(2006) 441–455www.elsevier.com/locate/gr

Gondwana Research 9

Electron microprobe dating of monazite from high-T shear zones in theSão José de Campestre Massif, NE Brazil

Zorano Sérgio de Souza a,⁎, Jean-Marc Montel b, Simone Maria Lima Costa Gioia c,Maria Helena Bezerra Maia de Hollanda c,1, Marcos Antonio Leite do Nascimento d,

Emanuel Ferraz Jardim de Sá a, Venerando Eustáquio Amaro d, Márcio Martins Pimentel c,Jean-Marc Lardeaux e, Michelle Veschambre f

a Pós-Graduação em Geodinâmica e Geofísica e Departamento de Geologia — Universidade Federal do Rio Grande do Norte, 59078-970 Natal, RN, Brazilb LMTG, Université Paul Sabatier, 39 allées Jules Guesde, 31000 Toulouse, France

c Instituto de Geociências, Universidade de Brasília, Laboratório de Geocronologia, 70919-900 Brasília/DF, Brazild Pós-Graduação em Geodinâmica e Geofísica — Universidade Federal do Rio Grande do Norte, 59078-970 Natal, RN, Brazil

e Laboratoire de Pétrologie et Tectonique, Université Claude Bernard, 69007 Lyon, Francef CNRS, UMR 6524, Magmas et Volcans, Département des Sciences de la Terre, Université Blaise Pascal, 5, Rue Kessler, 63038, Clermont-Ferrand cedex, France

Received 10 August 2005; accepted 21 November 2005Available online 15 March 2006

Abstract

The easternmost domain of the Borborema Province, northeastern Brazil, presents widespread, extensional-related high-temperaturemetamorphism during the Brasiliano (=Pan-African) orogeny. This event reached the upper amphibolite to granulite facies and provokedgeneralized migmatization of Proterozoic metapelitic rocks of the Seridó Group and tonalitic to granodioritic orthogneisses of the Archean toPaleoproterozoic basement. We report new geochronological data based on electron microprobe dating of monazite from metapelitic migmatiteand leuconorite within the high-T shear zones that make up the eastern continuation of the huge E–W Patos shear belt. These data were alsoconstrained by using the Sm–Nd isotopic systematic on garnet from a syntectonic alkaline granite and two garnet-bearing leucosomes. The resultssuggest an age of about 578 to 574Ma for the peak of the widespread high-T metamorphism. This event is best recorded by Sm–Nd garnet-wholerock ages. The U–Th–Pb isotopes on monazite of the metapelitic migmatite show a younger thermal event at 553±10Ma. When compared to theSm–Nd garnet-whole rock ages, the U–Th–Pb electron probe monazite ages seem to record an event of slightly lower temperatures after the peakof the high-T metamorphism. This may reflect the difference in the isotopic behavior of the geochronological methods employed. Otherwise, theU–Th–Pb ages on monazites could indicate an event not yet very well defined. In anyway, this paper reveals the partial or even complete re-opening and resetting of the U–Th–Pb isotopic system produced by the action of low-T Ca-rich fluid.© 2006 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved.

Keywords: Neoproterozoic; NE Brazil; Shear zones; Electron probe; Monazite dating

1. Introduction

The Brasiliano (=Pan-African) orogeny in the BorboremaProvince (Northeastern Brazil) is characterized by a huge amountof granitoid plutons and a complex system of transcurrent shearzones of crustal or lithospheric scale (Fig. 1). These shear zones

⁎ Corresponding author. Tel./fax: +55 84 32153831.E-mail addresses: zorano@geologia.ufrn.br (Z.S. de Souza), montel@cict.fr

(J.-M. Montel).1 Now at the IG/USP, São Paulo, SP, Brazil.

1342-937X/$ - see front matter © 2006 International Association for Gondwana Rdoi:10.1016/j.gr.2005.11.008

extended to West Africa and developed (and/or were reactivated)synchronously after the collision between the NeoproterozoicWest Africa, Congo and São Francisco cratons (Bertrand and Sá,1990; Caby et al., 1991; Castaing et al., 1994). In this region, aclose connection between the plutonism and the peak ofmetamorphism and deformation during the Brasiliano orogenyhas been previously demonstrated (Sá et al., 1986, 1987;Archanjo et al., 1992; Leterrier et al., 1994; Corsini et al.,1998). A number of syntectonic granitoid plutons are shear-related and seem to be affected during and after their cooling bysubsequent shearing (Vauchez et al., 1995). The shear zones

esearch. Published by Elsevier B.V. All rights reserved.

Fig. 1. Pre-drift reconstruction for West Africa and east South America (modified from Sá, 1994). Rectangle outlines approximate area of Fig. 2. WAC West AfricaCraton; AC Amazonian Craton; SFC São Francisco Craton; CC Congo Craton; BRMB Brasília and Ribeira Mobile Belts; BP Borborema Province; CS CameroonShield; NB Nigerian Belt; HS Hoggar Shield; PL Patos Lineament; PeL Pernambuco Linement; AdL Adamaoua Lineament.

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separate domains of variably strained supracrustal sequences ofassumed Paleo- to Mesoproterozoic ages, and a transpressionaltectonic setting has been suggested for the central part of theSeridó Belt (Sá et al., 1987; Archanjo and Bouchez, 1991;Corsini et al., 1991; Sá, 1994).

The age of the high-T Brasiliano metamorphism in the Seridóregion has been deduced mainly from Rb–Sr isochron and K/Ardata in the 700–500Ma range (Brito Neves et al., 1974; Sá et al.,1987). U–Pb zircon and Rb–Sr whole rock isochron dating ofdiorites and coeval granite plutons along the Seridó Belt led toestimate the age of the transpressional deformation and meta-morphic peak at 580±30Ma (Leterrier et al., 1994; Sá, 1994;Galindo et al., 1995; Dantas, 1997).

The aim of this paper is to better constrain the timing of hightemperature event in the São José de Campestre Massif, thenortheastern portion of the Borborema Province (Fig. 1), withnew data obtained by two geochronological techniques not yetused on those rocks: electron microprobe monazite dating andSm–Nd on garnet dating. Both techniques are especially effi-cient for dating poly-metamorphic rocks. We selected a mig-matitic metapelite (mylonitic, migmatitic micaschist) within theRemígio–Pocinhos Shear Zone (RPSZ) and a leuconorite in-truded along the Serrinha–Espírito Santo Shear Zone (SESSZ),

both displaying an extensional, strike-slip kinematic signature(Fig. 2).

