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

Gondwana Research 9 (2006) 441 – 455 www.elsevier.com/locate/gr

Electron microprobe dating of monazite from high-T shear zones in the São 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 Pós-Graduação em Geodinâmica e Geofísica e Departamento de Geologia — Universidade Federal do Rio Grande do Norte, 59078-970 Natal, RN, Brazil b 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, Brazil d 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, France CNRS, UMR 6524, Magmas et Volcans, Département des Sciences de la Terre, Université Blaise Pascal, 5, Rue Kessler, 63038, Clermont-Ferrand cedex, France

a

f

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

Abstract The easternmost domain of the Borborema Province, northeastern Brazil, presents widespread, extensional-related high-temperature metamorphism during the Brasiliano (=Pan-African) orogeny. This event reached the upper amphibolite to granulite facies and provoked generalized migmatization of Proterozoic metapelitic rocks of the Seridó Group and tonalitic to granodioritic orthogneisses of the Archean to Paleoproterozoic basement. We report new geochronological data based on electron microprobe dating of monazite from metapelitic migmatite and leuconorite within the high-T shear zones that make up the eastern continuation of the huge E–W Patos shear belt. These data were also constrained by using the Sm–Nd isotopic systematic on garnet from a syntectonic alkaline granite and two garnet-bearing leucosomes. The results suggest 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-whole rock ages. The U–Th–Pb isotopes on monazite of the metapelitic migmatite show a younger thermal event at 553 ± 10 Ma. When compared to the Sm–Nd garnet-whole rock ages, the U–Th–Pb electron probe monazite ages seem to record an event of slightly lower temperatures after the peak of the high-T metamorphism. This may reflect the difference in the isotopic behavior of the geochronological methods employed. Otherwise, the U–Th–Pb ages on monazites could indicate an event not yet very well defined. In anyway, this paper reveals the partial or even complete reopening 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 Borborema Province (Northeastern Brazil) is characterized by a huge amount of granitoid plutons and a complex system of transcurrent shear zones of crustal or lithospheric scale (Fig. 1). These shear zones ⁎ Corresponding author. Tel./fax: +55 84 32153831. E-mail addresses: [email protected] (Z.S. de Souza), [email protected] (J.-M. Montel). 1 Now at the IG/USP, São Paulo, SP, Brazil.

extended to West Africa and developed (and/or were reactivated) synchronously after the collision between the Neoproterozoic West Africa, Congo and São Francisco cratons (Bertrand and Sá, 1990; Caby et al., 1991; Castaing et al., 1994). In this region, a close connection between the plutonism and the peak of metamorphism and deformation during the Brasiliano orogeny has 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 shearrelated and seem to be affected during and after their cooling by subsequent shearing (Vauchez et al., 1995). The shear zones

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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 Africa Craton; AC Amazonian Craton; SFC São Francisco Craton; CC Congo Craton; BRMB Brasília and Ribeira Mobile Belts; BP Borborema Province; CS Cameroon Shield; NB Nigerian Belt; HS Hoggar Shield; PL Patos Lineament; PeL Pernambuco Linement; AdL Adamaoua Lineament.

separate domains of variably strained supracrustal sequences of assumed Paleo- to Mesoproterozoic ages, and a transpressional tectonic setting has been suggested for the central part of the Seridó 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 Seridó region has been deduced mainly from Rb–Sr isochron and K / Ar data in the 700–500Ma range (Brito Neves et al., 1974; Sá et al., 1987). U–Pb zircon and Rb–Sr whole rock isochron dating of diorites and coeval granite plutons along the Seridó Belt led to estimate the age of the transpressional deformation and metamorphic 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 high temperature event in the São José de Campestre Massif, the northeastern portion of the Borborema Province (Fig. 1), with new data obtained by two geochronological techniques not yet used on those rocks: electron microprobe monazite dating and Sm–Nd on garnet dating. Both techniques are especially efficient for dating poly-metamorphic rocks. We selected a migmatitic metapelite (mylonitic, migmatitic micaschist) within the Remígio–Pocinhos Shear Zone (RPSZ) and a leuconorite intruded 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), transtensional sites associated to high-T metamorphism appear to control the geometry 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 orthogneisses, similar to those of the Caicó basement complex in the central Seridó region (Hackspacher et al., 1990; Souza et al., 1993), are intruded by Brasiliano-age granitoids and overlain by supracrustal allochtons. All these units are separated from the central Seridó Belt by the NNE-trending Picuí–João Câmara Shear Zone (PJCSZ), which displays dextral strike-slip kinematics. The Remígio–Pocinhos schist belt (RPB), located in between Barra de Santa Rosa and Remígio villages (Fig. 3), was studied by Sá et al. (1993), Souza and Sá (1993) and Trindade et al. (1993). It displays a low-P metamorphism (named M3) grading from low amphibolite facies (andalusite + staurolite in metapelites) in its

