Scholarly article on topic 'U–Th–Pb behavior of zircons under high-grade metamorphic conditions: A case study of zircon dating of meta-diorite near Qixia, eastern Shandong'

U–Th–Pb behavior of zircons under high-grade metamorphic conditions: A case study of zircon dating of meta-diorite near Qixia, eastern Shandong Academic research paper on "Earth and related environmental sciences"

CC BY-NC-ND
0
0
Share paper
Academic journal
Geoscience Frontiers
Keywords
{Zircons / "U–Th–Pb system" / Reset / "High-grade metamorphism" / "SHRIMP dating" / "Early Precambrian"}

Abstract of research paper on Earth and related environmental sciences, author of scientific article — Yusheng Wan, Dunyi Liu, Chunyan Dong, Shoujie Liu, Shijin Wang, et al.

Abstract This paper carried out a study on U–Th–Pb behavior of zircons in a “dry” rock system during high-grade metamorphism in the Archean basement of eastern Shandong. The studied sample has a mineral assemblage of plagioclase + K-feldspar + clinopyroxene + biotite + quartz and its protolith is considered to be diorite. The zircons are stubby, equant or irregular in shape and show fir-leaf, sectorial, banded or oscillatory zoning. They contain inclusions, including mineral assemblages of clinopyroxene + orthopyroxene + hornblende + quartz and plagioclase + K-feldspar + biotite + quartz. Fifty SHRIMP analyses were performed on 34 zircon grains, which commonly yielded high Th/U ratios (mostly >0.5). Most analyses are distributed along concordia from 2.54 to 2.25 Ga, with the youngest age being ∼1.95 Ga. Compositions and ages show large variations even in a same zircon grain. Combined with early studies, conclusions can be drawn as follows: 1) the diorite underwent two episodes of high-grade metamorphism, at the end of the Neoarchean and the Paleoproterozoic (∼2.50 Ga and ∼1.95 Ga or slightly later); 2) high-grade metamorphism in a “dry” rock system may partially reset the U–Th–Pb system of zircons and, in this case, the ages between the oldest and youngest are chronologically meaningless; and 3) high Th/U ratios may be common features of zircons formed during high-grade metamorphic conditions.

Academic research paper on topic "U–Th–Pb behavior of zircons under high-grade metamorphic conditions: A case study of zircon dating of meta-diorite near Qixia, eastern Shandong"

GEOSCIENCE FRONTIERS 2(2) (2011) 137-146

available at www.sciencedirect.com China University of Geosciences (Beijing)

GEOSCIENCE FRONTIERS

journal homepage: www.elsevier.com/locate/gsf

ORIGINAL ARTICLE

U—Th—Pb behavior of zircons under high-grade metamorphic conditions: A case study of zircon dating of meta-diorite near Qixia, eastern Shandong

Yusheng Wan a,*? Dunyi Liu a, Chunyan Dong a, Shoujie Liu a, Shijin Wang Enxiu Yang b

aBeijing SHRIMP Center, Institute of Geology, Chinese Academy of Geological Sciences, Beijing 100037, China Shandong Geological Survey, Jinan 250013, China

Received 22 December 2010; accepted 22 February 2011 Available online 3 April 2011

KEYWORDS

Zircons;

U—Th—Pb system; Reset; High-grade metamorphism; SHRIMP dating; Early Precambrian

Abstract This paper carried out a study on U—Th—Pb behavior of zircons in a "dry" rock system during high-grade metamorphism in the Archean basement of eastern Shandong. The studied sample has a mineral assemblage of plagioclase + K-feldspar + clinopyroxene + biotite + quartz and its pro-tolith is considered to be diorite. The zircons are stubby, equant or irregular in shape and show fir-leaf, sectorial, banded or oscillatory zoning. They contain inclusions, including mineral assemblages of clinopyroxene + orthopyroxene + hornblende + quartz and plagioclase + K-feldspar + biotite + quartz. Fifty SHRIMP analyses were performed on 34 zircon grains, which commonly yielded high Th/U ratios (mostly >0.5). Most analyses are distributed along concordia from 2.54 to 2.25 Ga, with the youngest age being ~ 1.95 Ga. Compositions and ages show large variations even in a same zircon grain. Combined with early studies, conclusions can be drawn as follows: 1) the diorite underwent two episodes of high-grade metamorphism, at the end of the Neoarchean and the Paleoproterozoic (~2.50 Ga and ~1.95 Ga or slightly later); 2) high-grade metamorphism in a "dry" rock system may partially reset the U—Th—Pb system of zircons and, in this case, the ages between the oldest and youngest are

* Corresponding author. E-mail address: wanyusheng@bjshrimp.cn (Y. Wan). 1674-9871 © 2011, China University of Geosciences (Beijing) and Peking University. Production and hosting by Elsevier B.V. All rights reserved.

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

doi:10.1016/j.gsf.2011.02.004

chronologically meaningless; and 3) high Th/U ratios may be common features of zircons formed during high-grade metamorphic conditions.

© 2011, China University of Geosciences (Beijing) and Peking University. Production and hosting by Elsevier B.V. All rights reserved.

