Simplified geological map of the Song Ma-Song Da area Vietnamese and Laos territories, adapted and simplified from [Lepvrier et al., 2008] 1 Metamorphic formations of the Bu Kha.ng-Phu Hoa
Trang 1j ourna l h o me p ag e:h t t p : / / w w w e l s e v i e r c o m / l o c a t e / j o g
V ˜ u V Tícha, Ta Tro.ng Th ´˘anga,†
a VNU, University of Science, 334 Nguyen Trai, Thanh Xuan, Hanoi, Viet Nam
b Geowissenschaftliches Zentrum der Universität Göttingen, Goldschmidtstr 1-3, 37077 Göttingen, Germany
c Institute of Earth Sciences of Paris, UMR 7193, UPMC-CNRS, Université P&M Curie, 4 place Jussieu, 75252 Paris cedex 05, T45-46 E2, France
a r t i c l e i n f o
Article history:
Received 12 August 2010
Received in revised form 27 March 2012
Accepted 8 April 2012
Available online 16 April 2012
Keywords:
U/Pb
Sm/Nd ages
Ophiolite
Song Ma suture
Paleotethys
Vietnam
a b s t r a c t
TheSm/Ndisochronmineraldatingtechnique,appliedonlensesofophioliticrocksoftheSongMa suturezone,revealscrystallizationagesof387–313Mafortitanitesextractedfromthemaficcomponents (metagabbros,metabasalts)oftheophiolitesuite.Theseagescorrespondtoalargetimeintervalwithin theCarboniferousperiod.Suchresultsmeanthattheseblocksarelithosphericrelicsofaneasternbranch
ofthePaleotethys.TheyhoweverdonotexcludethatanolderearlyPaleozoicoceanhaspreviouslyexisted
inthearea.Theoverprintedmetamorphismthataffecttheserocksandthemetasedimentaryhostrocks, includinglocallyHPgranuliteandeclogitefaciesconditions(Nakanoetal.,2006,2008,2010),tookplace duringtheTriassicIndosinianorogenyafterclosureoftheocean,continentalsubductionandcollision, leadingtothesuturingoftheIndochinaandSouthChinablocks.U/PbandAr/Ardatarevealthatpeak metamorphicconditionswereattained266–265Maago,beingthenfollowedbycoolingat250–245Ma
© 2012 Elsevier Ltd All rights reserved
(Fig 1 is tectonicallyintercalated betweentheSamNua zone
(mainlyintheLaosterritory),madeofcontinentalassemblagesof
TriassicsedimentsandmostlyfelsicvolcanicsandtheSongDazone,
interpretedasacontinentalriftandcharacterizedbyLate
volcanics(Hanski et al.,2004).The SongMa zone is knownto
includetectoniclensesofultramaficandmaficrocks.Asearlyas
thefirsthalfofthe20thcentury,thiszonehasbeendocumented
byFrenchgeologists(Deprat,1914;Jacob,1921;Fromaget,1927,
1941,1952)andhasbeendescribedasthe“ThanhHoacicatrice”;
laterthezonehasbeenrecognizedasanoceanicsuturebetween
theIndochinablockandtheSouthChinablock(Hutchison,1975,
1989;Bachetal.,1982;BachandThang,1995;Sëngoretal.,1988;
Chuong,1995)
Geochemicalstudiesontheultramaficrocks(serpentinites)of
theSongMacomplexsupportanoceanicsignature(Trungetal.,
serpen-tinite body and in Bo Xing ultramafics(Thanh et al., 2011), in
∗ Corresponding author Tel.: +84 4 3 8585097; fax: +84 4 3 8583061.
E-mail addresses: vuongnv@vnu.edu.vn, vuongnv@gmail.com (N V Vuo.ng).
