THE GOIÁS MAGMÁTIC ARC The nature of the limits between the Archean/Paleoproterozoic terrains of the Goiás Massif and the Neoproterozoic (ca.900 -650 Ma) juvenile terrains of the Goiás Magmatic Are is still poorly understood.In this study we have investigated southwestern and southern áreas of the massif, in the region between the towns of Goiás, Jussara and Mossâmedcs.Several granitic, gneissic and mylonitic rock were recognized and dated (Rb-Sr and Sm-Nd methods) units in the transect between Goiás, situated in the Goiás Massif and Jussara, in the magmatic are.From east to west they are: (i) Uvá tonalitic gneiss (GT) (2564 ± 140 Ma; i.r.= 0.7017 ± 0.0003; T DM =3.0-3.5 Ga); (ii) Uvá granite (GU) (2851 ± 180 Ma; ε Nd (T) = +0.3;T DM = 3.0-3.l Ga); (iii) Itapirapuã mylonitic gneiss; (GMI) (iv) syn-tectonic coarse-grained porphyritic granite (GPG) (759 ± 62 Ma; i.r.= 0.7053 ± 0.0002; T DM = 2.0-2.3Ga); (v) Granitic gneiss (GG) (2601 ± 209 Ma; i.r.= 0.7016 ± 0.0007; T DM = 2.4 -2.5 Ga), (vi) Mylonitic gneiss (GM).GT and GU display predominantly E-W tectonic foliation, with a local and weak superimposed N-S deformation.N-S structures become progressively more important towards the west, with the development of wide dextral shear zones, which are typical in the rocks of the magmatic are.The structural and geochronologic data indicate that the E-W deformational fabric is Archaean or Paleoproterozoic in age, and the N-S fabric is Neoproterozoic.Felsic metavolcanic rocks from the Mossâmedes área, soutlvof the Goiás Archaean terrains, yielded Rb-Sr isochron ages of 1978 ± 55 Ma and 1582 ± 101 Ma, with initial 87 Sr/ 86 Sr ratios of ca.0.7023 and 0.7053, respectively.Therefore, these rocks are not part of the Neoproterozoic magmatic are, and are better interpreted as part of the southern edge of the Goiás Massif.The data presented suggest that the rocks investigated here are part of the southern and southwestern edges of the São Francisco-Congo continental plate.According to this reconstruction the Goiás massif represented an indentation of that plate during the continental collision that originated the Brasília Belt.
SIGNIFICANCE 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 (Brasíliano) orogenic region in central Brazil.The supracrustaJ belts are separated by terranes dominated by metatonalites and metadiorites and intruded by several post-orogenic granitic and dioritic bodies.This metavolcanic/metaplutonic rock association has been previously interpreted as a typical Archaean granite-greenstone terrane and, therefore, part of the sialic basement to the Brasíliano supracrustal fold belt.In this paper we present the first reliable geochronological and isotopic data for rocks of the Mara Rosa sequence and associated metaplutonic rocks.Samples of a felsic metavolcanic rock from the Posse gold mine, and of a metatonalite yielded U-Pb zircon ages of 862 ± 8 Ma and 856 +13/-7 Ma, respectively.These are interpreted as crystallization ages of the igneous protoliths.Titanite from the metavolcanic rock sample yielded a concordant recrystallization age of 632 ± 4 Ma Rb-Sr geochronological data for orthogneissic, metasedimentary and mylonitic rocks indicate the closure of Rb-Sr isotopic system at ca. 600 Ma after isotopic re-homogeneization.Sm-Nd isotopes for these rocks indicate primitive compositions, with ε Nd (T) of +4.6 and +3.7 and TDM model ages of ca.1.0 Ga.A syn-tectonic dioritic intrusion has an U-Pb crystallization age of 630 ± 6 Ma, and ε Nd (T) of +1.9 and T DM of ca.l .0Ga.Additional Sm-Nd analyses of metasedimentary rocks and post-orogenic granites of the Mara Rosa region showed TDM ages between ca.1.2 and 1.0 Ga.The isotopic data, combined with preliminary trace element results, suggest that the protoliths of the investigated metavolcanic and metaplutonic rocks formed at ca. 860 Ma in an island arc system, off the coast of the São Francisco-Congo continent.Recrystallization and deformation ages of ca.630 Ma could 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 ca.300 km to the southwest, in the Arenopolis area, proving the regional importance of the Neoproterozoic crustal accretion event in the central part of Brazil.