2. Geologic setting

In the São José de Campestre Massif (Fig. 2), transtensionalsites associated to high-T metamorphism appear to control thegeometry of supracrustal belts and granitoid emplacement(Souza and Sá, 1993; Sá et al., 1993; Sá, 1994; Sá et al., 1999).In this area, a small Archean block (Van Schmus et al., 1995;Dantas et al., 1996; Dantas, 1997) and Paleoproterozoic ortho-gneisses, similar to those of the Caicó basement complex in thecentral Seridó region (Hackspacher et al., 1990; Souza et al.,1993), are intruded by Brasiliano-age granitoids and overlain bysupracrustal allochtons. All these units are separated from thecentral Seridó Belt by the NNE-trending Picuí–João CâmaraShear Zone (PJCSZ), which displays dextral strike-slipkinematics.

The Remígio–Pocinhos schist belt (RPB), located in betweenBarra de Santa Rosa andRemígio villages (Fig. 3), was studied bySá et al. (1993), Souza and Sá (1993) and Trindade et al. (1993). Itdisplays a low-P metamorphism (named M3) grading from lowamphibolite facies (andalusite+staurolite in metapelites) in its

Fig. 2. Geological framework of the northeastern part of the Borborema Province, NE Brazil (modified from Sá et al., 1987). 1 — Archean to Paleoproterozoicbasement, SJCM São José de Campestre Massif; 2 — Meso- to Neoproterozoic supracrustals (SB Seridó Belt); 3 — Brasiliano-age granitoid; 4 — Brasiliano-ageshear zone (PSZ Patos, PJCSZ Picuí-João Câmara, RPSZ Remígio–Pocinhos); 5 — Phanerozoic cover. ● Town.

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northwestern part to upper amphibolite and even granulite facies(cordierite+garnet+prismatic sillimanite+mesoperthitic K-feld-spar in pelites; diopside+phlogopite+forsterite+spinel in mar-bles; orthopyroxene+clinopyroxene in metaplutonic rocks) withextensive migmatization in its southern counterpart. In thehighest-T zone, extensive partial melting of metapelitic protho-liths produced abundant centimeter to decimeter granitic totrondhjemitic garnet-bearing leucosomes. A moderate degree ofpartial melting is observed in tonalitic to granodioritic orthog-neisses of the Caicó Complex basement (Fig. 4a and b), whereasinterleaved amphibolites show a somewhat lesser degree ofmelting (Fig. 4a). In both cases the newly formed leucosomesfollow the S3 Brasiliano-age fabric. Brasiliano structures (D3) arecharacterized by schistosity (biotite±amphibole) or metamorphicbanding that varies from gently dipping in the northwestern,lower-T domain, to NE-trending, steeply SE-dipping whenreaching the RPSZ. This arrangement conforms to a negativehalf flower geometry for the RPB. In the high-T portion, severalslices of orthopyroxene-bearing orthogneiss and interleavedmafic granulites, as well as kilometer-scale andradite–ægirine-bearing alkaline granite sheets are also found (Nascimento, 2000).In both the low-T and high-T zones, a penetrative lineationchanges its direction from N–S or NNE–SSW in the extensionalNW domain to NE or ENE along the southern strike-slip zone.This high-T shear zone extends westward for hundreds of

kilometers as part of the major E–W Patos Shear Zone (PSZ),already characterized as a high-T structure (Corsini et al., 1991).

The SESSZ is an E–W sinistral–extensional high-T shearzone linked at its eastward tip to the prolongation of the RPSZ(Fig. 3). Along its strike, and especially in the northwesternborder of the Serrinha granite, abundant in situ partial melting ofthe basement tonalitic to dioritic orthogneiss marks the SESSZ.The trondhjemitic to granitic leucosomes were collected alongtension gashes and sinistral extensional C′ structures (Fig. 4b).Within the Serrinha granite, the same kinds of sites are filled byhectometer- to decameter-sized late granitic melts. In the sameway, shear bands with NE-direction are filled by small norite toquartz diorite bodies.

Up to this moment, no precise direct radiometric ages areavailable for this high-T metamorphism, any interpretationsbeingmademostly from structural and crosscutting relationshipsbetween granites and country rocks (Sá et al., 1986, 1999). Inthis respect the U–Pb zircon age of 579±14Ma (2σ) of the Acariand São João do Sabugi diorites, on the southweast of the areastudied, was interpreted as the peak of the thermal event and alsothe age of the transpressional deformation on the Seridó regionby Leterrier et al. (1994). However, the Acari granite, one of theseveral granitoid plutons considered as syntranspressional andspatially related to the diorites referred above (Sá, 1994) has aU–Pb zircon age of 555±10Ma (2σ; Legrand et al., 1991).

Fig. 3. Simplified geological map of the studied region (modified fromBarbosa et al., 1974; Sá, 1994;Dantas et al., 1996) and aNW–SE cross-section (A–B). 1—Archeanand 2 — Paleoproterozoic basement; 3 — Supracrustal sequences (SB Seridó Belt, RPB Remígio–Pocinhos Belt); 4 — Brasiliano-age granitoids and basic plutons (inblack); 5 — Phanerozoic cover; 6 — Brasiliano-age structures (SL/OE/OT=strike-slip/oblique extensional/oblique thrusting shear zones; S3/l3=planar/linear fabric).PJCSZ, RPSZ, SESSZ Picuí–João Câmara, Remígio–Pocinhos, Serrinha–Espírito Santo shear zones. Sample sites: VJ19B — metapelitic migmatite, VJ23A — maficgranulite, ES35B — leuconorite, ES31.2.D quartz diorite, MA01 — alkali granite, MA212 and ES156 — garnet-bearing leucosomes.

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As regards as the SESSZ, a recent Rb–Srwhole rock isochrondate of the Serrinha granite by Macedo et al. (1997) yielded anage of 603±22Ma (2σ). This age is at least 50Ma older than the540–550Ma ages obtained by the same method in the nearbyand structurally similar or younger Dona Inês and Monte dasGameleiras plutons (McMurry et al., 1986, 1987). On the otherhand, 40Ar–39Ar data of micas and amphibole of rocks in theeastern part of the PSZ revealed plateau ages spanning from 542to 491Ma (Corsini et al., 1998). Based on the known U–Pbzircon ages of syntectonic granites and their 40Ar–39Ar results,Corsini et al. (1998) suggested a homogeneous and unusual slowcooling history (3–4°C/Ma) and a rather slow uplift ratebetween 580 and 500Ma, followed by fast cooling over most ofthe Borborema Province around 500Ma.