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

northwestern part to upper amphibolite and even granulite facies (cordierite + garnet + prismatic sillimanite + mesoperthitic K-feldspar in pelites; diopside + phlogopite + forsterite + spinel in marbles; orthopyroxene + clinopyroxene in metaplutonic rocks) with extensive migmatization in its southern counterpart. In the highest-T zone, extensive partial melting of metapelitic protholiths produced abundant centimeter to decimeter granitic to trondhjemitic garnet-bearing leucosomes. A moderate degree of partial melting is observed in tonalitic to granodioritic orthogneisses of the Caicó Complex basement (Fig. 4a and b), whereas interleaved amphibolites show a somewhat lesser degree of melting (Fig. 4a). In both cases the newly formed leucosomes follow the S3 Brasiliano-age fabric. Brasiliano structures (D3) are characterized by schistosity (biotite ± amphibole) or metamorphic banding that varies from gently dipping in the northwestern, lower-T domain, to NE-trending, steeply SE-dipping when reaching the RPSZ. This arrangement conforms to a negative half flower geometry for the RPB. In the high-T portion, several slices of orthopyroxene-bearing orthogneiss and interleaved mafic granulites, as well as kilometer-scale andradite–ægirinebearing alkaline granite sheets are also found (Nascimento, 2000). In both the low-T and high-T zones, a penetrative lineation changes its direction from N–S or NNE–SSW in the extensional NW 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 shear zone linked at its eastward tip to the prolongation of the RPSZ (Fig. 3). Along its strike, and especially in the northwestern border of the Serrinha granite, abundant in situ partial melting of the basement tonalitic to dioritic orthogneiss marks the SESSZ. The trondhjemitic to granitic leucosomes were collected along tension gashes and sinistral extensional C′ structures (Fig. 4b). Within the Serrinha granite, the same kinds of sites are filled by hectometer- to decameter-sized late granitic melts. In the same way, shear bands with NE-direction are filled by small norite to quartz diorite bodies. Up to this moment, no precise direct radiometric ages are available for this high-T metamorphism, any interpretations being made mostly from structural and crosscutting relationships between granites and country rocks (Sá et al., 1986, 1999). In this respect the U–Pb zircon age of 579 ± 14 Ma (2σ) of the Acari and São João do Sabugi diorites, on the southweast of the area studied, was interpreted as the peak of the thermal event and also the age of the transpressional deformation on the Seridó region by Leterrier et al. (1994). However, the Acari granite, one of the several granitoid plutons considered as syntranspressional and spatially related to the diorites referred above (Sá, 1994) has a U–Pb zircon age of 555 ± 10 Ma (2σ; Legrand et al., 1991).

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Fig. 3. Simplified geological map of the studied region (modified from Barbosa et al., 1974; Sá, 1994; Dantas et al., 1996) and a NW–SE cross-section (A–B). 1 — Archean and 2 — Paleoproterozoic basement; 3 — Supracrustal sequences (SB Seridó Belt, RPB Remígio–Pocinhos Belt); 4 — Brasiliano-age granitoids and basic plutons (in black); 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 — mafic granulite, ES35B — leuconorite, ES31.2.D quartz diorite, MA01 — alkali granite, MA212 and ES156 — garnet-bearing leucosomes.

As regards as the SESSZ, a recent Rb–Sr whole rock isochron date of the Serrinha granite by Macedo et al. (1997) yielded an age of 603 ± 22Ma (2σ). This age is at least 50Ma older than the 540–550Ma ages obtained by the same method in the nearby and structurally similar or younger Dona Inês and Monte das Gameleiras plutons (McMurry et al., 1986, 1987). On the other hand, 40Ar–39Ar data of micas and amphibole of rocks in the eastern part of the PSZ revealed plateau ages spanning from 542 to 491 Ma (Corsini et al., 1998). Based on the known U–Pb zircon ages of syntectonic granites and their 40Ar–39Ar results, Corsini et al. (1998) suggested a homogeneous and unusual slow cooling history (3–4° C/Ma) and a rather slow uplift rate between 580 and 500Ma, followed by fast cooling over most of the Borborema Province around 500 Ma. In order to better constrain the age of the migmatization event and, by correlation, of the related extensional deformation, a sample of migmatized metapelites from the RPSZ (VJ19B in Fig. 3) and a leuconorite pluton intruded in a megascopic sinistral C′ structure along the SESSZ (ES35B in Fig. 3) were chosen for monazite electron microprobe dating.

Three other samples were studied by using the Sm–Nd mineral technique: i) an extensional-related garnet-bearing leucosome derived from partial melting of basement granitic gneiss (ES156 in Fig. 3); ii) an extensional-related garnet-bearing leucosome derived from partial melting of micaschists (MA212 in Fig. 3); and iii) a garnet-bearing alkali granite emplaced in a north–south-trending strike-slip shear zone (sample MA01 in Fig. 3). 3. Sample description and metamorphic P–T conditions 3.1. Leuconorite (ES35B) The mineral composition discussed below is based on average analyses from Table 1. The leuconorite is composed of plagioclase (An36), Fe-hypersthene (Wo2En49Fs49, mg# = 0.51) and diopside-enriched clinopyroxene (Wo 45 En 35 Fs 20 , mg# = 0.64) phenocrysts all variably transformed to euhedral polygonal mosaic indicative of high-T recrystallization. Abundant Ti-rich biotite (TiO2 = 5.51 wt.%, mg# = 0.51), zoned

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3.3. Metamorphic P–T conditions

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

allanite, pyrite, chalcopyrite, perthitic orthoclase (Or88) and interstitial quartz are also observed. Biotite is usually late magmatic, commonly including pyroxenes and plagioclase grains, and oriented in a well defined planar fabric (here called Sγ) together with newly recrystallized plagioclase and pyroxenes. In the field, this fabric parallels the NE-directed C ′ planes. 3.2. Metapelitic migmatite (VJ19B) This sample comprises a millimeter-scale banding of strongly pleochroic brown biotite rich (TiO2 = 2.04 wt.%, mg# = 0.48) (melanosome) and plagioclase (An20) + quartzrich bands (leucosome), which correspond to the D3 (S3) fabric on the field. Al- and Ti-rich muscovite (Al2O3 = 35.1 wt.%; TiO2 = 0.80 wt.%) mainly overgrowths the S3 fabric or crosscuts it, and is probably late tectonic with regard to biotite and plagioclase + quartz leucosome. Fine-grained biotite and muscovite lamellae follow discontinuous microshear zones at a small angle to the S3 fabric, representing a late ductile–brittle deformation episode or an incremental stage.

