Márcio Martins Pimentel

Neoproterozoic crustal accretion in central Brazil

Recent geochronological studies have demonstrated the existence of several Neoproterozoic orthogneiss and metavolcanic are suites (ca. 900 to 600 Ma) in the southern part of the Tocantins province, a major Neoproterozoic orogenic area in central Brazil, exposed between the Amazon and Sao Francisco cratons. Nd and Sr isotopic characteristics of these metamorphic suites are primitive: initial 87 Sr/ 87 Sr ratios vary from ∼0.7024 to ∼0.7042 and initial ϵ Nd values are positive, varying between +0.2 and +6.9. The most primitive initial Sr and Nd isotopic compositions are very similar to model depleted- mantle compositions at the time of formation of the original magmas. The mantle- like characteristics of these are suites in western Goias contradict previous ensialic evolution models for the Tocantins province and reveal that crustal accretion processes were important in large areas of central Brazil during Neoproterozoic time.

The osmium isotopic composition of convecting upper mantle deduced from ophiolite chromites

Chromites separated from the upper mantle or lower crustal portions of 18 ophiolites ranging in age from 900 Ma to 50 Ma are examined for Re-Os isotopic systematics. The ophiolites include both MORB and back arc types, although most are from supra-subduction zone (SSZ) settings. The chromites are robust indicators of the initial Os isotopic compositions of the systems sampled. There is very limited range in calculated initial γOs values, with the entire group averaging +1.31. Least squares linear regression of the age of chromite formation (in Ga) versus initial 187Os/188Os of a filtered suite yields a slope of −0.0058±0.0019 (2σ) and a present day intercept of 0.12809±0.00085 (2σ), equivalent to a γOs value of +0.9±0.6. Of the suite of 51 samples analyzed, 68% lie within ±1% of this evolution trajectory. Although most of the samples formed in SSZ environments, there is little evidence to suggest modification of the mantle Os isotopic composition via radiogenic melts or fluids derived from subducting slabs. The ophiolite data are interpreted as representative of the convecting upper mantle and suggest that the present isotopic composition of the convecting upper mantle averages approximately 1.2% less radiogenic than the estimated minimum composition of the primitive upper mantle of 0.1296±8 (Meisel et al., 2001). The most likely explanation for the difference is the formation, subduction and isolation of some portion of the mafic oceanic crust. Using models based on the assumption that the convecting upper mantle comprises 50% of the total mass of the mantle, and that the average isolation period for subducted oceanic crust is 1.5 to 2.0 Ga, it is estimated that approximately 2 to 3% of the total mass of the mantle is composed of subducted mafic oceanic crust that remains isolated from the convecting upper mantle. Because the isotopic compositions of the DMM and PUM overlap within uncertainties, however, the results do not require any isolated slab component.

The Sm-Nd isotopic method in the geochronology laboratory of the University of Brasília

Nd isotopes represent one of the best tools to investigate the processes involved in the evolution of the continental crust and mantle. This is due mainly to the similar geochemical behaviour of Sm and Nd, both light rare earth elements, which inhibits their fractionation during most varied geological processes. In order to carry out crustal evolution studies in central Brazil, the Sm-Nd isotopic method was implanted at the Geochronology Laboratory of the University of Brasília. The Sm-Nd separation methodology is basically that described in Richard et al. (1976), with the addition of some improvements. In this study we describe in detail the methodology used in Brasília. Precision and accuracy were checked with the international standards such as JB-3, BCR-1, BHVO-1 and La Jolla, and the following results were found: JB-3 (Nd=15.74 ppm and Sm=4.28 ppm), BCR-1 (143Nd/144Nd=0.512647+/-8, Nd=28.73 ppm and Sm=6.66 ppm), BHVO-1 (Nd=24.83 ppm and Sm=6.2 ppm) and La Jolla (143Nd/144Nd=0.511835+/-14).

