a

olycyclichibolitesum withgling ratesnappe

)ue Dansteacuteristiquesiquant unen acircge deLes tauxatique

C R Geoscience 335 (2003) 1081ndash1089

Geochemistry (Isotope Geochemistry)

An Early-Cambrian UndashPb apatite cooling age for thehigh-temperature regional metamorphism in the Piancoacute are

Borborema Province (NE Brazil) initial conclusions

Bruno Dhuimea Delphine Boscha Olivier Bruguierb Renaud CabyalowastCarlos Archanjoc

a Laboratoire de tectonophysique universiteacute Montpellier-2 UMR 5568ndashCNRSUMII place Eugegravene-Bataillon34095 Montpellier cedex 05 France

b Service ICPndashMS ISTEEM cc 056 universiteacute de Montpellier-2 place Eugegravene-Bataillon 34095 Montpellier cedex 05 Francec Instituto de Geociecircncias University of Satildeo Paulo Brazil

Received 28 February 2003 accepted 24 September 2003

Presented by Georges Peacutedro

Abstract

The Borborema Province (BP) of northeastern Brazil is a complex crustal assemblage which has undergone a pevolution during the Proterozoic In the Piancoacute-Alto Briacutegida belt a metamorphosed leucosome vein inserted in amphas a trace element pattern suggesting a T-MORB protolith Apatites yield a REE pattern indicating growth in equilibrigarnet thus pointing to its metamorphic origin UndashPb analyses yield an age of 540plusmn5 Ma interpreted as a cooling age followinamphibolite facies regional metamorphism associated with granitic emplacement at ca 580 Ma The resulting slow coo(ranging from ca 25 to 5C Maminus1) are consistent with underplating of mafic magmas or crustal thickening caused bystacking as possible processes governing the metamorphic evolution of the BPTo cite this article B Dhuime et al C RGeoscience 335 (2003) 2003 Acadeacutemie des sciences Published by Elsevier SAS All rights reserved

Reacutesumeacute

Acircge de refroidissement UndashPb sur apatite du Cambrien infeacuterieur pour le meacutetamorphisme de haute tempeacuterature dela reacutegion de Piancoacute province de Borborema (Nord-Est du Breacutesil) premiegraveres conclusions La Province Borborema (BPdans le Nord-Est du Breacutesil est un assemblage complexe ayant subi une eacutevolution polycyclique durant le Proteacuterozoiumlqla ceinture de Piancoacute-Alto Briacutegida un leucosome meacutetamorphiseacute associeacute agrave des amphibolites preacutesente des caracdrsquoeacuteleacutements en trace suggeacuterant un protolithe de type T-MORB Les apatites preacutesentent un spectre de terres rares indcroissance agrave lrsquoeacutequilibre avec le grenat ce qui traduit une origine meacutetamorphique Les analyses UndashPb fournissent u540plusmn 5 Ma interpreacuteteacute comme un acircge de refroidissement suivant le pic du meacutetamorphisme reacutegional dateacute agrave 580 Made refroidissement faibles compris entre 25 et 5C Maminus1 sont compatibles avec des processus de sous-placage magm

Corresponding authorE-mail addresses dhuimedstuuniv-montp2fr (B Dhuime) cabydstuuniv-montp2fr (R Caby)

1631-0713$ ndash see front matter 2003 Acadeacutemie des sciences Publisdoi101016jcrte200309012

hed by Elsevier SAS All rights reserved

1082 B Dhuime et al C R Geoscience 335 (2003) 1081ndash1089

ou drsquoeacutepaississement crustal pour expliquer lrsquoeacutevolution meacutetamorphique de la BPPour citer cet article B Dhuime et al C RGeoscience 335 (2003) 2003 Acadeacutemie des sciences Published by Elsevier SAS All rights reserved

Keywords Borborema Province apatite UndashPb dating cooling age

Mots-cleacutes province Borborema apatite datation UndashPb acircge de refroidissement

steacutero-for-do-ci-

e caqueas-[2po-faelir-blet dein dus-autesur

tracsul-dan

ABeacuteta-ub-rsquoacircgereacute-

na-t deineacute-ta-nesein

olendashteacutesord

di-6

ac-rconlasee

antembdeau15

UeacuteteacutepourneacuteelculctueacutereMSeur5]

treleacute-

rreslesT-

uea-e

Version franccedilaise abreacutegeacutee

1 Introduction

La province Borborema (BP) dans le Nord-Edu Breacutesil est constitueacutee par un socle paleacuteoprotzoiumlque incluant quelques noyaux archeacuteens et desmations meacutetaseacutedimentaires proteacuterozoiumlques Le laquomaine transversal raquo deacutelimiteacute par les deux grandssaillements dextres de Patos et de Pernambuco sracteacuterise par la preacutesence drsquoassemblages volcaniet plutoniques drsquoacircge Meacutesoproteacuterozoiumlque et de bsins turbiditiques du Neacuteoproteacuterozoiumlque terminal26] Lrsquoexistence de plutons synorogeacuteniques (Itaranga Campina Grande Acari Caraubas S RaFaz Nova ) dateacutes par la meacutethode UndashPb sur zcons autour de 580 Ma [614] indique que lrsquoensemdes structures et le meacutetamorphisme reacutegional datencette eacutepoque Ce travail se propose de dater au sedomaine de Piancoacute-Alto Briacutegida (PAB) le refroidisement associeacute au meacutetamorphisme reacutegional de htempeacuterature en utilisant le geacuteochronomegravetre UndashPbapatite combineacute avec des analyses drsquoeacuteleacutements enet de microscopie eacutelectronique agrave balayage Les reacutetats obtenus sont compareacutes avec ceux connusdrsquoautres parties de la BP

2 Cadre geacuteologique

Au sein du domaine transversal la ceinture P(Fig 1) comprend des meacutetaseacutediments des mvolcanites acides des orthogneiss agrave foliation shorizontale replisseacutee et des gneiss anatectiques dPaleacuteoproteacuterozoiumlque preacutesumeacute Le meacutetamorphismegional a culmineacute dans le faciegraves amphibolite avec atexie agrave proximiteacute des plutons granitiques La pluparces plutons se sont mis en place en conditions syncmatiques agrave 580 Ma [622] qui est lrsquoacircge du pic du meacutemorphisme reacutegional La roche eacutetudieacutee provient drsquoubande leucocrate de 30 cm drsquoeacutepaisseur situeacutee au

-s

e

s

drsquoune seacutequence meacutetabasique meacutesocrate agrave amphibclinopyroxegravene associeacutee agrave des niveaux calco-silicaElle est localiseacutee agrave une dizaine de kilomegravetres au ndu massif granitique drsquoItaporanga dateacute agrave 584plusmn 2 Ma[6] Lrsquoeacutechantillon renferme 50 de grenat almanneux 30 de quartz 11 de plagioclase calciquedrsquoamphibole verte et 1 de biotite Les mineacuterauxcessoires observeacutes sont les sulfures lrsquoapatite le ziet le sphegravene Le thermomegravetre amphibolendashplagioc[3] fournit une tempeacuterature drsquoeacutequilibre de lrsquoordre d640C agrave 4 kbar

3 Techniques analytiques

Les apatites ont eacuteteacute dissoutes sur plaque chauffen acide chlorhydrique 6 N La seacuteparation du ploet de lrsquouranium a eacuteteacute effectueacutee selon la meacutethodeacutecrite par Krogh [17] Les blancs de proceacutedurecours de la peacuteriode drsquoanalyse ont varieacute entreet 30 pg pour le Pb et moins de 5 pg pour lrsquoLa composition isotopique du Pb radiogeacutenique adeacutetermineacutee en retirant le blanc puis en supposantle Pb commun restant une composition deacutetermidrsquoapregraves le modegravele de Stacey et Kramers [24] Le cades points expeacuterimentaux et des acircges a eacuteteacute effeen utilisant le programme de Ludwig [18] La mesudes eacuteleacutements en traces a eacuteteacute reacutealiseacutee par ICPndashsuivant une meacutethode de calibration externe en viguagrave Montpellier et deacutecrite en deacutetails par Ionov et al [1

4 Reacutesultats et discussion

Lrsquoeacutechantillon de roche totale preacutesente un specplat (Fig 2) leacutegegraverement appauvri en terres raresgegraveres comparable au N-MORB Les teneurs en terares sont cependant eacuteleveacutees (environ 50 foischondrites) ce qui suggegravere une affiniteacute de typeMORB [11] Lrsquoanomalie neacutegative en Eu suggegravere qle liquide initial a subi un processus de cristallistion fractionneacutee Lrsquoeacutechantillon est interpreacuteteacute comm