For leuconorite and metapelitic migmatite, a low-P/high-T metamorphism is indicated by field observations and paragenetic relations described above as well as geothermometric and geobarometric calculations (data on Table 1). For a quartz diorite outcropping along the SESZ (sample ES31.2.D), the plagioclase–amphibole geothermometer (Blundy and Holland, 1990) and Al-in-amphibole geobarometer (Schmidt, 1992) gives, respectively, 680–730°C and 4.5–5.1 kbar. The amphibole is green to blue and was classified as Mg-hornblende (Leak, 1978), with mg# = 0.55, TiO2 = 0.87 wt.% and 6.61 b Si b 6.97pfu. The amphibole geothermobarometer (Blundy and Holland, 1990; Schmidt, 1992) for a mafic granulite (sample VJ23A) interleaved with the metapelitic migmatite gives T and P in the range 671–765 °C and 3.8–5.7 kbar. The amphibole (Mghornblende) is deep green to brownish, with 6.81 b Si b 6.92pfu, TiO2 = 0.92 wt.% and Mg-enriched (mg# = 0.68). In the mafic granulite, the low Al-contents of orthopyroxene (0.76 wt.%) and clinopyroxene (1.33 wt.%) and the absence (or rare occurrence) of garnet is consistent with a low-P metamorphism (Spear, 1993). The application of the two pyroxenes geothermometer proposed by Brey and Köhler (1990) yielded average temperatures of 800 °C for Ca-partitioning and 640 °C for Na-partitioning. These temperatures are slightly higher than the averages calculated for pyroxenes in the leuconorite (ca. 740 and 640°C, respectively) by the same geothermometer. The P–T results above (averaging of 724 °C/4.8 kbar) places the main metamorphic event at the transition between the upper amphibolite and the granulite facies, about 50 °C above the wet granite solidus (Holtz and Johannes, 1994, and reference therein). Experimental fluid-absent partial melting of metagreywackes (Le Breton and Thompson, 1988; Vielzeuf and Montel, 1994) and dehydration melting of tonalitic composition (Singh and Johannes, 1996) for P less than 10 kbar demonstrated that the biotite-out curve is in the range 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, the biotite completely disappears at 975 °C via incongruent melting forming orthopyroxene + oxides + granitic melt (Skjerlie and Johnston, 1993). Consequently, perthitic orthoclase in leuconorite and amphibole in quartz diorite and mafic to felsic granulites should have been re-equilibrated during the cooling stage of the M3 metamorphism. This assumption is supported by the relatively low TiO2 (0.94 wt.%, Ti = 0.1 pfu) contents of amphibole reported for granulite facies rocks (e.g., Raith and Raase, 1986; Schumacher et al., 1990), and the ubiquitous presence of associated ilmenite and Na-partitioning between ortho- and clinopyroxene. Under low-T conditions either in the vanishing stage of the metamorphic peak or in a post-tectonic event not yet very well constrained, the rocks studied (especially the migmatitic metapelite) were probably modified by retrometamorphic fluids, resulting in the crystallization of chlorite and fine grained white mica along the (001) biotite cleavage plane.

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4. Some comments about the electron microprobe dating of monazites The electron microprobe dating of monazite is now a widely used 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 main advantage of this technique is the possibility to rely the exact petrographic and textural position of the crystals to the age and the possibility to analyze even very small crystal. Additionally, the high spatial resolution (less than 3 μm in diameter) of the electron microprobe dating makes it possible to separate polygenetic portions of complex monazite and thus to avoid analytical mixing phenomena frequently interpreted as meaningless intermediate radiogenic ages. Extensive application of the electron microprobe method to polygenetic monazite (Montel et al., 1996; Braun et al., 1998; Cocherie et al., 1998; Finger et al., 1998; Williams et al., 1999, 2001; Montel et al., 2000; Williams and Jercinovic, 2002), as well as experimental resetting of the U–Th–Pb isotopic system of monazite (Smith and Giletti, 1997; Seydoux-Guillaume et al., 2002), have shown that resetting by diffusion is unlikely during metamorphic 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 assess by electron probe only (Seydoux-Guillaume et al., 2003; Gonçalves et al., 2004). 5. Analytical procedure Following Montel et al. (1994, 1996), normal polished thin sections prepared for conventional electron microprobe analysis

were used. Firstly, petrographic composition and textural relationships of monazites were described under a polarizing microscope. When included in biotite the monazite crystals can be recognized from their pleochroic haloes, which are normally thicker than the one produced by zircon. Otherwise it is difficult to clearly differentiate between zircon (that shows straight extinction and generally a better-shaped prisms, and may be concentrically zoned), monazite that has an oblique extinction angle (2–12°), a thin (100) cleavage and commonly rounded shape, and xenotime (with a good cleavage). To overcome these ambiguities, monazite crystals were definitively identified through qualitative analysis using a scanning-electron microscope (Cambridge-Leica Steroscan 360), equipped with an EDS analytical system, of the Université Blaise Pascal, ClermontFerrand, France. With this technique, even very small crystal (about 5 μm in diameter) can be detected, as well the distinction between monazite (which is extremely bright), zircon and xenotime are very well resolved. SEM work is very useful for revealing the internal structure of monazite and also for finding crystal not seen by optical microscopy. After being identified, the monazite crystals are then analyzed for U, Th and Pb with a Camebax Micro electron microprobe, with an accelerating voltage of 15 kV and a 90– 145nA probe current. U, Pb and Th are analyzed simultaneously with PET crystals using Mα lines for Th and Pb and Mβ lines for U. The spectral interference of YLγ line on PbMα was corrected by measuring the intensity of YLγ on synthetic YPO4. Extrapolation down to the Y contents of monazite (less than 2wt.%) shows that this creates a maximum overestimate of the Pb content of about 30ppm. The standards are ThO2, UO2, and a synthetic glass (CaO–Al2O3–SiO2) with 6200 ppm of Pb. The counting time for standards is 50s on peak and 100s on

Table 1 Average chemical composition (%wt) of selected minerals in the Remígio–Pocinhos and Serrinha–Espírito Santo shear zones Remígio–Pocinhos Shear Zone (RPSZ)

