O GREENSTONEBELTDO RIO ITAPICURU: UMA BACIA DO TIPO BACK-ARC FÓSSIL

Abstract

A model for the geotectonic evolution of the Rio Itapicuru granite-greenstone terrain is proposed herein, based on the integration of geochemical, metamorphic and structural data. Although spilitized, the basalts preserved geochemical signatures typical of ocean floor tholeiites (OFB). They show flat to enriched patterns of LREE, and flat patterns of HREE contrasting with negative and low values of epsilon Nd, thus, pointing to an environment compatible with a marginal basin. The andesitic and dacitic lavas and pyroclastics display a calc-alkaline affiliation, and were formed, at least in part, in a subaerial environment similar to a continental margin volcanic arc. The sintectonic granite-gneiss domes show calc-alkaline affiliation, with an origin in the mantle. The entire sequence was affected by one main deformational event related to the closure of the basin and the emplacement of the granite-gneiss domes resulting in a sinformal structure. The metamorphic grade varies from amphibolite at the margins grading to greenschist facies in the center. Geochronological studies point to the existence of an Archaean basement (2.9 Ga). The supracrustal sequence however, has An early Proterozoic age (2.5-1.8 Ga). These characteristics support the interpretation of the Rio Itapicuru greenstone belt as a supracrustal sequence accumulated in a back-arc basin, related to a collision/subduction process at the begining of the Proterozoic.