Mesozoic plutons of the Yidun Arc, SW China: U/Pb geochronology and Hf isotopic signature

1 (2007) 88–106www.elsevier.com/locate/oregeorev

Ore Geology Reviews 3

Mesozoic plutons of the Yidun Arc, SW China: U/Pb geochronologyand Hf isotopic signature

Anthony Reid a,⁎, Christopher J.L. Wilson a, Lui Shunb,Norman Pearson c, Elena Belousova c

a School of Earth Sciences, The University of Melbourne, Victoria, 3010, Australiab Department of Geology, Chengdu University of Technology, Chengdu, 56000, People's Republic of China

c ARC National Key Centre for Geochemical Evolution and Metallogeny of Continents (GEMOC), Department of Earth and Planetary Sciences,Macquarie University, NSW, 2109, Australia

Received 2 April 2003; accepted 18 November 2004Available online 12 June 2006

Abstract

The Yidun Arc is a Triassic volcanic arc located between the Songpan Garzê Fold Belt and the Qiangtang Block, southwest China.To constrain the age of a number of the major granitic plutons from the Yidun Arc, laser ablation ICP-MSU/Pb analysis of zircon wasconducted. Hafnium isotope data was also acquired through laser-ablation multicollector ICPMS analysis of zircon, with the aim ofgaining insight into the age and nature of the source region of the plutons. Three age groups have been identified from seven granitesamples: Early–Middle Triassic (∼ 245 to 229 Ma), Late Triassic (∼ 219 to 216 Ma) and Cretaceous (∼ 105 to 95 Ma). Hafniumanalysis shows the Triassic granites to have negative and variable εHf values and Mesoproterozoic (∼ 1.6 Ga) depleted-mantle modelages, which is interpreted to reflect derivation from an isotopically heterogeneous, largely crustal source. The Cretaceous graniteshows higher and less variable εHf values and slightly younger model ages (∼ 1.3 Ga), and is interpreted to be derived frommelting ofa more homogeneous crustal source. A depleted-mantle model age of∼ 1.5 Ga is calculated from the pooled Triassic and Cretaceoussamples. The source region for these magmas may be tentatively correlated with Mesoproterozoic material of the Yangtze Craton,which has been suggested to underlie the Yidun Arc; however, further work is necessary to demonstrate this suggestion.© 2006 Elsevier B.V. All rights reserved.

Keywords: Yidun Arc; U/Pb geochronology; Hf isotopes; Laser ablation-ICPMS analysis; Southwestern China

1. Introduction

A series of volcanic arcs developed between theYangtze Craton and the Qiangtang Block as these blocksmoved northwards towards Eurasia during the Permo–

⁎ Corresponding author. Current address: Geological SurveyBranch—Minerals and Energy Division, Primary Industries and ResourcesSouth Australia (PIRSA), GPO Box 1671, Adelaide SA 5001, Australia.Tel.: +61 8 8463 3039; fax: +61 8 8226 3200.

E-mail address: [email protected] (A. Reid).

0169-1368/$ - see front matter © 2006 Elsevier B.V. All rights reserved.doi:10.1016/j.oregeorev.2004.11.003

Triassic (Sengör, 1984; Yin and Harrison, 2000). Thelargest of these volcanic arc complexes is the YidunVolcanic Arc (Fig. 1). Despite its size, location relative tothese major crustal components of the Asian region, andits significance as part of the Sanjiang (Three Rivers)Tethyan Metallogenic Domain (Hou et al., 2007-thisvolume; Zaw et al., 2007-this volume), the Yidun Arcremains poorly documented in the international litera-ture. Bound to the west by the Jinsha Jiang Suture, and tothe east by the Garzê–Litang Suture, the Yidun Arc iscomposed of Triassic calc-alkaline volcanics with

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http://dx.doi.org/10.1016/j.oregeorev.2004.11.003

Fig. 1. Simplified stratigraphic map of the Yidun Arc (modified from Li et al., 1996), showing location of U/Pb geochronology samples. Underlinedsamples indicate those utilized for both U/Pb and Hf analysis. The Yidun Arc is bound by the Garzê–Litang Suture to the east and the Jinsha JiangSuture to the west. Inset shows location of the Yidun Arc relative to major tectonic blocks of Asia, in addition to the Tibetan Plateau within which theYidun Arc now lies. Note the Yangtze Craton is part of the South China Craton. Pluton abbreviations: BP = Baimaxue Shan Pluton; CP = Chuer ShanPluton; DP = Daocheng Pluton; GP = Garzê Pluton; HP = Haizi Pluton; SP = Suwalong Pluton.

89A. Reid et al. / Ore Geology Reviews 31 (2007) 88–106

intercalated flysch that overlie variably metamorphosedPalaeozoic rocks (Fig. 1). Voluminous granite andgranodiorite plutons occupy around 10% to 20% of thepresent outcrop and intrude deformed Palaeozoic andTriassic volcano-sedimentary sequences across theYidun Arc (Fig. 1). Obtaining accurate constraints onthe nature and age of these granites has particularsignificance for our understanding of the tectonics andmetallogeny of this northern Palaeotethyan domain.

Available radiometric age data for the granites of theYidun Arc suggest that there was more than one phase ofgranite intrusion (Hou et al., 2001a). K/Ar ages showconsiderable range, from Permian and Late Triassic toTertiary ages as young as 33.5 Ma, with large age dif-ferences present within a single pluton (SichuanBureau ofGeology and Mineral Resources, c. 1981). This apparentlack of consistency suggests the K/Ar data set should beinterpreted with caution. More robust age data comes

from a handful of granites in the west of the Yidun Arcthat show Early to Late Triassic Rb/Sr whole rock ages(Xu et al., 1992;Wang et al., 2000). Younger Rb/Sr wholerock ages between 77 and 85 Ma have also been reportedfrom granites of the central Yidun Arc (Guan, 1999). Thislimited data set shows a spectrum of largely Mesozoicages; however, the relationship between the timing ofdeformation and granitic plutonism across the Yidun Arcremains unclear.

Furthermore, the Late Triassic arc volcanism of theYidun Arc developed above thinned continental crust(Hou and Mo, 1993). It has been suggested that thiscontinental crust may be Proterozoic crystalline base-ment derived from the Yangtze Craton (Chang, 1997).Proterozoic basement rocks outcrop in the LongmenShan to the east (Fig. 1; Burchfiel et al., 1995; Chen andWilson, 1996; Zhou et al., 2002a; Li et al., 2003),although none has been identified within the Yidun

Table 1Description of samples used in this study

Sample Pluton Lat/long Rock description

457⁎ Suwalong 29.13; 99.07 Medium grained biotite granite;post-tectonic

460 n/a 29.10; 99.06 Medium grained, foliatedbiotite–hornblende granodiorite;late syn-tectonic

456b n/a 29.37; 99.07 Muscovite–pegmatite; foldedwith surrounding lecucocraticgneisses; late syn-tectonic

YA13 BaimaxueShan

28.44; 98.94 Medium grained K-feldspar-rich granite; post-tectonic

406b Garzê 31.62; 99.72 Medium grained, biotite–hornblende K-feldspargranodiorite; post-tectonic

YA05⁎ Daocheng 29.46; 100.19 Medium–coarse grained,biotite–hornblende K-feldspargranodiorite; post-tectonic

426 ChuerShan

31.94; 98.30 Medium grained biotite granite;post-tectonic

YA32⁎ Haizi 30.26; 99.44 Medium grained biotite granite;post-tectonic

Samples marked ⁎ used for both U/Pb and Hf isotope analysis.

90 A. Reid et al. / Ore Geology Reviews 31 (2007) 88–106

Arc. In addition, isotopic data from a number of plutonssuggests some may be mantle-derived and others to be ofcrustal origin (e.g., Hou, 1993; Wang et al., 2000).Constraints the age of the source material for the granitesof the Yidun Arc and their relationship to a possibleProterozoic crystalline basement are lacking.

This contribution presents the results of a combinedU/Pb and preliminary Hf isotope study of zircons from anumber of the major plutons of the Yidun Arc by laserablation ICPMS. The aim of this study has been two-fold: firstly, to constrain the emplacement ages of syn-and post-tectonic granitoids of the Yidun Arc, andsecondly, to infer the likely nature and age of the sourceregion for selected plutons. The results indicate thatintrusive phases of the Yidun Arc show considerable agevariation and that Mesoproterozoic crustal rocks are thelikely source material for the magmas.

2. Geological setting

The oldest rocks currently exposed in the Yidun Arcare Palaeozoic metasedimentary rocks that outcrop inthe western Yidun Arc that are commonly described asconstituting a ‘microcontinent’ known as the ZhongzaMassif (Fig. 1; e.g., Chang, 1997). These carbonate-richPalaeozoic successions have similar fossil assemblages tosediments and metasediments exposed in the LongmanShan Thrust Nappe Belt to the east (Chang, 1997). TheZhongza Massif is therefore thought to have rifted fromthe Yangtze Craton during the opening of the Garzê–Litang Ocean in the middle to late Palaeozoic, possiblyassociated with a major Permian phase regional extension(Zhang et al., 1998). The Palaeozoic metasediments of theZhongza Massif are separated from the Triassic coversequences of the Qiangtang Block to the west by theJinsha Jiang Suture (Fig. 1). Subduction along the JinshaJiang Suture is variably considered to have been west(e.g.,Mo et al., 1994) or east-dipping (Reid et al., 2005) inthe vicinity of the Yidun Arc. The Jinsha Jiang Suture isinterpreted to have closed by the Middle Triassic (e.g.,Wang et al., 2000; Zhong, 2000).

