Estratigrafia e geocronologia do greenstone belt de Guarinos, Goiás

Abstract

The Guarinos greenstone belt is one of the three low metamorphic grade supracrustal rocks assemblage rimmed by orthogneisses of Central Brazil's Archean Block northern limits. The investigation of the stratigraphy of those greenstone belts started by the end of 1970's and underwent improvements during the years as a result of detailed geological mapping. The latest and accepted stratigraphic model for the Guarinos greenstone belt refers its supracrustal rocks under the Guarinos Group, which is subdivided into the Serra do Cotovelo (metakomatiites), Serra Azul (metabasalts), Sao Patricinho (mafic metaturbidites), Aimbe (BIF) and Cabacal (carbonaceous phyllites and impure metarenites) formations. Detailed geologic mapping (1:10,000) and drill-cores during an exploration program by Yamana Desenvolvimento Mineral S.A. in the area allowed a better definition of the metasedimentary package of the Cabacal Formation, which is here formally proposed to be subdivided into a Lower Member of carbonaceous phyllites with basalt lava flows and gondite lenses interlayers, an Intermediate Member of gondite, iron formation, metachert and massive barite lenses, and an Upper Member of carbonaceous phyllites with minor metachert lenses. The impure metarenites, formerly considered as part of the Cabacal Formation, are proposed under the Mata Preta Formation, which is laterally interfingered with the Cabacal Formation. U-Pb LA-ICP-MS geochronological data of detrital zircon grains from the Sao Patricinho mafic metaturbidites and the impure metarenites of the Mata Preta formations indicate that the major source-area of the clastic load had a Siderian to Rhyacian age, with minor contribution from Archean rocks. From the rock assemblage of both units and their contact relationships it is concluded that the basin stage they represent evolved during the world-wide Anoxic Oceanic Event (AOE) that took place during the 2.2 to 2.06 Ga, represented by the carbonaceous phyllites of the Cabacal Formation, coeval with the erosion of a magmatic arc that derived the immature clastic load of the Mata Preta Formation.