U Pb geochronology of the southern Brasília belt (SE-Brazil): sedimentary provenance, Neoproterozoic orogeny and assembly of West Gondwana

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1 Precambrian Research 130 (2004) U Pb geochronology of the southern Brasília belt (SE-Brazil): sedimentary provenance, Neoproterozoic orogeny and assembly of West Gondwana Claudio M. Valeriano a,, Nuno Machado b,c, Antonio Simonetti b, Claudia S. Valladares a, Hildor J. Seer d, Luiz Sergio A. Simões e a TEKTOS Geotectonics Study Group, Universidade do Estado do Rio de Janeiro, Rua São Francisco Xavier 524/4006-A, Rio de Janeiro, RJ , Brazil b Centre de recherche en géochimie et géodynamique (GEOTOP-UQAM-McGill), Université du Québec à Montréal, Montréal, Que., Canada c Département des Sciences de la Terre et de l Atmosphère, Université du Québec à Montréal, CP 8888, Succ. Centre-Ville, Montréal, Que., Canada H3C 3P8 d Centro Federal de Educação Tecnológica de Minas Gerais, Av. Amazonas 807, Araxá, MG, Brazil e Departamento de Petrologia e Metalogenia, IGCE, Universidade Estadual Paulista, Caixa Postal 178, CEP , Rio Claro, SP, Brazil Received 13 February 2003; accepted 20 October 2003 Abstract The Brasília belt borders the western margin of the São Francisco Craton and records the history of ocean opening and closing related to the formation of West Gondwana. This study reports new U Pb data from the southern sector of the belt in order to provide temporal limits for the deposition and ages of provenance of sediments accumulated in passive margin successions around the south and southwestern margins of the São Francisco Craton, and date the orogenic events leading to the amalgamation of West Gondwana. Ages of detrital zircons (by ID TIMS and LA-MC-ICPMS) were obtained from metasedimentary units of the passive margin of the São Francisco Craton from the main tectonic domains of the belt: the internal allochthons (Araxá Group in the Áraxá and Passos Nappes), the external allochthons (Canastra Group, Serra da Boa Esperança Metasedimentary Sequence and Andrelândia Group) and the autochthonous or Cratonic Domain (Andrelândia Group). The patterns of provenance ages for these units are uniform and are characterised as follows: Archean Paleoproterozoic ages ( , , and Ga); Paleoproterozoic ages attributed to the Transamazonian event ( Ga, with a peak at ca Ga) and to the ca Ga Espinhaço rifting of the São Francisco Craton; ages between 1.6 and 1.2 Ga, with a peak at 1.3 Ga, revealing an unexpected variety of Mesoproterozoic sources, still undetected in the São Francisco Craton; and ages between 0.9 and 1.0 Ga related to the rifting event that led to the individualisation of the São Francisco paleo-continent and formation of its passive margins. An amphibolite intercalation in the Araxá Group yields a rutile age of ca. 0.9 Ga and documents the occurrence of mafic magmatism coeval with sedimentation in the marginal basin. Detrital zircons from the autochthonous and parautochthonous Andrelândia Group, deposited on the southern margin of the São Francisco Craton, yielded a provenance pattern similar to that of the allochthonous units. This result implies that Ga source Corresponding author. Tel.: ; fax: addresses: cmval@uerj.br (C.M. Valeriano), machado.nuno@uqam.ca (N. Machado), simonetti.antonio@uqam.ca (A. Simonetti), hildor@araxa.cefetmg.br (H.J. Seer) /$ see front matter 2003 Elsevier B.V. All rights reserved. doi: /j.precamres

2 28 C.M. Valeriano et al. / Precambrian Research 130 (2004) rocks must be present in the São Francisco Craton. They could be located either in the cratonic area, which is mostly covered by the Neoproterozoic epicontinental deposits of the Bambuí Group, or in the outer paleo-continental margin, buried under the allochthonous units of the Brasília belt. Crustal melting and generation of syntectonic crustal granites and migmatisation at ca. 630 Ma mark the orogenic event that started with westward subduction of the São Francisco plate and ended with continental collision against the Paraná block (and Goiás terrane). Continuing collision led to the exhumation and cooling of the Araxá and Passos metamorphic nappes, as indicated by monazite ages of ca. 605 Ma and mark the final stages of tectonometamorphic activity in the southern Brasília belt. Whilst continent continent collision was proceeding on the western margin of the São Francisco Craton along the southern Brasília belt, eastward subduction in the East was generating the Ma Rio Negro magmatic arc which collided with the eastern São Francisco margin at Ma, much later than in the Brasília belt. Thus, the tectonic effects of the Ribeira belt reached the southernmost sector of the Brasília belt creating a zone of superposition. The thermal front of this event affected the proximal Andrelândia Group at ca. 588 Ma, as indicated by monazite age. The participation of the Amazonian craton in the assembly of western Gondwana occurred at Ma in the Paraguay belt and ca. 500 Ma in the Araguaia belt. This, together with the results presented in this work lead to the conclusion that the collision between the Paraná block and Goiás terrane with the São Francisco Craton along the Brasília belt preceded the accretion of the Amazonian craton by million years Elsevier B.V. All rights reserved. Keywords: West Gondwana; U Pb geochronology; Detrital zircon; TIMS; LA-ICPMS; Neoproterozoic 1. Introduction and objectives The West Gondwana supercontinent was formed by the aggregation of Archean Paleoproterozoic continental blocks along Neoproterozoic mobile belts (Unrug, 1996). To unravel the diachronic history of this major event in the Earth history, addressing the detailed timing of ocean formation and closure is required for each orogenic belt. The Brasília belt occupies a key position in West Gondwana because of its central location (Fig. 1), and thus records the interaction between the Paraná, Goiás and São Francisco continental blocks (Strieder and Suita, 1999; Pimentel et al., 1999, 2000). The formation of this continental nucleus was followed, during late Neoproterozoic Ordovician, by development of the Paraguay and Araguaia belts in the West, and by the Ribeira Araçuaí belt in the East (Almeida et al., 2000). Later accretions took place during the Paleozoic along the western margin successively contributing to form West Gondwana (Ramos, 1988). The timing of passive margin build up along the southwestern border of the São Francisco Craton and of ocean closure Brazilide ocean of Dalziel (1997) and Goianides of Weil et al. (1998), however, are still poorly known and preclude detailed paleogeographic reconstructions. The purpose of this paper is to contribute to better define the formation of West Gondwana by studying a sector of the southern Brasília belt. Thus, age limits for the deposition of the passive margin successions (Araxá, Canastra and Andrelândia groups) and for orogenesis are presented. Dating detrital zircons from metasedimentary rocks has proved a particularly useful approach for constraining sedimentation ages (i.e. younger than the youngest dated detrital zircon) and provenance. New U Pb data were obtained by Isotope Dilution Thermal Ionisation Mass Spectrometry (ID TIMS) and by Laser Ablation Multi Collection Inductively Coupled Plasma Mass Spectrometry (LA-MC-ICPMS). The results yield a Neoproterozoic age for the sedimentation of the Araxá, Canastra and Andrelândia groups and reveal an unexpected variety of Mesoproterozoic sources, still undetected in the São Francisco Craton. 2. Geological setting 2.1. The Tocantins Structural Province The Tocantins Structural Province resulted from the interaction of three major paleocontinental blocks, whose present remnants are identified as the Amazonian, São Francisco and Paraná cratons (Strieder and Suita, 1999; Pimentel et al., 2000). The latter is completely covered by the Paraná basin, with rather