† Deceased.
chromitite, account for an original peridotite of spinel-bearing lherzolitic harzburgite type; fine-grained gabbros also indicate MORBaffinity
Xingcomplexisclearlyoceanic,indicatingthattheSongMasuture representsrelicsofapaleo-oceaniclithosphere,theprotolithage
ofthemagmaticrocks,whichcomposetheophioliticsuiteofSong
Ma,is notwellconstrained Eithertherocksareremnantsof a branchoftheeasternPaleotethysoceanfloor,whichclosed dur-ingtheTriassicIndosinianOrogeny,ortheyrepresentremobilized, disruptedfragmentsofanolderocean.Thesuturingtimeisthought
tohaveoccurredinthePaleozoic,duringtheSilurian-Early Devo-nian(Hutchison,1989;FindlayandTrinh,1997;Findlay,1998),a modelsupportedbypaleontologicalevidences(Janvieretal.,1994, 1997;Thanhetal.,1996).Mostauthorshoweverfavoureda col-lision of theIndochina and South Chinablocks that tookplace duringtheTriassicIndosinianevent,prior totheUpperTriassic unconformity(Tri,1977;Sëngoretal.,1988;Chungetal.,1999; Lepvrieretal.,2004,2008;Trungetal.,2006),amodelsupported
byAr–ArTriassicagesforthethermotectoniceventrecordedinthe TruongSonbeltandtheSongMazoneitself(Lepvrieretal.,1997; Maluskietal.,1999).AccordingtoCarteretal.(2001),Carterand Clift(2008),theIndosinianeventalongSongMaisnotrelatedtothe Indochina-SouthChinacollisionbutlinkedtoaTriassicreactivation
ofapreexistingsuturebycollisionoftheSibumasublockwiththe Indochina-SouthChinaone.Recently,CaiandZhang(2009)have
0264-3707/$ – see front matter © 2012 Elsevier Ltd All rights reserved.
Trang 2Fig 1. Simplified geological map of the Song Ma-Song Da area (Vietnamese and Laos territories), adapted and simplified from [Lepvrier et al., 2008] (1) Metamorphic formations of the Bu Kha.ng-Phu Hoa.t Tertiary dome; (2) Permo-Triassic granitic complexes (Song Ma, Dien Bien and other massifs); (3) Late Proterozoic to Early Paleozoic Nam Co and Song Ma Formations which include, as a tectonic mélange, various ophiolitic elements (ultrabasites, metabasalts, amphibolites, metagabbros and plagiogranites
of the Chieng Khuong Complex); (4) Upper Silurian and Lower Devonian Formations; (5) Middle Devonian to Carboniferous Formations; (6) Permian volcanics including komatiites; (7) Lower Triassic terrigenous to Middle Triassic mainly carbonate sediments; (8) Unconformable terrigenous and coal-bearing Norian-Rhetian sediments (Suoi Bang Formation); (9) Upper Cretaceous red beds of the Yen Chau Formation.
putforwardanewinterpretationofthezonewithSouthChinaand
Hainanislandcorrelations
petrologic,metamorphicandtectonicaspects(Lien,1980;Bachand
Thang,1995;Chuong,1995;FindlayandTrinh,1997;Findlay,1997,
1998;Lepvrieretal.,1997,2004,2008;Nakanoetal.,2008,2010)
Mostoftheisotopicageshavebeenobtainedonmetamorphicrocks
collectedintheNamCoFormationthroughthe40Ar/39Armethod
andallaregroupedintherangeof250–245Ma(Maluski etal.,
1999;Lepvrieretal.,1997,2004,2008).Uptonow,rarereliable
datahavebeenobtainedandpublishedonthecrystallizationage
ofthefragmentsofoceanicrocksfoundalongthesuture.Previous
