Sial A.N.,Federal University of Pernambuco |
Filho M.A.D.S.,Companhia de Pesquisa de Recursos Minerais CPRM |
Ferreira V.P.,Federal University of Pernambuco |
Pimentel M.M.,University of Brasilia |
And 3 more authors.
Precambrian Research | Year: 2010
Two cap carbonates overlying glaciogenic diamictites crop out extensively in the eastern Vaza Barris Domain of the Sergipano Belt, northeastern Brazil. They are represented by carbonates of the Jacoca Formation, resting on top of diamictite of the Ribeiropolis Formation, and by the Olhos D'Agua Formation (carbonates, organic-rich towards the top), which overlies diamictite of the Palestina Formation. These two sequences were deformed and metamorphosed at sub-greenschist facies-conditions during the Brasiliano cycle (650-600 Ma).In the western Vaza Barris Domain, dolostone of the Acauã Formation rest, in sharp contact, on diamictites of the Juetê Formation and are much less deformed than the Olhos D'Agua Formation.Values of (13C for the Jacoca and Acauã Formations cluster around -4 to -5‰. In the Olhos D'Agua Formation, however, negative values (around -5‰) in the shallow-marine base of the sequence are replaced up section by values close to zero and, abruptly, by positive values between +8 and +10‰ at the top. Strontium-isotope ratios for carbonates of these three formations are within the range of late Neoproterozoic seawater (0.7060-0.7090). The Acauã Formation displays 87Sr/86Sr ratios from 0.7072 to 0.7073, while the Jacoca and Olhos D'Agua formations display values within the range from 0.7077 to 0.7081. Strontium-isotopes coupled with available C-isotopes, detrital zircon U-Pb ages and structural data suggest that the Jacoca Formation and Acauã are probably correlative and mid Cryogenian in age, and that the Olhos D'Agua Formation is lowermost Ediacaran (Marinoan) in age.Mercury concentrations in carbonates of the Jacoca, Olhos D'Agua and Acauã formations are usually much higher than those observed in carbonates deposited not concomitantly with volcanic activities (<3ngg-1). They are, in fact, similar to those in carbonates deposited during volcanic activity (e.g. Tertiary Punta Rocallosa carbonates in Chile), with concentration values between 20 and 80ngg-1, suggesting that CO2 in the basal portion of these carbonate formations is mostly mantle-derived, accumulated in the atmosphere during Cryogenian-Ediacaran glacial events. © 2010.
Cordani U.G.,University of Sao Paulo |
Pimentel M.M.,University of Brasilia |
Araujo C.E.G.D.,Companhia de Pesquisa de Recursos Minerais CPRM |
Fuck R.A.,University of Brasilia
Brazilian Journal of Geology | Year: 2013
The assembly of West Gondwana was completed by the end of the Precambrian, when the Amazonian, West African, São Francisco- Congo, Kalahari and Rio de la Plata cratons, as well as the Saharan metacraton and the Parnaíba, Paranapanema and Luiz Alves cratonic fragments were united by means of the Brasiliano-Pan African orogeny, a geotectonic process that was active from the late Neoproterozoic to the early Paleozoic, related to the closure of a large oceanic domain, the Goiás- Pharusian Ocean. Several accretionary complexes and possible microcontinents were trapped within the Brasiliano-Pan African mobile belts, and they have been accommodated within a few hundred kilometers of the Transbrasiliano-Kandi tectonic corridor. The supercontinent was already formed at about 600 Ma, as indicated by the existence of a very large Ediacaran epicontinental sea covering large areas of west- central Brazil and southern Uruguay along the margins of the Amazonian and Rio de la Plata cratons, demonstrating the connection of both cratonic units at that time and making the idea of a collisional suture closing a supposed Clymene Ocean unsustainable. In the Cambrian, a major plate reorganization occurred, being responsible for the initiation of subduction of the oceanic lithosphere along an open and unconfined Pacific Ocean. The resulting Pampean Orogeny correlates nicely in time with the Saldania, Ross, and Tasmanian belts along the southern Gondwana margin. Simultaneously, extensional-type post-tectonic episodes occurred repeatedly along the Transbrasiliano-Kandi tectonic corridor.
