Isotopic signatures of Paleoproterozoic granitoids from the southern São Francisco Craton and implications for the evolution of the Transamazonian Orogeny

Journal of South American Earth Sciences 13 (2000) 225±239

Isotopic signatures of Paleoproterozoic granitoids from the southern SaÄo Francisco Craton and implications for the evolution of the Transamazonian Orogeny Carlos M. Noce a,*, Wilson Teixeira b, Jean J.G. QueÂmeÂneur a, Veridiana T.S. Martins b, EÂrica Bolzachini b a

CPMTC, Instituto de GeocieÃncias, Universidade Federal de Minas Gerais, Belo Horizonte MG, 31270-010, Brazil b CPGeo, Instituto de GeocieÃncias, Universidade de SaÄo Paulo, PO Box 11348, SaÄo Paulo SP, 05422-970, Brazil Received 1 July 1998; received in revised form 1 May 1999; accepted 1 December 1999

Abstract The southern SaÄo Francisco Craton, northeastern Brazil, consists of an Archean block surrounded by a Paleoproterozoic belt related to the Transamazonian Orogeny (ca. 2.0 Ga). A calc-alkaline plutonic arc developed within the belt and the granitoid plutons comprise two distinct groups. One group displays Archean TDM ages (3.07±2.62 Ga), eNd(t ) values between ÿ11.0 and ÿ3.8 and high initial 87Sr/86Sr values, and it consists mainly of peraluminous granites. TDM ages for the other group are Paleoproterozoic (2.43±2.27 Ga), and eNd(t ) values range between ÿ2.8 and +1.3; the plutons are metaluminous tonalites (trondhjemites) to granodiorites. The Transamazonian granitoids can be related to contrasting source-regions, from mantle- to crust-derived ones. A number of them are probably derived from mixing of Paleoproterozoic juvenile material and variable proportions of Archean crust material. Magmatism related to deep faulting, during the compressional stages of the Transamazonian Orogeny, is a plausible model for granitoid generation. The contribution of mantle-derived material to the granitoid sources supports the idea that a signi®cant episode of new crust formation occurred during the Transamazonian Orogeny. 7 2000 Elsevier Science Ltd. All rights reserved.

SumaÂrio A porc° aÄo meridional do CraÂton do SaÄo Francisco consiste de um nuÂcleo arqueano parcialmente circundado por um cinturaÄo paleoproterozoÂico, relacionado aÁ OrogeÃnese TransamazoÃnica (ca. 2,0 Ga). Um arco plutoÃnico de natureza caÂlcio-alcalina desenvolveu-se neste cinturaÄo, incluindo dois grupos de plutons granitoÂides. Um grupo, essencialmente composto por granitos peraluminosos, exibe idades TDM arqueanas (3,07±2,62 Ga), valores de eNd(t ) entre ÿ11,0 e ÿ3.8 e altas razoÄes iniciais de 87 Sr/86Sr. Para o outro grupo, que inclui tonalitos (trondhjemitos) e granodioritos de caraÂter metaluminoso, as idades TDM saÄo PaleoproterozoÂicas (2,43±2,27 Ga) e os valores de eNd(t ) variam entre ÿ2.8 e +1.3. Os granitoÂides TransamazoÃnicos podem ser relacionados a aÂreas-fonte distintas, de derivac° aÄo manteÂlica a crustal. Alguns plutons parecem ser produtos de mistura entre material juvenil paleoproterozoÂico e diferentes proporc° oÄes de material crustal arqueano. Sugere-se um modelo de granitogeÃnese ligada a falhamentos profundos, no contexto da fase compressiva da OrogeÃnese TransamazoÃnica. A indicac° aÄo de contribuic° aÄo manteÂlica para os plutons estudados reforc° a a hipoÂtese de que a OrogeÃnese TransamazoÃnica representou um importante episoÂdio de gerac° aÄo de crosta continental. 7 2000 Elsevier Science Ltd. All rights reserved.

* Corresponding author. Tel.: +55-31-499-5420; fax: +55-31-4995410. E-mail address: [email protected] (C.M. Noce).

1. Introduction The Transamazonian Orogeny (ca. 2.0 Ga) played

0895-9811/00/$ - see front matter 7 2000 Elsevier Science Ltd. All rights reserved. PII: S 0 8 9 5 - 9 8 1 1 ( 0 0 ) 0 0 0 1 9 - 5

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a major role in the agglutination of Archean crustal blocks, assembling a large continent from which the SaÄo Francisco Craton is a remnant in the tectonic framework of Western Gondwana. The largest exposed area of Archean-Paleoproterozoic basement rocks in this craton lies in Bahia State, where the study of Transamazonian belts revealed di€erent tectonic settings, including island-arc accretion (Figueiredo, 1989) and continent-continent collision (Sabate et al., 1990). Paleoproterozoic granitoid plutonism of varied composition is very voluminous within the belts (Conceic° aÄo and Otero, 1996). Based on geochronological inferences, Teixeira (1985) recognized the Transamazonian Mineiro Belt in the southernmost part of the craton in Minas Gerais State. Further studies described the granitoids related to the belt as part of a plutonic arc close to the cratonic margin (QueÂmeÂneur and Vidal, 1989; Teixeira

and Figueiredo, 1991; Pires and Barbosa 1993; Noce, 1995). This work presents a synthesis of the isotopic data for nine granitoid plutons of the Mineiro Belt, which, together with geochemical data, provided insights on the sources and tectonic settings of such plutons. In addition, the crustal growth processes that took place during the Transamazonian Orogeny are also addressed. We have established to what extent the granitoids represent juvenile crustal addition to the Archean continent, or re-working of this older crust. In particular, Nd and Sr isotopic signatures of the granitoids are used to understand the crust-mantle geodynamics. 2. Geological setting Archean rocks underlie most of the southern part of

Fig. 1. (a) Geological outline of the SaÄo Francisco Craton, southeastern Brazil (adapted from Alkmim et al., 1993): (1) Archean and Paleoproterozoic cratonic basement; (2) sedimentary cover (Meso- to Neoproterozoic plus Phanerozoic); (3) Brasiliano folded belts. QF=QuadrilaÂtero FerrõÂ fero; BH=Belo Horizonte; Sa=Salvador. Outlined is the study area enlarged in Fig. 1b. (b) Simpli®ed geological map of the Mineiro Belt. (I) Archean TTG gneiss and migmatite, granitoids, high-grade gneiss; (II) Archean greenstone belt; (III) Paleoproterozoic Minas Supergroup; (IV) Dioritic and gabroic intrusions; (V) Paleoproterozoic granitoid plutons (g1=Alto MaranhaÄo; g2=Ressaquinha; g3=Campolide; g4=RitaÂpolis; g5=Itutinga; g6=TabuoÄes; g7=Lavras; g8=Porto Mendes; g9=Alto JacarandaÂ); (VI) Meso- to Neoproterozoic SaÄo JoaÄo del Rei Group; CL=Conselheiro Lafaiete; ER=Entre Rios de Minas; Ca=CarandaõÂ ; R=Ressaquinha; Ba=Barbacena; Ti=Tiradentes; SJR=SaÄo JoaÄo del Rei; Ri=RitaÂpolis; RC=Rezende Costa; LD=Lagoa Dourada; Ja=Jacarandira; PT=Passa Tempo; MF=Morro do Ferro; BS=Bonsucesso; N=Nazareno; It=Itutinga; La=Lavras; Pe=PerdoÄes; SA=Santo AntoÃnio do Amparo; Ne=Nepomuceno; PM=Porto Mendes.