3 C.M. Valeriano et al. / Precambrian Research 130 (2004) Fig. 1. Simplified tectonic map of the southern Brasília belt (adapted from Valeriano et al., 2000). (a) Tectonic domains mentioned in this work and sample location (in part). Black arrows: direction of tectonic transport; (b) the Tocantins Structural Province, comprising the Brasília, Araguaia and Paraguay belts, in the context of West Gondwana (adapted from Almeida et al., 2000). AM, Amazonian Craton; PR, Parnaíba Block; SL-WA, São Luis-West African Craton; SF-Com, São Francisco-Congo Craton; PP, Parana block. speculative outlines, and may be connected to the Rio de la Plata craton (Almeida et al., 2000). The Tocantins Structural Province comprises three main branches: the Araguaia, the Paraguay and the Brasília belts. The first two display tectonic vergence toward the eastern and southeastern margins of the Amazon Craton, respectively, whilst the Brasília belt displays opposite vergence towards the São Francisco Craton. The southernmost Brasília belt is truncated by the NE SW structural fabric of the Ribeira belt which extends along the Atlantic coast The Brasília belt The Brasília belt has two segments (Fig. 1): the northern one trends NE and displays dextral

4 30 C.M. Valeriano et al. / Precambrian Research 130 (2004) transpressive kinematics (Fonseca et al., 1995), whilst the southern one trends SE and displays sinistral compression tectonics (Valeriano et al., 2000). The southern Brasília belt is divided into the following tectonic domains, from east to west: (a) the Cratonic Domain comprises the Archean Paleoproterozoic rocks of the São Francisco Craton, which is mostly covered by autochthonous and para-autochthonous pelitic carbonatic rocks of the Neoproterozoic Bambuí Group and older metasedimentary successions (Tiradentes and Lenheiro Sequences, Carandaí and Andrelândia groups). (b) The External Domain is composed of thrust systems of predominantly thick passive margin metasedimentary successions in low-metamorphic grade (Canastra, Ibiá and Paranoá groups) and subordinate Archean to Paleoproterozoic basement slivers. (c) Metamorphic nappes composed of distal passive margin and slope deposits (Araxá and Andrelândia groups) in intermediate to high-pressure metamorphic facies form the Internal Domain. The Araxá Group locally contains ophiolitic melanges, interpreted as remnants of subducted oceanic basin (Strieder and Nilson, 1992). (d) The Goiás Massif makes up a microcontinent of Archean and Paleoproterozoic basement composed of granite greenstone and migmatite-gneissic terrains and varied anorogenic Mesoproterozoic rock assemblages. These include the Juscelândia Sequence and large layered mafic ultramafic complexes (Niquelândia, Cana Brava and Hidrolina complexes) in granulite facies. The age of granulite metamorphism, ca. 780 Ma (Ferreira Filho et al., 1994), is the oldest record of an orogenic episode in the Tocantins Structural Province. (e) Finally, the Goiás magmatic arc consists of metavolcano-sedimentary and metaplutonic rocks of island arc affinity that started to develop at ca. 930 Ma as consumption of oceanic lithosphere initiated within the Goianides ocean (Pimentel et al., 1997) that formerly separated the Amazonian and São Francisco paleocontinents The southern Brasília belt The southern Brasília belt comprises three synformal allochthonous segments whose differential tectonic transport onto the foreland was accommodated by WNW-trending lateral ramps reactivated at a later time as sinistral strike slip fault zones. For convenience of description, the synformal segments are named northern, Furnas and southern (Fig. 1). Within each of these the Internal, External and Cratonic tectonic domains are well recognizable. However, as shown in Table 1, each segment has its own lithostratigraphic nomenclature, partly due to uncertainties of correlation Cratonic Domain The exposed basement in the southern part of São Francisco Craton consists of Archean granite greenstone terrains, in part reworked during the Transamazonian Orogeny (ca Ga), which also generated juvenile gneiss migmatite granitoid complexes and metavolcano-sedimentary successions. Late to post-orogenic granitoid magmatism is extensive and has been dated between 1.9 and 1.8 Ga (Teixeira et al., 2000). Statherian ( Ga) taphrogenesis in the São Francisco Craton is marked by Ma magmatism in the Espinhaço continental rift system (Machado et al., 1989). The ages of sedimentation of the Neoproterozoic supracrustal successions south and west of the São Francisco Craton are poorly constrained by Nd Table 1 The three segments of the Southern Brasília belt and names of lithostratigraphic units and tectonic elements Tectonic Domain Internal Domain External Domain Cratonic Domain Segment Thrust sheets Stratigraphic units Northern Araxá nappe Araxá Group Ibiá Group Canastra Group Bambuí Group Furnas Passos Nappe Araxá Group Canastra Group; granite greenstone basement (Piumhi Massif) Bambuí Group; Autochthonous basement Southern Guaxupé nappe; Luminárias nappe Andrelândia Group Andrelândia Group; Allochthonous basement Andrelândia Group (top); Carandaí Group Lenheiro/Tiradentes sequences Autochthonous basement

5 C.M. Valeriano et al. / Precambrian Research 130 (2004) model ages between 1.0 and 1.15 Ga on presumably syn-sedimentary amphibolite intercalations within the Andrelândia Group (Trouw et al., 1993). To the north, the anchimetamorphic pelitic and carbonatic rocks of the Bambuí Group, representing a post-glacial epicontinental marine environment, cover vast areas of the São Francisco Craton External Domain This domain is structurally sandwiched between the Internal and Cratonic Domains. It is segmented by WNW-trending transcurrent fault zones and comprises a series of thrust systems of imbricated psammite-dominated proximal passive margin successions which, in the south, also contain basement thrust sheets. The whole domain displays greenschist facies metamorphism. The three sectors of the External Domain considered here are located to the east of the Araxá, Passos and Luminárias nappes (Fig. 1) and, for simplicity, are here referred to as the northern, Furnas and southern segments, respectively. The northern segment comprises two main thrust sheets represented by the Ibiá (top) and Canastra groups, usually interpreted as proximal platform deposits. The Ibiá Group is predominantly composed of fine-grained rythmic chloritic phyllites and calcschists, and the Canastra Group comprises quartzitic metarenites and pelitic phyllite, where the increasing frequency of quartzites to the top suggest a regressive sequence. The Furnas segment displays a complex imbrication of metarenites and metapelites of the Serra da Boa Esperança Sequence (Canastra Group) with thrust sheets of basement rocks represented by fragments of an Archean granite greenstone lithologic association. The largest of these granite greenstone thrust sheets, south of the Piumhi locality (Fig. 2), contains predominantly komatiites and komatiitic to tholeiitic basalts, with locally preserved pillow and spinifex structure (Schrank, 1982). This basal volcano-sedimentary unit is intrude by younger calc-alkaline granitoid rocks which are dated in this work (see Section 6). The southern segment of the External Domain south of the São Francisco Craton is represented by SE-transported thrust sheets of platformal quartzite phyllite association, belonging to the Andrelândia Group, with slivers of the basement granite greenstone association. The Andrelândia Group, which occurs in the Internal, External and Cratonic domains, is interpreted as a passive margin succession developed along the southern border of the São Francisco Craton (Paciullo et al., 2000). Metamorphism developed under barrovian pressure regime Internal Domain The Internal Domain comprises three synformal nappes separated by WNW-trending lateral ramps: from north to south they are the Araxá, Passos and Luminárias nappes. The basal portion of the Araxá Nappe contains predominantly amphibolites of tholeiitic E-MORB composition and minor ultramafic schists. Metapelitic schists and metachert predominate to the top with subordinate intercalations of quartzitic metapsamites. This lithological association was interpreted by Seer (1999) as representative of ocean floor with deep marine sedimentary facies. To the north, pelitic schists and metachert with abundant metabasic and ultramafic disrupted lenses were characterised as ophiolitic mélange within the upper portions the Araxá Nappe (Strieder and Nilson, 1992). The Araxá Group in the Passos Nappe is composed of metasediments of shelf to slope facies, where nine lithostratigraphic units (Units A I, Fig. 2) can be individualised (Valeriano, 1992; Simões, 1995). The basal portion of the nappe contains a regressive sequence with carbonatic metapelites containing marble lenses, followed by a gradual increase in the number and thickness of quartzite intercalations (Unit A), culminating in a conspicuous and continuous layer of laminated micaceous quartzite of 50 m in thickness (Unit B, Furnas Quartzite). The overlying muscovite metapelitic layer (Unit C) grades to a distinct paragneiss layer (Unit D) which display relatively more intercalations of metabasic lenses of continental geochemical character (Valeriano and Simões, 1997). The upper units (E I) are composed mainly of metapelitic schists with intercalations of paragneiss and abundant calc-silicate rocks and amphibolite lenses. Major and trace element compositions of these metabasic rocks indicate they represent continental and mid-ocean ridge basalts. The distribution of the analysed samples shows that the MORB-type rocks tend to predominate to the top. However, the presence of continental basalts throughout the pile indicates that lithospheric thinning associated with the sedimentation of the Araxá Group in the area sampled