1. Introduction

In previous studies, it has been observed that the U—Th—Pb isotopic system of some zircons could be closed during highgrade metamorphism (high-amphibolite to granulite-facies). However, opposite examples are also common. Ashwal et al. (1999) reported 32 concordant ages ranging from 631 to 549 Ma from a zircon TIMS (thermal ionization mass spectrometer) dilute study on a granulite-facies metamorphosed leuconorite from Madagascar. Cathodoluminescence (CL) images show that the zircons underwent recrystallization to varying degrees with only minor overgrowth. Ages vary with grain sizes with older ones being larger in size. Another example is a zircon chronological study on a mafic granulite from the Canadian continental shield (Flowers et al., 2010). Two

generations of granulite-facies metamorphism have been identified using petrographic study; single-grain zircon TIMS-dilute analysis yielded upper- and lower-intercept discordant ages of 2555 Ma and 1869 Ma. Furthermore, SHRIMP zircon dating revealed that the age could change rapidly in different spots even within a same zircon grain, and this is uncoupled with the composition and structure of the zircons. This has been considered to be a result of heterogeneous Pb loss of non-decrystallization zircons under high-temperature conditions (Flowers et al., 2010). There are two ways to cause resetting zircon ages, namely solid state diffusion and recrystallization. The 'blocking temperature' for Pb in zircons is very high (~1000 °C) in solid state diffusion (Lee et al., 1997). However, usually Pb loss during recrystallization of imperfect zircons occurs at more normal crustal temperatures. Furthermore, fluid

Figure 1 Sketch map showing the Precambrian geology in the Qixia area, eastern Shandong (after Jahn et al., 2008). Also shown is the location of sample S0125-1 from this study.

Figure 2 Field photograph of meta-diorite (S0125-1) in the Qixia area, eastern Shandong.

activity can result in complete recrystallization of zircons, which causes complete resetting of zircon ages.

In this work, we present an example to show that partly resetting of the U—Th—Pb isotopic system might be a common feature of zircons in a dry rock system during high-grade metamorphism.

2. Geological background and petrology

The Neoarchean (2.7—2.5 Ga) supracrustal sequence and tona-lite-trondhjemite-granodiorite (TTG) rocks occur widely in the Qixia area, eastern Shandong Province, with minor rocks of 2.9 Ga (Fig. 1). SHRIMP zircon dating integrated with structural studies also revealed strong magmatism and metamorphism and deformation at the end of the Neoarchean (Tang et al., 2007; Jahn et al., 2008). This tectono-thermal event resulted in formation of granitoids and the granulite-facies metamorphism of the Jiaodong Group. The Paleoproterozoic Jinshan and Fen-zishan Groups in the area are similar in protolith (mainly sedimentary rocks and carbonate) and their deposition age (2.2—1.9 Ga) but they are different in the degree of metamorphism. The Jinshan Group has undergone high-amphibolite to granulite-facies metamorphism whereas the Fenzishan Group was subjected to greenschist to low-amphibolite-facies meta-morphism. Metamorphic zircons have yielded ages of 1.88—1.82 Ga (Wan et al., 2006; Zhou et al., 2008; Tam et al., 2010).

The studied sample, a high-grade dioritic rock, was collected near Dongbatai (Fig. 1). This meta-diorite occurs in a relatively small area and probably has an intrusive relationship with the surrounding (late Neoarchean) Jiaodong Group of high-grade metamorphic rocks. The rock is overall homogeneous across the

Figure 3 Photographs showing petrography of meta-diorite (S0125-1) in the Qixia area, eastern Shandong: A. Plane-polarized light; B. Cross-polarized light; C. Back-scattered electron (BSE) image; EMPA locations are shown with same numbers as in Table 1; D. BSE image, showing exsolution of clinopyroxene. Cpx = clinopyroxene, Fld = feldspar, Pl = plagioclase, Kf = K-feldspar, Bt = biotite, Zr = zircon.

outcrop, is fine-grained and has a gneissic structure with some leucosome being parallel to the gneissosity, showing anatectic and deformation features (Fig. 2).

It is composed mainly of plagioclase, K-feldspar, clinopyrox-ene, biotite and quartz. Plagioclase and K-feldspar are granular in shape and some plagioclase grains show polysynthetic twins. Biotite is ~5% in content, flaky in shape and brown in color with dark included needle-like minerals, probably ilmenite. Quartz is less than 10%. Pyroxenes and biotite are orientated within the gneissic fabric. Major minerals show equilibrium boundaries between each other, formed during high-temperature recrystalli-zation under directed stress (Fig. 3A—C). Some clinopyroxene grains exhibit exsolution (Fig. 3D) but the bands are too thin to determine their composition. Based on mineral assemblages, the rock is considered to have undergone high-amphibolite to granu-lite-facies metamorphism. Hornblendes occur only in a few pyroxenes as thin flakes that are parallel to the cleavage of the pyroxenes and green in color, suggesting only minor retrogression. Zircons occur in pyroxene, plagioclase, biotite and at the boundary of different minerals. The electron microprobe analysis (EMPA) chemical composition of major minerals is shown in Table 1.

Two analyses of plagioclase have values of 19.7 and 21.2. The rock has SiO2, P2O5 and TiO2 contents of 55.48%, 0.58% and 0.72%, respectively (Table 2). It is high in total REE (TREE = 601.5 ppm), and shows a strong differentiation in LREE and HREE ((La/Yb)n = 58.4) and a weak negative Eu anomaly (Eu/Eu* = 0.60) (Fig. 4A). It is enriched in LILE and depleted in Nb and Ti (Fig. 4B). All these features suggest that the

diorite formed by strong crystallization-differentiation of a mafic magma in an arc-related environment.

3. Analytical methods

Whole-rock major element analysis was performed with XRF and trace element and REE analyses were done using LA-ICP-MS, both in the National Research Center of Geoanalysis, Beijing. EPMA analysis was carried out on a CAMECA 51 at the Institute of Geology and Geophysics, Chinese Academy of Science, Beijing with a spot size of ~5 mm and a counting time of 10 s. Mineral inclusions inside the zircons were identified using a Laser Raman at the Key Laboratory of Continental Dynamics, Chinese Academy of Geological Sciences, Beijing.

