Zircon u pb ages and hf isotopic composi

On: 02 August 2012, At: 19:18Publisher: Taylor & Francis Informa Ltd Registered in England and Wales Registered Number: 1072954 Registered office: Mortimer House, 37-41 Mortimer Street,

On: 02 August 2012, At: 19:18

Publisher: Taylor & Francis

Informa Ltd Registered in England and Wales Registered Number: 1072954 Registered office: Mortimer House, 37-41 Mortimer Street, London W1T 3JH, UK

International Geology Review

Publication details, including instructions for authors and subscription information:

http://www.tandfonline.com/loi/tigr20

Zircon U–Pb ages and Hf isotopic compositions from the Sin Quyen Formation: the Precambrian crustal evolution of northwest Vietnam

Pham Trung Hieu ab , Fukun Chen a , Le Thanh Me c , Nguyen Thi Bich Thuy d , Wolfgang Siebel e & Ting-Guang Lan b

a Key Laboratory of Crust-Mantle Materials and Environments, School of Earth and Space Sciences, University of Science and Technology of China, Hefei, 230026, China

b Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing, 100029, China c

Department of Geology, Hanoi University of Mining and Geology, Hanoi, Vietnam d

Research Institute of Geology and Mineral Resources, Thanh Xuan, Hanoi, Vietnam e

Institut für Geowissenschaften, Universität Tübingen, 72074, Tübingen, Germany Version of record first published: 23 Dec 2011

To cite this article: Pham Trung Hieu, Fukun Chen, Le Thanh Me, Nguyen Thi Bich Thuy, Wolfgang Siebel & Ting-Guang Lan

(2012): Zircon U–Pb ages and Hf isotopic compositions from the Sin Quyen Formation: the Precambrian crustal evolution of northwest Vietnam, International Geology Review, 54:13, 1548-1561

To link to this article: http://dx.doi.org/10.1080/00206814.2011.646831

PLEASE SCROLL DOWN FOR ARTICLE

Full terms and conditions of use: http://www.tandfonline.com/page/terms-and-conditions

This article may be used for research, teaching, and private study purposes Any substantial or systematic reproduction, redistribution, reselling, loan, sub-licensing, systematic supply, or distribution in any form to anyone is expressly forbidden.

The publisher does not give any warranty express or implied or make any representation that the contents will be complete or accurate or up to date The accuracy of any instructions, formulae, and drug doses should

be independently verified with primary sources The publisher shall not be liable for any loss, actions, claims, proceedings, demand, or costs or damages whatsoever or howsoever caused arising directly or indirectly in connection with or arising out of the use of this material.

Vol 54, No 13, 10 October 2012, 1548–1561

Zircon U–Pb ages and Hf isotopic compositions from the Sin Quyen Formation: the Precambrian

crustal evolution of northwest Vietnam

Pham Trung Hieua,b, Fukun Chena*, Le Thanh Mec, Nguyen Thi Bich Thuyd, Wolfgang Siebeleand Ting-Guang Lanb

a Key Laboratory of Crust-Mantle Materials and Environments, School of Earth and Space Sciences, University of Science and Technology of China, Hefei 230026, China; b Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China;

c Department of Geology, Hanoi University of Mining and Geology, Hanoi, Vietnam; d Research Institute of Geology and Mineral Resources, Thanh Xuan, Hanoi, Vietnam; e Institut für Geowissenschaften, Universität Tübingen,72074 Tübingen, Germany

(Accepted 28 November 2011)

The location of the suture zone between the South China and Indochina blocks in northwest Vietnam has been under debate for decades Generally, the boundary between these blocks has been placed along (1) the Ailaoshan–Red River zone

or (2) the Song Ma zone The Sin Quyen Formation, lying between these zones, was previously regarded as a Palaeo-and Mesoproterozoic sequence It comprises its provenance Palaeo-and tectonic affinity We analysed detrital zircons from two paragneisses and one migmatite of the Sin Quyen Formation employing laser ablation inductively coupled plasma mass spectrometry U–Pb dating techniques U–Pb ages of these zircons show three main periods of zircon formation:∼2.7–3.0,

∼2.2–2.5, and ∼1.8 Ga, suggesting that Sin Quyen rocks were mainly derived from Palaeoproterozoic and Archaean base-ment units Inasmuch as the South China basebase-ment comprises rocks of similar ages, we conclude that the Sin Quyen Formation belongs to that block Our new data strengthen the view that the suture between the South China and Indochina blocks is located within the Song Ma zone In addition, zircons with U–Pb ages>3.0 thousand million years represent the

oldest minerals reported in northwest Vietnam so far, indicating the existence of Mesoarchaean crustal remnants in this region