6 32 C.M. Valeriano et al. / Precambrian Research 130 (2004) Fig. 2. (a) Simplified tectonic map of the Furnas segment of southern Brasília belt. Adapted from Valeriano et al. (2000); (b) Lithostratigraphic column of the Passos Nappe. Numbers refer to analysed samples.

7 C.M. Valeriano et al. / Precambrian Research 130 (2004) by the Passos Nappe was not extensive enough to generate oceanic crust (Valeriano and Simões, 1997). Thrusting and exhumation of the Passos Nappe was associated with the main (D 2 ) deformation characterized by the development of recumbent folding and penetrative foliation. The rapid exhumation of this nappe permitted the preservation of an inverted metamorphic gradient of medium to high-pressure, ranging from the greenschist facies (biotite zone) up to the amphibolite-granulite facies transition (Simões, 1995). Low grade retrogressive reactions are associated with late-d 2 nappe exhumation (Simões, 1995). To the south, the Luminárias Nappe displays lithology similar to that of the basal Passos Nappe. Metamorphism ranges from greenschist facies at the base, to high-pressure granulite facies in the upper structural units (Campos Neto and Caby, 1999). This segment of the belt is more complete since the Luminárias Nappe is overlain by the Socorro-Guaxupé Nappe, which is composed of low pressure granulitic rocks. Neodymium T DM ages of ca. 2.2 Ga for the Canastra Group fine-grained metasedimentary rocks were interpreted as indicating provenance from the São Francisco Craton. Metasedimentary rocks from the Ibiá and Araxá groups display a bimodal T DM age pattern, with peaks between and Ga. The latter group of ages was interpreted as indicating mixed provenance from the cratonic sources with juvenile ones, such as the the Neoproterozoic juvenile Goiás magmatic arc (Pimentel et al., 2001). The collisional stage of the orogen resulted in numerous small intrusions of syn-tectonic biotite muscovite leucogranites that usually contain xenoliths of the country rocks. Sm Nd mineral isochrons yielded metamorphic ages of 637 Ma (two-point isochron) and 596±32 Ma for the Araxá Group (Seer, 1999). 3. Analytical procedures Samples were crushed and pulverized with standard equipment under clean conditions and the heavy minerals concentrated by panning at LOPAG (Universidade Federal de Ouro Preto, Brazil) and at LGPA (Universidade do Estado do Rio de Janeiro, Brazil). Manual panning dispenses the use of a Wilfley table and minimises contamination. The heavy mineral concentrate was passed through a Frantz magnetic separator to extract monazite, titanite, rutile and zircon which was further separated into four magnetic and two diamagnetic fractions, the latter being preferred for hand picking. As much as possible, only the mineral grains free of alteration, inclusions and fractures were selected for analysis. The minerals were analysed by ID TIMS. Zircon grains larger than ca. 80ìm from some samples were also analysed by LA-MC-ICPMS. Analyses were performed in the GEOTOP-UQAM at Montreal, Canada ID TIMS The analysed zircon fractions were abraded (Krogh, 1982) between 72 and 120 h to reduce grain size by ca. 30%, which experience has shown to be necessary to eliminate most or all of the recent lead loss in Brazilian zircons and render them concordant or subconcordant. This requirement severely limits the amount of the usable grains because if these are smaller than m, the abraded grains are too small to be manipulated or sample weight becomes too low to yield reasonably precise analysis. Depending on the grain size, the abraded grains may still contain peripheral zones where the U Pb system remained open. Air pressure for abrasion was regulated in order to keep the zircon crystals intact during the procedure. Once the grains are broken, further abrasion removes the periphery but also the inside of the crystal where the U Pb system may have remained closed. Mineral dissolution, chemical extraction of U and Pb and mass spectrometric analysis were carried out following the procedures described in Machado et al. (1996a,b). Total procedural blanks average 10 pg Pb and 2 pg U for zircon analyses and 15 pg Pb and 5 pg U for titanite, rutile and monazite analyses. The uncertainties in isotopic ratios presented in this work (Table 2) were calculated with an error propagation program, which takes into consideration the analytical precision of the measured isotopic ratio. Regressions were calculated and concordia diagrams plotted using the Isoplot-Ex Version 2 (Ludwig, 2000). Errors are represented at the 1σ level but all ages are quoted at the 95% confidence interval.

8 Table 2 U Pb results by Isotope Dilution Thermal Ionisation Mass Spectrometry Sample Concentrations Atomic ratios Ages Number Min. a Weight ( g) U (ppm) b Pbrad. (ppm) b Pbcom. (pg) c 206 Pb/ 204 Pb d 208 Pb/ 206 Pb f 206 Pb/ 238 U f ±% (1σ) 207 Pb/ 235 U f ±% (1σ) 207 Pb/ 206 Pb f ±% (1σ) 206 Pb/ 238 U 207 Pb/ 235 U 207 Pb/ 206 Pb Disc. % Internal Domain Araxá Nappe Serra Velha Granite M Z(9) Z(6) Passos Nappe Araxá Group, Furnas Quartzite Unit B SZ SZ <1 477 e 115 e SZ SZ SZ SZ SZ M Passos Nappe Araxá Group, garnet plagioclase biotite schist Unit E M M SZ Passos Nappe Araxá Group, micaceous quartzite Unit E SZ SZ SZ SZ SZ SZ SZ Passos Nappe Araxá Group, micaceous quartzite Unit E SZ Passos Nappe Araxá Group, amphibolite Unit G R(50) R(14) Passos Nappe Araxá Group, micaceous quartzite Unit G M Passos Nappe Araxá Group, leucosome Unit G M < e 1807 e M Z(70) <1 104 e 12 e Z(54) Z(28) Z(45) Passos Nappe Araxá Group, orthoquartzite Unit H SZ SZ SZ SZ C.M. Valeriano et al. / Precambrian Research 130 (2004) 27 55

