The Tectonic Evolution of the Neoproterozoic Brasília Belt, Central
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DOI: 10.1590/2317-4889201620150004 ARTICLE The tectonic evolution of the Neoproterozoic Brasília Belt, central Brazil: a geochronological and isotopic approach A evolução tectônica da Faixa Brasília, Brasil central: uma abordagem geocronológica e isotópica Márcio Martins Pimentel1* ABSTRACT: The Brasília Belt is one of the most complete Neo- RESUMO: A Faixa Brasília é um dos mais completos orógenos Neopro- proterozoic orogens in western Gondwana. Rapid progress on the terozoicos no Gondwana Ocidental. Durante os últimos dez anos, o rápido understanding of the tectonic evolution of the belt was achieved progresso no entendimento da evolução tectônica desse orógeno foi propor- due to new U-Pb data, combined with Sm-Nd and Lu-Hf analyses. cionado pela produção de novos dados geocronológicos U-Pb SHRIMP e The evolution of the Brasília orogen happened over a long period LA-ICPMS, combinados com análises Sm-Nd e Lu-Hf. Os dados demon- of time (900 – 600 Ma) involving subduction, magmatism and ter- straram que a evolução da Faixa Brasília acontenceu ao longo de um rain accretion, as a result of the consumption of the Goiás oceanic prolongado intervalo de tempo (900 – 600 Ma), envolvendo subducção, lithosphere. Provenance studies, based on U-Pb zircon data, indica- atividade ígnea e acreção de terrenos, como resultado do consumo da li- te that the sedimentary rock units record different tectonic settings tosfera do oceano Goiás. Estudos de proveniência de sedimentos baseados and stages of the evolution of the orogen. The Paranoá and Canastra em geocronologia U-Pb de minerais detríticos indicam que as unidades groups represent passive margin sequences derived from the erosion sedimentares da Faixa Brasília também registram diferentes ambientes of the São Francisco Craton. The Araxá and Ibiá groups, however, tectônicos e estágios de evolução da Faixa. Os grupos Paranoá e Canastra have dominant Neoproterozoic detrital zircon populations, as young representam sequências de margem passiva, derivadas da erosão do Craton as 650 Ma, suggesting derivation from the Goiás Magmatic Arc. The do São Francisco. Por outro lado, as rochas sedimentares dos grupos Araxá Goiás Magmatic Arc represents a composite arc terrain, formed by e Ibiá contêm uma grande quantidade de zircões detríticos neoproterozo- the accretion of older (ca. 0.9 – 0.8 Ga) intraoceanic island arc(s), icos, tão jovens quanto 650 Ma, sugerindo proveniência a partir do Arco followed by more evolved continental arcs. It extends for several Magmático de Goiás. Os dados geocronológicos disponíveis até o momento thousand kilometers, from SW Goiás, through NE Brazil and into demonstram que o Arco Magmático de Goiás, na porção oeste do orógeno, Africa. Metamorphism took place between 650 – 630 Ma reflecting representa um terreno de arco formado pela acreção de arcos de ilha in- final closure of the Goiás Ocean and continental collision. traoceânicos mais antigos (ca. 0.9 – 0.8 Ga), seguida do desenvolvimento KEYWORDS: Brasília Belt; Neoproterozoic; U-Pb geochronology; de arcos continentais mais evoluídos. O arco magmático se extende por Sediment provenance. alguns milhares de quilômetros na direção NNE, desde o sudoeste de Goiás, através do Nordeste brasileiro, se prolongando para o continente africano. Idades de metamorfismo, muito embora não muito abundantes, indicam pico do metamorfismo entre 650 – 630 Ma, o que provavelmente corre- sponde à época de fechamento do oceano e colisão continental. PALAVRAS-CHAVE: Faixa Brasília; Neoproterozoico; Geocronologia U-Pb; Proveniência de sedimentos. 1Institute of Geosciences, Universidade de Brasília, Brasília (DF), Brazil. E-mail: [email protected] *Corresponding author. Manuscript ID: 20150004. Received in: 06/03/2015. Approved in: 11/27/2015. 67 Brazilian Journal of Geology, 46(Suppl 1): 67-82, June 2016 Evolution of the Brasília Belt INTRODUCTION the latest stages of evolution of the Neoproterozoic orogen (Jost et al. 2013). It underlies an area of about 18,000 km2 The Brasília Belt, in central Brazil, is one of the most (Fig. 2) and consists of typical Archean TTG granite-gneiss complete and complex Neoproterozoic orogens in western complexes (Queiroz et al. 2008, Jost et al. 2013), and nar- Gondwana. It formed during the convergence of the Amazonian, row greenstone belt sequences (Fortes et al. 1995, Tassinari São Francisco-Congo, Paranapanema (Rio de La Plata), as well et al. 2006, Santos et al. 2008, Jost et al. 2010). The gran- as smaller allochthonous blocks (Fig. 1). It is part of a wide and ite-gneiss terrains comprise orthogneisses which, in the long orogenic system extending for thousands of kilometers in northern portion of the terrain, comprise the Anta, Caiamar, central and northern Brazil and in NW Africa, in the Hoggar- Moquém and Hidrolina, and, in the south, the Caiçara and Pharusian and Dahomey belts. In Brazil, this orogenic area is Uvá complexes. The metaplutonic