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[en] In this paper, we report Rb-Sr and Ph isotopic data bearing on the age and origins of the granitic intrusions in the Sao Paulo area. The Morro Azul, Itu, and Nazare Paulista granites define Rb-Sr whole rock isochron dates of 525+-30,590 +-10, and 790 +-32 Ma, together with initial 87Sr86Sr ratios of 0.711+- 0.0006, 0.7066+- 0.0004, and 0.7132 +- 0.005, respectively. The Braganca Paulista and Cantareira granitoids yield preliminary whole rock Rb-Sr isochronic ages around 800 and 650 Ma respectively. In addition we report Pb isotopic analyses of feldspar separates separates from those massifs and the Moinho granitoid. The feldspar Pb analyses plot close to the evolution curve for the orogene in the plumbotectonics models, between 1.200 and 650 Ma. Most of the feldspar Pb data below the orogene curve in the 207Pb/204Pb versus 206Pb/204Pd diagram, and above it in the 208Pb/204Pb diagram, implying significant contributions from older lower continental crust. The Nazare Paulista data plot towards the upper crust curve. The combined evidence of Rb-Sr and Pb isotopic data suggests that the Moinho Intrusion is probably the oldest of the granites studied (Ca 1.200 Ma) and Morro Azul, the youngest (Ca 550 Ma). The main granitic activity in the Sao Paulo area is of Late Proterozoic age. Significant contributions to the parent magmas of the granites were derived by melting in the lower crust. (author)
[pt]
Neste trabalho foram realizadas analises isotopicas Rb-Sr e de Pb em granitoides da regiao de Sao Paulo com o objetivo de se determinar tanto suas idades como suas fontes magmaticas. Os granitoides de Morro Azul, Itu, e Nazare Paulista indicam, pelos diagramas isocronicos Rb-Sr em rocha total as idades de 525+-30, 590+-10 e 790+-32 Ma, com razoes iniciais de 0,7113 +- 0,0006, 0,7066 +- 0,0004 e 0,7132 +- 0,0005, respectivamente. Ja os corpos de Braganca Paulista e Cantareira sugeriram idades isocronicas Rb-Sr preliminares da ordem de 800 e 650 Ma. Adicionalmente, foram tambem realizadas analises isotopicas de Pb em feldspatos separados dos granitos acima citados e tambem do granitoide de moinho. Os pontos relativos a essas analises, quando plotados nos diagramas de plumbotectonica, situaram-se proximos a curva de evolucao do Pb para ambientes orogenicos, entre 1.200 e 650 Ma. Muitos dos feldspatos cairam abaixo da curva orogenica no diagrama Pb207/Pb204 versus Pb204/Pb204 e acima dela no diagrama Pb208/Pb204 versus Pb206/Pb204, o que surgere uma importante contribuicao de material proveniente da crosta continental inferior na composicao dos magmas parentais desses granitoides, cuja unica excecao seria o corpo de Nazare Paulista, que se supoe formado a partir de fusao parcial da costa continental superior. O uso combinado dos isotopos de Sr e Pb sugere que o granitoide do Moinho seria o mais velho dos corpos estudados com idade da ordem de 1.200 Ma, e Morro Azul o mais novo, como cerca de 550 Ma. A principal atividade granitica re regiao de Sao Paulo seria no Proterozoico Superior sendo os magmas parentais desses granitoides formados predominantemente por fusao parcial da crosta continental inferior. (autor)Journal
[en] The focussed area comprises five major different tectonic terranes separated by faults, which are named Alto Rio Grande Belt, Socorro-Guaxupe Nappe, Sao Roque, Embu and Costeiro Domains. The geological and geochronological history of these terranes show that the metamorphic episodes of crust-forming occurred involving both mantle-derived magmas and reworking of continental material since 3.4 Ga until 600 Ma. The post-tectonic granitic activities occurred within 1000-500 Ma range and in general, the rocks are progressively younger from the Socorro-Guaxupe Nappe (1000-850 Ma) in the NW towards the Costeiro Domain (550 Ma) in the SE. The Sr and Pb isotopic evidences, together with geological and geophysical informations, suggest that the proportions of the rock-forming processes through the geological time are: Archean, 10%; Lower Proterozoic, 10%; Middle Proterozoic, 38%; Late Proterozaic, 42%. Although the Mid and Late Proterozoic time were a period of a large amount of rocks were formed, they were not a major crustforming period, because these rocks are mainly constituted by recycled continental crust material. In our view, at end of the Early Proterozoic time, at least 85% of continetal crust, in this area, has accreted and differentiate. During the Middle and Late Proterozoic the continental crust grew at small rate. (author)