In order to better constrain the age of the migmatizationevent and, by correlation, of the related extensional deforma-tion, a sample of migmatized metapelites from the RPSZ(VJ19B in Fig. 3) and a leuconorite pluton intruded in amegascopic sinistral C′ structure along the SESSZ (ES35B inFig. 3) were chosen for monazite electron microprobe dating.

Three other samples were studied by using the Sm–Nd mineraltechnique: i) an extensional-related garnet-bearing leucosomederived from partial melting of basement granitic gneiss(ES156 in Fig. 3); ii) an extensional-related garnet-bearingleucosome derived from partial melting of micaschists (MA212in Fig. 3); and iii) a garnet-bearing alkali granite emplaced in anorth–south-trending strike-slip shear zone (sample MA01 inFig. 3).

3. Sample description and metamorphic P–T conditions

3.1. Leuconorite (ES35B)

The mineral composition discussed below is based on aver-age analyses from Table 1. The leuconorite is composed ofplagioclase (An36), Fe-hypersthene (Wo2En49Fs49, mg#=0.51)and diopside-enriched clinopyroxene (Wo45En35Fs20,mg#=0.64) phenocrysts all variably transformed to euhedralpolygonal mosaic indicative of high-T recrystallization. Abun-dant Ti-rich biotite (TiO2=5.51wt.%, mg#=0.51), zoned

Fig. 4. Field aspects of the high-T Serrinha–Espírito Santo shear zone. (a)Differentdegrees of partial melting of hornblende–biotite-bearing granodiorite (light grey)and amphibolite (dark grey to black). (b) Partial melting of hornblende–biotite-bearing granodiorite and collecting of melts in NE–SWextensional C′ sites and/oralong the S3 planar fabric. Both figures represent horizontal planes. lc=leucosome.

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allanite, pyrite, chalcopyrite, perthitic orthoclase (Or88) andinterstitial quartz are also observed. Biotite is usually latemagmatic, commonly including pyroxenes and plagioclasegrains, and oriented in a well defined planar fabric (here calledSγ) together with newly recrystallized plagioclase andpyroxenes. In the field, this fabric parallels the NE-directed C′ planes.

3.2. Metapelitic migmatite (VJ19B)

This sample comprises a millimeter-scale banding ofstrongly pleochroic brown biotite rich (TiO2=2.04wt.%,mg#=0.48) (melanosome) and plagioclase (An20)+quartz-rich bands (leucosome), which correspond to the D3 (S3)fabric on the field. Al- and Ti-rich muscovite (Al2O3=35.1wt.%; TiO2=0.80wt.%) mainly overgrowths the S3 fabricor crosscuts it, and is probably late tectonic with regard tobiotite and plagioclase +quartz leucosome. Fine-grainedbiotite and muscovite lamellae follow discontinuous micro-shear zones at a small angle to the S3 fabric, representing alate ductile–brittle deformation episode or an incrementalstage.

3.3. Metamorphic P–T conditions

For leuconorite and metapelitic migmatite, a low-P/high-Tmetamorphism is indicated by field observations and para-genetic relations described above as well as geothermometricand geobarometric calculations (data on Table 1). For a quartzdiorite outcropping along the SESZ (sample ES31.2.D), theplagioclase–amphibole geothermometer (Blundy and Holland,1990) and Al-in-amphibole geobarometer (Schmidt, 1992)gives, respectively, 680–730°C and 4.5–5.1kbar. The amphi-bole is green to blue and was classified asMg-hornblende (Leak,1978), with mg#=0.55, TiO2=0.87wt.% and 6.61bSib6.97pfu.

The amphibole geothermobarometer (Blundy and Holland,1990; Schmidt, 1992) for a mafic granulite (sample VJ23A)interleaved with the metapelitic migmatite gives T and P inthe range 671–765°C and 3.8–5.7kbar. The amphibole (Mg-hornblende) is deep green to brownish, with 6.81bSib6.92pfu, TiO2=0.92wt.% and Mg-enriched (mg#=0.68). Inthe mafic granulite, the low Al-contents of orthopyroxene(0.76wt.%) and clinopyroxene (1.33wt.%) and the absence (orrare occurrence) of garnet is consistent with a low-P meta-morphism (Spear, 1993). The application of the two pyroxenesgeothermometer proposed by Brey and Köhler (1990) yieldedaverage temperatures of 800°C for Ca-partitioning and 640°Cfor Na-partitioning. These temperatures are slightly higher thanthe averages calculated for pyroxenes in the leuconorite (ca. 740and 640°C, respectively) by the same geothermometer.

The P–T results above (averaging of 724°C/4.8kbar)places the main metamorphic event at the transition betweenthe upper amphibolite and the granulite facies, about 50°Cabove the wet granite solidus (Holtz and Johannes, 1994, andreference therein). Experimental fluid-absent partial meltingof metagreywackes (Le Breton and Thompson, 1988;Vielzeuf and Montel, 1994) and dehydration melting oftonalitic composition (Singh and Johannes, 1996) for P lessthan 10kbar demonstrated that the biotite-out curve is in therange 750–860°C, with orthopyroxene appearing at 805°C(melting of metagreywackes) and 720°C (melting of tonalite).In fluid-absent partial melting of tonalitic composition, thebiotite completely disappears at 975°C via incongruentmelting forming orthopyroxene+oxides+granitic melt (Skjer-lie and Johnston, 1993). Consequently, perthitic orthoclase inleuconorite and amphibole in quartz diorite and mafic to felsicgranulites should have been re-equilibrated during the coolingstage of the M3 metamorphism. This assumption is supportedby the relatively low TiO2 (0.94wt.%, Ti=0.1pfu) contentsof amphibole reported for granulite facies rocks (e.g., Raithand Raase, 1986; Schumacher et al., 1990), and the ubiquitouspresence of associated ilmenite and Na-partitioning betweenortho- and clinopyroxene. Under low-T conditions either in thevanishing stage of the metamorphic peak or in a post-tectonicevent not yet very well constrained, the rocks studied(especially the migmatitic metapelite) were probably modifiedby retrometamorphic fluids, resulting in the crystallization ofchlorite and fine grained white mica along the (001) biotitecleavage plane.