Zircon U—Th—Pb analyses were performed using the SHRIMP II at the Beijing SHRIMP Center, Institute of Geology, Chinese Academy of Geological Sciences. The principle and analyzing procedure have been described in detail in previous studies (Williams, 1998). The primary beam used was ~4 nA and the spot size was ~25 mm in diameter. Each spot was rastered for 150 s before analysis to remove common lead from the surface. Mass peaks were 90Zr216O+, 204Pb+, background, 206Pb+, 207Pb+, 208Pb+, 238U+, 232Th16O+ and 238U16O+. U content and 206Pb/238U were calibrated using the standards SL13 (U = 238 ppm) (Williams, 1998) and Temora (417 Ma) (Black et al., 2003), respectively. Data reduction and assessment were performed using the SQUID and ISOPLOT programs (Ludwig, 2001). The error of each single datum on Table 3 and in Fig. 7 is 1s.

Table 1 Electron-probe analysis of minerals from meta-diorite (S0125-1) in the Qixia area, eastern Shandong (%).

Nos. Minerals SiO2 TiO2 Al2O3 FeO MnO MgO CaO Na2O K2O Total Ab An Or

1 S0125-1- -Kf1 62.32 0.02 18.14 0.15 0.01 0.02 0.05 1.37 14.01 96.1 12.9 0.3 86.8

2 S0125-1- -Pl2 61.82 0.03 22.72 0.17 0.01 0.00 4.31 8.75 0.13 98.0 78.0 21.2 0.8

3 S0125-1- -Cpx3 52.24 0.12 1.20 8.53 0.31 13.64 21.46 0.65 0.03 98.2

4 S0125-1- -Bi4 37.73 3.88 13.99 14.49 0.11 14.96 0.00 0.11 9.74 95.2

5 S0125-1- -Pl5 62.20 0.00 22.26 0.08 0.00 0.02 4.06 8.88 0.36 97.9 78.2 19.7 2.1

6 S0125-1- -Kf6 64.32 0.04 18.34 0.16 0.00 0.00 0.00 3.03 12.23 98.1 27.4 0.0 72.6

7 S0125-1- -A-Kf1 62.87 0.10 17.88 0.03 0.01 0.00 0.01 0.97 14.04 95.9 9.5 0.1 90.5

8 S0125-1- -A-Pl2 62.90 0.02 22.22 0.01 0.02 0.00 3.54 9.43 0.14 98.3 82.2 17.0 0.8

9 S0125-1- -A-Kf3 62.38 0.00 17.96 0.00 0.01 0.00 0.04 0.92 14.56 95.9 8.7 0.2 91.1

10 S0125-1- -A-Cpx4 50.82 0.13 4.50 12.08 0.21 15.43 11.78 0.86 0.36 96.3

11 S0125-1- -A-Pl5 62.82 0.00 21.96 0.06 0.01 0.01 3.56 9.41 0.08 98.0 82.3 17.2 0.5

12 S0125-1- -B-Cpx1 51.04 0.06 1.40 7.80 0.27 13.71 21.75 0.70 0.00 96.8

13 S0125-1- -B-Opx2 51.91 0.03 0.55 23.40 0.73 21.57 0.32 0.01 0.00 98.5

14 S0125-1- -B-Hb3 44.95 1.64 8.23 11.27 0.14 14.44 11.14 2.04 0.34 94.3

15 S0125-1- -B-Hb4 44.98 1.62 7.97 11.00 0.12 14.24 11.16 1.83 0.35 93.3

16 S0125-1- -B-Hb5 46.76 1.44 7.58 10.88 0.17 15.50 11.18 1.90 0.23 95.7

17 S0125-1- -C-Pl1 61.43 0.00 23.07 0.07 0.00 0.02 4.51 9.11 0.15 98.4 77.9 21.3 0.8

18 S0125-1- -C-Ab2 66.94 0.04 19.38 0.08 0.02 0.03 0.61 11.32 0.13 98.5 96.4 2.9 0.7

19 S0125-1- -C-Bi3 37.63 0.49 15.59 16.09 0.15 13.62 0.05 0.05 9.91 93.6

20 S0125-1- -C-Kf4 62.67 0.00 18.37 0.11 0.03 0.01 0.40 0.80 14.72 97.1 7.5 2.1 90.5

21 S0125-1- -D-Pl1 62.08 0.00 22.53 0.02 0.00 0.00 3.81 8.85 0.16 97.5 80.0 19.0 1.0

22 S0125-1- -D-Kf2 62.20 0.06 18.10 0.07 0.00 0.00 0.03 0.71 15.15 96.3 6.6 0.2 93.2

23 S0125-1- -3-Pl 61.73 0.01 23.32 0.05 0.00 0.02 4.64 8.74 0.18 98.7 76.5 22.4 1.0

24 S0125-1- -3-Kf 61.88 0.09 18.14 0.01 0.02 0.00 0.02 0.86 14.64 95.7 8.2 0.1 91.7

Note: Nos. 1—6 are minerals in thin section with their numbers as in Fig. 3. Nos. 7—24 are mineral inclusions in zircon with the numbers of A—D as in Fig. 6. Pl = plagioclase, Kf = K-feldspar, Cpx = clinopyroxene, Opx = orthopyroxene, Hb = hornblende, Bt = biotite.

Table 2 Geochemical composition of meta-diorite (S0125-1) in the Qixia area, eastern Shandong (major element (%), trace and REE elements (ppm)).

