissic Anatectic biotite Muscovite-bearing Gneissic

granodiorite K-feldspar gneiss K-feldspar quartz schist quartz diorite

SiO2 66.54 63.06 79.77 61.56

TiO2 0.21 0.41 0.12 0.48

Al2O3 14.59 14.26 9.63 15.23

Fe2O3 2.13 1.40 1.05 1.64

FeO 1.74 3.79 0.52 4.58

MnO 0.08 0.04 0.05 0.11

MgO 0.80 2.45 0.70 2.17

CaO 4.84 1.25 0.13 2.48

Na2O 3.82 0.30 0.14 3.40

K2O 2.79 8.72 6.22 2.96

P2O5 0.10 0.14 0.05 0.16

H2O+ 1.00 2.68 0.84 2.64

CO2 0.70 1.25 0.08 2.50

Total 99.3 99.8 99.3 99.9

Cr 19 209 17 100

Ni 7 41 11 35

Sc 4 12 3 15

Rb 37 114 182 105

Ba 1040 2391 729 589

Sr 1413 157 86 271

Zr 76 119 92 137

Nb 1.6 5.3 4.2 6.9

Ta 0.1 0.4 0.4 0.6

Hf 1.8 3.1 2.4 3.8

Y 4.7 4.9 4.1 17.8

Th 0.1 5.6 4.1 8.2

U <0.05 0.7 1.7 2.5

La 28.00 24.60 9.26 30.80

Ce 42.70 49.40 18.80 59.70

Pr 4.11 5.51 3.02 6.92

Nd 13.90 19.50 11.30 25.20

Sm 1.96 3.00 1.98 4.41

Eu 1.39 0.76 0.45 1.12

Gd 1.59 2.05 1.16 3.67

Tb 0.18 0.23 0.16 0.56

Dy 0.89 1.05 0.73 3.16

Ho 0.18 0.19 0.14 0.63

Er 0.45 0.61 0.45 1.90

Tm 0.06 0.08 0.06 0.26

Yb 0.40 0.55 0.51 1.76

Lu 0.07 0.09 0.08 0.26

SREE 95.9 107.6 48.1 140.4

(La/Yb)n 46.1 29.5 12.0 11.5

Eu/Eu* 2.35 0.89 0.84 0.84

Note: major elements in %, trace elements in ppm.

region. The quartz diorite (J48-1) has high MgO + FeOT (8.23 wt.%) and Cr (100 ppm) contents with a weakly fractionated REE but high SREE pattern. It seems to have formed as a result of crystallization-differentiation of more mafic magma, possibly indicating a juvenile

addition of crust from a mantle source. On the other hand, the rock contains 2.6—2.7 Ga xenocrystic zircon cores, and the magmatic zircons have low eHf(t) values, not only suggesting that early Neo-archean rocks occur in the basement but also that these were involved in ~2.5 Ga magmatism. The ~2.5 Ga granodiorite and quartz diorite also occur in other areas of the NCC such as western Shandong and eastern Hebei, where ~2.5 Ga tonalite, trondhje-mite, monzogranite and syenogranite are widely distributed (Geng et al., 2006; Yang et al., 2008; Wan et al., 2010,2011b; Nutman et al., 2011). We speculate that these rock types may also occur in the basement beneath the CHB.

Most detrital zircons from the two metasedimentary samples have ages of ~2.5 Ga. This suggests that the source region is almost entirely composed of late Neoarchean rocks. However, detrital zircons from the muscovite-bearing K-feldspar quartzite (H5) show large variations in internal texture, and this may suggest a certain diversity of rock types and metamorphic grade in the source region. Many detrital zircons show strong lead loss due to high U and Th contents and have eHf (2.5 Ga) values ranging from 0 to 5. We suggest that the detrital material was derived from a source region that is mainly composed of granitoids derived from crustal reworking of late Mesoarchean to early Neoarchean continental material such as the Archean basement around the basin. In this study, no detrital zircons younger than ~2.5 Ga have been identified, and the metamorphic zircons are ~ 1.8 Ga in age. Therefore, we are able to constrain the time of deposition for the sedimentary rocks between ~ 1.8 and 2.5 Ga. More work is required to determine whether the sedimentary rocks were deposited during the early to middle Paleoproterozoic such as the Daqingshan supra-crustal rocks in the Daqingshan area (Wan et al., 2009; Dong et al., 2014) and the Shangtaihua Group in the Lushan area (Wan et al., 2006a), or during the late Paleoproterozoic such the khondalites which are widely distributed in the northern NCC (Wan et al., 2006b; Santosh et al., 2009; Zhao et al., 2010; Dong et al., 2012).