C.M. Noce et al. / Journal of South American Earth Sciences 13 (2000) 225±239

the SaÄo Francisco Craton and comprise TTG gneisses and migmatites, greenstone assemblages, and highgrade gneisses (Fig. 1). These rocks are part of a platform stabilized between 2700 and 2600 Ma (Machado et al., 1992; Noce, 1995). The Mineiro Belt evolved as a marginal belt to the Archean platform and includes a large area of reworked Archean basement. In addition, the belt embodies many Paleoproterozoic granitoid plutons and ma®c bodies, as well as supracrustal sequences (Fig. 1). The supracrustal rocks (Minas Supergroup) are restricted to the QuadrilaÂtero Ferrõ fero region and to a narrow N±NE belt linking the southwestern tip of the QuadrilaÂtero Ferrõ fero to the Bonsucesso ridge. The granitoid plutons form a string of bodies that extends nearly 300 km along the southern border of the SaÄo Francisco Craton (Fig. 1). A ma®c dike swarm, the emplacement age of which is constrained by a 40Ar/39Ar plateau age on amphibole of 1975 2 7 Ma (Pinese, 1997), is contemporaneous with the granitoid plutonism. Early stages of Mineiro Belt evolution are related to Minas Supergroup sedimentation. This sequence records the change from platformal to synorogenic sedimentation. Quartzite from the basal Moeda Formation (quartzite, conglomerate, phyllite) contains zircons with 207Pb/206Pb minimum ages of ca. 2600 Ma (Machado et al., 1996a), and marbles of the Gandarela Formation (dolomite, iron formation and phyllite) yielded a Pb/Pb whole rock isochron deposition age of ca. 2420 Ma (Babinski et al., 1995). The upper Sabara Formation (schist, phyllite, metagraywacke, metavolcanic, quatzite, conglomerate) and the overlying Itacolomi Group (mainly quartzite) contain zircons of 2125 Ma and ca. 2060 Ma, respectively. The latter age is identical to those of the regional metamorphism (Machado et al., 1996a). These two units were interpreted as a ¯ysh-molasse deposit of the belt (Dorr, 1969; Machado et al., 1996a). Tectonism within the Mineiro Belt overprinted the Archean crust, inducing isotopic resetting and giving rise to gneissic domes (Drake and Morgan, 1980; Teixeira, 1985; Chemale et al., 1994). U±Pb age determinations in sphenes from QuadrilaÂtero Ferrõ fero gneisses constrained the Transamazonian metamorphic peak at 2065±2035 Ma (Machado et al., 1992; Noce, 1995). Metamorphic grade increases eastward from the Bon®m Complex, where sphene ages are Archean (Machado and Carneiro, 1992), to the Bac° aÄo Complex where sphene U±Pb dates are concordant at 2059 Ma (Machado et al., 1992). In the Belo Horizonte Complex sphene analyses plot in a discordia line between 2860 Ma and 2041 Ma (Noce, 1995). East of the QuadrilaÂtero Ferrõ fero, the Transamazonian metamorphism reached granulitic grade at ca. 2000 Ma, as

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evidenced by concordant Rb±Sr and Pb±Pb wholerock isochron ages (Teixeira et al., 1987a). K±Ar amphibole ages in the Mineiro Belt range from 2.1 to 1.9 Ga and probably resulted from progressive uplift accompanying the tectonic stabilization. The majority of K±Ar biotite ages, however, are younger and variable. They re¯ect tectonothermal overprints associated with the Mesoproterozoic Espinhac° o intracratonic rift evolution as well as Neoproterozoic collision belts that developed marginal to the craton. 3. Chemical and isotopic characteristics of the granitoid plutons The Transamazonian granitoid intrusions comprise compositionally distinct plutons, from I-type tonalites to S-type granites, following a calc-alkaline trend in the AFM diagram (Fig. 2a), and plotting

Fig. 2. (a) AFM diagram showing the calc-alkaline trend of the granitoid plutons. The line between tholeiitic and calc-alkaline trends is from Irvine and Baragar (1971). (b) The granitoid plutons plotted on the Maniar and Piccoli (1989) diagram based on Shand's index.

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3.1. Alto MaranhaÄo

on metaluminous and peraluminous ®elds in the Maniar and Piccoli (1989) diagram based on Shand's index (Fig. 2b). Several alkaline plutons of similar age occur to the east and south of this plutonic arc (Pinto, 1995), within the marginal domain to the craton where superposed Neoproterozoic events are widespread. This study refers only to the calc-alkaline plutons. Whole-rock chemical compositions discussed in this paper come from many sources: Alto MaranhaÄo (Seixas, 1988; Noce, 1995); Ressaquinha and Campolide (Viana, 1991); RitaÂpolis, TabuoÄes, Itutinga, and Porto Mendes (QueÂmeÂneur, 1996). For the purpose of this study, major and some trace element analyses on samples from Porto Mendes and Lavras plutons were carried out at the LaboratoÂrio de Quõ mica e Plasma ICP-AES, Instituto de GeocieÃnces, Universidade de SaÄo Paulo. Interpretations based on chemical compositions must be considered with caution due to the fact that the data were obtained in di€erent laboratories, and for many samples only major elements plus a restricted set of trace elements were analysed. More complete sets of trace element and REE analyses are available for only three of the plutons (Alto MaranhaÄo, Ressaquinha and Itutinga). The available isotopic data for the nine plutons of the Mineiro Belt are presented in Table 1. All these data and the geologic, petrographic, and chemical characteristics of the plutons are summarized below.