Overlying the Palaeozoic of the Yidun Arc are Triassicsequences of clastic and intercalated arc volcanic rocks.The latter formed in response to westwards subduction ofthe so-called ‘Garzê–Litang Ocean’ during the LateTriassic (Chen et al., 1987; Hou and Mo, 1991; Hou,1993). Late Triassic bimodal volcanic suites and arc-typevolcanic rocks of the Yidun Arc are host to numerous syn-and epigenetic sulphide accumulations (Hou et al., 2003a),including the polymetallic Gacun volcanic-hosted mas-sive sulphide deposit (Hou and Mo, 1993; Hou et al.,2001b).

The Yidun Arc and its large eastern neighbor, theSongpanGarzê Fold Belt were deformed in the regionallyextensive deformation known as the Indosinian Orogeny,that resulted from the terminal accretion of the QiangtangBlock to the southern margin of Eurasia during the LateTriassic–Jurassic (e.g., Xu et al., 1992; Dirks et al., 1994;Burchfiel et al., 1995; Zhou and Graham, 1996;Harrowfield andWilson, 2005; Reid et al., 2005). Graniticplutons were emplaced into the (meta)sedimentary pileduring and after Indosinian deformation across the region.Since the collision of India with Asia, the Yidun Archas been incorporated into the eastern Tibetan Plateau(Fig. 1).

3. Samples

Across the Yidun Arc, three major N–S trending beltsof granitic plutons are observed (Fig. 1). In order to pro-vide a constraint on potential age variations between thesebelts, and to also constrain the timing of deformationwithin theYidunArc, eight samples of granitoids from theYidun Arc were collected for zircon U/Pb age determi-nation (Fig. 1; Table 1). These are: the Suwalong Pluton(457); a smaller granodiorite body (460) and a coarse-grained muscovite pegmatite (456b) both located near tothe Suwalong Pluton; the Baimaxue Shan Pluton (YA13),the Garzê Pluton (406b), the Daocheng Pluton (YA05),the Chuer Shan Pluton (426) and theHaizi Pluton (YA32).All samples, except 460 and 456b, are massive medium

Fig. 2. Cathodoluminescence images of selected zircons, showing the location of laser ablation sites from samples 457, 460, YA05, YA32 and 456b.Solid line represents U/Pb analysis, dashed line Hf analysis. Note samples 457, YA05 and YA32 show magmatic zonation with very little apparentcore material, whereas sample 460 shows strong metamictization. Also note the presence of needle shaped grain in sample YA32; no systematic agedifferences were found between needle shaped grains and more tabular shaped grains in this sample.


to coarse grained granitoids that intrude deformed andvariably metamorphosed country rock and are thus con-sidered to post-date the main ductile deformation event inthe Yidun Arc. Sample 460 preserves a foliation definedby aligned biotite and hornblende that is parallel to theaxial surface of late stage folds within a medium to high-grade metamorphic complex along the Jinsha Jiang Su-ture, and is thus considered to be syn-tectonic. Similarly,

sample 456b is a muscovite-bearing pegmatite intrusionthat is folded by late stage folds within the same high-grade metamorphic complex and is likewise syn-tectonic.

4. Methodology

Zircon separates for LAM-ICPMS U/Pb and Hfisotope analysis were obtained from crushed 1 to 2 kg-


size samples of fresh rock, using standard heavy liquidand magnetic separation techniques. Grains were handpicked under the binocular microscope and mountedinto epoxy resin discs that were then polished so as tojust expose the grains. Mean zircon grain size variedfrom 200 to 50 μm. The majority of zircons wereeuhedral and either tabular or needle shaped, exceptthose from sample 456b which were dominantly sub-rounded to rounded grains. Cathodoluminescence ima-ges, acquired for all analyzed grains, reveal small androunded cores overgrown by rims that showed oscilla-tory zonation, as is typical of magmatic zircon (Fig. 2).Laser ablation microprobe (LAM) analyses targetedmagmatic rim portions of the grains that were, whereverpossible, free of inclusions or cracks.

4.1. U/Pb age determination

U/Pb analyses were performed using the UV LAM-inductively coupled plasma mass spectrometer (ICPMS)at the Geochemical Analysis Unit, GEMOCNational KeyCenter, Macquarie University, Sydney, Australia. Thesystem comprises a New Wave Merchantek LUV213laser ablation system coupled to a Hewlett Packard 4500sICPMS. Operating conditions of the LAM-ICPMS aresimilar to those given by Belousova et al. (2001) and gavetypical ablation pit diameter of approximately 40 μm.Such spatial resolution enabled analysis of rims (mag-matic overgrowths) without contamination by olderinherited cores. Standards used were gem quality zircons;an in-house zircon, GJ-1, as the primary referencestandard and the Mud Tank and 91500 zircons asindependent controls on data accuracy and precision.Analytical runs consisted of analyses of ten unknownsamples bracketed by two or three analyses of the GJ-1standard and one each of the Mud Tank and 91500

Table 2Comparison of U/Pb data for standards used in this study with data obtained

Zircon No. of analyses 207Pb

GJ-1Corfu (TIMS)a 608This study 54 6

Mud TankKnudsen et al., 2001 (LA) 42 7Belousova et al., 2001 (LA) 23 7Black and Gulson, 1978 (TIMS) 5 7This study 17 7

91500Belousova et al., 2001 (LA) 30 10Wiedenbeck et al., 1995 (TIMS) 11 1065This study 14 10

All ages are quoted at the 2σ level. aTIMS analyses done at the University o

zircons. 206Pb/238U ages for the standards analyzed in thisstudy compared with those analyzed in previous studieshave ages lying within analytical error (Table 2). TheICPMS utilized is a quadropole mass spectrometer thatrecords counts per second for the five isotopes, 206Pb,207Pb, 208Pb, 232Th and 238U, by rapidly moving betweenpeaks for each isotope providing a ‘quasi-simultaneous’measurement of the five isotopes. A visual time-resolvedisotope abundance trace is displayed on the on-lineICPMS software, which permits identification of isotopicheterogeneity, such as may exist between rim and core.

Raw data was processed using the in-house on-linesoftware GLITTER which permits selection of optimaltime intervals from the analyses, enabling the screeningof the output trace for irregular isotope ratios such asmay occur if for example, inclusions are also ablated orcracks encountered in the ablation process. Agescalculated by GLITTER are 207Pb/206Pb, 206Pb/238U,207Pb/235U and 208Pb/232Th, where 235U is calculatedtheoretically using 235U=238U/137.88. A correction forthe presence of common lead and lead loss was thenapplied using a Microsoft Excel spreadsheet templatefollowing the methods of Andersen (2002). The cor-rection for common lead attempts to account for, andtherefore exclude from the age calculations, the per-centage of lead present within the raw analyses that isnon-radiogenic.

Methods such as secondary ionisation mass spectro-metry derive common lead corrections through mea-surement of non-radiogenic 204Pb. The LAM-ICPMStechnique, however, does not measure this isotope due toisobaric interference with the mercury contaminant inargon gas used in the analysis line (Griffin et al., 2000).Themethod ofAndersen (2002) overcomes this potentiallimitation of the LAM-ICPMS technique through thesolution of simultaneous equations that link the

from previous studies

/206Pb 206Pb/238U 207Pb/235U 208Pb/232U

.5±0.8 – – –06±7 602±3.8 603±3 610±20

24±12 731±3 729±3 722±1528±17 728±16 730±6 –34±10 – – –37±20 721±12 721±10 713±12

63±10 1060±7 1060±5 –.4±0.6 – – –61±14 1064±28 1064±16 1067±44

f Oslo by F. Corfu (unpublished data).


radiogenic and non-radiogenic lead of the total leadcontent, the initial crystallization age and a specified leadloss ‘event’ (t2). Since no information is available tosuggest that a lead loss event has occurred in any of thezircons used in this study, the age of the hypothetical leadloss event was varied between 200 Ma and 0 Ma in50 million year intervals to minimise the systematic errorin the resulting crystallization ages. It was found that themost consistent results were obtained if t2 was set to zero,the recommended value for most Phanerozoic zircons(Andersen, 2002). After application of the correctionprocedure, most analyses were observed to haveexperienced less than 1% lead loss and to typicallycontain less than 1% common lead, with all except twocontaining b5%. Concordia and inverse concordia plots,the latter of which are useful in discriminating the effectsof common lead or lead loss from individual analyses,along with intercept ages for corrected isotope ratioswere then processed using the software Isoplot 2.32(Ludwig, 2000). Weighted mean ages quoted herein areat the 95% confidence level and with error valuesreported at to the 2σ level.

4.2. Hafnium isotope signature

The 176Hf/177Hf ratio and depleted mantle model age(TDM) of an igneous mineral or rock yields informationon the composition of the magmatic source and the timeat which the precursor material for the sample departedfrom the bulk Earth Hf chondritic uniform reservoir(CHUR) (e.g., Patchett et al., 1981; Patchett, 1983;Stille and Steiger, 1991). Zircon is a particularly usefulmineral for determining Hf TDM model ages due to itshigh Hf content, durability and resistance to Hfcontamination (Patchett et al., 1981) and, as in thisstudy, the availability of U/Pb crystallization ages fromthe same grain. In this study three samples used for U/Pbanalysis, 457, YA05 and YA32, were selected for apreliminary Hf isotope investigation. These samplesoutcrop along an east–west ‘section’ across the YidunArc (see Fig. 1) and thus any potential spatial variationin source material for granitoids across the arc may bedetected in the Hf isotope data.