Sample Nos. S0125-1

Rock type Meta-diorite

SiO2 55.48

TiO2 0.72

M2O3 15.61

Fe2O3 2.12

FeO 3.75

MnO 0.12

MgO 5.65

CaO 7.84

Na2O 4.59

K2O 2.72

P2O5 0.58

H2O 0.74

CO2 0.36

Total 100.30

Cr 156.00

Ni 125.00

Co 23.00

Sc 12.00

Rb 53.00

Ba 1018.00

Sr 1805.00

Nb 5.00

Zr 217.00

Y 21.00

Th 7.00

La 129.50

Ce 278.00

Pr 27.20

Nd 115.60

Sm 20.03

Eu 3.59

Gd 15.80

Tb 1.64

Dy 5.20

Ho 0.79

Er 2.20

Tm 0.27

Yb 1.46

Lu 0.21

TREE 601.50

(La/Yb)n 58.40

Eu/Eu* 0.60

4. Zircon characteristics and dating results

Zircons are stubby, equant or irregular in shape and show complex structures in CL images, such as fir-leaf, sectorial, banded or oscillatory zoning (Fig. 5A—H). Some grains show strong luminescence in local domains probably due to recrys-tallization. In many cases, mineral inclusions can be observed in or near the recrystallization domains. Metamorphic rims are visible in some grains (e.g. Grain 18 in Fig. 5A, Grain 36 in Fig. 5E, Grain 10 in Fig. 5H). Laser Raman analysis and EMPA

revealed many different mineral inclusions in the zircons: K-feldspar (Kf), plagioclase (Pl), quartz (Qtz), biotite (Bt), hornblende (Hb), clinopyroxene (Cpx), rare orthopyroxene (Opx), epidote, apatite, calcite, pyrrhotite, rutile, and titanite. The mineral assemblages include: 1) Kf + Pl (Fig. 6A); 2) Cpx + Pl + Kf (Fig. 6A); 3) Cpx + Opx + Qtz + Hb (Fig. 6B); 4) Pl + Bt (Fig. 6C); 5) Kf + Ab (Fig. 6C); 6) Bt + Qtz + Pl + Kf (Fig. 6D); and 7) Pl + Qtz + Bt (Fig. 6E). Mono- or bi-phase mineral inclusions are more common. Apatite, rutile and titanite occur with major minerals. Fluid inclusions are also observed (Fig. 6E).

The Cpx + Opx assemblage is probably metamorphic in origin because crystallization of dioritic magma seldom produced this assemblage. The clinopyroxene co-existing with orthopyroxene (S0125-1-B-Cpx1, Fig. 6B) has the same composition with the one (S0125-1-Cpx3) in thin section. Another clinopyroxene (S0125-1-A-Cpx4, Fig. 6A) co-existing with K-feldspar and plagioclase shows a Fe-rich and Ca-poor feature in composition and seems to be transforming into orthopyroxene. The assemblage of Kf + Ab (Fig. 6C) is not one from the original magma but probably the product of partial melting during high-grade metamorphism. Only minor tiny flakes of retrograde hornblende replacing orthopyroxene are observed in thin section, therefore those seen as inclusions within the zircons were probably formed earlier, but their origin (whether from metamorphism or magmatism) is still unclear. Some of the biotite grains and tri-phase (Pl + Qtz + Bt) inclusions are round in shape, probably suggesting a crystallization origin of melt inclusions (Thomas et al., 2003; Nutman and Hiess, 2009). Plagioclase inclusions in the zircons have a similar composition to the one in the thin section with values of 17—22. Compared with the same minerals in thin section, the inclusions of biotite in the zircons are poorer in TiO2 whereas the K-feldspar is poorer in Na2O (Table 1).

A total of 50 analyses were performed on 34 zircon grains, which proved to have U contents and Th/U ratios of 46—406 ppm and 0.08—2.35. Four analyses have Th/U values of 0.08—0.28 while the rest have values higher than 0.46 (Table 3).

Most analyses are distributed along concordia and are mainly focused in the range of 2.54—2.25 Ga. Three analyzed spots (10.1, 21.1, 33.1) yielded the youngest age of ~ 1.95 Ga (Fig. 7). Some zircons show Pb loss. The large age variation is not only noticed from different zircon grains but also from different parts of a same zircon grain. In grain 18 (Fig. 5A), a rim, which seems to be metamorphic in origin, yielded an older age (2.52 Ga in analysis 18.2) than the core (2.37 Ga in analysis 18.1), whereas both have large Th/U values (>1.0) (Table 3). In grain 21 (Fig. 5G), four analyses yielded ages and Th/U values of 2.40—1.95 Ga and 0.28—1.55. Six analyses on grain 10 range in age from 2.42 to 1.97 Ga (Fig. 5H). Two spots on the apparent metamorphic rim have an older age of 2.42 Ga (10.4 and 10.5) with a low Th/U value of 0.08—0.09, whereas the core has younger ages of 2.35—1.97 Ga and higher Th/U values of 0.18—1.42 (Table 3).

