Centro Administrativo da Bahia

Salvador, Brazil

Centro Administrativo da Bahia

Salvador, Brazil
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Menezes Leal A.B.D.,Federal University of Bahia | Santos A.L.D.,Federal University of Bahia | Bastos Leal L.R.,Federal University of Bahia | Cunha J.C.,Centro Administrativo da Bahia
Journal of South American Earth Sciences | Year: 2015

The late Archaean Umburanas greenstone belt (UGB) is located in the São Francisco Craton, southwest of Bahia State, Brazil. The lower unit of UGB comprises basal komatiite lavas and tholeiitic basalts intercalated with felsic volcanic rocks. The regional crystalline basement rocks, the Gavião block, predominantly consist of granitic, granodioritic and migmatitic gneiss along with tonalite-trondhjemite-granodiorite (TTG) associations.Petrographic studies of UGB komatiites reveal characteristic spinifex igneous texture although primary mineralogy is rarely preserved. Based on textural relationships, komatiites are divided into cumulate, spinifex, and massive types. The MgO content varies in the range 31.5-40.4wt%. The MgO-SiO2 negative correlation in komatiites suggests olivine fractionation trend. The UGB komatiites are of Al-undepleted type, characterized by Al2O3/TiO2 (21-48) ratio, enriched in highly incompatible LILE relative to moderately incompatible HFSE and distinct negative Nb, Sr and Eu anomalies. Also shows depletion of light rare earths, convex-downward rare earth patterns typically not observed in komatiites world-wide, and primitive mantle normalized Gd/Yb (1.03-1.23) and La/Sm (2.36-4.99) ratios. The negative Eu anomaly is attributed to the circulation of H2O-rich fluid, whereas the negative Nb and Sr anomalies are attributed to contamination from granitic basement rocks of the Gavião block.The UGB komatiites are most likely derived from adiabatic decompressional melting of a mantle plume. The melting took place at liquidus temperatures in the range 1572-1711°C, which is consistent with mantle-plume origin invoked for several other komatiites in Archaean greenstone belts elsewhere. The melts were more likely generated at a depth shallower than 100km (pressure<2.5GPa) where garnet was absent in the source mineralogy. Geochemical characteristics suggest contamination of primary melts with granitic basement rocks either during ascent of melt or during emplacement of magma in a continental basin setting. Greenschist to low-T amphibolite facies metamorphism at ~2Ga may also have played a role in modifying the original komatiite petrography and composition. © 2015 Elsevier Ltd.

Cruz S.C.P.,Federal University of Bahia | Peucat J.-J.,CNRS Geosciences Laboratory of Rennes | Teixeira L.,Centro Administrativo da Bahia | Carneiro M.A.,Federal University of Ouro Preto | And 7 more authors.
Journal of South American Earth Sciences | Year: 2012

The Gavião Block comprises amphibolite- and granulite-facies gneisses and migmatites of tonalitic, granodioritic and granitic compositions and supracrustal sequences including volcanosedimentary layers metamorphosed up to the amphibolite facies. In the region of Abaíra-Jussiape (BA), two main igneous suites, called Caraguataí and Jussiape, are exposed in the core of an anticline. The Caraguataí suite encompasses alkali-feldspar granites, syenites and quartz syenites that contain biotite, magnetite/hematite, apatite, titanite, hastingsite/pargasite and zircon as accessory minerals that were adjusted to the amphibolite facies. White mica and epidote minerals are related to retrograde greenschist facies. These rocks were deformed in dextral to reverse-dextral shear zones, giving origin to protomylonites and augen-mesomylonites to ultramylonites. The ultramylonites have a prominent banding parallel to the main foliation of the rocks. Lithogeochemical studies revealed subalkaline to alkaline, metaluminous to peraluminous, Fe-rich protolith for instead of to these rocks associated with A2-type magmatism and partial melting of igneous continental crust. In situ U-Pb zircon dating using the Laser Ablation ICPMS method was carried out for five samples of the Caraguataí alkaline suite. The ages obtained for an isotropic syenite (SCP 1470: 2680±24Ma), a foliated syenite (SCP 2035: 2703±11Ma), a syenitic augen gneiss (SCP 2017: 2706±34Ma) and two ultramylonitic syenitic banded gneisses (SCP 1446: 2711±34Ma and SCP 1809: 2698±10Ma) fall in the same range. The average of the 62 concordant analyses obtained from the five samples allows to determine a mean 207Pb/ 206Pb age of 2696±5Ma (±2σ) interpreted as that of the alkaline plutonism. The geochronologic data obtained up to now have not helped to constrain an age for the metamorphism that affected the study area. The A2 type of magmatism, instead of and the T DM model ages (ca. 3.2-3.8Ga) and the corresponding negative ε(t) values (-4 to -6), suggest that the alkaline magmas are mainly derived from partial melting of Paleoarchean gneisses in an intraplate setting similar to those from the surrounding Aracatu region. The isotopic data obtained in this work together with published data suggest that in the southern sector of the Gavião Block an important event of crustal recycling occurred ca. 2.7Ga ago. Similar ages have been found in other parts of the São Francisco Craton. © 2011.

