Nd, Pb and Sr isotopes in the Identidade Belt, an Archaean greenstone beltof the Rio Maria region (Carajas Province, Brazil): implications for the Archaean geodynamic evolution of the Amazonian Craton

The Rio Maria granite-greenstone terrain (RMGGT), located in the Amazonian craton at about 250 km south of the Carajas Ridge, is a well preserved Meso archaean domain composed of metavolcanic rocks and several suites of granit oids 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 (t holeiites) and metadacites gives an age of 3046 +/- 32 Ma (MSWD of 1.6, 1 s igma). Nd model ages (T-DM) of the metadacites range between 3.04 and 3.11 Ga. The epsilon (Nd) 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 (1 sigma) obtained for the metadacite s. Rb-Sr systematic for the metadacites yielded an isochron age that is ca. 400 Ma youngest. The isotopic, geochemical and geological data suggest tha t oceanic lithosphere was the probable source of the ultramafic and mafic r ocks in such a way that partial melting of the depleted mantle generated ko matiites and low-K tholeiites. Partial melting of oceanic crust transformed into garnet amphibolite or eclogite generated the parental magmas of the m etadacites. Isotopic and trace element characteristics of both metavolcanic rocks and associated plutonics suggest intra-oceanic island arcs as the mo st probable tectonic setting of the RMGGT. Two main events of crustal accre tion are recognised at about 3.04-2.96 Ga and 2.87 Ga. Geological correlati on indicates that the oceanic island arcs were completely amalgamated at ar ound 2.76 Ga and acted as sources for the overlying sediments of the Rio Fr esco Group. (C) 2001 Elsevier Science B.V. All rights reserved.