The Alto MaranhaÄo pluton has a predominant tonalitic composition and is generally strongly foliated. The main components are oligoclase, quartz, biotite, and amphibole arranged in a granoblastic texture. It intruded and migmatized the Archean greenstone belt sequence of Conselheiro Lafaiete. This intrusion has a metaluminous character and contains an average SiO2 of 66.3%, CaO of 4.2%, Na2O of 4.6%, K2O of 1.8%, and high FeOt+MgO content (3.92±6.93%). Rb/Sr values are very low (around 0.06). U±Pb zircon and sphene analyses yielded an intrusion age of 212422 Ma (Noce, 1995). Sm±Nd data indicate a TDM age of 2.27 Ga. The calculated eNd(t ) value for the age of emplacement is +1.3 (Table 1). Rb±Sr analyses indicate isotopic disturbance caused by Neoproterozoic events. 3.2. Ressaquinha The Ressaquinha pluton is mostly granodioritic to tonalitic but includes diorites (Viana, 1991). Felsic rocks are weakly foliated, medium to coarse-grained, and the ma®c phase is biotite. Xenoliths of banded gneiss and granulite are a common local feature and probably were derived from the Archean country rocks. It is also a metaluminous intrusion, but it is richer in K2O (2.2±3.9%), poorer in CaO (2.0±3.7%) and has higher Rb/Sr values (0.11±0.29) when compared to the Alto MaranhaÄo pluton.

Table 1 Summary of isotopic data for the granitoid plutons Granitoid

Samples

RitaÂpolis

Lavras+Serrinhad Lavras Porto Mendes Cerr. Bandeira PerdoÄes Campolide Alto MaranhaÄo

GP-95-01 GP-95-02 GP-95-04 AP-WT-15a AP-WT-15 g GP-95-09 d Lav-1B-r SF-WT-07F-R GP-95-15j GP-95-14a GP-95-12 d HV-363 N-18n

Ressaquinha TabuoÄes

GP-95-05 GP-95-03

Itutinga Alto JacarandaÂ

a

Age (Ma) 1863244 01900 inferred 19002108

Method

MSWD

Rb/Sr

18.3

Sr/86Sr Initial age

eNd

(Ta)

eNd (0)

TDM (Ga)b

0.758420.0087

ÿ5.9 ÿ7.3 ÿ7.7 ÿ7.6 ÿ10.8 ÿ7.2d ÿ3.8 ÿ4.9 ÿ11.0 ÿ10.3 ÿ9.3 ± +1.3

(1.86) (1.86) (1.90) (1.90) (1.90) (1.94) (1.98) (2.06) (2.06) (2.06) (2.06) ± (2.12)

ÿ13.7 ÿ18.4 ÿ17.5 ÿ26.7 ÿ33.4 ÿ34.0 ÿ28.6 ÿ31.3 ÿ38.1 ÿ27.9 ÿ19.3 ± ÿ23.2

2.62c 2.71c 2.77c 2.85 2.95 2.62 2.48 2.62 3.03 3.07c 3.01c ± 2.27

ÿ1.3 ÿ2.8

(2.01) (1.96)

ÿ31.1 ÿ25.5

2.30 2.43

Rb/Sr

3.95

0.709620.0018

19402100 19822134 2061282 18552176±200

Rb/Sr Rb/Sr Rb/Sr Pb/Pb

0.70 0.61 6.75

0.7041720.00107 0.704120.0017 0.7040520.00362 m1=8.103

1998217 212422 664280 484237 2010252 1962220

Rb/Sr U/Pb Rb/Sre Rb/Sre Rb/Sr Rb/Sr

1.91

0.715720.0018

18.15 0.99 9.43 5.75

0.7073920.00030 0.7078820.00012 0.708620.0006 0.7024520.00005

T = emplacement age given by U/Pb and Rb/Sr ages. TDM values were calculated using DePaolo (1981) model parameters. c TDM double-stage model ag (Fig. 6). d Nd data for a Serrianha sample (Lavras pluton). e Rb/Sr isochrons interpreted as resetting ages. b

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A former Rb±Sr whole-rock isochron (Viana, 1991) was recalculated by adding new data and yielded 2010 252 Ma (MSWD=9.43) with initial 87Sr/86Sr=0.7086 2 0.0006. Sm±Nd analyses de®ned a TDM age of 2.30 Ga and the calculated eNd(t ) value is slightly negative (ÿ1.3; Table 1). 3.3. Campolide The Campolide pluton is a highly homogeneous granodiorite, locally displaying a porphyritic texture with K-feldspar megacrysts up to 5 cm. Biotite develops a weak tectonic orientation and the pluton is cut by narrow shear zones. Only one chemical analyses is available for this pluton and it indicates a more di€erentiated rock (higher in K and Rb) than the nearby Ressaquinha intrusion. A Rb±Sr whole rock isochron yielded 1998 2 97 Ma (MSWD=1.91) and high initial 87 Sr/86Sr=0.715720.0018 (Table 1). 3.4. RitaÂpolis The RitaÂpolis pluton is chie¯y composed of a weakly foliated, leucocratic, medium to coarse-grained granite. The rock-forming minerals are K-feldspar (30±40%), quartz (30±40%), plagioclase (15±30%) and biotite (3±5%). Accessory mineral are allanite, sphene, ilmenite, apatite and zircon. Chemical compositions reveal that the RitaÂpolis intrusion is a highly fractionated peraluminous granite with high SiO2 (average 73.9%) and low CaO (average 0.68%) and FeOt+MgO contents. It is poor in Ba, Li and V, and rich in U (average 50 ppm) and Th (average 55 ppm), and displays high Rb/Sr values (4.97±7.55). A Rb±Sr errorchron (MSWD=18.3) yielded 1863 2 44 Ma with a very high initial 87Sr/86Sr ratio (0.75842 0.0087). Sm±Nd TDM ages yielded 2.71 and 2.62 Ga, with calculated eNd(t ) of ÿ7.3 and ÿ5.9 (Table 1). 3.5. Itutinga The small Itutinga intrusion was deeply a€ected by the Neoproterozoic Brasiliano Orogeny, displaying a well-developed foliation and mylonitic textures. Less deformed sectors of the pluton display a coarsegrained granite, which is locally porphyritic with microcline megacrysts. The granite is peraluminous and has a chemical composition similar to the RitaÂpolis granite, except for some CaO enrichment (average 0.84%), U depletion (16±34 ppm), and lower Rb/Sr values (around 3). The Itutinga pluton presents a Sm± Nd TDM age of 2.77 Ga and a very negative eNd(t ) value of ÿ7.7 (Table 1).