The 176Hf/177Hf ratio for individual zircon crystalswas determined using laser ablation microprobe linkedto a multicollector ICPMS unit also at the GeochemicalAnalysis Unit, GEMOC National Key Center, followingthe methods of Griffin et al. (2002) and Knudsen et al.(2001). The system utilized combines a New WaveUP193A Excimer laser ablation microprobe with a NuPlasma MC-ICPMS. A time resolved analytical trace isdisplayed for each analysis and allows selection or

exclusion of inclusions or heterogeneity due to REEzoning within the zircon. Analysis of both Mud Tankand 91500 standard zircons was done at the beginningand end of each analytical run of 10 to 20 unknowns toprovide a control on data accuracy and precision. In-dividual zircon grains from the three samples that pro-duced concordant U/Pb ages and that were of suitablesize were selected for Hf analysis. Laser ablation pitsites were around 40 μm wide and located within aregion of magmatic zoning equivalent to that analyzedfor U/Pb dating. From the selected trace integrated177Hf/176Hf, 176Lu/177Hf and 176Yb/177Hf ratios werecalculated, from which an epsilon (ε; parts per 104

deviation from the chondritic 176Hf/177Hf evolutioncurve) Hf value was calculated based on the decayconstant for Lu of 1.94×10−11 yr−1 (Patchett et al.,1981; Tatsumoto et al., 1981). Importantly, for Phaner-ozoic zircons the present day 176Hf/177Hf is indistin-guishable from the initial 176Hf/177Hf (Stille and Steiger,1991) and measured values are therefore used in εHf andTDM calculations. Depleted mantle model ages arecalculated based on the measured 176Hf/177Hf comparedto a model depleted mantle with a present day 176Hf/177Hf=0.28325 and 176Lu/177Hf=0.0384 (after Griffinet al., 2002).

5. Results

5.1. U/Pb data

The U/Pb ages for seven of the eight samples fall intoone of three groups: Middle Triassic (∼ 245 to 229 Ma),Late Triassic (∼ 219 to 216 Ma) or Cretaceous (∼ 105 to95 Ma). Table 3 presents the common lead corrected agedata and Table 4 provides a summary of the U/Pb ageresults. Generally 206Pb/238U ages are considered themost reliable for zircons of this age range, while the207Pb/235U ages are less precise owing to the theoreticalcalculation. Concordia and inverse concordia (Tera-Wasserberg) plots are presented in Fig. 3. The eighthsample, 456b, showed a spectrum of ages from Archeanto Triassic and is discussed separately.

(1) Early Triassic ages were obtained from samples457, 460 and YA13 (Table 3). Both samples 457 andYA13 are from quartz–feldspar–biotite post-tectonicgranites located along the Jinsha River in the west of theYidun Arc (Fig. 1). A concordia plot of 20 analyses fromsample 457 shows a distribution of concordant toslightly negatively discordant ellipses, that range in agefrom ∼ 232 to ∼ 267 Ma (Fig. 3a). The large range inages for this sample probably reflects the incorporation

Table 3Common lead corrected U/Pb analytical results for each sample

Sample U–Th–Pb ratios Ages (Ma)

Analysis 207Pb/206Pb

1σ 207Pb/235U

1σ 206Pb/238U

1σ 208Pb/232Th

1σ 207/206 1σ 207/235

1σ 206/238

1σ 208/232

1σ

4571 0.05105 0.001 0.27072 0.0044 0.03846 0.0005 0.01062 0.0005 243 48 243 4 243 3 214 102 0.05099 0.0011 0.26716 0.0047 0.038 0.0005 0.01083 0.0002 240 50 240 4 240 3 218 33 0.05082 0.0005 0.25999 0.0031 0.0371 0.0005 0.01102 0.0001 233 13 235 2 235 3 222 34 0.05081 0.0011 0.25718 0.0045 0.03671 0.0005 0.01076 0.0002 232 51 232 4 232 3 216 45 0.05323 0.001 0.28221 0.0037 0.03845 0.0005 0.01193 0.0003 338 41 252 3 243 3 240 66 0.05228 0.0011 0.26842 0.0046 0.03724 0.0005 0.01123 0.0004 298 49 241 4 236 3 226 77 0.05167 0.0012 0.26069 0.0052 0.03659 0.0005 0.01146 0.0004 271 56 235 4 232 3 230 88 0.0518 0.0005 0.2645 0.0031 0.03703 0.0004 0.01153 0.0001 277 12 238 2 234 3 232 39 0.05254 0.001 0.27464 0.0042 0.03791 0.0005 0.01185 0.0002 309 45 246 3 240 3 238 510 0.05101 0.0019 0.26844 0.0094 0.03816 0.0005 0.01042 0.0011 241 89 241 8 241 3 210 2211 0.0533 0.0009 0.27938 0.0034 0.03801 0.0004 0.01188 0.0002 342 39 250 3 241 3 239 412 0.05121 0.0005 0.2912 0.0034 0.0412 0.0005 0.01151 0.0001 250 12 260 3 260 3 231 313 0.05176 0.001 0.28403 0.0042 0.0398 0.0005 0.01215 0.0004 275 44 254 3 252 3 244 714 0.05127 0.001 0.283 0.0045 0.04003 0.0005 0.00973 0.0003 253 46 253 4 253 3 196 715 0.05089 0.001 0.26165 0.0042 0.03729 0.0004 0.01013 0.0003 236 46 236 3 236 3 204 716 0.05335 0.0016 0.31086 0.0084 0.04226 0.0005 0.01209 0.0006 344 68 275 6 267 3 243 1117 0.05106 0.0005 0.27147 0.0031 0.03853 0.0004 0.01127 0.0001 244 12 244 2 244 3 227 318 0.05239 0.0005 0.28126 0.0031 0.03891 0.0004 0.01236 0.0001 302 12 252 2 246 3 248 319 0.051 0.001 0.26767 0.0043 0.03806 0.0004 0.0108 0.0004 241 46 241 3 241 3 217 720 0.05245 0.0008 0.2837 0.0033 0.03923 0.0004 0.01227 0.0002 305 37 254 3 248 3 246 5

4601 0.06349 0.002 0.37244 0.0106 0.04254 0.0005 0.01133 0.0009 725 68 321 8 269 3 228 192 0.05094 0.002 0.2644 0.0098 0.03764 0.0005 0.01163 0.0006 238 92 238 8 238 3 234 133 0.05808 0.0013 0.30085 0.006 0.03757 0.0004 0.01113 0.0002 533 51 267 5 238 3 224 54 0.05135 0.0008 0.27017 0.0032 0.03816 0.0004 0.0119 0.0004 256 38 243 3 241 3 239 85 0.05141 0.0008 0.27003 0.0032 0.0381 0.0004 0.01197 0.0003 259 38 243 3 241 3 241 6

YA131 0.05102 0.0012 0.26862 0.0051 0.03819 0.0005 0.01242 0.0003 242 54 242 4 242 3 249 62 0.05107 0.0006 0.2721 0.0038 0.03867 0.0005 0.01279 0.0002 244 14 244 3 245 3 257 33 0.05227 0.0014 0.27751 0.0063 0.03851 0.0005 0.01206 0.0003 297 60 249 5 244 3 242 54 0.05262 0.0006 0.27375 0.0037 0.03778 0.0005 0.01187 0.0002 312 14 246 3 239 3 239 35 0.05342 0.0006 0.28328 0.0039 0.03847 0.0005 0.01216 0.0002 347 14 253 3 243 3 244 36 0.05102 0.0014 0.26861 0.0067 0.03819 0.0005 0.01173 0.0003 242 66 242 5 242 3 236 77 0.05108 0.0013 0.27237 0.0057 0.03867 0.0005 0.01233 0.0003 245 58 245 5 245 3 248 58 0.05102 0.0006 0.27427 0.0038 0.03899 0.0005 0.01228 0.0002 242 14 246 3 247 3 247 39 0.05131 0.0006 0.27499 0.0037 0.03888 0.0005 0.01229 0.0002 255 14 247 3 246 3 247 310 0.05134 0.0006 0.27645 0.0039 0.03905 0.0005 0.01215 0.0002 256 14 248 3 247 3 244 311 0.05095 0.0013 0.26513 0.0057 0.03774 0.0005 0.01358 0.0007 239 59 239 5 239 3 273 1312 0.05119 0.0007 0.2758 0.004 0.03907 0.0005 0.01257 0.0002 249 15 247 3 247 3 252 413 0.05099 0.0014 0.26693 0.0062 0.03797 0.0005 0.01217 0.0003 240 63 240 5 240 3 244 514 0.05135 0.001 0.27238 0.0056 0.03858 0.0005 0.01202 0.0003 257 24 245 5 244 3 242 515 0.0517 0.0007 0.2761 0.0041 0.03872 0.0005 0.01267 0.0002 272 15 248 3 245 3 254 416 0.05071 0.0006 0.27465 0.0038 0.03928 0.0005 0.01203 0.0002 228 14 246 3 248 3 242 317 0.05076 0.0008 0.27563 0.0044 0.03944 0.0005 0.01211 0.0002 230 17 247 4 249 3 243 418 0.05149 0.0007 0.28166 0.0039 0.03969 0.0005 0.01217 0.0002 263 14 252 3 251 3 244 319 0.05002 0.001 0.27163 0.0057 0.03966 0.0005 0.01146 0.0003 196 25 244 5 251 3 230 520 0.05019 0.0007 0.27338 0.0042 0.03957 0.0005 0.01218 0.0002 204 16 245 3 250 3 245 4