Keywords: Precambrian; northwest Vietnam; Tethyan; zircon U–Pb ages; zircon Hf isotopes

Introduction

Northern Vietnam is part of the eastern Tethyan

oro-genic belt lying between the South China and Indochina

blocks It represents a key area for the study of the

collisional history of the eastern Tethyan orogen The

suture zone between the South China and Indochina

blocks is formed by the closure of the Palaeo-Tethyan

ocean during the late Palaeozoic to early Mesozoic

period (Tapponier et al 1981; Metcalfe 1988, 1998,

2002, 1996a, b; ¸Sengör et al 1988; Hutchison 1993,

Singharajwarapan and Berry 2000; Lepvrier et al 2008).

In western Yunnan, southwest China, two ophiolite belts

have been distinguished along the Jinshajiang–Ailaoshan

and Lancangjiang–Changning–Menglian zones, both being

interpreted as Palaeo-Tethyan sutures (Zhong 1998)

A strong debate has arisen concerning the

southeast-ward continuation of the Jinshajiang–Ailaoshan suture into

northern Vietnam Both the Ailaoshan–Red River zone

(Dovjikov 1965; Trinh et al 1996; Findlay and Trinh 1997)

and the Song Ma zone (Bunopas and Vella 1978; Tri 1979;

¸Sengör 1984; Maranate and Vella 1986; ¸Sengör et al 1988;

*Corresponding author Email: fkchen@ustc.edu.cn

Hutchison 1989a, b; Tung and Tri 1992; Metcalfe 2002;

Hieu et al 2009, 2010) have been advocated as the

con-tinuation of this suture (Figure 1) Hence, deciphering the formation time and geochemical features of the base-ment rocks between the Red River and Song Ma zones is essential to understand the tectonic evolution of northwest Vietnam and to place constraints on the tectonic boundary between the Indochina and South China blocks

In order to put constraints on the location of the suture zone, it is important to characterize the basement beneath northwest Vietnam Different geological terrains are often distinguishable in terms of crust formation ages and evolu-tion histories and bear unique tectonic and metamorphic imprints or special sedimentological and palaeontologi-cal features Recently, geochronologipalaeontologi-cal and geochemipalaeontologi-cal characteristics of detrital minerals from sedimentary rocks have been shown to provide a powerful tool to decipher the provenance and tectonic evolution of geological

ter-rains (e.g Valverde et al 2000; Nelson 2001; Chen et al.

2002, 2003, 2009; Cawood et al 2003, 2007; Dickinson

and Gehrels 2009) The closure of the Tethyan ocean and

ISSN 0020-6814 print/ISSN 1938-2839 online

© 2012 Taylor & Francis

http://dx.doi.org/10.1080/00206814.2011.646831

Figure 1 Sketch map of tectonic units in northwestern Vietnam.

subsequent orogenic processes had a profound influence

on the geology of northwest Vietnam Manifestations are

seen in major Mesozoic and Cenozoic tectonic,

metamor-phic, and igneous activities Only a few studies, however,

have shown direct or indirect evidence for the existence

of Precambrian basement rocks in this area (e.g Tri 1979;

Bodet and Schärer 2000; Lan et al 2000, 2001; Nam et al.

2001, 2002, 2003)

In this study, we report U–Pb ages and Hf isotopic

com-position of zircons from the Sin Quyen Formation,

north-west Vietnam Characterizing its basement and tectonic

affinity allows us to locate the suture between the South

China and Indochina blocks The discovery of detrital

zircons from the Sin Quyen Formation with U–Pb ages as

old as∼3.0 thousand million years and an Hf model age

of ∼3.3 thousand million years suggests that the

evolu-tion of northwest Vietnam started earlier than previously

thought Our results demonstrate that here crust formation

began during the late Archaean and early Proterozoic

This is essentially identical to the crustal evolution his-tory proposed for the South China block Based on these similarities, we postulate that the Song Ma zone repre-sents the suture between the South China and Indochina blocks

Geological background

Northwestern Vietnam is composed of three major tectonic zones, namely (1) the Red River zone (or Red River shear zone), (2) the Song Da zone, and (3) the Song Ma zone These zones, which extend into western Yunnan, played an important role during the evolution of the eastern Tethyan orogenic belt

The Red River zone is mainly composed of high-grade metamorphic rocks and deformed granitoids The Day Nui Con Voi massif, a large-scale antiformal ‘core complex’-type structure, bounded by the Red River fault and the Song Chay fault, records the shear activity of the

Red River zone in North Vietnam (Anczkiewicz et al.