9 SZ M M Passos Nappe Araxá Group, amphibolite Unit H R(60) External Domain Furnas segment Felsic metavolcanic unit (Costas schist) Z(5) Z(9) Z(12) Z(17) < e 276 e Serra da Boa Esperança Sequence: quartzite SZ SZ SZ Serra da Boa Esperança Sequence: quartzite SZ SZ Serra da Boa Esperança Sequence: metarkose SZ SZ Archean basement thrust sheet (Taquari granite) 71-1 T(50) Archean basement thrust sheet Hornblende orthogneiss Z(6) Z(7) Z(6) Z(10) Serra da Boa Esperança Sequence (Serra da Mamona unit) metaconglomerate SZ SZ External Domain Southern segment Andrelândia Group quartzites ITA3-2 M ITA2-1 M a The number within parentheses indicates number of analysed grains: SZ: single zircon; Z: zircon population; M: single monazite; T: titanite population; R: rutile population. b Concentrations are known to 20% for weights below 20 g. c Total common Pb present in analysis corrected for Pb in spike. d Measured ratio, corrected for fractionation only. e Sample weight below the sensitivity of the microbalance (below 1 g); therefore, listed concentrations are minimum values. f Ratios corrected for spike, fractionation, blank and initial common Pb. Errors quoted are in percentage at the 1σ confidence level. Maximum total blanks for zircon analyses are 15 pg for Pb and 2 pg for U. Isotopic composition of initial common Pb was calculated using the two-stage model of Stacey and Kramers (1975). C.M. Valeriano et al. / Precambrian Research 130 (2004)

10 36 C.M. Valeriano et al. / Precambrian Research 130 (2004) LA-MC-ICPMS The selected grains together with fragments of in-house standard zircon (UQ-Z8) were mounted on epoxy known to be devoid of Pb and U from previous analyses and manually polished on alumina (Al 2 O 3 )-lapping film of progressively smaller grain size from 9 to 0.3 m. The grain mount was successively ultrasonicated in distilled water, washed with sub-boiling HCl 6.2N and with sub-boiling H 2 O and left to dry under a class 100 clean hood. The results reported in this work were obtained with an excimer laser coupled to a multicollector ICPMS system. The laser system consists of a 193 nm Lambda Physik Compex 102-ArF laser delivering a maximum of 200 mj per pulse of 25 ns duration. The beam delivery system is designed by Merchantek-New Wave Research and closely resembles that described by Horn et al. (2000). Zircon was analysed using fluences of 10 ± 2Jcm 2 at 5 Hz laser frequency and beam diameters between 35 and 80 m. The laser ablation system is coupled to a Micromass Isoprobe, a multicollector mass spectrometer with an ICP source and an hexapole collision cell. Data was acquired in static, multicollection mode using six Faraday collectors, in the only configuration possible to encompass the large mass spread between 204 Pb and 238 U( 14%). Before ablation data is collected, the collector positions and gains are verified by aspirating a solution containing NIST 981Pb and U500 standards. Each analysis consists of a 50 s on-peak baseline measurement prior to the start of ablation, followed by two half-mass unit baseline measurements after ablation had commenced after which a block of fifty 1 s integrations is acquired. The on-peak baseline measurement compensates for 204 Hg present in the Ar gas. Although 204 Pb was not measured during the analyses presented here, during more recent work we obtained 206 Pb/ 204 Pb values never lower than 2000 and generally higher than Therefore, no common lead corrections were applied. After the laser frequency and energy and the beam diameter have been chosen, the in-house standard UQ-Z8 is ablated and the nebuliser (carrier) gas (Ar) flow rate adjusted to obtain a mean 206 Pb/ 238 Uas close as possible to that of the standard. The UQ-Z8 in-house standard is a zircon megacrystal from the same rock as UQ-Z1 reported on previously (Machado and Gauthier, 1996), which was dated by ID TIMS at 1143 ± 1 Ma. The 207 Pb/ 206 Pb and the 238 U/ 206 Pb values obtained for the standard during this work are precise to 0.1 and 1.3%, respectively. All analyses were corrected for U fractionation relative to 238 U/ 235 U of Age calculations and plotting were done with Isoplot-Ex Version 2 (Ludwig, 2000). The precision of the isotopic ratios is reported and plotted as standard error of the mean at the 1σ level but all ages are quoted at the 95% confidence interval (Table 3). Due to the poor precision of the measured 207 Pb/ 235 U values obtained for the youngest zircons (<1 Ga) ablated in an Ar atmosphere, the results are presented on 238 U/ 206 Pb vs. 207 Pb/ 206 Pb concordia diagrams (Tera and Wasserburg, 1972). The analytical results are displayed in Table 2 (ID TIMS) and Table 3 (LA-MC-ICPMS). 4. Results The results are presented in tectonic organisation from the Internal to the Cratonic Domain, following their geographical distribution from north to south. Each analysed sample and pertaining results are described separately, with TIMS data preceding those obtained by LA-MC-ICPMS. 5. Internal domain 5.1. Araxá nappe Serra Velha Granite The Serra Velha Granite (sample 486, Table 2) is one of a series of syn-tectonic granitoids emplaced into metasedimentary rocks and intercalated ortho-amphibolites of the Araxá Group. They are typical biotite muscovite-bearing collisional granites with geochemical characteristics indicating derivation from crustal melting (Seer, 1999). Quartz, orthoclase, microcline, plagioclase, white mica (phengite) and biotite are the main constituents, with accessory garnet, tourmaline, monazite, apatite and zircon. Secondary white mica, biotite, albite and chlorite grew under greenschist facies metamorphic conditions during the tectonic transport of the Araxá Nappe. The emplacement of the granitic bodies was controlled by gently dipping

11 C.M. Valeriano et al. / Precambrian Research 130 (2004) Table 3 U Pb results by LA-MC-ICPMS Group number 238 U/ 206 Pb a ±%(1σ) 207 Pb/ 206 Pb b ±%(1σ) Ages (Ma) Disc. (%) 238 U/ 206 Pb 207 Pb/ 206 Pb ± Ma (2σ) Internal Domain Passos Nappe Araxá Group, Furnas Quartzite; Unit B; sample Passos Nappe Araxá Group, micaceous quartzite; Unit E; sample

12 38 C.M. Valeriano et al. / Precambrian Research 130 (2004) Table 3 (Continued ) Group number 238 U/ 206 Pb a ±%(1σ) 207 Pb/ 206 Pb b ±%(1σ) Ages (Ma) Disc. (%) 238 U/ 206 Pb 207 Pb/ 206 Pb ± Ma (2σ) Passos Nappe Araxá Group, quartzite; Unit H; sample External Domain Northern segment Canastra Group, quartzite; sample HS

13 C.M. Valeriano et al. / Precambrian Research 130 (2004) Table 3 (Continued ) Group number 238 U/ 206 Pb a ±%(1σ) 207 Pb/ 206 Pb b ±%(1σ) Ages (Ma) Disc. (%) 238 U/ 206 Pb 207 Pb/ 206 Pb ± Ma (2σ) External Domain Furnas segment Serra da Boa Esperança Sequence; quartzite; sample Serra da Boa Esperança Sequence; meta-arkose; sample

14 40 C.M. Valeriano et al. / Precambrian Research 130 (2004) Table 3 (Continued ) Group number 238 U/ 206 Pb a ±%(1σ) 207 Pb/ 206 Pb b ±%(1σ) Ages (Ma) Disc. (%) 238 U/ 206 Pb 207 Pb/ 206 Pb ± Ma (2σ) Serra da Boa Esperança Sequence (Serra da Mamona unit); metaconglomerate; sample Andrelândia Group; quartzite; sample ITA

15 C.M. Valeriano et al. / Precambrian Research 130 (2004) Table 3 (Continued ) Group number 238 U/ 206 Pb a ±%(1σ) 207 Pb/ 206 Pb b ±%(1σ) Ages (Ma) Disc. (%) 238 U/ 206 Pb 207 Pb/ 206 Pb ± Ma (2σ) Andrelândia Group; quartzite; sample ITA a Values corrected for U fractionation relative to 238 U/ 235 U = b Measured ratios.