B Dhuime et al C R Geoscience 335 (2003) 1081ndash1089 1083

s al-s en

e derisseCecsese etpri-ab-e oudeacutefi-aly-atre

e-des-

reacute-

re-

ci-rentver-et dor-

subide

s aueste lableslesves

ues

for-ocieacutese-promeacutecag

auss deser-

sontani-ser-veacute-ique

bpreacute-reacuteteacuteurevecx de25Pd

ra-

andtheo-th-m-ter-eanro-i-

ingoic

Ga)ba-ud-so-ndashPbainwoog-Ma5]ina

ec-

un leucosome issu de la fusion partielle de basalteteacutereacutes similaire aux veines leucosomiques observeacuteedomaine oceacuteanique (par exemple [13]) Le spectrterres rares des apatites est marqueacute par un appauvment prononceacute en terres rares leacutegegraveres et lourdesindique une croissance agrave lrsquoeacutequilibre avec des phameacutetamorphiques majeures (telles que le plagioclasle grenat) qui incorporent ces eacuteleacutements de faccedilonvileacutegieacutee La structure interne des apatites reacutevegravele lrsquosence de figures magmatiques telles que zonagsurcroissance (Fig 3) Ces critegraveres permettent denir une origine meacutetamorphique pour les apatites anseacutees Dans le diagramme Concordia (Fig 4) les qufractions drsquoapatites deacutefinissent un acircge206Pb238U de540plusmn5 Ma (MSWD= 29) La tempeacuterature de fermture de lrsquoapatite vis-agrave-vis de la diffusion du Pb estlrsquoordre de 450ndash550C pour des vitesses de refroidisement comprises entre 1ndash10C Maminus1 [8] Dans cettepartie du domaine PAB le pic du meacutetamorphismegional culmine agrave environ 580 Ma et 640C Une baissede tempeacuterature depuis 640C agrave 580 Ma jusqursquoagrave 450ndash550C agrave 540 Ma permet de calculer des taux defroidissement compris entre 25 agrave 5C Maminus1 Ces va-leurs sont similaires agrave celles obtenues le long dusaillement de Patos par Corsini et al [10] et suggegraveune eacutevolution reacutetrograde et un refroidissement gouneacutes par des processus de remonteacutee isostatiquefaibles taux drsquoeacuterosion Les parties les plus meacutetamphiques et granitiseacutees de la ceinture PAB ont doncun refroidissement lent comparable et synchronecelui observeacute dans drsquoautres parties de la BP situeacuteenord du cisaillement de Patos [20] Cette eacutevolutioncependant diffeacuterente de celle connue pour le Sud dBP qui a subi des taux de refroidissement comparamais de faccedilon plus preacutecoce comme lrsquoindiquentacircges Ar de 580 Ma obtenus sur amphiboles par Neet al [21] Les taux de refroidissement eacutetant identiqsur lrsquoensemble de la BP (environ 3 agrave 5C Maminus1) cecisuggegravere un diachronisme avec migration de la deacutemation du magmatisme et du meacutetamorphisme assdu Sud vers le Nord Les faibles taux de refroidisment observeacutes ne sont pas compatibles avec descessus de deacutelamination et suggegraverent une eacutevolutiontamorphique lieacutee soit agrave des processus de sous-plade magmas basiques qui reacuteduisent lrsquouplift et reacutechauf-fent le systegraveme soit agrave un eacutepaississement crustal cpar lrsquoempilement de nappe Enfin de petits bassinmolasse continentale non meacutetamorphique sont preacute

-i

e

--e

eacute

veacutes le long du cisaillement de Patos Ces bassinscontemporains de la mise en place de plutons grtiques de type A drsquoacircge Cambrien supeacuterieur La preacutevation de lrsquoacircge UndashPb sur apatite indique que cet eacutenement nrsquoa pas eacuteteacute associeacute agrave une anomalie thermdrsquoampleur reacutegionale

5 Conclusions

Lrsquoacircge de 540plusmn 5 Ma obtenu par la meacutethode UndashPsur les apatites meacutetamorphiques drsquoun leucosomeleveacute au sein drsquoun complexe meacutetabasique est interpcomme celui du refroidissement lent de la ceintPAB au deacutebut du Cambrien Cet acircge fait le lien aceux connus au Nord et au Sud et indique des taurefroidissement lents (globalement compris entreet 5C Maminus1) identiques dans lrsquoensemble de la Bbien que le refroidissement soit plus preacutecoce au Su

1 Introduction

The Borborema Province (BP) of northeastern Bzil is a remote area of about 450 000 km2 locatedbetween the Satildeo Francisco Craton to the souththe Amazon Craton to the northwest It constitutescontinuation in South-America of the Pan-African mbile belts of Western Africa that occupied the norern part of Gondwana [7] (see Fig 1) This coplex domain is made up of several displacedranes which include large areas floored by Archand Proterozoic crustal units and Late Neoprotezoic turbiditic basins [25] Growing geochronologcal evidence suggests a polycyclic evolution durthe Paleoproterozoic (20ndash22 Ga) Mesoproteroz(095ndash110 Ga) and Neoproterozoic (058ndash064periods [226] The Paleoproterozoic event is probly the most important and corresponds in the stied area to a major period of crust formation asciated metamorphism and granite emplacement Uzircon dating of gneisses from the Transversal Domgave ages ranging from 197 to 220 Ga [222] Tmajor periods of felsic magmatism have been recnized around 15 Ga [22] and during the 1060ndash930time span (the so-called Cariris Velhos event) [2Syn- to late-kinematic granites (Itaporanga CampGrande Acari Caraubas S Rafael Faz Nova ) wereintruded around 580 Ma [614] suggesting that all t

1084 B Dhuime et al C R Geoscience 335 (2003) 1081ndash1089

ofs

tes

Fig 1A Location of the Borborema Province (NE Brazil) in a pre-drift reconstructionB Geological sketch map of the Transversal zonethe Borborema Province indicates location of the studied sample Neoproterozoic granites not represented Abbreviations as followAMAlto Moxoto AP Alto PajeuGAC GuyanandashAmazon CratonPAB Piancoacute-Alto BriacutegidaRC Rio CapibaribeSFC Satildeo Francisco CratonSLC Satildeo Luis CratonPaL Patos LineamentPeL Pernambuco Lineament

Fig 1 A Localisation de la province Borborema (Nord-Est du Breacutesil) dans une reconstruction ante-ouverture de lrsquoAtlantiqueB Cartegeacuteologique simplifieacutee de la Zone transversale de la province Borborema indique la localisation de lrsquoeacutechantillon eacutetudieacute Les granineacuteoproteacuterozoiques nrsquoont pas eacuteteacute repreacutesenteacutes Les abreacuteviations correspondent agrave AM Alto Moxoto AP Alto Pajeu GAC cratonGuyano-Amazonien PAB Piancoacute-Alto Briacutegida RC Rio Capibaribe SFC craton de Satildeo Francisco SLC craton Satildeo Luis PaL lineacuteamentde Patos PeL lineacuteament de Pernambuco

aticheaceor-

enton

tioninom

ofand

do-ro-vol-by

ina-

andal

ibo-

touchdesa-30-redlendashct

rnethi-itemi-

ther-ui-

theor-om

ralnd

tonic features as well as the regional syn-kinemmetamorphism are of Neoproterozoic age [7] Tpresent study combines SEM images ICPndashMS trelement analyses and UndashPb dating on apatite inder to date cooling down to ca 500C associated withthe last high-temperature regional metamorphic evthat affected this area Analyses were performedapatites from a metamorphosed mafic rock associaoutcropping in the Piancoacute-Alto Briacutegida (PAB) doma(Fig 1) and the results are compared with data frother parts of the BP

2 Geological setting

The PAB domain is located in the southern partthe BP between two major shear zones the PatosPernambuco lineaments [26] Rocks of the PABmain mainly consist of undifferentiated Mesoprotezoic and Late Neoproterozoic metasediments metacanics and polymetamorphic gneisses all affecteda recumbent foliation Most granitoids (about 30outcrop) were intruded during the regional deformtion as their thermal aureoles are syn-kinematiccoeval with the regional metamorphism [6] Regionmetamorphism grades from green-schist to amph

lite facies low-pressure anatexis being restricteddomains of coalescent Neoproterozoic plutons sas the Itaporanga granite the root of which inclusillimanite-bearing diatexites free of low temperture retrogression [1] The selected sample is acm-thick garnetiferous leucocratic band interlayewithin a melanocratic Fe- and Ca-rich amphiboclinopyroxene rock from the Tito Braz gold prospelocated few kilometres north of the 584plusmn 2 Ma Ita-poranga pluton [6] The sample is composed of ga(50) quartz (30) labradorite (11) green ampbole (6) biotite (1) Fe sulphides zircon titanand apatite occur as accessory minerals Electroncroprobe analyses and the amphibole-plagioclasemometer of Blundy and Holland [3] gave an eqlibration temperature of about 640C for an esti-mated pressure of 4 kbar which we interpret asmain stage of regional amphibolite facies metamphic overprint (data are available upon request frthe first authordhuimedstuuniv-montp2fr)

3 Analytical techniques

Apatites were separated by conventional mineseparation methods including high-density liquids a

1084 B Dhuime et al C R Geoscience 335 (2003) 1081ndash1089

ofs

tes

Fig 1A Location of the Borborema Province (NE Brazil) in a pre-drift reconstructionB Geological sketch map of the Transversal zonethe Borborema Province indicates location of the studied sample Neoproterozoic granites not represented Abbreviations as followAMAlto Moxoto AP Alto PajeuGAC GuyanandashAmazon CratonPAB Piancoacute-Alto BriacutegidaRC Rio CapibaribeSFC Satildeo Francisco CratonSLC Satildeo Luis CratonPaL Patos LineamentPeL Pernambuco Lineament

