ORÓGENO ARAÇUAÍ: SÍNTESE DO CONHECIMENTO 30 ANOS APÓS ALMEIDA 1977

Resumo

The Araçuaí Fold Belt was defined as the southeastern limit of the São Francisco Craton in the classicalpaper published by Fernando Flávio Marques de Almeida in 1977. This keystone of the Brazilian geologicliterature catalyzed important discoveries, such as of Neoproterozoic ophiolites and a calc-alkaline magmaticarc, related to the Araçuaí Belt and paleotectonic correlations with its counterpart located in Africa (the WestCongo Belt), that provided solid basis to define the Araçuaí-West-Congo Orogen by the end of the 1990thdecade. After the opening of the Atlantic Ocean in Cretaceous times, two thirds of the Araçuaí-West-CongoOrogen remained in the Brazil side, including records of the continental rift and passive margin phases ofthe precursor basin, all ophiolite slivers and the whole orogenic magmatism formed from the pre-collisionalto post-collisional stages. Thus, the name Araçuaí Orogen has been applied to the Neoproterozoic-Cambrianorogenic region that extends from the southeastern edge of the São Francisco Craton to the Atlantic coastlineand is roughly limited between the 15º and 21º S parallels. After 30 years of systematic geological mappingtogether with geochemical and geochronological studies published by many authors, all evolutionary stagesof the Araçuaí Orogen can be reasonably interpreted. Despite the regional metamorfism and deformation, thefollowing descriptions generally refer to protoliths. All mentioned ages were obtained by U-Pb method onzircon. The Macaúbas Group records rift, passive margin and oceanic environments of the precursor basinof the Araçuaí Orogen. From the base to the top and from proximal to distal units, this group comprises thepre-glacial Duas Barras and Rio Peixe Bravo formations, and the glaciogenic Serra do Catuni, Nova Auroraand Lower Chapada Acauã formations, related to continental rift and transitional stages, and the diamictitefreeUpper Chapada Acauã and Ribeirão da Folha formations, representing passive margin and oceanicenvironments. Dates of detrital zircon grains from Duas Barras sandstones and Serra do Catuni diamictitessuggest a maximum sedimentation age around 900 Ma for the lower Macaúbas Group, in agreement withages yielded by the Pedro Lessa mafic dikes (906 ± 2 Ma) and anorogenic granites of Salto da Divisa (875 ±9 Ma). The thick diamictite-bearing marine successions with sand-rich turbidites, diamictitic iron formation,mafic volcanic rocks and pelites (Nova Aurora and Lower Chapada Acauã formations) were depositedfrom the rift to transitional stages. The Upper Chapada Acauã Formation consists of a sand-pelite shelfsuccession, deposited after ca. 864 Ma ago in the proximal passive margin. The Ribeirão da Folha Formationmainly consists of sand-pelite turbidites, pelagic pelites, sulfide-bearing cherts and banded iron formations,representing distal passive margin to oceanic sedimentation. Gabbro and dolerite with plagiogranite veinsdated at ca. 660 Ma, and ultramafic rocks form tectonic slices of oceanic lithosphere thrust onto packagesof the Ribeirão da Folha Formation. The pre-collisional, calc-alkaline, continental magmatic arc (G1 Suite,630-585 Ma) consists of tonalites and granodiorites, with minor diorite and gabbro. A volcano-sedimentarysuccession of this magmatic arc includes pyroclastic and volcaniclastic rocks of dacitic composition datedat ca. 585 Ma, ascribed to the Palmital do Sul and Tumiritinga formations (Rio Doce Group), depositedfrom intra-arc to fore-arc settings. Detrital zircon geochronology suggests that the São Tomé wackes (RioDoce Group) represent intra-arc to back-arc sedimentation after ca. 594 Ma ago. The Salinas Formation, aconglomerate-wacke-pelite association located to northwest of the magmatic arc, represents synorogenicsedimentation younger than ca. 588 Ma. A huge zone of syn-collisional S-type granites (G2 Suite, 582-560Ma) occurs to the east and north of the pre-collisional magmatic arc, northward of latitude 20º S. Partialmelting of G2 granites originated peraluminous leucogranites (G3 Suite) from the late- to post-collisionalstages. A set of late structures, and the post-collisional intrusions of the S-type G4 Suite (535-500 Ma) andI-type G5 Suite (520-490 Ma) are related to the gravitational collapse of the orogen. The location of themagmatic arc, roughly parallel to the zone with ophiolite slivers, from the 17º30’ S latitude southwardssuggests that oceanic crust only developed along the southern segment of the precursor basin of the Araçuaí-West-Congo Orogen. This basin was carved, like a large gulf partially floored by oceanic crust, into the SãoFrancisco-Congo Paleocontinent, but paleogeographic reconstructions show that the Bahia-Gabon cratonicbridge (located to the north of the Araçuaí Orogen) subsisted since at least 1 Ga until the Atlantic opening.This uncommon geotectonic scenario inspired the concept of confined orogen, quoted as a new type ofcollisional orogen in the international literature, and the appealing nutcracker tectonic model to explain theAraçuaí-West-Congo Orogen evolution.