Claudio de Morisson Valeriano

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).

Deformational evolution of a Cretaceous subduction complex: Elephant Island, South Shetland Islands, Antarctica

Abstract New structural data from Elephant Island and adjacent islands are presented with the objective to improve the understanding of subduction kinematics in the area northeast of the Antarctic Peninsula. On the island, a first deformation phase, D 1 , produced a strong SL fabric with steep stretching and mineral lineations, partly defined by relatively high pressure minerals, such as crossite and glaucophane. D 1 is interpreted to record southward subduction along an E–W trench with respect to the present position of the island. A second phase, D 2 , led to intense folding with steep E–W-trending axial surfaces. The local presence of sinistral C′-type shear bands related to this phase and the oblique inclination of the L 2 stretching lineations are the main arguments to interpret this phase as representing oblique sinistral transpressive shear along steep, approximately E–W-trending shear zones, with the northern (Pacific) block going down with respect to the southern (Antarctic Peninsula) block. The sinistral strike-slip component may represent a trench-linked strike-slip movement as a consequence of oblique subduction. Lithostatic pressure decreased and temperature increased to peak values during D 2 , interpreted to represent the collision of thickened oceanic crust with the active continental margin. The last deformation phase, D 3 , is characterised by post-metamorphic kink bands, partially forming conjugate sets consistent with E–W shortening and N–S extension. The rock units that underlie the island probably rotated during D 3 , in Cenozoic times, together with the trench, from an NE–SW to the present ENE–WSW position, during the progressive opening of the Scotia Sea. The similarity between the strain orientation of D 3 and that of the sinistral NE–SW Shackleton Fracture Zone is consistent with this interpretation.

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

Mesozoic tectonic evolution of the south Orkney microcontinent, Scotia arc, Antarctica

The South Orkney Islands are the exposed part of a continental fragment on the southern limb of the Scotia arc. The islands are to a large extent composed of metapelites and metagreywackes of probable Triassic sedimentary age. Deformation related to an accretionary wedge setting, with associated metamorphism from anchizone to the greenschist facies, are of Jurassic age (176–200 Ma). On Powell Island, in the centre of the archipelago, five phases of deformation are recognized. The first three, associated with the main metamorphism, are tentatively correlated with early Jurassic subduction along the Pacific margin of Gondwana. D4 is a phase of middle to late Jurassic crustal extension associated with uplift. This extension phase may be related to opening of the Rocas Verdes basin in southern Chile, associated with the breakup of Gondwanaland. Upper Jurassic conglomerates cover the metamorphic rocks unconformably. D5 is a phase of brittle extensional faulting probably associated with Cenozoic opening of the Powell basin west of the archipelago, and with development of the Scotia arc. of this breakup are not well known. Since the SOM was situated in the region where disintegration started, its tectonic history may help to clarify the evolution of the Scotia arc as a whole.

The Southern Brasília Belt

This chapter presents a review of the main tectonic features of the Southern Brasilia belt with emphasis on its relationships with the southwestern margin of the Sao Francisco craton in the context of the Neoproterozoic Brasiliano orogenic collage of West Gondwana. The belt is characterized by a stack of W-vergent nappes of passive margin units bounded to the west by the Archean-Paleoproterozoic Goias microcontinent and the Neoproterozoic Mara Rosa, Goias and Arenapolis magmatic arcs. The protracted history of the passive margin development initiated with Mesoproterozoic rifting events that led to a final Tonian continental break up and development of a wide continental margin basin. Minimum sedimentation ages of the sedimentary units involved are given by subduction-related regional metamorphism between 650 and 610 Ma. An older sedimentary phase of the passive margin is represented by the Paranoa Group, the Jequitai Formation (Bambui Group), the Vazante and Canastra groups, the Cubatao Formation (Ibia Group) and part of the Araxa Group, whose youngest detrital zircons were dated between 1.0 and 0.9 Ga. The orogenic phase of the southern Brasilia belt started with progressive subduction of distal to proximal passive margin units at 650–630 Ma and development of recumbent folds associated with a regionally penetrative foliation and medium to high-pressure metamorphic parageneses. Post-metamorphic peak nappe exhumation took place around 610–605 Ma (monazite U-Pb) and final cooling at 600–580 Ma, as indicated by K-Ar ages on white mica and biotite. The syn-orogenic Neoproterozoic sedimentary units show detrital zircons as young as 600 Ma, with mixed cratonic/arc-related sedimentary provenance, represented by the Bambui Group, Rio Verde Formation (Ibia Group) and upper thrust sheets of the Araxa Group.

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.