Cordani U.G.,University of Sao Paulo |
Pimentel M.M.,Institute Geociencias Da University Of Braslia |
De Araujo C.E.G.,Companhia de Pesquisa de Recursos Minerais CPRM |
Basei M.A.S.,University of Sao Paulo |
And 2 more authors.
American Journal of Science | Year: 2013
Previous studies have proposed that a major suture resulted from the collision between the Amazonian and São Francisco-Congo cratons during the Cambrian, following the closure of a supposed Clymene Ocean. The proposal tentatively located this ocean along the Araguaia and Paraguay belts at the eastern margin of the Amazonian Craton, and its southern extension reached the Pampean belt in Argentina. In the present study we will argue that the existence of Ediacaran-Cambrian oceanic lithosphere in central South America is highly unlikely. West Gondwana was assembled during the convergence between the Amazonian, West African, São Francisco-Congo and Rio de La Plata cratons as well as the Saharan Metacraton, leading to the closure of the Goiás-Pharusian Ocean during the Neoproterozoic. Final closure and continental collision resulted in the development of the Transbrasiliano-Kandi mega-shear zone that cuts through several mobile belts, but leaves the cratonic areas totally untouched. Consistent results of radiometric dating along the Transbrasiliano (TB) mega-shear in South America and of metamorphic rocks of the Braślia Belt have indicated that the Neoproterozoic collision finished at ca. 620 Ma. After isostatic uplift, cooling, and denudation, between 590 and 500 Ma, emplacement of undeformed K-rich postorogenic granites represented the main tectonic event. At this time or afterwards, a series of small extensional sedimentary basins formed in graben troughs, most of which are within the TB tectonic corridor. They all were of extensional character, contrasting clearly with the convergent tectonics occurring within the coeval Pampean Orogen in Argentina. The main arguments showing that an Ediacaran to Cambrian oceanic closure in central Brazil is untenable include: (i) the assembly of West Gondwana was completed by ca. 600 Ma, when the convergence between the Amazonian, São Francisco and Rio de La Plata cratons had already ended. After this, there is no geological evidence of an oceanic lithosphere (for example, ophiolites, magmatic arcs, et cetera), ruling out the possible existence of an Ediacaran or Cambrian Clymene Ocean in Central Brazil; (ii) the Gurupi and Araguaia belts in Brazil, as well as the Bassaride and Rokelide belts in West Africa, are regarded as aulacogenic-type systems formed within an intraplate tectonic setting. Their tectonic history precedes the collision between the Amazonian and São Francisco-Congo cratons, as demonstrated by the linear structures of the Transbrasiliano megashear which truncate the N-S structural trends of the Araguaia Belt; (iii) there is a close correlation between the Corumbá Group of the Paraguay Belt in Brazil and the Arroyo del Soldado Group in Uruguay. These sedimentary sequences belonged to the same Ediacaran continental shelf and this is a powerful indicator for an Ediacaran connection between the Amazonian and Rio de La Plata cratons, which precludes the existence of a wide ocean (for example, the Clymene) between them. On the other hand, the tentative correlation between the Sierras de Cordoba and the Paraguay Belt cannot be accepted, because these are far apart and there is no similarity in lithology, metamorphism, or structural trends; (iv) the Puga paleopole is the most important evidence for the hypothesis of the Cambrian Clymene Ocean, however the age of about 600 Ma for this paleopole, taken on the basis of Sr and C isotopes, is loosely constrained. In addition this is located at low latitude, not far from the present pole, and therefore could be related to a younger remagnetization; (v) the Pampean Orogen is made up of medium-to high-grade metamorphic rocks constrained between 560 Ma and 520 Ma and therefore was tectonically active during most of the Cambrian. However, at this time, an oceanic lithosphere is not evident in the vicinity of the Paraguay belt, and in central Brazil extensional rather than convergent tectonic processes have been observed.