406b1 0.05519 0.0013 0.24755 0.0058 0.03254 0.0004 0.0103 0.0002 420 31 225 5 206 2 207 32 0.05036 0.0011 0.23396 0.0051 0.03364 0.0004 0.01018 0.0002 212 30 213 4 213 2 205 3


Table 3 (continued )



1σ 207Pb/235U

1σ 206Pb/238U

1σ 208Pb/232Th

1σ 207/206 1σ 207/235 1σ 206/238 1σ 208/232 1σ

406b3 0.05274 0.0007 0.24208 0.0034 0.0333 0.0004 0.01058 0.0001 318 15 220 3 211 2 213 24 0.05062 0.0012 0.2463 0.0049 0.03529 0.0004 0.00985 0.0002 224 54 224 4 224 2 198 45 0.05968 0.0013 0.28596 0.0061 0.03478 0.0004 0.01089 0.0002 592 26 255 5 220 3 219 36 0.05132 0.0011 0.24046 0.0042 0.03398 0.0004 0.01066 0.0002 255 48 219 3 215 2 214 47 0.0505 0.0014 0.23973 0.006 0.03443 0.0004 0.00948 0.0002 218 65 218 5 218 2 191 48 0.05112 0.0006 0.24497 0.0032 0.03476 0.0004 0.01071 0.0001 246 14 222 3 220 2 215 29 0.05073 0.0006 0.24334 0.0032 0.0348 0.0004 0.01055 0.0001 229 14 221 3 221 2 212 210 0.05105 0.0009 0.24351 0.0045 0.03461 0.0004 0.01048 0.0001 243 22 221 4 219 2 211 311 0.05072 0.0006 0.24236 0.0032 0.03466 0.0004 0.01037 0.0001 228 14 220 3 220 2 209 212 0.05018 0.0006 0.23846 0.0031 0.03447 0.0004 0.01052 0.0001 203 14 217 3 218 2 212 213 0.05079 0.0007 0.24617 0.0032 0.03516 0.0004 0.0105 0.0001 231 14 223 3 223 2 211 214 0.05035 0.0007 0.24063 0.0036 0.03468 0.0004 0.01016 0.0001 211 16 219 3 220 2 204 215 0.0507 0.0007 0.24314 0.0033 0.03479 0.0004 0.01012 0.0001 227 14 221 3 220 2 204 216 0.05206 0.0008 0.24225 0.0039 0.03375 0.0004 0.01047 0.0001 288 18 220 3 214 2 211 317 0.05047 0.0016 0.23809 0.0068 0.03421 0.0004 0.00964 0.0003 217 73 217 6 217 2 194 518 0.05048 0.0012 0.23847 0.0048 0.03426 0.0004 0.00986 0.0001 217 54 217 4 217 2 198 219 0.05099 0.001 0.24115 0.0045 0.03431 0.0004 0.0103 0.0001 240 24 219 4 217 2 207 320 0.05051 0.0025 0.23997 0.0114 0.03446 0.0004 0.00903 0.0005 218 115 218 9 218 3 182 10

YA051 0.05033 0.001 0.23022 0.004 0.03317 0.0004 0.00954 0.0002 210 48 210 3 210 2 192 32 0.05315 0.0012 0.25138 0.0051 0.03431 0.0004 0.01042 0.0002 335 53 228 4 217 2 209 43 0.05385 0.0006 0.25591 0.003 0.03447 0.0004 0.01075 0.0001 365 12 231 2 218 2 216 24 0.05328 0.0012 0.26029 0.005 0.03543 0.0004 0.01087 0.0002 341 51 235 4 224 2 219 45 0.05513 0.001 0.26005 0.0045 0.03422 0.0004 0.01077 0.0002 417 20 235 4 217 2 217 46 0.05042 0.0011 0.2349 0.0042 0.03379 0.0004 0.0103 0.0002 214 49 214 3 214 2 207 47 0.05133 0.0012 0.24322 0.0049 0.03436 0.0004 0.01077 0.0002 256 54 221 4 218 2 217 58 0.05045 0.001 0.2368 0.0041 0.03404 0.0004 0.01015 0.0002 216 48 216 3 216 2 204 39 0.06239 0.003 0.30824 0.014 0.03583 0.0006 0.01023 0.0006 687 105 273 11 227 3 206 1210 0.05137 0.0011 0.23815 0.0049 0.03364 0.0004 0.0106 0.0002 257 26 217 4 213 2 213 411 0.05527 0.001 0.25519 0.0047 0.0335 0.0004 0.01039 0.0002 423 22 231 4 212 2 209 412 0.05408 0.0011 0.25716 0.0045 0.03449 0.0004 0.01072 0.0002 374 48 232 4 219 2 216 413 0.05045 0.0013 0.23702 0.0056 0.03407 0.0004 0.00995 0.0002 216 62 216 5 216 2 200 414 0.0542 0.0012 0.24805 0.0046 0.03319 0.0004 0.01035 0.0002 379 50 225 4 211 2 208 515 0.05039 0.0012 0.23343 0.0051 0.0336 0.0004 0.01021 0.0002 213 58 213 4 213 2 205 316 0.05039 0.0014 0.23343 0.0057 0.0336 0.0004 0.00983 0.0002 213 64 213 5 213 2 198 417 0.05061 0.001 0.2458 0.0041 0.03522 0.0004 0.00864 0.0002 223 46 223 3 223 2 174 318 0.05179 0.0007 0.23912 0.0031 0.03351 0.0004 0.01056 0.0001 276 13 218 3 212 2 212 319 0.0559 0.0013 0.26157 0.0059 0.0339 0.0004 0.0107 0.0003 448 29 236 5 215 3 215 620 0.05342 0.0013 0.25392 0.0055 0.03448 0.0004 0.01077 0.0003 347 57 230 4 218 2 217 5

4261 0.04855 0.0006 0.11446 0.0015 0.01711 0.0002 0.0053 7E-05 126 14 110 1 109 1 107 12 0.04933 0.002 0.11118 0.0045 0.01636 0.0003 0.00544 0.0002 164 62 107 4 105 2 110 43 0.06688 0.002 0.15298 0.0039 0.01659 0.0002 0.00501 0.0002 834 62 145 3 106 1 101 34 0.0488 0.0009 0.11193 0.0022 0.01664 0.0002 0.0052 0.0001 138 24 108 2 106 1 105 25 0.04813 0.0013 0.10964 0.0027 0.01652 0.0002 0.00507 8E-05 106 65 106 2 106 1 102 26 0.05032 0.002 0.11396 0.0043 0.01643 0.0003 0.00524 0.0002 210 58 110 4 105 2 106 47 0.04811 0.0011 0.10871 0.0022 0.01639 0.0002 0.00517 0.0001 105 56 105 2 105 1 104 28 0.05147 0.0012 0.1136 0.0026 0.01601 0.0002 0.00511 0.0001 262 29 109 2 102 1 103 29 0.05334 0.0013 0.12437 0.0026 0.01691 0.0002 0.00527 0.0001 343 57 119 2 108 1 106 210 0.07997 0.0051 0.29472 0.0181 0.02673 0.0004 0.00427 0.0004 1196 129 262 14 170 3 86 811 0.04814 0.0012 0.11024 0.0023 0.01661 0.0002 0.00444 0.0001 106 58 106 2 106 1 90 212 0.04956 0.0013 0.11341 0.0026 0.0166 0.0002 0.00523 0.0001 174 60 109 2 106 1 105 213 0.0481 0.0012 0.1079 0.0023 0.01627 0.0002 0.00501 8E-05 104 59 104 2 104 1 101 214 0.04798 0.008 0.1014 0.0169 0.01533 0.0002 0.00415 0.0005 98 308 98 16 98 1 84 9

(continued on next page)





1σ 207Pb/235U

1σ 206Pb/238U

1σ 208Pb/232Th

1σ 207/206 1σ 207/235 1σ 206/238 1σ 208/232 1σ

15 0.05276 0.0016 0.11806 0.0034 0.01623 0.0002 0.00495 0.0001 318 72 113 3 104 1 100 216 0.04809 0.0016 0.10755 0.0034 0.01622 0.0002 0.00518 0.0002 104 79 104 3 104 1 105 317 0.04891 0.0009 0.11087 0.002 0.01645 0.0002 0.00517 8E-05 144 20 107 2 105 1 104 218 0.04982 0.0011 0.11466 0.0023 0.01669 0.0002 0.00521 0.0001 187 55 110 2 107 1 105 219 0.04863 0.0012 0.10399 0.0025 0.01553 0.0002 0.00482 0.0001 130 32 100 2 99 1 97 220 0.04802 0.0018 0.10375 0.0037 0.01567 0.0002 0.00438 0.0001 100 85 100 3 100 1 88 221 0.0488 0.002 0.10659 0.0041 0.01584 0.0002 0.00445 0.0002 138 94 103 4 101 1 90 322 0.04811 0.0011 0.10873 0.0022 0.01639 0.0002 0.00433 8E-05 105 57 105 2 105 1 87 223 0.04955 0.0006 0.10841 0.0015 0.01588 0.0002 0.00501 6E-05 174 14 105 1 102 1 101 124 0.04994 0.0012 0.11464 0.0024 0.01665 0.0002 0.0052 0.0002 192 57 110 2 106 1 105 425 0.04808 0.0022 0.10722 0.0047 0.01617 0.0002 0.00507 0.0002 103 103 103 4 103 1 102 326 0.04813 0.0012 0.1097 0.0024 0.01653 0.0002 0.00425 0.0001 106 58 106 2 106 1 86 327 0.04809 0.0011 0.10755 0.0021 0.01622 0.0002 0.00539 9E-05 104 53 104 2 104 1 109 2

