Toward a new tectonic model for the Late Proterozoic Araçuaí (SE Brazil)-West Congolian (SW Africa) Belt

Abstract

The Araçuaí Belt is a Late Proterozoic (Brasiliano Cycle) geotectonic unit which was developed along the southeastern margin of the São Francisco Craton (SE Brazil) and was formerly considered as being an ensialic orogen. It is correlated with the Pan-African West Congolian Belt (SW Africa) in many reports. In the western domain of the belt, the Macaúbas Group—the most important supracrustal sequence related to the evolution of the Araçuaí Belt —comprises the Terra Branca and Carbonita Formations, which consist of littoral glacial sediments to shelf turbidites. These formations grade upward and eastward to the Salinas Formation, consisting of distal turbidites related to submarine fans, pelagic sediments, and a rock association (the Ribeirão da Folha Facies) typical of an ocean-floor environment. Banded iron formations, metacherts, diopsidites, massive sulfides, graphite schists, hyperaluminous schists, and ortho-amphibolites, intercalated with quartz-mica schists and impure quartzites, characterize the most distinctive and restricted volcano-sedimentary facies yet found within the Salinas Formation. Ultramafic slabs were tectonically emplaced within the Ribeirão da Folha Facies. Eight whole rock samples of meta-ultramafic rocks and ortho-amphibolites yielded a SmNd isochronic age of 793 ± 90 Ma ( ϵ Nd(T) = +4.1 ± 0.6. MSWD = 1.76 ). The structures of the northern Araçuaí Belt are marked by a doen-dip stretching lineation (western domain) related to frontal thrusts which controlled tectonic transport from east to west; stretching lineation rakes decrease in the eastern tectonic domain, indicating dominant oblique to transcurrent motion; the northern arch of the belt is characterized by major high-dip transcurrent shear zones. Our tectonic model starts with marked fracturing, followed by rifting that took place in the São Francisco-Congo Craton around 1000 ± 100 Ma (ages of basic intrusions and alkaline anorogenic granites). A sinistral transfer zone was established at the north rn boundary of the belt, controlling the ellargement of the Araçuaí-West Congolian rift, with the neighbouring northern cratonic region remaining essentially unaffected by compressional stresses. A period of ocean-floor spreading took place at about 800 Ma. Rift closure started at about 750 Ma and led to the reversal of motion along former extensional structures. The main Brasiliano-Pan-African orogenic period took place between 700 and 550 Ma and was marked by regional metamorphism and deformation related to both thrusting and transcurrent movements, and emplacement of syntectonic grranites derived from anatectic melts trigered by collisional crustal thickening. In this period, ultramafic slabs were emmlaced within the Ribeirão da Folha Facies, and both may represent an ophiolite-type suite. Late- to post-tectonic (500 to 550 Ma) intrusive granites were generated and emplaced in the Brazilian side along a major zone of crustal thickening in response to the last stages of collision.