GLOSSÁRIO GEOLÓGICO ILUSTRADO
Pillows basálticas do greenstone belt Identidade, sul do Para
Estas pillows ocorrem na seqüência do greenstone belt Identidade, sul do Para, região de Rio Maria - Xinguara (07°12'S x 49°26'W), de idade em torno de 3.1 Ga (ver referencia e abstract abaixo). Os basaltos são toleitos de baixo potassio, fracamente metamorfisados (facies xisto verde), com preservação de texturas intersertal e amigdaloidal, e são considerados como originados da fusão parcial do manto empobrecido.
© Foto e legenda de Zorano Sérgio de Souza (reprodução autorizada citando-se a fonte e créditos)
Souza,Z.; Potrel,A.; Lafon,J.M.; Althoff,F.;
Pimentel,M.M.; D'Allagnol,R.; Oliveira,C.G. 2001. Nd, Pb and Sr of the Identidade
Belt, an Archaean greenstone belt of the Rio Maria region (Carajás Province):
implications for the Archaean geodynamic evolution of the Amazon. Precambrian
ABSTRACT: The Rio Maria granite-greenstone terrain (RMGGT), located in the Amazonian craton at about 250 km south of the Carajás Ridge, is a well preserved Mesoarchaean domain composed of metavolcanic rocks and several suites of granitoids ranging in age from ca. 2.96 to 2.87 Ga. The metavolcanics comprise a basal unit of komatiites and low-K tholeiites overlain by sodic metadacites. A Sm-Nd whole-rock reference isochron for the metabasalts, metagabbros (tholeiites) and metadacites gives an age of 3046±32 Ma (MSWD of 1.6, 1S). Nd model ages (TDM) of the metadacites range between 3.04 and 3.11 Ga. The ENd calculated at 3.0 Ga varies from +1.70 to +2.60 for the tholeiites and +0.30 to +3.22 for the metadacites. A Pb-Pb whole-rock linear array for the metabasalts yielded an anomalous older age of 3.41 Ga. The Sm-Nd age is coincident, within analytical error, with a Pb-Pb whole-rock errorchron age of 2944±88 Ma (1S) obtained for the metadacites. Rb-Sr systematic for the metadacites yielded an isochron age that is ca. 400 Ma youngest. The isotopic, geochemical and geological data suggest that oceanic lithosphere was the probable source of the ultramafic and mafic rocks in such a way that partial melting of the depleted mantle generated komatiites and low-K tholeiites. Partial melting of oceanic crust transformed into garnet amphibolite or eclogite generated the parental magmas of the metadacites. Isotopic and trace element characteristics of both metavolcanic rocks and associated plutonics suggest intra-oceanic island arcs as the most probable tectonic setting of the RMGGT. Two main events of crustal accretion are recognised at about 3.04-2.96 Ga and 2.87 Ga. Geological correlation indicates that the oceanic island arcs were completely amalgamated at around 2.76 Ga and acted as sources for the overlying sediments of the Rio Fresco Group.
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