Fig 1 A Localisation de la province Borborema (Nord-Est du Breacutesil) dans une reconstruction ante-ouverture de lrsquoAtlantiqueB Cartegeacuteologique simplifieacutee de la Zone transversale de la province Borborema indique la localisation de lrsquoeacutechantillon eacutetudieacute Les granineacuteoproteacuterozoiques nrsquoont pas eacuteteacute repreacutesenteacutes Les abreacuteviations correspondent agrave AM Alto Moxoto AP Alto Pajeu GAC cratonGuyano-Amazonien PAB Piancoacute-Alto Briacutegida RC Rio Capibaribe SFC craton de Satildeo Francisco SLC craton Satildeo Luis PaL lineacuteamentde Patos PeL lineacuteament de Pernambuco

aticheaceor-

enton

tioninom

ofand

do-ro-vol-by

ina-

andal

ibo-

touchdesa-30-redlendashct

rnethi-itemi-

ther-ui-

theor-om

ralnd

tonic features as well as the regional syn-kinemmetamorphism are of Neoproterozoic age [7] Tpresent study combines SEM images ICPndashMS trelement analyses and UndashPb dating on apatite inder to date cooling down to ca 500C associated withthe last high-temperature regional metamorphic evthat affected this area Analyses were performedapatites from a metamorphosed mafic rock associaoutcropping in the Piancoacute-Alto Briacutegida (PAB) doma(Fig 1) and the results are compared with data frother parts of the BP

2 Geological setting

The PAB domain is located in the southern partthe BP between two major shear zones the PatosPernambuco lineaments [26] Rocks of the PABmain mainly consist of undifferentiated Mesoprotezoic and Late Neoproterozoic metasediments metacanics and polymetamorphic gneisses all affecteda recumbent foliation Most granitoids (about 30outcrop) were intruded during the regional deformtion as their thermal aureoles are syn-kinematiccoeval with the regional metamorphism [6] Regionmetamorphism grades from green-schist to amph

lite facies low-pressure anatexis being restricteddomains of coalescent Neoproterozoic plutons sas the Itaporanga granite the root of which inclusillimanite-bearing diatexites free of low temperture retrogression [1] The selected sample is acm-thick garnetiferous leucocratic band interlayewithin a melanocratic Fe- and Ca-rich amphiboclinopyroxene rock from the Tito Braz gold prospelocated few kilometres north of the 584plusmn 2 Ma Ita-poranga pluton [6] The sample is composed of ga(50) quartz (30) labradorite (11) green ampbole (6) biotite (1) Fe sulphides zircon titanand apatite occur as accessory minerals Electroncroprobe analyses and the amphibole-plagioclasemometer of Blundy and Holland [3] gave an eqlibration temperature of about 640C for an esti-mated pressure of 4 kbar which we interpret asmain stage of regional amphibolite facies metamphic overprint (data are available upon request frthe first authordhuimedstuuniv-montp2fr)

3 Analytical techniques

Apatites were separated by conventional mineseparation methods including high-density liquids a

B Dhuime et al C R Geoscience 335 (2003) 1081ndash1089 1085

holeare

ur laes

erea

adtry

car-l Pb

15Theed

derof

s-e-in

ereci-ti-

plerete-ple

atite

de la

ith

ericighichnEuentgree

Ta4

Fig 2 Chondrite-normalized rare-earth element patterns for wrock and apatites from the PAB 50 sample Chondrite valuesfrom McDonough and Sun [19]

Fig 2 Spectre de terres rares normaliseacutees aux chondrites poroche totale et la fraction drsquoapatite de lrsquoeacutechantillon PAB 50 Lvaleurs des chondrites sont de McDonough et Sun [19]

a Frantz magnetic separator (eg [4]) Apatites wdissolved overnight in a Savillex teflon beaker onhot plate with 6 N HCl For isotopic analyses leand uranium were separated following the chemisdescribed by Krogh [17] and measurements wereried out on a VG Sector mass spectrometer Totablanks over the period of the analyses ranged fromto 30 pg and uranium blanks were less than 5 pgisotopic composition of radiogenic Pb was determinby subtracting the blank Pb and then the remainassuming a common Pb composition at the timeinitial crystallisation [24] Calculations were made uing the program of Ludwig [18] For trace element dtermination after dissolution samples were dilutedHNO3 shortly before analyses Concentrations wdetermined on a VG Plasmaquad II ICPndashMS (presion of 3 to 5) by external calibration using mulelement solutions prepared from 10 mg mlminus1 singleelemental solutions [15]

4 Results and discussion

41 Trace element analyses

Trace-element analyses of the whole rock samand of a small apatite fraction (ca 30 mg) areported in Table 1 and shown in the chondrinormalized diagram of Fig 2 The whole rock sam

Table 1ICPndashMS analytical data for the whole rock sample and the apfraction from the PAB 50 sample

Tableau 1Reacutesultats des analyses par ICPndashMS de la roche totale etfraction drsquoapatite de lrsquoeacutechantillon PAB 50

Sample name PAB 50

Analysis Whole rock ApatiteWeight (mg) 50 0032

Rb 20 031Sr 69 196Y 89 84Zr 22 44Nb 96 ndashCs 022 005Ba 31 40La 83 23Ce 28 134Pr 40 35Nd 22 309Sm 79 140Eu 26 24Gd 125 118Tb 22 92Dy 154 32Ho 31 33Er 89 50Tm 13 040Yb 83 14Lu 14 012Hf 067 ndashTa 071 ndashPb 32 14Th 062 013U 066 50

NbTa 134ThU 095 003(LaSm)CN 068 011(LaYb)CN 072 114EuEu 081 055

displays a flat to slightly LREE depleted pattern w(LaSm)CN and (LaYb)CN ratio close to 07 similar tonormal MORB derived from a depleted asthenosphmantle The rock however is characterized by hREE content of about 50 times the chondrites whsuggests affinity with a T-MORB protolith [11] Iaddition the pattern displays a slight negativeanomaly despite the high modal plagioclase contsuggesting the sample has undergone some deof differentiation and mineral segregation The Nbratio (134) is within error of the MORB value (14

1086 B Dhuime et al C R Geoscience 335 (2003) 1081ndash1089

ple

rons

tleas aingltserwsa

tescksE

fterhivelyts oof

usly

allainsclu-owNoer-ith

ednse--

50

de

read

phic

im-as itksveos-hichaler-

st-thelo-he

obeor-

gUndash-

Fig 3 SEM backscattered image of apatite from the PAB 50 sam

Fig 3 Image de microscopie eacutelectronique agrave balayage (eacutelectreacutetrodiffuseacutes) drsquoune apatite de lrsquoeacutechantillon PAB 50

plusmn17 after [16]) again suggesting a depleted mansource The sample is then tentatively regardedleucosome vein resulting from hydrous partial meltof altered basalts by similarity to leucocratic mefound within the roof zones of axial magma chamb[13 eg] The apatite REE-normalised pattern shoa pronounced depletion in LREE and HREE withnegative Eu anomaly The latter is typical of apatifrom various geodynamical settings and host ro[23] The marked depletion in both LREE and HREindicates that growth of the apatite occurred aor during crystallisation of LREE- and HREE-ricphases such as plagioclase and garnet respectThese phases are major metamorphic componenthe rock as they occur with a modal percentage11 and 50 respectively and can therefore seriocompete with apatite for REE during its growth

42 UndashPb analyses

Apatites occur as a uniform population of sm(100ndash150 microm) colourless translucent grains The grhave rounded shapes and are generally free of insions SEM images of polished surfaces (Fig 3) shthat they exhibit structureless internal domainscores oscillatory zoning patterns or magmatic ovgrowths have been observed which is consistent wa metamorphic origin The four fractions analyspresent low radiogenic Pb and U contents and coquently have low206Pb204Pb ratios (Table 2) Analy

f

Fig 4 UndashPb Concordia diagram for apatite fractions of the PABsample

Fig 4 Diagramme Concordia pour les fractions drsquoapatitelrsquoeacutechantillon PAB 50

ses are close to concordant (Fig 4) and show a spin the 207Pb235U ratios but identical206Pb238U ra-tios The four fractions provide a206Pb238U weightedmean age of 540plusmn 5 Ma (MSWD= 29) interpretedas our best estimate for the age of the metamorapatite

43 Implication on the post-metamorphic evolution

Although its application is limited by relatively lowU and high common Pb contents apatite is anportant accessory phase in UndashPb geochronologyis common in many igneous and metamorphic rocDue to its low ionic porosity apatite is thought to haa simple UndashPb system This contrasts with other phphatic phases such as monazites for example wmay contain multiple growth zones within individucrystals [5] and preserve an older age in an inhited core [9] Thus important information on the popeak metamorphic evolution can be obtained fromapatite results Cherniak et al [8] proposed a csure temperature for Pb diffusion in apatite in trange 450ndash550C for cooling rates of 1ndash10C Maminus1

and crystal sizes of 100ndash500 microm Electron microprstudy indicates that the sample underwent a metamphic overprint of about 640C therefore exceedinthe nominal closure temperature of apatite for thePb system The 540plusmn 5 Ma apatite age is thus in

BD

huime

etalCR

Geoscience

335(2003)

1081ndash10891087

Table 2UndashPb isotopic analyses for apatite fractions from the PAB 50 sample

Tableau 2Reacutesultats isotopiques UndashPb des fractions drsquoapatite de lrsquoeacutechantillon PAB 50