2007) Microfabrics document high temperatures between

700 and 500◦C during extensional and strike–slip

shear-ing (Anczkiewicz et al 2007) The Song Da terrane west

of the Red River zone is commonly interpreted as an

intra-continental rift represented by the Da Mai Formation

(Lepvrier et al 2008) The sedimentary succession in

this rift zone is dominated by Permian to lower Triassic

clastic rocks Mafic extrusive rocks of the Yen Duyet,

Cam Thuy, and Vietnam formations are interbedded in

this sedimentary succession (Hutchison 1989b; Lepvrier

et al 2008).

The Song Ma zone is located between the Truong Son

zone and the Song Da zone and is known as the Song

Ma anticlinorium in the Vietnamese literature (e.g Tri

1979; Krobicki et al 2008) The basement of the Song

Ma zone consists of low- to high-grade non-fossiliferous

schists, metagreywackes, greenschists, amphibolites, and

marbles intruded by late Palaeozoic granitoids According

to their metamorphic grade, the basement rocks in

north-western Vietnam are subdivided into two formations: the

lower grade Nam Co Formation and the high-grade Nam

Su Lu Formation (e.g Lepvrier et al 2008; Nakano

et al 2008) The Nam Co Formation is composed

dom-inantly of muscovite and garnet–phengite schists, with

intercalated layers of mafic and calc-silicate rocks The

Nam Su Lu Formation consists mainly of amphibolites

and interlayers of garnet–biotite and garnet–sillimanite–

biotite gneisses Late Palaeozoic high-pressure (HP)

meta-morphic rocks (eclogites and HP granulites) have been

reported and interpreted as a result of the Indochinese

orogeny (Nakano et al 2008, 2010) Serpentinite lenses

that outcrop within the metamorphic belt have been

regarded as relicts of former Palaeo-Tethyan lithosphere

(Hutchison 1989b)

Two tectonic events are generally considered to

have influenced the geological evolution of

northwest-ern Vietnam (e.g Lepvrier et al 2008) One of them

is the large-scale sinistral shear displacement along the

Red River zone that was active between 27 and 22 Ma

(Chung et al 1997) The other is the suturing between

the Indochina and South China blocks along the Song Ma

belt The time of the collision was previously placed in

the Silurian period on the basis of a greenschist

meta-morphic age of 455 million years obtained by the K–Ar

method (Tri 1979) Detailed Ar–Ar dating, however,

indi-cates metamorphic ages of 245 million years for the Song

Ma ophiolites (Lepvrier et al 1997) Recently, Hieu et al.

(2008) have reported U–Pb zircon ages between 254 and

262 million years for the Huoi Hao meta-basalt, the Chieng

Khuong granite, and the Song Ma granite along this suture

zone Such Permian to Triassic magmatic rocks are widely

distributed in northwest Vietnam, especially along the Song

Ma zone and the Truong Son zone, and provide evidence

for the closure of the Palaeo-Tethyan ocean and/or the

collision between the South China and Indochina blocks

(e.g Lepvrier et al 1997, 2004, 2008) during the late

Palaeozoic to early Mesozoic period

Analytical techniques

Rock samples, each of about 5 kg, were crushed for mineral separation using standard procedures including wet shak-ing table, magnetic separation, and heavy-liquid separation techniques More than 1000 zircon grains were obtained from two paragneisses and one migmatite Zircons of different sizes, colours, and morphologies were further hand-picked under a binocular microscope, mounted in epoxy resin, and polished to expose the grain interi-ors Prior to the U–Pb dating and Hf isotopic analysis, cathodoluminescence (CL) imaging of zircon grains has been undertaken to verify internal structures and for guid-ance in choice of analytical spot sites Electron microprobe analyses were performed on a CAMECA SX51 (CAMECA SAS, Gennevilliers, France) at the Institute of Geology and Geophysics, Chinese Academy of Sciences, using an accelerating voltage of 15 kV and a beam current of 15–20 nA