Serrinha–Espírito Santo Shear Zone (SESSZ)

Metapelitic migmatite

Leuconorite

Mafic granulite

(Sample VJ19B)

SiO2 Al2O3 TiO2 MgO FeO MnO CaO Na2O K2O Total mg# Ab An Wo En Fs

Sample VJ23A

Quartz diorite

Sample ES35B

Sample ES31.2.D

Bio (N = 7)

Musc (N = 7)

Pl (N = 14)

Opx (N = 3)

Cpx (N=4)

Hb (N = 9)

Pl (N = 11)

Opx (N = 11)

Cpx (N = 10)

Bio (N = 5)

Pl (N = 14)

Hb (N = 10)

Pl (N = 6)

37.10 21.35 2.04 8.42 16.27 0.46 0.04 0.2 9.77 95.65 0.48 – – – – –

45.5 35.1 0.8 0.78 1.34 0.01 0.04 0.62 10.07 94.26 0.51 – – – – –

63.25 22.88 0.03 0.01 0.06 0.02 4.14 8.97 0.18 99.54 – 78.8 20.1 – – –

52.63 0.76 0.04 21.67 22.67 0.9 0.37 0.01 0.02 99.07 0.63 – – 0.8 61.6 37.6

53.51 1.33 0.08 14.3 8.65 0.43 21.41 0.53 0 100.24 0.75 – – 44.2 41.1 14.7

47.17 8.9 0.92 14.43 12.08 0.18 11.26 1.39 0.76 97.09 0.68 – – – – –

59.48 24.97 0.04 0.01 0.18 0.02 6.86 7.26 0.34 99.16 – 64.2 33.7 – – –

51.28 0.65 0.07 16.83 29.16 0.99 0.81 0.01 0.01 99.81 0.51 – – 1.7 49 49.3

51.64 1.33 0.14 11.99 12.1 0.46 21.34 0.4 0.03 99.43 0.64 – – 44.7 34.9 20.5

35.98 13.71 5.51 11.05 18.97 0.05 0.14 0.09 9.74 95.24 0.51 – – – – –

59.06 25.3 0.09 0.02 0.13 0.07 7.34 6.94 0.32 99.27 – 61.9 36.2 – – –

45.57 8.28 0.87 11.16 16.76 0.37 11.86 1.24 1.02 97.13 0.55 – – – – –

63.4 23.86 – – 0.06 – 4.88 9.1 0.19 101.49 – 76.3 22.6 – – –

(Symbols for minerals: Bio Biotite, Musc Muscovite, Pl Plagioclase, Opx/Cpx Ortho/Clinopyroxene, Hb Amphibole.). See sample sites in Fig. 3. mg# (Mg-number) = (MgO) mol / (MgO + FeO) mol. ⁎Analysis made with a Camebax Micro electron microprobe at Institut de Géologie, Université Blaise Pascal, Clermont-Ferrand, France. ⁎⁎Analysis made with a Cameca SX50 electron microprobe at Instituto de Geociências, Universidade de Brasília, Brazil.

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Table 2 Analytical results for leuconorite (sample ES35B) A

Position

M

Th (ppm)

Error⁎

U (ppm)

Error⁎

Pb (ppm)

Error⁎

t (Ma)

Error

66 67 68 69 70 71 72 62 63 64 65 91 92 93 94 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90

See Fig. 4 “ “ “ “ “ “ C I R C R C R I I,F R R C C I C R I I I R C C I R R I,F

1 (HZ)

127,563 119,991 80,556 81,818 117,235 114,752 102,609 107,413 109,883 80,674 101,370 175,567 192,025 134,723 111,140 92,613 63,956 66,492 81,746 122,930 277,498 262,391 200,442 247,290 199,925 171,520 172,815 157,601 187,165 137,812 120,666 166,984 46,246

878 847 695 700 839 829 783 801 811 696 777 1046 1100 902 815 744 622 635 698 861 1372 1325 1131 1276 1129 1030 1039 984 1087 913 851 1016 542

4775 5271 4093 3891 4653 4293 4412 5014 5094 3669 4477 4332 4512 4247 4270 4391 3694 3581 3863 6414 6268 6310 5539 6311 4948 4562 5183 4671 5035 4639 5104 8687 3854

369 370 361 362 365 366 365 366 368 358 367 365 368 362 359 363 357 360 362 383 378 382 375 378 370 369 375 366 374 370 371 394 359

3514 3237 2216 2107 3168 3260 2815 3043 3714 2141 2888 4731 5088 3940 3580 2554 1767 1818 2677 3663 8150 7266 5297 6740 5341 4691 4673 4648 4812 4015 3096 4806 1100

318 310 291 292 308 309 298 306 311 294 301 334 342 319 310 297 285 285 297 319 396 382 352 374 348 336 335 332 341 321 307 340 273

546 525 525 496 532 563 535 547 650 514 553 554 547 589 634 531 517 517 629 566 607 570 539 559 550 559 548 597 526 583 502 547 418

56 57 78 78 59 61 65 63 64 80 66 45 42 55 64 70 94 92 81 57 34 35 41 36 41 46 45 49 43 54 57 44 114

2 (HZ)

3 (CZ)

5 (NZ)

6 (NZ) 7 (HZ)

8 (NZ)

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 = near fracture; HZ = crystal with a heterogeneous zoning, the brightest ones being richer in Th contents; CZ = crystal with regular concentric zoning, in which the inner portions are richer in Th contents; NZ = not zoned crystal; FC = fractured crystal.

background for ThO2 an UO2, and 300s on peak and 600s on background for Pb glass. For dating, the counting time was of 100 s at each point. For each measurement, the confidence interval for Th, U, Pb are determined, the age is computed from Eq. (1) of Montel et al. (1996), and the uncertainty on the age (95% confidence) is calculated by propagating the uncertainties on Th, Pb and U into Eq. (1) of Montel et al. (1996). So, each measurement yields an age with a statistical confidence interval. An additional check of the quality of the data is performed at the beginning of any session, through a complete procedure on a well-dated monazite sample that is used as an age standard. All the dates obtained are then statistically treated and the MSDW is calculated for one or more population following the procedure of Montel et al. (1996). Data are presented in two types of diagrams: the weighted-histogram representation in which each age is represented by a bell-shaped curve (Montel et al., 1994), and the age–U–Th diagram in which each age is plotted versus U and Th (Braun et al., 1998). The first one allows estimating how the age population is distributed. The second diagram allows discussing the relationships between the age and the chemical composition of the crystals.