Sample Weigth U Pb 206Pb204Pb 208Pb206Pb 206Pb238U plusmn 207Pb235U plusmn 207Pb206Pb plusmn Apparent age (Ma)

name (mg) (ppm) (ppm) (2σ error) (2σ error) (2σ error) 206Pb238U plusmnPAB 50

Ap1 088 78 28 362 101 00873 00011 0825 0098 00686 00078 539 7Ap2 243 144 49 380 094 00877 00005 0661 0060 00547 00047 542 3Ap3 595 45 17 352 105 00875 00006 0920 0070 00763 00054 541 4Ap4 704 115 48 336 108 00866 00006 0667 0077 00567 00061 535 4

1088 B Dhuime et al C R Geoscience 335 (2003) 1081ndash1089

n-ted

0ndashta-rate

guch

d bylowted

lateith

talthy0ndashm

andcelaron-ea-lingof

tes

ssi-hice

oustec-eob-a-lueso-m-ghichorre-

entro-

so-ofpre-po-

-te-2]thatig-

ted-e

w-ith

theug-n

ndtemil)

iera

lto

iatrib

anUndashelt1kllan

an-s da 3

o-inro-91

terpreted as recording a cooling point on the dowside of regional metamorphism and it is suggesthat the temperature has dropped from 640C at ca580 Ma (the age of peak metamorphism) to 45550C at 540 Ma This indicates that after peak memorphism the temperature decreased slowly at aof 25 to 5C Maminus1 Similar values (ranging from 3to 4C Maminus1) were calculated from Ar datings alonthe Patos shear zone and in the Seridoacute area [10] Sslow cooling rates suggest that denudation occurreprocesses consistent with isostatic recovery anderosion rates This contrasts with tectonically assisretrograde evolution associated for example withmetamorphic thrusting and nappe stacking and wthe differential uplift and exhumation of deep crusunits along major shear zones It is also noteworthat the 540 Ma apatite age is identical to the 54550 Ma Ar amphibole ages provided by rocks frothe western termination of the Patos lineamentfrom the Cedro belt north of this lineament [20] Sinthe amphibole closure temperature for Ar is simito the UndashPb closure temperature of apatite it is ccluded that crustal units on each side of the linment undergone the same post-metamorphic coohistory Conversely rocks from the southern partthe BP (Caruaru area) yielded identical cooling ra(ca 5C Maminus1) but cooling down to 500C occurredearlier at ca 580 Ma [21] This suggests a poble diachronous evolution for the tectonometamorpevent in the BP during the BrazilianoPanAfrican timThis period of tectonism metamorphism and igneemplacement represents a major period of platetonic activity resulting in the final assembly of thGondwana supercontinent The low cooling ratesserved in the whole BP except in the vicinity of mjor shear zones where cooling had reached vaof 20C Maminus1 (eg [20]) and the possible diachrnism are not compatible with delamination phenoena which are typically associated with uplift and hierosion rates Underplating of mafic magmas whreduces uplift while adding heat to the systemcrustal thickening caused by nappe stacking andsponsible for partial melting and granite emplacemare more likely processes to explain the Neoprotezoic metamorphic evolution of the BP and the asciated long-lived thermal anomaly Lastly relicsnon-metamorphic continental molassic basins areserved along the Patos lineament [7] Molasse de

sition was coeval with felsic volcanism and with emplacement of A-type hypo-volcanic granites of LaCambrian age in the northern part of the BP [1Preservation of the 540-Ma apatite age indicatesthis magmatic activity was not associated with a snificant thermal anomaly of regional extent

5 Conclusions

Conventional UndashPb analyses of apatite extracfrom a leucosome vein of T-MORB affinity outcropping in the Piancoacute-Alto Brigida (PAB) domain of thBorborema Province (BP) provide an age of 540plusmn5 Ma This value is interpreted as a cooling age folloing regional amphibolite facies metamorphism wcooling rates of about 25 to 5C Maminus1 These aresimilar to cooling rates obtained from other parts ofBP although cooling occurred earlier in the south sgesting a possible northndashsouth diachronic evolutio

References

[1] CJ Archanjo ER Da Siva R Caby Magnetic fabric apluton emplacement in a transpressive shear zone systhe Itaporanga porphyritic granitic pluton (northeast BrazTectonophysics 312 (1999) 331ndash345

[2] PR Bastos Leite J-M Bertrand ES Lima J LeterrTiming of granitic magmatism in the northern BorboremProvince Brazil a UndashPb study of granitoids from the APajeuacute Terrain J South Am Earth Sci 13 (2000) 549ndash559

[3] JD Blundy TJB Holland Calcic amphibole equilibrand a new amphibolendashplagioclase geothermometer ConMineral Petrol 104 (1990) 208ndash224

[4] D Bosch O Bruguier RT Pidgeon The evolution ofArchean Metamorphic Belt a conventional and SHRIMPPb study on accessory minerals from the Jimperding BYilgarn Craton West Australia J Geol 104 (1996) 695ndash71

[5] D Bosch D Hammor O Bruguier R Caby JM LucMonazite lsquoin-sitursquo 207Pb206Pb geochronology using a smageometry high-resolution ion probe Application to Archeand Proterozoic rocks Chem Geol 184 (2002) 151ndash165

[6] BB Brito Neves CR Passarelli MAS Basei EJ Stos Idades UndashPb em zircao de alguns granitos classicoprovincia Borborema Boletim Geol USP serie cientifica(in press)

[7] R Caby A Sial M Arthaud A Vauchez Crustal evlution and the Brasiliano orogeny in Northeast BrazilRD Dallmeyer J-P Leacutecorcheacute (Eds) The West African Ogens and Circum Atlantic Correlatives Springer-Verlag 19pp 373ndash397

B Dhuime et al C R Geoscience 335 (2003) 1081ndash1089 1089

onord

55

offor

udr-

lerlts)

s onteeas

f75ndash

onoof00)

ndashheir

en-ce

alpic73)

2y

rth

sil-

64tion

therian

eo-omazil

er-elypet-81ad

Sci

erthelu-

iniar

[8] DJ Cherniak WA Lanford FJ Ryerson Lead diffusiin apatite and zircon using ion implantation and Rutherfbackscattering techniques Geochim Cosmochim Acta(1991) 1663ndash1673

[9] P Copeland RR Parrish TM Harrison Identificationinherited radiogenic Pb in monazite and its implicationsUndashPb systematics Nature 333 (1988) 760ndash763

[10] M Corsini L Lambert de Figueiredo R Caby G FeacuteraG Ruffet A Vauchez Thermal history of the Boborema Province deduced from40Ar39Ar analysis Tectono-physics 285 (1998) 103ndash117

[11] JP Eissen T Juteau JL Joron B Dupreacute E HumA AlrsquoMukhamedov Petrology and geochemistry of basafrom the Red Sea axial rift at 18north J Petrol 30 (1989791ndash839

[12] AH Fetter UndashPb and SmndashNd geochronological constraintthe crustal framework and geologic history of the CearrsquostaNW Borborema Province NE Brazil implications for thassembly of Gondwana PhD Thesis University of Kans1999 145 p

[13] KM Gillis LA Coogan Anatectic migmatites from the rooof an ocean ridge magma chamber J Petrol 43 (1991) 202095

[14] IP Guimaratildees CN Almeida AF Da Silva FilhJM Arauacutejo Granitoids marking the end of the Brasilia(Pan-African) orogeny within the Central Tectonic Domainthe Borborema Province Rev Bras Geociecircncias 30 (20117ndash181

[15] DA Ionov L Savoyant C Dupuy Application of the ICPMS technique to trace element analysis of peridotites and tminerals Geostandards Newslett 16 (1992) 311ndash315

[16] F Kalfoun Geacuteochimie de NbndashTa contraintes sur la diffeacuterciation terrestre PhD thesis University Montpellier-2 Fran2001 300 p

[17] TE Krogh A low-contamination method for hydrothermdecomposition of zircon and extraction of U and Pb for isotoage determinations Geochim Cosmochim Acta 37 (19485ndash494

[18] KR Ludwig Users manual for ISOPLOTEX versionA geochronological toolkit for Microsoft Excel BerkeleGeochronology Center Spec Publ 1a (1999) 1ndash30

[19] WF McDonough SS Sun The composition of the EaChem Geol 120 (1995) 223ndash253

[20] P Monieacute R Caby MH Arthaud The Neoproterozoic Braiano orogeny in northeast Brazil40Ar39Ar and petrostruc-tural data drom Cearagrave Precambrian Res 81 (1997) 241ndash2

[21] SP Neves A Vauchez G Feacuteraud Tectono-thermal evolumagma emplacement and shear zone development inCaruaru area (Borborema Province NE Brazil) PrecambRes 99 (2000) 1ndash32

[22] J-M Saacute J-M Bertrand J Leterrier MHF Macedo Gchemistry and geochronology of the pre-Brasiliano rocks frthe Transversal Zone Borborema Province Northeast BrJ South Amer Earth Sci 14 (2002) 851ndash866

[23] LK Sha BW Chappell Apatite chemical composition detmined by electron microprobe and laser ablation inductivcoupled plasma mass spectrometry as a probe into graniterogenesis Geochim Cosmochim Acta 63 (1999) 3861ndash38