studiesprovideddistinctagegroupsof455and425Mathrough
K–Ardatingandanageof541±72Ma(ThangandTruong,1999;
Lien,1980)wasobtainedbytheRb–Srtechnique.Morerecently,
Phametal.(2008),usingLA-ICP-MS,havereportedU–Pbzircon
dataonsamplescollectedfromtheHuoiHaometabasalt,Chieng
yieldmostlyconcordantLatePermianage:251Maand253Mafor themetabasalt,258±4and260±5MaforSongMagraniteand
263±8MafortheChiengKhuongplagiogranite.AsimilarU–Pb zir-conageof271Mahasbeenobtainedonasampleofquartz-diorite
ofChiengKhuong(Liuetal.,inpress)
Clarifyingthetimeofformationoftheophioliticsuiteandthe timeofsuturingalongSongMaarecriticalpointsfor
continentalblocks
ThispaperpresentstheresultsofU/PbandSm/Nddatingsfrom
zoneandadiscussionoftheirgeologicalsignificances
VietnamandnortheasternLaos(Fig.1 parallelstotheSouththe CenozoicRedRiver(SongHong)shearzone.Itmarksthenorthern
Trang 3byaseriesofNW-trendingTriassicshearzones,formedduringthe
IndosinianOrogeny(Lepvrieretal.,1997)
inVietnamandLaos,beforebeingdeflectedclockwisebytheLai
Chau-DienBienPhuFaultzone(DBF),asaresultofpost-triassic
dextralmovements,whichtookplacealongthisfault.TheSongMa
Zone(SMZ)sensulatocomprisesseveralunits:
Trinh, 1997; Findlay, 1997) It is marked by the occurrence
HuoiHaoFormations).Theophioliticsuiteconsistsinultramafics
ofthePacNamFormationrepresentedbysmallblocksof
serpen-tinite,exceptintheThanhHoaareawheretheyformedthehuge
serpentinizedchromite-bearingperidotiteoftheNuiNuamassif
Maficcomponentsoftheophioliticcomplex,groupedintheBoXinh
Formation,consistinsmalllens-shapebodiesofmetagabbro,
includesgneissified,foliatedandelongatedbodiesofplagiogranite
suchastheChiengKhuongmassifandBanPhungmassif(Fig.1,
DGMV,2005a)
Allthesemagmaticrocks,aswellasthesedimentarycountry
rocksinclude: quartz–sericite–chlorite, feldspath–quartz–biotite
actinolite–epidote–chlorite and plagioclase–hornblende–epidote
(sphene),rarelyinapatite.Theultramaficrocksareoftenstrongly
serpentinizedwithdevelopmentofactinoliteandtalcminerals
as the Song Ma anticlinorium (Tri, 1977; Findlay, 1997), is in
faultedductilecontact,withtheSongMasuturezone.TheNam
DaFault(SDF).TherocksoftheNamCoFormation,attributedto
theProterozoicandLower-MiddlePaleozoic,displaymetamorphic
assemblagescorrespondingtotheamphibolitefacies.They
con-sist in peliticschists, quartz-sericiteschists, sericitic quartzites,
phyllites,garnet-bearing micaschists,locally containing kyanite
andstaurolite(Lien,1980;Findlay,1997).Pieces ofamphibolite
areintercalated inthe series.More totheeast, along SongMa
CoFormations(DGMV,2005a),high-pressuremetamorphicrocks
(Fig 1)have been discovered: high-pressuregranulite (Nakano
etal.,2008)andlow-temperatureeclogite(Nakanoetal.,2010)
Peliticschists,containinggarnetandphengite,withthepresence
of chloritoid,kyaniteand staurolite in more aluminous schists,
beenaffectedthemselvesbyeclogitefaciesmetamorphism
least2.1–2.2GPaand600–620◦C,priortoaclockwiseP–T
evolu-tionanda subsequentcooling;similarhigh-pressureconditions