6. Results obtained by the electron microprobe dating of monazite 6.1. Presentation of the results The analytical data are given in Tables 2 and 3, and the results 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 optical and scanning electron microscopy, and only one of them had monazite grains suitable to be analyzed with the microprobe. Monazite has intergranular habit (Fig. 5a) or is found as inclusions in pyroxene and plagioclase. Commonly, it includes small pyroxenes and feldspar grains. Consequently, they probably have been formed late in the course of crystallization of the norite magma. They are colorless or pale yellow, usually zoned with regard to Th contents, and have euhedral prismatic or rounded (rhombic) shape (Fig. 5a and b), sometimes with a moderate cleavage orthogonal to (001). They vary in size from

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

Position

M

Th (ppm)

Error⁎

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44

C C C I I,F I,F C I,F C,F C,F C C C,F C I,F I,F I R,F C I I C C I I I C C I I,F C I I C C C,F C,F C,F C I,F I,F C C I,F

1,F

32,864 35,532 33,802 29,932 33,872 35,242 28,934 36,957 32,586 33,105 28,570 25,473 33,182 30,646 32,932 31,589 35,337 32,802 35,524 33,628 32,925 34,313 35,029 32,439 31,474 35,058 32,133 36,719 33,265 33,483 32,195 33,185 34,955 31,993 36,797 39,975 34,355 29,512 31,428 31,668 32,693 21,985 29,163 34,923

470 484 478 457 477 486 449 493 469 466 443 431 473 458 472 464 486 473 486 476 473 477 480 465 458 478 462 487 469 470 464 469 480 463 487 505 475 450 459 463 470 406 449 480

2⁎

4 5⁎

6

10

9

11

7 8 12 13 14 15

U (ppm) 6169 6720 6770 4164 6802 8230 6020 5564 6452 8020 6618 2958 3628 3710 7604 8083 6452 6945 6173 6833 6690 7932 8250 7937 7130 8939 10,452 8145 7243 8635 7222 8181 8663 9630 7655 8391 7755 7290 8110 8508 7926 3351 3064 8567

Error⁎

Pb (ppm)

Error⁎

t (Ma)

Error

373 377 377 360 377 390 370 371 367 370 368 353 359 355 381 383 375 383 375 375 379 384 383 376 374 387 389 378 372 382 373 380 381 387 376 378 376 377 381 383 380 348 347 384

1092 1462 1539 1015 1015 860 1324 1185 499 1300 1162 817 831 1200 1215 1155 1598 676 1472 1588 1175 1520 1524 1793 1438 1574 1761 1715 1719 1551 1548 1452 1475 1246 1678 1389 1188 880 1403 1633 810 543 707 1280

270 271 278 268 271 274 268 273 270 267 265 263 267 269 275 275 278 279 279 288 276 272 271 272 266 268 271 269 272 265 264 265 269 270 269 269 269 270 267 272 273 258 262 270

460 565 609 519 405 312 603 479 210 489 515 517 413 621 469 445 627 275 586 628 478 560 546 677 582 544 589 600 666 558 614 539 519 439 601 460 445 370 538 608 311 370 404 455

125 118 124 153 118 106 138 122 119 111 130 186 145 158 117 116 124 120 125 128 124 113 109 118 121 104 102 107 121 107 119 110 105 104 109 98 111 123 115 114 112 190 163 106

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 = near fracture; ⁎ = crystal with a slightly concentric zoning.

20 to 150μm. Eventually, the monazite crystals show complete alteration to xenomorphic aggregates of apatite + allanite + thorite, indicating the influence of low-T Ca- and/or PO4enriched fluid (Cathelineau, 1987; Cuney and Friedrich, 1987). Table 2 shows the analytical data for monazite from leuconorite ES35B. The U and Th contents vary from 0.4% to 0.9% and from 4.6% to 27.7%, respectively, with the Pb content in the range 0.1% to 0.8%. The Th content is highly variable but the uranium content is rather constant (Fig. 6b). The calculated ages population looks homogeneous and unimodal (Fig. 6a and b), but the MSWD is too high (average age = 554 ± 9Ma, MSWD = 1.64). The high MSWD arise from few outliers (two older and one younger). Excluding those points yields a single

homogeneous population with average ages of 553 ± 10Ma and MSWD of 1.04. There is no significant age difference from center to rim or from one zone to another, within a single monazite crystal, and the age is independent from the chemical composition of the crystal (Fig. 5b). This indicates that the age obtained is highly reliable, and that the late Ca-rich fluid did not affect the crystals that were not destroyed. 6.3. Monazite of the metapelitic migmatite VJ19B For this rock, five samples were analyzed by both optical and scanning electron microscopy, and only one of them had monazite grains suitable to be analyzed with the microprobe.

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

Fig. 5. Backscattered electron (BSE) images of monazite from leuconorite (sample ES35B). (a) Textural relationships of monazite (M1) with plagioclase (Pl), clinopyroxene (Cpx), biotite (Bio) quartz (Qz) and magnetite (Mgt). (b) Variation in BSE intensity due to variable Th contents (see points 66 to 72 in Table 2).