rocks have U-Pb ages known as the Tocantins Province, which includes the Brasília ranging between ca. 3.04 and 2.71 Ga and a few Sm-Nd Belt, developed along the western margin of the São Francisco isotopic data obtained by Pimentel et al. (2003) from sam- Craton, as well as the Araguaia and Paraguay belts, formed ples of the southern Uvá and Caiçara complexes yielded along the continental margin of the Amazonian paleoconti- whole-rock model ages of ca. 3.0 Ga. The U-Pb discordias nent. They are all part of the amalgamation of West Gondwana presented by Queiroz et al. (2008) show patterns indicating during the neoproterozoic and eocambrian. strong Neoproterozoic Pb loss. Over the last ten years, a considerable amount of new The greenstone belts successions are made of a basal ultra- U-Pb and Sm-Nd geochronological data has been instru- mafic unit made of komatiites, locally presenting well-pre- mental to understand in greater detail the tectonic evolution served spinifex textures and pillow structures, covered by a of the Brasília Belt. It has been suggested in the literature metabasalt unit with intercalations of chemical and pelitic that the tectonic elements and geochronological data for the metasediments. At the top, a detrital unit made dominantly Brasília Belt record a long and complex history of deforma- of conglomerate, quartzite, metapelite and local marble is tion, metamorphism and accretion of terranes, including recognized. The ages of the komatiite-bearing lower units are island arc terranes, an ophiolitic mélange, high-grade ter- still poorly constrained. One whole-rock Sm-Nd isochron rains and a supracrustal fold-and-thrust belt, consisting of for komatiitic rocks of the Crixás greenstone belt yielded the juxtaposed metasedimentary sequences of various ages and age of ca. 3.0 Ga (Fortes et al. 1995). However, the prov- natures (for recent reviews see Pimentel et al. 2011, Araujo enance study of the upper metasedimentary sequences of et al. 2014, Cordani et al. 2013a, 2013b, Brito Neves et al. the belts revealed the presence of Paleoproterozoic detrital 2014, Brito Neves & Fuck 2014). zircon grains, suggesting that these upper sequences and the In the present study, the tectonic evolution of the Brasília associated gold deposits are not part of the Archaean geol- Belt was discussed mainly on the light of ID-TIMS, ion ogy of the greenstone belts (Jost et al. 2010). microprobe and LA-ICPMS U-Pb geochronological data Additionaly, a few Neoproterozoic granite intrusions have published mostly over the last ten years. Some key tectonic been identified within the TTG terrains. One of them, in the units will be highlighted: southern part of the terrain, was dated at ca. 625 Ma, sug- ■ the supracrustal sequences of the fold-and-thrust belt gesting that the allochthonous Archaean block had already and cratonic cover, docked at the western margin of the Brasília Belt at that ■ the neoproterozoic high-grade terrains (Uruaçu and time (Pimentel et al. 2003). Anápolis-Itauçu complexes), ■ three large Neoproterozoic mafic-ultramafic complexes Exposures of Paleoproterozoic of Barro Alto, Niquelândia and Canabrava, and asso- sialic basement ciated volcano-sedimentary sequences of uncertain age, Paleoproterozoic sialic basement of the Brasília Belt is ■ the Goiás Magmatic Arc. exposed either as small exposures in between younger rock units or as the large siderian/rhyacian region in the north- ern part of the belt, known as the Natividade-Cavalcante GEOLOGY AND GEOCHRONOLOGY crustal block (Fig. 3) (Fuck et al. 2014). This consists mainly OF THE BRASÍLIA BELT of volcano-sedimentary sequences (e.g. Almas-Dianópolis) and a great diversity of orthogneisses and granitic intrusions which have been dated between 2.4 and 2.04 Ga (Cruz & Goiás Archaean Block Kuyumjian 1998, Botelho et al. 2006, Fuck et al. 2013). Most This is considered to be an allochthonous microplate, of these granitoid rocks are juvenile although some of them accreted to the western margin of the Brasília Belt during present Nd models ages which are substantially older than 68 Brazilian Journal of Geology, 46(Suppl 1): 67-82, June 2016 Márcio Martins Pimentel 40 Atlantic Ocean 0 São Luiz Craton Rio Amazonas Parnaíba Rio ingu Basin BORBORMA PROVNC Amazon Rio Parnaíba Craton ARAGAA BLT 10S São Francisco Craton BRASLA BLT ARAA BLT PARAGA BLT Rio São Francisco Paran Basin 50 20S RBRA BLT Rio Paran Atlantic Ocean 0 km 500 Figure 1. The tectonic elements of central Brazil, from Valeriano et al. (2012). 69 Brazilian Journal of Geology, 46(Suppl 1): 67-82, June 2016 Evolution of the Brasília Belt 50 4 AR Amazon Parnaíba Craton Basin 12S 19 20 N Arraias Alto Paraíso de Gois PA São Francisco Craton Brasília 1 2 1S C.A.. naí Goiânia Arenpolis São Francisco Craton Vazante Gois Magmatic Arc Paran Arax Tapira 1 1 Basin Gois Massi Neoproterozoic 20S Passive Margin Piumhi Palaeo-to Passos Neoproterozoic Neoproterozoic 15 1 Luminrias 12 Archaean/ 9 10 11 Palaeoproterozoic N.S.G. 13 14 6 7 8 Archaean/Palaeoproterozoic 0 100 200 3 4 5 RB N.S.G.