16 42 C.M. Valeriano et al. / Precambrian Research 130 (2004) affected by greenschist facies metamorphism whilst the intermediate and the upper levels are at amphibolite and granulite facies, respectively. Results will be presented from the lower to the upper levels of the nappe. Fig. 3. Concordia diagram for the Serra Velha Granite (sample 486), Araxá Nappe. penetrative shear zones formed during the main compressive deformation phase (D 2 ), the late stages of which resulted in the transport of the Araxá Nappe. Zircons extracted from the Serra Velha Granite are small (<75 m), euhedral, vary in shape from equant to narrow, elongated, needle-like prisms and are generally cracked and contain inclusions. A group of six needle-shaped zircons (analysis 486-4) is concordant at 639 ± 1 Ma and a sub-idiomorphic monazite grain (analysis 486-1) yielded an identical within error age of / 12 Ma (Table 2, Fig. 3); a concordia age (Ludwig, 1998) for both analyses is 637 ± 1 Ma. This is the best estimate for the age of crystallisation of the Serra Velha Granite, and hence for the syn-collisional emplacement of the Araxá Nappe. A minimum age of 830 Ma was obtained for another fraction of nine similar zircons (486-2) indicating the presence of inheritance. A discordia between this analysis and 637 Ma points to 1031 Ma, the probable age of inheritance. Four other analyses (not shown) were carried out on inclusion-bearing and fractured zircons but high levels of common Pb rendered them useless for precise dating Passos Nappe Nine samples from the Araxá Group in this nappe were collected from the lower to the upper structural levels (named units A to I, Fig. 2). The lower level was Unit B Furnas Quartzite The Furnas Quartzite is a 50 m thick layer of white, fine-grained, laminated and micaceous metarenite, which forms the highest ridges of the Passos Nappe. Millimetric dark laminae in this unit contain an accumulation of heavy minerals (placers) and are represented by sample Except for subordinate small euhedral grains, detrital zircons from this sample are conspicuously well rounded and pitted and are devoid of metamorphic overgrowth. A total of 24 detrital grains were analysed, 8 by ID TIMS and 16 by LA-MC-ICPMS. Six of the zircons dated by ID TIMS (Table 2, Fig. 4a) yielded 207 Pb/ 206 Pb ages between 2144 and 1919 Ma and were most likely derived from rocks formed or metamorphosed during the Transamazonian Orogeny. Another one yielded an age of 2480 Ma the significance of which will be discussed in the following. The largest of the euhedral zircons yielded a concordant age of 1335 Ma with a relatively large error in the 207 Pb/ 206 Pb value, the / 7Ma 206 Pb/ 238 U age being more reliable. Concordant zircons of Mesoproterozoic age were also found in samples described in the following and are a significant finding. A rounded and pitted monazite grain, obviously detrital (1032-8M), yielded a 207 Pb/ 206 Pb age of 2117 Ma ( 1.8% discordant). This result indicates that low grade metamorphism, (biotite zone of the greenschist facies) at the base of the Passos Nappe did not disturb the U Pb system of detrital monazite. Thirteen zircons dated by LA-MC-ICPMS cluster in the Ga (Table 3, Fig. 4b) interval and are also taken to be derived from units related to the Transamazonian Orogeny. One zircon yielded a minimum age of 2388 Ma and two others yielded minimum ages of 3362 and 3381 Ma. Detrital zircons with ages in the Ga interval were also found in the Minas Supergroup (Quadrilátero Ferrífero area; Machado et al., 1996a,b) but an igneous or metamorphic event of this age is not known in the southern São Francisco Craton. It could also be argued that zircons of this age are Archean and were affected by Transamazonian

17 C.M. Valeriano et al. / Precambrian Research 130 (2004) Fig. 5. Concordia diagram pertaining to Unit E garnet plagioclase biotite schist (sample 1131), Araxá Group, Passos Nappe. Francisco Craton in the state of Bahia (Pinto et al., 1998; Nutman and Cordani, 1993; Martin et al., 1997; Leal et al., 2003). Fig. 4. Concordia diagrams for detrital zircon and monazite from the Furnas Quartzite (sample 1032, Unit B), Araxá Group, Passos Nappe. (a) ID TIMS results in normal concordia diagram (Wetherill, 1956); (b) LA-MC-ICPMS results in inverted concordia diagram (Tera and Wasserburg, 1972). metamorphism at ca Ga (Machado et al., 1992). However, the fact that some of them are concordant or close to concordia (e.g ) and above a Ga discordia, rather suggests that they were formed at ca Ga. Therefore, it can only be stated that an unknown source of this age is most likely present in the São Francisco Craton. The two Archean zircons (ages greater than 3381 and 3362 Ma) are older than the oldest documented rock in the southern São Francisco Craton, an anorthositic layer in a layered intrusion in the Piumhi greenstone belt ( / 7 Ma; Machado and Schrank, 1989). They are in the range of ages of zircon from detrital rocks of the Rio das Velhas Supergroup and the Minas Supergroup (Quadrilátero Ferrífero area; Machado et al., 1996a,b) in the southern sector of the craton, and for units of the São Unit E garnet plagioclase biotite schist Sample 1131 is representative of this unit which is in lower amphibolite facies. A euhedral monazite crystal yielded a minimum age of 615 Ma (1.2% discordant), whilst a rounded and frosted one yielded a minimum age of 624 Ma and is more discordant (2.5%; Table 2, Fig. 5). Given the morphological characteristics of the monazite grains, the 615 Ma age is interpreted as the approximate age of metamorphism. The age of the rounded monazite is taken to indicate that the mineral is older and was affected by the ca. 615 Ma metamorphism. These data indicate that the U Pb system in monazite may be open in amphibolite facies metamorphism, at ca. 764 C(Simões, 1995) in agreement with Heaman and Parrish (1991) estimate. A single prismatic detrital zircon has low U contents (60 ppm, Table 2), yields a 206 Pb/ 238 U age of 734 ± 4 Ma and a concordia age of 730 ± 3 Ma Unit E micaceous quartzite Sample 1040 was collected from a coarse-grained garnet biotite muscovite quartzite in amphibolite facies. The zircons extracted from this sample are rounded and pitted but smooth and display incipient pyramidal outgrowths. This is the lowest level of the Passos Nappe where zircon overgrowths are observed. Those features indicate that the metamorphic