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3.6. TabuoÄes The TabuoÄes pluton is made mostly of a weakly foliated, mesocratic to leucocratic, medium-grained trondhjemite. It contains 50±60% oligoclase, 20±30% quartz, 10% microcline, and 5±7% biotite. Based on Shand's parameters, TabuoÄes rocks can be both metaluminous and peraluminous. They contain moderate SiO2 (66.5±69.3%), high Al2O3 (15.5±16.5%), fairly constant CaO (3.2±3.8%), high Na2O (4.9±5.2%), and low K2O (1.0±1.4%) values. Rb/Sr values range between 0.04 and 0.07. A sixteen-point Rb±Sr whole rock isochron yielded 1962 2 13 Ma (MSWD=5.75) with initial 87 Sr/86Sr=0.70245 2 0.00005. One Sm±Nd analyses yielded a TDM age of 2.43 Ga and the calculated eNd(t ) value is ÿ2.8 (Table 1). 3.7. Lavras The Labras intrusion lies within the Brasiliano fold belt located marginally to the southern edge of the craton. Due to this tectonic overprint, the rock displays a well developed foliation and recrystallized (granoblastic) texture. The Lavras pluton comprises two compositionally di€erent intrusive phases. The dominant one is represented by medium- to coarse-grained granodioritic to granitic rocks containing about 73% SiO2, 1.12% CaO, 3.47% Na2O and 5.35% K2O. The other rock type is ®ne-grained and has a distinct composition, with lower SiO2 and K2O contents (average 71% and 2.25%, respectively) and higher CaO and Na2O (average 2.18% and 5.34%, respectively). In the Maniar and Piccoli (1989) diagram, the two rock-types plot close to or on the line between metaluminous and peraluminous ®elds. A Rb±Sr whole rock isochron yielded 1982 2 134 Ma (MSWD=0.61) with initial 87Sr/86Sr=0.70418 2 0.00107 (Teixeira, 1985; Heilbron et al., 1989). These data were regressed, together with additional whole rock analyses performed for this work. Eight analyses de®ne an age of 1940 2 101 Ma and initial 87 Sr/86Sr=0.70418 2 0.0017 (MSWD=0.71). However, three other analyses plot below this isochron and indicate disturbance by a younger event. Two Sm±Nd TDM ages are available for this pluton: 2.62 and 2.48 Ga, with eNd(t ) values of ÿ7.2 and 3.8 (Table 1). 3.8. Porto Mendes Porto Mendes is a large body intrusive into the southern edge of the Campo Belo Complex. It is essentially composed of a light gray medium- to ®ne-grained granite that is predominantly isotropic and locally shows ¯ow textures. Small satellite bodies occur in the proximity of the main intrusion. The granite displays

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equigranular hypidiomorphic texture and is slightly recrystallized. Biotite is the ma®c mineral and common accessory minerals are allanite, zircon, apatite and opaque phases. The Porto Mendes granite is slightly peraluminous and has a fairly constant chemical composition with SiO2 values ranging between 70.6% and 74,0%, an average CaO of 1.36%, and K2O/Na2O values ranging between 1.21 and 1.63. The Rb and Sr contents are moderate, averaging 230 ppm and 171 ppm, respectively. Published whole-rock Rb±Sr analyses (Teixeira et al., 1987b) were re-interpreted with the addition of new data, and a fourteen-point isochron yielded 2061 2 82 Ma (MSWD=6.75) and initial 87 Sr/86Sr=0.70405 2 0.00362. A preliminary ®ve-point whole-rock Pb±Pb isochron yielded 1855+176/ÿ200 Ma. If three additional Pb±Pb data are added to the regression line, an age younger than 1.7 Ga is obtained. The ®ve-point line has a m1=8.103 2 0.025 (isotopic measurements with 0.2% error). This is a ®rst-stage model m1 value (i.e., 238U/204Pb ratio from the magma source from which the granite could have been derived). Three Sm±Nd TDM ages available for this pluton are rather similar: 3.07, 3.03 and 3.01 Ga. One additional sample has a TDM age of 2.62 Ga. The calculated eNd(t ) values are ÿ11.0, ÿ10.3, ÿ9.3 and ÿ4.9, respectively (Table 1). Rb±Sr and Pb±Pb isochrons have not de®ned a precise age for the emplacement. In the case of the Rb/Sr method, the quality of the isochron is mainly dependent on the distribution of data, all of them plotting far from the 87Sr/86Sr initial ratio of the diagram. This situation is due to the homogeneous granitic composition of the body. On the other hand, a K±Ar age on muscovite from an enclave yielded 1915263 Ma. This favors the Rb±Sr age as a good estimate for the intrusion age.

4. Magmatism and source regions during the Transamazonian Orogeny Nine Transamazonian granitoid plutons were investigated (see above) along a broad longitudinal transverse; they were evaluated together in terms of chemical composition and isotopic signatures. Precise U±Pb dating is available only for the Alto MaranhaÄo tonalite (2124 Ma), precluding the establishment of a chronological sequence of plutonic emplacement. In addition, a small and weakly-deformed granitic stock north of the QuadrilaÂtero FerrõÂ fero yielded a U± Pb age of 2041 Ma (Noce, 1995), which allows a tectonic relationship with the nine plutons characterized here. The 2124±2041 Ma time-interval, therefore, is a plausible one for the magmatic event. Whole-rock Rb±

3.9. Alto Jacaranda The Alto Jacaranda granite is intrusive into Archean high-grade gneisses (Fiumari et al., 1985). In the outcrop studied, a pervasive shearing developed a foliation that is cut by late pegmatite veins. Detailed geologic and chemical data are not available for this pluton. A three point whole-rock Rb±Sr isochron yielded 1900 2 108 Ma (MSWD=3.95) with initial 87 Sr/86Sr=0.7096 2 0.0018 (Teixeira, 1985). Two Sm± Nd analyses yielded TDM ages of 2.95 and 2.85 Ga and the calculated eNd(t ) values are ÿ10.8 and ÿ7.6, respectively (Table 1).

Fig. 3. (a) Primitive mantle-normalized spidergram (norm values after Sun and McDonough, 1989) for the Alto MaranhaÄo pluton. (b) Primitive mantle-normalized incompatible elements spidergram (norm values after Sun and McDonough 1989) for the Ressaquinha pluton (chemical data from Viana, 1991).

C.M. Noce et al. / Journal of South American Earth Sciences 13 (2000) 225±239

Sr isochron ages for the Ressaquinha, Campolide, Lavras, and Porto Mendes plutons fall within error in the 2124±2041 Ma period. Those for TabuoÄes, Alto JacarandaÂ, and RitaÂpolis are younger, around 1900 Ma (Table 1). However, comparative Rb±Sr and U± Pb geochronology showed that, in many cases, Rb±Sr ages can be systematically younger than U±Pb crystallization ages, and also that disturbance of the Rb±Sr system was related to late and sometimes subordinate alteration events (Heaman et al., 1986; Beakhouse et al., 1988). As a whole, the plutons display rather distinct chemical compositions. This assertion is exempli®ed by three of them (Alto MaranhaÄo, Ressaquinha and Itutinga) whose chemical analyses sets are more complete. The Alto MaranhaÄo pluton spidergram shows a strong Nb depletion, Ba enrichment, and no Sr depletion (Fig. 3a). Comparatively, the Ressaquinha pluton shows a less important Nb depletion as well as no Ba enrichment; moreover, one of the curves displays Sr depletion (Fig. 3b). REE data for both plutons are similar (Fig. 4), yielding fractionated patterns with HREE depletion and slightly negative to positive Eu anomalies. REE contents are lower for the Ressaquinha pluton, and HREE depletion is stronger. The data suggest that a plagioclase-rich continental crust is not an important source for the Alto MaranhaÄo and Ressaquinha tonalites. This is probably also valid for the trondhjemitic TabuoÄes pluton. A di€erent conclusion arises from the chemical characteristics of the Itutinga pluton. The spidergram displays marked Ba, Sr, and Ti depletions (Fig. 5a). The REE patterns (Fig. 5b) have signi®cant negative Eu anomalies and there is no HREE fractionation. The data ®t well with a continental crust partial melting model, with the retention of signi®cant volumes of

Fig. 4. Chondrite-normalized REE patterns (norm values after Evensen et al. 1978) for the Alto MaranhaÄo pluton (*) and the Ressaquinha pluton (w); chemical data from Viana (1991).