Supracrustal rocks of the Araí Group, together with coeval A-type granites represent a ca. 1.77-1.58 Ga old continental rift in Brazil. Two granite families are identified: the older (1.77 Ga) group forms small undeformed plutons, and the younger granites (ca. 1.58 Ga) constitute larger, deformed plutons. Sr-Nd isotopic data for these rocks indicate that the magmatism is mostly product of re-melting of Paleoproterozoic sialic crust. Initial Sr ratios for both granite families are ca 0.726 and 0.720. Most TDM model ages are between 2.58 and 1.80 Ga. epsilonND(T) values are between +3.6 and -11.9. Araí volcanics are bimodal, with basalts and dacites/rhyolites interlayered with continental sediments. The felsic volcanics show Nd isotopic characteristics which are very similar to the granites, and are also interpreted as reworking of Paleoproterozoic crust. Detrital sediments of the Araí Group revealed T DM model ages between 2.4 and 2.16 Ga, indicating that they are the product of erosion of Paleoproterozoic crust. The data indicate that the Araí rift system was established on crust that had just become stable after the Paleoproterozoic orogeny.
Neste trabalho sao apresentados novos dados sobre a proveniencia das rochas metassedimentares do Grupo Ibia na sinforma de Araxa. A secao-tipo deste grupo esta localizada na regiao de Araxa, que faz parte do segmento meridional da Faixa Brasilia. A sinforma de Araxa e uma dobra regional com eixo caindo suavemente para WNW, com flancos de direcao WNW e mergulhos para NNE e SSW, ao longo dos quais afloram as rochas dos grupos Araxa, Ibia e Canastra. Estes grupos estao estruturados em tres lascas tectonicas, separadas entre si por zonas de cisalhamento. A lasca tectonica inferior (Grupo Canastra) e uma sequencia metassedimentar psamo-pelitica, metamorfisada em condicoes de facies xisto verde (zonas da clorita ate granada) com idades modelo Sm-Nd T DM = 2,2 Ga. Ela representa sedimentos depositados em ambiente marinho plataformal. A lasca tectonica superior (Grupo Araxa) compreende uma sequencia metaignea mafica com transicao para rochas metassedimentares, ambas metamorfisadas sob condicoes da facies anfibolito e cortadas por rochas granitoides. As rochas metassedimentares apresentam idades modelo Sm-Nd T DM = 1,9 Ga. Na lasca tectonica intermediaria (Grupo Ibia) uma sequencia metasedimentar pelitica esta arranjada em ritmitos finamente granulados. Tem idades modelo Sm-Nd T DM = 1,1 a 1,3 Ga e foi metamorfisada sob facies xisto verde (zona da clorita). Dados petrograficos e geoquimicos indicam que estes metassedimentos originaram-se a partir da erosao de arcos magmaticos. Os sedimentos foram provavelmente depositados por correntes de turbidez distais em uma bacia de retroarco. O Grupo Ibia e interpretado como um terreno tectonoestratigrafico, com estratigrafia propria e sem relacoes geneticas com os terrenos vizinhos representados pelos Grupos Araxa e Canastra.
ABSTRACT: The Brasília Belt is one of the most complete Neoproterozoic orogens in western Gondwana. Rapid progress on the understanding of the tectonic evolution of the belt was achieved due to new U-Pb data, combined with Sm-Nd and Lu-Hf analyses. The evolution of the Brasília orogen happened over a long period of time (900 - 600 Ma) involving subduction, magmatism and terrain accretion, as a result of the consumption of the Goiás oceanic lithosphere. Provenance studies, based on U-Pb zircon data, indicate that the sedimentary rock units record different tectonic settings and stages of the evolution of the orogen. The Paranoá and Canastra groups represent passive margin sequences derived from the erosion of the São Francisco Craton. The Araxá and Ibiá groups, however, have dominant Neoproterozoic detrital zircon populations, as young as 650 Ma, suggesting derivation from the Goiás Magmatic Arc. The Goiás Magmatic Arc represents a composite arc terrain, formed by the accretion of older (ca. 0.9 - 0.8 Ga) intraoceanic island arc(s), followed by more evolved continental arcs. It extends for several thousand kilometers, from SW Goiás, through NE Brazil and into Africa. Metamorphism took place between 650 - 630 Ma reflecting final closure of the Goiás Ocean and continental collision.