Zircon U–Pb dating was performed at the China University of Geoscience in Wuhan using the laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) technique (for detailed analytical techniques, see Liu

et al 2007) Analyses were done on an Agilent 7500

(Agilent Technologies, Inc., Santa Clara, CA, USA) with

a 193 nm ArF excimer laser Helium was used as the carrier gas to enhance transport efficiency of the ablated material The helium carrier gas inside the ablation cell was mixed with argon as a makeup gas before entering the ICP to maintain stable and optimum excitation condi-tions Analytical spot diameter was 40μm Analytical runs include a background acquisition of about 30 s and a signal acquisition time of about 80 s Standard silicate glass NIST SRM610 (National Institute of Standards and Technology, Gaithersburg, MA, USA) was used to calibrate and cal-culate U, Th, and Pb contents Raw data were processed using the GLITTER 4.0 software (Macquarie University, Macquarie Park, NSW, Australia) Zircon 91500 was used

as a reference standard Since large uncertainties are asso-ciated with the measurement of204Pb content by the LA-ICP-MS technique, common Pb correction was made using the EXCEL program ComPbCorr#3_151, assuming that the observed 206Pb/238U, 207Pb/235U, and 208Pb/232Th ratios of zircon can be inferred from the discordia curve

by a combination of lead loss at a defined time (Andersen 2002) Calculation of apparent U–Pb ages was done by the Isoplot software (Ludwig 2001) with 2σ errors.

Zircon Lu–Hf isotopic ratios were measured on

a Neptune multi-collector ICP-MS (Thermo Fisher Scientific, Waltham, MA, USA) at the Institute of Geology and Geophysics, Chinese Academy of Sciences The laser system delivers a beam of 193 nm UV light from a Geolas excimer laser ablation system Sampling spots had beam

diameters of 63 or 32 μm for small grains Ar and He

carrier gases were used to transport the ions derived from

the ablated material in the laser ablation cell via a

mix-ing chamber to the ICP-MS torch Analytical procedures

and the interference correction method are given in Xu

et al (2004) The output raw data of176Hf/177Hf were not

processed by isobaric interference corrections of176Lu on

176Hf due to extremely low176Lu/177Hf in zircon

(nor-mally <0.002) Isobaric interference of 176Yb on 176Hf

was, however, corrected using the mean fractionation index

proposed by Iizuka and Hirata (2005) The applied value

of 176Yb/172Yb was 0.5886 (Chu et al 2002; Vervoort

et al 2004) A175Lu/176Lu ratio of 0.02655 was used for

elemental fractionation correction An 176Hf/177Hf ratio

of 0.282295 ± 7 (2σ, n = 27) was obtained for zircon

91500, which is identical to values obtained by the

solu-tion method (Goolaerts et al 2004; Woodhead et al 2004).

InitialεHfvalues were calculated using chondritic Lu–Hf

data (Blichert-Toft and Albarède 1997) Parameters for the

calculation of depleted mantle model ages, TDM, are from

Griffin et al (2000).

Samples and analytical results

Two paragneiss samples and one migmatite were collected

from the Sin Quyen Formation about 40 km southeast of

Lao Cai town (coordinates 22◦0821 N 104◦1812E for

samples V0757 and V0758 and 22◦0900 N 104◦2029

E for sample V0761; Figure 2) The preferred orientation

of minerals can be observed both from a hand specimen and under the microscope (Figure 3) Sample V0758 was taken from a strongly migmatized gneiss section It con-tains quartz (∼50–60%), plagioclase (∼20–30%), biotite (∼5–10%), sillimanite (∼5–10%), and accessory phases, such as zircon, apatite, and opaque minerals This sam-ple contains abundant detrital zircons of different sizes that vary in colour from light to dark brown and have rounded habitus CL images of the zircons reveal variable internal structures (Figure 4), indicating complex origin and/or history Most of the grains preserve oscillatory

zoning of magmatic origin Despite strong migmatiza-tion, overgrowth and re-crystallization structures are hardly observed, especially in sample V0758

U–Pb analyses of 98 spots were performed on 85 zircon grains from the 3 samples Results are given in Table 1 and shown in 206Pb/238U versus 207Pb/235U concordia diagrams (Figure 5) Most of the zircons analysed give concordant or nearly concordant U–Pb apparent ages of about 1.8–3.0 thousand million years Part of them yield discordant and younger ages, indicating Pb loss during late overprint(s) Distribution histograms of207Pb/206Pb ages (Figure 6) show that these zircons mainly cluster in three age populations: late Palaeoproterozoic (∼1.8 thousand million years), early Palaeoproterozoic (∼2.2–2.5 sand million years), and Neoarchean (∼2.7–2.9 thou-sand million years) Zircons from sample V0757 record two phases of crystallization: one during the late Archaean and the other during the late Palaeoproterozoic (Figure 4)