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4. Some comments about the electron microprobe datingof monazites

The electron microprobe dating of monazite is now a widelyused technique (Suzuki and Adachi, 1991; Suzuki et al., 1994;Montel et al., 1994, 1996, Braun et al., 1998; Cocherie et al.,1998; Montel et al., 2000). In metamorphic rocks the mainadvantage of this technique is the possibility to rely the exactpetrographic and textural position of the crystals to the age andthe possibility to analyze even very small crystal. Additionally,the high spatial resolution (less than 3μm in diameter) of theelectron microprobe dating makes it possible to separate poly-genetic portions of complex monazite and thus to avoid analy-tical mixing phenomena frequently interpreted as meaninglessintermediate radiogenic ages. Extensive application of theelectron microprobe method to polygenetic monazite (Montelet al., 1996; Braun et al., 1998; Cocherie et al., 1998; Finger etal., 1998; Williams et al., 1999, 2001; Montel et al., 2000;Williams and Jercinovic, 2002), as well as experimental re-setting of the U–Th–Pb isotopic system of monazite (Smithand Giletti, 1997; Seydoux-Guillaume et al., 2002), haveshown that resetting by diffusion is unlikely during metamor-phic events unless fluids are involved (Cathelineau, 1987;Cuney and Friedrich, 1987). However, under those conditions,partial resetting of grains is possible, a feature difficult to assessby electron probe only (Seydoux-Guillaume et al., 2003; Gon-çalves et al., 2004).

5. Analytical procedure

Following Montel et al. (1994, 1996), normal polished thinsections prepared for conventional electron microprobe analysis

Table 1Average chemical composition (%wt) of selected minerals in the Remígio–Pocinho

Remígio–Pocinhos Shear Zone (RPSZ)

Metapelitic migmatite Mafic granulite

(Sample VJ19B) Sample VJ23A

Bio(N=7)

Musc(N=7)

Pl(N=14)

Opx(N=3)

Cpx(N=4)

Hb(N=9)

Pl(N=

SiO2 37.10 45.5 63.25 52.63 53.51 47.17 59.4Al2O3 21.35 35.1 22.88 0.76 1.33 8.9 24.9TiO2 2.04 0.8 0.03 0.04 0.08 0.92 0.0MgO 8.42 0.78 0.01 21.67 14.3 14.43 0.0FeO 16.27 1.34 0.06 22.67 8.65 12.08 0.1MnO 0.46 0.01 0.02 0.9 0.43 0.18 0.0CaO 0.04 0.04 4.14 0.37 21.41 11.26 6.8Na2O 0.2 0.62 8.97 0.01 0.53 1.39 7.2K2O 9.77 10.07 0.18 0.02 0 0.76 0.3Total 95.65 94.26 99.54 99.07 100.24 97.09 99.1mg# 0.48 0.51 – 0.63 0.75 0.68 –Ab – – 78.8 – – – 64.2An – – 20.1 – – – 33.7Wo – – – 0.8 44.2 – –En – – – 61.6 41.1 – –Fs – – – 37.6 14.7 – –

(Symbols for minerals: Bio Biotite, Musc Muscovite, Pl Plagioclase, Opx/Cpx Orthmg# (Mg-number)=(MgO) mol / (MgO+FeO) mol. ⁎Analysis made with a CamebaClermont-Ferrand, France. ⁎⁎Analysis made with a Cameca SX50 electron micropr

were used. Firstly, petrographic composition and texturalrelationships of monazites were described under a polarizingmicroscope. When included in biotite the monazite crystals canbe recognized from their pleochroic haloes, which are normallythicker than the one produced by zircon. Otherwise it is difficultto clearly differentiate between zircon (that shows straightextinction and generally a better-shaped prisms, and may beconcentrically zoned), monazite that has an oblique extinctionangle (2–12°), a thin (100) cleavage and commonly roundedshape, and xenotime (with a good cleavage). To overcome theseambiguities, monazite crystals were definitively identifiedthrough qualitative analysis using a scanning-electron micro-scope (Cambridge-Leica Steroscan 360), equipped with an EDSanalytical system, of the Université Blaise Pascal, Clermont-Ferrand, France. With this technique, even very small crystal(about 5μm in diameter) can be detected, as well the distinctionbetween monazite (which is extremely bright), zircon andxenotime are very well resolved. SEM work is very useful forrevealing the internal structure of monazite and also for findingcrystal not seen by optical microscopy.

After being identified, the monazite crystals are thenanalyzed for U, Th and Pb with a Camebax Micro electronmicroprobe, with an accelerating voltage of 15kV and a 90–145nA probe current. U, Pb and Th are analyzed simultaneous-ly with PET crystals using Mα lines for Th and Pb and Mβ linesfor U. The spectral interference of YLγ line on PbMα wascorrected by measuring the intensity of YLγ on synthetic YPO4.Extrapolation down to the Y contents of monazite (less than2wt.%) shows that this creates a maximum overestimate of thePb content of about 30ppm. The standards are ThO2, UO2, anda synthetic glass (CaO–Al2O3–SiO2) with 6200ppm of Pb. Thecounting time for standards is 50s on peak and 100s on

s and Serrinha–Espírito Santo shear zones

Serrinha–Espírito Santo Shear Zone (SESSZ)

Leuconorite Quartz diorite

Sample ES35B Sample ES31.2.D

11)Opx(N=11)

Cpx(N=10)

Bio(N=5)

Pl(N=14)

Hb(N=10)

Pl(N=6)

8 51.28 51.64 35.98 59.06 45.57 63.47 0.65 1.33 13.71 25.3 8.28 23.864 0.07 0.14 5.51 0.09 0.87 –1 16.83 11.99 11.05 0.02 11.16 –8 29.16 12.1 18.97 0.13 16.76 0.062 0.99 0.46 0.05 0.07 0.37 –6 0.81 21.34 0.14 7.34 11.86 4.886 0.01 0.4 0.09 6.94 1.24 9.14 0.01 0.03 9.74 0.32 1.02 0.196 99.81 99.43 95.24 99.27 97.13 101.49

0.51 0.64 0.51 – 0.55 –– – – 61.9 – 76.3– – – 36.2 – 22.61.7 44.7 – – – –49 34.9 – – – –49.3 20.5 – – – –

o/Clinopyroxene, Hb Amphibole.). See sample sites in Fig. 3.x Micro electron microprobe at Institut de Géologie, Université Blaise Pascal,obe at Instituto de Geociências, Universidade de Brasília, Brazil.