18 44 C.M. Valeriano et al. / Precambrian Research 130 (2004) for LA-MC-ICPMS analysis and the smaller ones for ID TIMS. Those zircons dated between 2053 and 1863 Ma are inferred to be from Transamazonian sources, and those dated 1756 Ma are most likely derived from anorogenic igneous rocks related to the Espinhaço rift system of the São Francisco Craton (Machado et al., 1989). The sources of zircons with Mesoproterozoic ages ranging from 1565 to 1224 Ma are intriguing since no rocks of these ages are known in the São Francisco Craton or in the surrounding areas. However, these ages are significant as they indicate for the first time the presence of Mesoproterozoic magmatism, possibly of anorogenic character. Although it could be argued that some of these zircons are older and were affected by Neoproterozoic metamorphism, the fact that some grains are concordant (e.g. 1565, 1506 and 1223 Ma) supports the inference that they are derived from rocks of these ages. The youngest concordant zircon, dated at ca. 907 Ma provides the first reliable maximum age of sedimentation for the Araxá Group. Fig. 6. Concordia diagrams pertaining to detrital zircon from Unit E quartzite (sample 1040), Araxá Group, Passos Nappe. conditions affecting the quartzite were compatible with the generation of new zircon. Seven detrital zircons were analysed by ID TIMS (Table 2, Fig. 6a). One yielded a concordant age of / 20 Ma, the large error being caused probably by high common Pb. The other six yielded ages between / 9 Ma and 1160 Ma, one of which is concordant at / 5 Ma. Twenty-one zircons were analysed by LA-MC- ICPMS (Table 3, Fig. 6b) which yielded 207 Pb/ 206 Pb ages between 2463 and 907 Ma. Nine are concordant at 1924 ± 11 Ma, 1806 ± 6 Ma, 1794 ± 11 Ma, 1756±11 Ma, 1756±9 Ma, 1565±5 Ma, 1506±8 Ma, 1224±9 Ma and / 72 Ma. It is observed that the variety of ages obtained by this method is greater than that obtained by ID TIMS. This is attributed in part to the greater number of grains analysed and in part to the fact that the largest grains were selected Unit E micaceous quartzite Sample 1041 is from a quartz-schist (impure metapsammite) intercalation within predominant garnet biotite muscovite metapelitic schists of Unit E, in the intermediate portion of the Passos Nappe. Zircon grains from this sample are also typically detrital rounded and pitted and also display incipient pyramidal outgrowths. In addition, whilst in the samples from lower grade rocks coloured zircons were common, in this sample all zircons are colourless to pale yellow, suggesting that metamorphic heating discoloured them, as has been observed in other areas (Machado, unpublished). A colourless, small zircon was analysed by ID TIMS (Table 2, Fig. 7a) and yielded a minimum age of 2152 Ma (3.2% discordant). Most of the 20 grains analysed by LA-MC-ICPMS (Table 3, Fig. 7b) yielded ages clustering in the Ma interval and are taken to be derived from rocks formed or metamorphosed during the Transamazonian Orogeny. The youngest grain yielded a minimum age of 1039 Ma (6.7% discordant) Unit G amphibolite Rutile was separated from one of several amphibolite layers intercalated in the predominantly detrital

19 C.M. Valeriano et al. / Precambrian Research 130 (2004) Fig. 8. Concordia diagram for rutile from Unit G amphibolite (sample 1036) and Unit H amphibolite (sample 1038), Araxá Group, Passos Nappe. Fig. 7. Concordia diagrams pertaining to detrital zircon from Unit E quartzite (sample 1041), Araxá Group, Passos Nappe. Araxá Group. It is therefore inferred that mafic magmatism was contemporaneous with sedimentation. A fraction of acicular, bright yellow rutile crystals extracted from sample 1038 yielded a 206 Pb/ 238 U concordant age / 45 Ma and another one of short, prismatic brown crystals an age of / 34 Ma (1038-2, ; Table 2, Fig. 8). Both fractions are very poor in U (1 and 0.3 ppm) and yield relatively high errors. The younger age is identical to the age of a rutile fraction from another amphibolite intercalation further up in the section (see in the following). The older age could indicate that the acicular rutile was part of the primary igneous assemblage in which case it may constrain the age of deposition of the Araxá Group. It is in the Ga range for ages of mafic magmatism in and around the São Francisco Craton (e.g. Chaves et al., 1997; Pedrosa-Soares et al., 1998; Machado et al., 1989) the significance of which will be discussed in the following (see Section 7). The ca. 595 Ma age for the short-prismatic rutile fraction suggests that this type of rutile grew during the retrometamorphism associated with the exhumation of the Passos Nappe. It falls in the Ma range of the youngest K Ar determinations on mica from the Passos Nappe (Valeriano et al., 2000). It is also noteworthy the presence of two types of rutile with different ages. This may indicate that metamorphic grade was not high enough or that duration of the metamorphism was too short-lived to reset the older rutile. The possibility should also be considered that the older rutile may have been included in other minerals and shielded from metamorphic reactions Unit G quartzite Sample 1039 represents a coarse-grained quartzite intercalated in predominant metapelitic schists of Unit G. One small, rounded and frosted monazite grain was analysed by ID TIMS (1039-2, Table 2, Fig. 9b) and yielded a minimum age of 630 Ma (5.5% discordant) Unit G leucosome in Araxá Group Sample 1081 is from one of many sigmoidal leucosome lenses within paragneiss and feldspathic garnet muscovite biotite schist from the upper portions of the Araxá Group. These lenses are centimetric to decimetric in size and are composed of K-feldspar, quartz, oligoclase, garnet and corundum. They are syn-tectonic to the main foliation (S 2 ), with indication of top to southeast tectonic transport given by sigmoidal trails (Fig. 9a). The D 2 deformation phase

20 46 C.M. Valeriano et al. / Precambrian Research 130 (2004) upper intercepts of 914, 863 and 794 Ma, and are similar to the ages of the youngest detrital zircons found in the metasediments. Fig. 9. (a) Photograph of sigmoidal leucosome veins (sample 1081) in Unit H schists, Araxá Group, Passos Nappe; (b) concordia diagram pertaining to zircon and monazite from sample is related with thrusting and exhumation of the Passos Nappe, when anatexis of metasediments took place in rocks of appropriate compositions. Two monazite crystals were analysed by ID TIMS, one euhedral, the other rounded. The former (1081-4, Table 2, Fig. 9b) yielded a concordant age of 631 ± 4 Ma, and the other (1081-1) a 206 Pb/ 238 U age of 606 ± 2 Ma. These ages are similar to other ages already mentioned for monazite and zircon and are considered to be related to two events of mineral growth (see Section 7). Four fractions of tiny, sub-equant to equant zircon are all discordant and yielded minimum ages between 730 (two analyses) and 686 Ma (Table 2). Assuming that the discordance is caused mainly by the ca. 630 Ma metamorphic event, the probable ages of inheritance can be estimated by discordia lines between the 631 Ma monazite and these fractions. They point to Unit H orthoquartzite at the top of Passos Nappe Sample 1042 is from a coarse-grained, well recrystallized orthoquartzite intercalation within paragneisses of Unit H, in the upper portions of the Passos Nappe. Metamorphic conditions attained the amphibolite to granulite facies transition. The predominant zircon morphology is represented by grains with well preserved rounded, presumably detrital cores and distinct euhedral overgrowths. Small equant, euhedral, multifaceted and colourless crystals typical of granulite facies rocks are less abundant. Grains containing two or three cores surrounded by new zircon (synneusis) are also observed. Five small equant grains were selected for ID TIMS analysis (Table 2). In spite of extended abrasion, all five analyses are discordant (12 44%) and yield minimum ages between 2057 and 1697 Ma (Fig. 10a). Cores were not observed when selecting these crystals under a binocular microscope but cathodoluminescence imaging of several crystals of this type carried out after the isotopic analysis, revealed the presence of cores and may explain the observed pattern of discordance. Even if it is postulated that the cores are most likely of detrital origin, it is worth to note that the three most concordant analyses define a reliable discordia with intercepts of 2123 ± 5 Ma and 656 ± 16 Ma. This is compatible with the possibility that these zircons are derived from a single source of Transamazonian age which underwent Neoproterozoic metamorphism. Two euhedral monazite crystals were analysed by ID TIMS: one is sub-concordant at / 2 Ma and the other is concordant at / 8Ma (Fig. 10a). These ages help constrain the metamorphic history of the Passos Nappe and will be discussed in the following. The detrital cores of 31 zircons were analysed by LA-MC-ICPMS and with the exception of an Archean grain they scatter along a discordia band between ca. 2.2 and 0.6 Ga (Fig. 10b). Similarly to the zircons analysed by TIMS, this indicates derivation from a Paleoproterozoic source and strong Pb loss due to Neoproterozoic high grade metamorphism.