YA321 0.04871 0.0005 0.10089 0.0012 0.01512 0.0002 0.0049 6E-05 134 12 98 1 97 1 99 12 0.05233 0.0023 0.10644 0.0044 0.01475 0.0002 0.00443 0.0003 300 102 103 4 94 1 89 63 0.04783 0.001 0.09371 0.0017 0.01421 0.0002 0.00427 0.0001 91 51 91 2 91 1 86 24 0.04778 0.002 0.09094 0.0036 0.0138 0.0002 0.00439 0.0002 88 94 88 3 88 1 89 35 0.04898 0.0014 0.09414 0.0025 0.01394 0.0002 0.00438 0.0001 147 69 91 2 89 1 88 26 0.05683 0.0021 0.1443 0.0049 0.01842 0.0003 0.00557 0.0002 485 83 137 4 118 2 112 37 0.05238 0.0008 0.1058 0.0017 0.01466 0.0002 0.00466 6E-05 302 17 102 2 94 1 94 18 0.04975 0.0006 0.10027 0.0014 0.01464 0.0002 0.00462 7E-05 183 14 97 1 94 1 93 19 0.04787 0.0017 0.09592 0.0032 0.01453 0.0002 0.00438 0.0002 93 81 93 3 93 1 88 310 0.05131 0.0014 0.10263 0.0027 0.01451 0.0002 0.00458 0.0001 255 35 99 3 93 1 92 211 0.04952 0.0006 0.10808 0.0014 0.01582 0.0002 0.00507 7E-05 173 14 104 1 101 1 102 112 0.04934 0.0006 0.10222 0.0014 0.01502 0.0002 0.00479 7E-05 164 14 99 1 96 1 97 113 0.04786 0.0033 0.09519 0.0064 0.01442 0.0003 0.00392 0.0002 92 155 92 6 92 2 79 414 0.04644 0.0009 0.09541 0.002 0.0149 0.0002 0.00461 0.0001 21 24 93 2 95 1 93 215 0.05016 0.0017 0.10274 0.0031 0.01486 0.0002 0.00467 0.0002 202 79 99 3 95 1 94 316 0.04796 0.0016 0.10039 0.0031 0.01518 0.0002 0.00492 0.0002 97 76 97 3 97 1 99 417 0.04797 0.0015 0.10123 0.003 0.0153 0.0002 0.00452 0.0002 98 74 98 3 98 1 91 418 0.04792 0.0017 0.09858 0.0032 0.01492 0.0002 0.00413 0.0002 95 80 95 3 95 1 83 319 0.04792 0.0023 0.09826 0.0045 0.01487 0.0002 0.00527 0.0002 95 107 95 4 95 1 106 320 0.04791 0.0045 0.09807 0.009 0.01484 0.0003 0.00534 0.0004 95 208 95 8 95 2 108 8

456b1 0.06599 0.0029 1.21163 0.0507 0.13316 0.0018 0.03937 0.0009 806 94 806 23 806 10 781 182 0.06415 0.0008 1.08544 0.0136 0.12276 0.0014 0.03595 0.0004 747 12 746 7 746 8 714 83 0.15681 0.0016 9.6587 0.1066 0.44687 0.005 0.11957 0.0013 2422 9 2403 10 2381 22 2283 234 0.05622 0.0008 0.57342 0.0081 0.07397 0.0009 0.02241 0.0003 461 14 460 5 460 5 448 65 0.11282 0.0012 5.20553 0.0582 0.33466 0.0038 0.09584 0.0011 1845 9 1854 10 1861 18 1850 196 0.13644 0.0016 7.40453 0.0905 0.39358 0.0046 0.11288 0.0015 2182 10 2162 11 2139 21 2162 287 0.10341 0.0017 3.47979 0.0402 0.24405 0.0028 0.07094 0.0016 1686 30 1523 9 1408 14 1385 318 0.10008 0.0023 3.1058 0.0684 0.2251 0.0033 0.06656 0.0014 1626 21 1434 17 1309 17 1302 279 0.11398 0.0012 4.76242 0.0544 0.30307 0.0034 0.08447 0.0011 1864 9 1778 10 1707 17 1639 2010 0.06555 0.0008 1.09997 0.0137 0.12173 0.0014 0.03689 0.0006 792 12 753 7 741 8 732 1211 0.10523 0.0026 4.43211 0.0961 0.30547 0.0037 0.07671 0.0014 1718 47 1718 18 1718 18 1494 2612 0.1242 0.0013 5.42849 0.0616 0.31721 0.0035 0.08763 0.0012 2017 9 1889 10 1776 17 1698 2213 0.11339 0.0012 4.97115 0.0556 0.31809 0.0035 0.08918 0.001 1854 9 1814 9 1780 17 1727 1914 0.10517 0.0021 4.42607 0.0715 0.30524 0.0033 0.0875 0.0011 1717 37 1717 13 1717 17 1695 2115 0.11412 0.0011 5.01849 0.0539 0.31901 0.0035 0.09093 0.0009 1866 9 1822 9 1785 17 1759 1716 0.13094 0.0013 6.75274 0.0732 0.37418 0.0041 0.10687 0.0011 2111 9 2080 10 2049 19 2052 2017 0.11403 0.0012 5.30141 0.0574 0.33731 0.0037 0.096 0.001 1865 9 1869 9 1874 18 1853 1818 0.11261 0.0012 5.14373 0.0597 0.33165 0.0037 0.09412 0.0011 1842 9 1843 10 1846 18 1818 20





1σ 207Pb/235U

1σ 206Pb/238U

1σ 208Pb/232Th

1σ 207/206 1σ 207/235 1σ 206/238 1σ 208/232 1σ

19 0.05084 0.001 0.25876 0.0044 0.03691 0.0004 0.0111 0.0002 234 47 234 4 234 2 223 320 0.0787 0.0017 2.14824 0.0385 0.19798 0.0022 0.05926 0.0017 1164 43 1164 12 1164 12 1164 3221 0.06085 0.0018 0.64241 0.0178 0.07657 0.0009 0.02349 0.0006 634 67 504 11 476 6 469 1122 0.11388 0.0012 4.71653 0.0541 0.30062 0.0033 0.08343 0.0011 1862 9 1770 10 1694 17 1620 2123 0.062 0.0012 0.94246 0.0154 0.11024 0.0012 0.02875 0.0004 674 43 674 8 674 7 573 724 0.05108 0.0013 0.27222 0.0061 0.03865 0.0005 0.00973 0.0003 244 60 244 5 244 3 196 5

Common lead correction after Andersen (2002).


in some of the laser analyses of older, core material. As aresult, determination of a single concordia age isimpossible. Excluding two analyses, 457-12 and 457-16, that lie outside the analytical error, a weighted mean206Pb/238U age of 240.9±6.2 Ma, (MSWD=4.3), iscalculated for this sample and is thus considered torepresent the main phase of zircon crystallization in theSuwalong Pluton.

Zircons from sample 460 showed strong metamictiza-tion (see Fig. 2) and consequently, of the grains mountedand imaged by cathodoluminescence, only a few showedregions of magmatic zonation that were consideredpotentially reliable for U/Pb age determination. Of thefive analyses carried out on zircons from sample 460,three are concordant and two show normal discordanceand a concordia age produced from this plot, 235±18Ma(MSWD=7.1; Fig. 3b). Up to 5% common lead wasdetected in the correction routine for the three discordantanalyses (Table 3). Excluding analysis 460-1, whichshows anomalously high percentage of common lead(∼ 6%) and is strongly discordant, aweightedmean age of239.5±5.8 Ma (MSWD=0.33) is taken to be the age ofzircon crystallization within this sample.

Table 4Summary of U/Pb results for each sample

Sample Weighted mean(Ma±2σ)

Lower interceptage (Ma±1σ)

Upper interceptage (Ma±1σ)

457 240.9±6.2(excl. 12 and 16)

232±14 2732±1200

460 239.5±5.8 (excl. 1) 235±18 –YA13 245.2±3.4 246.1±1.5 –406b 218.5±3.0

(excl. 1,3,5)213±10 1547±1600

YA05 215.2±3.4 (excl. 9) 210.3±5.5 2138±800426 104.7±2.0

(excl. 10 and 14)104.3±1.1(excl. 10 and 14)

–

YA32 94.4±2.4 (excl. 6) 94.3±1.8 (excl. 6) –

Analyses excluded from age calculations are indicated in brackets.

Zircons from sampleYA13were large grains (∼ 100 to200 μm in length) that showed strong magmatic zonation,minimal inclusions and very little inherited core. Thestrongly concordant analyses from sample YA13 range inage from∼ 239 to∼ 251 Ma and define a lower interceptage of 246.1±1.6 Ma (MSWD=0.89; Fig. 3c). This ageis identical within error to the weighted mean age of245.2±3.4 Ma (MSWD=1.5), which is considered a re-liable estimate for zircon crystallization in sample YA13.

(2) Late Triassic ages were recorded by zircons fromsamples 406b and YA05. These samples are from theGarzê and Daocheng hornblende-bearing quartz–feld-spar–biotite granodiorite plutons outcropping in the eastof the Yidun Arc (Fig. 1). These granitoids are the mostvoluminous in present outcrop and like the previoussamples are massive, preserving no tectonic foliation.