Table 2Analytical results for leuconorite (sample ES35B)

A Position M Th (ppm) Error⁎ U (ppm) Error⁎ Pb (ppm) Error⁎ t (Ma) Error

66 See Fig. 4 1 (HZ) 127,563 878 4775 369 3514 318 546 5667 “ 119,991 847 5271 370 3237 310 525 5768 “ 80,556 695 4093 361 2216 291 525 7869 “ 81,818 700 3891 362 2107 292 496 7870 “ 117,235 839 4653 365 3168 308 532 5971 “ 114,752 829 4293 366 3260 309 563 6172 “ 102,609 783 4412 365 2815 298 535 6562 C 2 (HZ) 107,413 801 5014 366 3043 306 547 6363 I 109,883 811 5094 368 3714 311 650 6464 R 80,674 696 3669 358 2141 294 514 8065 C 101,370 777 4477 367 2888 301 553 6691 R 3 (CZ) 175,567 1046 4332 365 4731 334 554 4592 C 192,025 1100 4512 368 5088 342 547 4293 R 134,723 902 4247 362 3940 319 589 5594 I 111,140 815 4270 359 3580 310 634 6473 I,F 5 (NZ) 92,613 744 4391 363 2554 297 531 7074 R 63,956 622 3694 357 1767 285 517 9475 R 66,492 635 3581 360 1818 285 517 9276 C 81,746 698 3863 362 2677 297 629 8177 C 6 (NZ) 122,930 861 6414 383 3663 319 566 5778 I 7 (HZ) 277,498 1372 6268 378 8150 396 607 3479 C 262,391 1325 6310 382 7266 382 570 3580 R 200,442 1131 5539 375 5297 352 539 4181 I 247,290 1276 6311 378 6740 374 559 3682 I 199,925 1129 4948 370 5341 348 550 4183 I 171,520 1030 4562 369 4691 336 559 4684 R 172,815 1039 5183 375 4673 335 548 4585 C 8 (NZ) 157,601 984 4671 366 4648 332 597 4986 C 187,165 1087 5035 374 4812 341 526 4387 I 137,812 913 4639 370 4015 321 583 5488 R 120,666 851 5104 371 3096 307 502 5789 R 166,984 1016 8687 394 4806 340 547 4490 I,F 46,246 542 3854 359 1100 273 418 114

The errors are for 95% confidence level.A=point analysed, M=monazite crystal number, C=center of the grain, I= intermediate position between rim and center of the grain, R=rim of the grain, F=nearfracture; HZ=crystal with a heterogeneous zoning, the brightest ones being richer in Th contents; CZ=crystal with regular concentric zoning, in which the innerportions are richer in Th contents; NZ=not zoned crystal; FC=fractured crystal.

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background for ThO2 an UO2, and 300s on peak and 600s onbackground for Pb glass. For dating, the counting time was of100s at each point. For each measurement, the confidenceinterval for Th, U, Pb are determined, the age is computed fromEq. (1) of Montel et al. (1996), and the uncertainty on the age(95% confidence) is calculated by propagating the uncertaintieson Th, Pb and U into Eq. (1) of Montel et al. (1996). So, eachmeasurement yields an age with a statistical confidence interval.An additional check of the quality of the data is performed at thebeginning of any session, through a complete procedure on awell-dated monazite sample that is used as an age standard. Allthe dates obtained are then statistically treated and the MSDW iscalculated for one or more population following the procedureof Montel et al. (1996). Data are presented in two types ofdiagrams: the weighted-histogram representation in which eachage is represented by a bell-shaped curve (Montel et al., 1994),and the age–U–Th diagram in which each age is plotted versusU and Th (Braun et al., 1998). The first one allows estimatinghow the age population is distributed. The second diagramallows discussing the relationships between the age and thechemical composition of the crystals.

6. Results obtained by the electron microprobe datingof monazite

6.1. Presentation of the results

The analytical data are given in Tables 2 and 3, and theresults graphically represented in (Figs. 6a, b, and 9a, b).

6.2. Monazite of the leuconorite ES35B

For this rock, four samples were analyzed by both opticaland scanning electron microscopy, and only one of them hadmonazite grains suitable to be analyzed with the microprobe.Monazite has intergranular habit (Fig. 5a) or is found asinclusions in pyroxene and plagioclase. Commonly, it includessmall pyroxenes and feldspar grains. Consequently, theyprobably have been formed late in the course of crystallizationof the norite magma. They are colorless or pale yellow, usuallyzoned with regard to Th contents, and have euhedral prismaticor rounded (rhombic) shape (Fig. 5a and b), sometimes with amoderate cleavage orthogonal to (001). They vary in size from

Table 3Analytical results for monazite from metapelitic migmatite (sample VJ19B)

A Position M Th (ppm) Error⁎ U (ppm) Error⁎ Pb (ppm) Error⁎ t (Ma) Error

1 C 1,F 32,864 470 6169 373 1092 270 460 1252 C 35,532 484 6720 377 1462 271 565 1183 C 33,802 478 6770 377 1539 278 609 1244 I 2⁎ 29,932 457 4164 360 1015 268 519 1535 I,F 33,872 477 6802 377 1015 271 405 1186 I,F 35,242 486 8230 390 860 274 312 1067 C 28,934 449 6020 370 1324 268 603 1388 I,F 36,957 493 5564 371 1185 273 479 1229 C,F 4 32,586 469 6452 367 499 270 210 11910 C,F 33,105 466 8020 370 1300 267 489 11111 C 28,570 443 6618 368 1162 265 515 13012 C 5⁎ 25,473 431 2958 353 817 263 517 18613 C,F 33,182 473 3628 359 831 267 413 14514 C 30,646 458 3710 355 1200 269 621 15815 I,F 32,932 472 7604 381 1215 275 469 11716 I,F 31,589 464 8083 383 1155 275 445 11617 I 35,337 486 6452 375 1598 278 627 12418 R,F 32,802 473 6945 383 676 279 275 12019 C 6 35,524 486 6173 375 1472 279 586 12520 I 33,628 476 6833 375 1588 288 628 12821 I 32,925 473 6690 379 1175 276 478 12422 C 10 34,313 477 7932 384 1520 272 560 11323 C 35,029 480 8250 383 1524 271 546 10924 I 32,439 465 7937 376 1793 272 677 11825 I 31,474 458 7130 374 1438 266 582 12126 I 9 35,058 478 8939 387 1574 268 544 10427 C 32,133 462 10,452 389 1761 271 589 10228 C 36,719 487 8145 378 1715 269 600 10729 I 33,265 469 7243 372 1719 272 666 12130 I,F 11 33,483 470 8635 382 1551 265 558 10731 C 32,195 464 7222 373 1548 264 614 11932 I 33,185 469 8181 380 1452 265 539 11033 I 34,955 480 8663 381 1475 269 519 10534 C 7 31,993 463 9630 387 1246 270 439 10435 C 36,797 487 7655 376 1678 269 601 10936 C,F 8 39,975 505 8391 378 1389 269 460 9837 C,F 34,355 475 7755 376 1188 269 445 11138 C,F 12 29,512 450 7290 377 880 270 370 12339 C 31,428 459 8110 381 1403 267 538 11540 I,F 13 31,668 463 8508 383 1633 272 608 11441 I,F 32,693 470 7926 380 810 273 311 11242 C 14 21,985 406 3351 348 543 258 370 19043 C 29,163 449 3064 347 707 262 404 16344 I,F 15 34,923 480 8567 384 1280 270 455 106

The errors are for 95% confidence level.A=point analysed, M=monazite crystal number, C=center of the grain, I= intermediate position between rim and center of the grain, R=rim of the grain, F=nearfracture; ⁎=crystal with a slightly concentric zoning.