[pt]
A area de estudo compreende cinco diferentes terrenos, separados por falhas e denominados de Faixa Alto Rio Grande, Nappe Socorro-Guaxupe, e dominios Sao Roque, Embu e Costeiro. A historia geologica e geocronologica destes terrenos (de 3,4 Ga ate 600 Ma) mostra que os episodios metamorficos de formacao de crosta envolveram nao so um retrabalhamento de material crustal, como tambem magmas de origem mantelica. Atividades graniticas pos-tectonicas ocorreram entre 1000 e 500 Ma e representam rochas progressivamente mais jovens a partir da Nappe Socorro-Guaxupe (1000-800 Ma) a NW, ate o Dominio Costeiro (550 Ma) a SE. Os dados isotopicos de Sr e Pb, junto com as informacoes geologicas e geofisicas, sugerem que as proporcoes de formacao de rocha atraves do tempo geologico foi: Arqueano, 10%; Proterozoico Inferior, 10%; Proterozoico Medio, 38%; Proterozoico Superior, 42%. A grande quantidade de rocha gerada quando do Proterozoico Medio-Superior representa uma reciclagem da crosta continental. O principal periodo de acrecao e diferenciacao da crosta continental se deu no Arqueano-Proterozoico Inferior (85%) e seu crescimento, durante o Proterozoico Medio-Superior, se deu em pequenas proporcoes. (autor)Journal
[en] On the shield areas of the South America continent (about 5,000,000 km2 large) there are about 17,000 geochronological determinations available. More than 75% of this amount has been carried out in the Centro de Pesquisas Geocronologicas da Universidade de Sao Paulo, CPGeo-USP, in the last 25 years. The majority of these data were accomplished through Rb-Sr (ca.60%) and K-Ar (ca.40%) methods. The author tried to work this amount of data on a coherent way according to the progress observed in the geologic and geotectonic knowledge during the last two decades. One of the fundamental goals was to outline an up-to-date subdivision for the Precambrian of this continent, having in mind a contribution to the International Commission on Stratigraphy (ICS), Subcommission of Precambrian Stratigraphy (SPS) of the IUGS. (author)
Journal
[en] Uranium distribution in granitoids, including gneisses and migmatites were investigated for the rocks of entire Brazilian territory. Uranium concentrations were determined in 1800 Rb-Sr whole rock isotopic analysed samples with known geological setting. The samples were plotted in Brazil's geotectonic outline, and those with high U contents (above 12 ppm) provide the characterization of probable uraniferous provinces. In addition the results were interpreted in relation to age, tectonic environment, lithologies, Rb contents and 97Sr/94Sr initial ratios. With respect to the lithology, the granites with alkaline compositions showed higher U contents than their host rocks which consist of gneissic-migmatic terranes, and other granitoids. In general the Uranium-enriched rocks are mainly related to the Mid-Proterozoic time. In terms of isotopic geochemistry, the higher 97Sr/94Sr ratio rocks (high Rb contents) showed a uranium enrichment trend when compared with those less differentiated material. On the other hand, rocks with low Sr initial ratio may present Uranium concentrations similar to those with high initial ratios, which suggest the primary Uranium enrichment process during the mantle-derived rock-forming process. (author). 30 refs, 9 figs
Source
International Atomic Energy Agency, Vienna (Austria); Nuclear Energy Agency, 75 - Paris (France); 258 p; ISSN 1011-4289; 
; Jun 1992; p. 13-22; Technical committee meeting on new developments in uranium exploration, resources, production and demand; Vienna (Austria); 26-29 Aug 1991
; Jun 1992; p. 13-22; Technical committee meeting on new developments in uranium exploration, resources, production and demand; Vienna (Austria); 26-29 Aug 1991
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