[24] JS Stacey JD Kramers Approximation of terrestrial leisotope evolution by a two-stage model Earth PlanetLett 26 (1975) 206ndash221

[25] WR Van Schmus BB Brito Neves P HackspachM Babinski UPb and SmNd geochronological studies ofEastern Borborema Province Northeast Brazil initial concsions J South Am Earth Sci 8 (1995) 267ndash288

[26] A Vauchez S Pacheco Neves R Caby M CorsM Egydio-Silva M Arthaud V Amaro The Borborema shezone system J South Am Earth Sci 8 (1995) 247ndash266

1086 B Dhuime et al C R Geoscience 335 (2003) 1081ndash1089

ple

rons

tleas aingltserwsa

tescksE

fterhivelyts oof

usly

allainsclu-owNoer-ith

ednse--

50

de

read

phic

im-as itksveos-hichaler-

st-thelo-he

obeor-

gUndash-

Fig 3 SEM backscattered image of apatite from the PAB 50 sam

Fig 3 Image de microscopie eacutelectronique agrave balayage (eacutelectreacutetrodiffuseacutes) drsquoune apatite de lrsquoeacutechantillon PAB 50

plusmn17 after [16]) again suggesting a depleted mansource The sample is then tentatively regardedleucosome vein resulting from hydrous partial meltof altered basalts by similarity to leucocratic mefound within the roof zones of axial magma chamb[13 eg] The apatite REE-normalised pattern shoa pronounced depletion in LREE and HREE withnegative Eu anomaly The latter is typical of apatifrom various geodynamical settings and host ro[23] The marked depletion in both LREE and HREindicates that growth of the apatite occurred aor during crystallisation of LREE- and HREE-ricphases such as plagioclase and garnet respectThese phases are major metamorphic componenthe rock as they occur with a modal percentage11 and 50 respectively and can therefore seriocompete with apatite for REE during its growth

42 UndashPb analyses

Apatites occur as a uniform population of sm(100ndash150 microm) colourless translucent grains The grhave rounded shapes and are generally free of insions SEM images of polished surfaces (Fig 3) shthat they exhibit structureless internal domainscores oscillatory zoning patterns or magmatic ovgrowths have been observed which is consistent wa metamorphic origin The four fractions analyspresent low radiogenic Pb and U contents and coquently have low206Pb204Pb ratios (Table 2) Analy

f

Fig 4 UndashPb Concordia diagram for apatite fractions of the PABsample

Fig 4 Diagramme Concordia pour les fractions drsquoapatitelrsquoeacutechantillon PAB 50

ses are close to concordant (Fig 4) and show a spin the 207Pb235U ratios but identical206Pb238U ra-tios The four fractions provide a206Pb238U weightedmean age of 540plusmn 5 Ma (MSWD= 29) interpretedas our best estimate for the age of the metamorapatite

43 Implication on the post-metamorphic evolution

Although its application is limited by relatively lowU and high common Pb contents apatite is anportant accessory phase in UndashPb geochronologyis common in many igneous and metamorphic rocDue to its low ionic porosity apatite is thought to haa simple UndashPb system This contrasts with other phphatic phases such as monazites for example wmay contain multiple growth zones within individucrystals [5] and preserve an older age in an inhited core [9] Thus important information on the popeak metamorphic evolution can be obtained fromapatite results Cherniak et al [8] proposed a csure temperature for Pb diffusion in apatite in trange 450ndash550C for cooling rates of 1ndash10C Maminus1

and crystal sizes of 100ndash500 microm Electron microprstudy indicates that the sample underwent a metamphic overprint of about 640C therefore exceedinthe nominal closure temperature of apatite for thePb system The 540plusmn 5 Ma apatite age is thus in

BD

huime

etalCR

Geoscience

335(2003)

1081ndash10891087

Table 2UndashPb isotopic analyses for apatite fractions from the PAB 50 sample

Tableau 2Reacutesultats isotopiques UndashPb des fractions drsquoapatite de lrsquoeacutechantillon PAB 50

Sample Weigth U Pb 206Pb204Pb 208Pb206Pb 206Pb238U plusmn 207Pb235U plusmn 207Pb206Pb plusmn Apparent age (Ma)

name (mg) (ppm) (ppm) (2σ error) (2σ error) (2σ error) 206Pb238U plusmnPAB 50

Ap1 088 78 28 362 101 00873 00011 0825 0098 00686 00078 539 7Ap2 243 144 49 380 094 00877 00005 0661 0060 00547 00047 542 3Ap3 595 45 17 352 105 00875 00006 0920 0070 00763 00054 541 4Ap4 704 115 48 336 108 00866 00006 0667 0077 00567 00061 535 4

1088 B Dhuime et al C R Geoscience 335 (2003) 1081ndash1089

n-ted

0ndashta-rate

guch

d bylowted

lateith

talthy0ndashm

andcelaron-ea-lingof

tes

ssi-hice

oustec-eob-a-lueso-m-ghichorre-

entro-

so-ofpre-po-

-te-2]thatig-

ted-e

w-ith

theug-n

ndtemil)

iera

lto

iatrib

anUndashelt1kllan

an-s da 3

o-inro-91

terpreted as recording a cooling point on the dowside of regional metamorphism and it is suggesthat the temperature has dropped from 640C at ca580 Ma (the age of peak metamorphism) to 45550C at 540 Ma This indicates that after peak memorphism the temperature decreased slowly at aof 25 to 5C Maminus1 Similar values (ranging from 3to 4C Maminus1) were calculated from Ar datings alonthe Patos shear zone and in the Seridoacute area [10] Sslow cooling rates suggest that denudation occurreprocesses consistent with isostatic recovery anderosion rates This contrasts with tectonically assisretrograde evolution associated for example withmetamorphic thrusting and nappe stacking and wthe differential uplift and exhumation of deep crusunits along major shear zones It is also noteworthat the 540 Ma apatite age is identical to the 54550 Ma Ar amphibole ages provided by rocks frothe western termination of the Patos lineamentfrom the Cedro belt north of this lineament [20] Sinthe amphibole closure temperature for Ar is simito the UndashPb closure temperature of apatite it is ccluded that crustal units on each side of the linment undergone the same post-metamorphic coohistory Conversely rocks from the southern partthe BP (Caruaru area) yielded identical cooling ra(ca 5C Maminus1) but cooling down to 500C occurredearlier at ca 580 Ma [21] This suggests a poble diachronous evolution for the tectonometamorpevent in the BP during the BrazilianoPanAfrican timThis period of tectonism metamorphism and igneemplacement represents a major period of platetonic activity resulting in the final assembly of thGondwana supercontinent The low cooling ratesserved in the whole BP except in the vicinity of mjor shear zones where cooling had reached vaof 20C Maminus1 (eg [20]) and the possible diachrnism are not compatible with delamination phenoena which are typically associated with uplift and hierosion rates Underplating of mafic magmas whreduces uplift while adding heat to the systemcrustal thickening caused by nappe stacking andsponsible for partial melting and granite emplacemare more likely processes to explain the Neoprotezoic metamorphic evolution of the BP and the asciated long-lived thermal anomaly Lastly relicsnon-metamorphic continental molassic basins areserved along the Patos lineament [7] Molasse de

sition was coeval with felsic volcanism and with emplacement of A-type hypo-volcanic granites of LaCambrian age in the northern part of the BP [1Preservation of the 540-Ma apatite age indicatesthis magmatic activity was not associated with a snificant thermal anomaly of regional extent

5 Conclusions

Conventional UndashPb analyses of apatite extracfrom a leucosome vein of T-MORB affinity outcropping in the Piancoacute-Alto Brigida (PAB) domain of thBorborema Province (BP) provide an age of 540plusmn5 Ma This value is interpreted as a cooling age folloing regional amphibolite facies metamorphism wcooling rates of about 25 to 5C Maminus1 These aresimilar to cooling rates obtained from other parts ofBP although cooling occurred earlier in the south sgesting a possible northndashsouth diachronic evolutio

References

[1] CJ Archanjo ER Da Siva R Caby Magnetic fabric apluton emplacement in a transpressive shear zone systhe Itaporanga porphyritic granitic pluton (northeast BrazTectonophysics 312 (1999) 331ndash345

[2] PR Bastos Leite J-M Bertrand ES Lima J LeterrTiming of granitic magmatism in the northern BorboremProvince Brazil a UndashPb study of granitoids from the APajeuacute Terrain J South Am Earth Sci 13 (2000) 549ndash559

[3] JD Blundy TJB Holland Calcic amphibole equilibrand a new amphibolendashplagioclase geothermometer ConMineral Petrol 104 (1990) 208ndash224

[4] D Bosch O Bruguier RT Pidgeon The evolution ofArchean Metamorphic Belt a conventional and SHRIMPPb study on accessory minerals from the Jimperding BYilgarn Craton West Australia J Geol 104 (1996) 695ndash71

[5] D Bosch D Hammor O Bruguier R Caby JM LucMonazite lsquoin-sitursquo 207Pb206Pb geochronology using a smageometry high-resolution ion probe Application to Archeand Proterozoic rocks Chem Geol 184 (2002) 151ndash165

[6] BB Brito Neves CR Passarelli MAS Basei EJ Stos Idades UndashPb em zircao de alguns granitos classicoprovincia Borborema Boletim Geol USP serie cientifica(in press)