MacHado R.,University of Sao Paulo |
Roldan L.F.,Tratch Mundi Gerenciamento Ambiental Ltda |
Jacques P.D.,Companhia de Pesquisa de Recursos Minerais CPRM |
Fassbinder E.,Federal University of Paraná |
Nummer A.R.,Federal Rural University of Rio de Janeiro
Revista Brasileira de Geociencias | Year: 2012
This work presents the multiscale study of structural lineaments that affect alkaline rocks associated to the Lages Dome, situated in the Eastern edge of the Paraná Basin, Santa Catarina state, Brazil. We analyzed satellite images (Landsat-TM) and Shuttle Radar Topography Mission (SRTM) project and carried out structural studies of alkaline rocks in quarries of the dome. The results show correlation between the lineaments extracted from the images and faults analyzed in quarries, especially around the NS directions (NNE to NNW) and EW (ENE to ESE) and, secondarily, NE and NW. These structures are largely related to reactivation of basement discontinuities. The first two directions correspond to dextral and sinistral transcurrent faults, respectively. Formation of these structures is compatible with a field of tension with axis oriented NE-SW, whose age ranges from the Late Cretaceous and Tertiary.
Applications and limitations of thermobarometry in migmatites and granulites using as an example rocks of the araçuaí orogen in southern bahia, including a discussion on the tectonic meaning of the current results
Moraes R.,University of Sao Paulo |
Nicollet C.,University Blaise Pascal |
Barbosa J.S.F.,Federal University of Bahia |
Fuck R.A.,University of Brasilia |
Sampaio A.R.,Companhia de Pesquisa de Recursos Minerais CPRM
Brazilian Journal of Geology | Year: 2015
In southern Bahia, there are outcrops of migmatites and granulites in the Jequitinhonha Complex, which is part of the northern portion of the Araçuaí Orogen. Migmatites (garnet-cordierite diatexite) dominate the metamorphic rocks and host lenses and layers of felsic garnet granulite. The conditions of temperature and pressure of metamorphism were calculated using conventional thermobarometry and the software THERMOCALC. Values around 850 °C and 7 kbar were obtained with THERMOCALC. The calculations for the garnet-cordierite diatexite were made considering aH2O equal to 1, but the best results of calculations for the granulites are obtained with aH2O values of 0.3. Pressure values obtained with GAPES resulted in consistent values with THERMOCALC, but the pair garnet-orthopyroxene always produces low values for temperature and high ones for pressure. The results are consistent with the presence of the pair garnet and cordierite in diatexite and orthopyroxene in felsic granulite. From the tectonic point of view, the setting in which metamorphism of these rocks occurred requires high heat flow with a thermal anomaly in mid continental crust, as indicated by values of 7 kbar. Recent studies have favored the closure of a back-arc basin for this tectonic setting, but it does not solve the problem that the time span between metamorphic peak and the end of granite intrusions, involving large bodies of charnockite, is more than 80 million yearsThe model of tectonic switching is suggested here as it can explain the maintenance of high temperatures for a more extended interval of time.
Perpetuo M.P.,University of Campinas |
Da Silva Amaral W.,University of Campinas |
Da Costa F.G.,Companhia de Pesquisa de Recursos Minerais CPRM |
Filho E.C.U.,CPRM |
De Sousa D.F.M.,University of Campinas
Brazilian Journal of Geology | Year: 2016
The Serra da Aldeia Suite is composed by circular or oval-shaped plutons, intrusive in meta-sedimentary and meta-volcanosedimentary rocks in the Riacho do Pontal Fold Belt, NE Brazil. The Serra das Melancias Pluton, belonging to Serra da Aldeia Suite, is located southeastern of Piaui state, near Paulistana city. Tese plutons represent a major magmatic expression in this area and contain important information about the late magmatic/collisional geologic evolution of the Brasiliano Orogeny. Based on petrographic and geochemical data, three facies were defined in the Serra das Melancias Pluton: granites, syenites and quartz monzonites. The rocks display high-K and alkaline to shoshonitic affinities, are metaluminous and show ferrous character. Tey are enriched in Light Rare Earth Elements and Large Ion Lithophile Elements, with negative anomalies in Nb, Ta and Ti. Teir high Ba, Sr, K/Rb, low Rb, relatively low U, T, Nb to very low Heavy Rare Earth Elements and Y resemble those of typical high Ba-Sr granitoids. The geochemical data suggest the emplacement of Serra das Melancias Pluton in a transitional, late to post-orogenic setting in the Riacho do Pontal Fold Belt during the late Brasiliano-Pan African Orogeny.