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20 to 150μm. Eventually, the monazite crystals show completealteration to xenomorphic aggregates of apatite+allanite+thorite, indicating the influence of low-T Ca- and/or PO4-enriched fluid (Cathelineau, 1987; Cuney and Friedrich, 1987).

Table 2 shows the analytical data for monazite fromleuconorite ES35B. The U and Th contents vary from 0.4% to0.9% and from 4.6% to 27.7%, respectively, with the Pb contentin the range 0.1% to 0.8%. The Th content is highly variable butthe uranium content is rather constant (Fig. 6b). The calculatedages population looks homogeneous and unimodal (Fig. 6a andb), but the MSWD is too high (average age=554±9Ma,MSWD=1.64). The high MSWD arise from few outliers (twoolder and one younger). Excluding those points yields a single

homogeneous population with average ages of 553±10Ma andMSWD of 1.04. There is no significant age difference fromcenter to rim or from one zone to another, within a singlemonazite crystal, and the age is independent from the chemicalcomposition of the crystal (Fig. 5b). This indicates that the ageobtained is highly reliable, and that the late Ca-rich fluid did notaffect the crystals that were not destroyed.

6.3. Monazite of the metapelitic migmatite VJ19B

For this rock, five samples were analyzed by both optical andscanning electron microscopy, and only one of them hadmonazite grains suitable to be analyzed with the microprobe.

Fig. 5. Backscattered electron (BSE) images of monazite from leuconorite(sample ES35B). (a) Textural relationships of monazite (M1) with plagio-clase (Pl), clinopyroxene (Cpx), biotite (Bio) quartz (Qz) and magnetite(Mgt). (b) Variation in BSE intensity due to variable Th contents (see points66 to 72 in Table 2).

Fig. 6. Weighted-histogram (a) and U–Th–Age (b) representations of data fromTable 2 (leuconorite ES35B). Each small bell-shaped curve represents theprobability density function for one measurement. The thick curve is the sum ofall individual bell-shaped curves. The thinner/dotted curve represents the agecalculated by the statistical procedure.

Fig. 7. Microscopic aspect of monazite grains from metapelitic migmatite(sample VJ19B) observed under plane polarized light (uncrossed nicols). Themonazite is recognized by its rhombic shape and thick pleochroic haloes whenincluded in biotite. M=monazite, Pl=Plagioclase, Bio=biotite.

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Monazite follows the (001) cleavage of biotite or muscovite, theformer displaying thick pleochroic haloes (Fig. 7). They aretypically unzoned or slightly concentrically zoned, euhedral tosubhedral, rhombic (Fig. 8a and b) to prismatic or even acicularin shape (Fig. 8c and d), varying from 15 to 80μm in size. Theyoften present needle-like terminations that invade the cleavageof micas (Fig. 8a to c). This demonstrates that monazite (as wellzircons, cf. Fig. 8d) crystallized mostly syntectonically withregard to the S3 fabric.

Table 3 shows the analytical data for monazite frommetapelitic migmatite VJ19B. The U and Th contents varyfrom 0.3% to 1.0% and from 2.2% to 4.0%, respectively, the Pbcontent being in the range 499ppm to 0.2%. Here the Th contentis low and homogeneous but the U content is variable. (Fig. 9b).The population looks heterogeneous, with a visible “tails”madeof young ages (Fig. 9a). The MSWD on the global population isvery high (=3.22) for and average age of 511±107Ma. Usingthe statistical procedure of Montel et al. (1996) three popu-

lations are necessary to get a statistically acceptable solution(301±50, 465±29 and 590±25Ma). This seems unnecessarilycomplex. As no simple geometrical relationship between theseage populations and the positions of the crystals can be foundwe think that those ages are meaningless. Actually, one homo-geneous population (542±20Ma, MSWD=1.39) can beobtained by excluding the 6 youngest ages, which are res-ponsible for the “tail” in Fig. 9a. Late Ca-rich fluids could beresponsible in that sample for partial lead loss in some grains,giving meaningless younger ages.

Fig. 9. Weighted-histogram (a) and U–Th–Age (b) representations of data fromTable 3 (metapelitic migmatite VJ19B). The thick curve is the sum of allindividual bell-shaped curves. The thinner/dotted curves represent the agescalculated by the statistical procedure.

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7. Sm–Nd data of garnet-bearing rocks

7.1. Analytical method

The samples were prepared by using the Sm–Nd method-ological procedure adopted in the Laboratory of Geochronologyof the University of Brasília (Gioia and Pimentel, 2000). Thisanalytical method consists in the determination of the Sm andNd contents by isotopic dilution and Nd isotopic compositionthrough Thermal Ionization Mass Spectrometry — TIMS.Approximately 100mg of powdered samples of whole rock andgarnet were weighted and an amount of mixed spike enriched in150Nd and 149Sm, in a proportion of around 5 :1 (whole rock)and 3 :1 (garnet), was added. The samples were submitted toacid digestion, which will aid to complete homogenizationbetween spike and sample. Three acid attack stages wereperformed in the samples. Garnet was taken in a solution inteflon bombs (procedure blanks around 200–68pg for Ndanalyses) using a mixture of distilled HF and HNO3, in a ratio of4 :1. The bombs were kept in an oven at 190°C for at least twodays. All the chemical attacks for whole rock was performed inSavilex® beakers (procedure blanks around 100–49pg for Ndanalyses) with the same acids in the 12 : 1 ratio, undertemperature around 60°C in hot plate. This first attack wasfollowed by total sample drying on hot metal plate to allow theH2O and SiF4 evaporation, both presenting reaction productsbetween samples and reagents. The second attack is made in a

Fig. 8. Backscattered electron (BSE) images of monazite from metapelitic migmatite (sample VJ-19B). (a) and (b) Rounded to rhombic shape, with recrystallized tailsalong (001) plane of micas. (c) Acicular habit of monazite within a homogeneous biotite crystal. (d) Prysmatic habit of monazite (M) and recrystallized zircon (Zir)along (001) cleavage plane of biotite. Analyses 42 to 43 (M14) in (a), 38 to 39 (M12) in (b), and 34 to 35 (M7) in (c) are given in Table 3.