5. Discussion and conclusions

The rock examined in this study is dioritic in composition and has a high Zr content (up to 217 ppm), and thus there must be some magmatic zircons in the rock. Mineral inclusions of plagioclase, K-feldspar, quartz, biotite and accessory minerals have been

La Pr Sm Gd Dy Er Yb

Figure 4 REE pattern (A) and trace element distribution pattern

Sr Rb Th Ce Zr Ti Yb Cr

(B) of meta-diorite (S0125-1) in the Qixia area, eastern Shandong.

observed within the zircons. The chemical compositions of K-feldspar and biotite inclusions are different from their counterparts in thin section, probably indicating their magmatic genesis. On the other hand, most zircons show fir-leaf, sectorial, banded, or weak oscillatory zoning, being similar to granulite-facies metamorphic zircons. Some zircon grains are irregular in shape and have inclusions of Cpx + Opx and anatexis-related Kf + Ab. These features support a metamorphic origin, consistent with the zircon age distribution modal and the high-grade meta-morphism of the rock. It should be noticed that the zircons which have the two pyroxene inclusions and show irregular shape did not yield the youngest age of 1.95 Ga, indicating that their formation was not because of high-grade metamorphism during the late Paleoproterozoic but in an earlier tectono-thermal event, although the evidence of the existence of an earlier event is difficult to identify in thin section due to the strong metamorphism and deformation of the late Paleoproterozoic.

The severe reworking of high-grade metamorphism during the late Paleoproterozoic led to the scattered distribution of the analyses near concordia. The ages between the youngest and oldest are not consistent with the time when any tectontothermal events occurred. Theoretically, the oldest and youngest ages may also be meaningless, as they may not represent the formation age of the zircons (the Neoarchean magmatic zircons in the present case) and the overprinting time (the late Paleoproterozoic highgrade metamorphism in the present case), because the U—Th—Pb isotopic system of the zircons might not have stayed totally closed or reset. However, for the reasons that 1) many of the analyzed spots yielded ages concentrating around 2.52 Ga; 2) an important magmatic event occurred in the late Neoarchean (~2.5 Ga) in this area (Tang et al., 2007; Jahn et al., 2008), the age of 2.52 Ga is considered to be the intrusive time of the diorite. On the other hand, considering that metamorphic ages of 2.50—2.49 Ga were recorded in many rocks in the area, we speculate that the early granulite-facies metamorphism of the diorite occurred around 2.50 Ga. It is a common feature for many areas of the North China Craton that TTG magmatism was soon followed by highgrade metamorphism at the end of the Neoarchean with a time span being only 20—10 Ma (Wan et al., 2005; Grant et al., 2009; Zhao et al., 2009). The late Neoarchean high-grade meta-morphism should have caused strong recrystallization of early magmatic zircons and the formation of new zircons. Both of them might originally have shown differences in CL structures, but those differences trailed off because of strong overprinting by late Paleoproterozoic metamorphism. Analysis 15.1 has the oldest 207Pb/206Pb age of 2.58 Ga but is markedly reverse discordant

(Fig. 7), and hence it is not considered reliable; it could for example contain small amounts of unsupported radiogenic Pb, due to differential movement of U versus Pb during the late event(s).

The structure and composition of the zircons indicate that the amphibolite-facies overprinting might have been immediately following the high-grade metamorphism at the end of the Neoarchean. Zircon grain 10 has Th/U values less than 0.1 in a well-defined metamorphic rim (10.4 and 10.5) and 0.18 in a recrystallization domain (10.3) (Fig. 5H, Table 3), which is different from typical granulite-facies metamorphic zircons with high Th/U values. Zircon grains 18 and 36 might also have the metamorphic rims, and they also formed prior to ~1.95 Ga (Fig. 5A, E). Most zircons are relatively high in Th/U value (0.69—1.25), for which there are two possibilities: 1) they were originally low in Th/U value as a result of retrograde meta-morphism of the late Neoarchean, but the late Paleoproterozoic recrystallization led to the higher Th/U value, or 2) they were originally high in Th/U value as a result of a combination of relict igneous zircon and late Neoarchean granulite-facies metamorphism. The latter may be more possible because the metamorphic rims are commonly narrow or absent in the zircons.

Granulite-facies metamorphic zircons are commonly relatively high in U/Th ratios, the reasons are that during any recrystallization/regrowth of zircon under these conditions, there are no co-existing high-Th phase, such as monazite, epidote and allanite, to accept preferentially Th over U. Therefore very low (<0.1) Th/U metamorphic and recrystallized zircon is not observed. This has been observed in numerous previous studies of granulite-facies zircons (Kinny, 1986; Grant et al., 2009; Santosh et al., 2009; Wan et al., 2009; Zhao et al., 2009). Another reason may be that high-grade metamorphism forces fluids out of the rock system, and U is easier to be flushed out with fluids because of being more compatible to fluids than Th in the process.

As indicated above, an amphibolite-facies retrograde meta-morphism might have overprinted earlier granulite-facies meta-morphism at the end of the Neoarchean. However, the internal structure and composition of the zircons indicate that the overprinting was not strong with only minor fluids being input from the out-of-rock system. Therefore, the late Paleoproterozoic highgrade metamorphism occurred within a relatively dry rock system, this might be the reason for the rarity of metamorphic overgrowth of this age. It was the high-grade metamorphism occurring in the 'dry' rock system that had led to the solid-phase recrystallization

Table 3 SHRIMP U-Pb data for zircons from meta-diorite (S0125-1) in the Qixia area, eastern Shandong.