[7] R Caby A Sial M Arthaud A Vauchez Crustal evlution and the Brasiliano orogeny in Northeast BrazilRD Dallmeyer J-P Leacutecorcheacute (Eds) The West African Ogens and Circum Atlantic Correlatives Springer-Verlag 19pp 373ndash397

B Dhuime et al C R Geoscience 335 (2003) 1081ndash1089 1089

onord

55

offor

udr-

lerlts)

s onteeas

f75ndash

onoof00)

ndashheir

en-ce

alpic73)

2y

rth

sil-

64tion

therian

eo-omazil

er-elypet-81ad

Sci

erthelu-

iniar

[8] DJ Cherniak WA Lanford FJ Ryerson Lead diffusiin apatite and zircon using ion implantation and Rutherfbackscattering techniques Geochim Cosmochim Acta(1991) 1663ndash1673

[9] P Copeland RR Parrish TM Harrison Identificationinherited radiogenic Pb in monazite and its implicationsUndashPb systematics Nature 333 (1988) 760ndash763

[10] M Corsini L Lambert de Figueiredo R Caby G FeacuteraG Ruffet A Vauchez Thermal history of the Boborema Province deduced from40Ar39Ar analysis Tectono-physics 285 (1998) 103ndash117

[11] JP Eissen T Juteau JL Joron B Dupreacute E HumA AlrsquoMukhamedov Petrology and geochemistry of basafrom the Red Sea axial rift at 18north J Petrol 30 (1989791ndash839

[12] AH Fetter UndashPb and SmndashNd geochronological constraintthe crustal framework and geologic history of the CearrsquostaNW Borborema Province NE Brazil implications for thassembly of Gondwana PhD Thesis University of Kans1999 145 p

[13] KM Gillis LA Coogan Anatectic migmatites from the rooof an ocean ridge magma chamber J Petrol 43 (1991) 202095

[14] IP Guimaratildees CN Almeida AF Da Silva FilhJM Arauacutejo Granitoids marking the end of the Brasilia(Pan-African) orogeny within the Central Tectonic Domainthe Borborema Province Rev Bras Geociecircncias 30 (20117ndash181

[15] DA Ionov L Savoyant C Dupuy Application of the ICPMS technique to trace element analysis of peridotites and tminerals Geostandards Newslett 16 (1992) 311ndash315

[16] F Kalfoun Geacuteochimie de NbndashTa contraintes sur la diffeacuterciation terrestre PhD thesis University Montpellier-2 Fran2001 300 p

[17] TE Krogh A low-contamination method for hydrothermdecomposition of zircon and extraction of U and Pb for isotoage determinations Geochim Cosmochim Acta 37 (19485ndash494

[18] KR Ludwig Users manual for ISOPLOTEX versionA geochronological toolkit for Microsoft Excel BerkeleGeochronology Center Spec Publ 1a (1999) 1ndash30

[19] WF McDonough SS Sun The composition of the EaChem Geol 120 (1995) 223ndash253

[20] P Monieacute R Caby MH Arthaud The Neoproterozoic Braiano orogeny in northeast Brazil40Ar39Ar and petrostruc-tural data drom Cearagrave Precambrian Res 81 (1997) 241ndash2

[21] SP Neves A Vauchez G Feacuteraud Tectono-thermal evolumagma emplacement and shear zone development inCaruaru area (Borborema Province NE Brazil) PrecambRes 99 (2000) 1ndash32

[22] J-M Saacute J-M Bertrand J Leterrier MHF Macedo Gchemistry and geochronology of the pre-Brasiliano rocks frthe Transversal Zone Borborema Province Northeast BrJ South Amer Earth Sci 14 (2002) 851ndash866

[23] LK Sha BW Chappell Apatite chemical composition detmined by electron microprobe and laser ablation inductivcoupled plasma mass spectrometry as a probe into graniterogenesis Geochim Cosmochim Acta 63 (1999) 3861ndash38

[24] JS Stacey JD Kramers Approximation of terrestrial leisotope evolution by a two-stage model Earth PlanetLett 26 (1975) 206ndash221

[25] WR Van Schmus BB Brito Neves P HackspachM Babinski UPb and SmNd geochronological studies ofEastern Borborema Province Northeast Brazil initial concsions J South Am Earth Sci 8 (1995) 267ndash288

[26] A Vauchez S Pacheco Neves R Caby M CorsM Egydio-Silva M Arthaud V Amaro The Borborema shezone system J South Am Earth Sci 8 (1995) 247ndash266

BD

huime

etalCR

Geoscience

335(2003)

1081ndash10891087

Table 2UndashPb isotopic analyses for apatite fractions from the PAB 50 sample

Tableau 2Reacutesultats isotopiques UndashPb des fractions drsquoapatite de lrsquoeacutechantillon PAB 50

Sample Weigth U Pb 206Pb204Pb 208Pb206Pb 206Pb238U plusmn 207Pb235U plusmn 207Pb206Pb plusmn Apparent age (Ma)

name (mg) (ppm) (ppm) (2σ error) (2σ error) (2σ error) 206Pb238U plusmnPAB 50

Ap1 088 78 28 362 101 00873 00011 0825 0098 00686 00078 539 7Ap2 243 144 49 380 094 00877 00005 0661 0060 00547 00047 542 3Ap3 595 45 17 352 105 00875 00006 0920 0070 00763 00054 541 4Ap4 704 115 48 336 108 00866 00006 0667 0077 00567 00061 535 4

1088 B Dhuime et al C R Geoscience 335 (2003) 1081ndash1089

n-ted

0ndashta-rate

guch

d bylowted

lateith

talthy0ndashm

andcelaron-ea-lingof

tes

ssi-hice

oustec-eob-a-lueso-m-ghichorre-

entro-

so-ofpre-po-

-te-2]thatig-

ted-e

w-ith

theug-n

ndtemil)

iera

lto

iatrib

anUndashelt1kllan

an-s da 3

o-inro-91

terpreted as recording a cooling point on the dowside of regional metamorphism and it is suggesthat the temperature has dropped from 640C at ca580 Ma (the age of peak metamorphism) to 45550C at 540 Ma This indicates that after peak memorphism the temperature decreased slowly at aof 25 to 5C Maminus1 Similar values (ranging from 3to 4C Maminus1) were calculated from Ar datings alonthe Patos shear zone and in the Seridoacute area [10] Sslow cooling rates suggest that denudation occurreprocesses consistent with isostatic recovery anderosion rates This contrasts with tectonically assisretrograde evolution associated for example withmetamorphic thrusting and nappe stacking and wthe differential uplift and exhumation of deep crusunits along major shear zones It is also noteworthat the 540 Ma apatite age is identical to the 54550 Ma Ar amphibole ages provided by rocks frothe western termination of the Patos lineamentfrom the Cedro belt north of this lineament [20] Sinthe amphibole closure temperature for Ar is simito the UndashPb closure temperature of apatite it is ccluded that crustal units on each side of the linment undergone the same post-metamorphic coohistory Conversely rocks from the southern partthe BP (Caruaru area) yielded identical cooling ra(ca 5C Maminus1) but cooling down to 500C occurredearlier at ca 580 Ma [21] This suggests a poble diachronous evolution for the tectonometamorpevent in the BP during the BrazilianoPanAfrican timThis period of tectonism metamorphism and igneemplacement represents a major period of platetonic activity resulting in the final assembly of thGondwana supercontinent The low cooling ratesserved in the whole BP except in the vicinity of mjor shear zones where cooling had reached vaof 20C Maminus1 (eg [20]) and the possible diachrnism are not compatible with delamination phenoena which are typically associated with uplift and hierosion rates Underplating of mafic magmas whreduces uplift while adding heat to the systemcrustal thickening caused by nappe stacking andsponsible for partial melting and granite emplacemare more likely processes to explain the Neoprotezoic metamorphic evolution of the BP and the asciated long-lived thermal anomaly Lastly relicsnon-metamorphic continental molassic basins areserved along the Patos lineament [7] Molasse de

sition was coeval with felsic volcanism and with emplacement of A-type hypo-volcanic granites of LaCambrian age in the northern part of the BP [1Preservation of the 540-Ma apatite age indicatesthis magmatic activity was not associated with a snificant thermal anomaly of regional extent

5 Conclusions

Conventional UndashPb analyses of apatite extracfrom a leucosome vein of T-MORB affinity outcropping in the Piancoacute-Alto Brigida (PAB) domain of thBorborema Province (BP) provide an age of 540plusmn5 Ma This value is interpreted as a cooling age folloing regional amphibolite facies metamorphism wcooling rates of about 25 to 5C Maminus1 These aresimilar to cooling rates obtained from other parts ofBP although cooling occurred earlier in the south sgesting a possible northndashsouth diachronic evolutio

References

[1] CJ Archanjo ER Da Siva R Caby Magnetic fabric apluton emplacement in a transpressive shear zone systhe Itaporanga porphyritic granitic pluton (northeast BrazTectonophysics 312 (1999) 331ndash345

[2] PR Bastos Leite J-M Bertrand ES Lima J LeterrTiming of granitic magmatism in the northern BorboremProvince Brazil a UndashPb study of granitoids from the APajeuacute Terrain J South Am Earth Sci 13 (2000) 549ndash559