The concordia plot for the 20 analyses of sample 406bshowsmainly concordant analyses, with three ellipses thatshow normal discordance (Fig. 3d). A lower intercept ageof 213±10 Ma (MSWD=2.6) is calculated from thisconcordia plot. Each of the discordant points, analyses406b-1, 406b-3 and 406b-5, show a systematic age-discordance pattern of 207Pb/206PbN 207Pb/235UN 206Pb/238UN 208Pb/232Th, with particularly anomalous 207Pb/206Pb ages between 300 and 600Ma, typical of grains thathave suffered lead loss after crystallization (Table 3). Aweighted mean average calculated omitting these threepoints yields an age of 218.5±3.0 Ma (MSWD=2.1),slightly higher than the concordia age for all twentyanalyses, which in view of the effect of lead loss on thethree outliers, is considered a more reliable estimate of thetiming of zircon crystallization within this sample.

A concordia plot for sample YA05 shows a clusterof concordant analyses, with a small number ofanalyses showing slight normal discordance (Fig. 3e).The lower-intercept concordia age from this plot is210.3±5.5 Ma (MSWD=2.7). The large outlying

Fig. 3. Selected concordia and inverse concordia diagrams for the eight samples; (a) 457, (b) 460, (c) YA13, (d) 406b, (e) YA05, (f) 426, (g) YA32, (h)456b. Inverse Concordia plots are shown for samples 426 and YA32 to highlight the common Pb present within the outlying analyses. Sample 456b isshown also as a cumulative probability diagram superimposed onto a histogram for the twenty four analyses. See text for discussion.


Table 5Results of Hf isotope analysis on samples 457, YA05 and YA32

Sample # 176Hf/177Hf 1s 176Lu/177Hf 1s 176Yb/177Hf 1s 206/238 Ma εHf 1s TDM Ga

457 1 0.282468 0.000011 0.001645 0.000067 0.066122 0.002100 243 −5.5 0.4 1.582 0.282480 0.000015 0.001144 0.000017 0.039117 0.000520 240 −5.1 0.5 1.553 0.282475 0.000013 0.000858 0.000023 0.027967 0.000470 235 −5.3 0.5 1.565 0.282486 0.000009 0.000953 0.000031 0.031770 0.001300 243 −4.8 0.3 1.536 0.282591 0.000011 0.000295 0.000019 0.009553 0.000840 236 −1.1 0.4 1.307 0.282418 0.000010 0.001330 0.000039 0.050000 0.001200 232 −7.5 0.4 1.6910 0.282477 0.000012 0.000711 0.000008 0.024045 0.000450 241 −5.1 0.4 1.5511 0.282456 0.000006 0.001255 0.000016 0.043799 0.000470 241 −5.9 0.2 1.6015 0.282464 0.000014 0.002106 0.000041 0.073082 0.001300 236 −5.9 0.5 1.5917 0.282464 0.000014 0.002106 0.000041 0.073082 0.001300 244 −5.7 0.5 1.5918 0.282483 0.000012 0.001113 0.000033 0.036019 0.001100 246 −4.8 0.4 1.5420 0.282424 0.000009 0.000966 0.000008 0.030305 0.000400 248 −7.5 0.3 1.68

YA05 2 0.282540 0.000014 0.001930 0.000130 0.067184 0.004500 217 −3.6 0.5 1.443 0.282447 0.000013 0.000940 0.000013 0.032197 0.000340 218 −6.7 0.5 1.635 0.282501 0.000019 0.001094 0.000048 0.039364 0.001700 217 −4.8 0.7 1.516 0.282623 0.000019 0.000936 0.000005 0.032548 0.000150 214 −0.5 0.7 1.257 0.282462 0.000023 0.000956 0.000030 0.033620 0.001200 218 −6.2 0.8 1.608 0.282503 0.000009 0.001095 0.000033 0.037391 0.000890 216 −4.8 0.3 1.5110 0.282450 0.000019 0.001275 0.000008 0.044732 0.000300 213 −6.7 0.7 1.6312 0.282612 0.000014 0.002252 0.000066 0.081658 0.002400 219 −1.0 0.5 1.2813 0.282540 0.000013 0.001528 0.000056 0.054145 0.001700 216 −3.5 0.5 1.4315 0.282471 0.000018 0.000942 0.000034 0.030888 0.001300 213 −5.9 0.6 1.5819 0.282516 0.000013 0.001694 0.000033 0.055514 0.001300 215 −4.4 0.5 1.4920 0.282477 0.000009 0.000814 0.000017 0.027162 0.000590 218 −5.6 0.3 1.56

YA32 2 0.282712 0.000020 0.000839 0.000077 0.026780 0.002800 94 0.0 0.7 1.133 0.282634 0.000008 0.001260 0.000030 0.044945 0.001200 91 −2.9 0.3 1.307 0.282602 0.000020 0.000850 0.000026 0.031240 0.001100 94 −3.9 0.7 1.368 0.282107 0.000025 0.000452 0.000004 0.015827 0.000130 94 −21.4 0.9 2.4212 0.282659 0.000019 0.001177 0.000099 0.041394 0.003400 93 −2.0 0.7 1.2413 0.282663 0.000014 0.000686 0.000021 0.023673 0.000920 96 −1.7 0.5 1.2316 0.282654 0.000023 0.001599 0.000160 0.058105 0.005800 92 −2.2 0.8 1.2617 0.282607 0.000029 0.001274 0.000050 0.043619 0.001200 97 −3.7 1.0 1.3518 0.282608 0.000012 0.000656 0.000014 0.020709 0.000490 95 −3.7 0.4 1.3520 0.282643 0.000012 0.000894 0.000023 0.030411 0.000830 95 −2.5 0.4 1.28

Mud Tank 0.282528 0.000013 0.000156 0.000000 0.006202 0.0000250.282527 0.000013 0.000108 0.000000 0.004263 0.000027

91500 0.282327 0.000014 0.000317 0.000000 0.011001 0.000072

Each analysis is from the grain with corresponding U/Pb analysis number. Also shown for reference are analyses of the Mud Tank and 91500 zircons.


ellipse, analysis YA05-9, shows an anomalously high207Pb/206Pb age and has large errors associated witheach of the age calculations (Table 3). Although thisgrain showed no evidence of a core in cathodolumines-cence imaging, a prominent dark inclusion is observednear the ablation pit, which, in the third dimension mayhave been incorporated into ablated material, thusyielding an anomalously high amount of commonlead. Omitting this analysis yields a weighted meanage of 215.3±3.4 Ma (MSWD=2.8), which is consid-ered a more reliable estimate of the time of crystalli-zation of zircon in the Daocheng Pluton.

(3) Cretaceous ages were obtained from samples 426and YA32, from the Chuer Shan and Haizi granitic

plutons respectively, located in the central Yidun Arc(Fig. 1). No tectonic foliation was recognised withineither pluton.

An inverse concordia plot for the 27 analyses of sample426 shows most to be concordant and clustering around100Ma, and highlights twomain outliers, analyses 426-10and 426-14 (Fig. 3f, Table 3). Analyses 426-10 and 426-14 show anomalously high common lead components,which, at over 20% are the highest encountered in thisstudy, consequently even the common lead correctedvalues for these analyses are considered unreliable. Theinverse concordia plot that includes all 27 analyses yields aconcordia age of 102.3±2.1 Ma (MSWD=4.1). Omittingthe strongly discordant analyses 426-10 and 426-14 from

Fig. 4. Plot of εHf versus age (Ma) for samples from the Suwalong(457), Daocheng (YA05) and Haizi (YA32) Plutons. See text fordiscussion.


the data yields a lower intercept age of 104.3±1.1 Ma(MSWD=4.8) for the inverse concordia plot and a similarweighted mean age of 104.72±2.0 (MSWD=4.6), thelatter of which is taken to be the best estimate for the agefor zircon crystallization in sample 426.

The inverse concordia for sample YA32, shown tohighlight the influence of common Pb on the outlyinganalyses, presents a cluster of concordant analysesabout ∼ 100 Ma and a lower intercept age of 93.1±2.3(MSWD=2.5; Fig. 3g). The outlying analyses, YA32-6,in addition to an anomalously low 238U/206Pb ratio, hasan elevated 207Pb/206Pb age of 485±83 Ma (Table 3)suggesting common Pb is included within this analyses.The location of the ablation pit nearer the centre of thistabular grain suggests some core material may havebeen sampled, giving an unreliable mixed age betweenthe core and magmatic rim. Removing this outlier fromthe inverse concordia plot yields ages of a lower in-tercept age of 94.3±1.8 Ma (MSWD=2.1), which,being indistinguishable from the weighted mean age of94.4±2.4 (MSWD=4.8), is taken as the most reliableestimate for crystallization of zircon in sample YA32.

(4) Sample 456b yields a scatter of mostly concordantages, ranging from the Archean to Mesozoic (Table 3).Zircons from this sample were typically sub-rounded torounded, with some showing regions of magmaticzonation and others metamorphic overgrowths undercathodoluminescence (see Fig. 2). This spread of agesand the rounded zircon morphology imply sample 456bcontains inherited zircons scavenged from the sourceregion of the pegmatite intrusion. A small number of

euhedral prismatic zircons with magmatic zonation werealso located within this sample, which yield Mesozoicages; 234±2 and 244±3 Ma for analyses 456b-19 and456b-24 respectively (Table 3). These ages are similar tothe ages of both the post tectonic Suwalong Granite(sample 457) and that of the syn-tectonic granodioritelocated nearby (sample 460), confirming the timing ofdeformation in the western Yidun Arc to lie in this Earlyto Middle Triassic interval.