(ppm) 238U

Discordant

(ppm) 206Pb* 235U 238U correction Age (Ma) Age (Ma) (%)

29 0.1463 1.2 8.33 2.0 0.4130 1.6 0.80 2229 ±30 2303 ±21 3

32 0.14007 0.66 7.32 1.7 0.3792 1.6 0.92 2073 ±28 2228 ±11 7

50 0.1532 0.71 9.39 1.7 0.4447 1.5 0.91 2372 ±31 2382 ±12 0

50 0.1679 0.73 9.38 1.8 0.4050 1.6 0.91 2192 ±30 2537 ±12 14

68 0.15307 0.48 9.20 1.6 0.4360 1.5 0.96 2333 ±30 2380 ±8 2

66 0.15321 0.43 9.08 1.6 0.4300 1.5 0.96 2306 ±30 2382 ±7 3

67 0.16597 0.58 10.80 1.6 0.4720 1.5 0.94 2492 ±32 2517 ±10 1

43 0.16539 0.56 11.06 1.7 0.4852 1.6 0.94 2550 ±33 2512 ±9 -2

40 0.16178 0.56 10.64 1.7 0.4771 1.6 0.94 2514 ±33 2474 ±9 -2

56 0.15666 0.47 9.66 1.6 0.4473 1.6 0.96 2383 ±31 2420 ±8 2

25 0.1500 1.1 8.37 2.1 0.4045 1.8 0.85 2190 ±34 2346 ±19 7

68 0.16158 0.43 10.23 1.6 0.4593 1.5 0.96 2437 ±31 2472 ±7 1

27 0.1209 0.89 5.77 1.8 0.3465 1.6 0.87 1918 ±26 1969 ±16 3

32 0.1413 0.74 7.84 1.9 0.4028 1.8 0.92 2182 ±33 2243 ±13 3

82 0.1503 0.77 8.92 1.9 0.4302 1.8 0.92 2307 ±34 2349 ±13 2

153 0.15635 0.39 9.47 1.6 0.4395 1.5 0.97 2348 ±30 2416 ±7 3

85 0.15670 0.59 9.92 1.6 0.4593 1.5 0.93 2437 ±31 2420 ±10 -1

39 0.14532 0.62 7.19 1.7 0.3588 1.6 0.93 1976 ±26 2292 ±11 14

50 0.16500 0.50 10.80 1.6 0.4746 1.6 0.95 2504 ±32 2508 ±9 0

33 0.1586 0.65 9.84 1.8 0.4497 1.6 0.93 2394 ±33 2441 ±11 2

46 0.16655 0.56 10.84 1.8 0.4719 1.7 0.95 2492 ±36 2523 ±9 1

80 0.15833 0.41 9.91 1.6 0.4540 1.5 0.97 2413 ±31 2438 ±7 1

48 0.1663 0.63 10.37 1.7 0.4524 1.6 0.93 2406 ±31 2520 ±11 5

51 0.1659 0.68 9.92 1.7 0.4334 1.6 0.92 2321 ±30 2517 ±11 8

50 0.17250 0.51 12.06 1.8 0.5070 1.7 0.96 2644 ±38 2582 ±9 -2

28 0.1641 0.76 10.53 2.0 0.4652 1.9 0.93 2462 ±39 2499 ±13 1

44 0.16860 0.54 11.22 1.7 0.4828 1.6 0.95 2539 ±33 2544 ±9 0

28 0.1521 0.82 9.09 2.5 0.433 2.4 0.95 2321 ±46 2370 ±14 2

63 0.1658 0.78 11.00 1.8 0.4810 1.6 0.90 2532 ±33 2516 ±13 -1

55 0.14908 0.53 8.60 1.6 0.4183 1.6 0.95 2253 ±30 2335 ±9 4

19 0.1661 0.93 10.72 2.2 0.4680 2.0 0.91 2475 ±41 2519 ±16 2

62 0.1445 0.79 8.40 1.8 0.4218 1.6 0.89 2268 ±30 2281 ±14 1

54 0.11927 0.62 5.586 1.7 0.3397 1.5 0.93 1885 ±25 1945 ±11 3

36 0.1550 1.1 9.36 1.9 0.4381 1.6 0.83 2342 ±31 2401 ±18 2

18 0.1342 1.6 5.71 2.6 0.3085 2.0 0.79 1734 ±31 2154 ±28 20

51 0.14443 0.61 8.15 1.7 0.4091 1.6 0.93 2211 ±29 2281 ±10 3

53 0.1628 0.81 10.73 2.2 0.4778 2.0 0.93 2518 ±42 2485 ±14 -1

30 0.1607 0.99 9.95 1.9 0.4493 1.6 0.85 2392 ±32 2463 ±17 3

53 0.1623 0.64 10.27 1.8 0.4592 1.7 0.93 2436 ±33 2479 ±11 2

38 0.1619 0.85 9.99 1.8 0.4477 1.6 0.88 2385 ±32 2476 ±14 4

33 0.1653 0.69 10.69 1.8 0.4689 1.6 0.92 2479 ±34 2511 ±12 1

46 0.1654 0.78 10.73 1.8 0.4707 1.6 0.90 2487 ±33 2511 ±13 1

40 0.1686 0.80 11.11 1.8 0.4779 1.7 0.90 2518 ±35 2543 ±13 1

63 0.1600 0.70 9.58 1.7 0.4344 1.6 0.91 2325 ±31 2455 ±12 5

31 0.1209 1.3 5.46 2.1 0.3275 1.7 0.78 1826 ±26 1969 ±23 7

62 0.1644 0.69 10.46 1.7 0.4617 1.6 0.92 2447 ±32 2501 ±12 2

32 0.1493 0.98 8.29 2.0 0.4029 1.7 0.87 2183 ±32 2338 ±17 7

89 0.1350 0.94 6.87 1.8 0.3689 1.5 0.85 2024 ±26 2164 ±16 6

29 0.1342 1.1 6.04 2.0 0.3262 1.7 0.83 1820 ±26 2154 ±19 16

42 0.1330 1.9 6.89 2.5 0.3759 1.6 0.65 2057 ±29 2138 ±33 4

S0125-1-S0125-1-S0125-1-S0125-1-S0125-1-S0125-1-S0125-1-S0125-1-S0125-1-S0125-1-S0125-1-S0125-1-S0125-1-S0125-1-S0125-1-S0125-1-S0125-1-S0125-1-S0125-1-S0125-1-S0125-1-S0125-1-S0125-1-S0125-1-S0125-1-S0125-1-S0125-1-S0125-1-S0125-1-S0125-1-S0125-1-S0125-1-S0125-1-S0125-1-S0125-1-S0125-1-S0125-1-S0125-1-S0125-1-S0125-1-S0125-1-S0125-1-S0125-1-S0125-1-S0125-1-S0125-1-S0125-1-S0125-1-S0125-1-S0125-1-