231

mineral phases like plagioclase and Fe-Ti minerals in the source-region. Among the plutons that are essentially granitic in composition, the highly fractionated and peraluminous RitaÂpolis pluton is chemically quite similar to the Itutinga pluton. Others, like the Lavras and Porto Mendes rocks, are chemically less evolved. The conclusion that the granitoid plutons are related to contrasting source regions is con®rmed by isotopic data, as discussed below. The analytical aspects and uncertainties of the Sm±Nd data are presented in the Appendix, along with the age calculation models adopted herein. The Nd isotopic evolution of the plutons is shown in Fig. 6. Most display typical crustal 147Sm/144Nd values (Table 2) between 0.0805 and 0.1180 ( fSm/Nd between ÿ0.59 and ÿ0.40), indicating that there was little or no Sm/Nd fractionation during magma genesis. The isotopic evolution of these unfractionated

Fig. 5. (a) Primitive mantle-normalized incompatible elements spidergram (norm values after Sun and McDonough, 1989) for the Itutinga pluton. (b) Chondrite-normalized REE patterns (norm values after Evensen et al., 1978) for the Itutinga pluton (chemical data from QueÂmeÂneur, 1996).

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Fig. 6. Nd evolution for the granitoid intrusions through time; for explanation, see text.

materials versus time is represented by the dashed lines shown in Fig. 6. These lines represent isotope systems that have characteristically low present-day eNd values between ÿ38 and ÿ23 (Table 1). Also plotted along these lines are the eNd values at the time of emplacement (full dots). Five samples were clearly fractionated, as shown by 147 Sm/144Nd values higher than 0.1269 (Table 2). This can lead to overestimated model ages because the basic assumption, that Sm/Nd values for the granites were the same as those for the protolith, is incorrect. Indeed, infracrustal di€erentiation processes involving partial melting and crustal fractionation can modify Sm/Nd ratios in granitic rocks (Vidal, 1987; Pimentel and Charnley, 1991). It is important to stress, however,

that eNd parameters in fractionated systems remain undisturbed and can be useful for estimating the Nd compositions of the magma sources (Pimentel and Charnley, 1991). The fractionated samples came from the RitaÂpolis (GP95-01, -02) and Itutinga (GP95-04) plutons and two satellites of the Porto Mendes pluton (named Bandeira and PerdoÄes, samples GP95-14a and GP95-12 d, respectively). All these samples are from highly evolved granites where accessory minerals enriched either in LREE (sphene and allanite) and middle to heavy REE (zircon) are common phases. Also, REE patterns for Itutinga samples (Fig. 5b) display a certain LREE disturbance that might be indicative of Sm/Nd fractionation.

Table 2 Isotopic parameters for the granitoid plutonsa Granitoid

Samples

87

87

RitaÂpolis

16.838

Alto MaranhaÄo

GP-95-01 GP-95-02 GP-95-04 AP-WT-15a AP-WT-15 g GP-95-09 d Lav-1B-r SF-WT-07F-R GP-95-15j GP-95-14a GP-95-12 d N-18n

Ressaquinha TabuoÄes

GP-95-05 GP-95-03

Itutinga Alto Jacaranda Lavras Serrinha Porto Mendes

Rb/86Sr (m)

Sr/86Sr (m)

eSr(2.0Ga)

fSm/Nd

147

Sm/144Nd

143

Nd/144Nd

eNd(2.0Ga)

1.19777

150.36

5.664 2.125

0.86584 0.767453

7.50 58.23

0.7975 0.777 3.033

0.727175 0.72615 0.79989

29.22 22.94 147.8

0.263 0.223 1.091 1.175

0.70987 0.70941 0.73944 0.73586

2.09 11.95 83.47 ÿ2.21

ÿ0.17 ÿ0.24 ÿ0.21 ÿ0.40 ÿ0.47 ÿ0.55 ÿ0.50 ÿ0.51 ÿ0.52 ÿ0.34 ÿ0.19 ÿ0.46

0.163820.0001 0.149720.0001 0.156220.0001 0.118020.0001 0.103320.0001 0.087820.0003 0.098420.0001 0.096020.0001 0.093720.0003 0.129620.0004 0.158620.0005 0.106420.0001

0.51193320.000033 0.51169320.000026 0.51174020.000024 0.51127020.000023 0.51092520.000024 0.51089320.000032 0.51117220.000036 0.51103120.000037 0.51068720.000034 0.51120820.000041 0.51165020.000036 0.51144920.000043

ÿ5.4 ÿ6.4 ÿ7.2 ÿ6.6 ÿ9.6 ÿ6.3 ÿ3.5 ÿ5.7 ÿ11.8 ÿ10.8 ÿ9.6 ÿ0.2

ÿ0.59 ÿ0.46

0.080520.0001 0.105820.0001

0.51104320.000045 0.51132920.000023

ÿ1.5 ÿ2.3

a Note: 87Rb/86Sr (m) and 87Sr/86Sr (m) are mean values from data used in isochron age calculation (see text for details). Sm/Nd fractionated samples (see text for details).

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233

JacarandaÂ, Lavras, and Porto Mendes plutons) display Archean TDM ages (3.07±2.62 Ga), except for one Lavras sample that yields a TDM age of 2.48 Ga, as already mentioned (to be discussed below); eNd(t ) values range between ÿ11.0 and ÿ3.8. Group ``B'' comprises the less evolved tonalitic (trondhjemitic) to granodioritic plutons (Alto MaranhaÄo, Ressaquinha, and TabuoÄes plutons), which display Paleoproterozoic TDM ages (2.43, 2.30 and 2.27 Ga) and higher eNd(t ) values (+1.3, ÿ1.3 and ÿ2.8; Table 1).