of1.9–2.3GPaat580–600◦Chaveaffectedthehostpeliticrocks
(Nakanoetal.,2008,2010)
TheSongMagraniticcomplex, totheSouth,extendingover
80km,liesalongtheSongMaFault,whichseparatestheSamNua
por-phyriticgraniteandgranodiorite(DGMV,2005a).Thiscalcalkaline
complexisgeneticallylinkedwiththevolcanosedimentary mate-rial oftheAnisian DongTrau Formation.Isotopic Rb/Srages of
232±11Mahavebeenreportedfromthismaterial(DGMV,2005b)
260±5Ma(Phametal.,2008).MoretotheWNW,butina sim-ilar tectonic position,theDien DienPhu undeformedmassif is
acompositecalcalkalinegraniticcomplexmadeofseveral intru-sivephasesofPermian-Triassicages,(DGMV,2005b).U/Pbzircon
quartz–monzoniteandporphyriticgranodiorite,respectively(Liu
etal.,inpress).Thecoal-bearingSuoiBangUpperTriassic Forma-tionunconformablycoverstheSongMaandDienBienPhuintrusive massifs
complicatedmulti-stagedeformation,includingthesuperimposed developmentofplanarandlinearstructuresandtheexistenceof severalphasesoffoldingwithinitialisoclinalfoldsandsheathfolds and lateruprightfolds associatedwithsteepcrenulation cleav-ageandkink-bandsdevelopment(Findlay,1997,1998;Findlayand Trinh,1997;Lepvrieretal.,1997;Hai,2009).Suchoverprintingand refoldinghaveobviouslyaffectedtheinitialgeometryofthesuture andexplainedtheratherbroadspatialdistributionoftheophiolitic rocksfragments(Hai,2009)
AlongSongMaRiver,intheaxialpartofthezone,theSong
MaFormationdisplaysasteeptoverticalfoliationcoupledwith highanglefoldaxes.Theophioliticfragmentsaretransformedinto elongatedlenses.NW-trendingmylonitesandultramylonitesbear
astretchinglineationwithangleofplungevaryingfrom5◦to20◦ eastward.Kinematiccriteria,includingtheindicationgivenby sig-moidalporphyroclasts,revealadextralsenseofshearing(Lepvrier
etal.,1997,2008).IntheNamCoFormation,asaresultofafinal stageofuprightfolding,theoriginalflat-lyingfoliationhasgiven risetoasuccessionofantiformsandsynforms(SongMa anticlino-rium,Tri,1977),examplified,alongthecross-sectionfromMaiSon
toChiengKhuong,bythelargeNW-trendingNamCoantiform.In theareaofCoMa,theLowerDevonianlimestones,transformedinto marblesexhibittightisoclinalfolding;equivalentmarbles,along theroadfromTuanGiaotoDienBienPhu(MuongAng)area,are affectedbyaboudinageinthenortherndirection
3.1 Samplespositionsandpetrographicdescriptions
Inthepresentstudy,severalrocksampleshavebeencollected anddatedbySm/NdandU/Pbdatingtechniques(Table1).VN34-00
isametagabbro-amphibolitecollectedinthesouthernpartofthe NamCoanticline,nearbyChiengKhuong.VN18-00isagneissified
amphi-bolite.OthersamplesweregatheredalongtheSongMaRiver:a
grain-sizerangingfrom0.3to1mm,plagioclasecrystals
clinopyroxenprisms.Titanite wasfoundasthemainassociated mineral,theother,lessfrequentmineralswererutileandcalcite 3.2 Mineralseparation
Mineralseparation,enrichmentandanalysiswerecarriedout
atthelaboratoryofisotopegeologyattheUniversityof Goettin-gen,Germany,followingthetechniquesdescribedinKrogh(1973)
Trang 4Table 1
Samples position, rock types, dating methods and ages.