1.1 2.1

3.2 4.1

4.2 6.1

7.1 8.1

8.2 9.1 10.1 10.2 10.3

83 97 131 143 180 177 165 103 97 146 72 171 91 94 220

10.4 406

10.5 215

10.6 11.1 11.2 12.1 13.1

14.2 15.1 16.1 17.1 18.1

127 124 86

113 205 123 136

114 69

106 76

18.2 152 19.1 153

20.1 21.1 21.2 21.3 21.3 26.1

27.1 28.1

29.2 30.1 31.1 32.1 33.1 34.1 35.1 36.1 37.1 38.1

46 171 186 95 67 146 130 78 134 99 81 113 97 169 109 156 92 279 102 129

87 145 276 212 80 261 172 143

131 200

76 250 122

119 38 34 17

175 182

158 166 370 179 310 162

87 145

83 184 224

68 112 50 142 42

159 164

76 213 130 105 166

120 281

84 177 108 187

132 138

1.09 1.55 2.18 1.53 0.46 1.52 1.08 1.44

1.40 1.42 1.08

1.51 1.38 1.31 0.18 0.09 0.08 1.42

1.52 1.91

1.52 1.87

2.35 1.47 1.29

1.41 1.13 1.25

1.53 0.67 0.28 1.55 0.65 1.13 1.31 1.01 1.64

1.36 1.35

1.52 1

1.71 0.79 1.17 1.21 0.69 1.34 1.11

±% 207Pb*/

±% 206Pb / ±% Error

206Pb/238U

207Pb/206Pb

of zircons, resulting in the U—Th—Pb isotopic system of the zircons opening and re-adjusting to various degrees and, therefore, the scattered distribution of age data along concordia. The analyses are distributed almost continuously from 2.54 to 2.25 Ga, but seldom from 2.25 to 1.95 Ga (Fig. 7). This may represent the real

case because of the large quantity of analyses (up to 50) and because of the partial resetting of the zircon U—Th—Pb isotopic system during 'dry' high-grade metamorphism. Presence of fluids is crucial to the U—Th—Pb behavior of zircons. In the case where fluids occur, even low-grade metamorphism could lead to the

Figure 5 Cathodoluminescence images for zircons from meta-diorite (S0125-1) in the Qixia area, eastern Shandong: A—H, cath-odoluminescence images of representative zircons, showing banded, fir-leaf, sectorial or oscillatory zoning. Fld = feldspar, Ap = apatite, RC = recrystallization domain. Ellipses (~ 25 mm) show positions of SHRIMP analytical sites with their identification numbers and ages (in Ga) as in Table 3.

alteration in zircon structure, composition and age (Bomparola et al., 2007; Geisler et al., 2007). Three analyzed spots (10.1, 21.2, 33.1) yielded ages around 1.95 Ga and had relatively high Th/U values (0.28—1.38). Petrographically, analysis 33.1 is probably on a metamorphic rim (Fig. 5B) whereas analyses 10.1 and 21.2 fall into recrystallization domains (Fig. 5G, H). The age of 1.95 Ga is considered to be the oldest age of the high-grade metamorphism in the late Paleoproterozoic, and the 'true' age should be slightly younger than 1.95 Ga. Metamorphic ages of 1.88—1.82 Ga have been reported from previous studies in this area (Wan et al., 2006; Zhou et al., 2008; Tam et al., 2010).

Acknowledgments

We thank Hua Tao for mount-making, Chun Yang and Hui Zhou for help with zircon CL images, and Yuguang Ma and Qian Mao for assistance with the EPMA analysis. We express our acknowledgment to Ian Williams and Lance Black for providing zircon standards. We are grateful to Allen Nutman, Chunming Wu and Tiannan Yang for their kind help in the research. This work is supported by the Key Program of the Ministry of Land and Resources of China (Grant Nos. 1212010811033, 1212010711815).

Figure 6 Mineral inclusions in zircons from meta-diorite (S0125-1) in the Qixia area, eastern Shandong: A—D, BSE images; E—F, plane light; Fld = feldspar; Pl = plagioclase; Kf = K-feldspar; Qtz = quartz; Ap = apatite; Cpx = clinopyroxene; Opx = orthopyroxene; Hb = hornblende; Bt = biotite; Fi = fluid inclusion. Crosses show positions of EMPA analytical sites with their identification numbers as in Table 1.

207pb/235u

Figure 7 Concordia diagram of SHRIMP U—Pb data for zircons from meta-diorite (S0125-1) in the Qixia area, eastern Shandong.

References

Ashwal, L.D., Tucker, R.D., Zirnner, E.R., 1999. Slow cooling of deep crustal granulites and Pb-loss in zircon. Geochimica et Cosmochimica Acta 63, 2839—2851.

Black, L.P., Kamo, S.L., Allen, C.M., Aleinikoff, J.N., Donald, W.D., Korsch, R.J., Foudoulis, C., 2003. TEMORA 1: a new zircon standard for Phanerozoic U—Pb geochronology. Chemical Geology 200, 155—170.