All dated rocks of this study underwent metamorphism and deformation. The gneissic quartz diorite sample (J48-1) recorded a metamorphic age of ~2.5 Ga. The recrystallized dark domains of zircons from the gneissic granodiorite sample (XG1-1) have 207Pb/206Pb ages ranging from 2.4 to 2.5 Ga. The metamorphic age of ~2.4 Ga recorded in sample XG1-1 has also been identified elsewhere in the NCC, but its geological meaning is still uncertain (Dong et al., 2014, references therein). No precise ages for meta-morphic zircons have been obtained for the metasedimentary rocks (MG1, H5) that were deposited during the Paleoproterozoic. However, they underwent a tectono-thermal event at the end of the Paleoproterozoic, as discussed above.

We summarized the common features of the NCC basement in the introduction. It is evident that the basement beneath the CHB is similar in many aspects to the basement elsewhere in the NCC.

Table 4

LA-ICPMS Hf isotope data for zircons from early Precambrian rocks from drill cores beneath the Central Hebei Basin, North China Craton.

No. Age Discordant (%) 176Yb/177Hf 176Lu/177Hf 176Hf/177Hf ±2s £Hf(0) ^Hf(t) ±2s tDM1 ±2s tDM2(CC) ±2s fLu/Hf

Gneissic granodiorite (XG1-1)

1-1RC(d) 2489 4 0.035412 0.000776 0.281237 0.000015 -54.29 0.25 0.52 2792 40 2890 64 -0.98

3-1MA 2500 -3 0.043179 0.000941 0.281265 0.000019 -53.30 1.21 0.68 2766 52 2850 83 -0.97

4-1RC(l) 2422 -3 0.029061 0.000608 0.281287 0.000017 -52.53 0.79 0.60 2713 45 2808 73 -0.98

6-1RC(d) 2386 6 0.050176 0.001087 0.281321 0.000023 -51.30 0.44 0.80 2699 62 2797 98 -0.97

7-1RC(l) 2570 -4 0.012013 0.000235 0.281249 0.000018 -53.85 3.47 0.63 2737 47 2794 77 -0.99

8-1RC(d) 2404 -1 0.039241 0.000847 0.281276 0.000023 -52.91 -0.38 0.83 2744 64 2853 102 -0.97

8-2RC(l) 2298 -5 0.016614 0.000320 0.281249 0.000018 -53.86 -2.90 0.62 2743 47 2893 76 -0.99

9-1MA 2488 -1 0.041452 0.000842 0.281315 0.000022 -51.54 2.89 0.77 2691 58 2756 93 -0.97

10-1RC(d) 2432 3 0.061930 0.001216 0.281315 0.000022 -51.51 1.04 0.78 2717 60 2804 95 -0.96

11-1RC(l) 2484 -9 0.016299 0.000317 0.281305 0.000021 -51.87 3.35 0.73 2668 55 2730 89 -0.99

12-1RC(d) 2189 36 0.051758 0.001217 0.281365 0.000023 -49.76 -2.61 0.81 2649 63 2790 99 -0.96

14-1MA 2501 -1 0.042799 0.000887 0.281274 0.000021 -52.97 1.66 0.74 2749 56 2828 90 -0.97

15-1RC(d) 2413 2 0.062073 0.001248 0.281303 0.000022 -51.94 0.13 0.79 2736 61 2834 97 -0.96

Anatectic biotite K-feldspar gneiss (MG1)

2-2D 2634 23 0.087679 0.001479 0.281193 0.000024 -55.84 0.71 0.85 2904 66 2984 103 -0.96

4-2D 2402 27 0.112868 0.002080 0.281312 0.000027 -51.63 -1.16 0.97 2784 77 2890 119 -0.94

5-1D 2485 20 0.048654 0.000892 0.281352 0.000024 -50.23 4.05 0.84 2645 64 2695 103 -0.97

6-1D 2508 16 0.057438 0.001074 0.281235 0.000025 -54.34 0.12 0.91 2816 69 2911 110 -0.97

9-1D 2461 58 0.170546 0.002988 0.281365 0.000031 -49.74 0.49 1.12 2777 90 2855 136 -0.91

11-1D 2386 47 0.145312 0.002741 0.281352 0.000029 -50.22 -1.16 1.04 2778 84 2877 127 -0.92

14-1D 2446 44 0.152903 0.003835 0.281306 0.000029 -51.85 -3.34 1.05 2931 86 3035 127 -0.88

16-1D 2482 29 0.071434 0.002006 0.281291 0.000022 -52.38 -0.05 0.78 2808 62 2899 95 -0.94

18-1D 2329 52 0.125614 0.002430 0.281366 0.000023 -49.73 -1.38 0.83 2735 66 2842 101 -0.93

19-1RC 1636 27 0.073282 0.002219 0.281364 0.000030 -49.80 -15.87 1.08 2722 85 3016 131 -0.93

Muscovite-bearing K-feldspar quartzite (H5)