[3] JD Blundy TJB Holland Calcic amphibole equilibrand a new amphibolendashplagioclase geothermometer ConMineral Petrol 104 (1990) 208ndash224

[4] D Bosch O Bruguier RT Pidgeon The evolution ofArchean Metamorphic Belt a conventional and SHRIMPPb study on accessory minerals from the Jimperding BYilgarn Craton West Australia J Geol 104 (1996) 695ndash71

[5] D Bosch D Hammor O Bruguier R Caby JM LucMonazite lsquoin-sitursquo 207Pb206Pb geochronology using a smageometry high-resolution ion probe Application to Archeand Proterozoic rocks Chem Geol 184 (2002) 151ndash165

[6] BB Brito Neves CR Passarelli MAS Basei EJ Stos Idades UndashPb em zircao de alguns granitos classicoprovincia Borborema Boletim Geol USP serie cientifica(in press)

[7] R Caby A Sial M Arthaud A Vauchez Crustal evlution and the Brasiliano orogeny in Northeast BrazilRD Dallmeyer J-P Leacutecorcheacute (Eds) The West African Ogens and Circum Atlantic Correlatives Springer-Verlag 19pp 373ndash397

B Dhuime et al C R Geoscience 335 (2003) 1081ndash1089 1089

onord

55

offor

udr-

lerlts)

s onteeas

f75ndash

onoof00)

ndashheir

en-ce

alpic73)

2y

rth

sil-

64tion

therian

eo-omazil

er-elypet-81ad

Sci

erthelu-

iniar

[8] DJ Cherniak WA Lanford FJ Ryerson Lead diffusiin apatite and zircon using ion implantation and Rutherfbackscattering techniques Geochim Cosmochim Acta(1991) 1663ndash1673

[9] P Copeland RR Parrish TM Harrison Identificationinherited radiogenic Pb in monazite and its implicationsUndashPb systematics Nature 333 (1988) 760ndash763

[10] M Corsini L Lambert de Figueiredo R Caby G FeacuteraG Ruffet A Vauchez Thermal history of the Boborema Province deduced from40Ar39Ar analysis Tectono-physics 285 (1998) 103ndash117

[11] JP Eissen T Juteau JL Joron B Dupreacute E HumA AlrsquoMukhamedov Petrology and geochemistry of basafrom the Red Sea axial rift at 18north J Petrol 30 (1989791ndash839

[12] AH Fetter UndashPb and SmndashNd geochronological constraintthe crustal framework and geologic history of the CearrsquostaNW Borborema Province NE Brazil implications for thassembly of Gondwana PhD Thesis University of Kans1999 145 p

[13] KM Gillis LA Coogan Anatectic migmatites from the rooof an ocean ridge magma chamber J Petrol 43 (1991) 202095

[14] IP Guimaratildees CN Almeida AF Da Silva FilhJM Arauacutejo Granitoids marking the end of the Brasilia(Pan-African) orogeny within the Central Tectonic Domainthe Borborema Province Rev Bras Geociecircncias 30 (20117ndash181

[15] DA Ionov L Savoyant C Dupuy Application of the ICPMS technique to trace element analysis of peridotites and tminerals Geostandards Newslett 16 (1992) 311ndash315

[16] F Kalfoun Geacuteochimie de NbndashTa contraintes sur la diffeacuterciation terrestre PhD thesis University Montpellier-2 Fran2001 300 p

[17] TE Krogh A low-contamination method for hydrothermdecomposition of zircon and extraction of U and Pb for isotoage determinations Geochim Cosmochim Acta 37 (19485ndash494

[18] KR Ludwig Users manual for ISOPLOTEX versionA geochronological toolkit for Microsoft Excel BerkeleGeochronology Center Spec Publ 1a (1999) 1ndash30

[19] WF McDonough SS Sun The composition of the EaChem Geol 120 (1995) 223ndash253

[20] P Monieacute R Caby MH Arthaud The Neoproterozoic Braiano orogeny in northeast Brazil40Ar39Ar and petrostruc-tural data drom Cearagrave Precambrian Res 81 (1997) 241ndash2

[21] SP Neves A Vauchez G Feacuteraud Tectono-thermal evolumagma emplacement and shear zone development inCaruaru area (Borborema Province NE Brazil) PrecambRes 99 (2000) 1ndash32

[22] J-M Saacute J-M Bertrand J Leterrier MHF Macedo Gchemistry and geochronology of the pre-Brasiliano rocks frthe Transversal Zone Borborema Province Northeast BrJ South Amer Earth Sci 14 (2002) 851ndash866

[23] LK Sha BW Chappell Apatite chemical composition detmined by electron microprobe and laser ablation inductivcoupled plasma mass spectrometry as a probe into graniterogenesis Geochim Cosmochim Acta 63 (1999) 3861ndash38

[24] JS Stacey JD Kramers Approximation of terrestrial leisotope evolution by a two-stage model Earth PlanetLett 26 (1975) 206ndash221

[25] WR Van Schmus BB Brito Neves P HackspachM Babinski UPb and SmNd geochronological studies ofEastern Borborema Province Northeast Brazil initial concsions J South Am Earth Sci 8 (1995) 267ndash288

[26] A Vauchez S Pacheco Neves R Caby M CorsM Egydio-Silva M Arthaud V Amaro The Borborema shezone system J South Am Earth Sci 8 (1995) 247ndash266

1088 B Dhuime et al C R Geoscience 335 (2003) 1081ndash1089

n-ted

0ndashta-rate

guch

d bylowted

lateith

talthy0ndashm

andcelaron-ea-lingof

tes

ssi-hice

oustec-eob-a-lueso-m-ghichorre-

entro-

so-ofpre-po-

-te-2]thatig-

ted-e

w-ith

theug-n

ndtemil)

iera

lto

iatrib

anUndashelt1kllan

an-s da 3

o-inro-91

terpreted as recording a cooling point on the dowside of regional metamorphism and it is suggesthat the temperature has dropped from 640C at ca580 Ma (the age of peak metamorphism) to 45550C at 540 Ma This indicates that after peak memorphism the temperature decreased slowly at aof 25 to 5C Maminus1 Similar values (ranging from 3to 4C Maminus1) were calculated from Ar datings alonthe Patos shear zone and in the Seridoacute area [10] Sslow cooling rates suggest that denudation occurreprocesses consistent with isostatic recovery anderosion rates This contrasts with tectonically assisretrograde evolution associated for example withmetamorphic thrusting and nappe stacking and wthe differential uplift and exhumation of deep crusunits along major shear zones It is also noteworthat the 540 Ma apatite age is identical to the 54550 Ma Ar amphibole ages provided by rocks frothe western termination of the Patos lineamentfrom the Cedro belt north of this lineament [20] Sinthe amphibole closure temperature for Ar is simito the UndashPb closure temperature of apatite it is ccluded that crustal units on each side of the linment undergone the same post-metamorphic coohistory Conversely rocks from the southern partthe BP (Caruaru area) yielded identical cooling ra(ca 5C Maminus1) but cooling down to 500C occurredearlier at ca 580 Ma [21] This suggests a poble diachronous evolution for the tectonometamorpevent in the BP during the BrazilianoPanAfrican timThis period of tectonism metamorphism and igneemplacement represents a major period of platetonic activity resulting in the final assembly of thGondwana supercontinent The low cooling ratesserved in the whole BP except in the vicinity of mjor shear zones where cooling had reached vaof 20C Maminus1 (eg [20]) and the possible diachrnism are not compatible with delamination phenoena which are typically associated with uplift and hierosion rates Underplating of mafic magmas whreduces uplift while adding heat to the systemcrustal thickening caused by nappe stacking andsponsible for partial melting and granite emplacemare more likely processes to explain the Neoprotezoic metamorphic evolution of the BP and the asciated long-lived thermal anomaly Lastly relicsnon-metamorphic continental molassic basins areserved along the Patos lineament [7] Molasse de

sition was coeval with felsic volcanism and with emplacement of A-type hypo-volcanic granites of LaCambrian age in the northern part of the BP [1Preservation of the 540-Ma apatite age indicatesthis magmatic activity was not associated with a snificant thermal anomaly of regional extent

5 Conclusions

Conventional UndashPb analyses of apatite extracfrom a leucosome vein of T-MORB affinity outcropping in the Piancoacute-Alto Brigida (PAB) domain of thBorborema Province (BP) provide an age of 540plusmn5 Ma This value is interpreted as a cooling age folloing regional amphibolite facies metamorphism wcooling rates of about 25 to 5C Maminus1 These aresimilar to cooling rates obtained from other parts ofBP although cooling occurred earlier in the south sgesting a possible northndashsouth diachronic evolutio

References

[1] CJ Archanjo ER Da Siva R Caby Magnetic fabric apluton emplacement in a transpressive shear zone systhe Itaporanga porphyritic granitic pluton (northeast BrazTectonophysics 312 (1999) 331ndash345

[2] PR Bastos Leite J-M Bertrand ES Lima J LeterrTiming of granitic magmatism in the northern BorboremProvince Brazil a UndashPb study of granitoids from the APajeuacute Terrain J South Am Earth Sci 13 (2000) 549ndash559

[3] JD Blundy TJB Holland Calcic amphibole equilibrand a new amphibolendashplagioclase geothermometer ConMineral Petrol 104 (1990) 208ndash224

