How many oceans closed during the Brasiliano Cycle in northeastern Brazil? Implications for the amalgamation of western Gondwana

Abstract

In a pre-Mesozoic drift reconstitution, the Borborema Province in northeastern Brazil connects with the Central African Fold belt to the East and with the Nigerian and Tuareg shields to the North. Therefore, the Borborema Province is an important component to consider in the context of the amalgamation of western Gondwana. The tectonic setting of the Borborema Province has been debated, with some workers advocating for the accretion of allochthonous terranes and others arguing for the decratonization of a large continental landmass followed by the re-accretion of the fragments, either involving or not involving formation of large oceanic domains between them. Resolving the tectonic setting of the Borborema Province has implications for correlations with other Brasiliano-Pan-African belts and supercontinent reconstructions. This paper discusses proposed evidence for oceanic subduction, arc magmatism, and oceanic basin closure based on a comprehensive literature review. This information is integrated with data from the African counterparts of the Borborema Province to provide a geodynamic model for the Neoproterozoic evolution of this portion of western Gondwana. The main evidence for subduction is provided by ultra-high-pressure rocks and c. 660–640 Ma-old intermediate metaigneous rocks with relatively young (1.4–1.0 Ga) whole-rock SmNd and zircon Hf model ages in the northwestern part of the Borborema Province. However, a key aspect of the Brasiliano Orogeny in most of the Borborema Province is its synchronous nature. Deposition of predominantly siliciclastic sediments throughout the province ended slightly before or partly coeval with c. 640–610 Ma tectonothermal activity and early orogenic magmatism. Plutons and batholiths emplaced during this age interval comprise predominantly alkali-calcic monzogranites and syenogranites with a large contribution of Paleoproterozoic sources. Together with the absence of juvenile oceanic crust remnants and calc-alkaline metavolcanic rocks, it is thus concluded that (i) widespread crustal extension was almost immediately followed by contractional deformation and regional metamorphism, and (ii) crustal reworking was the dominant process, with limited formation and consumption of oceanic lithosphere. A similar conclusion applies to the Nigerian Shield and the central and southern parts of the Central African Fold Belt. In contrast, large volumes of juvenile crust are found in the Tuareg Shield and in the northern part of the Central African Fold Belt. The model envisions that no large oceanic domain separated the São Francisco/Congo Craton from the Amazonian/West Africa Craton in the middle Neoproterozoic, which were part of the Atlantica paleocontinent since c. 2.0 Ga. Extensional conditions in the Borborema Province and its African counterparts are attributed to far-field stresses transmitted to the interior of Atlantica by an outwardly, N-dipping subduction system. The onset of the contractional phase resulted from the accretion of the different components of the Tuareg Shield to the northern part of Atlantica (whose E-W suture zone is now hidden by the Saharan Desert), and the combined SE-directed and SW-direct indentation of the West African and Saharan cratons, respectively.