Figure 2 Geological map of the study area showing the sample localities

Figure 3 Thin section photographs of rock samples (A) Sample V0757, crossed palars (B) Sample V0761, crossed palars (C and D) Sample V0758, crossed palars and plane-polarized light Qz: quartz; Bi: biotite; Si: sillimanite; Pl: plagioclase

Figure 4 Cathodoluminescence (CL) images of zircon grains

Table 1 Zircon U–Pb age data of zircons from the Sin Quyen Formation.

Sample (ppm) (ppm) Th/U 206Pb/238U 207Pb/235U 207Pb/206Pb 206Pb/238U 207Pb/235U 207Pb/206Pb

Paragneiss sample V0757

−1 334 1017 0.14 0.2911± 31 4.332± 46 0.1079± 11 1647± 15 1699± 9 1765± 9

−2 545 1531 0.15 0.3142± 33 4.838± 50 0.1117± 11 1761± 16 1792± 9 1827± 9

−3 277 1681 0.21 0.1422± 15 2.072± 22 0.1057± 11 852± 8 1103± 9 1639± 32

−4 194 576 0.20 0.2930± 31 4.400± 48 0.1089± 12 1653± 15 1699± 10 1755± 31

−5 371 1273 0.15 0.2550± 27 3.891± 42 0.1107± 12 1457± 14 1579± 10 1747± 30

−6 525 1502 0.17 0.3079± 32 4.733± 49 0.1115± 11 1730± 16 1773± 9 1824± 9

−7 404 1655 0.15 0.2162± 23 3.209± 33 0.1076± 11 1259± 12 1446± 9 1732± 29

−8 139 363 0.24 0.3281± 35 5.193± 57 0.1148± 12 1825± 17 1833± 11 1843± 31

−9 459 1252 0.15 0.3234± 34 5.055± 52 0.1134± 11 1803± 16 1817± 9 1833± 28

−10 456 1422 0.15 0.2841± 30 4.334± 46 0.1106± 11 1610± 15 1692± 9 1796± 29

−11 591 1740 0.14 0.3027± 32 4.682± 49 0.1121± 11 1705± 16 1764± 9 1834± 9

−12 458 1295 0.14 0.3138± 33 4.847± 51 0.1120± 11 1757± 16 1782± 10 1812± 28

−13 430 1369 0.13 0.2810± 30 4.254± 49 0.1098± 12 1595± 15 1680± 10 1788± 30

−14 492 1593 0.15 0.3152± 33 4.887± 51 0.1124± 12 1763± 16 1786± 10 1813± 29

−15 509 1490 0.13 0.3051± 32 4.738± 49 0.1126± 11 1717± 16 1774± 9 1842± 9

−16 238 647 0.22 0.3207± 33 4.960± 53 0.1122± 12 1793± 16 1813± 9 1835± 9

−17 563 1553 0.19 0.3183± 33 5.038± 53 0.1148± 12 1779± 16 1817± 10 1861± 29

−18 444 1333 0.14 0.2981± 31 4.618± 48 0.1124± 11 1682± 15 1752± 9 1838± 9

−19 149 203 0.65 0.5206± 56 14.79± 7 0.2061± 23 2679± 25 2762± 14 2823± 31

−20 452 1107 0.04 0.3395± 35 8.403± 88 0.1795± 18 1880± 17 2265± 9 2634± 24

−21 237 660 0.02 0.3160± 35 5.142± 74 0.1180± 17 1766± 17 1826± 12 1894± 33

−22 615 1280 0.13 0.3994± 42 8.510± 89 0.1546± 16 2163± 19 2278± 10 2384± 26

−23 616 1797 0.08 0.2909± 30 5.766± 60 0.1438± 14 1645± 15 1939± 9 2270± 26