Table 4Mineral Sm–Nd and whole rock for samples from high-T shear zones

Sample Description Sm(ppm)

Nd(ppm)

147Sm/144Nd 143Nd/144Nd

Leucosome 1MA-212 WR Whole rock 5.727 27.24 0.116952 0.512308MA-212 Gar Garnet 6.466 19.703 0.1984 0.512634MA-212 Bio Biotite 15.13 83.24 0.109878 0.512317

Leucosome 2ES-156 WR Whole rock 12.32 87.5 0.08511 0.510956ES-156 Gar Garnet 8.008 2.311 2.0951 0.518534

Alkali granite(1)

MA-01 WR Whole rock 1.191 6.049 0.1196 0.511271MA-01 Gar Garnet 84.6 152.22 0.33597 0.51209MA-01 Cpx Clinopyroxene 39.6 97.2 0.24750 0.511753

See sample sites in Fig. 3. The errors on 147Sm/144Nd and 143Nd/144Nd ratiosare of, respectively, 0.15% and 0.003% (2σ).(1) Nascimento et al. (2001).

Fig. 10. Sm–Nd isochrones for two garnet-bearing leucosomes (a, b) and theCaxexa alkaline granite (c). See sample sites in Fig. 3.

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similar way, as the first attack, but the bombs/Savilex® staylonger in the oven/hot plate (at least five days). The driedresidue is taken into solution in 5mL 6N HCl, for at least 2days,and it further evaporated, and subsequently, it taken again in0.5mL 2.5N HCl for the cationic separation stage.

For chemical extraction of the REE group ion exchangecolumns packed with BioRad AG 50W-X8 cationic resin wereused. Sm and Nd were extracted by reverse-phase chromatog-raphy (teflon powder impregnated with di-(2-etilexi) phospho-ric acid). Sm and Nd aliquots were loaded onto double Reevaporation filaments, and the isotopic ratio and concentrationsmeasurements were made on a Finnigan-MAT262 massspectrometer in static mode. Nd ratios were normalized toa146Nd / 144Nd ratio of 0.7219. The reproducibility of the isoto-pic ratios is controlled with systematic measurements of in-ternational La Jolla standard, with mean value of 143Nd / 144Ndof 0.511858±7 (2σ). The external precision to 147Sm/ 144Ndratio is of 0.15% (2σ) and for 143Nd / 144Nd ratio is of 0.003%(2σ).

7.2. Results obtained from Sm–Nd data on garnet-bearingrocks

Table 4 reports the Sm–Nd data for two granitic leucosomes(samples MA-212 e ES-156) and a syntectonic garnet-bearingalkaline granite (sample MA-01) emplaced in an extensionalshear zone (see Fig. 3 for sample sites). The results are dis-cussed below.

7.2.1. Leucosome 1 (MA-212)This sample was collected from a migmatitic aureole around

a gabbro–noritic pluton about 9km southweast of Casserenguetown, Paraíba state. The protolith of the leucosome are garnet–cordierite-bearing biotite schist of the Proterozoic Seridó Belt.The leucosomes have trondhjemitic to leucotonalitic composi-tion, medium- to coarse-grained texture and millimeter-sizedporphyroblasts of (almandine-rich) garnet, andalusite andbrown biotite (Nascimento, 2000). Some chlorite and muscovite

lamellas may be observed along the (001) biotite cleavage or infractures affecting the garnet crystals. An isochron with threepoints (whole rock, biotite, garnet) yielded an age of 574±67Ma (2σ) and MSWD of 2.688 (Fig. 10a). This age isinterpreted as the age of the thermal event due to the gabbro–noritic intrusion. The elevated error on the age may result fromincomplete re-equilibration of the Sm–Nd system of garnet(which contain relict biotite lamellas) and biotite (partiallytransformed into chlorite, muscovite and/or iron oxides)porphyroblasts.

7.2.2. Leucosome 2 (ES-156)This sample was picked up from migmatites on the south of

the Serrinha Espírito Santo shear zone (Fig. 3). The protoliths ofthe leucosome are biotite-bearing granitic to granodioriticgneiss of the Precambrian basement. The partial melting ofthis gneiss generated a number of leucosomes that werechanneled into 45–55°Az-directed sinistral, extensional shear

Table 5Summary of the U–Th–Pb electron microprobe monazite and Sm–Nd garnet-whole rock ages obtained in this paper and the U–Pb zircon, Rb–Sr whole rock andmineral 40Ar–39Ar ages discussed in the text

Sampleanalysed

U–Th–Pb (monazite) Sm–Nd garnet–whole rock U–Pb zircon Rb–Sr whole rock 40Ar–39Ar

Serrinha granite 603±22(6)

Leuconorite ES-35B 550±10(1) 599±16(8)

Metapelite VJ-19B(1) 542±20Caxexa granite MA-01(2) 578±14Leucosome MA-212(1) 574±67Leucosome ES-156(1) 574±2Diorites(3) 579±14Acari granite (4) 555±10Caxexa granite (2) 536±4Gameleira granite (5) 544±16Amphibole and micas (7) 542±3 to 491±3

All dates are in Ma within 2σ range.(1) This paper. (2) Nascimento et al. (2001). (3) Leterrier et al. (1994). (4) Legrand et al. (1991). (5) McMurry et al. (1987). (6) Macedo et al. (1997). (7) Corsini et al.,(1998). (8) Dantas (1997).

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bands. The leucosome has granitic composition, medium- tocoarse-grained texture and is composed of millimeter-sizedalmandine-rich garnet, perthitic microcline, oligoclase, brownTi-rich biotite, green hornblende and brown tourmaline. Anisochron with two points (whole rock, garnet) yielded an age of574±2Ma (Fig. 10b). This age is interpreted as the age of thepeak of the thermal event related to the partial melting of thegneiss. As the leucosomes fill shear bands, that age of 574Ma ishere assumed as the age of the extensional shear deformation.