Granites and the geodynamic history of the neoproterozoic Brası́lia belt, Central Brazil: a review

Abstract Recent field and geochronological studies have demonstrated the importance of granitic magmatism in the evolution of the Neoproterozoic Brasilia Belt, in Central Brazil. This is an orogenic belt developed in response to the convergence between the Amazon, Sao Francisco–Congo and Parana continental blocks. The presence of Neoproterozoic juvenile arc rocks and syn-collisional peraluminous granites challenged previous intracontinental evolution models for the belt. The granitoid intrusions reviewed in this paper record the different stages of evolution of the orogen and their field and isotopic characteristics can be used to reconstruct the tectonic history of the belt. The main field and isotopic characteristics of four granite suites associated with the Brasilia Belt are reviewed: (i) 1.77–1.58 Ga old rift related A-type granite intrusions, (ii) ca. 0.8–0.7 syn-collisional granitoids, (iii) arc metatonalites and metagranodiorites (ca. 0.9 to 0.63 Ga), and (iv) bimodal post-orogenic suite ranging in age from ca. 0.59 to 0.48 Ga. These rocks suggest that during most of the Neoproterozoic the western margin of the Sao Francisco continent faced a large oceanic basin, where subduction and oceanic lithosphere consumption started at ca. 0.9 Ga, roughly coeval with the initial stages of the break up of Rodinia. Final ocean closure happened at ca. 0.63–0.60 Ga with crustal thickening, uplift and erosion. Post-orogenic extension-related magmatism took place between ca. 0.6 and 0.5 Ga and was partially contemporaneous with the deposition of the Paraguay and Tucavaca sedimentary successions, resulting from the rifting event related to the break up of Laurentia from southwestern Gondwana.

High spatial resolution analysis of Pb and U isotopes for geochronology by laser ablation multi-collector inductively coupled plasma mass spectrometry (LA-MC-ICP-MS)

A determinacao de idades atraves do decaimento dos isotopos radioativos 235U e 238U para os isotopos radiogenicos 207Pb e 206Pb, respectivamente, utilizando o mineral zircao (ZrSiO4), e amplamente aplicada para decifrar processos geologicos. Um novo metodo tem sido desenvolvido nos ultimos anos, a ablacao a laser multi-coletor espectrometria de massas com plasma indutivamente acoplado (LA-MC-ICP-MS), superando o laborioso trabalho anteriormente necessario em outros metodos, na preparacao de amostras, e permite obtencao de razoes isotopicas com alta resolucao espacial de micrometros. O presente estudo descreve os procedimentos analiticos e os metodos usados na reducao de dados que estao sendo aplicados no Laboratorio de Geocronologia da Universidade de Brasilia. Explora-se a precisao e exatidao do metodo atraves da analise de tres padroes internacionais de zircao. Observa-se uma precisao entre 1,9 a 3,7% (2σ desvio padrao) e uma exatidao de 0,6 a 3,8% (2σ desvio padrao) para as razoes isotopicas de Pb e U dos padroes. Tambem foram obtidas idades pelo metodo LA-ICP-MS de duas amostras de zircoes naturais, que ja foram datadas anteriormente por outros metodos analiticos. A comparacao dos resultados mostra uma boa conformidade das idades obtidas, dentro dos limites de erro.Os dados demonstram o grande potencial do metodo analitico para analises isotopicas rapidas, precisas e exatas de U-Pb, numa escala de micrometros.

The Mara Rosa Arch in the Tocantins Province: further evidence for Neoproterozoic crustal accretion in Central Brazil

The Mara Rosa volcano-sedimentary sequence consists of several NNE-trending belts of metavolcanic (metabasalts to metarhyolites) and metasedimentary rocks (micaschists, quartzites, cherts), exposed over large areas of the Tocantins Province, a Neoproterozoic (Brasiliano) orogenic region in the central part of Brazil. The supracrustal belts are separated by terrains dominated by metatonalites and metadiorites and intruded by several post-orogenic granitic and dioritic bodies. These volcano-sedimentary and associated metaplutonic rock associations have been previously interpreted as a typical granite-greenstone terrain of the Archaean basement. In this paper we present geochronological and isotopic data for rocks of the metavolcanic-metaplutonic rock association. Samples of a felsic metavolcanic rock from the Posse gold mine, and a metatonalite yielded UPb zircon ages of 862±8 Ma and 856−7+13 Ma, respectively. These are interpreted as crystallization ages of the igneous protoliths. Titanites from the same metavolcanic rock sample yielded a concordant recrystallization age of 632±4 Ma. SmNd isotopes for these rocks indicate primitive compositions, with ϵNd(T) of +4.6 and +3.7 and TDM model ages of ∼1.0 Ga. A syn-tectonic dioritic intrusion has an UPb crystallization age of 630±6 Ma, and ϵNd(t) of +1.9 and TDM of ∼1.0 Ga. Additional SmNd analyses on metasedimentary rocks and post-orogenic granites of the Mara Rosa region yield TDM ages between ∼1.2 and 1.0 Ga. The isotopic data, combined with preliminary trace element results, suggest that the protoliths of the metavolcanic and metaplutonic rocks investigated were formed in an island arc system, off the coast of the Sao Francisco-Congo continent at around 860 Ma. Recrystallization and deformational ages of ∼630 Ma may represent the timing of final ocean closure and continental collision. The rock associations, structural pattern, geochronological and isotopic characteristics of the rocks from the Mara Rosa region are very similar to those of Neoproterozoic arc terrains exposed ∼ 300 km to the southwest, in the Arenspolis area, proving the regional importance of the Neoproterozoic crustal accretion event in the central part of Brazil.