[4] D Bosch O Bruguier RT Pidgeon The evolution ofArchean Metamorphic Belt a conventional and SHRIMPPb study on accessory minerals from the Jimperding BYilgarn Craton West Australia J Geol 104 (1996) 695ndash71

[5] D Bosch D Hammor O Bruguier R Caby JM LucMonazite lsquoin-sitursquo 207Pb206Pb geochronology using a smageometry high-resolution ion probe Application to Archeand Proterozoic rocks Chem Geol 184 (2002) 151ndash165

[6] BB Brito Neves CR Passarelli MAS Basei EJ Stos Idades UndashPb em zircao de alguns granitos classicoprovincia Borborema Boletim Geol USP serie cientifica(in press)

[7] R Caby A Sial M Arthaud A Vauchez Crustal evlution and the Brasiliano orogeny in Northeast BrazilRD Dallmeyer J-P Leacutecorcheacute (Eds) The West African Ogens and Circum Atlantic Correlatives Springer-Verlag 19pp 373ndash397

B Dhuime et al C R Geoscience 335 (2003) 1081ndash1089 1089

onord

55

offor

udr-

lerlts)

s onteeas

f75ndash

onoof00)

ndashheir

en-ce

alpic73)

2y

rth

sil-

64tion

therian

eo-omazil

er-elypet-81ad

Sci

erthelu-

iniar

[8] DJ Cherniak WA Lanford FJ Ryerson Lead diffusiin apatite and zircon using ion implantation and Rutherfbackscattering techniques Geochim Cosmochim Acta(1991) 1663ndash1673

[9] P Copeland RR Parrish TM Harrison Identificationinherited radiogenic Pb in monazite and its implicationsUndashPb systematics Nature 333 (1988) 760ndash763

[10] M Corsini L Lambert de Figueiredo R Caby G FeacuteraG Ruffet A Vauchez Thermal history of the Boborema Province deduced from40Ar39Ar analysis Tectono-physics 285 (1998) 103ndash117

[11] JP Eissen T Juteau JL Joron B Dupreacute E HumA AlrsquoMukhamedov Petrology and geochemistry of basafrom the Red Sea axial rift at 18north J Petrol 30 (1989791ndash839

[12] AH Fetter UndashPb and SmndashNd geochronological constraintthe crustal framework and geologic history of the CearrsquostaNW Borborema Province NE Brazil implications for thassembly of Gondwana PhD Thesis University of Kans1999 145 p

[13] KM Gillis LA Coogan Anatectic migmatites from the rooof an ocean ridge magma chamber J Petrol 43 (1991) 202095

[14] IP Guimaratildees CN Almeida AF Da Silva FilhJM Arauacutejo Granitoids marking the end of the Brasilia(Pan-African) orogeny within the Central Tectonic Domainthe Borborema Province Rev Bras Geociecircncias 30 (20117ndash181

[15] DA Ionov L Savoyant C Dupuy Application of the ICPMS technique to trace element analysis of peridotites and tminerals Geostandards Newslett 16 (1992) 311ndash315

[16] F Kalfoun Geacuteochimie de NbndashTa contraintes sur la diffeacuterciation terrestre PhD thesis University Montpellier-2 Fran2001 300 p

[17] TE Krogh A low-contamination method for hydrothermdecomposition of zircon and extraction of U and Pb for isotoage determinations Geochim Cosmochim Acta 37 (19485ndash494

[18] KR Ludwig Users manual for ISOPLOTEX versionA geochronological toolkit for Microsoft Excel BerkeleGeochronology Center Spec Publ 1a (1999) 1ndash30

[19] WF McDonough SS Sun The composition of the EaChem Geol 120 (1995) 223ndash253

[20] P Monieacute R Caby MH Arthaud The Neoproterozoic Braiano orogeny in northeast Brazil40Ar39Ar and petrostruc-tural data drom Cearagrave Precambrian Res 81 (1997) 241ndash2

[21] SP Neves A Vauchez G Feacuteraud Tectono-thermal evolumagma emplacement and shear zone development inCaruaru area (Borborema Province NE Brazil) PrecambRes 99 (2000) 1ndash32

[22] J-M Saacute J-M Bertrand J Leterrier MHF Macedo Gchemistry and geochronology of the pre-Brasiliano rocks frthe Transversal Zone Borborema Province Northeast BrJ South Amer Earth Sci 14 (2002) 851ndash866

[23] LK Sha BW Chappell Apatite chemical composition detmined by electron microprobe and laser ablation inductivcoupled plasma mass spectrometry as a probe into graniterogenesis Geochim Cosmochim Acta 63 (1999) 3861ndash38

[24] JS Stacey JD Kramers Approximation of terrestrial leisotope evolution by a two-stage model Earth PlanetLett 26 (1975) 206ndash221

[25] WR Van Schmus BB Brito Neves P HackspachM Babinski UPb and SmNd geochronological studies ofEastern Borborema Province Northeast Brazil initial concsions J South Am Earth Sci 8 (1995) 267ndash288

[26] A Vauchez S Pacheco Neves R Caby M CorsM Egydio-Silva M Arthaud V Amaro The Borborema shezone system J South Am Earth Sci 8 (1995) 247ndash266

B Dhuime et al C R Geoscience 335 (2003) 1081ndash1089 1089

onord

55

offor

udr-

lerlts)

s onteeas

f75ndash

onoof00)

ndashheir

en-ce

alpic73)

2y

rth

sil-

64tion

therian

eo-omazil

er-elypet-81ad

Sci

erthelu-

iniar

[8] DJ Cherniak WA Lanford FJ Ryerson Lead diffusiin apatite and zircon using ion implantation and Rutherfbackscattering techniques Geochim Cosmochim Acta(1991) 1663ndash1673

[9] P Copeland RR Parrish TM Harrison Identificationinherited radiogenic Pb in monazite and its implicationsUndashPb systematics Nature 333 (1988) 760ndash763

[10] M Corsini L Lambert de Figueiredo R Caby G FeacuteraG Ruffet A Vauchez Thermal history of the Boborema Province deduced from40Ar39Ar analysis Tectono-physics 285 (1998) 103ndash117

[11] JP Eissen T Juteau JL Joron B Dupreacute E HumA AlrsquoMukhamedov Petrology and geochemistry of basafrom the Red Sea axial rift at 18north J Petrol 30 (1989791ndash839

[12] AH Fetter UndashPb and SmndashNd geochronological constraintthe crustal framework and geologic history of the CearrsquostaNW Borborema Province NE Brazil implications for thassembly of Gondwana PhD Thesis University of Kans1999 145 p

[13] KM Gillis LA Coogan Anatectic migmatites from the rooof an ocean ridge magma chamber J Petrol 43 (1991) 202095

[14] IP Guimaratildees CN Almeida AF Da Silva FilhJM Arauacutejo Granitoids marking the end of the Brasilia(Pan-African) orogeny within the Central Tectonic Domainthe Borborema Province Rev Bras Geociecircncias 30 (20117ndash181

[15] DA Ionov L Savoyant C Dupuy Application of the ICPMS technique to trace element analysis of peridotites and tminerals Geostandards Newslett 16 (1992) 311ndash315

[16] F Kalfoun Geacuteochimie de NbndashTa contraintes sur la diffeacuterciation terrestre PhD thesis University Montpellier-2 Fran2001 300 p

[17] TE Krogh A low-contamination method for hydrothermdecomposition of zircon and extraction of U and Pb for isotoage determinations Geochim Cosmochim Acta 37 (19485ndash494

[18] KR Ludwig Users manual for ISOPLOTEX versionA geochronological toolkit for Microsoft Excel BerkeleGeochronology Center Spec Publ 1a (1999) 1ndash30

[19] WF McDonough SS Sun The composition of the EaChem Geol 120 (1995) 223ndash253

[20] P Monieacute R Caby MH Arthaud The Neoproterozoic Braiano orogeny in northeast Brazil40Ar39Ar and petrostruc-tural data drom Cearagrave Precambrian Res 81 (1997) 241ndash2

[21] SP Neves A Vauchez G Feacuteraud Tectono-thermal evolumagma emplacement and shear zone development inCaruaru area (Borborema Province NE Brazil) PrecambRes 99 (2000) 1ndash32

[22] J-M Saacute J-M Bertrand J Leterrier MHF Macedo Gchemistry and geochronology of the pre-Brasiliano rocks frthe Transversal Zone Borborema Province Northeast BrJ South Amer Earth Sci 14 (2002) 851ndash866

[23] LK Sha BW Chappell Apatite chemical composition detmined by electron microprobe and laser ablation inductivcoupled plasma mass spectrometry as a probe into graniterogenesis Geochim Cosmochim Acta 63 (1999) 3861ndash38

[24] JS Stacey JD Kramers Approximation of terrestrial leisotope evolution by a two-stage model Earth PlanetLett 26 (1975) 206ndash221

[25] WR Van Schmus BB Brito Neves P HackspachM Babinski UPb and SmNd geochronological studies ofEastern Borborema Province Northeast Brazil initial concsions J South Am Earth Sci 8 (1995) 267ndash288

[26] A Vauchez S Pacheco Neves R Caby M CorsM Egydio-Silva M Arthaud V Amaro The Borborema shezone system J South Am Earth Sci 8 (1995) 247ndash266