21 C.M. Valeriano et al. / Precambrian Research 130 (2004) Fig. 11. Concordia diagram for detrital zircon from Canastra Group quartzite (sample HS41), External Domain. Fig. 10. Concordia diagrams pertaining to zircon and monazite from Unit H quartzite (sample 1042), Araxá Group, Passos Nappe Unit H amphibolite An amphibolitic intercalation in the Araxá Group similar to that sampled at lower metamorphic grade (Unit G, sample 1038) yielded orange, short prisms of rutile with low U contents and a 206 Pb/ 238 U age of / 13 Ma (Table 2, Fig. 8). This age is similar to the ca. 595 Ma age found for rutile of sample 1038 and is interpreted as dating the retrometamorphism associated with the exhumation of the Passos Nappe. 6. External Domain 6.1. Northern segment The area east of the Araxá Nappe is in the classical region where the Canastra Group was originally defined by Barbosa (1954). A fine-grained orthoquartzite with millimetric dark layers of heavy minerals, metamorphosed under greenschist facies conditions, was collected from the upper portion of the Canastra Group (sample HS41). Colourless to dark pink-purple zircons are sub-rounded to well rounded, pitted and devoid of metamorphic overgrowth. The ages of 20 grains analysed by LA-MC-ICPMS (Table 3, Fig. 11) range between 2875 ± 3Ma and 1226 ± 8 Ma. Archean and Paleoproterozoic sources have already been referred to above for the Araxá Group metasediments. The Ma sub-concordant ages aare similar to those found in the Araxá Group and confirms the previous inference for the occurrence of Mesoproterozoic sources in the São Francisco Craton. The youngest zircon (1226 ± 8 Ma) provides an upper limit for the sedimentation of the Canastra Group Furnas segment The External Domain to the east of the Passos Nappe consists of an intensely imbricated thrust system (Ilicínea-Piumhi thrust system, Valeriano et al., 1995) comprising from bottom to top sheets of felsic metavolcanic rocks (Costas schist), metasedimentary sequences (Serra da Boa Esperança Sequence; Valeriano, 1992), Archean granite greenstone basement (Piumhi greenstone belt, Schrank, 1982) and chromitite-bearing serpentinites, which are in turn overlain by an upward-coarsening metasedimentary sequence (Serra da Mamona unit). The age of the metasedimentary sequences is poorly constrained by ca. 600 Ma K Ar ages on mica (Valeriano et al.,

22 48 C.M. Valeriano et al. / Precambrian Research 130 (2004) ). All rock units are in the chlorite zone of the greenschist facies. Results are presented from the lower to the upper structural levels Felsic metavolcanic unit (Costas schist) This unit forms a tectonic lens located along the basal thrust of the External Domain, sandwiched between the autochthonous Bambuí Group and the Serra da Boa Esperança metasedimentary sequence. It is a homogeneous felsic metavolcanic rock (sample 1130) composed of quartz, plagioclase, muscovite, epidote and chlorite. Zircons extracted from this sample belong to a single type characterized by euhedral, colourless to pale yellow prisms with aspect ratio ranging from 1:2 to 1:4. Three of four analyses define a discordia with intercepts at 1721 ± 9 Ma and 655 ± 4Ma (Table 2, Fig. 12). One analysis falls below the discordia possibly because of insufficient air abrasion. The upper intercept age agrees with previous ages for felsic magmatism related to the Statherian taphrogenetic events such as those documented for the Espinhaço Supergroup (Machado et al., 1989) and Araí Group (Pimentel et al., 1991). The 655 ± 4 Ma age is inferred to date a metamorphic event, and is remarkably coincident with a K Ar age of 659 ± 8Ma that was obtained from coarse muscovite from the same outcrop (Valeriano et al., 2000) Metasedimentary sequence (Serra da Boa Esperança) The basal thrust sheet of this sequence comprises grey, coarse-grained micaceous metarenite from which Fig. 12. Concordia diagram for zircon from felsic metavolcanic rock (Costas schist, sample 1130), External Domain. Fig. 13. Concordia diagrams pertaining to detrital zircon from quartzite from Serra da Boa Esperança Sequence (sample 1044), External Domain. two samples were collected (1044 and 1046). Zircons extracted from the first sample are colourless or exhibit different shades of pink and display typically detrital morphological features. Two analyses by ID TIMS yielded minimum ages of 2081 and 1946 Ma (both 2% discordant), a third one is concordant at 1445 ± 2Ma (Table 2, Fig. 13a). Twenty four analyses by LA-MC- ICPMS cluster in the following age groups (Table 3, Fig. 13b). An Archean group ( 207 Pb/ 206 Pb ages of Ma); three Paleoproterozoic groups ( and Ma and concordant analysis at 1728 ± 28 Ma); a Mesoproterozoic group with six grains between 1397 and 1331 Ma and a group of Neoproterozoic grains the two most concordant yielding 207 Pb/ 206 Pb ages of 107 and 989 Ma. Sample 1046 was collected from another outcrop of the same lithology and tectonic position as