TDM ages for these ®ve samples were calculated assuming a double-stage model (DePaolo et al., 1991; Sato, 1997 Ð see Appendix for details) in which the Sm±Nd fractionation took place at early stages of the magma crystallization, in the presence of the cited accessory minerals. This calculation provides TDM ages comparable to those calculated for the non-fractionated samples (Table 1). The isotope evolution of the highly fractionated samples in Fig. 6 is distinguished from that of the unfractionated materials by heavy lines that have characteristically lower present-day eNd values ranging from ÿ38 to ÿ14 (Table 1 and Fig. 6). These lines in¯ect to the second evolutionary stage at the respective eNd(t ) value because of its undisturbance, as stated above (Fig. 6). These second stage lines were drawn assuming a 147Sm/144Nd mean ratio=0.11 ( fSm/ Nd=0.43), which has been empirically determined for the continental crust of the Brazilian Shield (Sato, 1997; see also Appendix). It is noteworthy that the fSm/ Nd=0.43 value is within the range of those calculated herein for the Archean Campo Belo rocks (Table 3). The plutons investigated can be divided into two groups according to their chemical and Nd isotopic characteristics. Group ``A'' plutons that are essentially granitic in composition (RitaÂpolis, Itutinga, Alto

4.1. Group ``A'' plutons Initial 87Sr/86Sr values range from 0.704 (Porto Mendes and Lavras) to 0.7096 (Alto JacarandaÂ). An anomalously high value of 0.7584 was obtained for the RitaÂpolis granite. A ®rst approximation based on Nd and Sr parameters suggests a crustal (Archean) source for these plutons. This seems to be the case for the RitaÂpolis, Itutinga, and Alto Jacaranda granites. However, a rather complex picture is more likely for Porto Mendes and Lavras plutons. Three of the eNd(t ) values for the Porto Mendes granite range between ÿ11.0 and ÿ9.3 and suggest an Archean source region. A less negative eNd(t ) value of ÿ4.9 points to a subordinate Paleoproterozoic com-

Table 3 Isotopic parameters of the basement rocksa Samples

Unit/rock

TDM

eNd (T = 2.0

840/JC-87j 843/JC-24 d 844/JC-71e D-150 D-142 WT-04 SF-WT-12-1b WT-06 WT-08 11.1 120 243.6 625F 656C 11.15 11.18 DJ8/B DJ8/1 DJ8/2 DJ8/3 DJ8/5 DJ8/7 19 60 45 100

CBC/grl CBC/grl CBC/grl CBC/grl CBC/grl CBC/grl CBC/grl CBC/grn CBC/gns BC/gns BC/gns BC/gns BC/ton BC/ton BC/grn BC/grn BC/gns BC/gns BC/gns BC/gns BC/gns BC/gns ma®c dikes ma®c dikes ma®c dikes ma®c dikes

3.25 2.93 3.03 3.20 3.16 2.90 3.01 3.08 3.21 3.15 3.12 3.24 2.94 3.09 2.94 3.10 ± ± ± ± ± ± 3.06 3.38 2.66 3.13

ÿ10.7 ÿ9.3 ÿ11.9 ÿ12.7 ÿ11.9 ÿ10.6 ÿ11.1 ÿ10.3 ÿ12.1 ÿ8.9 ÿ3.3 ÿ6.1 ÿ9.1 ÿ10.0 ÿ9.3 ÿ10.6 ± ± ± ± ± ± ÿ1.4 ÿ1.1 0.9 ÿ1.8

Ga)

87

Rb/86Sr

87

Sr/86Sr

± ± ± 0.078 3.074 0.470 0.896 0.700 0.410 ± ± ± ± ± ± ± 0.748 0.866 0.997 1.519 0.661 1.899 0.161 0.174 0.179 0.224

± ± ± 0.70526 0.81579 0.72909 0.74774 0.73450 0.71760 ± ± ± ± ± ± ± 0.73165 0.73439 0.73779 0.75815 0.72861 0.76647 0.70693 0.70793 0.70736 0.70790

eSr (T = 2.0 ± ± ± 12.30 357.29 190.86 281.70 173.55 51.84 ± ± ± ± ± ± ± 113.26 103.88 98.55 174.36 105.66 136.95 2.11 11.11 0.61 ÿ9.96

Ga)

References Carneiro (1992) Carneiro (1992) Carneiro (1992) Pinese (1997) Pinese (1997) Teixeira et al. (1996) Teixeira et al. (1996) Teixeira et al. (1996) Teixeira et al. (1996) Teixeira et al. (1996) Teixeira et al. (1996) Teixeira et al. (1996) Teixeira et al. (1996) Teixeira et al. (1996) Teixeira et al. (1996) Teixeira et al. (1996) Teixeira et al. (1996) Teixeira et al. (1996) Teixeira et al. (1996) Teixeira et al. (1996) Teixeira et al. (1996) Teixeira et al. (1996) Pinese (1997) Pinese (1997) Pinese (1997) Pinese (1997)

a Note: 87Rb/86Sr and 87Sr/86Sr are measured values; TDM model ages were recalculated using DePaolo (1981) model parameters. CBC=Campo Belo Complex; BC=Bon®m Complex; grl=granulite; grn=granite; gns=gneiss.

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ponent. A heterogenic isotopic feature is similarly shown by the samples analysed from the Lavras pluton since they yielded eNd(t ) values of ÿ7.2 and ÿ3.8. Such variation in Nd parameters might be expected when granitoid plutons are derived from the mixing of variable proportions of mantle and crust material. This model is enhanced by the wide range of TDM ages for both plutons (3.07±2.48 Ga). Therefore, the youngest ages (2.48 and 2.62 Ga) obtained for these two nearby intrusions (Lavras and Porto Mendes) are probably hybrid ages re¯ecting the variable contribution of Paleoproterozoic material during magma genesis. Additional information about the Porto Mendes pluton comes from Pb and Sr data. The Pb model m1 value of 8.1020.02 is slightly higher than the depleted mantle m1 value of ca. 8.0 during the Proterozoic (Moorbath and Taylor, 1981). The initial 87Sr/86Sr value of 0.7040 is also slightly higher than values expected for the mantle at the time. Both parameters indicate a source enriched in U and Rb in relation to a Paleoproterozoic depleted mantle. Therefore, an older and U-depleted lower crust can be ruled out as a possible source for the Porto Mendes intrusion. On the other hand, Pb isotope data could result from contamination from U-enriched upper crust material, although higher initial 87Sr/86Sr values would be expected if a signi®cant contribution from older Archean material was envisaged. In conclusion, the Porto Mendes pluton would result from a mixture of mantle-derived material and anatectic melts derived from upper crust material, mostly of Archean age. This model may also apply to the Lavras pluton that displays comparable Nd and Sr isotopic data. 4.2. Group ``B'' plutons The initial 87Sr/86Sr values fall within a wide range, from 0.7024 (TabuoÄes) to 0.7086 (Ressaquinha). The isotope signature of Alto MaranhaÄo tonalite, given by positive eNd(t ) (+1.3) and very low initial 87Sr/86Sr values recalculated from Noce (1995) for T = 2.12 Ga (0.70191 and 0.70266), is indicative of a mantle-derived source. Concerning the Ressaquinha and TabuoÄes plutons, negative eNd(t ) values (ÿ1.3 and ÿ2.8, respectively) suggest a contribution of crustal material. However, the contribution to these rocks from Archean crustal sources was very limited. Even the observed high initial 87Sr/86Sr ratio (0.708) of the Ressaquinha pluton when associated with its slightly negative eNd(t )=ÿ1.3 indicates derivation from Paleoproterozoic upper crustal material rather than from Archean material. The genesis of I-type tonalites like the Alto MaranhaÄo is often ascribed to a two-stage model relating the magma source region to a subcrustal ma®c underplate