1-1D 2483 43 0.033560 0.000721 0.281306 0.000018 -51.86 2.66 0.63 2695 48 2764 76 -0.98

2-1D 2319 72 0.071698 0.001333 0.281386 0.000024 -49.03 0.84 0.85 2628 66 2722 104 -0.96

3-1D 2607 2 0.035969 0.000694 0.281312 0.000025 -51.63 5.74 0.91 2685 69 2710 110 -0.98

4-1D 2509 2 0.027983 0.000550 0.281284 0.000022 -52.63 2.76 0.77 2713 58 2780 94 -0.98

5-1D 2526 20 0.013242 0.000268 0.281319 0.000023 -51.39 4.87 0.81 2646 61 2687 99 -0.99

6-1D 2498 -3 0.015315 0.000322 0.281280 0.000022 -52.75 2.77 0.79 2701 59 2770 96 -0.99

7-1D 2460 48 0.040991 0.000769 0.281334 0.000020 -50.86 3.05 0.70 2660 53 2725 86 -0.98

8-1D 2452 57 0.109854 0.001921 0.281359 0.000029 -49.99 1.84 1.03 2708 81 2780 126 -0.94

9-1D 2438 64 0.061044 0.001120 0.281344 0.000027 -50.49 2.36 0.96 2670 74 2742 117 -0.97

10-1D 2516 34 0.037189 0.000695 0.281325 0.000023 -51.18 4.13 0.83 2667 63 2716 101 -0.98

12-1D 2459 63 0.093736 0.001735 0.281404 0.000026 -48.39 3.91 0.93 2632 73 2681 114 -0.95

13-1D 2539 32 0.030138 0.000587 0.281316 0.000022 -51.51 4.51 0.77 2672 58 2716 94 -0.98

Gneissic quartz diorite (J48-1)

1-1MA 2368 37 0.024617 0.000567 0.281253 0.000011 -53.73 -1.57 0.39 2756 29 2883 47 -0.98

2-1C 2690 4 0.074362 0.001388 0.281244 0.000021 -54.03 3.94 0.73 2827 56 2868 89 -0.96

4-2MA 2487 11 0.029414 0.000686 0.281290 0.000016 -52.39 2.27 0.56 2713 43 2786 68 -0.98

5-1RC(d) 1973 67 0.057452 0.001372 0.281331 0.000017 -50.94 -8.75 0.62 2706 48 2926 75 -0.96

7-2RC(d) 2220 61 0.054240 0.001252 0.281341 0.000017 -50.60 -2.80 0.60 2684 46 2825 73 -0.96

8-1C 2576 1 0.034594 0.000669 0.281160 0.000019 -57.01 -0.33 0.69 2888 52 2989 84 -0.98

9-1MA 2489 15 0.108535 0.001966 0.281302 0.000026 -51.97 0.57 0.91 2789 72 2873 111 -0.94

11-1RC(d) + R 1646 86 0.088098 0.001690 0.281342 0.000021 -50.55 -15.84 0.74 2713 58 3022 90 -0.95

12-2MA 2516 35 0.036388 0.000805 0.281287 0.000016 -52.51 2.60 0.59 2726 45 2793 71 -0.98

14-1MA 2503 6 0.066984 0.001304 0.281348 0.000021 -50.34 3.64 0.75 2678 58 2730 91 -0.96

15-1MA 2482 9 0.030570 0.000740 0.281283 0.000016 -52.66 1.79 0.58 2727 44 2806 71 -0.98

17-1MA 2405 43 0.069090 0.001329 0.281344 0.000027 -50.50 1.27 0.95 2685 74 2770 116 -0.96

18-1RC(d) 2484 24 0.041602 0.000905 0.281226 0.000022 -54.66 -0.45 0.78 2815 59 2920 95 -0.97

20-1MA 2460 26 0.041136 0.000906 0.281310 0.000023 -51.69 1.99 0.83 2702 63 2778 101 -0.97

26-1MA 2474 4 0.030802 0.000717 0.281265 0.000019 -53.29 1.02 0.67 2750 51 2839 81 -0.98

29-1MA 2451 32 0.062967 0.001218 0.281400 0.000017 -48.51 4.47 0.62 2601 48 2646 75 -0.96

30-1RC(d) 2422 35 0.050939 0.001223 0.281324 0.000019 -51.21 1.11 0.66 2705 51 2792 80 -0.96