Monazite follows the (001) cleavage of biotite or muscovite, the former displaying thick pleochroic haloes (Fig. 7). They are typically unzoned or slightly concentrically zoned, euhedral to subhedral, rhombic (Fig. 8a and b) to prismatic or even acicular in shape (Fig. 8c and d), varying from 15 to 80μm in size. They often present needle-like terminations that invade the cleavage of micas (Fig. 8a to c). This demonstrates that monazite (as well zircons, cf. Fig. 8d) crystallized mostly syntectonically with regard to the S3 fabric. Table 3 shows the analytical data for monazite from metapelitic migmatite VJ19B. The U and Th contents vary from 0.3% to 1.0% and from 2.2% to 4.0%, respectively, the Pb content being in the range 499 ppm to 0.2%. Here the Th content is low and homogeneous but the U content is variable. (Fig. 9b). The population looks heterogeneous, with a visible “tails” made of young ages (Fig. 9a). The MSWD on the global population is very high (= 3.22) for and average age of 511 ± 107Ma. Using the statistical procedure of Montel et al. (1996) three popu-

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

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

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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 methodological procedure adopted in the Laboratory of Geochronology of the University of Brasília (Gioia and Pimentel, 2000). This analytical method consists in the determination of the Sm and Nd contents by isotopic dilution and Nd isotopic composition through Thermal Ionization Mass Spectrometry — TIMS. Approximately 100 mg of powdered samples of whole rock and garnet were weighted and an amount of mixed spike enriched in 150 Nd and 149Sm, in a proportion of around 5 : 1 (whole rock) and 3 : 1 (garnet), was added. The samples were submitted to acid digestion, which will aid to complete homogenization between spike and sample. Three acid attack stages were performed in the samples. Garnet was taken in a solution in teflon bombs (procedure blanks around 200–68 pg for Nd analyses) using a mixture of distilled HF and HNO3, in a ratio of 4 : 1. The bombs were kept in an oven at 190 °C for at least two days. All the chemical attacks for whole rock was performed in Savilex® beakers (procedure blanks around 100–49 pg for Nd analyses) with the same acids in the 12 : 1 ratio, under temperature around 60°C in hot plate. This first attack was followed by total sample drying on hot metal plate to allow the H2O and SiF4 evaporation, both presenting reaction products between samples and reagents. The second attack is made in a

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

Fig. 8. Backscattered electron (BSE) images of monazite from metapelitic migmatite (sample VJ-19B). (a) and (b) Rounded to rhombic shape, with recrystallized tails along (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.

Z.S. de Souza et al. / Gondwana Research 9 (2006) 441–455 Table 4 Mineral Sm–Nd and whole rock for samples from high-T shear zones Sample

Description

Sm (ppm)

Nd (ppm)

147

Sm / 144Nd

143

Nd / 144Nd

Leucosome 1 MA-212 WR Whole rock MA-212 Gar Garnet MA-212 Bio Biotite

5.727 6.466 15.13

27.24 0.116952 19.703 0.1984 83.24 0.109878

0.512308 0.512634 0.512317

Leucosome 2 ES-156 WR Whole rock ES-156 Gar Garnet

12.32 8.008

87.5 0.08511 2.311 2.0951

0.510956 0.518534

Alkali granite(1) MA-01 WR Whole rock 1.191 6.049 0.1196 MA-01 Gar Garnet 84.6 152.22 0.33597 MA-01 Cpx Clinopyroxene 39.6 97.2 0.24750

0.511271 0.51209 0.511753

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

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lamellas may be observed along the (001) biotite cleavage or in fractures affecting the garnet crystals. An isochron with three points (whole rock, biotite, garnet) yielded an age of 574 ± 67Ma (2σ) and MSWD of 2.688 (Fig. 10a). This age is interpreted as the age of the thermal event due to the gabbro– noritic intrusion. The elevated error on the age may result from incomplete re-equilibration of the Sm–Nd system of garnet (which contain relict biotite lamellas) and biotite (partially transformed 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 of the leucosome are biotite-bearing granitic to granodioritic gneiss of the Precambrian basement. The partial melting of this gneiss generated a number of leucosomes that were channeled into 45–55°Az-directed sinistral, extensional shear

similar way, as the first attack, but the bombs/Savilex® stay longer in the oven/hot plate (at least five days). The dried residue is taken into solution in 5 mL 6N HCl, for at least 2days, and it further evaporated, and subsequently, it taken again in 0.5 mL 2.5N HCl for the cationic separation stage. For chemical extraction of the REE group ion exchange columns packed with BioRad AG 50W-X8 cationic resin were used. Sm and Nd were extracted by reverse-phase chromatography (teflon powder impregnated with di-(2-etilexi) phosphoric acid). Sm and Nd aliquots were loaded onto double Re evaporation filaments, and the isotopic ratio and concentrations measurements were made on a Finnigan-MAT262 mass spectrometer in static mode. Nd ratios were normalized to a146Nd / 144Nd ratio of 0.7219. The reproducibility of the isotopic ratios is controlled with systematic measurements of international La Jolla standard, with mean value of 143Nd / 144Nd of 0.511858 ± 7 (2σ). The external precision to 147 Sm / 144 Nd ratio is of 0.15% (2σ) and for 143 Nd / 144Nd ratio is of 0.003% (2σ). 7.2. Results obtained from Sm–Nd data on garnet-bearing rocks Table 4 reports the Sm–Nd data for two granitic leucosomes (samples MA-212 e ES-156) and a syntectonic garnet-bearing alkaline granite (sample MA-01) emplaced in an extensional shear zone (see Fig. 3 for sample sites). The results are discussed 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 Casserengue town, Paraíba state. The protolith of the leucosome are garnet– cordierite-bearing biotite schist of the Proterozoic Seridó Belt. The leucosomes have trondhjemitic to leucotonalitic composition, medium- to coarse-grained texture and millimeter-sized porphyroblasts of (almandine-rich) garnet, andalusite and brown biotite (Nascimento, 2000). Some chlorite and muscovite