7.2.3. Alkaline granite (MA-01)This sample was collected from a syntectonic alkaline body

(the Caxexa pluton) emplaced in the mylonitic interface betweenthe Precambrian basement and micaschists of the Proterozoicbelt, some kilometers on the northweast of the high-T Remígio–Pocinhos Shear Zone (see Fig. 3). The Caxexa pluton wasdescribed in detail by Nascimento (2000) and Nascimento et al.(2001). It is a hololeucocratic, white to pink alkali-feldspargranite, with andradite-rich garnet, albite (An0–9), clinopyrox-ene (ægirine–augite or hedenbergite) and Ti-poor magnetite. Anisochron with three points (whole rock, clinopyroxene, garnet)yielded an age of 578±15 (2σ) and MSWD of 0.05 (Fig. 10c).This age was interpreted as the age of the main stage ofcrystallization of the alkaline magma by Nascimento (2000) andNascimento et al. (2001). However, closer examinations of thetextural description from these authors suggest that this age maybe ascribed to the high-T recrystallization of those minerals inthe solid state during a late- to post-magmatic deformational andthermal event. This is corroborated by the abundance ofgranoblastic aggregates of feldspar and clinopyroxene as wellas by porphyroblasts of garnet.

8. Discussions

Table 5 summarizes the U–Th–Pb electron probe monaziteand Sm–Nd garnet–whole rock ages obtained in this work.Other relevant geochronologic U–Pb, Rb–Sr and 40Ar–39Ardata are also reported for discussion and comparison with ourU–Th–Pb and Sm–Nd data.

The results obtained for a metapelitic migmatite within theRemígio–Pocinhos Shear Zone (RPSZ), and a leuconorite bodyalong the Serrinha–Espírito Santo Shear Zone (SESSZ) andclose to the southwestern border of the Serrinha granite, led to aminimum age estimate of the pluton emplacement and the high-T metamorphism observed in the region.

The metapelitic migmatite shows an age of 542±20Ma,similar to the age of the leuconorite. As the monazite from themetapelitic migmatite crystallized syntectonically with regardsto the S3 Brasiliano planar fabric, it is interpreted to correspond tothe thermal metamorphic peak and the related extensionalBrasiliano deformation (M3/D3) in the São José de CampestreMassif. This U–Th–Pbmonazite age is slightly younger than theSm–Nd ages of garnet from i) a peraluminous leucosome (574±67Ma), ii) a granitic leucosome (574±2Ma), and iii) anandradite–clinopyroxene-bearing alkaline granite (578±14Ma), which agrees very well with the U–Pb zircon age ofdiorite plutons of the Seridó belt with ca. 579±14Ma (seeTable 5).

Two explanations can be proposed for this difference. Thiscould reflect the dissimilarity in the behavior of the U–Th–Pbsystem of monazite and the Sm/Nd system in garnet. Here themonazite could record the age of hydrothermal events at the endof the metamorphism, while Sm/Nd could record the progradeformation of garnet or the peak of metamorphic conditions.Alternatively, the monazite ages could be a distinct event; forexample the age of the Ca-rich fluid hydrothermal event.Anyway, this event should occur in a tectonic context similar tothe formation of S3 planar fabric because monazite is clearlyinvolved in the S3 structure. We think that the first hypothesis issimpler and more coherent. The lower limit for the Brasilianoevent in this area is ca. 500Ma, as shown by 40Ar–39Ar ages(Corsini et al., 1998). Monazite would record an intermediatestage between the peak or prograde conditions indicated by Sm/Nd in garnet and U/Pb in zircon and the end of the metamorphicevent indicated by Ar–Ar ages.

One may question if the transpressional regime in the centralpart of the Seridó belt is synchronous with the extensional–transtensional setting in the São José de Campestre Massif. A

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somewhat similar context is described by Carson et al. (1997)from Larsemann Hills (Antarctica). In that case, extensivepartial melting during decompression produces a large numberof syntectonic granites and pegmatites, but the transpressionalevent is slightly earlier than the extensional one, respectively,515 and 500Ma. Crustal thickening caused by transpressionwithin the central part of the Seridó belt should be balanced bysubsequent or concomitant extensional deformation andcrustal thinning elsewhere. In this way, the extensionalstructures in the São José de Campestre Massif may beregarded as mechanically complementary sites of exhumationand decompression, concomitant with generalized partialmelting, and laterally adjacent to regions dominated by crustalthickening (cf. Carson et al., 1997). However, in the Seridóbelt, the available data do not display a clear time differencebetween the transpressional and the extensional–transtensionalregimes.

Field and textural observation suggest a late Brasilianoincremental deformation shown by late extensional low-T C′planes, oblique axial surfaces folding sheet-like Brasilianoalkaline granites within the Remígio–Pocinhos Belt, and low-T(greenschist facies) mylonites affecting the previous high-Tfabrics (Sá et al., 1993; Souza and Sá, 1993; Trindade et al.,1993). The sequence of events described can be easilycorrelated to uplift and exhumation of deeper levels of theorogen (Sá, 1994), like in the late Pan-African of Madagascar(Nicollet et al., 1997) and in the Phanerozoic analogues (Vanceet al., 1995; McGrath and Tarney, 1995). Consequently, theBrasiliano orogeny has been active until the end of theCambrian, then probably restricted to reworking alongnarrower, low-T shear zones. These ones could have channeledthe low-T Ca-rich fluids responsible for reopening the U–Th–Pb isotopic system of some monazite grains. For this, thefollowing interpretations may be posed: a) a brittle deformationevent under greenschist (or even lower) metamorphic gradeduring the Ordovician; b) a meaningless, partial resetting relatedto the Mesozoic Gondwana break up or a hypothetical mid-Paleozoic tectonic event.

9. Conclusions

The results obtained on monazite dating from a leuconoriteand a metapelitic migmatite, both related to high-T extensionalor transtensional shear zone point to a long-lived Brasilianoorogenic event in the São José de Campestre Massif, north-eastern Borborema Province. It is inferred that the peak of thehigh-T metamorphism (from upper amphibolite to hydratedgranulite facies) and ductile extensional or transtensionaldeformation may be placed at about 578–574Ma. This age isbest recorded by the Sm–Nd on garnet (granitic leucosome,garnet-bearing alkaline granite) and U–Pb on zircon (diorites).The 540–550Ma age recorded on monazites may representanother phase under slightly lower temperatures before theending of the Brasiliano event. This suggests that the U–Th–Pbisotope system in monazite may register events different thanthe ones recorded by the U–Pb zircon and Sm–Nd garnetsystematics. Late Brasiliano, low-T reworking along narrow

shear zones appear to have been active up to the Cambrian asdeduced from 40Ar–39Ar ages in the literature. Ages youngerthan 500Ma (up to 300Ma) could be related to a low-T tectonicevent of Ordovician or younger age not yet recognized in theregion.

Acknowledgements

Z. S. S. and V.E.A. acknowledge CAPES scholarship (3070/95-11) and CAPES/COFECUB project (177/95) for supportinglab work in France. Field work in NE Brazil and lab work at theUniversidade de Brasília have been financed by PADCT/FINEPand CNPq.

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