Bomparola, R.M., Ghezzo, C., Belousova, E., Griffin, W.L., O'Reilly, S.Y., 2007. Resetting of the U—Pb zircon system in Cambro-Ordovician intrusives of the deep freeze range, northern Victoria Land, Antarctica. Journal of Petrology 48, 327—364.

Flowers, R.M., Schmitt, A.K., Grove, M., 2010. Decoupling of U—Pb dates from chemical and crystallographic domains in granulite facies zircon. Chemical Geology 270, 20—30.

Geisler, T., Schaltegger, U., Tomaschek, F., 2007. Re-equilibration of zircon in aqueous fluids and melts. Elements 3, 43—50.

Grant, M.L., Wilde, S.A., Wu, F.Y., Yang, J.H., 2009. The application of zircon cathodoluminescence imaging, Th—U—Pb chemistry and U—Pb

ages in interpreting discrete magmatic and high-grade metamorphic events in the North China Craton at the Archean/Proterozoic boundary. Chemical Geology 261, 155—171.

Jahn, B.M., Liu, D.Y., Wan, Y.S., Song, B., Wu, J.S., 2008. Archean crustal evolution of the Jiaodong peninsula, China, as revealed by zircon SHRIMP geochronology, elemental and Nd-isotope geochemistry. American Journal of Science 308, 232—269.

Kinny, P.D., 1986. 3820 Ma zircons from a tonalitic Amitsoq gneiss in the Goldthab district of southern West Greenland. Earth and Planetary Science Letters 79, 337—347.

Lee, J.K.W., Williams, I.S., Ellis, D.J., 1997. Pb, U and Th diffusion in natural zircon. Nature 390, 159—161.

Ludwig, K.R., 2001. Squid 1.02: A User's Manual. Berkeley Geochro-nology Centre, Special Publication, pp. 21—19.

Nutman, A.P., Hiess, J., 2009. A granitic inclusion suite within igneous zircons from a 3.81 Ga tonalite (W. Greenland): restrictions for Hadean crustal evolution studies using detrital zircons. Chemical Geology 261, 77—82.

Santosh, M., Wan, Y.S., Liu, D.Y., Dong, C.Y., Li, J.H., 2009. Anatomy of zircons from an ultra-hot orogen: the amalgamation of the North China Craton within the supercontinent Columbia. The Journal of Geology 117, 429—443.

Tam, P.Y., Zhao, G.C., Liu, F.L., Zhou, X.W., Sun, M., Li, S.Z., 2010. Timing of metamorphism in the Paleoproterozoic Jiao-Liao-Ji Belt: new SHRIMP U—Pb zircon dating of granulites, gneisses and marbles of the Jiaobei massif in the North China Craton. Gondwana Research 19 (1), 150—162.

Tang, J., Zheng, Y.F., Wu, Y.B., Gong, B., Liu, X.M., 2007. Geochro-nology and geochemistry of metamorphic rocks in the Jiaobei terrane: constraints on its tectonic affinity in the Sulu orogen. Precambrian Research 152, 48—82.

Thomas, J.B., Bodnar, R.J., Shimizu, N., Chesner, C.A., 2003. Melt inclusions in zircon. In: Hanchar, J.M., Hoskin, P.W.O. (Eds.), Zircon. Mineralogical Society of America, Washington, pp. 63—87.

Wan, Y.S., Liu, D.Y., Dong, C.Y., Xu, Z.Y., Wang, Z.J., Wilde, S.A., Yang, Y.H., Liu, Z.H., Zhou, H.Y., 2009. The Precambrian Khondalite Belt in the Daqingshan area, North China Craton: evidence for multiple metamorphic events in the Palaeoproterozoic era. In: Reddy, S.M., Mazumder, R., Evans, D.A.D., Collins, A.S. (Eds.), Palaeoproterozoic Supercontinents and Global Evolution. Geological Society, London, Special Publication, 323, pp. 73—97.

Wan, Y.S., Song, B., Liu, D.Y., Wilde, S.A., Wu, J.S., Shi, Y.R., Yin, X.Y., Zhou, H.Y., 2006. SHRIMP U—Pb zircon geochronology of Palae-oproterozoic metasedimentary rocks in the North China Craton: evidence for a major Late Palaeoproterozoic tectonothermal event. Precambrian Research 149, 249—271.

Wan, Y.S., Song, B., Yang, C.H., Liu, D.Y., 2005. Zircon SHRIMP U—Pb geochronology of Archaean rocks from the Fushun-Qingyuan area, Liaoning Province and its geological significance. Acta Geologica Sinica 79, 78—87 (in Chinese with English abstract).

Williams, I.S., 1998. U—Th—Pb geochronology by ion microprobe. In: McKibben, M.A., Shanks, W.C., Ridley, W.I. (Eds.), Applications of Microanalytical Techniques to Understanding Mineralizing Processes. Reviews in Economic Geology, 7, pp. 1—35.

Zhao, Z.R., Song, B., Shen, Q.H., Song, B., 2009. The petrogeochemical characters and SHRIMP U—Pb zircon ages of the meta-mafic rocks from the Yushui Complex, in Yishui County, Shandong Province. Geological Review 55, 286—299 (in Chinese with English abstract).

Zhou, X.W., Zhao, G.C., Wei, C.J., Geng, Y.S., Sun, M., 2008. EPMA U—Th—Pb monazite and SHRIMP U—Pb zircon geochronology of high-pressure politic granulites in the Jiaobei massif of the North China Craton. American Journal of Science 308, 328—350.