5.2. Hf isotope data

As all zircons analyzed for Hf from granites of theYidun Arc show concordant or nearly concordant U/Pbages, little pre- or post-emplacement contamination ofthe zircons by non-radiogenic Hf can be assumed to haveoccurred, hence the observed Hf ratios are taken to berepresentative of those of the parent melt (cf. Stille andSteiger, 1991). Sample 457 shows εHf values between−1.1 and −7.5, an average εHf value of −4.8 (Table 5;Fig. 4). Notably a single outlier (analysis 6) yields arelatively high εHf value of−1.1, only slightly lower thanthe CHUR reference value (Table 5; Fig. 4). Thecorresponding TDM model ages from sample 457 rangefrom 1.29 to 1.69 Ga and average 1.56 Ga (Table 5).Sample YA05 shows a similar range of εHf values (−0.5to −6.7) a similar average εHf value (−4.5) and virtuallyidentical TDM model ages, ranging from 1.25 to 1.63 Ga(average 1.49 Ga), to that of sample 457 (Table 5).

The Cretaceous sample YA32 yields a range of εHfvalues from 0.0 to −3.9, with a significant outlier(analysis 8) having a εHf value of −21.4 (Table 5). Thisvery negative εHf value reflects the ablation of an older,inherited zircon core in addition to the Cretaceous rim inthis analysis. As the age is a mixed signal between the rimand core analysis, this analysis is not considered further.Omitting analysis 8 from the data set gives an average εHfvalue of −2.5 for sample YA32, slightly more positivethan for the Triassic aged zircons. This trend ismirrored inthe younger TDM model ages, which range from 1.23 to1.36 Ga and average 1.28 Ga (Table 5).

6. Discussion

6.1. Triassic plutonism and closure of the Jinsha Jiangand Garzê–Litang sutures

The Palaeozoic metasedimentary rocks outcropping inthe western region of the Yidun Arc were deformed du-ring the subduction–accretion processes related to theclosure of the Jinsha Jiang Suture (e.g.,Wang et al., 2000).The timing of this deformation is constrained by Wang


et al. (2000) using a Rb–Sr whole rock analysis of afoliated granite that yields an isochron age of 238±18Ma.In this study U/Pb ages of syn- and post-tectonic graniteplutons that intrude these Palaeozoic metasedimentsyield similar, but more precise ages. The syn-tectonicgranodiorite, sample 460, with a mean 206Pb/238U age of239.5±5.8 Ma, is indistinguishable in age from both thepost-tectonic Suwalong Pluton, sample 457 at 240.9±3.1Ma, and the Baimaxue Shan Pluton, sample YA13 at245.2±3.4 Ma (Table 4). Thus, the emplacement of syn-and post-tectonic granitoids during the close of the Early-Middle Triassic constrains the cessation of deformationassociated with the accretion along Jinsha Jiang Suture,and thus the time at which the suture is considered to haveclosed (Fig. 6).

The easternmost granites, the Garzê and DaochengPlutons (samples 406b and YA05 respectively) bothrecord Late Triassic U/Pb ages. The timing of emplace-ment of these large granitic bodies into eastern YidunArc places a limit on the age of both the country rockitself and also of deformation within the surroundingvolcano-sedimentary pile. The ages of the plutons fromthe east of the Yidun Arc do not overlap with those of thewest, being spaced by some 10 to 20 million years. Thistime delay represents the interval during which a roughly5 km thick sequence of Late Triassic flysch and arcvolcanic rocks were deposited into the Yidun Arc (Fig. 6;Chen et al., 1987).

The data presented here clearly shows thatdeformation observed within the Triassic rocks of theYidun Arc occurred subsequent to the deformation thatis observed in the Palaeozoic metasediments exposedin the western region of the Yidun Arc. The U/Pb agesfor these granitoids therefore provide evidence ofcontinuous Triassic tectonic activity in the Yidun Arc,beginning with Early-Middle Triassic collision alongthe Jinsha Jiang Suture, followed by Middle-LateTriassic sedimentation and volcanism in the easternregion of the Yidun Arc and finally, Late Triassicdeformation of these Middle-Late Triassic volcano-sedimentary sequences and granite intrusion (see Reidet al., 2005). These constraints suggest that the Garzê–Litang Suture had closed by the Late Triassic as partof the progressive closure of the Songpan Garzê Basin,the northern-most relict of the Palaeotethys (Fig. 6).

6.2. Cretaceous plutons: a product of intra-continentalextension?

The youngest plutons sampled in the Yidun Arc arethe Cretaceous aged Chuer Shan (sample 426) and Haizi(sample YA32) plutons. These granites intrude deformed

Middle to Late Triassic sediments along a belt in thecentral Yidun Arc. Cretaceous plutons are also reportedfrom the Tibetan region including, the Songpan GarzêFold Belt (Roger et al., 2004), the central-easternQiangtang Block (Li et al., 1999), the south-easternLhasa Block (Li et al., 1999), the Transhimalaya orGangdese Batholiths of the central-western Lhasa Block(Schärer et al., 1984) and in the Kohistan and Lhadakregions to the north-west of the Himalaya (Searle et al.,1999). These Cretaceous magmas are generally inter-preted to have formed in either an Andean arc-typesetting or due to extension in the overriding plate, bothof which resulted from the subduction of the TethysOcean underneath the southern margin of Asia in theTertiary. Similarly, Cretaceous magmatism in southeastAsia, particularly the Western Granitoid Province ofThailand (Schwartz et al., 1995) and the Mogokmetamorphic belt in Myanmar (Barley et al., 2003), iscorrelated with intra-continental extension. It is sug-gested that the Cretaceous granites of the Yidun Arcmay also be the result of extension due to thenorthwards subduction of Tethys underneath the Asiancontinent.

6.3. Inherited zircon ages: correlations with the YangtzeCraton

Zircons from pegmatite sample 456b are interpreted tobe largely inherited grains based on their morphology andthe wide range in ages observed (see Section 5.1). Theseinherited zircons were apparently scavenged fromPalaeozoic metasediments that experienced partial melt-ing during the Early Triassic within the western region ofthe Yidun Arc. When plotted on a cumulative frequencydiagram the ages for the 24 analyses show fourmain peakscorresponding to ∼ 240, ∼ 500, ∼ 750 and ∼ 1800 Ma,with lesser peaks between ∼ 1160 and ∼ 1400 Ma (seeFig. 3h). The ∼ 750 Ma peak is similar to the age ofextensive Neoproterozoic magmatism within the westernYangtze Craton (Li et al., 2003), and the large peak at∼ 1800Ma also correlates with Mesoproterozoic materialdocumented from the Yangtze Craton (Sichuan Bureau ofGeology and Mineral Resources, 1991). Late Triassicsediments of the Songpan Garze Fold Belt also showpopulations of∼ 750 Ma and∼ 1800Ma detrital zircons,whichwere considered to reflect sediment derivation fromthe Yangtze Craton (Bruguier et al., 1997). Being similarto known ages from the Yangtze Craton, the spectrum ofinherited zircon ages present within sample 456b supportsfossil evidence that the Palaeozoic sediments of the YidunArc did indeed rift from the Yangtze Craton (Chang,1997). The ages from sample 456b represent a starting

Fig. 5. Plot of 176Hf/177Hf versus 206Pb/238U Age (Ma) for analyses from samples 457. Ya05 and YA32, showing location of Depleted Mantle Curve(assuming present day 176Hf/177Hf=0.28325 and 176Lu/177Hf=0.0384 after Griffin et al., 2000). The slope of the line between the analyses and thedepleted mantle curve is determined by the assumed value of the crustal 176Lu/177Hf ratio; in the case of the individual Triassic and Cretaceous ages, anaverage crustal value of 0.015 is used (e.g., Patchett et al., 1981). The Triassic samples, 457 andYA05 give similar TDMmodel ages andwhen combinedgive an average TDM model age of ∼ 1.6 Ga. The Cretaceous sample YA32, yields a slightly younger average TDM model age of ∼ 1.3 Ga. A pooledTDMmodel age for all samples is calculated if the crustal 176Lu/177Hf ratio is set to 0.018, giving a TDMmodel age of∼ 1.52 Ga. See text for discussion.


point for future detrital zircon comparison between theYidun Arc and the Yangtze Craton, which may furtherclarify the links between these two terranes.

6.4. Source regions of the plutons of the Yidun Arc

As 176Hf is partitioned into a silicate melt morestrongly than its parent isotope 176Lu, a melt derived frompreviously undepleted mantle will be more enriched in176Hf than the residue. Consequently the melt and residuecomponents will diverge in their 176Hf/177Hf ratio, withthe depleted mantle residue being more radiogenic andthus having a higher 176Hf/177Hf ratio, while the melt willhave a correspondingly lower 176Hf/177Hf ratio. Ingeneral the continental crust preserves a lower 176Hf/177Hf ratio than the mantle as a result of this fractionationprocess (Patchett et al., 1981). The zircons analyzed fromthe granites of the Yidun Arc all possess low 176Hf/177Hfand hence negative εHf values, and are inferred to bederived from a 177Hf enriched source such as may beexpected of the continental crust.

Both Triassic samples show a range of εHf valuessuggesting the samples were derived from source(s)that possess a non-uniform Hf composition. Non-uniform isotopic composition within a magma can bedeveloped in a number of ways including but notlimited to, progressive fractional-crystallization withinan evolving magma or, the mixing of two or moremagmas that possessed a different initial isotopiccomposition. An alternative cause of isotopic hetero-

geneity is the derivation of a magma from anisotopically heterogeneous source, such as a subductedsedimentary package (Woodhead et al., 2001).

