STRUCTURE, PETROLOGY, GEOCHEMISTRY AND ZIRCON U/Pb AND Pb/Pb GEOCHRONOLOGY OF THE SYNKINEMATIC ARCHEAN (2.7 Ga) A-TYPE GRANITES FROM THE CARAJÁS METALLOGENIC PROVINCE, NORTHERN BRAZIL

Abstract : The Archean geological evolution of the Carajás metallogenic province is marked by A-type granite emplacement coevally with a regional shortening episode at 2.7 Ga. These A-type granites intruded metavolcano-sedimentary series of the Itacaiúnas Supergroup. We studied three massifs: Igarapé Gelado, Estrela and Serra do Rabo. The granites present low Al2O3 values, high FeOt/(FeOt + MgO) and (K2O + Na2O)/CaO values, along with high contents of Zr, Y, Nb and rare-earth elements. The Sm–Nd model ages (2.97 Ga to 3.2 Ga) and the slightly negative {varepsilon}Nd(t) values (–0.38 to –2.06), obtained in the Estrela granite complex suggest that the magmas were most probably produced by partial melting of ca. 3.0 Ga rocks. Coexistence of ilmenite, ferropargasite, hastingsite and hedenbergite suggest crystallization of high-temperature melts under low fugacity of oxygen. The granites induced high-temperature ductile deformation in the contact aureole. The structural patterns of the granites suggest that their emplacement and deformation result from a continuous evolution under decreasing temperature due to the interference between inflation and horizontal shortening. The A-type granites, when first discriminated, were considered to be restricted to anorogenic or rift-related environments. Our data corroborate the fact that A-type granites can abridge a wide family of rocks formed in several tectonic regimes. The characteristics of the Carajás A-type granites point to an orogenic history considered to be related to an environment of magmatic arc.