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

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Table 5 Summary 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 and mineral 40Ar–39Ar ages discussed in the text Sample analysed Serrinha granite Leuconorite ES-35B Metapelite VJ-19B(1) Caxexa granite MA-01(2) Leucosome MA-212(1) Leucosome ES-156(1) Diorites(3) Acari granite (4) Caxexa granite (2) Gameleira granite (5) Amphibole and micas (7)

U–Th–Pb (monazite)

Sm–Nd garnet–whole rock

U–Pb zircon

Rb–Sr whole rock

40

Ar–39Ar

603±22(6) (1)

(8)

550 ± 10 542 ± 20

599 ± 16 578 ± 14 574 ± 67 574 ± 2 579 ± 14 555 ± 10

536 ± 4 544 ± 16 542 ± 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).

bands. The leucosome has granitic composition, medium- to coarse-grained texture and is composed of millimeter-sized almandine-rich garnet, perthitic microcline, oligoclase, brown Ti-rich biotite, green hornblende and brown tourmaline. An isochron with two points (whole rock, garnet) yielded an age of 574 ± 2Ma (Fig. 10b). This age is interpreted as the age of the peak of the thermal event related to the partial melting of the gneiss. As the leucosomes fill shear bands, that age of 574 Ma is here 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 between the Precambrian basement and micaschists of the Proterozoic belt, some kilometers on the northweast of the high-T Remígio– Pocinhos Shear Zone (see Fig. 3). The Caxexa pluton was described in detail by Nascimento (2000) and Nascimento et al. (2001). It is a hololeucocratic, white to pink alkali-feldspar granite, with andradite-rich garnet, albite (An0–9), clinopyroxene (ægirine–augite or hedenbergite) and Ti-poor magnetite. An isochron 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 of crystallization of the alkaline magma by Nascimento (2000) and Nascimento et al. (2001). However, closer examinations of the textural description from these authors suggest that this age may be ascribed to the high-T recrystallization of those minerals in the solid state during a late- to post-magmatic deformational and thermal event. This is corroborated by the abundance of granoblastic aggregates of feldspar and clinopyroxene as well as by porphyroblasts of garnet. 8. Discussions Table 5 summarizes the U–Th–Pb electron probe monazite and Sm–Nd garnet–whole rock ages obtained in this work. Other relevant geochronologic U–Pb, Rb–Sr and 40Ar–39Ar data are also reported for discussion and comparison with our U–Th–Pb and Sm–Nd data.

The results obtained for a metapelitic migmatite within the Remígio–Pocinhos Shear Zone (RPSZ), and a leuconorite body along the Serrinha–Espírito Santo Shear Zone (SESSZ) and close to the southwestern border of the Serrinha granite, led to a minimum age estimate of the pluton emplacement and the highT 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 the metapelitic migmatite crystallized syntectonically with regards to the S3 Brasiliano planar fabric, it is interpreted to correspond to the thermal metamorphic peak and the related extensional Brasiliano deformation (M3/D3) in the São José de Campestre Massif. This U–Th–Pb monazite age is slightly younger than the Sm–Nd ages of garnet from i) a peraluminous leucosome (574 ± 67 Ma), ii) a granitic leucosome (574 ± 2 Ma), and iii) an andradite–clinopyroxene-bearing alkaline granite (578 ± 14Ma), which agrees very well with the U–Pb zircon age of diorite plutons of the Seridó belt with ca. 579 ± 14Ma (see Table 5). Two explanations can be proposed for this difference. This could reflect the dissimilarity in the behavior of the U–Th–Pb system of monazite and the Sm/Nd system in garnet. Here the monazite could record the age of hydrothermal events at the end of the metamorphism, while Sm/Nd could record the prograde formation of garnet or the peak of metamorphic conditions. Alternatively, the monazite ages could be a distinct event; for example the age of the Ca-rich fluid hydrothermal event. Anyway, this event should occur in a tectonic context similar to the formation of S3 planar fabric because monazite is clearly involved in the S3 structure. We think that the first hypothesis is simpler and more coherent. The lower limit for the Brasiliano event in this area is ca. 500 Ma, as shown by 40Ar–39Ar ages (Corsini et al., 1998). Monazite would record an intermediate stage between the peak or prograde conditions indicated by Sm/ Nd in garnet and U/Pb in zircon and the end of the metamorphic event indicated by Ar–Ar ages. One may question if the transpressional regime in the central part 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, extensive partial melting during decompression produces a large number of syntectonic granites and pegmatites, but the transpressional event is slightly earlier than the extensional one, respectively, 515 and 500 Ma. Crustal thickening caused by transpression within the central part of the Seridó belt should be balanced by subsequent or concomitant extensional deformation and crustal thinning elsewhere. In this way, the extensional structures in the São José de Campestre Massif may be regarded as mechanically complementary sites of exhumation and decompression, concomitant with generalized partial melting, and laterally adjacent to regions dominated by crustal thickening (cf. Carson et al., 1997). However, in the Seridó belt, the available data do not display a clear time difference between the transpressional and the extensional–transtensional regimes. Field and textural observation suggest a late Brasiliano incremental deformation shown by late extensional low-T C′ planes, oblique axial surfaces folding sheet-like Brasiliano alkaline granites within the Remígio–Pocinhos Belt, and low-T (greenschist facies) mylonites affecting the previous high-T fabrics (Sá et al., 1993; Souza and Sá, 1993; Trindade et al., 1993). The sequence of events described can be easily correlated to uplift and exhumation of deeper levels of the orogen (Sá, 1994), like in the late Pan-African of Madagascar (Nicollet et al., 1997) and in the Phanerozoic analogues (Vance et al., 1995; McGrath and Tarney, 1995). Consequently, the Brasiliano orogeny has been active until the end of the Cambrian, then probably restricted to reworking along narrower, low-T shear zones. These ones could have channeled the low-T Ca-rich fluids responsible for reopening the U–Th– Pb isotopic system of some monazite grains. For this, the following interpretations may be posed: a) a brittle deformation event under greenschist (or even lower) metamorphic grade during the Ordovician; b) a meaningless, partial resetting related to the Mesozoic Gondwana break up or a hypothetical midPaleozoic tectonic event. 9. Conclusions The results obtained on monazite dating from a leuconorite and a metapelitic migmatite, both related to high-T extensional or transtensional shear zone point to a long-lived Brasiliano orogenic event in the São José de Campestre Massif, northeastern Borborema Province. It is inferred that the peak of the high-T metamorphism (from upper amphibolite to hydrated granulite facies) and ductile extensional or transtensional deformation may be placed at about 578–574 Ma. This age is best recorded by the Sm–Nd on garnet (granitic leucosome, garnet-bearing alkaline granite) and U–Pb on zircon (diorites). The 540–550 Ma age recorded on monazites may represent another phase under slightly lower temperatures before the ending of the Brasiliano event. This suggests that the U–Th–Pb isotope system in monazite may register events different than the ones recorded by the U–Pb zircon and Sm–Nd garnet systematics. Late Brasiliano, low-T reworking along narrow