Position
Sample number GPS coordinates Type of rock Dating method and
mineral fraction
Age (Ma)
103 ◦ 58 32.2
Titanite
313 ± 32
103 ◦ 59 39.4
Chieng Khuong Granite
U–Pb Zircon
222 ± 4
104◦0014.5
Zircon
241.3 ± 5.3
103 ◦ 33 34.0
Amphibole, Clinozoisite
387 ± 56
103◦3726.5
Garnet bearing Metagabbro
Sm–Nd Amphibole, Pyroxene
338 ± 24
103 ◦ 50 04.4
Amphibole
322 ± 45
103 ◦ 59 39.4 Amphibolite Sm–Nd
Titanite, Amphibole
315 ± 92
U–Pb Titanite
265 ± 4
Sm–Nddating.The1.5kgweightedsamplewaspulverizedtoabout
0.25mm grainsize The mineralgrains withthe size less than
0.25mm,whichweremainlyplagioclaseandamphibole,were
sep-aratedbymeansofaWilfleyTMtable.Strongmagneticamphibole
wasenrichedwithanisodynamicFrantzTMseparator.Weak
mag-neticgreenamphiboleswerecollectedunderamicroscope.Titanite
waschosenaswell-formedcracklesssinglecrystals.Singlecrystals
ofamphiboleandtitanitewerethencleanedultrasonicallyinweak
HNO3solutionforhalfanhourandthenre-cleanedwithdistilled
water.ZirconswereextractedfromsampleVN34-00andVN18-00
forU–Pbdating
3.3 Analyticalprocedure
mixed150Nd–149Smspikesolutionspriortodissolutionina
system.Thesolutionswereprocessedbystandardcation-exchange
techniquesforpurificationoftheSmandNdfractions.Forthe
2.5NHClonpre-conditioneddoubleRefilaments.Measurementsof
isotopicratioswereperformedonaFinniganMAT262RPQ+©mass
spectrometerinstaticmode(GZGGöttingen,Dept.ofIsotope
Geol-ogy).RepeatedmeasurementoftheLaJollaNdstandardyieldeda
143Nd/144Ndratioof0.511840±3(n=13,2␦)overthecourseofthis
study.TheobtainedNdisotopicratiosofthesampleswere
normal-izedtoa146Nd/144Ndratioof0.7219.Totalprocedureblankswere
consistentlybelow150pg.forSmandNd.All143Nd/144Ndratios arereportedwiththeir2␦internalprecisionplustheuncertainties resultingfromthespikecorrection.ForfurtherdetailsseeTütken
etal.(2002) SamplesforU/Pbanalyseswerecrushedinajaw-crusherand subsequently milled before sieving (<250m) Zircon concen-trates wereobtained usinga Wilfley© table, heavy liquids and
aFranz© magneticseparator.Theisolatedmineralsweresorted according to their grain size and analysed as multigrain frac-tionsfollowingtheanalyticalproceduregiven byKrogh(1973) Leaddatawerecorrectedformass-fractionation,initialleadand blank.Initialleadisotopiccompositionwascalculatedaccording
calcu-latedusingthedecayconstantsrecommendedbyI.U.G.S.(Steiger and Jaeger, 1977).Calculation of errors and correlations ofthe
206Pb/238Uand207Pb/235UratiosfollowtheprocedureofLudwig (2011)consideringtheuncertaintiesofthemeasurementof iso-toperatios,uncertaintiesoftheU/Pbratiointhespike,summation
of errors due to thespike/sample ratio as well as errors from theblankandcommonleadcorrection,seealsoSchneideretal (2004)
4.1 U/Pbagesoftitaniteandzircons The isotopic composition in U and Pb are given in Table 2 Thedeterminedagesareillustratedinconcordiadiagram(Fig.2)
theconcordialine,withanaverageageof265.4±3.7Ma.Asthe
Table 2
U–Pb isotope data of titanite (VN34-00) and zircon fractions (VN19-00 and VN18-00).