Teixeira J.B.G.,Federal University of Bahia | Silva M.D.G.D.,Federal University of Bahia | Silva M.D.G.D.,Centro Administrativo da Bahia | Misi A.,Federal University of Bahia | And 2 more authors.
Journal of South American Earth Sciences | Year: 2010

This paper aims at establishing a tectonic and temporal framework to characterize the metallogenic processes that contributed to the origin of the mineral provinces in the northern São Francisco Craton. Many Archean mineralizations (eg. massive sulfide zinc, lead, zinc and copper, besides magnesite-talc, iron-titanium-vanadium, iron, chromite and manganese) were generated before the assembly of the Craton. Deposits of chromite, nickel, gold and emerald were produced during the Paleoproterozoic orogenic cycle, when the Craton was amalgamated into the Atlantica paleocontinent. An extension event is recorded in the Neoproterozoic, during the breakup of Rodinia, associated with deposits of phosphorite and uranium. Kimberlite diamond and gold mineralization were generated during the Brasiliano orogenic cycle, coeval with the amalgamation of West Gondwana. A long-lasting and rather uniform crustal stress is recorded in the area during the Cambrian period. Resetting of the isotopic and magnetic systems that affected the Neoproterozoic sediments of the Irecê Basin at about 520 Ma was attributed to the regional-scale fluid migration and mineralization in the aftermath of the Brasiliano orogenic cycle. Este artigo visa estabelecer um quadro tectônico e temporal para caracterizar os processos metalogenéticos que contribuíram para a origem das províncias minerais no setor norte do Cráton São Francisco. Muitas mineralizações arqueanas (por ex. sulfeto maciço de zinco, chumbo, zinco e cobre, além de magnesita-talco, ferro-titânio-vanádio, ferro, cromo e manganês) foram geradas antes da formação do Cráton. Depósitos de cromo, níquel, ouro e esmeralda foram produzidos durante a orogênese paleoproterozóica, quando o cráton foi amalgamado dentro do paleocontinente Atlântica. Um evento extensional é registrado no Neoproterozóico, durante a fragmentação do paleocontinente Rodinia, associado a depósitos de fosforita e urânio. Kimberlitos diamantíferos e mineralizações de ouro foram geradas durante o ciclo orogenético Brasiliano, contemporâneo com a aglutinação do paleocontinente Gondwana Ocidental. Um regime de tensão crustal bastante uniforme e de longa duração foi registrado na região durante o período Cambriano. A reconfiguração dos sistemas isotópico e magnético que afetou os sedimentos neoproterozóicos da bacia de Irecê há cerca de 520 Ma foi atribuída ao evento tectônico de escala regional, à migração de fluidos e mineralização, como consequência do ciclo orogenético Brasiliano. © 2010 Elsevier Ltd.

Dos Santos E.S.M.,Federal University of Bahia | Silva R.W.S.,Centro Administrativo da Bahia | Sampaio E.E.S.,Federal University of Bahia
Exploration Geophysics | Year: 2012

On October 2008, the large extent of dissolution of limestone in the Ireĉ Basin resulted in the occurrence of minor ground shakes and the enhancement of fractures along the surface of the ground and on the walls of buildings in the urban and periurban area of the city of Lapo. After preliminary geological and geotechnical studies to analyse the danger of collapse of underground caves, we have carried out gravity and electro-resistivity profiles in order to map the concealed caves and to help the determination of the level of groundwater within them. The interpretation of the results provided useful information both for the immediate safety of the population and for subsequent hydrological and geotechnical work. We have defined five risk areas based on the variations of the Bouguer and of the electrical resistivity data, by correlating the distribution of gravity lows with the presence of caves and the less resistive zones with the presence of brine water underground. © 2012 ASEG.

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