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

1992, Journal of South American Earth Sciences

Arafuaf BeR is a Late Proterozoic (BrasUiano Cycle) ~eotectonic unit which was developed along the southeastem margin of the Sac 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 repefts. In the we, stem domain of the belt, the Macafbas Group-the most important supracrostal sequence related to the evolution of the Araquef 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 Ribeirao da Folha Facies) typical of an ocean-floor environment. Banded iron formations, metacberts, diopsidites, massive anltides, graphite schists, hyperalnminous schists, and ortho-amphibolites, intercalated with quartz-mica schists and impure quartzires, characterize the most distinctive and restricted volcano-sedimentary facies yet found within the Salines Formation. Ultramallc slabs were teotonicatly emplaced within the Ribeirao da Folha Facies. Eight whole rock samples of meta-ultramaflc rocks and ortho-amphibolites yielded a Sm-Nd isuchronic age of 793 + 90 Ma (ENd(T) = 4'4.1:1: 0.6, MSWD ffi 1.76). The structures of the northern Araquaf Belt are marked by a down-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 beR is characterized by major high-dip transcurrent shear zones. Our tectonic model starts with marked fracturing, followed by rifling that took place in the SAo Francisco-Congo Crston around 1000 + 100 Ma (ages of basic intrusions and alkaline anorogenic granites). A sinistral transfer zone was established at the northem boundary of the belt, controlling the enlargement of the Araquaf-West Congoiian rill with the neighbouring northern oratonic region remaining essentially unaffected by compressional stresses. A period of ocesn-flour spreading took place at about 800 M_a. Rift closure started at about 750 Ma and led to the reversal of motion aloug former extensional structures. The main Braslliano-Pan-African orogenic period took place between 700 and 550 Ma and was marked by regional metamorphism and deformation related to both thrusting and lranscurrent movements, and emplacement of synteotonic granites derived from anatectic melts triggered by collisional crustal thickening. In this period, uRramafic slabs were emplaced within the Ribeirlto 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.