Of these explanations for isotopic variability, simplefractional-crystallization is considered unlikely to causesuch variation in a population of zircon crystals thatpossess such homogeneous crystallization ages. Mag-matic mixing may have occurred as both crustal andmantle derived melts were present in the region duringthe Triassic, as suggested by the presence of crustal andmantle derived granites in the western Yidun Arc (Wanget al., 2000), and arc-magmatism during the Late Triassic(e.g., Chen et al., 1987). Irrespective of the mechanismthat produced the observed isotopic variability, thesimilarity of Hf composition of the Triassic plutonssuggests that despite their spatial separation anddifference in age, their source material was isotopicallysimilar and therefore potentially continuous underneaththe Yidun Arc.

The Cretaceous granite sample YA32, in contrast tothe Triassic samples, shows absolute εHf values closerto the CHUR reference value, suggesting there hasbeen less crustal input into these magmas or that therehas been the addition of a greater depleted mantle-derived component to the parent magma compared tothe Triassic samples. Hou et al. (2001a) report a rangeof initial 87Sr/86Sr ratios from the same granites andlikewise suggest a potential mixing of crust and mantlesource regions. The more limited range of less negativeεHf values also suggest that the pluton may be derived


from a more isotopically homogeneous source than theTriassic granites.

6.5. Hf model ages and possible Mesoproterozoicbasement underneath the Yidun Arc

The TDM model ages for the Triassic granites of theYidun Arc indicate that the source material for thesemelts departed from the CHUR reference during theMesoproterozoic ∼ 1.6 Ga (Table 5; Fig. 5). BothTriassic granites show a significant crustal componentin their εHf values and the TDM model ages cantherefore be interpreted as showing the approximateage of the continental crust from which the melts werederived. The continental source material for theTriassic granites may be either, the Palaeozoicmetasediments of the Zhongza Massif, the inferredcrystalline basement upon which these metasedimentslie, or, in the case of the Late Triassic plutons in theeast, the sedimentary material subducted along theGarzê–Litang Suture. The variability in the Hf isotopes

Fig. 6. Tectonic reconstruction of the Palaeotethyan region during the PermMetcalfe, 1996). Note that an alternative model for the geometry of subductionto which the reader is referred to for a detailed discussion of regional PermoGarzê–Litang Ocean, I = Indochina Block, JJS = Jinsha Jiang Suture, L =Sibumasu Block, SC = South China Craton, SGB = Songpan Garzê Basin, SGZ = Zhongza Massif. The Permian reconstruction (a) shows the Zhongza MaLitang Ocean, within the broader tectonic context of the Palaeotethys and Me(b), the Jinsha Jiang Suture had closed between the Qiangtang Block and the Zthis combined continental mass was also initiated during the Middle–Late Trand South China Block (including the Yangtze Craton) had initiated, thus formGarzê Fold Belt was derived (indicated by arrow; e.g., Zhou and Graham, 199Zhongza Massif–Sibumasu–Indochina–South China collage, closed the Garzregion of the Yidun Arc, and the Songpan Garzê Fold Belt at large (indicated band Garzê–Litang Suture during the Early and Late Triassic is purely schem

for these Triassic granites suggests magmatic mixingcannot be ruled out and hence the model ages mayrepresent the contamination of crustal material of olderProterozoic age by a younger mantle-derived meltgiving a mixed model age.

The Cretaceous zircons yield TDM model ages slightlyyounger than the Triassic samples (Table 5; Fig. 5), whichmay reflect the melting of a more juvenile source region.Pooling both Triassic and Cretaceous samples enables thecalculation of an average TDM model age of ∼ 1.5 Ga,based on a slightly higher source region 176Lu/177Hf valueof 0.018 (Fig. 5).We note that Roger et al. (1995) obtainedNd depletedmantlemodel ages ranging from1.2 to 1.5Gafrom the Konga Shan granite (Fig. 1), which theyinterpreted to reflect derivation from a mixed sourceincluding material from the Yangtze basement. Thecorrespondence of the Mesoproterozoic Hf TDM modelages for Triassic and Cretaceous plutons from the YidunArc, the presence of Mesoproterozoic material within thecrystalline basement of the Yangtze Craton (SichuanBureau ofGeology andMineral Resources, 1991), and the

ian, early Triassic and Late Triassic (adapted from reconstructions ofacross the Jinsha Jiang Suture has been presented by Reid et al. (2005)-Triassic tectonic reconstructions. GLS = Garzê–Litang Suture, GO =Lhasa Block, NC = North China Craton, Q = Qiangtang Block, S =FB = Songpan Garzê Fold Belt, YA = Yidun Arc, YC = Yangtze Craton,ssif rifted from the Yangtze Craton through the opening of the Garzê–sotethys Oceans and the supercontinent of Pangea. In the Early TriassichongzaMassif. The subduction of the Garzê–Litang Ocean underneathiassic. Note also the onset of collision between the North China Blocking the Qinling Orogen, from which much of the flysch of the Songpan6). In the Late Triassic (c) the northwards movement of the Qiangtang–ê–Litang Suture and resulted in terminal deformation across the easterny dashed lines). Note the positioning of the Zhongza Massif Yidun Arcatic.


Nd model ages of Roger et al. (1995), make it tempting tosuggest that some portion of the Yangtze Craton formedthe source region for the Mesozoic plutons and may thusunderlie the Yidun Arc.

6.6. Metallogenic implications

The Late Triassic volcanic rocks and post-tectonicintrusives formed apparently as a result of thesubduction of the Garzê–Litang Ocean westwardsunderneath the Zhongza Massif (Fig. 6), and theformer are associated with numerous VHMS sulphidedeposits (e.g., Hou and Mo, 1993; Zaw et al., 2007-this volume; Hou et al., 2001a, 2007-this volume).However, no significant Triassic Cu–Au porphyry orepithermal Au deposits have yet been discovered in theYidun Arc, despite the presence of a significantTertiary Cu–Au porphyry belt in the QiangtangBlock to the west (Hou et al., 2003b) and mineralizedgranite belts of Triassic age in other terranes of south-east Asia (e.g., Schwartz et al., 1995). Tectonicreorganization, including subduction reversal, hasbeen strongly correlated with the occurrence ofmodern magmatic–hydrothermal deposits in the Pacif-ic region (e.g., Solomon, 1990; Barley et al., 2002)and may be an important factor controlling mi-neralization in the Triassic tin-bearing granite belts ofThailand and Myanmar (e.g., Mitchell, 1992). How-ever, the dominantly crustal source for the granitessampled in this study is negatively correlated with theformation of such deposits, which may often be relatedto melting of a metasomatized mantle (e.g., Sillitoe,1997).

Cretaceous magmatism in the Yidun Arc isassociated with local skarn-type Sn–W mineralization(Guan, 1999; Hou et al., 2007-this volume). Creta-ceous magmatism elsewhere in Asia is associatedwith both Sn and Au mineralization. For example,extensive Sn mineralization is associated withcrustally-derived Cretaceous magmas of the WesternGranitoid Province in Thailand (Schwartz et al.,1995) and widespread Au mineralization is associatedwith Cretaceous magmatism related to lithosphericthinning within the North China Craton (Zhou et al.,2002b). Both of these examples of Cretaceousmineralization share similarities with the crustally-derived and presumably extension related granites ofthe Yidun Arc. However, it is worth noting thatunlike those of the Yidun Arc, the North Chinamagmas are temporally and spatially associated withmafic intrusives (Yang et al., 2003), hence furtherinvestigation may be required to determine the

potential for the Yidun Arc granites to host similarmineralization.

7. Conclusions

The deformed Palaeozoic and Triassic sequence ofthe Yidun Arc are intruded by granitoid plutons that fallinto three age groups: Early–Middle Triassic (∼ 245 to229 Ma), Late Triassic (∼ 219 to 216 Ma) and Cre-taceous (∼ 105 to 95 Ma). The granites of Early-Middle Triassic age intrude Palaeozoic metasedimentsalong a N–S trending belt in the western Yidun Arc.Late Triassic aged granites intrude the Late Triassicflysch and volcanics in the eastern Yidun Arc. Theemplacement of these Triassic granites places an upperlimit on the age of deformation within the western andeastern Yidun Arc respectively and shows deformationwas diachronous across the arc. Cretaceous granitesintrude the central region of the Yidun Arc and arepossibly related to the subduction of the Tethyanoceanic crust underneath Asia and correspondingregional extension.

Both of the Triassic plutons studied have similar Hfisotope systematics suggesting they share a commonMesoproterozoic source. The variability in the Hf iso-topic signature of these granites is interpreted to reflectderivation from either a mixed magma source or asubducted sedimentary package, consistent with theirorigin within a magmatic arc setting. The Cretaceousgranite shows slightly more homogeneous Hf system-atics; however, the similar depleted mantle model agesfor both the Cretaceous and Triassic samples suggests apooled Mesoproterozoic age is applicable and hencethe source region for these plutons may well becontinuous underneath the Yidun Arc. This pooledmodel age is tentatively correlated with the Mesopro-terozoic of the Yangtze Craton, however further workwill be necessary to conclusively demonstrate thiscorrelation.

Acknowledgements

This work was funded by ARC grant A1002030Zto CW. AR acknowledges support of a MelbourneUniversity Science Faculty Research Scholarship. G.Holm, A. Raza and A. Fowler are thanked for helpduring mineral separation and sample preparation. A.Priymak is acknowledged for help with cathodolumi-nescence imaging. S. Elhlou is thanked for assistancewith U/Pb dating. Helpful reviews by Z.X. Li and M.Barley are also gratefully acknowledged. This isPublication 337 from the ARC National Key Centre


for Geochemical Evolution and Metallogeny ofContinents (GEMOC).

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Mesozoic plutons of the Yidun Arc, SW China: U/Pb geochronology and Hf isotopic signature

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