Monazite ID-TIMS U-Pb geochronology in the LAGIR laboratory, Rio de Janeiro State University: protocols and first applications to the assembly of Gondwana supercontinent in SE-Brazil

The chemical and spectrometric procedures of the U-Pb geochronology method on monazites, recently installed in the LAGIR laboratory, are described in detail. In addition, preliminary results on monazite samples from the Brasilia and Ribeira belts are reported and discussed in the context of the regional geology. Several experiments for calibration of ion exchange chromatographic columns with the AG-1x8 resin, were performed with HCl, using dissolved natural monazite samples. The Pb blanks of reagents are ~0.5 pg/g in acids and ~1 pg/g in H2O. The total Pb blanks in chemical procedures were below 22 pg. Preliminary results are presented from three case studies related to Brasiliano orogenic belts of SE-Brazil, which correlate very well with previous age determinations from literature: two sub-concordant grains from an Araxa Group quartzite (southern Brasilia belt) define a concordia age of 602.6 ±1.4 Ma; a -0.8% discordant grain from a quartzite of the Sao Fidelis Group (Costeiro Domain, central Ribeira belt) yielded a concordia age of 535.3 ± 2.4 Ma; two 0.4 % and 1.3 % discordant monazite grains from the post-collisional Itaoca Granite (Costeiro Domain, central Ribeira belt) define a concordia age of 476.4 ± 1.8 Ma.

The Varre-Sai chondrite, a Brazilian fall: petrology and geochemistry

ABSTRACTThe Varre-Sai meteorite fell along the border of the states of Espirito Santo and Rio de Janeiro, Brazil; on 19 June 2010 at 5:40 pm. Petrography and X-ray powder diffraction (XRD) indicate...ABSTRACT The Varre-Sai meteorite fell along the border of the states of Espirito Santo and Rio de Janeiro, Brazil; on 19 June 2010 at 5:40 pm. Petrography and X-ray powder diffraction (XRD) indicate that the rock is an L5 S4 chondrite, with blastoporphyritic texture that has not been previously described. Geochemical data based on major and rare-earth elements (REEs) show that Varre-Sai is highly similar to the other L chondrites. In Harker diagrams, Varre-Sai, L, and LL chondrites form a single group, suggesting no significant chemical differences between them and contributing to the long-standing debate of whether LL chondrites form a distinct group or whether they are a subset of the L group. Harker diagrams also define a trend from E to H and L/LL chondrites, similar to the cosmochemical trends suggested by other authors. The behaviour of Fe2O3t and NiO indicates a relationship with Fe-Ni alloys, and their trend in the diagram suggests some chemical differentiation in the ordinary chondrite parental bodies. The REE content in Varre-Sai, normalized to C chondrites, falls in the field of L chondrites and others, but with slight REE enrichment. The chemical differences in chondrites, mainly in REEs, Fe2O3t and NiO could be alternatively interpreted as variations in the inherited agglutinated materials as chondrules, Ca–Al-rich inclusions and Fe–Ni nodules.

Litogeoquímica do Complexo Campos Gerais e granitoides intrusivos, Arqueano/Paleoproterozoico, Brasil

O Complexo Campos Gerais localiza-se proximo a borda sudoeste do Craton do Sao Francisco Meridional, na Faixa Brasilia. Seu conjunto litologico e composto por ortognaisses mesoarqueanos migmatizados (ortognaisses Campos Gerais e Mandembo). Esse conjunto e intrudido por granitoides neoarqeanos, alem de granitos e tonalitos, provavelmente paleoproterozoicos. O Ortognaisse Campos Gerais apresenta estrutura migmatitica estromatica, com mesossoma de composicao tonalitica a monzogranitica, compondo uma suite subalcalina, calcioalcalina, peraluminosa a metaluminosa. Idade modelo (T DM ) encontrada foi de 3,2 Ga ( e Nd : -4,6). O Ortognaisse Mandembo compreende um conjunto predominantemente diatexitico nebulitico, de composicao tonalitica a sienogranitica, subalcalina, calcioalcalina, peraluminosa de alto potassio, com idades T DM em 3,4 e 3,2 Ga ( e Nd ≈ -8). O Metagranitoide Grao Mogol e intrusivo no Ortognaisse Campos Gerais, apresenta composicao tonalitica a monzogranitica e compoe uma serie de rochas de afinidade subalcalina, calcioalcalina, peraluminosa, de alto potassio. Corpos circunscritos de metatonalito intrudem as unidades anteriormente descritas, com idade modelo (T DM ) muito proxima das idades absolutas (2,85 a 2,77Ga) e e Nd proximo a zero. O Granito Itapixe apresenta-se como um granito isotopico, levemente foliado, com afinidade subalcalina, calcioalcalina, peraluminosa, de alto potassio. O Granito Corrego do Sapateiro e uma rocha isotropica, de granulometria media a grossa e coloracao rosada. Pertence a serie subalcalina, calcioalcalina, peraluminosa, de alto potassio. Os dados litogeoquimicos sugerem que os protolitos do Complexo Campos Gerais sao relacionados a fusao de crosta oceânica metamorfisada (Ortognaisse Campos Gerais) e anatexia de crosta TTG (Ortognaisse Mandembo).