Júlio César Horta de Almeida

Correlation of Neoproterozoic terranes between the Ribeira Belt, SE Brazil and its African counterpart: comparative tectonic evolution and open questions

Abstract Four main classes of tectonic entities may be considered for the Ribeira Belt and southwest African counterparts: (1) cratonic fragments older than 1.8 Ga and their passive margin successions, (2) reworked basement terranes with Mesoproterozoic and/or Neoproterozoic deformed cover, (3) magmatic arc associations, (4) terranes with Palaeoproterozoic basement and deformed Neoproterozoic back-arc successions. Based on comparative investigation, a tectonic model of polyphase amalgamation is proposed with c. 790 and 630–610 Ma major episodes of intra-oceanic and cordilleran arc magmatism along both sides of the Adamastor Ocean. Subsequent diachronous collision of the arc terranes and small plates followed at c. 630, 600, 580 and 530 Ma. The tectonic complexity reflects an accretionary evolution from Cryogenian to Cambrian times. The São Francisco–Congo and Angola palaeo-continents did probably not behave as one consolidated block, but rather may have accommodated considerable convergence during the Brasiliano/Pan-African episodes. The final docking of Cabo Frio and Kalahari in the Cambrian was coeval with the arrival of Amazonia on the opposite side, resulting in lateral reactivation and displacement between the previously amalgamated pieces. The transition between the Cambrian and the Ordovician is marked by the extensional collapse of the metamorphic core zones of the orogens.

Tectonic evolution of the Brasília Belt, Central Brazil, and early assembly of Gondwana

Abstract The Brasília Belt comprises terranes and thrust-sheets that were tectonically transported towards the western passive margin of the São Francisco–Congo palaeocontinent during an orogenic episode resulting from collision of the Paranapanema and Goiás blocks and the Goiás magmatic arc against São Francisco–Congo at 0.64–0.61 Ga. The tectonic zones of the belt are, from east to west: a foreland zone with Archaean–Palaeoproterozoic granite–greenstone basement covered by Neoproterozoic anchimetamorphic sedimentary rocks (Bambuí Group); a low metamorphic grade thrust-fold belt of proximal shelf successions, mostly siliciclastic, containing rare basement slivers; metamorphic nappes in upper greenschist to granulite facies of distal shelf and slope metasediments and subordinate tholeiitic metabasalts; the Goiás massif, possibly a microcontinent; and the Goiás magmatic arc. The accretion of these terranes against the western margin of the São Francisco–Congo palaeocontinent took place during an early phase of Gondwana supercontinent amalgamation, when terranes accreted around São Francisco–Congo to create a proto-West Gondwana landmass, around which subsequent collisional and accretionary events followed, such as those in the Borborema–Trans-Saharan province (c. 0.62–0.60 Ga); in the Ribeira–Araçuaí belt (c. 0.58 Ga); along the Araguaia and Paraguay belts (collision of Amazonia, c. 0.54–0.52 Ga); and the accretion of Cabo Frio terrane in the Ribeira Belt (c. 0.53–0.50 Ga).

Pre-rift tectonic scenario of the Eo-Cretaceous Gondwana break-up along SE Brazil–SW Africa: insights from tholeiitic mafic dyke swarms

Abstract This chapter presents a synthesis of the pre-break-up plate tectonics of western Gondwana and the pre- and syn-rift magmatism in the SW South Atlantic margin (Brazil, Uruguay and Argentina) and the conjugate African continental margin (Angola, Namibia and South Africa). An updated database of the geographical locations of the primary available radiometric ages is also included in this work. A systematic analysis of the K–Ar and Ar/Ar ages from outcrops and boreholes shows a marked Mid to Late Jurassic peak in the southernmost segment of the South Atlantic, related to the emplacement of the Karoo volcanics in South Africa and in Argentina (including the Falkland Islands), and an important Early Cretaceous peak with age distributions that are related to the Gondwana break-up and formation of rift basins along the incipient continental margins. In both the southern Brazilian and Argentinian margins, as well as in the conjugate Namibian and South African margins, several igneous centres and basaltic lava flows are suggestive of the influence of mantle plumes in the Early Cretaceous, which were heralded by mafic dyke swarms in Argentina, Brazil, South Africa and Namibia. Supplementary material: A complete table with Radiometric ages of Jurassic–Early Cretaceous magmatism in the southern portion of West Gondwana is available at: www.geolsoc.org.uk/SUP18589

The Ribeira Belt

The generation of the NW-verging Ribeira belt that fringes the Sao Francisco craton to the south resulted from collisional episodes dated at 620–605, 605–565, and 535–510 Ma. During these collisional events, micro-continents and magmatic arcs converged and accreted to a continental margin system formed along the previously amalgamated Sao Francisco-Paranapanema landmass. The Sao Francisco-Paranapanema collision at around 640–620 Ma led to the development of the Southern Brasilia belt. The Ribeira orogenic front overprinted the pre-existent southern end of east-verging Brasilia belt, thereby creating a tectonically very complex interference zone. The Ribeira belt as whole comprises four tectono-stratigraphic terranes: the Occidental, Paraiba do Sul, Oriental, and Cabo Frio terranes. The Occidental terrane, representing the external sector of the belt, involves the reworked cratonic basement (Archean and Paleoproterozoic orthogneisses and orthogranulites), Mesoproterozoic intra-cratonic basins and a Neoproterozoic passive margin unit (Andrelândia Group). The uppermost unit of the Andrelândia Group is regarded to be deposited in an active margin setting and sourced by a magmatic arc installed in the Paranapanema plate. The Paraiba do Sul terrane, composed of an Archean-Paleoproterozoic basement, Neoproterozoic metasedimentary units, and a continental magmatic arc (the 640–595 Ma Serra da Bolivia complex), accreted to southeastern sector of the craton margin between 620 and 605 Ma. Afterwards, the juvenile to immature magmatic arc of the Oriental terrane (860–620 Ma Rio Negro and Serra da Prata complexes) collided to proto-Ribeira belt. The crustal thickening resulting from these collisions gave rise to widespread generation of I, S and hybrid granites that intrudes both basement and cover units in the most deformed sectors of the belt. The Cambrian (535–510 Ma) docking of the Cabo Frio terrane (an Angola craton fragment) reworked the previous accreted terranes, generating large scale folds and dextral transpresional shear zones that reached the SFC margin. Finally, a vigorous bimodal magmatic event associated with transtensional deformation episodes characterizes the orogenic collapse of the belt and marks its stabilization in the interior of the Gondwana supercontinent.