(Pitcher, 1993). A TDM of 2.27 Ga suggests that this underplated protolith developed ca. 150 Ma before the tonalitic intrusion (2124 Ma). The TDM for Ressaquinha (2.31 Ga) and TabuoÄes (2.45 Ga) probably records the presence of a crustal component. 4.3. Sr±Nd isotope modelling The fact that the granitoids yielded Archean and Paleoproterozoic TDM ages indicates that Nd data can discriminate the genesis of the plutons. In order to constrain such a process we have assumed a mean emplacement age of 2 Ga. Fig. 7 presents the Nd signature of the granitoids investigated in conjunction with data compiled from contemporaneous ma®c dikes (Lavras ma®c dike swarm) and Archean country rocks (Campo Belo and Bon®m Complexes). All the eNd values have been recalculated for T = 2.0 Ga (Tables 2 and 3). The assumption of a mean emplacement age was necessary because the Rb±Sr isochron ages, between 2060 and 1860 Ma, have large uncertainties (Table 1), and only the Alto MaranhaÄo intrusion has been dated by the U±Pb method. Group ``B'' plutons contrast with those from group ``A'' in having higher eNd(2.0Ga) values (ÿ2.8 to +1.3), which are similar to eNd(2.0Ga) values for the Lavras ma®c dike swarm (ÿ1.8 to +0.9). Moreover, group ``B'' samples tend to have a narrow range of eNd values compared with those of group ``A'' (Fig. 7). This corroborates the idea that the main source regions of the magmas for these plutons (Alto MaranhaÄo, Ressaquinha, and TabuoÄes) were not Archean, implying the presence of a Paleoproterozoic juvenile accreted crust in the southern border of the SaÄo Francisco Craton. Group ``A'' samples are characterized by an inherited crustal signature, as indicated by their wider range of eNd(2.0Ga) values (ÿ3.8 to ÿ11.0; Table 2). This range is compatible with the isotopic signature of the Archean crust (indicated by heavy lines in Fig. 7), and with the assesment that granitic magmas formed mainly by melting of typical crustal rocks have initial eNd values that cluster at the value for the mean crustal rocks from the basement (Bennett and DePaolo, 1987). For example, two samples from the Alto Jacaranda granite, intrusive into the Archean Bon®m Complex, yield eNd(2.0Ga) values that plot within the range of eNd values calculated for di€erent rocks from this complex (Fig. 7 and Table 3). A binary mixing model (mantle- and crust-derived components) provides additional details for the general isotopic characteristics of the granitoids (Fig. 8). In this model we assume that the mantle-derived source is represented by the 2.0 Ga Lavras dike swarm investigated by Pinese (1997), whereas the Archean com-

C.M. Noce et al. / Journal of South American Earth Sciences 13 (2000) 225±239

235

Fig. 7. Variation of eNd values (for T = 2.0 Ga) of the granitoid plutons compared with eNd values of Archean complexes and ma®c dikes.

ponents are represented by the Campo Belo and Bon®m Complexes. The calculated eNd and eSr values for T = 2.0 Ga (Table 3) for both Archean rocks and ma®c dikes are plotted together with the isotopic signature of the depleted mantle (DM in Fig. 8), compared to the bulk Earth. It is noteworthy that all the samples lie to the right and mainly below the bulk earth value. Moreover, the main feature of this diagram (Fig. 8) is that the granitoid data tend to form a broad trend between the ®eld of the ma®c dikes and the ®elds of Campo

Belo and Bon®m rocks. This seems to con®rm the idea that the granitoids derived from di€erent proportions of mixing between a mantle source (isotopically distinct from DM) and Archean components. The Alto MaranhaÄo sample plots within the ®eld of the ma®c dikes, which are enriched compared to the bulk earth (Pinese, 1997). On the other hand, the data from the Alto Jacaranda granite plot between the enriched ®eld and the Bon®m Complex ®eld. Therefore, this granite is clearly derived from partial melting of Archean material, as already seen.

Fig. 8. Sr±Nd mixing model of two components applied to granitoid plutons, ma®c dikes, and country rocks of the southern SaÄo Francisco Craton. The delineated ®eld of the Bon®m Complex was extrapolated from the range in eNd and eSr values based upon the data from di€erent samples (Table 3). The DM ®eld was plotted using the Nd parameters of DePaolo (1981) and the following Rb/Sr isotopic ratios: 87 Rb/86Sr=0.046 and 87Sr/86Sr=0.7026 (Taylor and McLennan, 1985) which yielded eSr(2.0Ga)=ÿ12.4; and 87Rb/86Sr=0.049 and 87 Sr/86Sr=0.7023 (Ben Othman et al., 1984) which yielded eSr(2.0Ga)=ÿ18.1.

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Fig. 9. Regional di€erences in Sm±Nd TDM ages (Ga) for Archean country rocks (underlined) and Transamazonian granitoid plutons (italics) within the southern SaÄo Francisco Craton: (1) Archean TTG gneiss and migmatite (a=Archean cratonic area); (2) Archean greenstone belt; (3) Paleoproterozoic Minas Supergroup; (4) Paleoproterozoic granitoid plutons; (5) Meso- to Neoproterozoic SaÄo JoaÄo del Rei and Araxa Groups; (6) Neoproterozoic Bambuõ Group.