−24 124 173 0.56 0.5302± 55 14.30± 15 0.1956± 20 2742± 23 2770± 10 2790± 8

−25 450 1389 0.18 0.3110± 32 4.841± 50 0.1129± 11 1746± 16 1792± 9 1846± 9

−26 262 534 0.15 0.3992± 42 9.270± 98 0.1684± 17 2160± 19 2351± 10 2522± 26

−27 133 194 0.56 0.5250± 56 13.95± 16 0.1928± 21 2720± 24 2746± 11 2766± 8

−28 226 408 0.15 0.4541± 48 10.09± 11 0.1611± 17 2412± 21 2439± 10 2462± 26

−29 160 248 0.31 0.4947± 52 12.95± 14 0.1898± 20 2582± 23 2657± 11 2716± 27

−30 185 302 0.17 0.4910± 51 11.93± 12 0.1760± 18 2572± 22 2591± 10 2606± 26

−31 763 2626 0.21 0.2327± 24 5.073± 52 0.1582± 16 1342± 13 1807± 10 2397± 27

−32 46 59 0.61 0.5569± 77 15.98± 32 0.2080± 42 2854± 32 2875± 19 2890± 16

−33 356 1039 0.16 0.2869± 30 4.593± 49 0.1161± 12 1617± 15 1710± 10 1826± 29

−34 587 13330 0.52 0.0372± 04 0.537± 06 0.1048± 11 235± 2 437± 4 1711± 9

−35 445 1707 0.25 0.2158± 22 3.281± 34 0.1103± 11 1260± 12 1477± 8 1804± 9

−36 541 1448 0.15 0.3146± 33 4.889± 51 0.1127± 12 1758± 16 1777± 10 1801± 29

−37 426 1716 0.23 0.2698± 28 4.380± 49 0.1177± 13 1522± 14 1630± 12 1773± 33

−38 460 1243 0.13 0.3128± 33 4.941± 54 0.1145± 12 1750± 16 1791± 10 1839± 29

Paragneiss sample V0761

−1 207 218 0.63 0.4262± 44 8.683± 92 0.1478± 15 2288± 20 2305± 10 2320± 8

−2 110 136 0.45 0.3952± 42 7.735± 91 0.1420± 17 2141± 20 2183± 13 2224± 33

−3 187 215 0.32 0.4271± 45 8.693± 100 0.1476± 17 2292± 20 2306± 10 2319± 9

−4 623 1632 0.52 0.1846± 19 2.439± 26 0.0958± 10 1076± 11 1150± 13 1291± 45

−5 661 1475 0.22 0.2299± 24 3.446± 36 0.1087± 11 1334± 12 1515± 8 1778± 9

−6 455 570 0.17 0.4046± 44 7.919± 103 0.1419± 18 2190± 20 2222± 12 2251± 10

−7 735 1480 0.68 0.2352± 24 3.748± 39 0.1156± 112 1361± 13 1582± 8 1889± 9

−8 274 272 0.53 0.4654± 48 10.46± 11 0.1631± 17 2463± 21 2477± 10 2488± 8

−9 112 130 0.51 0.4119± 47 8.088± 123 0.1424± 22 2223± 21 2241± 14 2257± 12

−10 131 158 0.43 0.4040± 56 7.745± 185 0.1391± 34 2187± 26 2202± 21 2215± 23

−11 150 234 0.03 0.3335± 35 5.649± 61 0.1228± 13 1854± 16 1919± 9 1989± 26

−12 182 275 0.04 0.3450± 36 5.742± 65 0.1207± 13 1910± 17 1936± 9 1963± 27

−13 593 954 0.22 0.3130± 32 5.631± 59 0.1305± 13 1749± 16 1899± 10 2067± 28

−14 242 288 0.25 0.4206± 45 8.577± 107 0.1479± 18 2263± 20 2294± 11 2322± 10

−15 453 676 0.24 0.3370± 35 6.146± 70 0.1323± 15 1870± 17 1990± 11 2116± 29

(Continued)

Table 1 (Continued).