Neoproterozoic backarc basin: Sensitive high-resolution ion microprobe U-Pb and Sm-Nd isotopic evidence from the Eastern Pampean Ranges, Argentina

The Eastern Pampean Ranges comprise high-grade supracrustal sequences with linear belts of mafic-ultramafic bodies representing ophiolite remnants. New U-Pb and Nd isotopic data suggest that the tectonic evolution of the Pampean Ranges started ca. 640 Ma with the deposition of supracrustal sequences in a backarc basin between a Neoproterozoic magmatic arc to the east and the Pampia terrane to the west. Ophiolite remnants of this backarc basin yielded a whole-rock isochron indicating the age of 647 ± 77 Ma (2σ) and ϵNd (initial time [T]) of +5.2. Sensitive high-resolution ion microprobe U-Pb data for detrital metasediments show provenance patterns with two main age populations: the older 1.1–0.9 Ga, and the younger between ca. 0.7 and 0.6 Ga. The Neoproterozoic population is relatively more abundant in sediments of the easternmost units of the Eastern Pampean Ranges and becomes less abundant toward the west. Depleted mantle ages show a similar pattern, with ages generally increasing from east (1.42 Ga) to west (1.76 Ga), suggesting the presence of Neo-proterozoic sources to the east of the ranges. The provenance data do not support previous evolution models for the Eastern Pampean Ranges, according to which the supracrustal sequence represents the passive margin of the Rio de la Plata craton. Early Cambrian collision and high-grade metamorphism mark the final stages of evolution of the belt and were shortly followed by calc-alkaline metaluminous and peraluminous granitic magmatism ca. 530–514 Ma. The results suggest that the geological evolution of the Eastern Pampean Ranges took place between ca. 640 and 514 Ma, coeval with other Brasiliano orogens in Brazil (e.g., the Paraguay and Araguaia fold belts).

Post-Brasiliano (Pan-African) high-K granitic magmatism in Central Brazil: the role of late Precambrian-early Palaeozoic extension

Abstract In western Goias, Brazil, the emplacement of large, high-K postorogenic granites and associated small gabbro-dioritic intrusions, followed immediately after the last deformational events of the Brasiliano-Pan-African orogeny at ∼600 Ma. Well-fitted whole-rock RbSr isochrons indicate ages which suggest two discrete intrusive events: the older between ∼588 and 560 Ma and the younger between ∼508 and 485 Ma. The older granites display general petrographic and geochemical characteristics of highly differentiated calc-alkaline I-type granitoids, whereas the younger intrusions are more alkaline, similar to A-type granites. Initial 87 Sr 86 Sr ratios vary from ∼0.703 to 0.710 and initial Nd isotope ratios yield ϵNd(T) values in the range between −4.0 and +3.0. There are no major differences in initial isotopic compositions between the two granite groups, suggesting that the parental magmas for both groups of rocks mostly originated by refusion of crustal sources isotopically similar to the ∼940-640 Ma basement arc-type metatonalites-metagranodiorites and associated arc metavolcanics. The major and trace element compositional differences between the two granite groups is explained in terms of modifications in the melting conditions within the crust, with younger melts being produced by the refusion of anhydrous, depleted crustal sources left after the extraction of previous batches of more hydrated calc-alkaline magmas. The heat input required to promote extensive remelting of the continental crust was, most likely, provided by mantle-derived mafic magmas that invaded and probably underplated the crust, during uplift and extension. The two intrusive events are bimodal in nature and are interpreted as shallow-level extension-related events associated with regional uplift and denudation occurring just after two orogenic pulses at the end of the Proterozoic and early Palaeozoic, the older at ∼600 Ma and the younger between ∼550 and 510 Ma. The onset of the granite magmatism at ∼590 Ma, shortly post-dating the Brasiliano orogeny (∼600 Ma), is broadly coeval with the initial stages of sedimentation of the terrigenous and carbonatic rocks of the ensialic Paraguay Belt in Brazil and its correlative in Bolivia, the Tucavaca Belt, which probably correspond to rift deposits related to the break up of Laurentia from Gondwana at the end of the Proterozoic and beginning of the Palaeozoic. Granites of the younger group cut the deformational structures of the Paraguay Belt metasediments and pre-date, by between ∼40 and 20 Ma, the initial stages of subsidence and sedimentation of the Parana Basin.