The Porto Mendes and RitaÂpolis granitoids show a more complicated signature since they have a wider range of eNd(2.0Ga) values coupled with characteristically very high positive eSr(2.0Ga) values (+150). This wide range is diagnostic of Archean components Ð represented here by the data from the Campo Belo Complex Ð that have participated in the genesis of these plutons, besides subordinate Paleoproterozoic materials (see previous discussion). The Ressaquinha granitoid sample plots away from the whole group of data. This con®rms that the Sr±Nd signature of this pluton cannot be associated with the presence in the magma source of an Archean component because of the distinct high eNd(2.0Ga) value (ÿ1.5), as already seen. Besides, the moderate eSr(2.0Ga) suggests that crustal rocks presenting an inherited radiogenic Sr signature (Paleoproterozoic sediments?) have played an important role in magma genesis. This isotopic feature also implies that a simple binary mixing model is not adequate to explain the isotopic data available for all the granitoids of the Mineiro Belt. 5. Conclusions The contrasting chemical and isotopic characteristics of the granitoid plutons can be related to distinct

source regions and suggest di€erent degrees of interaction between mantle-derived magmas and continental crust. The range of TDM ages for granitoids and country rocks of the southern part of the SaÄo Francisco Craton is plotted in Fig. 9. The geochronologic patterns of the eastern part of the Mineiro Belt (granitoids with TDM ages between 2.27 and 2.30 Ga) and of the Archean terrains of the Campo Belo and Bon®m Complexes (TDM ages typically between 2.90 and 3.25 Ga) are distinct. The observed gradational change in TDM ages, getting younger from the Archean platform eastward, can be interpreted as resulting from Paleoproterozoic crust-forming processes at the edge of the older domain. In particular, the Transamazonian plutonism investigated herein appears to be the culmination of a tectonic process that led to the accretion of the Mineiro Belt to the Archean platform. Neoarchean to early Paleoproterozoic TDM ages, between 2.62/2.48 and 2.43 Ga for the TabuoÄes and Lavras plutons can be interpreted in a similar way, suggesting that Paleoproterozoic juvenile crust accretion also occurred at the central and southern parts of the belt. The fact that mantle-derived material played an important role in the generation of some of the Mineiro Belt granitoids implies that a signi®cant crust accretion

C.M. Noce et al. / Journal of South American Earth Sciences 13 (2000) 225±239

237

episode occurred in connection with the Transamazonian Orogeny. Processes of oceanic crust subduction and ma®c magma underplating are likely to have occurred at early stages of the orogen evolution. Magmatism related to deep faulting is a plausible mechanism to generate granitoid plutons with close spatial/temporal relationships and contrasting source regions (Cobbing, 1996). Actually, the emplacement of many of the plutons (Alto MaranhaÄo, Ressaquinha, Campolide, TabuoÄes, RitaÂpolis, and Itutinga) has recently (Endo, 1997) been ascribed to strike-slip faulting in a compressional tectonic setting. Finally, the study area consists of a preserved remnant of a much wider Paleoproterozoic orogenic belt strongly reworked during the Brasiliano Orogeny. It makes modelling of the Mineiro Belt a very dicult task. Granitoids at the southern SaÄo Francisco Craton were emplaced at 2124±2041 Ma, but considerably older granitoid rocks have been described as forming basement thrust sheets within the Brasiliano Ribeira Belt. In this concern, two gneissic bodies associated with the substratum of this belt along the coastal area of southeastern Brazil yielded crystallization U±Pb ages of 2169 2 3 and 2185 2 8 Ma (Machado et al., 1996b). This fact implies, therefore, that granitoid plutonism during the Transamazoniam Orogeny spanned more than 150 million years.

the Sm±Nd analyses: ®rst, an ion exchange resin was employed for separation of the REE, and then an HDEHP-coated Te¯on powder column was used for separation of Sm and Nd elements. Details of the analytical procedures are as reported in Sato et al. (1995). The laboratory blanks for Nd and Sm elements yield maximum values of 70 and 30 pg, respectively. The average measured 143Nd/144Nd values of the La Jolla Nd and BCR-1 standards are 0.51184920.000025 and 0.51266220.000027, respectively, with errors at the 1s level. Uncertainties for 143Nd/144Nd and 147Sm/144Nd ratios (propagated from isotope dilution isotopic data) are 0.008% and 0.3% or better, respectively. in the granitoid samples (Table 2). Published TDM model ages were recalculated using DePaolo (1981) model parameters (Table 1): a = 0.25; b=ÿ3; c = 8.5, and 146Nd/144Nd=0.7219 to normalize the isotopic ratios [143Nd/144Nd (Chur)0=0.512638 and 147Sm/144Nd (Chur)0=0.1967]. The eNd values were calculated using the simpli®ed equation eNd (T )=eNd (0)ÿQfSm/NdT, with the CHUR(0) values above and QNd=25.09 (Rollinson, 1993). Double-stage TDM model ages (DePaolo et al., 1991) were calculated for ®ve samples (Table 1) that show fractionation of the Sm/Nd systematics (Table 2), using their isotopic ratios (Sato, 1997) and applying the following equation age:

Acknowledgements

TDM2 ˆ l ÿ1 ln f1 ‡ …143 Nd=144 Nd †DM

This work was funded by FAPEMIG (C. M. Noce and J. QueÂmeÂneur), Fapesp 95/4652-2 and 95/6721-1, and CNPq 523486-94.4 and PIBIC 80.3070/87-0 (W. Teixeira). We thank Angela Pinto for digitizing the geological map. We are grateful to W. R. Van Schmus, M. M. Pimentel, and N. J. Snelling for their critical reviews. Appendix All the isotopic analyses presented herein were carried out at the Geochronological Research Center (CPGeo) of the University of SaÄo Paulo, Brazil. The Rb±Sr data and part of the Sm±Nd data available for the Mineiro Belt granitoids have been already published, as presented in the text and summarized in Tables 1 and 2. The published Rb±Sr whole rock isochrons were ®tted following the principles of Williamson (1968) in which the experimental variance was used to weight each point. The precision of the age and initial 87 Sr/86Sr ratios are quoted at the 95% (2s) con®dence level. In the CPGeo, a two-column technique was used for

ÿ ‰…143 Nd=144 Nd †s ÿ …elT… fe † ÿ 1†‰147 Sm=144 Nd †s ÿ …147 Sm=144 Nd †f1 ŠŠ=‰…147 Sm=144 Nd †DM ÿ …147 Sm=144 Nd †f1 g, where ``fe'' is the age of the fractionation event; (147Sm/144Nd)f1 is the average value for the crustal rock source, which is 00.11 for granitic rocks; ``s'' is the sample ratio; (147Sm/144Nd)DM=0.2188; and (143Nd/144Nd)DM=0.513151 (Millisenda et al., 1994).

References Alkmim, F.F., Brito-Neves, B.B., Castro-Alves, J.A., 1993. Arcabouc° o tectoÃnico do CraÂton do SaÄo Francisco. In: Dominguez, J.M.L., Misi, A. (Eds.), O CraÂton do SaÄo Francisco. SBG-BA/SGM/CNPq, Salvador, Brazil, pp. 45±62. Babinski, M., Chemale, F., Van Schumus, W.R., 1995. The Pb/Pb age of the Minas Supergroup carbonate rocks, QuadrilaÂtero FerrõÂ fero, Brazil. Precambrian Research 72, 235±245. Beakhouse, G.P., McNutt, R.H., Krogh, T.E., 1988. Comparative Rb±Sr and U±Pb zircon geochronology of late- to post-tectonic plutons in the Winnipeg River Belt, northwestern Ontario, Canada. Chemical Geology 72, 337±351. Bennett, V.C., DePaolo, D.J., 1987. Proterozoic crustal history of

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