[m]
Sample [mg]
VN-19-00
VN18-00
Trang 5240
280
320
> 18 0
80 - 10 0
0,03 0 0,03 4 0,03 8 0,042 0,04 6 0,05 0 0,054
207 Pb/ 235 U
VN-34 -00
Concordia Age = 265 4 ±3.7 Ma (2s, de cay-const err s included ) MSWD (of concorda nce) = 0.28 , Probabili ty (of concorda nce) = 0.60
VN-34 -00
X-Y Weighted Mea n:
X = 0.297 5±0.0068 2s
Y = 0.0420 4±0.0005 9 X-Y err or corr elation = +0.65 3 MSWD = 0.39 , Probabili ty =0.68
6 Pb
8 U
280 290 300 310
VN-19 -00
240
260
45 - 63
< 45
0,036 0,038 0,040 0,042 0,044 0,046
207 Pb/ 235 U
VN-18-00
Model 1 Solution without [with] decay-const err s on 3 points Lower intercep t: 22 1.7 ± 3.8 [±3.9] Ma Upper intercep t: 1500 ± 88 Ma MSWD = 0.69 , Proba bili ty of fit = 0.41
closuretemperatureoftitaniteisabout660–700◦C(Scottand
St-Onge,1995)forthepairU/Pb,theageobtainedbythismethod
isgenerallyregardingasreflectingpeakmetamorphicconditions
temperatureorhigh-temperaturecoolingshortlyafter,whenthe
crustwas still thickand hot This U/Pb agecoincides with the
266±4MacoolingagefortheVN9sample,obtainedby40Ar/39Ar methodonhornblende(Lepvrieretal.,1997),whichhas, inthe Ar/Arsystem,asimilarclosingtemperature
Trang 6WR
0,51252
0,51256
0,51260
0,51264
0,51268
0,51272
147 Sm/ 144 Nd
0,51 28 0,5129 0,51 30 0,51 31 0,51 32 0,5133
147 Sm/ 144 Nd
0,51 298
0,51 302
0,51 306
0,51 310
0,51 314
0,51 318
0,51322
147 Sm/ 144 Nd
0,5127 5 0,5128 5 0,5129 5 0,5130 5 0,5131 5
0,17 0,19 0,21 0,23 0,25 0,27 0,29 0,31 0,33
147 Sm/ 144 Nd
0,5128 6 0,5128 8 0,5129 0 0,5129 2 0,5129 4 0,5129 6 0,5129 8 0,5130 0
0,17 5 0,185 0,19 5 0,20 5 0,21 5 0,22 5 0,235
147 Sm/ 144 Nd
Fig 3.Sm–Nd isochrone ages.
AslightlyyoungerU/Pbzirconageof241Mahasbeenobtained
fromtheamphibolitesampleVN19-00.Thisageiscomparablewith
mostoftheAr–Aragesof245–240Ma,whichhavebeenpreviously
(Lepvrieretal.,1997,2004)
obtainedinourstudyonthefoliatedandfoldedVN18-00granitic
(2011) have noted that zircon grains could be partly reset
reportedfromtheChiengKhuongplagiogranite:263±8Mamean agebyPhametal.(2008)and(271)MabyLiuetal.(inpress)should
beinterpretedasmetamorphicagesratherthanprotolithages
IntheNamSuLuFormation,thegarnet-clinopyroxenegranulite,
aswelltheeclogiteblocks,wasimpossibletodateintheabsence
thehighpressurepeliticgneissthat enclosetheseHPgranulite andeclogiteblocks,yieldequivalentEarlytoMiddleTriassicages U–Th–Pbweightedaverageageof233±5Ma(Nakanoetal.,2008) and243±4Ma(Nakanoetal.,2010)havebeendetermined.They
Trang 7Table 3
Sm/Nd data.