23 C.M. Valeriano et al. / Precambrian Research 130 (2004) sample A pink and a colourless rounded detrital zircons were analysed by ID TIMS and yielded minimum ages of 1948 and 1396 Ma (Table 2, Fig. 13a). Sample 1128 is a meta-arkose that belongs to a unit of tectonic mélange formed along the sole thrust of the External Domain (near the locality of Santo Hilário), where the basal thrusts sheets of the Serra da Boa Esperança metapsammites are thrusted over the autochthonous Bambuí Group. This unit is heterogeneously deformed, with intensely sheared granite and quartzite fragments within a groundmass of metapelite or feldspathic metarenite. These characteristics suggest syn-orogenic sedimentation related to the exhumation of the external allochthons, followed by their erosion and incorporation of the debris along the sole thrust. The results of two single-grain analyses by ID TIMS (Table 2, Fig. 13a) and of 25 by LA-MC-ICPMS (Table 3, Fig. 14) point to different proportions of the same age groups as was the case for the previous samples. One of the grains is concordant at 1234 ± 7 Ma and the youngest is 1086 Ma old (8.7% discordant). The Archean and Paleoproterozoic provenance ages are similar to those found for the Araxá Group and can be attributed to the sources previously mentioned. It is worth noting the Ga old zircons were also found in the Araxá Group and further substantiate the occurrence of a magmatic (or metamorphic) event in that age range. The 1086 and 989 Ma old zircons bracket the upper limit for the sedimentation of the Serra da Boa Esperança Sequence Archean basement The Piumhi thrust sheet is a fragment of a typical Archean granite greenstone association (Schrank, 1982; Machado and Schrank, 1989). Two units were sampled: a coarse-grained alkali-granite (Taquari granite, sample 71-1) that outcrops in the north of the thrust sheet and a hornblende-bearing orthogneiss (sample 1129) with granodioritic composition occurring in the south (Fig. 2). The granite displays intrusive contact relationships with the Piumhi mafic ultramafic volcanic sequence. In the least deformed and metamorphosed samples the primary mineralogy can be observed to include K-feldspar, quartz, biotite (altered to chlorite) and plagioclase. Fluorite, zircon and titanite are found as accessory minerals. An ID TIMS analysis of brown titanite (sample 71-1T) is 10% discordant with a 207 Pb/ 206 Pb age of 3019 Ma (Table 2, Fig. 15). Zircon extracted from the hornblende-orthogneiss is pink, euhedral and vary in shape from subequant to 2:1 prisms. Three analyses (Table 2, Fig. 15) define a discordia with an upper intercept 2935±13 Ma which is interpreted as the crystallisation age of the protolith. This age is younger than the 3116 Ma age for a mafic layered intrusion (Machado and Schrank, 1989), and indicates that Archean magmatic activity in the Piumhi Fig. 14. Concordia diagram for detrital zircon from meta-arkose from Serra da Boa Esperança Sequence (sample 1128), External Domain. Fig. 15. Concordia diagram for zircon from a hornblende gneiss (sample 1129) and for titanite from the Taquari granite (sample 71), both samples from the Piumhi granite greenstone thrust sheet, External Domain.

24 50 C.M. Valeriano et al. / Precambrian Research 130 (2004) granite greenstone belt lasted for at least 280 million years, considerably longer than previously thought Serra da Mamona unit The uppermost thrust sheet of the External Domain is separated from those below by a tectonic lens composed of chromitite-bearing serpentinites and talc-schists. It is represented the Serra da Mamona Unit, an upward-coarsening metasedimentary unit (Serra da Mamona unit; Valeriano, 1992) with basal intercalations of banded iron formation (BIF) within carbonaceous metapelite which grade upwards to quartzitic metarenite and overlying metaconglomerates. Characteristically rounded quartz pebbles of the latter indicate the degree of sedimentary maturity of this unit. Abundant metapelite and BIF intraclasts are also present. Detrital zircons extracted from a metaconglomerate layer (sample 1084) are typically pink-coloured, rounded and pitted. Two ID TIMS analyses (Table 2, Fig. 13a) yielded minimum ages of 2976 Ma (15% discordant) and 2817 Ma (2.2% discordant). The ages of 24 zircons dated by LA-MC-ICPMS (Table 3, Fig. 16) are between 3060 ± 7 Ma and 2685 Ma, including another concordant zircon at 2906±2 Ma. The exception is a grain with a minimum age of 2197 Ma (20% discordant) with a probable age between 2.2 and 2.3 Ga (depending on whether it was affected or not by Neoproterozoic metamorphism at 0.6 Ga) indicating that deposition of the Serra da Mamona unit could be Paleoproterozoic or younger. Fig. 16. Concordia diagram for detrital zircon from metaconglomerate from Serra da Boa Esperança Sequence (sample 1084), External Domain Southern segment The southern segment of the External Domain constitutes a transition zone between the autochthonous southernmost São Francisco Craton and the thin skinned thrust system of the Brasília belt (Fig. 1). It is composed of both autochthonous and allochthonous units of the predominantly metasedimentary Andrelândia Group and its Archean Paleoproterozoic basement. The age of sedimentation of the Andrelândia Group is limited by the Paleoproterozoic age of its basement and by a ca. 567 Ma discordia lower intercept (Söllner and Trouw, 1997). Samples ITA-2 and ITA-3 are from a thin thrust sheet of the Andrelândia Group whilst ITA-1 is from the underlying intensely folded but autochthonous Andrelândia Group. The three samples are orthoquartzites representing typical units of this group and are in lower amphibolite facies. The morphological features of the zircons extracted from this sample indicate that they are typical detrital grains. Thirty zircon analyses by LA-MC-ICPMS (sample ITA-1, Table 3, Fig. 17a), yielded ages clustering in the same Archean and Paleoproterozoic groups already mentioned. A series of concordant zircons in the Ma range was also found. Given that the unit sampled is autochthonous and that zircons shown no overgrowth, these ages irrefutably document the presence of Mesoproterozoic sources in the São Francisco Craton. In addition, zircons with ages in the Ga range, including a concordant one at / 40 Ma, are the best estimate for the upper limit for the sedimentation of the Andrelândia Group. Zircons from the other sample from the allochthonous Andrelândia Group (ITA-3) analysed by the same method yielded a similar pattern of ages (Table 3, Fig. 17b). Two rounded monazite grains with rough surface were analysed from the same klippe of Andrelândia Group quartzites. One yielded a minimum age of 588 Ma (1.7% discordant, ITA3-2) and the other 2027 Ma (16% discordant, ITA2-1, Table 2, Fig. 17c). A discordia line constructed with the two grains yields an upper intercept at 2104 Ma and indicates that the latter is probably detrital and was affected by Neoproterozoic metamorphism. It is noted that in spite of the growth of new monazite in sample ITA3, the detrital zircons analysed do not show Pb loss related to the Neoproterozoic metamorphism.

25 C.M. Valeriano et al. / Precambrian Research 130 (2004) Discussion and conclusions 7.1. Provenance ages and implications The Araxá, Canastra and Andrelândia groups are consensually interpreted as part of the sedimentary succession of the Neoproterozoic passive margin bordering the western and southern São Francisco Craton (Dardenne, 2000; Trouw et al., 2000). Because of the lack of constraints on the age of sedimentation and reasonable stratigraphic correlation, the Serra da Boa Esperança Sequence was individualised (Valeriano et al., 2000). A common feature of the clastic rocks from these units is the similarity of zircon provenance ages. As already mentioned, the Archean and Paleoproterozoic ages are compatible with erosion of the São Francisco Craton. Particularly interesting are the Mesoproterozoic provenance ages between ca. 1.2 and 1.6 Ga, with a cluster at ca. 1.3 Ga (Fig. 18). It is worth noting that among zircons with ages in this range, several are concordant to sub-concordant, indicating that these ages are not due to the effect of Neoproterozoic metamorphism on Archean Paleoproterozoic grains. Therefore, one of the main conclusions of this work is that Mesoproterozoic sources must be present in the São Francisco Craton that have yet to be identified. These sources may be located underneath the Bambuí Group, which covers most of the southern São Francisco Craton. The occurrence of Mesoproterozoic anorogenic magmatism is compatible with pre-bambuí rift structures recognised by seismic methods (Braun et al., 1993; Fig. 17. Concordia diagrams pertaining to Andrelândia Group quartzites. (a) Detrital zircon from autochthonous domain quartzite (sample ITA1); (b) detrital zircon from allochthonous domain quartzite (sample ITA3); (c) monazite from allochthonous domain quartzites (samples ITA2 and ITA3), External Domain. The data for this sample thus bracket the sedimentation of the Andrelândia Group in the Neoproterozoic between the age of the youngest detrital zircon ca Ma and the age of metamorphism at ca. 588 Ma. Fig. 18. Histogram depicting the 207 Pb/ 206 Pb ages for detrital zircons from clastic rocks from the southern Brasília belt. Only concordant ages are plotted.