Sample (ppm) (ppm) Th/U 206Pb/238U 207Pb/235U 207Pb/206Pb 206Pb/238U 207Pb/235U 207Pb/206Pb

Migmatite sample V0758

−1 730 2332 0.31 0.2540± 26 4.433± 46 0.1266± 13 1453± 14 1691± 11 2000± 30

−2 306 516 0.35 0.4643± 48 10.428± 11 0.1628± 17 2458± 21 2473± 10 2484± 8

−3 160 197 1.01 0.5431± 58 15.12± 17 0.2019± 23 2783± 27 2800± 17 2812± 35

−4 236 698 0.09 0.2884± 30 5.349± 56 0.1345± 14 1631± 15 1868± 9 2144± 26

−5 96 124 0.67 0.5546± 61 15.68± 20 0.2050± 26 2844± 25 2857± 12 2867± 9

−6 235 387 0.74 0.4410± 46 9.288± 102 0.1528± 16 2355± 21 2367± 10 2377± 8

−7 341 572 0.40 0.4619± 48 10.22± 11 0.1604± 17 2448± 21 2454± 10 2460± 8

−8 157 250 0.11 0.5117± 57 13.29± 18 0.1884± 25 2664± 24 2700± 13 2728± 10

−9 248 568 0.12 0.3681± 38 6.901± 72 0.1360± 14 2015± 18 2082± 10 2149± 27

−10 131 200 0.10 0.5281± 55 14.58± 16 0.2003± 21 2732± 23 2786± 10 2826± 25

−11 105 282 0.39 0.2974± 31 4.541± 49 0.1107± 12 1667± 16 1689± 12 1717± 35

−12 333 773 0.73 0.3175± 33 4.918± 51 0.1124± 11 1767± 17 1763± 16 1759± 41

−13 183 293 0.09 0.5091± 54 12.96± 15 0.1847± 20 2650± 22 2672± 10 2688± 26

−14 287 812 0.02 0.3121± 32 4.933± 52 0.1146± 12 1751± 15 1807± 9 1873± 26

−15 228 332 0.37 0.5202± 55 13.93± 16 0.1942± 21 2689± 24 2724± 12 2750± 28

−16 333 598 0.34 0.4337± 45 9.632± 100 0.1611± 16 2313± 20 2376± 11 2432± 28

−17 31 43 0.96 0.4758± 51 12.34± 15 0.1881± 23 2468± 25 2545± 18 2607± 39

−18 118 154 0.59 0.5456± 57 15.30± 16 0.2034± 21 2787± 25 2799± 13 2808± 29

−19 91 105 1.14 0.5549± 59 15.77± 17 0.2061± 22 2846± 24 2863± 11 2875± 8

−20 42 62 0.66 0.4776± 51 12.40± 14 0.1884± 21 2491± 23 2583± 15 2656± 33

−21 212 328 0.32 0.4919± 51 13.26± 14 0.1956± 20 2571± 22 2684± 11 2770± 26

−22 372 906 0.58 0.3115± 32 4.850± 50 0.1129± 11 1739± 17 1755± 14 1775± 37

−23 466 772 0.77 0.4359± 45 9.141± 95 0.1521± 16 2332± 20 2352± 10 2370± 8

−24 143 236 0.30 0.4768± 49 10.80± 11 0.1642± 17 2513± 22 2506± 10 2500± 8

−25 47 60 0.32 0.5883± 63 18.54± 21 0.2286± 25 2983± 25 3018± 11 3042± 8

−26 414 980 0.87 0.3138± 33 4.797± 50 0.1109± 11 1759± 16 1784± 9 1814± 9

−27 540 1334 0.74 0.3070± 32 4.759± 51 0.1124± 12 1719± 17 1751± 16 1789± 41

−28 164 471 0.01 0.3169± 33 4.954± 53 0.1134± 12 1775± 16 1811± 9 1852± 27

−29 319 517 0.14 0.5087± 54 13.44± 15 0.1916± 24 2646± 23 2702± 11 2744± 26

−30 336 925 0.03 0.3254± 34 5.250± 54 0.1170± 12 1808± 16 1829± 9 1852± 27

−31 183 333 0.23 0.4558± 48 10.06± 11 0.1601± 16 2421± 21 2441± 10 2457± 8

−32 333 595 0.36 0.4533± 47 9.936± 102 0.1590± 16 2410± 21 2429± 9 2445± 8

−33 312 1383 0.05 0.2774± 29 4.150± 49 0.1085± 13 1576± 14 1656± 9 1758± 29

−34 69 88 0.49 0.5899± 63 18.45± 20 0.2269± 24 2989± 25 3014± 11 3030± 8

−35 641 1058 1.03 0.4347± 45 8.939± 92 0.1492± 15 2327± 20 2332± 9 2336± 8

−36 98 112 1.16 0.5496± 58 15.39± 17 0.2031± 22 2823± 24 2839± 10 2851± 8

−37 268 689 0.49 0.3184± 33 4.968± 54 0.1132± 12 1782± 16 1814± 9 1851± 9

−38 225 543 0.25 0.5098± 69 15.20± 29 0.2163± 42 2656± 29 2828± 18 2954± 15

−39 74 122 0.54 0.4641± 49 12.19± 14 0.1905± 21 2458± 22 2619± 10 2746± 8

−40 25 29 0.81 0.5427± 59 15.03± 18 0.2009± 24 2795± 25 2817± 11 2834± 9

−41 286 582 0.73 0.4334± 45 8.983± 92 0.1503± 15 2321± 20 2336± 9 2350± 8

−42 288 562 0.10 0.4607± 48 11.54± 12 0.1817± 19 2443± 21 2568± 10 2668± 8