6.54
5.30
14.73
5.12
correspondtotheageofthepeak-pressuremetamorphism.The
almostsimilarityoftheU–Th–PbageswiththeAr/Aragesimplya
rapidexhumationandsubsequentcooling
4.2 Sm/Ndisochronages
TheisotopiccompositionandcontentofSmandNdaregiven
inTable3.TheSm/Ndmineralisochronesobtainedonmaficrocks,
mainlygabbroandamphiboliteareofratherpoorqualityandall
theseSm/Ndresultsdisplaybigerrors.However,thesmallvalue
ofMSWDallowsconsideringthattheseisochroneageshave
313Mahasbeenobtained(Table1andFig.3):313±32Ma
(VN05-00),322±45Ma(VN32-00),338±24Ma(VN22-00),387±56Ma
ageofVN34-00,apreviousageof414MareportedinVuongetal
(2006)hasbeenre-appreciated:whenwefavourafive-point
min-eralisochronfor titanite,amore comparableageof315±92 is
obtained Omittingthesmalltitanitefractions 60–80minthe
calculation,theageincreasesto331Maandtheerrorto140Ma
Clearly,themetamorphicoverprint,undermetamorphic
amphibo-litefacies,determinedwithU/Pbontitaniteattheageof265.4Ma,
hasnotbrokentheequilibriumofSm/Ndisotopes.However,the
U/Pbzirconageof251±6and252±3MaobtainedbyPhametal
(2008)onasampleofmetabasaltcanbeinterpretedastheresult
ofametamorphicoverprint
Alltheseages,obtainedbySm/Ndisochronmineraldatingon
showmetamorphicandcrystallizationages,respectively.Thelarge
discrepancyofthecrystallizationagescanbesimplyexplainedby
thefactthattheformationoftheoceaniclithosphereisa
continu-ousprocessoveralongtime.Inthisstudy,theycorrespondtothe
Carboniferousperiod.Theageof387Maistheminimumagefor
theonsetofoceanicaccretion.TheyoungestCarboniferousageof
313Mameansthattheoceaniccruststillwasformedatthattime,
whateverthesubductionprocesshasalreadystartedornot
Althoughthedataareratherimprecise,theSm/Ndisotopicages,
onmetabasitesrocksfromtheSongMasuture,establishthatthe
ophioliticrocksalongSongMadefinitelyrepresentrelicsofthe
Paleotethyslithosphere.Theseresults,however,donotexcludethe
possibleexistenceofpiecesofanearlyPaleozoiclithosphere,but
noisotopicevidencehasbeenfoundinthepresentstudy
Thepaleotethyancrusthasbeenthenconsumedbymeanofan oceanicsubduction,whichlikelyshouldhavebeensouth-directed, accordingtotheexistenceofcalcalkalineplutovolcanicrockslying
tothesouthofthesuturezone.ThePermianoceanicsubduction hasbeenfollowed duringtheTriassicby a continental subduc-tion,whichexplainsthegenerationofhigh-pressuregranuliteand eclogite,formedatadepthofmorethan70km(Nakanoetal.,2008,
2010).Theseblocksarenowfoundastectonicblockswithinthe garnet-phengitebearinghostschists(NamSuLuformation),which havebeensubmittedtothesameHPconditions.Thefinal conti-nentalcollisionofIndochinaandSouthChinahasbeentheresultof
anobliqueconvergence,accordingtotheductiledextralstrike-slip structuresobservedinthemetamorphicrocksoutcroppingalong thesuturezone(Lepvrieretal.,1997,2004).TheHProckshave beenexhumedataratherfastratetoexplainthefactthattheAr/Ar coolingagesobtainedintheNamCoFormationarenotsignificantly differentoftheageofthepeakmetamorphism.TheU/Pbagesof
270and263MaobtainedontheChiengKhuonggranitebyLiuetal (inpress)andPhametal.(2008)respectivelyandc.250Maon metabasalt(Phametal.,2008)donotrepresentcrystallizationages butlikelycorrespondtotheindosinianmetamorphicoverprint
Acknowledgments
ThisworkwaspartlysupportedbyNationalFoundationfor
and105.01.53.09).WewouldliketothankGZGforanalytical sup-ports.NguyenVanVuongisgratefulforthesupportbytheLower SaxonyMinistryofScienceandCulture,Germanyforavisitinggrant (ZN887)toGöttingen.TheauthorswouldliketothankF.Wilskyfor reprocessingthedata.ClaudeLepvrierhasbeenpartlysupported
bytheInstituteofEarthSciencesofParis(UMR7193UPMC-CNRS)
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