32-1MA 2381 45 0.054541 0.001075 0.281338 0.000023 -50.70 0.95 0.80 2675 62 2767 98 -0.97

1-1MA 2368 37 0.024617 0.000567 0.281253 0.000011 -53.73 -1.57 0.39 2756 29 2883 47 -0.98

2-1C 2690 4 0.074362 0.001388 0.281244 0.000021 -54.03 3.94 0.73 2827 56 2868 89 -0.96

4-2MA 2487 11 0.029414 0.000686 0.281290 0.000016 -52.39 2.27 0.56 2713 43 2786 68 -0.98

5-1RC(d) 1973 67 0.057452 0.001372 0.281331 0.000017 -50.94 -8.75 0.62 2706 48 2926 75 -0.96

7-2RC(d) 2220 61 0.054240 0.001252 0.281341 0.000017 -50.60 -2.80 0.60 2684 46 2825 73 -0.96

8-1C 2576 1 0.034594 0.000669 0.281160 0.000019 -57.01 -0.33 0.69 2888 52 2989 84 -0.98

9-1MA 2489 15 0.108535 0.001966 0.281302 0.000026 -51.97 0.57 0.91 2789 72 2873 111 -0.94

11-1RC(d) + R 1646 86 0.088098 0.001690 0.281342 0.000021 -50.55 -15.84 0.74 2713 58 3022 90 -0.95

12-2MA 2516 35 0.036388 0.000805 0.281287 0.000016 -52.51 2.60 0.59 2726 45 2793 71 -0.98

(continued on next page)

Table 4 (continued )

No. Age Discordant (%) 176Yb/177Hf 176Lu/177Hf 176Hf/177Hf ±2s £Hf(0) *Hf(t) ±2s tDM1 ±2s tDM2(CC) ±2s fLu/Hf

14-1MA 2503 6 0.066984 0.001304 0.281348 0.000021 -50.34 3.64 0.75 2678 58 2730 91 —0.96

15-1MA 2482 9 0.030570 0.000740 0.281283 0.000016 -52.66 1.79 0.58 2727 44 2806 71 -0.98

17-1MA 2405 43 0.069090 0.001329 0.281344 0.000027 -50.50 1.27 0.95 2685 74 2770 116 -0.96

18-1RC(d) 2484 24 0.041602 0.000905 0.281226 0.000022 -54.66 -0.45 0.78 2815 59 2920 95 -0.97

20-1MA 2460 26 0.041136 0.000906 0.281310 0.000023 -51.69 1.99 0.83 2702 63 2778 101 -0.97

26-1MA 2474 4 0.030802 0.000717 0.281265 0.000019 -53.29 1.02 0.67 2750 51 2839 81 -0.98

29-1MA 2451 32 0.062967 0.001218 0.281400 0.000017 -48.51 4.47 0.62 2601 48 2646 75 -0.96

30-1RC(d) 2422 35 0.050939 0.001223 0.281324 0.000019 -51.21 1.11 0.66 2705 51 2792 80 -0.96

32-1MA 2381 45 0.054541 0.001075 0.281338 0.000023 -50.70 0.95 0.80 2675 62 2767 98 -0.97

Note: (1) In-situ Hf isotopic analyses were carried out on the same spots as zircon dating; (2) age in Ma; (3) MA, D, C, RC(d) and RC(l) represent magmatic, detrital and core zircons and recrystallized dark and light domains, respectively.

Figure 6. Age versus eHf(t) diagrams for zircons from early Precambrian rocks obtained from drill cores beneath the Central Hebei Basin, North China Craton. (a) Gneissic granodiorite (XG1-1); (b) anatectic biotite K-feldspar paragneiss (MG1); (c) muscovite-bearing K-feldspar quartzite (H5); (d) gneissic quartz diorite (J48-1). MA, D, C, RC(d) and RC(l) represent magmatic, detrital and core zircons and recrystallized dark and light domains, respectively.

7. Conclusions

1) The basement beneath the CHB contains ~2.5 Ga magmatic and Paleoproterozoic metasedimentary rocks.

2) Zircons of different origins yielded eHf (2.5 Ga) values and Hf crustal modal ages ranging from 0 to 5 and 2.7 to 2.9 Ga, respectively.

3) The analyzed samples recorded two tectono-thermal events during the late Neoarchean to early Paleoproterozoic and at the end of the Paleoproterozoic, respectively.

4) The early Precambrian basement beneath the CHB is similar in many aspects to that elsewhere in the NCC.

Acknowledgments

This paper expresses our appreciation to the distinguished geologists Guowei Zhang and Alfred Kroner who have made significant contributions to the geology of China. We thank Jianhui Liu for help with SHRIMP data collection, Chun Yang and Weilin Gan for making the zircon mounts, and Liqing Zhou and Ning Li for CL imaging. We thank two anonymous reviewers for their valuable comments. This project was financially supported by the Major State Basic Research Program of the People's Republic of China (Grant No. 2012CB416600), the National Natural Science Foundation of China (Grant No. 40672127) and the Key Program of the

Ministry of Land and Resources of China (Grant Nos. 1212010811033,12120113013700).

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