|
Roberto Dall'Agnol, Federal University of Para (Brazil)
Marcelo Oliveira, Federal University of Para (Brazil)
Jose Almeida, Federal University of Para (Brazil)
Davis Oliveira, Federal University of Para (Brazil)
Fernando Althoff, Federal University of Para (Brazil)
Carlos Barros, Federal University of Parana (Brazil)
Albano Leite, Companhia Vale do Rio Doce (Brazil)
|
|
The Carajás Metallogenic Province (CMP), the more important Archean province of the Amazonian craton, is divided in two major tectonic domains: the Rio Maria Granite-Greenstone Terrane (RMGGT) and the Carajás Basin (CB). These domains are separated by a Transition zone, originally similar to the RMGGT but intensely affected by the magmatic and tectonic events recorded in the CB. There are important differences between the RMGGT and the CB: (1) The supracrustal sequences of these domains differ in age (2.97 to 2.9 Ga in the RMGGT vs. ca. 2.76 Ga in the CB) and petrologic characteristics. (2) The rocks exposed in the RMGGT were formed in between 2.98 and 2.86 Ga and were affected by a shear event at ∼2.87 Ga. In the CB, the main stratigraphic units formed in the period of 2.76 to 2.70 Ga and the last Archean deformational and metamorphic event has been dated at 2.58 to 2.50 Ga. The tectonic stabilization of the RMGGT preceded that of the CB. (3) The Archean granitoids of the RMGGT are similar to those found in classical Archean terranes and yielded ages between 2.98 and 2.86 Ga, while in the CB, 2.75 Ga to 2.57 Ga old, subalkaline A-type granites are dominant. (4) In the RMGGT, a back-arc setting was admitted for the evolution of the mafic-ultramafic sequences of the greenstone belts (2.97 Ga). It evolved to a subduction-related magmatic arc, during the generation of the 2.87 Ga granitoids. In the CB, two models are proposed: the first model admits that the supracrustal sequences were related to a continental rift tectonic setting; the second assume a continental margin setting, which evolution was related to the subduction of an oceanic crust, followed by a continent-continent collision. Nd isotope data from the Paleoproterozoic granites of the CB favor an ensialic evolution. Six Archaean granitoid groups (3.0 to 2.57 Ga) and one Paleoproterozoic anorogenic granite group (1.88 to 1.86 Ga) were distinguished in the CMP. The magmatism of the RMGGT is similar to that recorded in Archean terranes. The Neoarchean magmatism of the CB is very peculiar reflecting its distinct tectonic evolution. The dominance in the CB of subalkaline granites with A-type geochemical signature is remarkable. The anorogenic Paleoproteorozoic A-type magmatism is found in both tectonic domains and is associated with an extensional tectonic setting.
|