U–Pb detrital zircon ages and Sm–Nd isotopic features in low-grade metasedimentary rocks of the Famatina belt: implications for late Neoproterozoic–early Palaeozoic evolution of the proto-Andean margin of Gondwana

Abstract: The Famatina belt, Central Andes, is part of an ancient accretionary margin built along Western Gondwana in the early Palaeozoic. U–Pb ion microprobe analysis of detrital zircons and Sm–Nd whole-rock analysis of two early Palaeozoic low-grade metasedimentary units record the early evolution of this region. Detrital zircons in the Negro Peinado and Achavil formations have ages ranging from Palaeoproterozoic to Cambrian, consistent with derivation from Gondwanan sources. TDM ages suggest that the sedimentary rocks were derived from a composite source area, which separated from the mantle during the Palaeoproterozoic (c. 1.8–1.6 Ga). Constraints from the youngest detrital grains indicate accumulation in a Mid- to Late Cambrian foreland basin adjacent to the inboard Pampean orogenic tract. The dominance of Cambrian ages in the Negro Peinado Formation suggests derivation principally from the eastern Pampean belt whereas the dominance of late Neoproterozoic ages in the Achavil Formation suggests that input from the Pampean belt was overwhelmed by older sources. The paucity of Palaeoproterozoic ages argues against direct input from older areas such as the Río de la Plata craton. The predominance of Meso- and Neoproterozoic ages over older sources suggests that a Brasiliano-age magmatic arc developed on a Mesoproterozoic basement, probably a southern extension of the Arequipa–Antofalla massif.

Tectonic evolution of the Brasília Belt, Central Brazil, and early assembly of Gondwana

Abstract The Brasília Belt comprises terranes and thrust-sheets that were tectonically transported towards the western passive margin of the São Francisco–Congo palaeocontinent during an orogenic episode resulting from collision of the Paranapanema and Goiás blocks and the Goiás magmatic arc against São Francisco–Congo at 0.64–0.61 Ga. The tectonic zones of the belt are, from east to west: a foreland zone with Archaean–Palaeoproterozoic granite–greenstone basement covered by Neoproterozoic anchimetamorphic sedimentary rocks (Bambuí Group); a low metamorphic grade thrust-fold belt of proximal shelf successions, mostly siliciclastic, containing rare basement slivers; metamorphic nappes in upper greenschist to granulite facies of distal shelf and slope metasediments and subordinate tholeiitic metabasalts; the Goiás massif, possibly a microcontinent; and the Goiás magmatic arc. The accretion of these terranes against the western margin of the São Francisco–Congo palaeocontinent took place during an early phase of Gondwana supercontinent amalgamation, when terranes accreted around São Francisco–Congo to create a proto-West Gondwana landmass, around which subsequent collisional and accretionary events followed, such as those in the Borborema–Trans-Saharan province (c. 0.62–0.60 Ga); in the Ribeira–Araçuaí belt (c. 0.58 Ga); along the Araguaia and Paraguay belts (collision of Amazonia, c. 0.54–0.52 Ga); and the accretion of Cabo Frio terrane in the Ribeira Belt (c. 0.53–0.50 Ga).