−43 458 967 0.33 0.3919± 40 7.822± 80 0.1448± 15 2125± 19 2191± 11 2254± 29

−44 183 478 0.16 0.4124± 43 8.085± 88 0.1422± 15 2226± 20 2241± 10 2254± 8

−45 43 60 1.01 0.5502± 68 15.38± 20 0.2027± 27 2826± 26 2839± 13 2848± 10

∗Radiogenic Pb.

It is widely argued that magmatic and metamorphic

zircons can be distinguished by their Th/U ratio Magmatic

zircons from various rock types commonly have high Th/U

ratios, while the zircons grown during metamorphism show

low Th/U ratios (<0.1; e.g Williams and Claesson 1987;

Kinny et al 1990) The analysed zircon grains of the three

gneiss samples have variable Th/U ratios ranging from

0.02 to 1.14, but about 90% of them have Th/U ratios >0.1

(Table 1), indicating a magmatic origin, consistent with the evidence from the internal structures shown in the CL images (Figure 4)

Fifty-four zircon grains of sample V0758 were anal-ysed for Lu–Hf isotopic composition Analytical data are given in Table 2 Initial Hf ratios were calculated back to the ages obtained from U–Pb dating for those grains on which U–Pb and Lu–Hf analysis were equally performed

Figure 5 Concordia diagrams showing zircon analyses from (A) migmatitic gneiss sample V0758; (B) paragneiss sample V0757; and (C) paragneiss sample V0761

16

12

~1.8 Ga

2.2–2.5 Ga

2.7–2.9 Ga

Number 8

4

0

Figure 6 Histogram of 207Pb/206Pb isotopic ages of zircon

grains of three gneiss samples showing three age peaks around

1.8, 2.2–2.5, and 2.7–2.9 thousand million years

and back to 1800 or 2500 million years for grains without

U–Pb ages All analytical spots yield Hf model ages older

than 2.5 thousand million years Hafnium TDM1 ages of

detrital zircons range from about 2600 to 3400 million

years with age peaks around 2700, 3000, and 3300

mil-lion years (Figure 7A), while hafnium TDMages vary from about 2900 to 4000 million years and are mostly older than 3200 million years (Figure 7B) 176Hf/177Hf ratios

of the zircons vary significantly from 0.28083 to 0.28136, corresponding to initialεHfvalues in the range−20.4 and 6.2, but largely between−14 and −5 (Figure 7C) In the

εHf(t) versus age diagram (Figure 7D), it can be observed

that some 2.7–3.0 Ga detrital zircons have positiveεHf(t)

values, while all late Archaean to late Palaeoproterozoic detrital zircons have negativeεHf(t) values and plot below

the evolution line of 3.0 Ga old crustal material

Discussion

It has been shown that combined U–Pb and Lu–Hf in situ analyses on zircon can provide time-resolved information

on juvenile crust formation and recycling of ancient crustal

material (Kemp et al 2006; Zhang et al 2006; Yu et al.

2007) The U–Pb age spectrum in Figure 6 testifies to strong Neoarchean and Palaeoproterozoic magmatic activ-ity in northwestern Vietnam In the plot zircon U–Pb crys-tallization age versus Hf isotope composition (Figure 7D), the majority of zircons lie below the 3.0 Ga crustal evolu-tion line The negative εHf(t) values (Figure 7C) indicate

that magmatism was characterized by recycling of ancient crust and the source of the magmas experienced a long

Table 2 Zircon Hf isotopic composition of sample V0758.

Spot no

Age (million years) 176Lu/177Hf 176Hf/177Hf Error (2σm) 176Hf/177Hf(t) εHf (t) Error (2σ)

TDM1 (million years)

TDMC (million years) fLu/Hf