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shear zones appear to have been active up to the Cambrian as deduced from 40Ar–39Ar ages in the literature. Ages younger than 500 Ma (up to 300 Ma) could be related to a low-T tectonic event of Ordovician or younger age not yet recognized in the region. Acknowledgements Z. S. S. and V.E.A. acknowledge CAPES scholarship (3070/ 95-11) and CAPES/COFECUB project (177/95) for supporting lab work in France. Field work in NE Brazil and lab work at the Universidade de Brasília have been financed by PADCT/FINEP and CNPq. References Archanjo, C.J., Bouchez, J.-L., 1991. Le Seridó, une chaîne transpressive dextre au Protérozoïque supérieur du Nord-Est du Brésil. Bull. Soc. Géol. Fr. 162, 637–647. Archanjo, C.J., Olivier, P., Bouchez, J.-L., 1992. Plutons granitiques du Seridó (NE du Brésil): écoulement magmatique parallèle à la chaîne révélé par leur anisotropie magnétique. Bull. Soc. Géol. Fr. 163, 509–520. Barbosa, A.J., Braga, A.P.G., Bezerra, M.A., Gomes, J.A.V., 1974. Projeto leste da Paraíba e Rio Grande do Norte, Mapa Geológico, escala 1 : 250.000. Companhia de Pesquisa e Recursos Minerais- Ministério das Minas e Energia / Departamento Nacional da Produção Mineral, Recife, Brazil. Bertrand, J.-M., Sá, E.F.J., 1990. Where are the Eburnean–Transamazonian collisional belts? Can. J. Earth Sci. 27, 1382–1393. Blundy, J.D., Holland, T.J.B., 1990. Calcic amphibole equilibria and a new amphibole–plagioclase geothermometer. Contrib. Mineral. Petrol. 104, 208–224. Braun, I., Montel, J.-M., Nicolet, C., 1998. Electron microprobe dating of monazites from high-grade gneisses and pegmatites from the Kerala Kondalite Belt, southern India. Chem. Geol. 146, 65–85. Brey, G.P., Köhler, T., 1990. Geothermobarometry in four-phase lherzolites. II. New thermobarometers, and practical assessment of existing thermobarometers. J. Petrol. 31, 1353–1378. Brito Neves, B.B., Vandoros, P., Pessoa, D.A.R., Cordani, U.G., 1974. Revaliação dos dados geocronológicos do Precambriano do nordeste Brasileiro. Proceedings, Congr. Bras. Geol., 28th, Porto Alegre, vol. 6, pp. 261–271. Caby, R., Sial, A.N., Arthaud, M.H., Vauchez, A., 1991. Crustal evolution and the Brasiliano Orogeny in northeast Brazil. In: Dallmeyer, R.D., Lécorché, J.P. (Eds.), The West African Orogens and Circum-Atlantic Correlatives. Springer-Verlag, Berlin, pp. 373–397. Carson, C.J., Powell, R., Wilson, C.J.L., Dirks, P.H.G.M., 1997. Partial melting during tectonic exhumation of granulitic terrane: an example from the Larsemann Hills, east Antarctica. J. Metamorph. Geol. 15, 105–126. Castaing, C., Feybesse, J.L., Thiéblemont, D., Triboulet, C., Chèvremont, P., 1994. Paleogeographical reconstructions of the Pan-African/Brasiliano orogen: closure of an oceanic domain or intracontinental convergence between major blocks? Precambrian Res. 69, 327–344. Cathelineau, M., 1987. U–Th–REE mobility during albitization and quartz dissolution in granitoids: evidence from south-east French Massif Central. Bull. Minéral. 110, 249–259. Cocherie, A., Legende, O., Peucat, J.-J., Kouamelan, A.N., 1998. Geochronology of polygenetic monazites constrained by in situ electron microprobe Th–U–total lead determination: implications for lead behaviour in monazite. Geochim. Cosmochim. Acta 62, 2475–2797. Corsini, M., Vauchez, A., Archanjo, C.J., Sá, E.F.J., 1991. Strain transfer at continental scale from a transcurrent shear zone to a transpressional fold belt: the Patos–Seridó system, northeastern Brazil. Geology 19, 586–589. Corsini, M., Figueiredo, L.L., Caby, R., Féraud, G., Ruffet, G., Vauchez, A., 1998. Thermal history of the Pan-African/Brasiliano Borborema Province of northeast Brazil deduced from 40Ar / 39Ar analysis. Tectonophysics 285, 103–117.

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